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LIBKAKY I ~
UNITED STATES GEOLOGICAL SURVEY
CHARLES I). WALCOTT, DIRECTOR
A
TREATISE ON METAMORPHISM
BY
CHARLES RICHARD VAN HISE
WASHINGTON
GOVERNMENT PRINTING OFFICE
1904
CONTENTS.
Page.
I .KTTKU OF TRAXSMITTAL -
CHAPTER I. INTRODUCTION 31
General nature of alterations 31
Classification of metamorphism 39
Geological factors affecting the alterations of rocks 40
Composition 40
Structures and textures 40
Porosity 40
Water and gaseous content 41
Climatic and geographic conditions 41
Time 41
Environment 42
Degree of movement 42
Depth.... 43
CHAITER II. THE FORCES OF METAMORPHISM 45
Chemical energy 45
Gravity 46
Mechanical action * 46
Molecular mechanical action 47
Mass mechanical action 49
Permanent strain without openings 49
Permanent strain with openings 49
Permanent strain with closing of openings and welding 50
Water action 50
Heat and light 51
Sources of heat and light 51
The sun as a source of heat and light :'. 51
Heat derived from within the earth by conduction or convection through water
or magma 53
Mechanical action as a source of heat 54
Chemical action as a source of heat 54
Effects of heat and light on alterations of rocks 54
Direct effects of heat and light 55
Mechanical effects 55
Chemical effects 56
indirect effects of heat and light 56
General statements 57
5
6 CONTENTS.
/
Page.
CII.MTER III. THE AGENTS OK METAMORPHISM ">S
General statement 5s
Part I. Gaseous solutions 59
Section 1 . Chemical ami physical principles controlling the action of gases 60
Gases | iresent CO
The pressure 61
The temperature 61
Section 2. Geological work c >f gases 6L'
Part II. Aqueous solutions and solids 63
General considerations 63
Section 1. Chemical and physical principles controlling the action of ground water.. 65
Principles of solutions applicable to ground water 65
Solution of gases in ground waters 68
Gases present 68
The pressure 70
The temperature 72
Solids in solution 72
Solution of solids in ground water 72
Compounds present 76
Relations of solution and pn ssure 77
Relations of solution and temperature 79
Speed of solution 79
Quantity of material which may be held in solution 79
Relations of solution to absorption and liberation of heat 81
Diffusion 82
Principles of chemical reactions applicable to ground waters 84
General statement , 84
Definitions 84
Dissociation 84
Hydrolysis 86
Reactions 87
Equilibrium 90
Homogeneous and heterogeneous systems 90
Nature and speed of reactions 91
The compounds 91
Strength of the solutions 94
Mechanical action 95
Speed of chemical action 95
Direct deformation effect 95
SI rain without rupture '. 95
Strain with rupture 98
Readjustment of particles 98
Indirect heat effect 99
Nature of the chemical reactions 100
CONTENTS. 7
CII.UTKR III. THE AGENTS OF METAMOUI-IIISM Continued. page.
Part II. Aqueous solutions ami solids Continued.
Sirtion 1. Chemical and physical principles, etc. Continue*,
Principles ill chemical reactions applicable to ground waters Continued.
Nature and speed of reactions Continued.
Mechanical action Continued.
Nature of the chemical reactions Continued.
Smaller rock volume as the result of solution and deposition
without change in chemical composition 101
Recrystallization and condensation without change of min-
erals 101
Recrystallization and condensation with change of minerals. 10:2
Smaller volume as the result of solution and redeposition with
change in chemical composition 103
Crystallization and condensation of amorphous compounds. 103
Recrystallization and condensation of crystallized com- J
pounds 103
General statements 104
Heat 105
Relations of chemical action, mechanical action, and heat 110
Precipitation 113
I 'nvipitation by change of pressure 1 14
Precipitation by change of temperature 1 15
1'recipitation by reactions between aqueous solutions 116
Precipitation by reactions between aqueous solutions and gases 1 Hi
I recipitation by reactions between solutions and solids 120
Section 2. Circulation and work of ground water 123
I'niversal presence of water in rocks 123
Pore space of rocks 125
Circulation of ground water 129
Openings in rocks 129
Form and continuity of openings 129
Sixc of openings 134
Supereapillary openings .__: 137
Capillary openings 138
Rubcapillary openings 143
Percentage of openings, or pore space 146
Forces producing water circulation 1 46
( ! ravit y 146
Heat 147
Gravity and heat 148
Mechanical action 149
Molecular attraction 150
Vegetation 152
General statements.. 152
8 CONTENTS.
CHAPTER III. THE AGENTS OK METAMORI-HISM Continued. Page.
Part II. Aqueous solution." ami solids Continued.
Section 2. Circulation and work of ground water Continued.
Circulation of ground water Continued.
The factor opposing water circulation 153
General statements 154
Geological work of ground water ' 156
CHAPTER IV. THE ZONES AMI BELTS OF METAMOKIMIISM 151)
< M-neral considerations 159
Zone of katamorphism 160
Belt of weathering 163
Belt of cementation 164
Belts of weathering and cementation contrasted : 166
Zone of anamorphism .' 167
Relations of zones of katamorphism and anamorphism 170
General considerations 186
Relations of zones of katamorphism and anamorphism to zones of fracture and flowage . . 187
Upper limit of zone of flowage 187
CHAPTER V. MINERALS 192
Section 1. Chemical and mineral composition of the known crust of the earth 192
Section 2. General nature of alterations 202
Alteration without change in chemical composition _ . 202
Molecular rearrangement 202
Simple recrystallization 202
Alteration with change in chemical composition 202
Alteration without addition or subtraction of material 203
Alteratipn with addition or subtraction of material 2u:>
General statements 206
Section 3. Rock-making minerals 207
Manner of treatment 207
General statements 207
Native elements 212
Graphite 212
Occurrence 212
Alterations 21 2
The sulphides 212
Pyrrhotite, pyrite, and marcasite 213
Occurrence 213
Alterations 214
The fluorides 216
Fluorite _ 216
Occurrence 216
Alterations 216
The oxides 217
Quartz 21 7
Occurrence 21 7
CONTENTS. 9
CHAPTER V. MINERALS Continued. y Page.
Section 3. Rock-making minerals Continued.
The oxides Continued.
Quartz Continued.
Modifications 218
Tridymite 220
Occurrence 220
Modifications 220
Opal 221
Occurrence 221
Modifications 221
Chert, chalcedony, etc 222
Hematite group 223
Corundum, hematite, and ilmenite 223
Corundum 223
Occurrence 223
Alterations 223
Hematite 225
Occurrence 225
Alterations 226
Ilmenite 227
Occurrence 227
Alterations 227
Spinel group 228
Spinel, magnetite, and chromite 228
Spinel 228
Occurrence 228
Alterations 228
Magnetite 229 '
Occurrence 229
Alterations 229
Chromite 229
Occurrence 229
Alterations 230
Entile group 230
Rutile, octahedrite, and brookite 230
Occurrence 230
Alterations 230
Diaspore group 231
Diaspore and limonite 231
Diaspore 232
Occurrence 232
Alterations 232
Limonite '. 232
Occurrence 232
Alterations 233
i
10 CONTENTS.
CII.UTEK V. MINERALS Continued. Page
Section 3. Rock-making mineral.-' Continued.
The oxides Continued.
Ilrucite group
Brucite and gibbsite
Brucite '-*
Occurrence 235
Alterations -'''
Gibbsite 235
Occurrence 235
Alterations 2:!5
The carbonates 236
Calcite group 237
Calcite, dolomite, ankerite, parankerite, magnesite, and siderite 237
Calcite 237
Occurrence 237
Alterations
Dolomite 240 .
Occurrence '-- 240
Alterations -41
Ankerite and parankerite 242
Occurrence 242
Alterations 242
Magnesite - - - 243
Occurrence 243
Alterations 243
Siderite 244
Occurrence 244
Alterations 244
Aragonite group 245
Aragonite 245
Occurrence 245
A Iterations 245
The silicates 24(>
Glass 246
Occurrence 247
Evidence that devitrification takes place 247
Scale of devitrification 247
Rate of devitrification 248
Devitrification in the two zones 24i>
Minerals produced <- 251
Heat and volume relations 251
Feldspar group
Munoclinic or peeudtimonoclinic 253
Orthoclase, microcline, and anorthoclase 253
Orthoclase and microcline 253
CONTENTS. 1 1
CHAPTER V. MINERALS Continued. Page.
Sections. Rock-making minerals Continued.
The silicates Continued.
Feldspar group Continued.
Monoclinic or pseudonoonoclinic Continued.
Oltboclase, microcline, and anorthoclase C'oiitinued.
Orthoclase and microcline Continued.
Occurrence 253
A Iteral ions'. 253
Anorthoclase 257
Occurrence L'.">7
Alterations 258
Triclinic LTi!)
Albite, oligoclase, andesine, labradorite, bytownite, and anorthite 259
Occurrence 259
A Iterations 260
Leucite group 266
Leucite 26(3
Occurrence 266
Alterations 266
Pyroxene group 267
Orthorhombic pyroxenes 267
Enstatite, bronzite, and hypersthene 267
Occurrence 268
Alterations 268
Monoclinic pyroxenes 271
IMopside, sahlite, hedenbergite, augite, acmite, spodumene, wollastonite,
and pectolite 27 i
Occurrence 272
Alterations of the diopside-augite series 273
Alterations of pyroxenes other than the diopside-augite series 280
Amphibole group 281
Orthorhombic amphiboles. 281
Anthophyllite and gedrite -. 281
Occurrence 281
Alterations ; 282
Monoclinic amphiboles 283
Tremolite, actinolite, cummingtonite, griinerite, hornblende, glauco-
phane, riebeckite, and arf vedsonite 283
Occurrence 28'5
Alterations ^s;,
lolite (cordierite) 291
Occurrence 291
Alterations 291
Nephelite group 2112
Nephelite and cancrinite 2UL>
12 CONTENTS.
CHAPTER V. MINERALS Continued. i^,,.
Section 3. Rock-making minerals Continued.
The silicates Continued.
.Jsephelite group Continued.
Nephelite and cancriniti Continued.
Nephelite 292
Occurrence 292
Alterations 292
Cancrinite 2ii4
Occurrence 294
Alterations 294
Sodalite group '. 295
S< idalite, haiiynite, and noselite 295
Sodalite . 295
Occurrence 295
Alterations 2!)5
Haiiynite and noselite 297
Occurrence 2H7
Alterations 298
Garnet group 299
Grossularite, pyrope, almandite, spessartite, melanite, and uvarovite 299
Occurrence 300
Alterations 302
Chrysolite group 308
Forsterite, olivine, and fayalite 308
Occurrence 308
Alterations 308
Srapolite group 311
Mcionite, wernerite, and inarialite 311
Occurrence 312
Alterations 312
Melilite 314
Occurrence 314
Alterations 314
( iehlenite 3] 4
Occurrence 314
Alterations 314
Vesuvianite ;;15
Occurrence 315
Alterations 315
Zircon gn nip 315
Occurrence 315
Alterations 315
Aluminum-silicate group 316
Topaz, andalusite, sillimanite, and oyanite :;i ii
Occurrence 316
Alterations 31g
CONTENTS. 13
ClI.UTER V. MlXKK.U.S Continued. Page.
Section 3. Rock-making minerals Continued.
The silicates ( 'ontinued.
Epidiite group 320
Zoisite, epidote, piedmontite, and allanite 320
Occurrence 320
Alterations 322
Axinite 323
Occurrence 323
Alterations 324
Prehiiite 324
Occurrence 324
Alterations 324
Humite group 325
Chondrodite, humite, and clinohumite 325
Occurrence 325
A Iterations 325
Tourmaline 326
Occurrence 326
A Iterations 326
Staurolite 327
Occurrence 327
Alterations 327
Zeolite group 329
Thomsonite, hydronephelite, natrolite, mesolite, scolecite, analcite, apophyl-
lite, epistilbite, heulandite, stilbite, phillipsite, harmotome, gismondite,
chabazite, gmelinite, and laumontite 329
Occurrence 331
Alterations 333
Mica group 336
Muscovite, paragonite, biotite, and phlogopite 336
Muscovite 336
Occurrence 336
Alterations -.' 337
I 'arugonite 338
Occurrence 338
Alterations 338
Biotite - 339
( >ccurrence 339
Alterations 339
Phlogopite '. 343
Occurrence 343
A Iterations 343
Clintonite group 344
Margarite, chloritoid, and ottrelite 344
Occurrence :'.44
Alteration- . . 345
14 CONTENTS.
CHAPTER V. MINERALS Continued. Page.
Section 3. Rock-making minerals Continued.
The silicates Continued.
Chlorite group 345
Ameaite, corundophilite, prochlorite, clinochlore, and penninite 345
Occurrence 346
Alterations 347
Serpentine-talc group
Serpentine and talc 348
Serpentine
Occurrence 349
Alterations 349
Talc 350
Occurrence 350
Alterations 351
Glauconite 351
^ Occurrence 351
Alterations 351
Kaolin group 352
Occurrence 352
Alterations :. 352
Summary of alteration of silicates 352
The titanates 354
Titanite and perovskite 354
Titanite 354
Occurrence 354
Alterations 355
Perovskite 355
Occurrence 355
Alterations 355
The phosphates 356
Apatite 356
Occurrence 356
Alterations 356
The sulphates 357
Anhydrite and gypsum 357
Anhydrite 357
Occurrence 357
Alterations 357
Gypsum 357
Occurrence 357
Alterations 358
Section 4. General statements 359
Physical-chemical factors upon which nature of alterations depends 359
Chemical composition 359
CONTENTS. 15
CHAPTER V. MINERALS Continued.
Section 4. < ieneral statements Continued.
Physical-chemical factors upon which nature of alterations depends Continued.
Chemical composition 359
Chemical composition of adjacent minerals 359
Chemical composition of circulating solutions 359
Specific gravity 360
Symmetry 360
Specific gravity and symmetry 361
Heat reactions 362
Pressure and volume 363
Reversible reactions 366
Sections. Tables 369
Table A. Sources of minerals 369
Table B. Alteration products of minerals 372
Table C. Chemical reactions and volume changes ._. 375
Table D. Classification of alterations, with volume changes 395
CHAPTER VI. THE BELT OF WEATHERING 409
Belt of weathering denned -, 409
Form of level of ground water 411
Amount and source of water in belt of weathering 413
The circulation 416
Downward movements of water 417
Upward movements of water 419
Molecular attraction 419
Vegetation 422
Variation in level of ground water 428
Precipitation, seepage, and evaporation 423
Uplift and subsidence 426
Denudation and valley tilling 426
Influence of man 427
Barometric pressure 428
Temperature
General statements 429
Metamorphism in the belt of weathering 429
Variable materials and conditions of belt of weathering 429
Mechanical work 431
Water, ice, and wind 432
Change in temperature ;
Change from water to ice 440
Plants 444
Lichens, mosses, etc
Cacti 445
Grasses, grains, and vegetables 445
Shrubs and trees 445
16 CONTENTS.
C'HAJTER VI.- THE BELT OF WEATHERING Continued. Page.
Metamorphism in the belt of weathering Continued.
Mechanical work Continued.
Animals 447
Earthworms 44S
Aiits, termites, and other insects 448
The larger burrowing animals 449
Man : 450
General statements 451
Chemical work 4.51
The agents 45:.'
Plants. . 451'
Plants, alive 452
Plants, dead, and bacteria 455
Animals 456
Animals, alive 456
Animals, dead, and bacteria 457
Work of solutions 457
Joint work of agents of weathering 461
Oxidation 461
Oxidation of organic compounds i 461
( )xidation of carbon and hydrogen 461
Oxidation of nitrogen 465
Oxidation of inorganic compounds 466
Iron 467
Sulphur 468
General statements 469
Carbonation 473
Hydration and dehydration 481
Oxidation, carbonation, and hydration 483
Solution 484
Deposition . 487
General statements 487
Contact metamorphism 488
Direct contact effect 489
Indirect contact effect, or work of fumaroles and solfataras 490
Relations of disintegration to decomposition and solution 494
Regions favorable to prominence of disintegration ' 496
Arid regions 496
Regions of high latitude 498
Regions of marked topographic relief 499
Regions of sparse plants and animals 500
Regions near the sea 500
Regions favorable to prominence o f decomposition 501
Humid regions 501
Regions of low latitude 502
CONTENTS. 17
CHAPTER VI. THE BELT OF WEATHERING Continued. Page.
Metamorphism in the belt of weathering Continued.
Relations of disintegration to decomposition and solution Continued.
Regions favorable to prominence of decomposition Continued.
Regions of moderate topographic relief 502
Regions of abundant plants and animals 503
Regions remote from the sea 504
General statements 504
Change in chemical composition of the rocks 507
Order of decomposition of the minerals, and the end products 518
Total gains and losses in weathering, and changes in volume 522
Emphasis and retention of structures and textures 524
Obliteration of structures and textures 526
Surfaces of weathering 527
Depth and degree of weathering 529
Rate of weathering 532
Forces and agents at work in weathering 532
Materials weathered 532
Chemical composition 532
Mineral composition 533
State of aggregation 533
Thickness of the belt of weathering 534
Stage of weathering 535
Distribution of dissolved materials ' 536
Material abstracted by plants 537
Material transferred to belt of cementation 538
Material permanently abstracted by run-off 538
Material dissolved, transported, and reprecipitated in belt of weathering 539
Concentration at and near the surface 543
Concentration by underground circulation 543
Concentration by circulation mainly confined to belt of weathering 544
Concentration by circulation extending into belt of cementation 550
Concentration by overground circulation 551
Distribution of residual materials 554
Relations of belt of weathering to sedimentary rocks 555
Belt of weathering the source of sedimentary rocks 555
Material transported in suspension 556
Material transported in solution 557
Material transported in suspension and solution 558
Rocks produced from material of belt of weathering without transportation to the .
sea 559
Transition between belt of weathering and belt of cementation 560
CHAPTER VII. THE BELT OP CEMENTATION 562
Belt of cementation denned 562
Boundaries of belt of cementation 565
MON XLVII 03 2
18 CONTENTS.
CHAPTER VII. THE BELT OP CEMENTATION Continued. Pa ce
Condition of water in belt of cementation iiiid
Amount of water in belt of cementation "xi'.i
Circulation of water in belt of cementation 571
Vigor of circulation 57!
Character of circulation ">72
Limiting formations 57t>
Gravity 578
Increase of temperature with depth 57s
Relative lengths of vertical and horizontal components 57!i
Preferential use of large channels 580
Resultant circulation 582
General statement 589
Temperature of entering and issuing water, and transfer of heat 5s;i
Variable materials and conditions of belt of cementation 594
Work in belt of cementation 59 1
Mechanical work 594
Consolidation 5ii5
Strain within elastic limit 597
Strain beyond elastic limit 5!i'.i
Chemical work (id-
Chemical changes ( 602
Oxidation 604
Carbonation 608
Hydration 612
Solution and deposition 612
Quantitative relations between solution and deposition 613
Resultant processes (>l 7
Cementation 617
Cementing substances 621
Oxides 622
Silica 622
Iron oxides 623
Aluminum oxides 624
Carbonates 624
Calcite and dolomite 624
Siderite 625
Silicates 625
Sulphides 627
Distribution of elements in cementing minerals 627
Distribution of cementing minerials 628
Causes of cementation <;L".<
Expansion reactions 631
Contributions from igneous rocks ...'. 634
Selective precipitation 634
Diffusion 636
Conclusion .. 639
CONTENTS. 19
VII. THE BELT OF CEMENTATION Continued. Pag e .
Work in licit of cementation Continued.
Chemical work Continued.
Resultant processes Continued.
Metasomatism 640
Definition 640
Extent of process 640
Conditions favorable to metasomatism 641
Minerals produced 642
(irowth of large individuals and preservation of textures 643
Segregation of individual minerals 645
Igneous work 646
Injection 646
Combinations and relations of mechanical work, chemical work, and igneous work.. 653
Changes of chemical composition 655
CHAPTER VIII. THE /O.N-E OF ANAMORPHISM 657
Definition of z< me 657
Condition of water 65S)
i Quantity of water 661
Circulation of water 661
Variable materials and conditions 6(58
Work in zone of anamorphism 670
Mechanical work 670
Welding 670
Strain within elastic limit 671
Strain beyond elastic limit 673
Chemical work 675
Chemical changes 676
Deoxidation 676
S i 1 i cation 677
Dehydration i;7!>
Solution and deposition / 680
Resultant processes 681
Cementation _ 681
Metasomatism 682
Minerals formed 683
Alterations in connection with mass-mechanical action 685
Recrystallization 686
Facts of recrystallization 686
Theory of reerystallization ' 690
Recrystallization lags behind deformation 696
Conclusion 698
Alterations under mass-static conditions 698
Development of porphyritic textures 699
Regeneration of mineral particles 705
20 CONTENTS.
t
( HAITEH VIII. TitK /ONE OF ANAMORPHISM Continued. Page.
\\"<>rk in /om- of anamorphism Continued.
Igneous work 707
Injection 707
Manner of intrusion 708
Eesultant metamorpbiam 7]]
Factors controlling metamorphism 711
Size of intrusive masses 711
The temperature 712
Amount of water present 712
Composition of intrusive and intruded rocks 713
Metamorphic effects 716
Structures 716
The minerals 717
Pegmatites _ 720
Fusion and absorption 728
Combinations and relations of the various processes 736
Relations of granulation and recrystallization 737
Character of material 738
Temperature 740
Pressure and rapidity of deformation 741
Water content 741
Rock flowage 748
Meaning of rock flowage 74S
Conclusion 759
Meaning of rock cleavage 760
Effect of rock flow on textures and structures 760
Rock flowage and mashing 762
Changes in chemical composition 764
Relations of zone of anamorphism to zone of katamorphism 766
Comparative energy required for deformation in zones of katamorphigm and ana-
morphism - 769
Conclusion 774
CHAPTER IX. ROCKS 77")
Use of some general terms applied to metamorphic rocks 77ti
Meta 776
Apo 776
Slate and schist 778
Slate 778
Schist .' 779
Gneiss 782
General statements 783
Sedimentary rocks 784
Nonfragmental class ' 787
Nitrate order 787
Niter familv 7S7
CONTENTS. 21
CHAPTER IX. ROCKS Continued. Page.
Sedimentary rocks Continued.
Nonf ragmen tal class Continued.
Sulphate order 788
(iypsnin and anhydrite family 788
Chloride order 789
Uork-salt family 789
Carbonate order 791
Calcium-magnesium carbonate family and metamorphosed equivalents 791
Limestones 791
Source of material of limestones 791
Organic precipitates 792
Chemical precipitates 793
Springs and streams 793
Inland seas with no outlets 793
Possible chemical precipitates in the ocean or in seas con-
nected with the ocean 793
Metamorphigm of organic and chemical calcium carbonate deposits. . 795
Dolomite 798
Origin of dolomite 798
Dolomite due to replacement of calcium by magnesium 798
Conclusion 802
How and why dolomitizatioii occurs 802
Dolomitization before limestone emerges from the sea 802
Dolomitization after limestone emerges from the sea 804
Marble 808
Cherty limestones, cherty dolomites, and eherty marbles 816
Silicated marbles 820
Silicate rocks 822
General statements 823
Iron-bearing carbonate family and metamorphosed equivalents 823
Siderite, ankerite, and parankerite 823
Origin of siderite, ankerite, and parankerite 824
Ferruginous shales, ferruginous cherts, and jaspilites 829
Ferruginous shales 830
Ferruginous cherts 830
Jaspilites 831
Actinolitic and griineritic marbles 833
Actinolite-magnetite-quartz rocks and grunerite-magnetite-quartz rocks. 834
General statements 841
Oxide order 842
Iron-oxide family 342
Limonite 842
Hematite 843
Magnetite 845
22 CONTENTS.
CHAPTER IX. ROCKS Continm-cl. !;,,.,.
Sedimentary rocks Continued.
Nonfragmental class Continued.
Oxide order Continued.
Silica family 847
Chert 847
Rearrangement of chert sin
Fragmental class 85:!
Psephite order s:>.;
Pebble, gravel, and bowlder deposita sr>:;
Conglomerates S.">5
Schist-conglomerate and gneiss-pgephite, or conglomerate-schist and
]isc| ihitc-jruciss s:>7
Psammite order x ii i
Quartz-sand family M;O
Quartz-sand mck 860
Sandstone sii4
Quart/.ite sii5
Schist-quartzite or quartzite-schist SliS
Quartz-feldspar-sand family 870
Quartz-feldspar sand S70
Arki ise 874
Schist-arkose and gneiss-arkose, or arkose-schist and arkose-gneiss 875
Ferromagnesian-sand family S77
Ferromagnesiao sands 877
Grits , 879
Gray wacke 880
Slate-gray vvacke, schist-graywacke, ami iznoiss-graywacke; orgraywacke-
. slate, gray wacke-schist, and graywacke-gneiss 883
Pelite order 886
Mud family 886
Shale family 8WL'
Slate-pelite, schist-pelite, and gneiss-petite; or pelite-slate, pelite-schist,
and pelite-gneiss s'.i 1
^Development of minerals of slates 898
Development of minerals of schists and gneisses SIM
Igneous rocks 1104
CHAPTER X. THE RELATIONS OF METAMOHPHISM TO STRATIOKAPHV !)07
Introductory !)()7
Discrimination between metamorphosed sedimentary and metamorphosed igneous rocks. 908
Cases of confusion 909
Criteria for discrimination '.Hi'
Relations of metamorphic sedimentary rocks to stratigraphy !U 7
Variation in metamorphism : 917
Upon what variations are dependent 917
Resulting variations 918
CONTENTS. 23
CHAPTER X. THK RELATIONS OK MBTAMORPHISM TO STRATIGRAPHY Continued. p s e
Ui 'hit inns of igneous roeks to stratigraphy 922
Relations of rock flowage to mountain making 924
< 'IIAITER XI. RELATIONS OF METAMOKPHISM TO THK DISTRIBUTION OF THE CHEMICAL ELEMENTS. 932
Composition of the iithoephere 932
Table of analyses of igneous and crystalline rocks 934
Table giving symbols, atomic weights, and proportions of the twenty-three most
abundant elements in the outer 10 miles of the earth 936
Table showing the amounts of the eleven most common oxides of the lithosphere, ;is
estimated in 1891 and 1900 937
Composite analyses of sedimentary rocks 938
Constituents of meteorites 94."i
Redistribution of the chemical elements 947
Oxygen 948
Sulphur 957
Silicon !i.V.i
Carbon s 962
Amount of carbon 962
Segregation of carbon 964
Segregation by carbonation 964
Segregation in carbonaceous deposits 966
Sources of segregated carbon 967
Titanium 974
. Phosphorus H~r>
Chlorine : 978
Nitrogen 980
Hydrogen 981
Aluminum (is:;
I ron 986
Manganese 989
Calcium 990
Magnesium 992
Sodium 996
Potassium 999
Barium, strontium, chromium, nickel, lithium, fluorine, bromine 1002
General statements 1 1002
CHAPTER XII. THE RELATIONS OK METAMORPHISM TO ORE DEPOSITS 1004
Part I. General principles 1004
Introductory 1004
Classification of ore deposits 1005
Deformation of the lithosphere 1005
Zone of fracture, or zone of katamorphism 1005
Openings of zone of fracture 1006
Form and continuity of openings 1007
Size of openings 1008
Volume of openings 1008
24 CONTENTS.
CHAPTER XII. THE RELATIONS OF METAMORPHISM TO ORE DEPOSITS Continued. Page.
Part I. General principles Continued.
Deformation of the lithosphere Continued.
Zone of fracture, or zone of katamorphisin Continued.
Chemical reactions 1008
Zone of combined fracture and flowage 1009
Zone of flowage, or zone of anamorphism 1011
Openings of zone of flowage 1012
Reactions of zone of flowage 1012
Relations between zones of deformation 1012
Effects of deformation and chemical changes upon temperature 1013
Volcanism 1014
Circulation and work of solutions 1017
Circulation of gaseous solutions 1018
Circulation in belt of weathering 1018
Circulation in belt of cementation 1019
Circulation in zone of anamorphism 1020
Circulation of aqueous solutions 1021
Circulation in zone of fracture, or zone of katamorphism 1022
Belt of weathering 1023
Belt of cementation 1024
Circulation in zone of combined fracture and flowage 1 028
Circulation in zone of flowage, or zone of anamorphism 1029
Source of the metals.. 1030
i
Part II. Segregation of ores 1036
General statements 1036
Division A. Ores produced by processes of sedimentation 1037
Ores formed by chemical precipitation 1037
Ores formed by mechanical concentration 1038
Metamorphic alterations of sedimentary ores 1039
Division B. Ores produced by igneous processes 1043
Division C. Ores produced by processes of metamorphism 1052
Group A. Ores deposited by gaseous solutions 1052
Group B. Ores deposited by aqueous solutions . . . 1058
I. Source of aqueous solution* 1065
II. Source of metals for ores deposited from aqueous solutions 1069
III. Work of aqueous solutions in segregating ores 1072
Subclass 1. Ores precipitated from ascending aqueous solutions 1072
Solution of the metals 1073
Transportation of the metals 1075
Precipitation of the metals 1081
Precipitation by decrease of temperature and pressure 1081
Precipitation by mingling of solutions 1082
Precipitation by reactions between solutions and solids 1086
General statements . . 1088
CONTENTS. 25
\
CHA"PTER XII. THE RELATIONS OK METAMORPHISM TO ORE DEPOSITS Continued. Page.
Part II. Segregation of ores Continued.
Division C. Ores produced by processes of metainorphism Continued.
< ; n >up B. Ores deposited by aqueous solutions Continued.
III. Work of aqueous solutions in segregating ores Continued.
Sulirlass 1. Ores precipitated from ascending aqueous solutions Cont'd.
Compounds deposited by ascending solutions 1088
Metals 1089
Gold 1089
Solution 1089
Precipitation .' 1091
Silver 1099
Solution 1099
Precipitation 1100
Copper 1101
Solution 1101
Precipitation 1101
Sulphides 1104
Solution of sulphides 1106
Precipitation of sulphides 1108
Precipitation of sulphides transported as such 1109
Precipitation of sulphides transported as oxidized salts. 1110
General statements 1117
Tellurides 1119
Oxides 1125
Magnetite 1126
Zincite 1126
Franklinite . 1126
Hematite 1126
Cassiterite 1127
Carbonates 1128
Silicates 1129
Criteria for discriminating deposits of the deep circulation 1 132
General statements _.- 1138
Subclass 2. Ores precipitated from ascending and descending aqueous
solutions 1139
Association of lead, zinc, and iron compounds , 1144
Facts of occurrence 1144
Second concentration 1147
Oxidized ores 1147
Sulphide ores 1 148
Galena 1148
Sphalerite 1151
Mareasite and py rite 1152
General statements . . 1153
26 CONTENTS.
CHAJTKH XII. THE RELATIONS OF MKT vMoi:i'iiis\i TO OKI-: PKPOSITS Continued, r'n:. .
Part II. Segregation of ore- ( 'ontinued.
Division ('. Ores produced l>y processes of metamorphism Continued.
Group 15. Ores deposited l>y aqueous solutions Continued.
III. Work of aqueous solutions in segregating ori's Continued.
Subclass 2. Ores precipitated from ascending and descending aqueous
solutions Continued.
Association of copper and iron compounds 1 1 ~>S
Association of silver and gold with base metals 1 Kit!
Silver IKili
(i.)ld 116!)
Concentration by reaction upon sulphides compared with metallur-
gical concentration 11 74
Other reactions of descending solutions .' 117")
Second concentration favored by large openings near the surface ... 1177
Depth of effect of descending waters 117!)
Illustrations of secondary enrichment and diminution of richness
with depth 1182
General statements 1189
Subclass 3. Ores precipitated from descending aqueous solutions 1193
Iron ores 119:;
Manganese ores 1 198
IV. Special factors affecting the concentration of ores 119!)
Variations in porosity and structure 1200
Distribution and size of openings 1201
. Complexity of openings 1202
Preexisting channels and replacements 1203
Impervious strata at various depths 1207
Pitching troughs and arches 1211
General statements 121(5
Character of topography 1217
Effect of vertical etement 1217
Effect of horizontal element 1218
Physical revolutions 1 221
V. General statements 122:>
VI. Ore shoots 122:;
General statements 1230
Summary and conclusion 1232
INDEX 1 245
I L L US T R A T I N S .
Page.
I'I.ATJO I. Fail-view Dome, Sierra Nevada, from the north 43l>
II. -1, Parallel veins of i-alcite, (ireat Basin; B, Biotitie granite, showing garnet
surrounded by lenticular areas deficient in in. n-bearing minerals 700
III. Photomicrographs of metamorphic textures , 704
IV. Textures of limestones and marble 796
V. Textures of metamorphosed marbles 810
VI. Photomicrographs nf limestone and marbles 814
Vlf. Photomicrographs of iron-bearing rocks SMii
VIII. A, Unaltered Newark conglomerate from Virginia, after Keith; B, Schist conglom-
erate from Felch Mountain district, Michigan 858
IX. Photomicrographs of sandstone and quartzitoa 872
X. Photomicrographs of gray \vackes 888
XI. Photomicrographs of pelites 902
XII. .1, Vein quart/, Banner mine, California; B, Secondary galena and blende in ores
from Missouri 1156
XIII. Iron-ore deposits in pitching troughs 1196
FK;. 1. Change of volume resulting from solution, and relations of solution and pressure 78
J. Quantitative relations between solution and temperature 80
:>. Triangular en >s- sections of pore space 132
4. Spheres packed in the most compact manner possible 133
.">. Relations of level of ground water to topography and to surface drainage 410
6. Effect of unequal heating of the surface of a rock 434
7. Ideal horizontal section of the flow of ground water from one well to another 570
8. Ideal vertical section of the llo\v of ground water from one well to another 571
!'. Ideal vertical section of the flow of ground water entering at one point on a slope and
issuing at a lower point 573
10. Ideal vertical secii. m of the flow of ground water entering at three points and issuing
at a single point _ 574
11. Ideal vertical section of the How of ground water entering at many points along a
slope and issuing at a single point at a lower elevation ">7.i
12. Ideal section illustrating the chief requisite conditions of artesian wells 577
13. Part of a thin section of a quartz-schist showing liquid- and gas-filled cavities of a
secondary nature; Black Hills, South Dakota 620
14. Enlargement of feldspar fragment 626
15. Enlargement of hornblende fragment 626
16. Clastic quart/ penetrated by serpentine 643
27
28 ILLUSTRATIONS.
Pagr.
FIG. 17. Granulation of feldspar and gradation between undulatory extinction and granu-
lation 674
18. Granulation of quartz in a rock in which the feldspar is but little affected i>74
19. Liquid-filled cavities extending across several quartz individual* without change of
direction 746
20. Diagram showing possible relation of old and new grains of recrystallized rocks 752
21. Diagrams illustrating mass deformation of a rock 769
22. Sketch of oval irregular grains of calcite with longer diameters parallel 810
23. Graywacke undergoing serpentinization along cracks
24. Conglomerate deposited in depression produced by erosion of basic dike through
gneiss !'-''
25. Diagrams illustrating the manner in which deformation in the zone of flowage may
concentrate crustal shortening in the zone of fracture 928
26. Ideal vertical section of the flow of water entering at a number of points on a slope
and passing to a valley below through a homogeneous medium interrupted by two
open vertical channels, one on the slope and one in the valley 1076
27. Ideal section showing underground circulation in which no water anywhere ascends
before issuing at the surface 1080
28. Cross section of banded vein near the London shaft, Mineral Point, Colorado 1135
29. Diagrammatic section of Enterprise mine, Colorado, and its blanket pay shoot 1208
30. Diagram illustrating mingling of circulations of two limestones separated by a shale. 1209
31. Ideal vertical section of flow of underground water in the Galena limestone of the
upper Mississippi Valley 1210
32. Ore deposit in limestone beneath impervious shale, Elkhorn mine, Montana 1214
LETTER OF TRANSMITTAL.
DEPARTMENT OF THE INTERIOR,
UNITED STATES GEOLOGICAL SURVEY,
SECTION OF PRE-CAMBRIAN AND METAMOKPHIC GEOLOGY,
Madison, Wis., April 30, 1903.
SIR: I transmit herewith the manuscript of a treatise on metamorphism,
to be published as a monograph.
This treatise is an attempt to reduce the phenomena of metamorphism
to order under the principles of physics and chemistry, or, more simply,
under the laws of energy. The first nine chapters treat of metamorphism;
the last three chapters, of the relations of metamorphism to stratigraphy,
to the redistribution of the elements, and to ore deposits.
In the preparation of this monograph I have had important assistance
from various sources. The late Prof. George H. Williams, of Johns
Hopkins University, before his death had begun to accumulate material
and notes upon metamorphism. He had made a careful abstract of the
most important literature on the subject; also a draft, consisting of about
twenty pages of manuscript, of a first chapter. All of this material
Mrs. Williams turned over to me. The summary of literature prepared
by Dr. Williams has been of very great service. To this dear friend, the
first great teacher of petrology in this country, I dedicate this volume.
In the actual preparation of the manuscript I have had the assistance
of a number of men, and of a considerable number of advanced students.
In the earlier work Dr. C. K. Leith aided me much by looking up and
summarizing literature and by offering many valuable suggestions. Later
Mr. W. N. Smith continued this work. To the discriminating judgment of
Dr. Leith and Mr. Smith I am greatly indebted. Mr. A. T. Lincoln has
made all the numerical computations in reference to the volume relations of
original and secondary minerals, and Mr. R. M. Chapman has verified Mr.
Lincoln's work. While these are the men who have assisted me most, a
number of graduate students, both at the University of Wisconsin and at
the University of Chicago, have helped in various ways.
29
30 LETTER OF TKANSMITTAL.
Geologists who write in other languages than English will have just
cause for complaint because of scant reference to publications on ineta-
morphism in such languages, but the arduous work of preparing this treatise
lias taxed my eyes to the utmost without going exhaustively through
foreign literature.
If this attempt to treat one phase of the phenomena of geology from
the point of view of energy proves successful, I shall hope that it will lead
to similar treatment of other parts of this great science.
Very respectfully, your obedient servant,
CHARLES RICHARD VAN HISE.
Hon. CHARLES D. WALCOTT,
Director of United States Geological Survey.
A TREATISE ON METAMORPHISM.
By CHARLES RICHARD VAN HISE.
CHAPTER I.
INTRODUCTION.
Following Powell, I shall regard the earth as composed of four
spheres the atmosphere, the hydrosphere, the lithosphere, and the ceutro-
spliere." The terms atmosphere and hydrosphere need no definition. The
term lithosphere, as here used, will be confined to that portion of the outer
part of the earth which is within the limits of observation. How far below
the surface observation extends is somewhat uncertain; but it is certain that,
in consequence of deformation and denudation, we may observe rocks
which have been several thousands of meters below the surface. Clarke
suggests that the zone of observation be defined as extending to a depth of
10 miles (Ifi kilometers) below the level of the sea. 6 Whatever distance
be taken as the limit of the zone of observation, it is certain that such
distance is but a very small fraction of the radius of the earth. All the
earth below this fraction will be considered as the centrosphere ; but no
hypotheses are advanced in respect to any essential difference in character
between the material of the lower part of the lithosphere and that of the
upper part of the centrosphere.
GENERAL NATURE OF ALTERATIONS.
The data of geology have become so numerous as to be almost unman-
ageable. Not many decades ago it was possible for a geologist to have a
a Powell, J. W., Physiographic processes: Nat. Geog. Mon., vol. 1, No. 1, 1895, p. 1.
^Clarke, F. W., The relative abundance of thechemical elements: Bull. U. S. Geol. Survey No.
78, 1891, p. 34.
31
32 A TREATISE ON MKTAMORPHISM.
reasonably full and satisfactory knowledge, not only of the known principles
of geology, l)ut of the observed phenomena in the parts of the world which
had been studied. While by many years of work a geologist may still be
able to learn the important facts concerning various provinces, it is no
longer possible for one man to have anything like complete information as
to the local geology of many parts of the world. Not only is this so, but
no one geologist can know all the important discovered facts concerning a
particular branch of geology. Moreover, in recent years the accumulation
of facts has gone on much faster than the development of geological theory.
Nowhere is this more true than in the branch of geology known as
petrology, and in petrology it is perhaps more true of the phenomena of the
alterations of rocks than of any other. Scarcely a paper on petrology
appears that does not contain some account of the alterations of minerals or
of rocks, but in most cases there is 110 serious attempt to arrange the observed
phenomena in order tinder recognized principles. Indeed, there is 110
general set of recognized principles under which the phenomena can be
reduced to order.
Some years ago, finding myself lost in the vast accumulation of data,
I began to formulate principles applicable to the alterations of rocks. The
result of this work is the present treatise, which is an attempt to reduce the
phenomena of metamorphism to order under the principles of physics and
chemistry, or, more simply, under the laws of energy. It is but a part of
the larger task of reducing to order under the same laws the entire subject
of physical geology.
/ As a result of the development of the science of petrology, especially
microscopical petrology, it has been ascertained that changes are continu-
ally occurring within the rocks constituting the outer part of the earth.
This statement is equally applicable to the most porous rocks at the surface
of the earth and to the densest rocks as deep below the surface as observa-
tion gives exact knowledge. All changes, by whatever forces, agents, and
processes caused, and in whatever classes of rocks occurring, whether
solidified magmas, chemical precipitates, organic deposits, or mechanical
deposits, may be called metamorphism.
Metamorphism, as here used, means any change in the constitution of
any kind of rock.
It will be shown that at any given time and place, under any given
ROCKS ADAPTED TO ENVIRONMENT. 33
set of conditions, minerals tend to form which remain permanent under
those conditions. This tendency is more potent with minerals crystallizing
from magmas than with minerals which constitute the sedimentary rocks
or with the secondary minerals which form by metamorphism. The reason
for this is that adjustment to existing conditions is so much more readily
accomplished in fluids than in solids; but the tendency to form minerals
which are permanent under the existing conditions controls in the solid
rocks, although there is a great amount of lag in the process of modifica-
tion. If adjustment be reached in a given case and if the conditions
remain the same, the minerals formed do not again alter, but may remain
the same through eons. This is illustrated by the meteorites, the minerals
of which may persist without change during the evolutions of stellar sys-
tems. However, when an important change of conditions occurs, as when
a meteor gives up its separate existence in the interstellar spaces and joins
a planet, as the earth, readjustment begins at once.
Although the changes of conditions upon the earth are not so great as
the change when a meteor falls to the earth, the range of conditions upon
the earth is large and varied. The conditions may be those of ordinary
pressure and temperature at or near the surface of the earth, or they may
be those of very high pressure and temperature, such as exist well below
the surface of the earth. A rock mass may alternately be subject to each
of these sets of conditions and to various intermediate conditions. Changes
of physical conditions result from surficial transfer of material by epigene
agents bringing rocks to the surface here, burying them there from
igneous intrusions, from erogenic movement, and from other causes. The
changes upon the earth are therefore profound, although usually slow.
During the changes the rocks are always modified in the direction of
adjustment to the new conditions. Such modification of rocks has led to
the idea of adaptation to their environment. As conditions change, species
of plants and animals are so rapidly modified that at first sight adaptation
seems almost perfect. Indeed, so sensitive are plants and animals to their
environment that since the theory of evolution gained ascendancy the fact
of approximate adaptation is taken for granted. The variety and complexity
of the structures, colors, etc., of life forms resulting from adaptation to
environment is a constant source of wonder. Almost daily some remarkable
structure or form is described, and its existence explained by showing how
MOX XLVII 04 3
34 A TREATISE ON METAMORPHISM.
it is advantageous to the animal under the conditions in which it lives.
Since adaptation is an assumed law, in those cases where there seems to be
lack of adaptation, as where some peculiar structure is present which
apparently is not of advantage to an animal or plant, it is believed that the
facts are not fully known or that the structure was once useful and is a
survival. However, the very idea of survival shows that to a certain degree
the development of plants and animals lags behind their changing environ-
ment. Upon a priori grounds it would be certain that this is the case; and
the existence of rudimentary organs, such as the muscles for moving the
human ear, which at one time may have had a use, is positive evidence of
the lag of organic species during their adaptation to changing environment.
Likewise it is believed that minerals constantly tend to change to
forms that are relatively stable under existent conditions. This, however,
is accomplished by granulation or recrystallization or some analogous
process, and is adaptation only in the sense that the old particles break
up into smaller particles or develop into new mineral particles which
conform to the existent conditions. Some minerals are stable under a
considerable variety of conditions, and therefore are less sensitive to
change than are others. For instance, quartz develops directly from an
igneous rock, and it also forms as a deposit from water. It persists under
both quiescent and dynamic conditions. Other minerals require rather
definite conditions for their existence. Such are leucite and olivine, which
abundantly form as original minerals in igneous rocks of certain composi-
tion, but which readily change under new conditions to other minerals.
However, no mineral persists without reference to its environment, and so
it may be said that there is a tendency in all mineral substances to form
minerals adjusted to the conditions under which they exist. Rocks are
composed of aggregates of different minerals. Therefore rocks, like
minerals, have a tendency toward adjustment to their environment.
Even if the chemical composition of a small mass, say a cubic milli-
meter, remains exactly the same, the mineral constituents of the mass
may greatly change. At the end of the change the original minerals may
not be in the same proportions as before and minerals which did not
originally exist in the rock may have formed. But the adjustment of rocks
is not confined to redistribution of the elements present in a small space.
There may be a change in the average chemical composition of rocks.
ADAPTATION TO ENVIRONMENT SLOW. 35
Material may be intruded, or may be brought in by water solutions, or
may be abstracted by water solutions. By any one of these processes or
by any combination of them a considerable change in the chemical
composition of a rock may take place.
While it may be safely asserted that all rocks, under all conditions,
at all times, are being adapted to their environment, the change in a rock
goes on so slowly that its lag behind the change in the environment may
be measured by millions of years. Often the lag is so great that the con-
ditions again change before the process of adjustment has made much
advancement, and, therefore, before one set of changes is near completion
another set is begun. Indeed, a later change in conditions may be a
reversal of an earlier change, and, therefore, in the process of adaptation,
work done in an early stage may be reversed at a later stage. But even in
such a case it is clear that the principle of adaptation applies, just as in the
case of many plants and animals, although there may be, in fact, little more
than a tendency toward adaptation to existent conditions.
Because the adjustment of rock to environment is so slow, in order that
it may be approximately complete it is necessary that a rock remain under
substantially the same conditions for a very long time. This has happened
in some regions in which important mechanical movements have not
occurred for a period or an era and the rocks of which have remained
buried to a moderate depth for most of the time. Such were the conditions
of the ancient volcanics of certain parts of the Lake Superior region. These
have escaped important mechanical movement since the beginning of Pale-
ozoic time. They were buried under Paleozoic sediments to a moderate
depth. Denudation since the beginning of Cretaceous time brought them
to the surface. Finally glacial erosion removed a skin of weathered
material and exposed the volcanic rocks, approximately adapted to their
past environment, that of the belt of cementation under quiescent condi-
tions. (See Chapter VII, pp. 594 et seq.) So far as they have reached the
surface they are subject to a new set of conditions; and a new cycle of
change, begun at the end of the Glacial epoch, but not far advanced, is in
progress.
In considering metamorphism, the fundamental hypothesis of geology
will be applied as in other branches of the subject. That is to say,
the Huttonian principle, that the present is the key to the past, is
36 A TREATISE ON METAMORPHISM.
assumed. Where certain phenomena are now produced by certain com-
binations of forces and agents, and by these only, and similar phenom-
ena are found in the rocks long since formed, it is assumed that the like
phenomena, present and past, are due to essentially the same combina-
tions of forces and agents. For instance, if alterations of a certain
kind are now being produced by a complex set of. geological factors, and
by these only, where similar alterations are found in ancient rocks it is
assumed that they are due to practically the same combination of the
forces and agents of alteration.
But the above statement does not imply that the changes are now
taking place with the same speed as that with which they occurred in the
past, as might have been held by Lyell; nor is it assumed that the various
forces and agents have the same relative values. Indeed, it is believed to
be highly probable that there have been changes in the rate of alteration
of rocks and in the nature and effectiveness of the factors producing the
alterations.
While the Huttonian principle is of service in the study of metamor-
phisrn, the alterations of rocks take place so slowly that it does not have
nearly the value that it has in the study of the work of the epigene agents,
such as air and water and ice; nor the value it has in the study of such
hypogeue agents as volcanoes and earthquakes.
Many of the rock alterations are now taking place under conditions
which can not be directly observed, but must be inferred from the records
of the change. This statement applies to all changes below a inile in
depth, and it is very largely applicable to all but the mere outer film of the
rocks, for most excavations and cuttings are not deeper than a few score or
a few hundred feet and the deepest shafts are but little over a mile. For-
tunately it frequently happens that in a rock formation now at the surface
the results of various stages of change under deep-seated conditions are
preserved, so that the character of the alterations and the nature of the
forces and agents which have produced them may be inferred from a close
study of the different stages of alteration.
In such cases, instead of observing the forces and agents accomplishing
certain results, and of reasoning that similar results produced in the past
are due to these forces and agents, we observe the results at various stages
of development and infer from them the nature of the forces and agents
producing them; we then infer that similar forces and agents are at work
METHOD OF REASONING. 37
beyond the zone of observation, accomplishing at the present time similar
results. This is a complete reversal of the Huttonian method. Hence, in
treating of metamorphism we must argue both from the present to the past
and from the past to the present. By studying the action of the forces and
agents now at work in the zone of observation and the stages of alteration
o o
preserved in the rocks brought into the zone of observation we are able to
push the boundaries of the known for a certain distance into the domain of
the unknown, and infer with considerable certainty the nature of the
changes which have taken place in the far-distant past and of those which
are now taking place but which we can not directly observe.
It will be generally agreed that the majority of the altered rocks,
including a large portion of the schists and gneisses, have been metamor-
phosed from aqueous and igneous rocks like those now being produced.
This is in accordance with the Huttonian principle. But some may hold
that the most ancient of the schists and gneisses, those of the so-called
Basement Complex, had a different origin. For instance, it has been held
by some that these ancient rocks are direct precipitates in a primeval
ocean. On later pages it will be seen that the most ancient schists and
gneisses are in all respects like those produced from more recent rocks by
the processes of alteration, and therefore that the probable, but not certain,
inference is that they were produced from rocks not fundamentally unlike
those now being formed by processes of change not .radically different from
those now at work. But in ascertaining the forces and agents and their
method of work in both the ancient and the modern rocks, we must for the
most part follow the reversal of the Huttonian principle i. e., argue from
past results as to the nature and method of work of present forces and
agents.
Whatever the origin of rocks whether solidifications from magmas,
chemical precipitates, organic deposits, or mechanical deposits as already
noted, they may be altered so as to modify their structures, so as to change
their mineral composition, and so as to change their chemical composition.
In place of the original characteristic structures of the igneous rocks, such
as flowage structure and massive structure, and in place of the original
structures of the sedimentary rocks, such as bedding, there may be pro-
duced secondary structures, such as cleavage, fissility, joints, slatiness,
schistosity, and gneissosity. In place of the original textures of igneous
rocks, such as granolitic, porphyritic, ophitic, and poikilitic, and in place
38 A TREATISE ON METAMORPHISM.
of the original textures of sedimentary rocks, such as granular and oolitic,
there may be produced textures characteristic of the metamorphic rocks,
such as cataclastic, parallel orientation, etc. The alteration may result in
the change to minerals all of which may wholly differ from any of the
original minerals, or it may take place by recrystallization without change
in mineral character, as in the case of the formation of marble from lime-
stone. Chemical change may result in the addition of constituents, as in
the case of oxidation and hydration of compounds already existing, or in
the deposition of additional material in the interstices, or in the abstraction
of material. Any given mineral may gain additional elements, or a greater
proportion of some of the elemeiits; it may lose a part or all of some of its
elements, or it may be wholly replaced by another mineral. While the
chemical composition of the rock may be greatly affected by such changes,
in other cases the alterations may result merely in a redistribution of the
elements without affecting the average composition of the rock, as in the
case of marmorization, some cases of devitrification, various cases of
metasomatism, etc.
After one set of changes has taken place, or while they are in progress,
a change of physical conditions may come about in consequence of which
a different set of changes may be set up. Thus rocks may be partly modi-
fied under mass-static conditions and subsequently modified under mass-
mechanical conditions. They may be modified near the surface of the
earth, and as a result of burial be later modified at much greater depth;
or they may be modified at great depth, and as a result of erosion be
brought near the surface and there be again modified. Therefore one set
of changes may be superimposed upon another. In many cases it is cer-
tain that rocks have gone through several very complex sets of modifica-
tions. * For instance, a rock may be modified under conditions at the
surface, afterwards be buried under other strata and thus pass into a deep
zone, where it may be modified in a different manner, and still later, as a
result of denudation, be brought to the surface and in the passage undergo
successive alterations in intermediate belts, and when it reaches the surface
once more be altered by the same forces and agents as at first. Substan-
tially this history has been gone through by the jaspilites of the Lake
Superior region. (See pp. 831-833.) Many other rocks have had an
equally intricate but very different history.
AGENCIES CONCERNED IN ROCK CHANGES 39
CLASSIFICATION OF METAMOBPIIISM.
The forces of metamorphisin are chemical energy, gravity, and heat
and light. The agents of metamorphism are gases, liquids, and organic
compounds.
A critical examination of the published classifications of metamorphism
shows that the kinds of metamorphism recognized are based upon the idea
that one force or agent or process is dominant in the production of a
particular kind of rock. But in all of the various kinds of metamorphism
ordinarily recognized in classifications, such as thermo-metamorphism,
hydro-metamorphism, chemical metamorphism, static metamorphism, pres-
sure metamorphism, dynamo-metamorphisni, regional metamorphism, and
contact metamorphism, all of the forces above mentioned are required, and
also the chief agent, water. There is no metamorphism of a rock without
the presence of heat, and hence all metamorphism is partly thermo-
metamorphism; there is no metamorphism without the presence of water
solutions, and hence all metamorphism is partly hydro-metamorphism; there
is no metamorphism in which chemical action does not enter, and hence all
metamorphism is partly chemical metamorphism; there is no metamorphism
without motion, and hence, in an exact sense, all metamorphism is dynamic.
In the alterations of rocks the forces of metamorphism in each case produce
atomic, molecular, and mechanical changes." When it is realized that in
all the varieties of metamorphism mentioned chemical action, heat, and
dynamic action enter as important factors, and that water is present and
active wherever metamorphism occurs, it becomes self-evident that the
classifications ordinarily given are not satisfactory. Moreover, the classifi-
cations involve different factors not belonging to the same category, some
being physical, some chemical, some geological, some referring to an agent,
others to a cause. For instance, thermo-metamorphism refers to heat;
hydro-metamorphism refers to the presence of water; chemical metamor-
phism refers to the action of chemical forces; static metamorphism and
pressure metamorphism refer to quiescent conditions; dyuamo-metamorphism
refers to conditions of motion; regional metamorphism refers to the extent
If this be true, it is clear that a classification of metamorphism into paramorphism, metatrophy,
and metataxis, restricting these terms to atomic, molecular, and mechanical changes, respectively,
as proposed by A. Irving, is wholly impracticable. Irving, A., Metamorphism of rocks, London.
1389, pp. 4-5.
40 A TREATISE ON METAMORPHISM.
of the alterations; and contact metamorphism refers to the contiguity of
an igneous rock.
As a matter of fact, all of these different kinds of metamorphism are
related in the most intricate manner, and certain metamorphic results
which have been attributed to one of these forces, agents, or processes
could equally well be attributed to another. For instance, in many cases
metamorphism known as thermo-metamorphism might just as well be
called hydro-metamorphism, or regional metamorphism be called dynamic
metamorphism, or contact metamorphism be called thermo-metamorphism
or chemical metamorphism.
It follows from the above that a satisfactory classification of meta-
morphism based upon chemical forces alone, or physical forces alone, or
individual processes, is quite out of the question. It appears to me that
the only workable classification of metamorphism is geological. (See
pp. 43-44.)
GEOLOGICAL FACTORS AFFECTING THE ALTERATIONS OF ROCKS.
The more important geological factors affecting the alterations of rocks
are: Composition; structures and textures; porosity; water and gaseous
content; climatic and geographic conditions ; ^ time ; environment; degree
of movement; depth. Many physical factors enter into each of these
geological factors.
At present only general statements will be made with reference to
these factors, but on later pages the effect of each of them will more
clearly appear.
composition I n so far as rocks are composed of minerals which are
permanent under the existing conditions, or are composed of minerals
which may exist under a wide variety of conditions, this is favorable to
stability.
structures and textures J n so f ar as there are coarse structures and textures,
this is favorable to permanency, for it will be seen that fine material is
more readily altered than coarse material.
porosity Porosity has a very important influence upon the rapidity of
change. In proportion as rocks are porous the agents of alteration, gases
and water, may enter and rapidly circulate. In proportion as they are
dense, the amount of water present is small and the circulation is slow.
GEOLOGICAL FACTORS AFFECTING ROCK ALTERATION. 41
Hence porosity is favorable to rapid change; density is favorable to
stability.
water and gaseous content Jn proportion as rocks contain water and gas
they are readily altered. In proportion as water and gas are absent they
are stable.
Climatic and geographic conditions TllC gp66Cl of alteration of 1'OcltS is affected
by their geographical position. The alteration of surface rocks is more
rapid in tropical than in arctic regions; it is more rapid in humid than in
arid regions; it is more rapid on steep than on gentle slopes; it is more
rapid along coasts than in the interior. In short, the nature of the altera-
tions of the upper belt of rocks varies with every varying factor of climate
and geography.
Time Time is a factor of the very highest importance in metamor-
phism. Time can not be included among the forces or the agents of meta-
morphism, but the amount of metamorphism is a function of the time.
Where a given set of forces and agents is at work under a given set of
conditions, increase of time increases the metamorphism, but not in a direct
ratio, for in proportion as adjustment to environment is approached the
alterations decrease in speed. The importance of time in geology can not
be too strongly emphasized, for a comparatively weak force or agent
working through a great length of time may accomplish an almost incred-
ible amount of work. We are accustomed to judge of the efficiency of a
force or agent by observations in the chemical or physical laboratory, but
the time through which an experiment may be continued in the laboratory
is an almost infinitely small fraction of the time through which the forces
and agents have been at work in nature. To illustrate, in the chemical
laboratory the amount of crystallized silica which can be dissolved in water
and transported to another place within the time during which an ordinary
experiment is carried on is so small as to be immeasurable, and yet it is
certain that in nature water has dissolved and transported to other places
enormous quantities of silica. (See Chapter VII, pp. 622-623.) This illus-
tration enforces the fact that the geologist has very much more time at his
command than has the chemist or the physicist. If the geologist ignores
this fact, and reasons in reference to the potency of forces and agents in
metamorphism as a chemist or physicist would in the laboratory in refer-
ence to the same forces and agents, he is certain to fall into very serious
42 A TREATISE ON METAMORPHISM.
error. The importance of the time factor lias been recognized by most
geologists with respect to erosion and many of the other geological
processes, but it is of even greater importance in metamorphism. Most of
the metamorphic processes are very slow indeed, but the amount of time
available in a single geological period is great, and the metamorphic results
are often stupendous.
In general it may be said that in proportion as rocks are old they are
likely to have been greatly altered; in proportion as they are young they
are likely to have been little altered. While time is a most important
factor in the amount of alteration, time alone, without the other necessary
conditions for change, is not sufficient to insure important metamorphic
results. Further, when the other conditions are very favorable to change,
extensive alteration may take place in a comparatively short time, consid-
ering this factor from a geological point of view. It follows, because of
variations in other factors than time, that in some regions very ancient
rocks may be little modified and in other regions comparatively young
rocks may be greatly modified.
Environment I n many cases environment may be important. If the
rocks surrounding a given rock be porous, this condition readily permits the
entrance of the agents of alteration water and gases and therefore much
more profound change may occur than if the rock were surrounded by
comparatively impervious material. This is illustrated by the diabase dikes
of the Penokee series of Michigan, which where surrounded by the broken
rocks of the iron-bearing formation are completely altered, but which
where surrounded by the impervious black slates are comparatively
unaltered. A further very important factor in environment is the
presence of intruded igneous rocks. Igneous rocks, by conduction, may
directly heat the adjacent rocks; but of even greater importance is the
fact that igneous rocks may furnish solutions to the adjacent rocks or heat
the solutions which percolate through them. These illustrations show that
the alteration of a rock may be greatly affected by the surrounding rocks.
Degree of movement One of the most important of the factors affecting
alterations is movement; indeed, the factor of movement is so important
that it has frequently been made a basis for a classification of metamorphism.
Changes of rocks take place with comparative slowness under conditions
of quiescence and take place with comparative rapidity under conditions of
DEPTH THE MOST IMPORTANT GEOLOGICAL FACTOR. 43
movement, Furthermore, the alterations which occur under dynamic con-
ditions are far more profound than those which take place under static
conditions. For instance, very ancient sedimentary rocks which have been
undisturbed by orogenic movements may be in almost the original condition
in which they were deposited. On the other hand, rocks of comparatively
recent age which have been in mountain-making areas and been deeply
buried may be profoundly modified. Little metamorphosed rocks of great
age are illustrated by the St. Peter sandstone of Wisconsin and the uncon-
solidated Cambrian sands of Russia. Profoundly metamorphosed rocks
of comparatively recent age are illustrated by the Eocene and Neocene
rocks of the Coast Range of California and the Eocene of the Alps.
Depth. Rocks at or near the surface of the earth are ordinarily under
conditions of slight pressure and low temperature. Rocks at some depth
below the surface are under conditions of considerable pressure and
temperature. It will be shown that the alterations of a given rock under
these varying conditions are very different. Therefore depth is a matter of
great consequence in the consideration of metamorphism. Indeed, depth
is believed to be the most important of the influences which determine the
character of the alterations of rocks. Therefore the geological factor
which in this treatise will serve as the primary basis for a classification of
metamorphism is the dominant factor of depth. On this basis metamor-
phism will be classified into (1) alterations in the zone of katamorphism
and (2) alterations in the zone of anamorphism. The zone of katamor-
phism is subdivided into (a) the belt of weathering and (b) the belt of
cementation. The zone of katamorphism may be defined as the zone in
which the alterations of rocks result in the production of simple com-
pounds from more complex ones. The zone of anamorphism may be
defined as the zone in which the alterations of rocks result in the pro-
duction of complex compound's from more simple ones. The belt of
weathering is the belt which extends from the surface to the level
of ground water. The belt of cementation is the belt which extends
from ground-water level to the zone of anamorphism.
It is to be noted not only that this classification is geological, but that
the factor is one which is universally applicable. Geological factors of
different kinds, such as movement, contact action, etc., are not introduced.
It is therefore clear that the proposed classification follows one law of all
44 A TREATISE ON METAMORPHISM.
good classifications, viz, that a factor or factors of the same class shall be
used throughout as a primary basis. While the primary classification of
metamorphism will be based upon depth, it is recognized that there are no
sharp dividing lines between the zones and belts. In metamorphism, as in
every other branch of geology and of science, there is complete gradation
between the phenomena of the various classes.
However, it has been seen that depth is not the only geological factor
of consequence in metamorphism. It is recognized that various other
geological factors enter into the alteration of a given rock. Moreover, these
various factors overlap. In the discussion of the zones of metamorphism
the geological factors of subordinate importance will be given proper
consideration.
Before considering the general alterations in the zones of katamor-
phism and anamorphism, and the alterations of the individual minerals
and rocks in these zones, it is necessary to consider the forces and the
agents of metamorphism from chemical and physical points of view.
It should therefore be recalled that the forces of metamorphism are
chemical energy, gravity, and heat and light, and that the agents of
metamorphism are gases, liquids, and organic compounds. The rocks
are the materials upon which these forces and agents work. The forces
of metamorphism are considered in Chapter II, the agents of metamor-
phism in Chapter HI, and the work of these forces and agents upon the
rocks in the later chapters.
CHAPTER II.
THE FORCES OF METAMORPHISM.
As already seen, the important forces of metamorphism are chemical
energy, gravity, and heat and light.
CHEMICAL ENERGY.
When different compounds are brought together molecular interchange
may occur between them. As a result the compositions of the compounds
are mutually changed. Such interchange is chemical action. Chemical
action usually involves expenditure of chemical energy, which is one of the
main original sources of energy; but it will be seen that other forms of
energy may be transformed into chemical energy, and chemical action in
this way be promoted.
Chemical action may take place between gas and gas, gas and liquid, gas
and solid, liquid and liquid, liquid and solid, and solid and solid. Chemical
action, or molecular interchange, involves movement between the atoms and
molecules. Chemical action therefore never takes place without dynamic
action. So far as we know chemical action never takes place without the
presence of heat. Under the conditions obtaining in the crust of the earth
chemical action is usually promoted by heat and by mechanical action. As
chemical action always produces a heat effect, positive or negative, such
action may result in the liberation or in the absorption of heat. The heat
effect may hasten or retard further chemical action. In so far as chemical
action results in the liberation of heat, it usually hastens further chemical
action, and therefore promotes metamorphism; in so far as chemical action
results in the absorption of heat, it usually retards further chemical action,
and therefore stays metamorphism. It is shown (pp. 170-186) that both
classes of reactions take place on a very extensive scale.
In consequence of chemical action material may be added to or sub-
tracted from a given mineral. A mineral may alter into two or more other
minerals with the simultaneous addition or subtraction of material. Two
or more minerals may unite to produce a single mineral. Either of these
45
46 A TREATISE ON METAMORPHISM.
changes may take place without addition of material, or added material
may be derived from some other particle or particles near or remote.
Material subtracted from any given mineral particle may be added to
another mineral particle at a greater or less distance. Illustrating the above
are the alterations of feldspar into muscovite and quartz, and of olivine
into serpentine, magnesite, magnetite, and quartz. Chemical action is in
most cases accomplished through solutions. Therefore its detailed discus-
sion is considered in connection with the agents of metamorphism, gaseous
solutions, and aqueous solutions. (See Chapter III.)
GRAVITY.
Gravity is now the great dominating force of the universe. Indeed,
it is a main original source of energy. Certainly it is the source of energy
which has largely controlled the development of the solar system, including
the sun and all the planets and satellites. The transformations of gravity
into chemical energy, heat, light, and other forms of energy are important
factors in the development of the solar system, including the earth. More-
over, gravity still remains as the great dominating force which controls
earth movements," both vertical and horizontal, and also the circulation of
the water, both overground and underground. By earth movements are
meant all movements of the solids or rocks of the earth not in solution. In
this broad sense the movement of glaciers is an earth movement.
The direct work of gravity in metamorphism may be considered under
two headings mechanical action and water action.
MECHANICAL ACTION.
Rocks may be stressed within the elastic limit, or the stress may
extend beyond the resisting power of the material. In either case the
rocks are strained. Strain may occur with or without chemical action.
Strain is always accompanied by some transfer of energy into heat.
When the rocks are strained the molecules are moved with reference to
one another. If the strain be within the elastic limit and chemical change
does not take place, the molecules are only slightly farther apart or closer
together, and when the stress is removed they may return to their original
"Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11,
1898, pp. 512-514.
MECHANICAL ACTION. 47
positions, or nearly so. If under the stress chemical interchange also takes
place between the molecules, when the stress is removed the body may still
return to nearly its original form. But if the strain extends beyond the
elastic limit the form of the body is notably changed, as when a piece of
wrought iron or steel is drawn out or when a piece of cast iron is crushed.
Mechanical action may therefore be considered as molecular or mass.
Bv molecular mechanical action is meant differential movements of the
molecules. By mass mechanical action is meant differential movements of
lar<;e mas>i-s of the rocks. Molecular movement also frequently involves
differential movements of the atoms. Metamorphisin by molecular move-
ment has generally been called static metarnorphisni. But molecular
mechanical action is always accompanied in some degree by mass
mechanical action, though this process may be subordinate. The term
"dvuauiic metamorphism'' has usually been restricted to alterations in con-
nection with mass deformation. But mass mechanical action is always
accompanied by molecular mechanical action as an important and essential
concomitant, although this invariable relation has not always been recog-
nized. Further, as mass movement becomes important molecular move-
ment, instead of becoming less important, is likely to be of even greater
consequence. There is therefore gradation between molecular mechanical
action and mass dvnamic action.
MOLECULAR MECHANICAL ACTION'.
Molecular mechanical action involves various degrees of movements.
Presumably the lesser movements are the cases of change in crystalline
form and of strain within the elastic limit. In the change of a substance
from one crystalline form to another as, for instance, of aragonite to cal-
cite the movement of the molecules may not involve more than a rear-
rangement of those which are adjacent. In the case of substances strained
within the elastic limit, the molecules are simply pressed slightly closer
together or pulled slightly farther apart, and yet these very slight adjust-
ments may have a profound effect upon the physical properties of the
materials. For instance, amorphous glass when strained but slightly and
well within its elastic limit becomes an anisotropic substance. Leucite
crystallizes in the isometric system at high temperatures. As the mineral
cools it passes at once into an anisotropic form. The transformation from
48 A TREATISE ON METAMORPHISM.
one to the other may be seen by alternately heating and cooling this
mineral under the microscope. In the foregoing cases, while we can not
doubt that movement occurs, the readjustment is molecular, and it is there-
fore beyond the power of the microscope to determine its character.
It might at first be supposed that such slight movements as are involved
in strains within the elastic limit are unimportant, but it is to be remembered
that strains of this kind not only affect every mineral particle, but displace
the individual molecules with reference to one another, so that the strained
masses are affected throughout. While, therefore, it requires polarized
light to detect the strained condition in minerals, it is certain that the effect
is pervasive. It will be seen (pp. 95-98) that such state of strain is of
fundamental importance in the matter of solution and deposition through
the agency of solutions.
In a second class of movements there is molecular interchange between
substances by which the compounds are modified in composition. Such
interchanges involve chemical action. The motions which occur during
chemical changes in solids are commonly for such short distances that the
naked eye does not discover the relations of the original and secondary
minerals. Such movements are microscopic. Chemical interchange may
be mainly accomplished by chemical forces and the movement be an
incident of this process. On the other hand, mechanical action may be the
inciting cause which leads to chemical action. And, finally, the purely
chemical and mechanical forces may interact, each promoting the other.
The more important chemical reactions resulting from mechanical action
are accomplished through the agency of solutions, and hence are treated
in Chapter III. But Prof. Walther Spring" has shown that chemical
changes may be induced by mechanical action alone, without the presence
of solutions. For instance, when barium carbonate and solid sodium
sulphate were mixed in equal molecular proportions and subjected to a
pressure of 6,000 atmospheres a change took place by which 80 per cent
of the barium carbonate and sodium sulphate were changed to barium
sulphate and sodium carbonate, respectively; and conversely, when barium
sulphate and sodium carbonate were mixed together in equal molecular
proportions and subjected to a like pressure about 20 per cent was changed
Professor Spring on the physics and chemistry of solids, review by C. F. Tolman, jr.: Jour.
Geol., vol. 6, 1898, p. 323.
MASS MECHANICAL ACTION. 49
to barium carbonate and sodium sulphate." In all such changes the
fundamental principle controlling is that reactions shall take place which
result in smaller volumes. Spring 6 found that in the case of dry reactions
induced by mechanical action time is a very important factor, the reactions
taking place much more slowly than when compounds are moist and water
is an intermediate agent.
MASS MECHANICAL ACTION.
Mass mechanical action (a) may permanently strain the rocks without
openings, (b) may strain the rocks with rupture and openings, and (c) may
close the openings in rocks and produce welding.
permanent strain without openings. In order that permanent strain beyond
the elastic limit without openings may take place in the rocks it is nec-
essary that deformation shall occur while the rocks are under a sufficient
pressure in all directions to hold the molecules so close together that the
molecular attraction is effective. This will be true only where the pressure
is greater in all directions than the crushing strength of the rocks. It is
well illustrated by Adams and Nicolsou's experiment on the deformation
of marble while under pressure in all directions/ The molecules were
held close to one another, and the deformed marble retained considerable
strength.
Later we shall see that the process of readjustment may be mechanical
or chemical or partly each. When the process is mechanical the mineral
particles are usually granulated that is, finely fractured. When the
process is chemical the particles are recrystallized. Also the process of
readjustment may be accomplished by any combination of granulation and
recrystallization (See pp. 737-748.) Under natural conditions, in order
that the pressure in all directions shall be greater "than the crushing strength
of a rock, it is necessary that it be in the zone of flowage for that rock.
permanent strain with openings. When the rocks are strained beyond the elastic
limit and the pressure is not greater in all directions than the crushing
strength of the rocks, rupture and openings are produced. The ruptures
may be regular or irregular. The regular ruptures may be of great extent
oNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p. 390.
6 Spring, op. cit., p. 322.
Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble:
Philos. Trans. Royal Soc. London, ser. A, vol. 195, 1901, pp. 363-401.
>ION XLVII 04 -i
50 A TREATISE ON METAMORPHISM.
and wide apart, as in the case of faults; or of moderate extent and width,
as in the case of joints and bedding partings; or close together, as in the
case of fissility. The irregular ruptures may be continuous and the open-
ings wide, as in the case of the coarse breccias; or discontinuous and the
openings small; or so minute as to affect the individual particles, and thus
grade into deformations without openings or granulation.
Under natural conditions, in order that the pressure shall not exceed
the crushing strength of a rock in all directions, it is necessary that it shall
be in the zone of fracture for that rock.
Permanent strain with closing of openings and welding. Mechanical action may cloS6
openings in rocks and weld the separated parts. In this case there is a
diminution of volume due to bringing the particles closer to one another.
In order that welding shall take place there must be sufficient pressure in
all directions to bring the particles so close together that the molecular
attractions are effective, or the pressure in all directions must be greater
than the crushing strength of the rock.
Of course, the pressure required to satisfy the above conditions tor
welding depends very greatly upon the character of the material. Moder-
ate pressure may be sufficient to weld material composed of small and
weak particles. For instance, moderate pressure of clay may bring many
of the minute particles of kaolin so close to one another as to place them
within the limits of effective molecular attraction. When the clay is dried
the mass becomes harder. This hardening is doubtless due in part to the
precipitation of the dissolved material contained by the water and the conse-
quent cementation of the particles, as explained on pages 617-621. In pro-
portion as the particles are coarse, strong, and large, and have relatively
few points of contact, the pressure necessary to produce welding increases.
To produce deformation with welding of the separated large particles of
the strong minerals considerable pressure is necessary.
WATER ACTION.
The movement of water under the force of gravity is of the utmost
importance in metamorphism. It is, indeed, the great agent of transporta-
tion of material both overground and underground, and is the dominating
agent through which metamorphism is accomplished. Its work is fully
considered in Chapter III, on "The agents of metamorphism."
FORCES OF METAMORPHISM. 51
HEAT AND LIGHT.
Heat and light are form's of energy of the first importance. It has
already been noted that their ultimate source is largely gravity. Heat
is always present as a factor in metamorphism, for nowhere upon the surface
of the earth nor within the earth is the temperature absolute zero. Other
things being equal, the higher the temperature the more rapidly do alterations
of rocks take place. Light also affects all parts of the earth at the surface.
In metamorphism heat and light should be considered from two points of
view (1) sources of heat and light, and (2) effect of heat and light upon
the alterations of rocks.
SOURCES OF HEAT AND LIGHT.
Heat and light agential in the alteration of rocks are derived (a) from the
sun, (b) from deep within the earth by conduction or by convection through
water or magma, (c) from mechanical action, and (d) from chemical action.
The heat from all these sources is important; light, however, is derived
chiefly from the sun, that from the other three sources being of little
consequence.
THE SUN AS A SOURCE OF HEAT AND LIGHT.
The heat and light of the sun are forces of the first order of magnitude
in the alterations of rocks. The effect of these forces needs to be considered
in four cycles the cycle of the solar system, that of the seasons, that of
the cyclone, and that of the day.
The solar-system cycle is the most important. This cycle involves
two factors the absolute temperature and change in temperature.
As to the absolute temperature, were it not for the heat and light of
the sun it is certain that the temperature of the surface of the earth would
not greatly exceed that of the interstellar spaces. Probably it would be
200 C., or even lower. At the present time the temperature of the
surface of the earth averages 10 C. (283 C. absolute) or more. Therefore
the temperature of all the upper zone of the earth is 200 C., or more,
greater than it would be without the heat from the sun. Were it not for
this heat the water in the outer zone of the earth would be congealed, and
the atomic and molecular energy would be greatly diminished. As a
comparatively slight increase of temperature over that prevalent at the
surface of the earth increases greatly the speed of alteration of rocks, it is
52 A TREATISE ON MKTAMORPHISM.
to be presumed that under such low temperatures changes in rocks would
be so slow as to be negligible.
How deep below the surface of the earth the heat of the sun produces
an effect can not be accurately determined. It is highly probable that it
has an important effect to a depth of thousands of meters, probably beyond
the limits of the zone of observation. If the sun were not furnishing heat
to the earth, and the increment of increase in temperature were the same
as at present (1 C. for 30 meters), and the temperature at the surface were
200 C. lower it would be necessary to penetrate to a depth of 6,000 meters
to reach a temperature as high as that at the surface under the present
conditions. Below 6,000 meters the temperature would increase approxi-
mately as it does now from the surface downward. However, it is not to
be supposed that the effects of metamorphism would be the same as those
in the outer 6,000 meters at the present time, for the conditions of pressure
would be very different. (See Chapter I, p. 43, and Chapter IV, pp. 159-
160.) The assumption that the increment of temperature would remain
the same were not the sun giving heat to the earth is only approximately
true; but when it is remembered that 6,000 meters is an exceedingly small
fraction of the earth's radius, it seems probable that the increment of
increase of heat with depth in the outer part of the crust of the earth
would not be greatly different, even if the sun had long ceased to be a
source of heat; but if it were not for the heat of the sun, the temperature
of that part of the lithosphere directly under observation would be so low
that all chemical clianges would be very slow, if indeed they were not
inappreciable.
The absolute temperature at the surface is also dependent upon
latitude. The average temperature at the warmest tropical regions is about
300 C. absolute, or, stated in the ordinary scale, 27 C.; the average
temperature of the coldest polar region where observations have been made
(latitude 81 44') is 252.9 absolute, or, in the ordinary scale, 20.1 C."
At intermediate latitudes there are all gradations between these extremes.
At any place the temperature may be presumed to increase with depth from
these surface temperatures at the rate of 1 C. per 30 meters.
It would be fruitless to attempt a discussion of the changes of the
temperature of the outei' part of the earth due to the solar cycle. So far
"Hann, Julius, Handbuch <ler Klimatologie, J. Engelhorn, Stuttgart, 1883, p. 733.
CHANGES IN TEMPERATURE. 53
as rock alterations now taking place are concerned, the sun may be regarded
as furnishing to the earth a uniform amount of heat.
The seasonal changes of temperature are very important, at the surface
rano-ino- from 30 C. or less to as much as 80 C. However, the depth to
O C *
which the seasonal change produces an effect is not great, probably about
15 meters.
The cyclonic changes of temperature may be very great, ranging from
a few degrees to about 70, but the depth to which these changes extend is
slight, probably less than 3 meters.
The diurnal changes in temperature are scarcely less than the cyclonic,
ranging from to 50 C. or more; but the depth to which the diurnal
changes extend is insignificant, probably but a fraction of a meter.
From the foregoing it is plain that the heat and light derived from the
sun are of very great direct importance in the chemical and mechanical
changes that rocks undergo. It will also be seen that the various changes
of temperature as well as the absolute temperatures are of great consequence.
Moreover, the heat and light of the sun exert a very important indirect
influence upon metamorphism by reason of their being the sole source
of the energy which produces plants and animals, and these agents will be
seen to have a far-reaching effect upon the alterations of rocks. The effects
of the heat and light derived from the sun are fully considered in Chapters
VI, VII, and VIII, on "The belt of weathering," "The belt of cementation,"
and "The zone of anamorphism."
HEAT DERIVED FROM WITHIN THE EARTH BY CONDUCTION OR CONVECTION THROUGH
WATER OR MAGMA.
The amount of heat derived by the crust of the earth from the interior
depends upon the conductivity of the various rocks and upon the convec-
tional movements of magma and water.
The heat conductivity of the majority of rocks is between 0.4 and 0.6,
silver having a conductivity of 100. It is apparent that the conductivity
of rocks is very low as compared with that of the metals, but it can not be
doubted that there is a steady but slow flow of heat by conduction from the
interior of the earth to the zone of observation.
The amount of heat derived by the crust of the earth from intrusions
of igneous rocks is very great. So far as this heat passes into the adjacent
54 A TREATISE ON METAMORPHISM.
rocks by conduction, the coefficients are the same as in the transfer of heat
from the interior of the earth. The transfer of the heat of magma to adjacent
rocks is probably largely accomplished by convection. The magmas fre-
quently furnish heated solutions. Ordinary circulating waters approach or
come in contact with the igneous rocks; they thus became heated. The
heated waters move through the rocks controlled by the laws of under-
ground circulating waters (see pp. 146-153), and give up a part of their heat
to the surrounding rocks. The important metamorphosing effects of the
great igneous masses through water convection may extend several miles.
Contact metamorphisni is sometimes restricted to the very marked effects
due to high temperature immediately adjacent to the igneous rock. How-
ever, the alterations thus produced by high temperature as the result of
direct conduction are probably small, compared with the widespread effects
resulting from the dispersal of heat and material by means of underground
waters.
MECHANICAL ACTION AS A SOURCE OF HEAT.
It is a well-known principle that when work is done involving strain
of solids within the elastic limit, or subdivision of solids, or differential
movement between solids in contact, the energy is partly transformed to
heat. Hence strain within the elastic limit, subdivision of the rocks, and
differential movement between rock masses and particles and within the
particles raise the temperature of the rocks, and this greatly increases the
speed and extent of the chemical reactions. Heat developed by mechanical
action is therefore an important factor in the metamorphism of rocks.
Indeed, the resultant metamorphic products are very different under con-
ditions of movement and under conditions of quiescence; but heat is only
one of the factors entering into the differences. (See pp. 685707.)
CHEMICAL ACTION AS A SOURCE OF HEAT.
Chemical action always produces a positive or negative heat effect,
and thus promotes or retards metamorphism.
EFFECTS OF HEAT AND LIGHT ON ALTERATIONS OF ROCKS.
The relations between metamorphism and heat and light may be gener-
ally stated as follows: The kinetic energy of the molecules of substances,
whether in the form of gas, liquid, or solid, is increased by heat and light
EFFECTS OF HEAT AND LIGHT. 55
The speed of metamorphism is therefore largely dependent upon the amount
of heat and light present, especially the former.
In rock alteration heat and light produce direct effects and indirect
effects.
DIRECT KFFECTS OF HEAT AND LIGHT.
The more important direct effects may be either mechanical or
chemical.
Mechanical effects. The mechanical effects are desiccation, baking, and
fusion. At the surface of the earth the heat of the sun frequently results
in evaporating the moisture and desiccating the rocks; an attendant result
is induration. This process is especially important in the clay sediments,
and occurs to the greatest extent in the hot and arid regions, although
desiccation is not unimportant in the colder regions. The details of the
process especially concern the belt of weathering and are treated in the
chapter on that subject. (See Chapter VI, pp. 541-550.) Where igneous
rocks as a consequence of volcanism are brought into contact with other
rocks the latter may be baked for a longer or shorter distance from the
igneous rocks. The process of baking as here used is restricted to modifi-
cations similar to those which take place in the baking of bricks; that is, to
effects which are mainly due directly to the heat. This process is restricted
to the belt above the level of underground water the belt of weathering
and is therefore treated in detail in the chapter on that subject. (See
Chapter VI, pp. 488-494.) Below the belt of weathering the rocks are
saturated with water and the heat effects are mainly produced through that
agent. Even in the belt of weathering the baking effect is not wholly due
to heat, but is partly accomplished through the agency of the contained
water, precisely as is the transformation of clay to brick by burning; for all
rocks under natural conditions contain gas and water, and usually consid-
erable quantities. During the baking process the original molecules are
brought nearer together, but there are also important chemical changes.
Where the masses of the igneous rocks are very great, and especially
where adjacent rocks are included in masses of igneous rocks, the rocks
may be softened by the heat or even absorbed by the magma. Where
the rocks are softened they are likely to be very greatly changed, perhaps
recrystallized. Where they are absorbed by the magma they are lost as
original rocks and become a part of the magma by which they are absorbed.
56 A TREATISE ON METAMORFHISM.
When the modified magma crystallizes it takes the form of an ordinary
igneous rock, and may show no evidence of the fact that previously
solidified rocks have contributed material.
chemical effects. In proportion as the temperature is high chemical reactions
are likely to take place between solids. This is illustrated by the case-
hardening of iron. When soft iron is placed in contact with pulverized
charcoal and the temperature is raised to a red heat, but not to the point
of fusion, some of the carbon unites with the iron, transforming the outer
part of it into steel. Thus it is casehardened. Just how the union takes
place between the iron and the carbon is uncertain. It is supposed to be due to
the direct union of the solids, but we can not be quite sure that the result
is not accomplished through the agency of a gas. The carbon may be partly
oxidized, and thus be transformed to the gas carbon monoxide. This may
penetrate the iron, which may reduce the carbon monoxide to carbon again.
The reduced carbon may at the instant of reduction unite with the iron,
forming the carbide, or steel. While it is certain that high temperature is
favorable to the mutual chemical reactions of solids, when the temperature
becomes so high as to transform the solids to liquids the chemical reactions
are those of liquids rather than those of solids.
INDIRECT EFFECTS OF HEAT AND LIGHT.
The indirect effects of heat and light are accomplished through the
agents of metamorphism gases, water, and organic forms. The move-
ments of the atmosphere and hydrosphere are the conjoint effect of heat
and light and gravity. It has already been noted that the movements of
these bodies are the agents which do the main work of epigene transfer
of material. Not only do gas and water act as agents of transfer, but they
act as agents for chemical changes. It has already been seen that chem-
ical action may be a direct result of heat. However this may be, it is
certain that by far the more important, indeed the dominant, effects which
heat and light have upon chemical reactions are accomplished through the
agency of gases and water and organic forms. Of the forces heat and
light, the former is the important one in the reactions accomplished through
the agency of gases and water solutions ; but light is very important in the
production of organic agencies. The indirect effects of heat and light and
.all other conjoint forces are considered in connection with the agents of
alteration in Chapter III.
EFFECTS OF HEAT AND LIGHT. 57
GENERAL, STATEMENTS.
From the foregoing it is apparent that the effects of chemical energy,
gravity, and heat and light are not independent of one another; on the
contrary, they are most intricately interlocked. To a considerable degree
any one of the forms of energy may be transformed into the others. Con-
sequently the action of one almost always produces an effect upon the
action of the others. Moreover, one almost never acts without the action
of the others. Frequently all of the forces of metamorphism are important
simultaneous factors in the results; again, one or two of the forces may be
prominent, or even dominant, the others -playing a subordinate part. But,
in every transformation of metamorphism, if all the energy factors of the
entire system affected be taken into account, some of the energy is changed
into the lowest form of energy, heat, and at least a portion of this heat is
dissipated."
Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 51.
CHAPTER III.
THE AGENTS OF METAMORPHISM.
GENERAL STATEMENT.
The agents through which the alterations of rocks take place are
gaseous and liquid solutions and organisms. Solutions are the special
subject of this chapter. Organisms are influential only in the belt ot
weathering, and their action is therefore considered in connection with that
belt. (See Chapter VI.)
The circulation and work of solutions involve a consideration of the
circulation and work of the gases of the earth, of which the atmosphere is
the dominant portion, and a consideration of the circulation and work of the
water of the earth, of which the ocean is the dominant portion. While the
circulation and work of the atmosphere and of overground water may from
a purely theoretical point of view be considered as a part of a treatise on
metamorphism, the work of these epigene agents is the subject of that
division of geology which has been named physiography, and as the work
of the atmosphere and overground water is so fully dealt with in connection
with that subject, this branch of metamorphism will not be discussed here
at all. Hut the circulation and work of underground gas and water solu-
tions are of fundamental importance in metamorphism and must be some-
what fully considered.
Gas and water below the surface in the openings of the rocks will be
called ground gas and ground water, to discriminate them from gas and
water above the lithosphere.
Solutions "are homogeneous mixtures which can not be separated into
their constituent parts by mechanical means."" The properties of solutions
vary continuously and regularly with the concentration. 6 Under the
"Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., London,
1891, p. 1.
^Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. No. 64, Field
Operations of the Division of Soils, 1899, U. S. Dept. of Agric., 1900, pp. 142-143.
58
CHARACTER OF THE SOLUTIONS. 59
definition, solutions may be made by mingling gases and gases, gases and
liquids, gases and solids, liquids and liquids, liquids and solids, and solids
and solids. The solutions resulting from these various combinations may
be gases, liquids, or solids, or partly two or all. Gaseous solutions may be
formed by the mingling of gases and gases, of gases and liquids, and
of gases and solids." Liquid solutions may be formed by the mingling of
gases and gases, of gases and liquids, of liquids and liquids, of solids and
liquids, and of gases, liquids, and solids. Solid solutions may be formed
by the mingling of gases and solids, of liquids and solids, of solids and
solids, and of gases, liquids, and solids. But however complex the origin
and however numerous the components, the compounds with which the
geologist has to deal are gases, liquids, and solids. The two common
combinations which he has to consider are gaseous solutions and solids,
and liquid solutions and solids. The liquid solutions are universally
aqueous. The solids are the rocks. The combinations gaseous solutions
and solids, and aqueous solutions and solids will be treated under Parts I and
II of this chapter.
PART I. GASEOUS SOI/UTIONS.
Since the geological work of gases and vapors can not be practically
discriminated, the term gas is here used to cover both gases and vapors.
The gases which are important in rock alteration are oxygen (O 2 ),
sulphur (S 8 to S 2 ), water gas (H 2 O), ammonia (NH 3 ), carbon dioxide (CO 2 ),
sulphurous oxide (SO 2 ), boric acid (H 3 BO 3 ), hydrochloric acid (HC1), and
hydrofluoric acid (HF).
Never is one of these chemical compounds at work alone upon the
rocks; at the place of action there are always solutions of several gases.
Mineralizers in rocks, according to the original definition, are substances
which act in the gaseous condition; 6 but it will be seen (pp. 490-494)
that the term has been practically restricted to peculiar gases under special
circumstances. Notwithstanding the definition of the term, the action of
water solutions containing certain compounds, which if alone would be
gaseous, has been spoken of as due to mineralizers. The term miner-
alizers, if it is to serve any useful purpose, should be definitely restricted
aDaniell, Alfred, A text-book of the principles of physics. 3d ed., Macmillan Co., New York,
1895, p. 330.
6 Expression "Agents mineralisateurs " first used by Elie de Beaumont and defined by H. Ste.-
Claire Deville: Comptes rendus des Stances de I'Acad&nie des Sciences, vol. 52, 1861, pp. 920, 1264.
60 A TREATISE ON METAMORPHISM.
to the action of some particular compound or compounds, or else to some
form of compound, such as gases.
Gaseous solutions require consideration from two points of view the
chemical and physical principles controlling the action of gases and the
geological work of gases.
SECTION i. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION
OF GASES.
The chemical and physical principles controlling the work of gases
may be considered under (1) the gases present, (2) the pressure, and (3)
the temperature.
Gases present. The law of greatest importance controlling the chemical
action of gaseous solutions is: The properties of a homogeneous mixture or
solution of various gases are the sum of the properties of the constituents of
the mixture. To illustrate, when carbon dioxide (C0 2 ) and oxygen (0 2 ) are
mixed the properties and activities of each are the same as if the same
quantity of each were free from the other and occupied the same space.
Therefore, in the belt of weathering, where gases are active, the carbon
dioxide and oxygen are both doing their work, the one that of carbonation
(see pp. 473-480), the other that of oxidation (see pp. 461-473), as if the
other were not present. It is clear, therefore, that the properties of gaseous
mixtures are additive.
Slight deviations from this law have been noted under certain condi-
tions, but these mainly concern the exact physics of the gaseous solutions
rather than their geological work, and hence are not here considered. In
applying the law, however, we must be sure that the gases do not unite
chemically and produce a new compound. To illustrate, while the law is
certainly applicable to the case mentioned, that of a mixture of carbon
dioxide and oxygen, it is not certain that this is the case when water gas
(H 2 O) and carbon dioxide or sulphurous oxide (S0 2 ) are mixed, for these
compounds may unite with water gas, producing carbonic acid (H 2 C0 3 )
and sulphurous acid (H 2 SO 3 ) gases. Certainly the law will not apply to a
mixture of the gases ammonia (NH 3 ) and water, for these gases will largely
unite and produce ammonium hydroxide (NH 4 OH), which may exist in the
form of gas. In case a gas be formed by the union of two or more gases,
PRESSURE AI\ 7 D TEMPERATURE OF THE GASES. 61
the law controlling- the action of gases is applicable to the new compound,
and to the other gases with which it is mingled but does not unite
chemically.
As to the relative importance of the gases, it might at first be thought
that the strong acids, such as hydrochloric and hydrofluoric, are of greater
consequence than the much less active compounds, carbon dioxide and
oxygen; but it should be remembered that carbon dioxide and oxygen are
everywhere at work upon the surface of the earth, whereas the presence
of the strongly active compounds in more than minute quantities is excep-
tional. It therefore follows that the action of the universally present
weaker agents, such as carbon dioxide and oxygen, is of immeasurably
greater geological importance than the action of the stronger but much less
abundant gases.
The pressure. Increase of pressure increases the chemical activity of a gas.
This law follows from the fact that the number of molecules which act upon
a given space is directly as the pressure. The varying atmospheric pressure
may be taken as illustrating this principle. When the pressure increases,
say, by .05, this means that 1.05 times as many molecules of gas are actively
at work upon a given area as before.
One of the best illustrations of the increased activity of gases in accom-
plishing chemical work when under pressure is that of carbon dioxide. As
shown in another place (pp. 175-176), carbon dioxide is capable of decom-
posing many silicates at ordinary temperatures; but Struve and Mueller
have shown that when carbon dioxide is under pressure its effect in decom-
posing silicates is very much greater than under ordinary conditions. This
is in accordance with the law of mass action. In proportion as the pressure
increases the number of active molecules increases, and therefore the
geological work increases in proportion.
The temperature. The activity of gases increases with increase of temper-
ature. In proportion as the temperature is high, the kinetic molar energy
of the molecules of gases is great. The absolute temperature of a perfect
gas is believed to be a direct measure of its kinetic molar energy. By
molar kinetic energy is meant the energy of translation of the molecule of
a gas, and not the vibratory or rotary motions of the molecules themselves.
"Mueller, Richard, Untersuchungen tiber die Eimvirkung des kohlensiiurehaltigen Wassers auf
einige Mineralien und Gesteine: Tschermaks mineral. Mittheil., vol. 7, 1877, p. 47.
62 A TREATISE ON METAMORPHISM.
The kinetic energy of a moving body is the product of one-half of its mass
into the square of its velocity. When a gas is very dense its molecules
are closely crowded, and on account of the molecular attraction there is
an appreciable decrease in the theoretical pressure, which is a measure of
the kinetic molar energy. Since the kinetic energy of the gaseous molec-
ular projectiles increases as the squares of the velocities, this may explain
why a slight increase of temperature often greatly increases the chemical
reactions of the gases in contact with the solids of the earth's crust, for
the likelihood of a chemical union depends, among other things, upon the
energy with which the particles of a gas come in contact with the minerals
of the rocks.
SECTION 2. GEOLOGICAL WORK OF GASES.
The observable geological work of gases is mainly above the level of
ground water, or in the belt of weathering. In the belt of cementation,
below the level of underground water, the rocks are saturated with water
solutions. Gaseous substances, if present, would be in solution in water,
and their action would therefore fall under water solutions, treated on later
pages.
In the belt of weathering oxygen and carbon dioxide are immeasura-
bly the most important of the mineralizers, because they are present in the
interstices of the rocks in this belt throughout the laud areas. However,
in volcanic districts any or all of the geologically important gases may be
present and have a very marked metamorphosing effect upon the rocks.
But of these gases that of water is of vastly the greatest consequence.
The consideration in detail of the effects of these various mineralizers and
of their action in conjunction with other agents properly falls in Chapter VI
on "The belt of weathering."
In the deep-seated zone of anamorphism water itself is mainly above
its critical temperature (see pp. 659-661), and is therefore in the form of a
gas. On account of the great pressure the gases are dense. Under these
conditions most or all of the substances held in solution would also be in
the form of gases. The active substances would be solutions of gases in
gases. One would expect that the action of water gas holding in solution
other gases under such conditions of pressure and temperature would be
different from the action of highly heated water, in that its viscosity would
ACTION OF THE GASES. 63
be very small. It would therefore have a greater penetrating power than
water, and would be more highly energetic in its action. Under these
conditions the minutest spaces would be somewhat readily traversed. The
rocks of the deep zone in which action of this kind has taken place can
reach the surface only by passing through the zone in which water is in the
liquid form. Therefore the effects which were produced by the mineralizers
in the deepest zone will have been modified by the action of water solu-
tions during the long time the rocks were in the belt of cementation. The
details of the effect of water gases in the zone of rock flowage will be con-
sidered in Chapter VIII, on "The zone of anamorphism."
All of the gases may act in either of two ways: (1) By their presence
they may influence crystallization or recrystallization without entering into
combination. (2) They may enter into the combinations forming oxides,
hydroxides, carbonates, sulphates, etc. The first of these actions is spoken of
as that of crystallizers, and the second as that of mineralizers. In the meta-
morphic rocks it is ordinarily difficult to prove the past action of gases, not
in water solutions. Occasionally the materials of volcanic cones have been
rendered porous and the rocks altered in consequence of the action of
gaseous exhalations. In such cases the gases usually have united to some
extent with the materials through which they have passed, and in this way
furnish evidence of their past action.
PART II. AQUEOUS SOLUTIONS AISTD SOLIDS.
GENERAL CONSIDERATIONS.
The one liquid through which the greater part of the alterations of
rocks occur is water solution. Indeed, this is so- profoundly true that the
water of the earth has been compared with the blood of an organism. And it
is certainly true that the transformations of tissues by the blood are scarcely
more far-reaching than those of the lithosphere by the agency of water. It
has been determined by laboratory experiments that pure water at ordinary
temperatures is capable of dissolving all compounds to some extent. Cor-
responding with this fact, analyses of ground waters show that they contain
in solution all of the elements which occur in nature. The solutions may
vary from very dilute to rather strong. So far as the gases are dissolved in
water, their action is to be treated under water solutions, not under gases.
64 A TREATISE ON METAMORFHISM.
In the belt of weathering, above the free surface of ground water,
gaseous solutions and liquid solutions work together. In this belt the rocks
are not ordinarily saturated with water, but on the average contain a con-
siderable amount of water held by adhesion between the liquid and the solid
mineral particles. It is believed that in this belt the gases act upon the
rocks chiefly through water solutions. As evidence of this is the small
amount of decomposition of the disintegrated rocks in arid regions. (See
pp. 496-498.) It therefore appears that the dominant agents of alterations
in the belt of weathering are aqueous solutions.
In the belt of cementation below the free surface of ground water the
rocks are practically saturated, and in this belt aqueous solutions are the
chief agents of alterations.
Water solutions are also a chief agent in the transportation of material
from one place to another.
At this point it is necessary to understand that the places of interaction
of aqueous solutions and solids are the contacts between the two. It will
be seen later that, on account of the molecular attraction between water and
rock, a thin filni of water adheres to the solid particles with which it is in
contact. This film is not in active circulation, yet it is the part of the
agent, water, which is immediately concerned in the transfer of mineral
material from the rocks to the solutions and from the solutions to the rocks.
The contact film may take material of the rock into solution. From
this film the materials taken into solution migrate to other parts of the
solution. Probably the migration from the contact film to the free water is
largely by diffusion (see pp. 82-83) ; but, once beyond the contact film, the
migration is largely accomplished by convectional movements. Material
may be supplied to the contact film by migration of material from the free
parts of the solution. From the contact film material may be deposited in
the rocks.
In this connection it is interesting to note that in the portions of the
solutions near the contact with solids "there is often a concentration of
the dissolved material. This phenomenon has been called adsorption.""
The phenomena of adsorption seem to show with great clearness, not only
that the contact film is the active agent in transfer between the free solu-
Cameron, Frank K., Application of the theory of solutions to the study of soils: Report No. 64,
Field Operations of Division of Soils, 1899, U. S. Dept. of Agric., 1900, p. 142.
ACTION OF AQUEOUS SOLUTIONS. 65
tions and the solids, but that in this film the migration of the dissolved
material is to some extent stayed by the molecular attraction of the
crystals.
Aqueous solutions as a geological agent require consideration from two
points of view the chemical and physical principles controlling the action
of ground water, and the circulation and geological work of ground water.
These are treated in the following sections I and II, respectively:
SECTION i. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION
OF GROUND WATER.
The work of ground water, like any other work, requires the expendi-
ture of energy. The energy by which the water accomplishes its work is
derived from chemical action, heat, and mechanical action.
In order to comprehend the processes of alteration of rocks it will be
necessary to summarize the important conclusions of physical chemistry as
to solutions and chemical reactions. The principles here contained are
mainly taken from the works of Ostwald and Nernst.
Chemical action will be considered under the headings, "Principles of
solutions applicable to ground waters," and "Principles of chemical reactions
applicable to ground waters."
PRINCIPLES OF SOLUTIONS APPLICABLE TO GROUND WATERS.
While the consideration of the principles of solution logically falls
under general chemical action, and, perhaps, ought to be treated as a special
case under the general treatment of chemical reactions, it seems advisable,
because the subject of solutions is somewhat simple as compared with the
interactions of complex chemical compounds, to take up this subject first,
after which the general laws controlling chemical reactions will be given.
The water of rocks, whether at ordinary temperatures and pressures or
at higher temperatures and pressures, may take any of the substances
with which it comes in contact into solution; it may deposit substances
from solution; it may combine with substances forming hydroxides, as in the
case of many of the zeolites and limonite; it may part with its hydrogen in
exchange for bases, thus at the same time changing the composition of the
rock and taking the bases replaced into solution. This is illustrated by the
alteration of enstatite to talc. (See Chapter y, p. 268.) There may be
MON XLVII--04 5
66 A TREATISE ON METAMORPHISM.
reactions as a result of different substances being- taken into solution at
different times; there may be reactions as a result of different solutions
conning together, and thus mingling; there may be reactions between
substances in solution and the solid material with which the water is in
contact; there may be reactions as a result of changing temperature and
pressure. All these changes are in the nature of chemical action. There-
fore by chemical action through solutions is meant the taking of material
into solution, the deposition of material from solution, the interchange
between materials in solutions, the interchange between materials in solu-
tions and adjacent solids, and, finally, the interchange of the adjacent solid
particles, for such an interchange is usually accomplished through the
medium of a separating film of water. In this case the apparently simple
reaction between solids is really accomplished by transfers through sepa-
rating solutions. In all these interchanges the materials pass through a
stage of solution.
Salts are combinations of the metals and the acid radicals. Thus
Na,S0 4 is a combination of Na 2 and S0 4 , and KC1O 3 of K and C10 3 .
Faraday called these constituents ions. This term will be used as defined
by Faraday without any implication that a compound in solution separates
into its constituent ions or is dissociated.
According to many chemists" salts in various solutions are at least
partly separated into their ions. Such supposed separation has been called
electrolytic dissociation. If electrolytic dissociation takes place to a consid-
erable extent, the properties of the compounds are practically the sum of
the properties of their separated ions. In its power of dissociation of
dissolved salts water is held to exceed all other solvents. Water itself is
held to be slightly dissociated, or the H 2 O separates into the ions OH and
H. According to the theory of dissociation the presence of free ions in
water solutions is therefore universal. By the advocates of the theory it
is held that it is by the interaction of these free ions that chemical
interchanges are accomplished. But dissociation is held to be very imper-
fect in strong solutions, relatively far advanced in dilute solutions, and in
very dilute solutions nearly or quite complete. As the greater portion of
"Nernst, W., Theoretical chemistry, trans, by C. S. Palmer, Macmillan & Co., London, 1895,
p. 307. Ostwald, W., Outlines of general chemistry, trans, by James Walker, Macmillan & Co., Lon-
don, 1895, pp. 266-290.
FORM OF SILICA IN SOLUTIONS. . . 67
underground solutions are very dilute, at least where somewhat free circu-
lation is the rule, if the theory of dissociation be true we may suppose
that the salts held in solution are largely separated into their ions. While
the theory of dissociation and the explanation of chemical reactions by
interchange of free ions (see pp. 84-85) have a strong foothold in theoretical
chemistry, they have never gained universal support; and recently the
theory has been strongly attacked by Kahlenberg, who not only holds
that the theory is unnecessary to explain chemical reaction, but brings
together many facts which appear to controvert it." He has shown, more-
over, that instantaneous chemical changes take place in solutions that are
the best of insulators. 6
Until recently it has not been known how the most important of the
geological compounds, the silicates, behave when dissolved. However,
Kahlenberg and Lincoln c have shown that when dilute solutions of sili-
cates are made the silica exists in such solutions in the form of colloidal
silicic acid. To illustrate: If a sufficiently dilute solution of sodium
silicate be made, but much more concentrated than ordinarily occurs in
underground waters, the compound breaks up into NaOH and colloidal
silicic acid. From this fact it would not be supposed that the silicic acid is
a chemically active compound, and it is not active near the surface of the
earth at ordinary temperatures and pressures; but on subsequent pages it
will be seen that at considerable depth, where the pressure and temperature
are much above the normal, silicic acid is a most active compound.
Before the ionic theory of solutions gained recognition it was cus-
tomary in the published analyses of underground waters to suppose that
the bases and acids of the dissolved materials are united in a definite way.
For instance, chlorine was ordiuarilv considered as united with the potas-
sium, sodium, or calcium. The sulphuric oxide radical S0 4 was supposed
to be united with the oxides of potassium, magnesium, calcium, and sodium.
The carbon dioxide radical CO 3 was supposed to be united with the oxides
of iron, magnesium, sodium, and calcium. The aluminum and silica were
Kahlenberg, L., The theory of electrolytic dissociation as viewed in the light of facts recently
ascertained: Bull. Univ. of Wisconsin No. 47, 1901, pp. 299-351; also Jour. Phys. Chem., vol. 5, 1901,
pp. 339-392.
6 Kahlenl>erg, L., Instantaneous chemical reactions and the theory of electrolytic dissociation:
Jour. Phys. Chem., vol. 6, 1902, p. 1.
c Kahlenberg, L., and Lincoln, A. T., Solutions of silicates of the alkalies: Jour. Phys. Chem.,
vol. 2, 1898, pp. 88-90.
68 A TREATISE ON METAMORPHISM.
usually regarded as oxides, although in some cases the aluminum was
treated as united with the chlorine." However, results of recent analyses
have ordinarily been given on the basis of ions. 6
. In a solution, under the law of mass action, each of the bases is to be
considered as divided between all acids, and under the theory of disso-
ciation there are also present in the solutions the free ions of both the
bases and the acids. For example, suppose a strong underground water
solution to contain three bases and three acid radicals; as, for instance, the
bases sodium, calcium, and magnesium, and the radicals of carbonic,
sulphuric, and hydrochloric acid; then the following nine compounds are
present, Na 2 CO 3 , Na,S0 4> NaCl, CaCO 3 , CaSO 4 , CaCl 2 , MgC0 3 , MgSO 4 ,
MgCl 2 , and also the six free ions, Na, Ca, Mg, C0 3 , S0 4 , and 01, making
altogether fifteen separate combinations of the elements. However, under
the theory of dissociation, if the solutions be so weak that the substances in
solution are wholly ionized the nine compounds first mentioned will not be
present. If the dissociation theory be rejected, under the law of mass
action in all cases all of the nine compounds will be present, but not the
free ions.
Under the principles of solutions it is necessary to consider the cases
of (1) the solution of gases in ground waters, (2) the solution of solids in
ground waters, and (3) diffusion.
SOLUTIOX OF C1ASES IX OROUJID WATERS.
The quantity of gases which can be dissolved in underground water
depends upon the gases present, the pressure, the temperature, and the
solids in solution.
oases present A.11 the natural gases may be dissolved in water or may
unite with water. In the latter case the resultant compounds are dissolved.
In both cases solutions are formed.
Since below the level of the free surface of underground water it is
clear that the gases enter into solution either by absorption or by combina-
tion, it follows that the more far-reaching effects of these substances in
metamorphism are not as gases, but as aqueous solutions. The gases are
a Peale, A. C., Lists and analyses of the mineral springs of the United States: Bull. U. S. Geol.
Survey No. 32, 1886, pp. 43, 115, 133.
''Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods, U. 8. Geol.
Survey, 1880-1896: Bull. U. S. Geol. Survey No. 148, 1897. Clarke, F. W., Analyses of rocks,
laboratory of the U. 8. Geol. Survey, 1880-1899: Bull. U. S. Geol. Survey No. 168, 1900.
GASES IN SOLUTIONS. 69
therefore important factors in the c "lien of ground waters, but they are of
course only a small portion of the substances which ground waters carry.
The more important of these gases which pass into ground waters are:
Oxygen (O 2 ), carbon dioxide (CO 2 ), hydrosulphuric acid (H 2 S), sulphur-
ous oxide (SO 2 ), hydrochloric acid (HC1), hydrofluoric acid (HF), boric
acid (H 3 BO 3 ), and ammonia (NH 3 ). Sulphur and boric acid as gases occur
mainly in connection with volcanic action. If the above-mentioned or
other gases unite with the water the laws below given as to solubility do
not hold; thus carbon dioxide unites with water, forming carbonic acid
(C0 2 + H 2 = H 2 CO 3 ); sulphurous oxide unites with water, producing sul-
phurous acid (SO 2 +H 2 H 2 SO 3 ); ammonia unites with water, producing
ammonium hydrate (NH 3 + H 2 0=NH 4 OH). In some of these cases, for
instance, that of ammonia and sulphurous oxide, the water may unite with
many times its volume of the gas, with increase of volume; thus water at
C. and atmospheric pressure absorbs 1,050 volumes of ammonia as a
result of the union of the two. What portion of C0 2 contained in ground
water remains as CO 2 in solution, and what part unites with water, forming
carbonic acid, is uncertain, but it is definitely known that much of the CO 2
contained in the ground water is in the form of the so-called bicarbonates
for instance, such salts as Na 2 C0 3 +H 2 CO 3 or 2NaHCO 3 and therefore is
united with the water.
When new compounds are formed by the union of the gases with the
liquids, the substances held in solution are the new compounds. When
these new compounds are gases the laws below given concerning the solu-
tion of gases in liquids apply only to the new compound, not to the original
gas. Where the compound is a solid as, for instance, a bicarbonate the
laws for the solution of gases in water do not apply, but such compounds
are held under the laws controlling the solution of solids in liquids. (See
pp. 72-82.)
In some cases in nature a part of a gas may unite with a substance in
solution and make a new compound and a part may unite with water and
be dissolved in this form. If both the compounds be gases the laws for the
solution of gases in liquids hold. If the new compound formed be a solid
salt the laws for the solution of solids in liquids apply to it, and the laws
for the solution of gases in liquids apply to the uncombined gas. This case
is illustrated by carbon dioxide, already mentioned.
70 A TREATISE ON METAMORPHISM.
The pre.sure. "The quantity of a gas dissolved by a specified quantity of
a liquid is proportional to the pressure of the gas." This statement is true
of each gas without reference to whether a gas be alone or mixed with other
gases. Thus the solubility of each of a number of mixed gases is controlled
by the pressure exerted by that gas, not by the total pressure exerted by
the mixture. It is therefore clear that under natural conditions the press-
ure of that part of any gas which is in the atmosphere and the pressure of
that part which is held in solution in the water immediately adjacent are
the same when the two are in equilibrium, and the water is therefore just
saturated.
So far as ground waters are concerned, there are two cases; first, the
waters of the belt of weathering, or those to the level of ground water;
and second, those below the level of ground water, or the belt of satura-
tion. In the belt of weathering the pressure is atmospheric. Changes of
pressure are barometric. In so far as the atmospheric pressure varies and
this is by fractions up to one-fifteenth the solubility of the natural gases
in the water of the belt of weathering also varies directly as the pressure of
each of the gases varies, without reference to the pressure and solubility of
the other gases.
In the belt of saturation, just at the level of ground water, the amount
of gases held in solution is proportional to atmospheric pressure; but
at greater depths higher degrees of concentration of gases are possible,
although it might at first be thought that the atmospheric pressure or vapor
pressure at the free surface of the water would determine the concentration
of the solution. The pressure which really is determinative as to the
amount of gas which may be held in solution is that of a column of water
extending to the free surface, plus the atmospheric pressure. Since,
however, water is so much heavier than the atmosphere, at considerable
depths below the level of ground water the atmospheric pressure may be
neglected; and the pressure, and therefore the solubility of underground
gases in water, is almost directly proportional to the depth below the level
of ground water. For instance, at a depth of only 100 meters below the
level of ground water the pressure of the atmosphere is only one-tenth
that of the water pressure; at a depth of 1,000 meters it is only one
"Ostwald, W., Solutions, translated by M. M. Pattiaon Muir; Longmans, Green & Co., New York,
1891, p. 9.
RELATIONS OF PRESSURE AND SOLUTION. 71
one-hundredth, and at still gi.ater depths the fraction of pressure due to
the atmosphere is insignificant.
But in order that saturation for any gas corresponding to the pressure at
any given depth shall occur, it is necessary that a sufficient amount of gas
shall there exist. Gases may be produced below the level of ground water
by the chemical reactions, as by the liberation of carbon dioxide in the
process of silication. Later it will be seen (see Chapter VIII, pp. 677-679)
that this is one of the fundamental processes of the lower physical-chemical
zone. It follows from the above that at depth the amount of carbon
dioxide or other gas in solution per unit of water may be many score times
greater than near the surface. The pressure of carbon dioxide at the
surface is only about 0.0006 of an atmosphere. The water pressure at a
depth of 1,000 meters is almost 100 atmospheres; therefore the amount of
free carbon dioxide which may be held in solution, if pressure were the
only factor concerned, might be 166666 times as great as that held in
solution in the belt of weathering.
But it must be remembered that, as shown below, the increase of
temperature due to increase of depth somewhat reduces this multiple.
It should be remembered also that carbon dioxide combines with
water, producing carbonic acid, and the amount of this compound which
may be held in solution at the surface of ground water is not dependent
upon the pressure of the atmospheric carbon dioxide. But it is evident
that deep ground waters, where the pressure is great, may hold a vastly
greater quantity of carbon dioxide than can be held in solution near the
level of ground water.
As already pointed out, the law which obtains in reference to geological
work is that the activity of the carbon dioxide increases in direct ratio with
its quantity.
The theoretical conclusion that the action of carbon dioxide would be
increased by pressure, and consequent greater quantity, has been experi-
mentally verified by Mueller" and Struve, who found that strong pressure
increased the action of carbon dioxide in the decomposition of the silicates
more than did increase of time.
a Mueller, Richard, Untersuehungen iiber die Einwirkung des kohlensiiurehaltigen Wassers auf
einige Mineralien und Gesteine: Tschermaks mineral. Mittheil., vol. 7, 1877, p. 47.
72 A TREATISE ON METAMORPHISM.
The temperature. Increase of temperature generally results in decrease of
solubility of a gas." Increase in temperature with depth, or because of
volcanism, lessens the solubility of gases in ground water, and to this
extent works against the effect of increased pressure.
solids in solution. There is still another factor which enters to a slight
extent into the solubility of gases. Water holding solids in solution, in
most cases, absorbs less of a gas at a given pressure than does pure water. 6
However, the solutions near the surface are ordinarily so dilute that this
law is probably not important, but at depth it may be of some consequence
in working against the effect of increased pressure.
SOU;TI< OF SOLIDS IN OROUXD WATER.
Where a solid is placed in a liquid some or all of it dissolves, and thus
forms a homogeneous mixture composed of the two, or a solution.
It has been found that if a liquid be placed in a vessel having two com-
partments separated by a membrane through which the solvent but not the
dissolved substance may pass, when a soluble compound for instance,
sugar is dissolved in the liquid in one of the compartments, pressure
against the membrane is produced. This pressure has been called osmotic
pressure, to distinguish it from ordinary gas pressure, known as vapor
pressure. According to vau't Hoff, the osmotic pressure "is independent
of the nature of the solvent, and in general obeys the laws of gases." That
is to say, "the osmotic pressure is proportional to the concentration; the
osmotic pressure is proportional to the absolute temperature; the same
osmotic pressure can be obtained by equimolecular quantities of the most
various substances in the same solvent; the osmotic pressure is exactly the
same as the gas pressure which would be observed if the solvent were
removed and the dissolved substance were left filling the same space in the
gaseous state at the same temperature." These somewhat sweeping state-
ments need various modifications. For instance, where the solutions are
very concentrated the molecules in solution are believed to be so close to
aQstwald, W., Outlines of general chemistry, translated by James Walker, Macmillan & Co.,
London, 2d ed., 1895, p. 121.
&Ostwald, op. cit., p. 121.
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 134-137. Otwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co..
New York, 1891, pp. 112-117.
CONDITIONS OF SOLIDS IN SOLUTION. 73
one another that molecular attraction produces an effect, and in this case
the osmotic pressure does not vary directly as the concentration. But,
Cameron says, in so far as the molecules in solution are sufficiently sepa-
rated so that they may act as a gas, "the volume, pressure, and temper-
ature relations are dependent only upon the number of molecules involved."
Since all of these relations are the same as the laws controlling the
behavior of gases, it is held by many physical chemists that when a solid
passes into solution it is transformed to a gas. Under this explanation the
osmotic pressure is a gaseous pressure. "The kinetic energy of the
molecules of the dissolved substance is equal to that of the gas at the same
temperature ; and, moreover, as the kinetic energies of the molecules of the
dissolved subtance and of the solvent must agree, because these molecules
are in immediate contact, it follows also that the kinetic energy of the
molecules of the liquid must, on the whole, be the same as that of gaseous
molecules at the same temperature.'"'
If the above theory be correct, it follows that the solution of solids in
liquids is similar to that of gases in liquids; for in both cases the compound
when dissolved is in the form of a gas; and the geological work of under-
ground water, whether the solutions be produced by a mingling of gases
and water, solids and water, or the three combined, could be considered as
a unit. (See pp. 63-64.)
In case a salt dissolved in water be an electrolyte, under the dissocia-
tion theory it is separated into ions to some extent. If this be so, the
number of dissolved particles is represented by the number of ions plus the
number of undissociated molecules. Therefore in very dilute solutions,
where the dissociation is held to be complete, the number of dissolved
particles and consequently the osmotic pressure, is doubled in the case of
a salt of a monad acid with a monad base. Thus the law of equal gaseous
pressure for equal number of molecules is believed by many to still hold
good. For instance, if NaCl dissociates into the ions Na and Cl, or KOH
into the ions K and OH, thus giving twice as many molecules as in the
case of a compound which does not ionize, under the law the osmotic
pressure is twice as great as that of the compound which does not dissociate.
"Cameron, F. K., Application of theory of solutions to the study of soils: Report No. 64, Field
Operations of Division of Soils, 1899, U. S. Dept. of Agric., 1900, p. 144.
&0stwald, op. cit., p. 148.
74 A TREATISE ON METAMORPHISM.
The conclusions of van't Hoff, Ostwald, and others in reference to
osmotic pressure being due to gaseous pressure of tht dissolved substances
have never been accepted by Mendeleeff, and have recently been strongly
opposed by Kahlenberg. Certainly there are many discrepancies between
the observations made as to the amount of osmotic pressure and the amount
which the pressure should be under the gas law. But. so far as the
observations of geology show, I see nothing that controverts or confirms
van't Hoff's theory. In studying the work of underground solutions I
have been unable to discover any criteria which will separate the work of
gases in water solutions from the work of solids in water solutions. So
far as geology is concerned, solutions of gases in water and solutions of
solids in water can not be discriminated. It has been held by some that
the presence of fluorite and other minerals is evidence of gaseous action,
but, as yet, I have not been able to find valid evidence offered by any
author for this conjecture. It may be that gases dissolved in water and
solids dissolved in water are held in solution in consequence of chemical
affinity, as held by Mendeleeff, or they may be in solution as gases, as
held by van't Hoff, but in either case the manner of action of the two is
the same, and therefore there is no warrant for attributing the development
of fluorite, tourmaline, etc., to the presence of " miueralizers " in the sense
that these compounds are the products of the action of gases as opposed to
water solutions.
When a soluble solid is placed in a liquid solvent it at once begins to
dissolve. The temperature and pressure remaining constant, if an excess
of the solid be present after a sufficient time there is no further decrease
in the amount of the solid present, nor is there any increase. When this
state is reached the solution is saturated.
When a solid is in a saturated solution, and therefore constant in
amount, even if temperature and pressure remain constant it does not follow
that no interchange takes place between the dissolved and solid salt. The
kinetic theory of solutions leads to the conclusion that many molecules are
released from the solid to the solution, and pass from the solution into the
solid, but these amounts balance. This is well illustrated by sugar solu-
tions. If finely pulverized sugar be placed in the bottom of a saturated
sugar solution and sugar-covered threads be suspended in the solution,
sticks of rock candy will be formed. The crystals of the candy grow at
GROWTH OF LARGE CRYSTALS. 75
the expense of the sugar below, which is being constantly taken into solu-
tion and deposited as crystals about the string 1 ; and, therefore, although the
solution is continuously saturated, there is continuous solution and deposi-
tion. Even if no sugar-coated strings were placed in the sugar, after a
time it would be found to be coarser grained or to have recrystallized.
Thus the constant interchange between a saturated solution and that of an
adjacent solid is certain.
The change occurs under the law by which large crystals grow at the
expense of small ones. In order that crystals shall grow in a solvent, it is
necessary that the solutions shall be saturated or supersaturated at the
immediate place of crystal growth. Since underground there is always a
superabundance of many materials as compared with the amount of water,
we may suppose that at a moderate depth below the surface, and especially
in the smaller spaces, where movement is very slow (see pp. 138-146), the
solutions are often saturated. It is well known that the growth of larger
crystals at the expense of smaller ones, under conditions of saturation and
superabundance of material, goes on more rapidly in proportion as the
temperature is high and the pressure is great. The principle is taken
advantage of in the chemical laboratory in the production, before nitration, of
a coarse precipitate by boiling or other means. During the process the finer
particles of the precipitate are dissolved and the coarser ones are enlarged
at their cost. The growth of the large crystals at the expense of the small
ones is due to the fact that the smaller crystals are somewhat more soluble
than the larger. The explanation of this change, as given by Ostwald, a
lies in the "surface tension which exists on the boundary surfaces between
solids and liquids, as on those between liquids and gases the so-called
free surfaces of liquids. This tension acts so that the surfaces in question
are reduced in size, with the consequent enlargement of individual crystals
(the total amount of precipitate remaining practically unaltered), i. e., with
the coarsening of the grains." During the change, for a given volume of
solid the lessening of the total surface of the crystals, and consequently the
lessening of the surface tension, results from the fact that the surfaces are
small in proportion as the individuals are large. For a given volume of a
substance the surfaces of the crystals are inversely as their diameters. (See
a Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan,
Macmillan & Co., London, 1895, p. 22.
76 A TREATISE ON METAMORPHISM.
p. 98). The increase in the size of the crystals, lessening the surface
tension, may be considered as a transfer of potential into kinetic energy.
This passes into heat and is dispersed under the apparently general law of
the dissipation of energy. Why the tendency to the transformation of all
forms of energy into heat and the dissipation of heat should be a law of
nature it is not my purpose here to discuss. But such the law seems to
be, and in its application we carry the causal sequence as far as we are
now able.
The growth of large individuals at the expense of small ones in ground
water is of the most profound significance in the metamorphism of rocks.
It is illustrated by the secondary enlargement of minerals and by the por-
phyritic crystals which frequently develop in schists and gneisses, such as
the porphyritic crystals of feldspar, hornblende, garnet, staurolite, etc.
(See pp. 643-644, 699-700.)
The above principle in reference to the growth of large crystals at the
expense of small ones is very clearly applicable to the growth of segregations
of minerals of a certain kind as compared with smaller segregations. If,
for instance, at one place there be a mineral aggregate, this, so far as the
surface tension and the free surface of liquids are concerned, acts as a unit
and tends to draw to itself the material of smaller aggregates or of individual
mineral particles. For aggregates which do not have crystal boundaries
the form which would be assumed under ideal conditions is spherical.
This principle of the growth of large aggregates at the expense of small
ones is illustrated by chert nodules. (See pp. 816-818.)
The quantity of a solid which can be dissolved in aqueous solutions
depends upon the compounds present, the pressure, and the temperature.
When the limit of solubility is reached the solution is said to be saturated.
COMPOUNDS PRESENT.
Theoretically all compounds are soluble to some extent in water. This
statement applies to all natural compounds; that is, the minerals of nature
are elements, oxides, or salts which are soluble in water. No substance is
wholly insoluble in the ground solutions, even at the ordinary temperatures
and pressures. Tliis statement is illustrated by the solution of quartz and
the more refractory silicates at the surface." Under surface conditions
a Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol. 8,.
1S97, pp. 214-217.
MUTUAL INFLUENCE OF COMPOUNDS. 77
quartz grains are sometimes etched by meteoric waters, and the decompo-
sition and partial solution of the refractory silicates is universal. Under
conditions of deep-water circulation solution of quartz and the refractory
silicates may be accomplished with relative rapidity. This is illustrated
by the Calumet and Hecla conglomerate, many of the pebbles of which
have been partly or even completely dissolved and the space once occupied
by them taken by copper.
Since underground solutions always contain a number of compounds,
and often many, the influence of one compound upon the solubility of
another is of consequence in various ways. For instance, when several
compounds are present, a unit quantity of water will not dissolve as much
of a given salt as it would if it were alone. But if a number of units of
water are each saturated with a single salt, and the solutions are mingled
without chemical reaction, the mixture is capable of taking additional quan-
tities of the salts into solution. In other words, a unit of solution simul-
taneously saturated with each of several compounds contains a greater
total of solids than a unit of solution saturated with fewer of these com-
pounds, but less of any individual salt than it would were it saturated with
that salt alone. 6
In the ground solutions the different compounds frequently react upon
one another, and therefore important modifications in the above statement
are necessary, as is explained under "Precipitation," pp. 113-123.
RELATIONS OK SOLUTION AND PRE&Sl'RE.
In general, the volume of the solvent plus that of the dissolved
compound is greater than that of the solution. For a given quantity of
the solid the contraction is greater the more of the solvent is used. c In
some cases, however, the volume of the dissolved compound and solvent is
less than that of the solution, or expansion results from dissolving the solid.
Ammonium chloride in water is an illustration of this case. From the fore-
going relations we obtain a rule as to the relations of pressure to solubility.*
In the common case in which the volume of the solution is less than that of
Pumpelly, R., The paragenesis anil derivation of copper and its associates on Lake Superior: Am.
Jour. Sci., 3d ser., vol. 2, 1871, p. 34.
0stwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., lle-.v York,
1891, pp. 83, 84.
"Ostwald, W., op. cit., p. 82.
<*Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macuiillan & Co., London,
1895, p. 567.
78
A TREATISE ON METAMOKPHISM.
solvent and solid, pressure increases solubility; for in that case solution
tends to bring the molecules nearer together and works in conjunction with
the pressure. A mixture of water and ice furnishes an excellent illustration
of this principle. At any moment the volume of the water is less than
that of the equivalent water and ice. Hence pressure promotes solution and
prevents freezing, or in other words, crystallization. In the reverse case,
that in winch the volume of the solution is greater than that of solvent and
solid, pressure decreases the solubility, the reason being the reverse of that
of the previous case.
The above law may be illustrated by fig. 1. A given amount of salt,
say 10 cc. in volume, may be supposed to be placed in 90 cc. of water, and
the salt be of such a nature as to saturate the water at that
temperature and pressure. Before solution begins the
space occupied is 100 cc. After solution this space may
be greater or less than 100 cc., say 105 cc. or 95 cc. ; that
is, the water surface instead of being at aa will be at cc or
Hb. If it be at bl, where the volume is less, and the pres-
sure be increased, an additional amount of salt ma} be
added and taken into solution. If it be at cc, and the
pressure be increased, a part of the salt already in solution
will be precipitated from the solution.
It is well known that the solubility of calcium car-
bonate and of some other carbonates is increased by pres-
sure." It is a fair inference from Barus's work that the
solubility of the silicates is also increased by pressure.
Barus 6 found that when soft glass is dissolved in water at temperatures
above 210 C., the volume is 20 to 30 per cent less than the two sepa-
rately. This glass is one which contains alkalies, alkaline earths, and
lead, and therefore is somewhat similar in composition to many natural
silicates. The carbonates and the silicates are the dominant compounds
in underground solutions. The solubility of many other salts, besides
the carbonates and silicates, occurring underground is increased by pres-
sure. Therefore, in the majority of the complex underground solutions
"Lindgren, W., Gold-quartz veins of Nevada City and Grass Valley, California: Seventeenth
Ann. Kept. U. 8. Geol. Survey, pt. 2, 1896, pp. 176-178.
6 Barus, C., Hot water and soft glass in their therinodynarnic relations: Am. Jour. Sci., 4th
ser., vol. 9, 1900, p. 173.
FIG. 1. Change of vol-
ume resulting from so-
lution, and relations of
solution and preaiure.
RELATION OF SOLUTION AND TEMPERATURE. 79
the totals of the salts in solution are in general increased by pressure,
and the volumes of the solution are less than those of the salts and solvents
separately.
RELATIONS OF SOLUTION AND TEMPERATURE.
The relations of solution and temperature have three phases; first, the
speed of solution; second, the quantity of material which may be held in
solution; third, the relations of solution to absorption and liberation of heat.
speed of solution. The speed of solution is commonly increased greatly by
rise of temperature." 1 A slight increase in temperature may increase the rate
of solution out of all proportion to the absolute change in temperature. At
temperatures above 100 C., and especially above 185 C., the activity of
water may increase to an amazing degree. The rapid solution of glass, by
Barus, 6 at temperatures about 185 C. illustrates this. At any temperature
solution will continue until the point of saturation is reached, but this state
will be attained at high temperatures in but a small fraction of the time
required at low temperatures. For instance, to saturate an underground
solution with the refractory silicates or sulphides at ordinary temperatures
might require months, or even years, while to saturate them at temperatures
above 185 C. might require only an equal number of minutes, or at most,
hours. The capacity of water for action at high temperatures combined
with pressure, considered above, is adequate to explain the complete
recrystallizatiori of great volumes of rock. (See pp. 749-751.)
Quantity of material which may be held in solution. The effect of temperature UpOll
the quantity of material which may be held in-solution does not admit of a
simple general statement." For most substances moderate increase of
temperature gives greater capacity for solution; but for many substances
there exists a temperature at which there is the maximum capacity for
solution, and the amount of material which may be held in solution at
higher and lower temperatures is less than this maximum. The quanti-
tative relations of solution and temperature at ordinary pressure between
oNernst, W., Theoretical chemistry, translated by C. 8. Palmer, Macmillan & Co., London, 1895,
p. 568.
* Barus, C., Hot water and soft glass in their thermodynamic relations: Am. Jour. Sci., 4th ser.,
vol. 6, 1898, p. 270, and vol. 9, 1900, pp. 167-168.
''Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New-
York, 1891, pp. 55-77.
80
A TREATISE ON METAMORPHISM.
C. and 100 C. are shown by fig. 2, taken from Ostwald." For various
substances the maximum capacity for solution lies between 60 and 140
C., and for many substances it is probably below 200 C. It therefore
follows, in respect to underground solutions, that a general statement can
not be made as to how change of temperature may affect solubility.
However, it is highly probable that up to temperatures of 100 C., and
therefore under normal conditions to depths of 3,300 meters, increase of
temperature increases the average capacity of underground water to hold
material in solution; and it is probable that the average capacity of
ground water increases to temperatures considerably above 100 C., and
therefore to depths greater than 3,300 meters. But when water passes
downward to the deeper parts of the zone of fracture the increase in temper-
ature may lessen the average capacity for holding material in solution,
provided the joint effect of pressure be
barred. But it has been seen that increas-
ing pressure with increasing depth pro-
motes solubility. It is almost certain that
high temperature and pressure combined
greatly increase the capacity of water for
10 20 30 40 so 60 7o 80 90 100 solution. This is proved by the experi-
Temprature.
,j j, _
Fie. 2.-Quantitative relations between solution and m 6ntS OI O&VUS UpOll the Solubility Of
glass. He has shown that at temperatures
above 185 C. and below 200 C it is possible "to impregnate glass
with water to such an extent as to make it fusible below 200 C. The
solution occurs with contraction of bulk relatively to the ingredients and
increasing compressibility." . . . "If these solutions are sufficiently
concentrated they coagulate at ordinary temperature and the congealed
aqueous glass is not different in general appearance from common glass.
The melting point of the coagulated aqueous silicate frequently lies below
200 C., probably above 1 50 C., depending on the glass." And he con-
cludes that "Glass as a colloid is miscible in all proportions with water." b
Since glass is one of the important silicate rocks which occur in nature,
these statements are directly applicable to one set of rocks. They may
"Ostwald, W., Grundlinien der anorganischen Cheinie, Engelmann, Leipzig, 1900, p. 222..
'>Barun, C., Remarks on colloidal glass: Am. Jour. Sci., 4th ser., vol. 6, 1898, p. 270. See also Am.
Jour. Sci., 4th ser., vol. 9, 1900, pp. 161-175.
GRADATION BETWEEN LIQUIDS AND SOLIDS. 81
not be applicable to the same extent to crystallized silicate rocks, but it
seems to me highly probable that they apply in large measure to many.
In so far as Barus's final conclusion is applicable, there may result all grada-
tions, from solutions in which the water is the dominant constituent to
those in which it is the subordinate constituent. This principle of the
increased quantity of material which may be held in solution as a result of
combined high pressure and temperature is believed to possess very great
significance in alterations in the zone of anamorphism, and to be of impor-
tance in alterations in the belt of cementation. (See pp. 602-603, 659-661.)
Relations of solution to absorption and liberation of heat. As already explained, wlieil
material passes into solution the molecules are separated and acquire
kinetic energy, and are believed by many to change from the solid to the
gaseous form. This process absorbs heat. On the other hand, where the
volume of the solution is less than the volume of the solvent and salt sepa-
rately, the molecules of the solvent and salt combined are brought closer
together and heat is therefore liberated. In the reverse case, where the
volume of the solution is greater than that of the solvent and salt separately,
the molecules are pushed farther apart, and heat is absorbed. If the com-
pounds in solution separate into ions this process is believed to be usually
attended by liberation of heat." Whether there is a rise or fall of tempera-
ture of the solution will depend upon the relative values of these factors.
In the common case where there is decrease in the volume as a result of
solution, the heat thus liberated by change in volume plus the supposed
heat of ionization are together preponderant, and there is, therefore, libera-
tion of heat and a rise in temperature. However, in the case where there
is increase in the volume as a result of solution, the heat thus absorbed and
the heat absorbed in changing the salt from the state of a solid to that of a
gas is greater than that supposed to be liberated by dissociation. The first
two factors are dominant, and there is usually a marked absorption of heat
and, consequently, a fall in the temperature of the solution. This is illus-
trated by the solution of ammonium chloride in water. The volume is
considerably decreased and the fall in temperature is very decided.
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co.. London, 1895,
p. 562.
MON XLVII 04 6
82 A TREATISE OX METAMORPHISM.
1HFFI'SIO>.
It has been seen that the molecules of gases and of solids when dis-
solved in water are distributed through the solution. When the material
dissolved is not evenly distributed the molecules are more abundant here
and less abundant there. If the theory be true that the dissolved solids are
gaseous the molecules would exert a greater pressure where more closely
packed. Under these conditions molecules where more closely packed
move toward places where they are less closely packed. This move-
ment is regarded by many as the explanation of osmotic pressure.
Kahlenberg, however, does not accept this explanation, but regards osmotic
pressure as due to the "mutual attraction between solvent and dissolved
substance."" Without reference to either theory the more important
conclusions in reference to diffusion may be summarized.
The force which drives the dissolved substances from place to place,
and the velocity with which a dissolved substance wanders in a solvent, is
proportional to the degree of concentration. 6 Therefore, "the quantity of
a salt which diffuses through a given area is proportional to the difference
between the concentrations of two areas infinitely near one another."" In
other words, diffusion is proportional to the difference in strength. The
quantity diffused is proportional to the square root of the time of diffusion,
and the distance over which a determinate concentration extends is also
proportional to the square root of the time of diffusion.* 1 Several salts in a
solution diffuse almost independently of one another, each at its own specific
rate.' At 20, according to Ostwald, there is twice as much diffusion as at
0, and at 40 twice as much as at 20/ When a solution is in equilibrium
the concentration of the solution varies inversely as the temperature. It
follows that when the temperature of the solution varies, equilibrium is
obtained not by equal distribution of the solutes, but by unequal distri-
bution. If the temperature be the same throughout a solution with equal
Kahlenberg, Louis, The theory of electrolytic dissociation as viewed in the light of facts recently
ascertained: Bull. Univ. of Wisconsin No. 47, 1901, p. 349.
&Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 143-144.
c Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York.
1891, p. 120.
<* Solutions, cit., p. 135.
Solutions, cit, p. 139.
/Solutions, cit., p. 13C.
SLOWNESS OF DIFFUSION. 83
distribution of the dissolved compounds, a deviation from uniformity in the
temperature of the solution will disturb the equilibrium and result in
unequal distribution of the dissolved substances.
The values of the coefficient of diffusion (D) of certain substances in
water solutions at various temperatures are given by the following table
from Nernst: 6 (The table gives the number of grams of the dissolved
substance which will pass in one day through a section of 1 sq. cm. when
the difference in concentration of the cross section 1 cm. apart amounts to
1 gram in a cubic centimeter.)
Hates of diffusion of certain substances in water solutions at various temperatures.
Temp. D.
Hydrochloric acid 0. 1.4
Do 11.0 1.84
Nitric acid 9. 1. 75
Sulphuric acid 7. 5 1. 04
Acetic acid 14.0 .81
Potassium hydroxide 13. 5 1. 66
Sodium hydroxide 8. 1. 96
Ammonium hydroxide 4. 5 1.06
Sodium chloride 6. .75
Ammonium chloride 17.5 1.31
Potassium chloride 9. .66
Barium chloride 8. .65
Potassium nitrate 7. .92
Sodium nitrate 13.0 .90
Silver nitrate 7. 5 .90
Lead nitrate 12. .70
Urea 7. 5 .81
Chloral hydrate ; 9. .55
Mannite 10. .38
This table shows that diffusion is extremely slow. The slowness
with which diffusion occurs is due, according to Nernst, to "the resistant
friction experienced by the dissolved substance in its movement through
the solvent." c This friction is very great, because the molecules themselves
are exceedingly small.
Later it will be seen that the process of diffusion is of very considerable
importance in the migration of compounds in ground water. (See pp.
636-639.) This is illustrated by the very important process of solution
and deposition or recrystallization.
Ostwald, Solutions, cit., pp. 150-151.
& Nernst, W., Theoretical chemistry, translated by 0. S. Palmer, Macmillan & Co., London, 1895,
p. 144.
c Nernst, op. cit., p. 145.
84 A TREATISE ON METAMOKPHISM.
PRINCIPLES OF CHEMICAL REACTIONS APPLICABLE TO GROUND WATERS.
GENERAL STATEMENT.
DEFINITIONS.
Before taking up chemical reactions it is advisable to give a number
of elementary definitions.
"Compounds whose aqueous solutions contain the hydrogen ion (H)
are termed acids, and those which contain the hydroxyl ion (OH) bases.""
To illustrate, HC1 is an acid; NaOH is a base. When the hydrogen ion
united with one or more uonmetallic elements is mingled in solution with
the hydroxyl ion united with a metal a double reaction occurs, resulting
in the union of the hydrogen ions with the hydroxyl ions, forming water,
and the union of the nonmetallic parts of the compound with the metallic
parts. This latter union forms a salt. For example
HCl+NaOH=NaCl+H 2 O.
H 2 CO s +2NaOH=Na 2 C0 3 +2H 2 0.
The acids and salts which contain only a single nonmetallic element are
called binary compounds. The acids and salts which contain two non-
metallic elements are called ternary compounds. For example, HC1 is a
binary acid; NaCl is a binary salt; H 2 CO 3 is a ternary acid; Na 2 CO 3 is
a ternary salt. Compounds having the composition of acids, bases, and
salts may be separated from solution as solids, and of course all of these
solids may pass into solution.
Some salts also contain a certain amount of acid, and such salts are
called acid salts. For instance
NajCO,+H 2 CO 5 =2NaHCO 8 .
The latter compound is acid sodium carbonate. On the other hand, some salts
contain some additional base, and such salts are called basic. For example,
Fe 2 (S0 4 ) 3 may be united with Fe 2 (OH) 6 , producing mFe 2 (SO 4 ) 3 .nFe 2 (OH) 6 .
This compound is basic ferric sulphate.
DIHSOCUTION.
In explaining chemical reactions the theory of dissociation as advocated
by Arrhenius, Ostwald, Nernst, and others is followed for the most part
"Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan,
Macmillan & Co., London, 1895, p. 117.
THEORY OF DISSOCIATION. 85
This theory is firmly placed in the text-books. No opinion is expressed
by me as to its correctness. Indeed, I have no right to any opinion on the
subject. As already pointed out, this theory has been vigorously opposed
by Kahlenberg, but as yet that author has offered no constructive theory
to take its place. I therefore follow the theory of the standard text-books
so far as necessary to show how it would apply to the work of ground solu-
tions if it prove to be true ; but so far as practicable I make the statements
in such form that they will be correct even if the theory of free ions and
reactions between such ions is finally abandoned.
Under the theory of dissociation the superiority of water as a solvent
for chemical interchanges is regarded as largely due to the fact that the
dissolved substances are separated into their ions to a greater degree than
in any other solvent. To the fact of active reactions in water, whatever
their cause, are very largely due the profound changes which occur in rocks
through the medium of water solutions. '
Under the theory of dissociation water solutions, acids, bases, and salts
separate into their ions. For instance, HC1 separates into the free ions H
and Cl; NaOH separates into the free ions Na and OH; and NaCl into the
free ions Na and Cl. However, in solutions the dibasic acids are supposed
to separate into free ions somewhat differently from what might be expected.
For instance, it might be expected that H 2 C0 3 would separate into the free
ions H 2 and CO 3 , but it is supposed to separate thus :
Other dibasic acids are thought to dissociate in a similar manner. However,.
if the dibasic acid be very strong the compound ion may again break up.
Thus, H 2 SO 4 is thought to first break up into the free ions H and HSO 4 ,
and the latter to break up into the free ions H and SO 4 , so that these would
be the ions present in the water. But in the case of the weak acid, carbonic,
it is thought that the last change does not take place, and that the free ions
remain H and HCO 3 .
Ostwald regards the absence of the second stage of dissociation as the
explanation of the peculiar characteristics of carbonic acid." Since carbonic
acid is, next to silica, the most important rock-making acid, the manner in
which it breaks up is of great consequence in metamorphism.
"Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, pp. 276-278,.
397-398.
86 A TREATISE ON METAMORPHISM.
HYDROLYSIS.
Under the theory of dissociation, not only do acids, bases, and salts
separate into ions in water solutions, but the water itself is believed to
dissociate to a very small extent, according to the equation H 2 0=rH-f-OH,
thus simultaneously forming free hydrogen and hydroxyl. If this be true
the hydrogen ions and the hydroxyl ions coexist and water solutions to a
small extent contain free acids and free bases at the same time. The
excellence of water as an agent for reactions between the substances it
holds in solution is held to be partly due to hydrolysis.
When strong bases and acids are in solution the amount of their dis-
sociation is believed to be so much greater than that of water that the
dissociation of the latter is of little consequence. But if a very strong base
be united with a weak acid the solution will give an alkaline reaction, and
this is regarded as showing the presence of free hydroxyl ions or of
Hydrolysis. For instance, if the strong base, sodium, be united with the
weak acid, carbonic, and a water solution be made, it is held that hydrolysis
will take place to some extent, thus:
Na 5 CO s +H 2 O=NaHCO s +NaOH.
It is supposed that NaHCO 3 breaks up into the ions Na and HC0 3 , and
the NaOH into the ions Na and OH. Therefore, in a solution of Na 2 CO 3
in water the coexistent ions are thought to be H, HC0 3 , Na, and OH.
Since the base, NaOH, is stronger than the acid, HC0 3 , the separation into
the ions is thought to be the explanation of the alkaline reaction.
Cameron has shown that sodium silicate in solution gives an alkaline
reaction, and his explanation is that this compound is hydrolized in a man-
ner precisely similar to that of sodium carbonate. 6 Not only do solutions
of sodium silicate give alkaline reactions, but Clarke has shown" that many
natural mineral silicates, when treated with pure water, show an alkaline
reaction. The following gave permanent alkaline reactions: Phlogopite,
oligoclase, albite, cancrinite, sodalite, analcite, natrolite, pectolite, apophyl-
lite, segirite. The following gave more or less distinct colorations to the
phenolphthaleiu indicator, but in time faded: Muscovite, lepidolite, ortho-
"Ostwald, W., Grundlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, pp. 254-257.
^Cameron, F. K., Application of the theory of solutions to the study of soils: Rept. No. 64, Field
Operations of the Division of Soils, 1899, U. 8. Dept. of Agric., 1900, p. 169.
Clarke, F. W., Alkaline reaction of some natural silicates: Jour. Am. Chem. Soc., vol. 20, 1898,
pp. 739-742.
HYDROLYSIS OF COMPOUNDS. 87
clase, leucite, nephelite, spodumene, scapolite, laumontite, stilbite, chabazite,
heulandite, thomsonite." If the theory of dissociation be true, this shows that
the silicates are hydrolized, thus :
R 2 SiO 4 +4HOH =2R(OH ) 2 +H 4 Si0 4 .
However, according to Kahlenberg and Lincoln, the H 4 SiO 4 does not dis-
sociate into the radicals H and SiO 4 , but forms colloidal silicic acid. 6 Thus
this compound is inert and the reaction reverses only to a small extent and
under favorable conditions, and the hydrate of the alkali metal gives an
alkaline reaction.
In a similar manner hydrolysis is held to occur in water solutions of the
strong base sodium with the weak acid hydrosulphuric, thus:
Na 2 S+H 2 O=NaHS+NaOH.
Cameron further states that hydrolysis is to be expected in the case of the
aluminates and ferrates/ When a strong acid is united with a weak base
the solution gives an acid reaction, and this is also explained by dissociation,
the free acid supposed to result from hydrolysis being stronger than the
weak base. d
Since the three most abundant acids of nature are silicic, carbonic, and
hydrosulphuric, all weak, hydrolysis, if true, is a reaction of fundamental
importance in metamorphism.
REACTIONS.
When, after a number of chemical substances are brought together,
and especially when they are united by a solvent, interactions between
them may occur which after a time appear to cease. When the conditions
have become such that there is no increase or decrease in the amount of
any one of the chemical compounds, the system is in a condition of
chemical equilibrium. 8 When two substances in solution, A and B, react
upon each other so as to produce two other substances, C and D, if
solutions of C and D are mixed they in turn will react upon each other to
Clarke, cit., pp. 740-741.
6 Kahlenberg, L., and Lincoln, A. T., Solutions of silicates of the alkalies: Jour. Phys. Chem., vol.
2, 1898, pp. 77-90.
e Cameron, cit., p. 169.
<*Ostwald, Grundlinien, cit., pp. 276-278, 397.
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 35-356.
88 A TREATISE ON METAMORPHISM.
produce more or less of the substances A and B. That is, the reaction is
reversible to a greater or less degree. To illustrate, if two solutions, one
of them containing MgSO 4 and the other Na 2 CO 3 , come together, the ions
are Mg, Na, SO 4 , and CO 3 . A part of the Mg will unite with the CO 3 ,
producing MgCO 3 , and a part of the Na will unite with the SO 4 , producing
Xa 2 SO 4 . Vice versa, if solutions of Na 2 S0 4 and MgC0 3 are mingled in a
similar manner, MgSO 4 and Na-jCOs will be produced. The reversible
reaction may be briefly expressed thus:
MgSO,-
The sign 71 means that the equations may be read from left to right
or from right to left.
These are the facts: The ions do interchange between compounds
whenever a chemical reaction takes place. Just how and why they inter-
change is another matter, upon which there is not agreement. Under the
theory of dissociation the interchange takes place through the medium of
the free ions. The free ions of a compound A are held to collide with the
free ions of the compound B, and thus produce the compound C and D,
and vice versa. Or, in the specific case above given, of MgSO 4 and Na^COs,
the free Mg ions collide with the free CO 3 ions and produce MgCO 3 , and
the SO 4 ions collide with the Na ions and produce Na^SO^ From the
MgCO 3 and Na 2 SO 4 , MgSO 4 and Na 2 CO 3 are reproduced in a similar manner.
Whether the theory of chemical reactions through free ions be of any value or
not, it seems probable that free ions exist for a moment when the interchange
takes place. To illustrate, it seems hardly probable that the Mg is united
to the SO 4 and the C0 3 at the same time. If the Mg lets go of the S0 4 to
attach itself to the C0 3 , for that instant the ion Mg is free. The same is
true of each of the other ions, Na, SO 4 , and C0 3 , at the instant of inter-
change. Hence the question at issue is the cause of the interchange. Does
it take place as the result of contact of free ions produced by dissociation,
or does the chemical affinity of the Mg for the CO 3 cause a portion of it to
leave the stronger acid radical SO 4 for the weaker acid radical CO 3 , etc.?
But this is a question for chemists to settle. The problem is stated here
because it is one of such fundamental importance in metamorphism.
But whatever the cause, reversible reactions are a certainty, and it will
oNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 356-357.
REVERSIBLE REACTIONS. 89
be seen that certain reactions of the zones of katamorphism are reversed in
the zone of anamorphism, and vice versa. But it should be remembered
that to close the cycle in any case, or, in other words, to make a trans-
formation from left to right and then from right to left, thus completely-
reversing any reaction, requires the expenditure and dissipation of energy.
The most general law controlling chemical systems is expressed by Le
Chatelier as follows: "Every change of one of the factors of an equilibrium
occasions a rearrangement of the system in such a direction that the factor
in question experiences a change in a sense which is contrasted with the
original change."
Nernst remarks that this law reminds one of the principle of action and
reaction. Put in another way, it may be said that any chemical change, by
the mere fact of its occurrence, sooner or later renders the conditions less
favorable for its continuance. To illustrate, if the increase in volume
demanded by the reaction becomes too great, this may stay the reaction.
For example, if calcium acetate and copper acetate be placed together in a
very strong vessel and but little additional space be left, the reaction resulting
in the expansion of volume will go on until the pressure becomes so great as
to stay the reaction. 6 Also, if in a closed vessel a large amount of calcium
carbonate be heated it will give off carbon dioxide. But as the amount of
CO 2 increases, and the pressure therefore accumulates, the reaction will be
retarded and finally cease. At this stage the CO 2 formed unites with the
CaO, producing CaC0 3 , as fast as CaCO 3 decomposes and produces CaO
and CO 2 .
If the heat as the result of a reaction becomes too great, this will stay
the reaction. For instance, at low temperatures CO will completely unite
with 0, producing CO 2 ; but if the temperature becomes too high, as a
result of the change the reaction will be stayed or cease altogether. A
case of much greater geological consequence is that of hydration and
dehydration. A comparatively low temperature is favorable to hydration
of minerals. However, a very moderate temperature anything above
110 C. at ordinary conditions of pressure is likely to stay the reaction
of hydration, or even to reverse this process and produce dehydration.
Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p. 567.
6 Jones, H. C., On the increasing importance of inorganic chemistry: Science, new ser., vol. 8,
1898, p. 930.
90 A TREATISE ON METAMORPHISM.
Van't Hoff states the general law controlling chemical systems in
another way: "A transformation will take place of itself only in case it is
in a position to do a positive amount of work. If the amount of work
done is negative, the transformation can take place of itself only in the
opposite sense. If the work done is zero, it can take place in neither
sense." "
EQUILIBRIUM.
When the ions of any compound, A, unite with the ions of another
compound, B, so as to produce C and D, just as fast as the ions of C unite
with the ions of D to produce A and B the conditions are those of equi-
librium, or of chemical statics. When either change takes place faster
than the other, these are the conditions of chemical reactions, or of chemical
kinetics. 6 When the two solutions, A and B, were first mingled, the con-
ditions would be those of chemical kinetics for a time that is, until a
certain amount of C and D had been produced. However, when the
amount of C and I) is sufficiently great, so that they react upon each other
to produce A and B as fast as A and B react to produce C and 1), the
conditions are those of chemical statics, or equilibrium. But it is plain
that this does not mean that chemical activity has ceased or that there is
real quiescence. Interchange is taking place all the time ; but as this inter-
change is compensatory, no heat effect is produced, and the total quantity
of each of the compounds present remains the same. The equilibrium is
therefore really dynamic.
HOMOGENEOUS AND HETEROGENEOUS SYSTEMS.
A chemical system is homogeneous " when it has the same physical and
chemical nature at every point." When this is not the case it is heteroge-
neous." A solution is therefore homogeneous. If a solid substance be also
present, the system is heterogeneous. A heterogeneous system consists " in
the intimate association of different complexes, each of which is homoge-
neous in itself, such as solid salts and saturated solutions.'" 2 Each of these
complexes is called a phase of the system. "The condition of equilibrium
of a heterogeneous system is independent of the relative quantity by weight
"Jones, H. C., On the increasing importance of inorganic chemistry; Science, new series, vol. 8,
1898, p. 930.
&Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
pp. 358-360.
Nernst, cit., p. 357.
t, cit, p. 391.
HETEROGENEOUS SYSTEMS. 91
in which each phase is present in the system."" To illustrate, if an excess
of salt be in a solution, so that it is saturated, and an additional amount
of salt be added, this does not in the least change the quantity of salt
held in a given volume of the solution. Therefore the equilibrium in a
saturated solution is independent of the amount of undissolved salt in the
solution. It follows that in a heterogeneous system " the condition of
equilibrium is independent of the relative mass of each of the phases." 6 A
simple case of heterogeneous equilibrium is that between ice and liquid
water, or between liquid water and water vapor. " For a definite external
pressure there corresponds a definite temperature at which the two systems
can exist beside each other ; thus ice and water are coexistent at atmos-
pheric pressure at C.; and liquid water and water vapor, at atmospheric
pressure and at 100 C. If we change the external pressure, at a tempera-
ture which is kept constant, or if we change the temperature, at an external
pressure which is kept constant, then the reaction advances'" 1 to equilibrium
in one direction or the other. " The process is ended as soon as the
expansive force of the evaporating or dissolving substance is held in equi-
librium by the gas pressure of the vaporized molecules or by the osmotic
pressure of the dissolved molecules, respectively."
NATURE AND SPEED OF REACTIONS.
The fundamental principle of chemical dynamics is that chemical action
is proportional to the active mass. d This is the law of mass action.
The speed of a chemical reaction which occurs under any given con-
ditions depends upon the compounds, the strength of the solutions, the
mechanical action, and the heat. Hence each of these features requires con-
sideration.
THE COMPOUNDS.
The reactions depend upon the compounds present, or, in other words,
upon the nature of the ions composing them ; for the conditions under which
two ions, A and B, unite may be different from those under which one of
these ions will unite with a third, as A with C, or different from those under
which two other ions, C and D, unite. In order that ions shall unite in
solution they must meet or come within the limits of molecular attraction of
"Nernst, cit.,pp. 391-392.
fcNernst, cit., p. 393.
"Nernst, cit., p. 403.
rfOstwald, W., Outlines of general chemistry, translated by Jamea Walker, Macmillan& Co., 2d ed.,
London, 1895, p. 292.
92 A TREATISE ON METAMORPHISM.
one another under certain definite conditions which are peculiar to each sub-
stance. Therefore not every time such a meeting occurs are compounds
formed. The ratio between meeting and union in the case of any two com-
pounds is a constant, which can be compared with the constant of any
other two compounds, each pair of which lias its constant. This is merely
another statement of the old law that different substances have different
affinities for one another, and it is well known that the chemical affinities
are developed only when the molecules are in immediate contact with one
another.
The ions which are present in ground waters in any given case
largely depend upon the character of the adjacent rocks. In a lime-
stone region, for instance, the water may quickly take into solution all
the calcium and magnesium it can hold, considering the acids present.
Under such circumstances the acid ions will be mainly balanced by the cal-
cium and magnesium. The other substances, such as sodium and potas-
sium, perhaps in more readily soluble forms than the calcium and magnesium,
will be largely kept from going into solution, or if in solution will be partly
thrown down, because these substances are obliged to compete for the acid
radicals with the vastly greater number of calcium and magnesium molecules.
Is it not possible that the agricultural advantage of having calcium and mag-
nesium abundantly in the soil is largely, or at least partly, due to the fact
that the presence of these soluble substances in abundance prevents the
solution and washing out of the elements potassium and sodium which the
plants need!
Ostwald divides the bases into strong, moderately strong, and weak."
The alkalies and alkaline earths, with the exception of magnesium, are
strong bases; magnesium is a moderately strong base; iron and aluminum
are weak bases of the two aluminum is the weaker. It follows that,
other things being equal, in underground solutions the alkalies and alkaline
earths, with the exception of magnesium, largely take possession of the acids.
To a less extent this is true of magnesium, and to a still smaller degree of
iron and aluminum. Thus we have the partial explanation of the relative
solubilities of the bases in the belt of weathering. In this belt the alkalies
are dissolved to the greatest extent; next in order comes calcium, then
magnesium, and finally iron and aluminum. (See p. 518.)
a Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan,
Macmillan & Co., London, 1895, pp. 55-56.
WEAKNESS OF ACIDS COMPENSATED BY QUANTITY. 93
Ostwald divides the acids into strong, moderately strong, weak, and
very weak. The acids H 2 SO 4 , HC1, and HNO 3 , are strong acids. The
acids H 2 S0 3 and H 3 PO 4 are moderately strong. The acids H 2 S, H 3 B0 3 ,
and H 2 CO 3 are weak acids. The acids of silica are very weak."
The strong acids H 2 SO 4 , HC1, HNO 3 , when present in ground solu-
tions, as they sometimes are, of course take possession of the bases in
proportion to their quantity. However, in the crust of the earth strong
acids are not abundant on the average, although under exceptional con-
ditions, as in volcanic districts, they may be rather plentiful. Also the
moderately strong acids H 2 SO 3 and H 3 PO 4 are not abundant, although
phosphoric acid is rather widespread. Of the weak acids H 2 S and H 3 B0 3
are not plentiful. The two great acids of nature are carbonic and silicic
acids, and the major contest in the rocks, so far as the acids are concerned,
is between the weak carbonic acid and the very weak silicic acid. These
two acids are everywhere very abundant in the rocks. While, therefore,
the moderately strong and the strong acids play a relatively important
part in proportion to their quantity, one weak and one very weak acid,
because of their dominant quantity, under the law of mass action play
the greatest part in rock alterations; and in the contest the very weak
acid, silicic, holds its own against the weak acid, carbonic, partly because
its far greater abundance compensates for its relative weakness. The fact
of the formation of carbonates and the simultaneous decomposition of
the silicates under surface conditions the world over is well known.
(See pp. 163, 473-486.) The partial explanation of the phenomena is the
relative abundance of carbonic acid under the conditions in the zone of
katamorphism. As shown in another place (see p. 479), the reaction is also
one which liberates heat, and this is a favorable factor in the process.
In the zone of anamorphism, where the pressure is great, the reaction
of the upper zone is reversed. (See pp. 173-178, 677-679.) The replace-
ment of carbonic by silicic acid results in decrease in volume (see p. 177).
Therefore, under the great pressures of the zone of anamorphism,
the relative volumes of the original and secondary compounds is a most
important, probably dominant, factor in the process. But also it is
probable that at the high temperatures and pressures which obtain in the
lower zone silicic acid gains strength as compared with carbonic acid.
Foundations, cit. , p. 55.
94 A TREATISE ON METAMORPHISM.
It may under these conditions be a stronger acid than at the surface,
and if this were the case the reactions would be partly explained.
Bearing in this direction is the experiment of Bischof, who has shown
that at 100 C. silicic acid, when present in abundance, may partially
replace carbonic acid of carbonates."
Ostwald's explanation of the varying strength of the bases and acids is
based on the varying amount of supposed dissociation.
The velocity of a reaction is proportional to the masses of the active
components, and according to Ostwald these are the free ions. Therefore
the speed depends upon the number of free ions which are acting. But
the number of free ions which are present is dependent upon the degree
of dissociation, and in this matter different compounds vary greatly.
Therefore the degree of electrolytic dissociation of the various bases and
acids determines their respective strengths and is " the measure of the
reaction capacities of all substances." b
From this it follows that an acid or base which is strongly dissociated
is stronger than, or, in other words, is able to largely replace, an acid or
base which is but slightly dissociated; for the number of free ions of the
stronger compound far exceeds that of the weaker. It therefore becomes
important, from Ostwald's point of view, to know the comparative strength,
or the relative amounts of dissociation, of the abundant bases and acids
which occur in the rocks. According to Ostwald the strong bases and
strong acids may be largely dissociated; the moderately strong bases and
acids under ordinary conditions are dissociated to a much less extent; the
weak acids, carbonic, hydrosulphuric, and boric, are usually not dissociated to
the extent of 1 per cent; silicic acid under ordinary conditions is scarcely
dissociated at all.
STRENGTH OF THK SOLUTIONS.
Saturated and strong solutions are more active than weaker solutions;
for the amount of the active compound increases with the concentration,
but not in a simple ratio. Weak solutions are relatively more active than
strong solutions, and by those who believe in dissociation this is attributed
to their nearer approach to complete dissociation; but the greater relative
activity of weak solutions never compensates fully for the greater dilution.
Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison & Sons, London, 1854, vol. 1, p. 6.
^Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p. 440.
EFFECT OF QUANTITY OF ELEMENTS. 95
Although, as just seen, strong bases and acids have a great advantage
over weak bases and acids, the quantity of an element present is a very
important factor in the final result of the action of the solutions on the solids.
If a certain element is abundant in the ground solutions, it may to a large
extent replace another element in the solids, an element of the solid going
into solution at the same time. This may take place to a large extent even
if the element in solution is weaker than the one it replaces in the solid.
For instance, the relatively weak base, magnesium, when abundant in
solutions, is known to replace the stronger base, calcium, on a large scale
in calcium carbonate, thus changing limestone to dolomite. In this reaction,
while the abundance of magnesium is a very important factor, a number of
others enter; and therefore its detailed consideration is given under the
process of rock dolomitization. (See pp. 802-808.)
MECHANICAL ACTION.
It has already been seen that no changes in rocks take place without
movements of material, small or great, for long or short distances. Even
in the case of a mineral passing from one form to an allotropic form, there
is movement of the molecules. In short, wherever there is rearrangement
of the elements there must be movements.
Mechanical action alone is one of the processes of metamorphism of
the utmost importance. (See pp. 46-50.) However, the effect of mechan-
ical action in the promotion of chemical action is even more important
than mechanical action alone.
Mechanical action influences chemical action in two general ways the
speed is promoted, and the nature of the reaction is modified.
SPEED OF CHEMICAL ACTION.
The speed of chemical action is promoted directly by the deformation,
and indirectly by the heat liberated.
DIRECT DEFORMATION EFFECT.
As already shown (pp. 49-50), mechanical action produces deformation
in three different ways by producing strain without rupture, strain with
rupture, and readjustment of the particles
strain without rupture. When material is strained without rupture, even if
the amount of deformation be slight, a great change in the molecular con-
stitution may be involved. This is well shown by a common experiment
96 A TREATISE ON METAMORPHISM.
on glass. If a piece of glass, free from stress, be placed under the micro-
scope with crossed nicols, the light is cut off because the glass is isotropic.
If, however, the glass.be slightly flexed, well within the elastic limit, it
immediately becomes anisotropic, and brilliant colors flash out. So far as
light is concerned and this is one of the best agents for giving an insight
into the molecular constitution of bodies the strained glass behaves wholly
different from unstrained glass. Evidently when glass is alternately strained
and freed from strain it undergoes a profound change in molecular consti-
tution. The greatness of the molecular change in material when strained
within the elastic limit is dwelt upon to show that such changes might
greatly affect chemical action; and it will be seen below that the facts
correspond to this expectation.
Barus has shown" in the case of metals strained to the point of rupture
that a considerable per cent of the energy expended in straining them is
potentialized; in "glass-hard" steel 50 per cent, in brass 40 percent, in
copper 25 per cent. A larger percentage of the energy was potentialized
in the earlier stages of strain than in the later stages. By stating that
energy is poteutialized is meant that the mechanical equivalent in heat of
the work done on the metals was only partially developed ; the remainder
of the energy is stored up in the strained metals. Now, considering a brittle
substance which is analogous in physical characters to rocks, Prince Rupert
drops, the explosion of a drop when a point is broken shows tiiat a large
amount of energy is potentialized, or that the glass is in a high state of
strain. The experiments of Barus and the condition of the Rupert drop
show that in strained materials energy is probably potentialized. If this
be true, must it not be the case that the atoms and molecules of a strained
body are in a more than ordinarily favorable condition for chemical action?
Bodies in which energy is potentialized are believed to be in an
exceptionally favorable condition for chemical action. For instance, if a
strained metal, in which on that account more than the usual amount of
energy is stored, be dissolved in an acid, less than the usual amount of
chemical energy is expended, for the resultant salts in the solution have the
same energy of combination in each case. But in the strained metal work
has been done, the equivalent of which has not escaped as heat during
strain, and is therefore stored energy. Therefore this energy is available
o Barus, C., The mechanism of solid viscosity: Bull. U. S. Geol. Survey No. 94, 1892, pp. 107-108.
STRAINED MINERALS EASILY DISSOLVED. 97
to assist the chemical reaction. That it is utilized is shown by the fact
that the heat of combination of the resultant chemical compound must
be the same whatever the condition of the metal. Hence less chemical
energy is required for the solution of a strained metal, and the reaction is
promoted by the state of the strain.
The validity of this reasoning is dependent upon the principle of the
conservation of energy. As a result of my studies in the phenomena of
recrystallizatioii," I became convinced that strained minerals are more
readily acted upon by underground solutions than unstrained minerals.
(See pp. 690692.) Barus's experiments already cited suggested the above
explanation. I then predicted that experiments would show that strained
metals are more readily acted upon chemically than unstrained ones, and
asked that this prediction be tested experimentally. This Mr. Hambuecheu
has done in reference to iron, with the following results:
The application of stress to metals causes an increase in chemical activity, this
increase being especially marked after the elastic limit has been reached.
It is possible to get a curve showing the relation of electro-motive force to
strain which is similar to that of stress to strain.
There is a definite relation between the electrical potential of iron toward an
electrolyte and the amount of energy stored up in the metal through the application
of stress. 6
Thus complete experimental confirmation of this prediction is made so
far as iron is concerned; and it can hardly be doubted that this illustrates
the general principle above given.
Applying the above principles to strain and chemical action, it may be
said that in so far as minerals are strained either within or beyond the
/
elastic limit, this potentializes energy and puts such minerals into a
condition more favorable for chemical reactions than unstrained minerals.
All rocks, except at the very surface of the earth, are under stress, and
therefore strained to some extent at all times. It is true that the amount
of stress may not be great within a few meters of the surface; but with
increase of depth the average amount of stress becomes more important.
In most cases of ordinary horizontal rocks near the surface it is customary
"Compare Van Hise, C. R., Metamorphism of rocks and rock flowage: Bull. Geol. Soc. America,
vol. 9, 1898, p. 300.
* Hambuechen, Carl, An experimental study of the corrosion of iron under different conditions:
Bull. Univ. of Wisconsin No. 42 (Engr. ser., vol. 2, No. 8), 1900, p. 255.
MON XLVII (M 7
98 A TREATISE ON METAMORPHISM.
to regard them as practically free from stress and strain. However, not
infrequently rapid deformation by uplift of an arch or by fracture when a
few meters of load is removed, as at the Chicago drainage canal and at
the combined lock of Appleton," shows that such rocks are under very con-
siderable stress, and therefore must be strained.
Not only are rocks generally under stress, but because of the com-
plexity and variability of rock compositions, structures, and textures,
wherever rocks are under stress the amount of stress and therefore of strain
continually varies with changing direction and changing position. Variable
amount of strain is therefore a universal law. In so far as any mineral
particle is strained to a greater degree than an adjacent mineral particle of
the same kind similarly strained, the paiticle under greater strain is more
rapidly altered by chemical action. In so far as any portion of a mineral
particle is strained to a greater degree than another portion of the same
particle similarly strained, the part under greater strain is t more rapidly
altered by chemical action. Finally, for the same mineral particle or some
part of the same the strain varies continually during deformation.
From the foregoing it follows that the almost universal state of strain,
and the not less universal variability in the amount of strain, are of the
most profound significance in metamorphism. (See Chapters VI, VII, VIII.)
strain with rupture Where deformation produces rupture, another feature
enters, also favorable to chemical action. Rupture is favorable to chemical
action since thereby the surface exposed to the underground waters is
inversely as the average diameter of the mineral particles. Granulation
very greatly increases the surface of action.
Readjustment of panicles The readjustment of the rock particles with refer-
ence to one another can hardly fail to give better opportunities for the
chemical action of the ground waters; for during the adjustment the
water will necessarily be moving and will come in contact with a succession
of mineral particles, and thus promote chemical interchange. Hence I
conclude that mechanical action is favorable to metamorphism by chemical
action, whether the deformation be strain without rupture, with rupture, or
merely readjustment of the rock particles, or, finally, any combination of
these.
Cramer, Frank, On the rock fracture at the Combined Locks mill, Appleton, Wis. : Am. Jour.
Sri., 3d sen, vol. 41, 1891, pp. 432^34.
HEAT PRODUCED BY MECHANICAL ACTION. 99
INDIRECT HEAT EKFEIT.
It is a well-known law that mechanical action develops an equivalent
amount of heat, except for the part of the energy which is potentialized
It has already been seen that heat is ordinarily favorable to chemical
action. Therefore mechanical action promotes chemical action, because it
develops heat and raises the temperature. Indeed, the heat developed by
mechanical action is frequently one of the most important favorable con-
ditions for metamorphism. It will be shown (Chapter VIII, p. 740) that
where mechanical action is strong the complete recrystallizatioii of rocks
may occur much nearer the surface than under quiescent conditions. This
result is largely attributed to the rise in temperature due to deformation,
which results in vastly greater efficiency of the water as an agent of
chemical action.
It therefore becomes of the utmost importance to consider to what
extent the temperature is raised in the rocks by mechanical action.
The heat, as already intimated, is produced by the transformation of
work into heat as a result of straining the rock particles within the elastic
limit, by rupturing them, and by their frictional movements over one
another. Mallet" has held that the heat thus developed may be sufficient
to liquefy rocks by aqueo-igneous fusion. He thus accounts for the crys-
tallized cores of many mountain ranges. He even holds that the material
fused by mechanical action may intrude the adjacent solid rocks. LeConte
follows Mallet in this belief. It may be theoretically possible that rock
material can be ground so fine as to develop sufficient heat to fuse it.
However, as explained (Chapter VIII, pp. 728-732), we have no evidence in
the field that this has occurred. It is shown (Chapter VIII, pp. 690-696),.
that when the temperature of water-saturated rocks rises a certain amount,
readjustment occurs, not by mechanical subdivision and grinding of the
particles over one another, but by recrystallization. The process is thus
chemical, not mechanical, and the expenditure of energy and the conse-
quent development of heat are far less than by the former process. How-
ever, it is probable that, as a result of the interior kneading of rocks, the
temperature may be materially increased, perhaps several hundred degrees
beyond the normal temperature which obtains as a result of the depth of
"Mallet, Robert, Volcanic energy; an attempt to develop its true and cosmical relations: Philos.
Trans. Royal Soc. London, vol. 163, 1873, pp. 147-227.
100 A TREATISE ON METAMORPHISM.
burial. And it is certain that the temperature can be very materially
increased, and therefore that the chemical activity is enormously increased.
NATURE OF THE CHEMICAL REACTIONS.
Pressure influences chemical reactions under the following law: If a
chemical system be compressed at a constant temperature, there follows a
displacement of the equilibrium in that direction, which is associated with
a diminution of volume. This law in relation to pressure and chemical
activity may be stated in a more general form, as follows: "Those chemical
forces are strengthened by compression which condition a diminution of
volume; and those chemical forces are weakened by compression which
condition an increase in volume.'' In other words, so far as pressure
influences chemical reactions, changes go on in directions which produce
smaller volumes. Therefore pressure at all times and places is influencing
chemical reactions in the direction of the production of more condensed
systems. It has been seen (Chapter II, pp. 4849) that pressure alone,
without the presence of solutions, may produce reactions under this law.
However, in nature, the vast majority of reactions under the law are
accomplished through the agency of water. The importance of water in
this connection is well illustrated by Spring's experiments upon the con-
solidation of clay when dry and wet. By pressure upon moist clay
confined in a cylinder he was able to consolidate the clay into a body as
compact as a piece of shale indeed, so compact that it was difficult to
scratch it with the finger nail. But using the same pressure upon dry clay
he produced a substance so little consolidated that it was easily scratched
with the finger nail. In the case of the moist clay, he attributed the consoli-
dation to the escape of the plastic material about the piston, and to the
precipitation of material from solution at the moment of escape. 6 Spring's
explanation therefore does not introduce chemical readjustment of the com-
pounds. However, it will be seen that pressure does promote chemical
interchange, producing compounds which are, on the average, denser than
the original ones. This, as will be shown on the following pages, is
believed to be a dominant process for a great many chemical reactions
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 1895,
p. 567.
&Tolman, C. F., jr., Professor Spring on the physics and chemistry of solids: Jour. Geol., vol. 6,
1898, p. 323.
CHANGE OF VOLUME BY RECRYSTALLIZATION. 101
resulting from pressure as the chief motive force; and it may.be that
chemical interchange is one of the processes which explain the consolidation
of the clay in Spring's experiment,
In the rocks a smaller volume may result in either of two ways:
Material may be taken into solution and deposited in a more compact
form without change in chemical composition, or with change in chemical
composition.
SMALLER ROCK VOLUME AS THE RESULT OF SOLUTION AND DEPOSITION WITHOUT CHANGE IN CHEMICAL COMPOSITION.
It has already been explained (pp. 7778) that pressure promotes solu-
tion in case the volume of the solution is less than that of the solvent and
solid, and that pressure promotes precipitation in case the volume of the
solution is greater than that of the solvent and solid. Thus the solubility
of a salt increases with pressure, provided the dissolving is associated with
a contraction of the volume of the solution plus the salt; and, conversely,
the solubility decreases if the separation of the salt (from the solution) is
associated with a diminution of the volume of the system." In ground
solutions the general law is that the volume of the solution is less than that
of the substances dissolved and the water. It follows from this law that
pressure in rocks, the interstices of which are filled with water, promotes
recrystallization and condensation.
The production of a smaller rock volume without change in chemical
composition may occur where the recrystallization and condensation take
place without change of minerals, and where the recrystallization and
consolidation take place with change of minerals.
Recrystallization and condensation without change of minerals. As aU illustration of the
principle, we may consider a stratum of unconsolidated crystallized calcium
carbonate over which is a layer of water saturated with calcium carbonate.
Inasmuch as the calcium carbonate is porous, the water in the rock is free
to move and is under the pressure of the hydrostatic column above it.
The particles of CaC0 3 are under this pressure, and also that of the solid
above All the water in the crevices and pores small enough to hold water
by capillarity is under both the pressure of the water and in part that of
the rock. This water is saturated under this pressure, and it can hold more
substances in solution than the water under less pressure. An interchange
"Nerast, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 1895,
p. 567.
102 A TREATISE ON METAMORPHISM.
is constantly carried on between the free and the capillary water, and as the
capillary water becomes free it is supersaturated and deposits some of its
load in the interstices of the rock. But gravity ever pulls the material
downward, and although this process is not rapid, it is continuous, and in
course of time the particles are cemented. A solidified and recrystallized
limestone is produced. Evidently the greater the pressure the more rapid
and complete is this change.
Another example of solidification without change in mineral composi-
tion is the change of snow or separate ice crystals where mingled with
water to solid ice, as at the head of glaciers. Ice has its melting point low-
ered by pressure. Where the granules are under more than the average
pressure some of them melt. The water flows out into the free spaces and
is again frozen. Or, as expressed above, under more pressure more of the
ice is dissolved in the water than under less pressure. When the pressure
is relieved in the more open spaces the ice is reprecipitated." As the
process goes on the particles are finally cemented. This process, like that
of the recrystallization of limestone, is continuous, and finally the separated
snow granules are transformed to continuous ice.
Recrystallization and condensation with change of minerals ReCl'yStallizatioil and CO11-
densation with change of minerals but without change in chemical composi-
tion may take place by precisely the same processes as already given. The
resultant minerals, where the inducing cause is pressure, are more compact
than the original minerals. Illustrating this principle, pressure induces
the transformation of amorphous calcium carbonate to calcite. Similarly,
pressure may induce the transformation of many other amorphous substances
to crystalline forms. Pressure also induces minerals to change to forms
having higher specific gravities. Thus pressure tends to transform tridymite,
sp. gr. 2.28-2.33, to quartz, sp. gr. 2.653-2.660; and marcasite, sp. gr.
4.85-4.90, to pyrite, sp. gr. 4.95-5.10. (See pp. 220-221, 215.)
We may also safely argue that, where the pressure is great, minerals
are not likely to crystallize in forms having low specific gravities. Thus
under great pressure it is to be expected that silica will crystallize as
quartz and not as tridymite. Doubtless this principle explains why
quartz is always found in the plutonic rocks, and why tridymite often is
a Le Chatelier, in Theoretical chemistry, by W. Nernst, p. 654. Zeitechr. phys. Chemie, vol. 9,
1892, p. 335.
VOLUME DECREASED BY RECKYSTALLIZAT1ON. 103
found in the volcanic rocks. The plutonic rocks crystallize under condi-
tions of great pressure, while the volcanic rocks crystallize under conditions
of moderate or slight pressure. It would be interesting to know the-
relations of quartz and tridymite in the matter of depth in the lavas, and
therefore in reference to pressure at the time of crystallization.
SMALLER VOLUME AS THE RESULT OF SOLUTION AND REDEPO8ITION WITH CHANGE IN CHEMICAL COMPOSITION.
Pressure inducing chemical reactions involving changes in chemical
composition may produce crystallization and condensation of amorphous
compounds and recrystallization and condensation of crystallized com-
pounds.
Crystallization and condensation of amorphous compounds. 111 general the amOrpllOUS
compounds occupy more volume than their complex crystalline equivalents.
"Therefore, since the crystallized state is generally that which takes the
smallest volume, pressure aids crystallization." a According to Delesse, in
passing from the crystalline to the glassy state, granite decreases in density
9 to 11 per cent, syenite 8 to 9 per cent, diorite 6 to 8 per cent, dolerite 5
to 7 per cent, and trachyte 3 to 5 per cent. 6 Thus glass occupies from 3 to
1 1 per cent more volume than the equivalent crystallized rocks. It there-
fore follows that pressure is one of the potent forces which result in the
devitrification of glass. In general it may be said that rocks near the
surface, whether original magmas, sediments, or schists and gneisses partly
altered in the belt of weathering, very frequently contain amorphous prod-
ucts; whereas rocks which have been altered while deeply buried rarely
contain any considerable quantity of amorphous material. It is believed
that the explanation of the difference is largely due to difference in pres-
sure. At depth where pressure is forceful the amorphous products which
occupy more space than their crystallized equivalents either have not
formed or if formed at the surface and deeply buried have become crystal-
lized, the pressure being one of the important forces in the process.
Recrystallization and condensation of crystallized compounds. PreSSUl'e ma} 7 llldllCe cliem-
ical action upon crystallized compounds, producing recrystallized products
of a different kind and with more compact molecules, and therefore of
greater specific gravity. In some cases the recrystallization has occurred
f'Tolman, C. F., jr., Professor Spring on the physics and chemistry of solids: Jour. Geol., vol. 6,
1898, p. 320.
6 See Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, p. 265.
104 A TREATISE ON METAMORPHISM.
at least twice. After one set of compounds was produced recrystallization
again occurred, producing heavier compounds. The first change may be
illustrated by the rearrangement of minerals which constitute mud so as to
produce mica, quartz, and feldspar, and the second stage may be illustrated
by the development from the latter rock of the still heavier minerals, garnet,
stjuirolite, etc, (See p. 685.) However, the process of recrystallizatioii
in nature works in connection with more rapid solution of minerals where
strained (see pp. 95-98) and with other forces. Its full consideration is
therefore deferred to Chapter VIII (pp. 686-698).
GENERAL STATEMENTS.
Where pressure is unimportant, as near the surface of the earth, the
chemical reactions are ordinarily controlled by other factors than pressure;
but as the pressure increases, due to depth below the surface or other causes,
it becomes a more and more important factor in the reactions which occur.
But it is shown in Chapter VI, on " Weathering," that pressure may be an
important factor in chemical reactions comparatively near the surface. This
is illustrated by granitic rocks in the District of Columbia, described by
Merrill," which when brought to the surface underwent rapid disintegration,
hydration, and expansion. The pressure of a few feet of rock was appa-
rently sufficient to prevent the completion of these reactions, and thus it is
clear that the adjustment between chemical reaction and pressure may be
very delicate.
However, as explained in Chapter VI, chemical reactions near the sur-
face do extensively take place with expansion of volume, and therefore in
spite of some pressure. But it is also shown (Chapter VIII) that the
pressure becomes a more and more potent factor in controlling the reac-
tions; and, finally, that there exists a lower zone of anamorphism in which
this is the dominant force. In this zone the chemical changes so take place
as to lessen the volume of the compounds, and therefore to produce heavy
minerals. Moreover, the reactions which occur in this lower zone are
frequently just the reverse of those which take place in the upper zone,
where pressure is una"ble to control, or the reactions in the two zones
reverse each other.
"Merrill, G. P., Disintegration of the granitic rocks of the. District of Columbia: Bull. Geol. Soc.
America, vol. 6, 1895, pp. 322-332.
THE HEAT OF SOLUTION. 105
HEAT.
Heat is a very important factor in chemical action. In the heat factor
two points are involved: first, the general effect of heat; and, second, the
effect of change in temperature in consequence of the reactions.
As to the first of these, in the lithosphere the higher the temperature in
general the more rapid the alteration. To this law there may be excep-
tions, but none are positively known to me.
As to the second point, the chemical effect due to the change in tem-
perature in consequence of a reaction is much more complicated.
In considering whether heat be liberated or absorbed as a result of a
chemical reaction it is necessary to take into account the heat changes in
solution, the heat changes in precipitation, the heat changes in mixing
solutions, and the heat effects of chemical reactions.
"By the heat of solution is meant the quantity of heat produced by
the solution of 1 gram molecule of a substance in a large quantity of the
solvent." 11 It has already been seen that in general the volume of the
solvent and salt is greater than that of the solution, and that in this case
there is usually liberation of heat and consequently rise in temperature;
but in exceptional cases the volume of the salt and solvent is less than that
of the solution, and in this case there is generally absorption of heat, and
consequent fall in temperature. The total effect as to the liberation or
absorption of heat depends upon whether the total of the factors, change
in volume, change of the solid to its dispersed form in the solution, and
the heat factor of dissociation, provided this occurs, is plus or minus.
Decrease of volume tends to liberate heat; increase of volume tends to
absorb heat. The change from the solid to the dispersed state of solution
absorbs heat. The supposed dissociation of a substance into its ions is
regarded as attended with either a liberation or an absorption of heat,
though liberation is held to occur more frequently.''
In precipitation the heat effect is just the opposite from that of solution
and is equivalent to the heat effect of the solution of an equal amount of
the like salt. "In general, in comparing substances which are chemically
analogous and soluble with difficulty, the heat of precipitation (the
negative value of the heat of solution) is the greater the more insoluble
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London,
1895, p. 503.
bNernst, cit, p. 562.
106 A TREATISE ON METAMORPHISM.
the substance is." If this law be applicable to quartz and to silicates it is
of great importance in metamorphism, because these are the substances
most largely dissolved and deposited by the ground water, with the pos-
sible exception of the carbonates.
As to the heat relations when two solutions are mixed, Ostwald states
that in mixing solutions heat is produced by the work between the hetero-
geneous molecules, and heat is used in separating and spreading out the
homogeneous molecules. The sum of these may be positive or negative,
but in most cases the former is the case, and hence the two liquids usually
become warmer when they are mixed. 6 Upon the same point Nernst says:
"No heat phenomena result from the mixture of salt solutions [provided
that no precipitate (and no volatile compound) is produced]."
When chemical reactions occur there is a certain amount of heat of
formation of the compounds. "By the 'heat of formation' of a chemical
compound is meant the quantity of heat which is given off in the formation
of the compound from its respective ingredients."'* "The 'heat- toning' of a
reaction is equal to the sum of the resulting heats of formation minus the
sum of the heats of formation of the vanished molecules." d In whatever
way a chemical result is accomplished, and however many the stages of
process of the change, "the energy differences (and therefore the heat dif-
ferences) between two identical conditions of the system must be the same,
independently of the way by which the system is transferred from one
condition to the other."'
This last is an important law so far as the work of ground waters is
concerned, for in most cases we know only the opening and closing stages
of the processes of alteration, and can ascertain whether the heat effect of
a reaction is plus or minus only by comparing the heat required for the
production of the original minerals with that required for the production of
the secondary minerals, and their gaseous and fluid by-products.
While the above gives the conclusions as to the heat effect of indi-
vidual reactions, the reaction which is likely but not certain to obtain at
moderate pressure and temperature is covered by the rule of Berthelot.
He says, "Every chemical change gives rise to the production of those
Nernst, \V., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London 1895
p. 504.
Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Oo. "New York
1891, p. 308.
cNernst, cit., p. 508. <*Nernst, cit., p. 505. Nernst, cit, p. 496.
CHEMICAL REACTION AND HEAT. 107
substances which occasion the greatest development of heat." And there-
fore, "other things being equal, there is the more chance that a substance
can be formed, the greater its heat of condensation." 6 While these are the
usual rules, they are not broad enough to cover the reactions of meta-
morphism under all pressures and temperatures. A more general statement
of the law as to the relations of heat and chemical reactions is that of vau't
Hoff: "On the whole, the preponderating chemical reactions at lower tem-
peratures are the combinings (associations) which take place with a devel-
opment of heat, while the reactions preponderating at higher temperatures
are the cleavings (dissociations) which take place with the absorption of
heat."" The meaning of this law may be illustrated by the following
reactions: At ordinary temperatures CO combines with O, producing CO 2 ,
with great liberation of heat; at very high temperatures CO 2 dissociates
into CO and O, with very great absorption of heat. This illustration
makes it clear, as stated by Nernst, that to cover all cases van't Hoff's
law must replace that of Berthelot, above given. Still more general laws
as to the relations of heat and chemical reactions are the following: "If
we heat a chemical system, at constant volume, then there occurs a
displacement of the state of equilibrium, and in that direction towards
which the reaction advances with absorption of heat." d "Those chemical
forces which condition a development of heat will always be weakened by
an increase of temperature; and, conversely, those which condition an
absorption of heat will be strengthened by such an increase in tempera-
ture; and it is this fact which, primarily, gives the preceding proposition
its universal validity ."' d "If we heat the system, therefore, the reaction
which takes place will be accompanied by absorption of heat; if we cool
the system, the corresponding reaction will develop. heat." e
Now that the general laws covering the mutual influences of heat and
chemical action have been given, we may consider in more detail their
meaning. The speed of the reaction is commonly increased much more
rapidly than the increase in absolute temperature. Thus, the speed of
reaction of two similar solutions, one of which is at higher temperature
Nernst, cit., p. 581, quoting Berthelot.
6 Nernst, cit, pp. 585-586.
e Nernst, cit., p. 583.
a Nernst, cit., p. 566.
Ostwald, W., Outlines of general chemistry, translated by James Walker, 2d ed., Macmillan &
Co., London, 1895, p. 312.
108 A TREATISE ON METAMORPHISM.
than the other, may be far greater in the solution at high temperature than
would be calculated from the relative absolute temperatures of the solutions.
Indeed, the velocity of a chemical reaction commonly increases
enormously with moderate increase of temperature. The partial explana-
tion of the phenomena lies in the fact that in most cases the reactions
themselves, as already seen, develop heat, which immediately reacts
to increase the kinetic energy of the remaining molecules, and this again
increases the kinetic energy of the molecules, and so on, there being
continual action and reaction between the chemical activity and the rising
temperature.
Another illustration of the very important way in which increase of
temperature increases chemical action is the increased activity of substances
which at low temperatures are relatively inert. While at ordinary tempera-
tures carbon dioxide replaces silica in silicates, at temperatures of 100 C.
silica, if present in abundance, may replace carbon dioxide in carbonates."
While this is explained in part by the increase of activity of silicic acid
with increase of temperature, it doubtless in part is explained by the law
of mass action and the increased volatility of carbon dioxide at higher
temperatures.
If the dissociation theory be true, a third factor which may have some
effect in producing speed of reaction with increase of temperature is the
increase in the amount of hydrolysis with increase of temperature. This is
illustrated by ferric chloride, which at low temperatures is regarded as but
little hydrolized, but at high temperatures is believed to be hydrolized to a
perceptible extent according to the equation:
Fe 2 Cl+6H,O=Fe 2 (OH) 6 +6HCl.
The presence of the feme hydroxide is shown by the color of the solu-
tion. 6 In a similar manner, the carbonates and silicates are believed to be
hydrolized to a much greater extent at high temperatures than at low
temperatures. This is illustrated by calcium carbonate, which in solution
at high temperatures gives a strong alkaline reaction of calcium hydroxide,
and this is regarded as evidence of strong hydrolysis. It is possible that
hydrolysis is an important factor in the reactions which take place in the
different zones of metamorphism.
"Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drum-
mond, Harrison & Sons, London, vol. 1, 1854, p. 6.
*Ostwald, W., Gmndlinien der anorganischen Chemie, Engelmann, Leipzig, 1900, p. 583.
CHEMICAL CHANGES ACCELERATED BY HEAT. 109
While in general, speed of chemical change is promoted by rise of
temperature, as indicated by the second part of van't HofFs law, there is a
limit to the increase of speed due to action and reaction between chemical
change and heat, for when the temperature becomes too high a reverse
tendency is set up, since the compounds formed by the chemical reactions
frequently can not exist at very high temperatures. In such cases the rate
of reaction may cease to increase with increase of temperature, and, indeed,
the reactions which obtain at lower temperatures may be reversed.
Just as a slight increase of temperature may enormously increase the
speed of chemical reactions, so a slight decrease of temperature may very
greatly lessen the speed of reactions. Therefore, if the reaction be one
which itself absorbs heat, and thus lowers the temperature, the slight
decrease in the kinetic energy of the molecules may greatly retard the
speed of the reaction.
At the very moderate temperatures which generally prevail within the
outer part of the crust of the earth the heat resulting from the chemical
changes does not become so great as to stay the reactions. Therefore, it
may be said that the chemical reactions which take place with liberation of
heat promote metamorphism, and those which take place with absorption
of heat retard metamorphism. The great importance of these two tenden
cies, as applied to rocks, will be shown on subsequent pages in connection
with the discussion of the zones of katamorphism and anamorphism.
On subsequent pages it will be seen that in the zone of katamorphism
the first part of van't Hoff's law or*the rule of Berthelot generally prevails
in the alterations of rocks for a considerable distance from the surface.
That is to say, on the whole the preponderating chemical reactions are
those which take place with the liberation of heat. Moreover, as a
consequence of increase of heat with depth, at a very moderate depth
the temperature is rather high. Also, igneous rocks give high temperatures
to the surrounding rocks and solutions. As a result of any of these causes,
water may reach the moderate temperature of 100 to 200 C., and such
temperatures increase the activity of water in an amazing degree. (See
pp. 79-81.) Thus we see that in the zone of katamorphism the heat of
chemical action, and that derived from the interior of the earth through
conduction and convection by means of magma and water, all work
together to increase the speed of chemical action, and therefore to hasten
metamorphism.
110 A TREATISE ON METAMORPHISM.
However, it will also be seeu that in the zone of anamorphism, with
pressure as a dominant factor, reactions very generally occur with the
absorption of heat under the second part of van't Hoff's law. Thus, in this
zone the heat effect of 'the chemical reactions is to stay metamorpbism.
But while the reactions which occur at depth are very generally those
which absorb heat, it must be remembered that in the zone of anamorphism
the amount of heat available, due to increase of heat with depth and to the
difficulty with which the heat escapes from intrusive rocks, is very great.
Therefore, notwithstanding the fact that the chemical reactions themselves
absorb heat, the temperature is much higher than in the upper zone.
Consequently one would expect that the chemical activity would be
greater in the zone of anamorphism than in the upper zone of katamorphism;
and with these expectations the facts correspond. (See pp. 660-661, 690-
692, 749-751.)
RELATIONS OF CHEMICAL ACTION, MECHANICAL ACTION, AND HEAT.
All transformations of material upon the earth, provided all the energy
factors be taken into account, involve the expenditure of energy and the
dissipation of part of it as heat. If this were not true it would be possible
to manufacture an engine by means of which an equal or greater amount
of energy is available for work than is expended in driving the engine, and
perpetual motion would be possible. In metamorphism of rocks, in order
that the above general statement as to the expenditure of energy shall be
true, it is necessary to take into account the chemical force, mechanical
force, and heat which promote the transformations. In those cases where a
transformation of material does not at first sight appear to demand the
expenditure and dissipation of energy, this is due to the fact that some of
the energy factors are overlooked.
It has been noted that chemical actions are reversible, and it will be
seen subsequently that chemical reactions which take place on a large scale
in the zone of katamorphism are reversed in the zone of anamorphism.
When a chemical reaction takes place, and laier that reaction is reversed
and the cycle is repeated, exterior energy must have been expended and
dissipated. To illustrate, let us consider the reversible reaction
FeA+.SHsO^FeA. 3H 2 O.
The reaction may advance from left to right by the expenditure of
chemical energy alone, and as a result of the process heat is liberated.
CHEMICAL ACTION, MECHANICAL ACTION, AND HEAT. Ill
However, to reverse the reaction or to advance it from right to left
requires the expenditure of a greater amount of external energy than the
chemical energy expended in the first reaction. In the reaction given
the available external energy may be from one of two sources heat or
mechanical action. The ferric hydrate may be broken into ferric oxide
and water by heating. Also, if the pressure be very great and water have
a chance to escape the same transformation may take place by the
expenditure of mechanical energy. Doubtless in nature in many cases
both of these forces unite in the process, but whether the dehydration takes
place as a result of the expenditure of heat energy alone or mechanical
energy alone, or the two combined, a greater amount of energy must be
expended than the cnemical energy expended in the hydration of the iron.
Hence, when hydration takes place in the zone of katamorphism energy is
expended. This is potential chemical energy. When dehydration takes
place in the zone of anamorphism, reversing the first process, energy is also
expended. This is either potential mechanical energy or the energy of
heat, or the two together. I say potential mechanical energy, for I have
held in another place that all earth movements, provided all the factors are
taken into account, result in bringing the material moved nearer the center
of the earth, and therefore the energy expended is the potential gravitative
energy of position."
The reasoning applied to the case of hydration and dehydration of
ferric oxide is applicable to every other reversible reaction in metamor-
phism; hence, when we take all the energy factors into account, at the end
of the process energy has been expended. Furthermore, a part of this
energy during the process has been transformed to heat and dissipated; for
in all transformations of energy there is an inevitable tendency for some
of the energy to run down into the lowest form, heat, a portion of which
is lost. 6
In conclusion, therefore, in the zone of katamorphism, while chemical
reactions frequently take place which liberate heat and expand the volume,
and in the zone of anamorphism chemical reactions take place which absorb
heat and condense the volume, in both zones alike when all of the energy
Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11,
1898, pp. 487, 488, 512-514.
h Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 51.
112 A TREATISE ON METAMORPHISM.
factors are taken into account the reactions take place in such a way as to
demand the expenditure of energy and the loss of a part of it.
Where chemical force, mechanical force, and high temperature work
together, with an abundance of water, as an agent of metamorphism, the
speed of rock metamorphism is very great .as compared with the slow
alterations which occur at the surface of the earth. For instance, Barus
finds that water at temperatures above 185 C. and under high pressure is
capable of very rapidly uniting with glass, forming a new compound, which
at these temperatures is liquid, and which he calls water glass. In a retort
he combined 210 grams of glass and 50 grams of water in twelve hours
at a temperature of 210 C. into water glass, which was liquid at that
temperature, but became a clear solid at ordinary temperatures."
Not only amorphous compounds but crystalline minerals also are acted
upon rapidly at such temperatures and pressures. At 180 C., with
pressure sufficient to keep the water in the liquid form, Lemberg 6 has
completely dissolved zeolites in pure water. Under similar conditions
it has also been shown that pure water acts rapidly upon powdered
anhydrous silicates. For instance, Forchhammer showed that water under
these conditions dissolves potassium silicate from powdered orthocla,se. c
Within the zone of rock flowage temperatures and pressures higher
than those with which these experiments have been made are available, and
it is therefore to be supposed that in the zone of anamorphism there is rapid
transformation of the minerals to forms which are relatively stable under
the conditions obtaining at any given time and place. So far as substances
have not a compact state of aggregation energy is potentialized. Pressure
being a very potent factor, the transformations would of course be into
condensed systems, or into minerals having high specific gravity and
probably complex molecular structure. It is evident that in the forces of
chemical action, mechanical action, and heat, and the agent, water, we have
adequate causes for the crystallization of amorphous compounds, for the
recrystallization of strained minerals, and for the recrystallization of highly
Barus, C., The compressibility of liquids: Bull. U. S. Geol. Survey No. 92, 1892, pp. 78-84.
Hot water and soft glass in their thermo-dynamic relations: Am. Jour. Sci., 4th ser., vol. 9, 1900,
pp. 164-65.
& Doelter, C., Allgemeine chemische Mineralogie, Wilhelm Engelmann, Leipzig, 1890, p. 189.
Forchhammer, G., Ueber die Zusammensetzung der Porcellanerde und ihre Entstehung aus
dem Feldspath: Poggendorff, Annalen, vol. 35, 1835, p. 354.
NATURE OF PRECIPITATION. 113
potentialized minerals to lower potentialized forms. These changes are
illustrated by the passage of glass to a crystalline form, by the passage of
minerals from a strained to an unstrained condition, and by the passage
of minerals of low specific gravity to minerals of higher specific gravity.
PRECIl'ITA TION.
From solutions, by changing conditions, solids may separate. This
process is called precipitation. Since precipitation from ground water
solutions is of the utmost importance in metamorphism, it is necessary to
consider fully the conditions under which precipitation takes place. It has
already been seen that in solutions the ingredient which is present in excess
is called the solvent and the ingredients which are subordinate are the sub-
stances dissolved. When from solutions the substance in excess, or the
solvent, separates, this is called a freezing of the solution. When in the
solution the substances dissolved first separate, this is called crystallizing
out of the materials dissolved. "The processes of freezing and of crystal-
lizing out are both to be considered from the same point of view ; and when
we are not dealing with dilute solutions where one ingredient is present in
large excess, but with a mixture, where both ingredients are present in
about the same proportions, then we would be in actual doubt whether the
separation should be regarded as a freezing (of the solvent) or a crystal-
lizing out (of the substance dissolved), or perchance of both processes.""
The necessary condition for precipitation is supersaturation; for if a
solution be not saturated it will take more material into solution ; but if a
solution be sufficiently supersaturated some of the material must be thrown
down or be precipitated. If solids are present similar to the compounds
in solution, considerable supersaturation does not occur. This is very
frequently the case with ground solutions. Under such circumstances
the salts in solution separate out upon the minerals already present, or the
minerals grow. At any given pressure and temperature, provided the
changes occur slowly, equilibrium is nearly retained by this continuous
adjustment. This relation between minerals already present and solutions
is one of the most important factors which control the growth of minerals
which are present. If, for instance, in a complex solution containing
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York,
1895, p. 414.
MON XLVII 04 8
114 A TREATISE ON METAMORPHISM.
various ions there are also various crystalline minerals, the moment that
the solution becomes supersaturated with reference to several ions which
may unite to produce one of the solids present, this union will take place,
the material will be precipitated upon the minerals of that kind, and thus
they will grow.
This process of mineral growth applies alike to minerals in magmas and
to minerals in sedimentary rocks. If, for instance, in a magma plagioclase
and pyroxene individuals once begin to form, they may grow to large size
and produce a gabbro. In a sedimentary rock in which quartz and feld-
spar particles are present and the solutions are of a kind which furnish
constituents for their growth, these particles are likely to be enlarged.
Supersaturation, and consequently precipitation, may result in various
ways, of which the following are the more important: (1) Precipitation
by change of pressure, (2) precipitation by change of temperature, (3)
precipitation by reactions between aqueous solutions, (4) precipitation
by reactions between aqueous solutions and gases, and (5) precipitation by
reactions between solutions and solids.
I'RKCIPITATION BY CHAMJK OF PRESSl'UK.
Change of pressure may result in supersaturatiou, and therefore in
precipitation. Where the volume of the solution is less than that of the
solvent and substance dissolved, decrease of pressure is favorable to
precipitation. Where the volume of the solution is greater than that of
the solvent and substance dissolved, increase of pressure is favorable to
precipitation. The volume relations are opposite in the cases of the
crystallization of minerals from solutions of ground water and the crystal-
lization of minerals from magmas. In the case of substances dissolved in
ground solutions the volumes of the solutions are commonly less than those
of the solvent and the substances dissolved; therefore decrease of pressure
is favorable to precipitation. But in the case of crystallization from
magmas the volume of the solution is greater than that of the crystallized
minerals; therefore pressure is favorable to crystallization.
In another connection it is suggested that under certain conditions water
and magma are miscible in all proportions. (See Chapter VIII, p. 723.)
In other words, there is every gradation from, water containing compounds
in solution to magmas containing subordinate amounts of water. If this
LAWS OF PRECIPITATION. 115
be so, ideally there must be a neutral point in which the volume of the
material is the same whether as a solution or as a solid. In this case
pressure would have no effect upon precipitation. However, the precipita-
tion of any part of the material from a solution modifies the character of
the remainder of the solution, and it is not to be supposed that a case is
likely to occur in which crystallization of material takes place without there
being any pressure effect.
Where circulating waters are descending the pressure is increasing,
and where ascending the pressure is decreasing. Therefore, in the case of
ordinary ground-water solutions the direction of water circulation which is
favorable to precipitation is ascension.
PRECIPITATION BY ( II \\(,l. OF TEMPERATI'KE.
Change of temperature may result in supersaturation, and therefore in
precipitation. In general, in ground solutions increase in temperature
increases solubility. (See pp. 79-81.) Therefore decrease in temperature
is favorable to supersaturation and precipitation. While this statement is
true for most substances at temperatures below 100 C., and is correct for
many substances at temperatures considerably higher than this, at very
high temperatures the conditions are reversed for some substances. (See
p. 79.) In the common case, that of precipitation with decrease of
temperature, the freezing point of the solution is lower than that of the
solvent." Apparently the amount of lowering is proportional to the
molecular weights, and is stated by Raoult as follows: "One molecule of
any compound when dissolved in 100 molecules of a liquid lowers the
solidification point of the liquid by an amount which is nearly constant,
viz, 0.62;" or, the molecular depression, when the solvent is to the
solute as 1:100, is 0.62 . 6 In dilute solutions "of salts in water, the
molecular depression may be larger than this, in which case the substance
is regarded by many as dissociated." It was by the application of the
principle of molecular depression that Kahlenberg and Lincoln were able
to reach the conclusion already given (p. 87), that silica goes into
solution as colloidal silicic acid. When the silicates are dissolved and
Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New
York, 1891, p. 199 et geq.
Ost\vald, Solutions, tit., p. 208.
r Ostwald, Solutions, cit. p. 214.
116 A TREATISE ON METAMOKPHISM.
decomposed by hydrolysis into colloidal silicic acid and metallic hydroxides,
the latter (or, according to the dissociation theory, their ions) caused the
molecular depression, which was unaffected by the colloidal silicic acid.
In precipitation from complex mixtures the substances do not solidify
at the same time. The compounds crystallize in such order, and the
separated solid is of such a character, that "the freezing point of the
remaining liquid is lowered." After one compound has separated another
follows, which again lowers the freezing point, and finally a liquid is left
with the lowest freezing point, and this liquid is the last compound to
crystallize.
Change in temperature is the rule for underground circulating waters.
The waters which are passing to lower levels are, on the average, becoming
warmer.' Waters which are rising to higher levels are, on the average,
becoming colder. Also there are changes of temperature, both positive
and negative, due to varying local conditions; for instance, the presence of
intruded igneous rocks. Ascending waters are, on the whole, precipitating
material, because they are losing heat. The increase in the capacity to
hold material in solution with rising temperature, and the simply enormous
increase in this capacity as the temperature becomes very high, have
already been pointed out. (See pp. 79-81.) During the upward journey
of the water the temperature continuously falls, and if the journey be long
the total loss of heat is great, and the amount of precipitation is correspond-
ingly large. Since the upward course of the water is likely to be in the
larger openings (see p. 583). such as the spaces of porous sandstones,
faults, joints, etc., we have the partial explanation of the filling of these
openings in the belt of cementation. However, this general statement
needs various modifications, dependent upon many variable factors. (See
pp. 629-640.)
PHECIPITATIOX BY REACTIONS BETWEEN AQUEOUS SOLUTIONS.
It has already been seen that when solutions containing various salts
are mixed the resultant solution will contain all the salts which can be
made by the various combinations of their positive and negative ions.
(See p. 68.) The first law of precipitation may be stated thus: When any
combination of the various ions in a solution can form to a sufficient extent
" Si-met, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York,
1895, p. 111.
LAWS OF PRECIPITATION. 117
to be insoluble in the liquid present, such compound will be produced and
precipitated. To illustrate, if a solution of BaCl 2 be added to a solution
if Na^SOj, the ion Ba can unite with the ion SO 4 and produce the insoluble
compound BaSO 4 , which will be precipitated.
The above is a statement of the empirical facts. The explanation of
hese facts under the theory of dissociation is given by Ostwald as follows:
In any given case there is a constant relation between the amount of a
ompound which can be held in solution and the number of free ions of
hat compound. Upon this statement are based the laws of precipitation
rom solutions. Says Ostwald:
In solutions a state of equilibrium subsists between the ions of the electrolyte
ncl the nondissociated portion. To take the simplest possible case, if we have a
inary electrolyte C, which can break up into ions A and B', and if a, b, and c
present the concentrations of these three constituents in a given solution, then the
>1 lowing simple formula holds good: ab=kc.
Now, the two kinds of ions are produced in equivalent quantities, in the above
ase, hence a=b. If, further, the total amount of the electrolyte =1, and a repre-
nts the ionized portion, then a=b=- and c= ,v being the volume of the solu-
v v
m in which unit quantity (a molecular weight in grammes) of the electrolyte is
mtained. By carrying out the substitution we get the formula -r- r=kv, which
xpresses the state of ionisation of an electrolyte at the dilution v. a
In the saturated aqueous solution of an electrolyte we have a complex equilib-
ium. On the one hand the solid is in equilibrium with the nonionised portion of
welf which is in solution, while on the other hand this nonionised portion is in
luilibrium with the dissociated part i. e., with the ions of the same substance,
'he first equilibrium comes under the law of proportional concentration, or, since
v; are dealing here with a substance of unalterable concentration on the one hand,
te concentration of the nonionised portion in the solution must have a perfectly
cfinite value. For the second equilibrium we have in the simplest case i. e., when
te ions of the compound are monovalent ab=kc, a and b representing the concen-
rions of the ions and c the concentration of the nonionised portion.
Now, since c is constant at a given temperature, as we have already seen, kc,
ad therefore ab, must be constant also. Equilibrium is thus established between a
pecipitate and the liquid above it when the product of the concentrations of the two
ins, into which the precipitate falls, has a definite value. This product may be
tcrned the solubility product for the sake of brevity.
" Ostwald, W., Foundations of analytical chemistry, translated by George McGowan, Macmillan &
( , London, 1895, p. 59.
118 A TREATISE ON MET AMORPHISM.
If the electrolyte consists of polyvalent ions in the proportion mA:nB, the
solubility product takes the form: a m b n = constant.
From the foregoing follows Ostwald's statement of the first law of pre-
cipitation, already given: "Whenever in any liquid the solubility product
of a solid is exceeded, the liquid is supersaturated with respect to that solid,'"
and therefore precipitation of the salt follows. Of the various salts which
may be precipitated from a solution, that one will be precipitated first
whose solubility product exceeds its constant of solubility.
Ostwald illustrates this by the cases already cited: If a solution of
BaCl 2 be added to Na 2 S0 4 , BaSO 4 will be precipitated. According to
Ostwald's view, this happens because the solubility product of the ions in
BaSO 4 is very small.
The second law of precipitation follows from the fact that "the solu-
bility of one salt is depressed in the presence of another having a common
ion."" This is equivalent to saying that "the solubility of each molecular
species in a mixture is always smaller than for the particular species when
alone.'" 1 Hence, when to a solution containing certain ions a solution is
added which has an ion in common with one of those already in the solu-
tion, supersaturation and precipitation are promoted. An example of this
is the addition of HC1 to a solution of BaCl 2 . The chlorine ion is common,
and if the solution is near saturation before the HC1 is added, BaCl 2 will be
precipitated. Again, if one adds a saturated solution of NaC10 3 to a satu-
rated solution of KC1O 3 , an abundant precipitate of the latter salt will form.
The above law is a general statement which includes the rule that "The
addition to a solution of a liquid which is able to form a homogeneous whole
with the solution causes precipitation of more or less of the substance in
solution if that substance is insoluble in the liquid which is added." 6 This
rule is illustrated by the same examples. It follows from this that "in
order to precipitate a substance completely from its solution, an addition of
an excess of the precipitant is an advantage." 7
The converse of the second law of precipitation is: The solubility of a
salt increases on the addition of a second salt containing no ion in common.
a Ostwald, W., Foundations of analytical chemistry, translated by George McGowan, Macmillan
& Co., London, 1895, p. 76.
* Ostwald, Foundations, cit., pp. 76-77.
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York,
1895, p. 446.
<*Nernst, cit, p. 453.
' Ostwald, Solutions, p. 90.
/Xernst, cit., p. 449.
LAWS OF PRECIPITATION. 119
To illustrate: "If one adds some KXO 3 to AgBr0 3 , a number of molecules
of AgNO 3 and also of KBr0 3 will be formed. This will result in a diminution
of the number of the molecules of AgBrO 3 , which must be replaced from
the solid salt," or the solubility will be increased."
Cameron gives two illustrations of this converse which are of great
importance in ground solutions :
Gypsum, which is essentially the salt calcium sulphate containing some water,
is sparingly soluble in water. But the addition of an electrolyte with no common
ion, such as sodium chloride, will considerably increase the solubility of the gypsum.
Some experiments made in this laboratory have shown that in moderatelv strong
brines containing only sodium chloride gypsum can be regarded as a soluble salt.
The reason for this is readily seen when the substances which are formed are con-
sidered, both the calcium chloride and the sodium sulphate being very soluble salts.
The transportation of large quantities of lime by the drainage and ground waters in
arid regions where these salts arc found is readily explicable from this point of view.
Calcium carbonate, so abundant and so important in nature, is dissolved in a
precisely similar way; but the ionization of carbonates being relatively small, the
effect is not so striking and relatively much less lime is transported in the solution.
Treadwell and Reuter* have recently published investigations on this point and find
the solubilit3 r of calcium carbonate in sodium chloride solutions does not become
markedly large until considerable concentrations of the latter salt are reached. The
effect of carbon dioxide in forming the more soluble bicarbonate of lime undoubtedly
is an important element in this connection, but as the ionization is but little affected
by its presence its influence must be small in the presence of such a salt as sodium
chloride/
PRECIPITATION BY REACTIONS BETWEEN AiJUEOtS SOLUTIONS AND GASES.
Another case of precipitation occurring in nature follows as a result
of mixing solutions, one of which is a gas which acts upon the compounds
in the aqueous solution, producing ions of a different kind from those before
present, and in some cases forming compounds, the solubility of which is
so small that precipitation results. Perhaps the most important case of
this kind is the mixing of oxygen with a solution containing salts of iron
protoxide. As a result of this the iron is changed from ferrous to ferric
form, and the latter is precipitated as a sesquioxide or hydrosesquioxide of
iron. In the latter case hydration occurs simultaneously with oxidation.
Nerast, cit., p. 450.
6 Treadwell, F. P., and Reuter, M., Ueber die Loslichkeit der Bikarbonate des Calciums und
Magnesiums: Zeitschr. fiir anorgan. Chemie, vol. 17, 1898, p. 170.
c Cameron, F. K., Application of the theory of solutions to the study of soils: Kept. No. 64, Field
Operations of Division of Soils, 1899, U. S. Dept. of Agric., 1900, pp. 150-151.
120 A TREATISE ON METAMORPHISM.
i t
PRECIPITATION BT REACTIONS BETWEEN SOLUTIONS AND SOLIDS.
"If one pours a solution of KBr over solid AgCl, . . . the bromine
existing in the solution will be largely replaced by chlorine, because as
AgBr is much less soluble than AgCl an equivalent quantity of AgCl will
be changed into AgBr. This is also established by experiment. If one
knows the solubilities of AgCl and AgBr, then for a given concentration of
KBr we may state the point of equilibrium which the system strives to
reach."" Hence we conclude that if a salt, A, is treated with a saturated
solution of another salt, B, a greater or less part of the salt B may separate
out, the salt A being taken into solution at the same time. In this case
"the active mass of the solid substance is a constant." The meaning of
this is that if any of a solid salt is present after the reaction has ceased
there was sufficient to produce equilibrium between the salt and the
solution.
An excellent case illustrating precipitation from solution in nature by
the action of a solid, one of the most fundamental importance, is the partial
dolomitization of the calcium carbonate of shells and corals by the sea
waters, which contain both calcium and magnesium salts. In this case,
under the law of chemical equilibrium, there is constant action and reaction
between the magnesium salts in solution and the solid CaCO 3 . The
magnesium and calcium partially interchange, the calcium going into
solution by uniting with the ions before combined with the magnesium, and
the magnesium simultaneously uniting with the C0 3 ion before united with
the calcium and thus being thrown down as MgC0 3 . Thus the calcite is
partially dolomitized.
This case of dolomitization well illustrates the principle that simul-
taneously with the precipitation of one element or mineral another element
or mineral may be dissolved, one being conditioned upon the other. There
are very numerous complicated cases of this kind which need investigation.
(See pp. 203-206.)
The solids present exert an important influence in precipitation
independently of the passage of elements of the solids into the solutions.
That is to say, if there be solids present, even if none of the elements of
any of such compounds pass into solution, these solids may influence the
"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 1895,
p. 452.
''Nernst, cit., p. 450.
LAWS OF PRECIPITATION. 121
nature of the precipitation. This statement is applicable both to com-
pounds present in solutions before precipitation begins and to compounds
formed by precipitation itself. Once any precipitate begins to form, par-
ticles of that precipitate are present and influence further precipitation,
precisely as do other solids which were present before the precipitation
began. .The proof of the influence of the solids present is furnished by
the very well-known tendency to the enlargement of mineral particles
already existing in preference to the formation of new individuals.
The growth of mineral particles already present is probably con-
nected with the phenomenon of adsorption, described on pages 6465. It is
there noted that the contact film of solutions with solids contains more than
an average amount of material in solution.- It may be suggested that this
is due to the molecular attraction of the crystal for the molecules in solu-
tion, just as the adherent film of the liquid itself is due to the molecular
attraction between the solids and liquids. As the particles in solution move
about they continually impinge against the solids in the solutions. These
particles thus come within the limits of the molecular attraction of the solids
and are to a certain extent held, and hence the concentration. It would
follow that the adherent films of liquid are likely to become supersaturated
in advance of the remainder of the solutions. Under these circumstances the
moment supersaturation is reached with reference to the compounds forming
a given particle, these materials will be deposited upon the particle, 'and
will grow. Precipitation immediately follows supersaturation of the con-
centrated film because of the orienting and selecting power of the mineral
particle already existing. It is probable in the case of a given mineral that
for compounds other than those which can unite to produce the mineral
supersaturation can take place to some extent, and that from this slightly
supersaturated adherent film this material may escape into the free solution.
However, when such solutions become supersaturated in the presence of a
mineral which could use them they would be thrown down. By this process
is explained the selective power by which each mineral particle is able to
take from solution material like itself and add it to itself; and also the fact
that particles once formed abstract materials like themselves from solutions
in preference to the formation of new particles." The presence of any
a For explanation of adsorption see Ostwald, W., Grundlinien der anorganischen Chemie,
Engelmann, Leipzig, 1900, pp. 387-389.
122 A TREATISE ON METAMORPHISM.
mineral species will prevent considerable supersaturation of the solution, so
for as the compounds of that species are concerned. The result is that if
there be materials in a solution which can unite to produce mineral species
which are present they will do so. In this way the minerals control or
guide to a considerable extent the character of the solids which are deposited,
since when a certain mineral is absent, before that mineral can begin to be
precipitated supersaturation must occur with reference to the chemical
combination which composes it.
Therefore the mineral species which are present in a solution have
an advantage over other kinds of minerals which are absent, To a less
degree, minerals which are abundantly present have an advantage over
those which are sparse. To illustrate, if quartz be present and the solu-
tions contain ions of silica, it will be apt to abstract the silica from
the solutions the moment supersaturation occurs. In the same way, if
feldspar be present and there are ions of sodium, calcium, aluminum, and
silica in proper proportions, these are likely to be grouped together to
produce feldspar. Moreover, it appears to be the case that the feldspar
may so nearly control that a closely analogous feldspar is produced, and
twinning and other phenomena characteristic of the original grains be
continued in the secondary growth. The same statements apply to
hornblende, tourmaline, calcite, and, in fact, to all minerals in which a
secondary growth has been noted. Of course, in a rock in which there
are present a large number of mineral particles, the particular mineral
which is formed will depend upon the various ions in the solution, their
relative proportions, and the relative insolubility of the salts. For instance,
tourmaline can not form unless the boric acid ions are present; horn-
blende can not be produced unless there are in the solution all the bases
demanded by that mineral in sufficient abundance. Thus the particular
mineral which forms depends upon a complicated adjustment of the mineral
particles present, the ions present in the solution, their relative proportion,
and the solubility of the mineral particles.
In the above chemical principle lies a partial explanation of the strange
fact that minerals are so firmly cemented by material like the dominant
original mineral. In quartzose sandstone the chief cement is silica; in feld-
spathic sandstone the chief cement is likely to be feldspar; in strongly horn-
blendic rocks one of the chief cements is hornblende, and so on. Another
GROUND WATER UNIVERSAL. 123
important factor in the process is the extension of the rock masses from
the places of solution to the places of deposition. For instance, in any
rock which extends from the belt of weathering to the belt of cementation
the water at the places of solution (especially the belt of weathering)
would obtain material adapted to the enlargement of the minerals of the
same rock at the place of deposition (especially the belt of cementation).
Consequent upon the two factors above given, rocks in many instances
are cemented by minerals like those present before cementation began.
SECTION 2. CIRCULATION AND WORK OF GROUND WATER.
UNIVERSAL PRESENCE OF WATER IN ROCKS.
It has already been explained at the opening of this chapter that water
is the great dominating agent through which the greatest transformations
are accomplished. Free water is present to some extent in all rocks within
the zone of observation. That it is abundant in porous rocks is well
known. Water has also the power to slowly penetrate the apparently solid
rocks. Between the mineral particles there is space sufficient for water to
make its way, and a small amount of water is found in the most massive
and relatively impervious rocks.
Besides the free water in rocks, there is always present water in a
combined form. The combined water varies from a small fraction of 1 per
cent to several per cent. Commonly the combined water does not fall
below 0.50 per cent, and seldom is higher than 8 per cent. It therefore
appears that all rocks contain water, both in the free and in the combined
form. The amounts of each of these are very variable. Bischof many
years ago noted the penetration of basalt by water." The permeation of
apparently solid rocks by water is well illustrated by the readiness with
which agate, chalcedony, and such materials are affected by a staining
solution. When agates are boiled in colored solutions, the liquid makes its
way through the minute subcapillary spaces so small that the microscope
can not detect them, and the bands are differently tinted, the amount
of deposited coloring material depending upon the relative sizes of the
minute openings.
Bischof, Gustav, Chemical and physical geology, translated by Paul and Drummond, Harrison
& Sons, London, vol. 1, 1854, p. 10.
124 A TREATISE ON METAMORPHISM.
The water in rocks may completely or partly fill the openings. Where
the openings of a rock are completely filled, the rock is saturated. Unless
all the openings in a rock are subcapillary it will remain saturated only
so long as it is surrounded or partly surrounded by the saturating liquid.
If withdrawn from the saturating liquid, all the water may be drawn
off by ordinary physical means except that adhering to the walls of the
openings. This residual amount of water is called the water of imbibition.
The difference between the water of saturation and that of imbibition,
which, as will be seen, is the water which may flow somewhat readily, may
be called the water of hygrometricity. In the rocks having subcapillary
openings (see pp. 143-146) the attraction extends from wall to wall, and
therefore the entire film of water in the spaces adheres to the rock particles,
or is water of imbibition. In the rocks having subcapillary pores only, the
water of imbibition and saturation is the same.
The next question which arises is as to the source of the ground water.
On pages 661-668 reasons are given for the belief that the circulation in the
zone of anamorphism, which corresponds to the zone of rock flowage, is
very slow indeed. In this deep-seated zone decarbonation, dehydration,
and to some extent deoxidation of the rocks take place. It is shown (see
pp. 764-766) that with these exceptions, excluding igneous rocks, the compo-
sition of the rocks metamorphosed in the zone of anamorphism closely
corresponds with their original composition, contrasting greatly in this
respect with the rocks metamorphosed in the zone of fracture. From these
and other facts it is certain that the circulation of water in the zone of
anamorphism is very slow. However, it is probable that a large portion of
the carbon dioxide and water liberated slowly makes its way into the zone
of fracture. It is also explained that some water may join the zone
of fracture through the agency of igneous rocks which enter this zone.
But the amount of these supplies of water at any one time is small indeed,
insignificant compared with the amount required to keep up the active
circulation which we know exists in the zone of fracture. Since, then, it can
not be shown that any considerable fraction of the water of circulation of
the zone of fracture is derived from the zone of rock flowage, we can only
suppose that this water is derived from precipitation. The subterranean
water is therefore predominantly of meteoric origin.
POKE SPACE IN ROCKS. 125
i
POKE SPACE OF ROCKS.
The pore space of rocks varies from a small fraction of 1 per cent to 50
per cent, or more. The pore space in compact, strong, igneous rocks is
exceedingly small. For instance, in fresh, strong granites the percentage
of water absorbed by the dry rock varies from 0.08 to 0.20 per cent, which
corresponds to a pore space of 0.20 to 0.50 per cent. The more compact
limestones also contain very little pore space. Some of them absorb as
smarll an amount as 0.20 per cent by weight of water, which corresponds to
a pore space of about 0.55 per cent.
Ordinary compact limestones used for building material, when satu-
rated, contain from 1 to 5 per cent of water by weight, and this corre-
sponds to a pore space of about 2.5 to 12.5 per cent. The more porous
limestones are capable of absorbing 10 per cent or more of water by weight.
Sandstones are ordinarily very porous, holding from about 2 or 3 to 15
per cent of water by weight. This corresponds to a pore space of from
about 5 to 28 per cent Capacity to hold about 10 per cent by weight, and
therefore a pore space of about 20 per cent, is very common in sandstones.
The extreme of porosity for sandstones yet reported is the Dunnville sand-
stone of Wisconsin, which, according to Buckley, contains a fraction more
than 28 per cent of air space when dry," and therefore when saturated is
capable of having 28 per cent of its volume occupied by water. According
to Merrill, 6 chalk may contain as much as 20 per cent by weight of water.
Supposing the specific gravity of the chalk to be 2.8, this corresponds to a
pore space of about 41 per cent. However, in coherent rocks, pore spaces
of more than 25 per cent are rather uncommon.
In unconsolidated rocks where cementation has not taken place at all,
and in products of the belt of weathering, the pore space may be even
greater than the above amounts. If grains of sand are spherical, of uniform
size, and " are arranged in the most compact manner possible, each grain
will touch the surrounding grains at twelve points." c In this case the pore
space will be 25.95 per cent. d If the pai-ticles be spherical, of uniform
"Buckley, E. K., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat.
Hist. Survey, No. 4, 1898, p. 225.
6 Merrill, G. P., Rocks, rock- weathering, and soils, Macmillan Co., New York, 1897, p. 198.
"Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, p. 306.
^Slichter, cit., p. 310. Becker, G. F., Geology of the quicksilver deposits of the Pacific coast:
Mon. U. S. Geol. Survey, vol. 13, 1888, p. 399.
126 A TREATISE ON METAMORPHISM.
size, and arranged "so that the lines joining their centers form cubes," this
will be the most open possible arrangement, In this case the pore space
will be 47.64 per cent. 6
King has made a number of experimental determinations of the pore
space of unconsolidated sands, of broken rocks, and soils, the material being
packed as closely as he was able to pack it." Where quartz sand com-
prising materials varying greatly in coarseness was used, a pore space as
low as 25.43 per cent was obtained/ But " well-rounded grains of nearly
uniform diameter tend to give a pore space which lies between 32 and 40
per cent. * * * For simple sands with angular grains the pore space is
much larger than it is for the rounded sands of the same size of grains, and
in the case of the crushed glass, whose grains are more angular than those
of the crushed limestone, which have a tendency to be cuboidal in form,
the pore space is the largest of all." 8
Seelheim found that clays when allowed to settle in water have a pore
space of 50 to 79 per cent, and that there is no sensible reduction of this
space under a pressure of 30 meters of water/
In clay loams and clays pore spaces as high as 48 to 52 per cent were
obtained by King." He suggests that the high pore space of clays may
possibly be partly explained by the angularity of the grains, it being well
known that the very fine mechanical sediments are largely composed of
angular particles.*
It. is evMent from these experimental results of King's that the grains
of sands and soil are not packed by nature in the most compact manner
possible; otherwise the pore spaces would run lower, rather than higher,
than Slichter's minimum pore space (25.95 per cent); for the natural grains
oSlichter, cit, p. 308.
*Slichter, cit., p. 309.
In order to get the closest packing, the material was added " in small lots at a time and gently
tamped with a broad, flatrfaced pestle until the vessel was filled. . . . The vessel, after being filled
by tamping, was ' struck off ' with a piece of plate glass, then held firmly while with ligh't blows the
walls of the tubes were struck gently, but repeatedly, as long as any reduction in volume could be
produced." King, F. H., Principles and conditions of the movements of ground water: Nineteenth
Ann. Kept. U. S. Geol. Survey, pt. 2, 1899, p. 208.
''King, cit., p. 211.
''King, cit., p. 215.
/Seelheim, Zeitschr. fiir anal. Chemie, vol. 19, p. 387; cited in King, F. H., Principles and con-
ditions of the movements of ground water: Nineteenth Ann. JJept. U. S. Geol. Survey, pt. 2, 1899, p. 78.
?King, cit, pp. 213-215.
A King, cit., pp. 217-218.
PORE SPACE IN ROCKS. 127
of soil and sand are not spherical in shape, or of uniform size. In so far as
the grains vary from regular forms and uniform magnitude the pore space
would be less than calculated; but in so far as the method of packing is
not the most compact possible the pore space would be greater than calcu-
lated. Thus these two factors neutralize each other to a considerable
degree, and we are obliged to turn to experiment to ascertain approxi-
mately the facts. It is probable that King's experimental results" on sands
composed of well-rounded grains of nearly uniform diameters, where the
pore space was between 32 and 40 per cent, represent approximately the
original pore space in the coarser assorted mechanical sediments. The
more porous sandstones, where the pore space, as ascertained by Buckley, b
varies from 18 to 28 per cent, have a crushing strength varying from 172
to 413 kilograms per square centimeter; indeed, are strong enough to serve
for building stones. It is clear that a considerable amount of cementing
material has been added, and that the pore space measured is much less
than the original space in the sands before cementation. Hence it appears,
both from experimental work by King and by deductions from actual
measurements of the space in partially cemented sandstones, that the
original pore space in clean, well-assorted sands probably varies from one-
fifth to as much as two-fifths, with a probable average of about one-third.
It is much more difficult to give a statement as to the average pore
space of the lavas. Some of these rocks are rather dense and had orig-
inally a very small amount of pore space ; others are exceedingly vesicular
and originally had pore spaces amounting to 50 to 75 per cent, or even
more. It is rather probable that where a succession of thin-bedded basic
lavas are piled up one on the other, as in the Keweenawau of the Lake
Superior region, the pore space averages as much as in ordinary sandstones;
but from this maximum the average runs down as the lava flows become
thicker and as they become more acid. Therefore the average pore space
of the vesicular lavas is probably not more than one-third to one-half as
great as in the mechanical sediments.
It is even more difficult to make an estimate of the amount of pore
space due to fractures in the rocks, such as faults, joints, fissility, the open-
King, cit, pp. 147-157.
6 Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat.
Hist. Surv. No. 4, 1898, pp. 393-395, 402-403.
128 A TREATISE ON METAMOHPHISM.
ings of autoclastic rocks, etc. In the case of some breccias the pore space
is certainly as large as in the mechanical sediments, and such breccias in
some places are present in considerable volume. From this maximum
amount the pore space of course varies to a fraction of 1 per cent.
I am therefore wholly unable to give any general averages of the
amount of pore space, taking the world as a whole. But Shaler has esti-
mated that the amount of igneous and vein material of certain regions of
the New England coast is from 3 to 5 per cent of the superficial area."
Since the volumes are as the cubes of the dimensions, if the amount of vein
material were the same in other directions this would involve a filled pore
space of from 0.52 to 1.12 per cent.
From the foregoing it is plain that, while it is easy to ascertain the
amount of pore space in a given rock, it is very difficult indeed to make any
estimate of the average amount of pore space in the zones of katamorphism
and anamorphism. It is shown on pages 187191 that these zones corre-
spond, respectively, to the zones of fracture and flowage. It is certain that
the pore space in the zone of fracture is far greater than in the zone of flow-
age. It is also equally certain that the pore space in the belt of weather-
ing is vastly greater than in the belt of cementation. When these various
zones and belts are discussed it will be shown that both the unconsolidated
materials and the coherent rocks of the belt of weathering are exceedingly
open and have a very large pore space. It will further be seen that in
passing downward from the belt of weathering to the belt of cementation
there is a sudden diminution in the amount of pore space available, the
rocks becoming almost at once far less open. Doubtless on the average
the amount of pore space in the belt of cementation steadily diminishes
from the upper to the lower part; and in the zone of anamorphism the pore
space is almost certainly but a fraction of 1 per cent.
It is to be remembered that below the comparatively thin belt of
weathering, the rocks, with unimportant exceptions, are saturated. Dana
estimates the average amount of water contained in the rocks as 2.67 per
cent of their weight. 6 Supposing that the specific gravity of the crust is 2.7,
this would mean a pore space of 6.89 per cent of the volume of the rocks;
or, if the rocks were saturated, about 69 liters of water in every cubic
a Shaler, N. 8., The crenitic hypothesis and mountain building: Science, vol. 11, 1888, p. 281.
* Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, pp. 205, 311.
CHARACTER OF OPENINGS IN ROCKS. 129
meter. Supposing the pore space for the upper part of the zone of fracture
to be one-fifth of that suggested by Dana and to diminish to zero at the
lower part of that zone, this would give an average pore space for that
zone of 0.69 per cent. Supposing that the zone of fracture extends to a
depth of 10,000 meters and that the pore space is saturated, the amount of
contained water, if concentrated to the exclusion of rock, would make a
sheet 69 meters thick, extending throughout the continental areas. This
calculation is of course made upon an hypothetical basis (see pp. 569-571),
but it shows that the underground water is truly a great subterranean sheet.
This subterranean sheet may be compared to the blood of an organism,
and the comparison has force to the degree that it is the chief medium
through which the transformations of the rocks are accomplished.
CIRCULATION OF GROUND WATER.
Subterranean water must be considered from two points of view-
its circulation and its work.
The actual ground-water circulation depends upon the openings in the
rocks, the forces producing water circulation, and the forces opposed to
circulation.
OPENINGS IN ROCKS.
The rate and amount of flowage of water is largely dependent upon
the openings in rocks. The openings in rocks in reference to flowage
need to be considered from the following points of view: The form and
continuity of the openings, the size of the openings, and the percentage of
openings, or pore space.
FORM AND CONTINUITY OF OPENINGS.
For a given cross section, in proportion as an opening approaches a
circular form that is, as it approaches a minimum of wall area per unit of
volume the flow increases, because the friction between the moving water
and the film of fixed water upon the walls is less per unit volume. In
proportion as the openings are continuous in rocks the flow increases.
The openings in rocks include (1) those which are of great length and
breadth as compared with their width, and thus are essentially flat parallel-
epipeds ; (2) those in which the dimensions of the cross sections of the
openings are approximately the same, and therefore resemble tubes of
various kinds; and (3) irregular openings.
5ION XLVII 04 9
130 A TREATISE ON METAMORPHISM.
(1) The openings which have great length and breadth as compared
with their width are those of bedding partings, of faults, of joints, and of
fissility. It is recognized that many of the fractures are exceedingly
complex. They are, indeed, in many instances a series of parallel or
intersecting fractures, forming a zone of brecciatiou. However, for such a
a zone, as a whole, the statement still holds that the openings have great
length and depth as compared with their width.
Bedding partings are parallel to the layers. Since ground waters
very frequently follow formations, the bedding partings become important
factors in the promotion of flowage parallel to the formation This is
especially true of the contacts of formations of different character. These
contacts are places of maximum differential movements, of consequent com-
plex fracturing, and therefore of important openings and large circulation.
In position the fault, joint, and fissile openings ordinarily have an
important vertical element, or at least traverse the beds. Frequently they
are nearly vertical, or traverse layers or formations at right angles. In
consequence of this they are very important factors in the vertical move-
ments of ground water.
As to continuity, bedding partings are likely to be the most continuous ;
faults come next in continuity, joints next, and fissile openings are those
that are least continuous.
Bedding partings are likely to be continuous for long distances, and
because of this and their size (considered on pp. 137-138), they are fre-
quently important factors in the flowage of ground water."
Faults may have very great continuity. Thrust faults of 15 kilometers
and more along the dip are known; and along the strike faults may extend
for even hundreds of kilometers, although ordinarily their extent is much
less. From their great persistence and from the fact that they are likely to
cut across formations, thus frequently severing and displacing impervious
strata and consequently connecting porous strata separated by impervious
strata with one another, faults are of very great consequence in the flowage
of ground water.
Joints are less extensive than faults, but they may extend across an
entire formation, or even across two or more contiguous formations. The
"King, F. H., Principles and conditions of the movements of ground water: Nineteenth Ann.
Kept. U. S. Geol. Survey, pt. 2, 1899, p. 126.
NATURE OF OPENINGS. 131
extent of joints along- the strike may be many kilometers. While joints
are less extensive than faults, they are far more numerous. Probably their
number, as compared with faults, more than compensates for their lack of
extent. Joints are therefore of very great importance in the flowage of
ground water. On the average the}' may be of even greater importance
than faults. Joints, like faults, may connect separated porous strata, but
very frequently the joints do not pass through the relatively plastic
separating- impervious strata, and therefore in this respect are of less
consequence than faults.
Fissility openings usually have less extent than bedding partings,
faults, or joints; and the openings are small. While they doubtless have an
important influence in water flowage. they are not of such consequence as
bedding partings, faults, or joints.
(2) Openings in which the dimensions of the cross sections are
approximately the same are those of the mechanical deposits, including
conglomerates, sandstones, soils, tuffs, etc.
The openings of mechanical sediments have a strong tendency to a
definite form, and are continuous. The forms of these openings have been
fully discussed by Slichter. The openings alternately narrow and widen.
At the wider parts their sections are roughly polygonal, the polygons
having more than three sides, and these curved. At their narrowest places
the cross sections of the openings approximate triangles, and where the
grains are of equal size the triangles are equilateral. The form of the
tubes at their minimum is due to the contact of three grains in a plane,
the space between which is nearly triangular. (Fig. 3.)
Professor Slichter has further shown that there are various possible
natural systems of packing of particles. In nature one system of packing
may hold for a certain distance, and then be replaced by another system.
Within any system of packing all the openings are connected with one
another by straight or curved tubes, triangular at their minimum cross
section, and no opening is shut off from any other opening. Slichter has
shown that in the various natural systems of packing of the particles there
is at least one direction in which the tubes are straight; in other words,
there is one direction in which a straight wire may be thrust without coming
Slichter, C. S., Theoretical investigation of the motion of ground water: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, pp. 305-323.
132 A TREATISE ON METAMORPHISM.
in contact with any grain. (Fig. 4.) In any other than the one direction,
where the grains are naturally arranged, the tubes are ordinarily interrupted
In any case the continuity of the tubes in straight lines persists so far as the
arrangement of grains is by one system of piling. Slichter has shown that
the openings in the directions in which the tubes are not straight may be
neglected so far as the flowage of water is concerned; he therefore con-
cludes the quantity of flowage to be dependent upon the continuous straight
or nearly straight tubes. These of course vary in size, but the water
may be reckoned as passing through continuous tubes of the minimum size,
made by the cross section between three grains arranged in a plane at right
angles to the direction of the tubes. Of course it is understood that any
FIG. 3. Triangular cross sections of pore space. After Slichter,
one system of arrangement does not extend indefinitely, and that where
one system of packing changes into another there are, ordinarily, bends in
the tubes.
Slichter further shows that the amount of space in mechanical sediments
before cementation takes place is largely dependent upon the system of
packing. It is also dependent upon the regularity of, the grains and their
variation in size. The more nearly spherical the grains and the more
nearly uniform the size, the greater is the pore space.
Ordinarily the continuous tubes of mechanical sediments are limited
by the boundaries of a stratum or formation. However, a porous formation
may extend for hundreds of kilometers and have a thickness of hundreds
IMPORTANCE OF OPENINGS IN SANDSTONES.
133
of meters. This is well illustrated by the strata bearing artesian water,
many of which certainly transmit great quantities of water for hundreds of
kilometers. An excellent illustration of porous strata of this class is the
Dakota sandstone. This sandstone yields great quantities of water along
the James River Y alley, and the nearest feeding area, so far as known, is
in the Black Hills, 400 kilometers distant. The volume of water which issues
FIG. 4. Spheres packed in the most compact manner possible, showing continuous open-
ing* in one direction. After Sliehter.
from sandstone strata in artesian basins shows how important is the class of
openings tinder consideration.
Since the continuous openings of sediments are commonly limited to
a formation, it is plain that such openings are very favorable to the flowage
of water along a formation, but are less potent in the transference of water
from one stratum or formation to another.
(3) Irregular openings are those of the vesicular lavas and of the
134 A TREATISE ON METAMORPHISM.
irregular fractures of rocks. In rocks where the openings are exceedingly
irregular in form the flowage of water is limited by the continuous openings,
however small they may be.
Irregular openings may be of any form. In the lavas they are fre-
quently spherical or ovoid. In the compact rocks they are confined to the
very minute, exceedingly irregular interspaces between the mineral par-
ticles, which apparently are in perfect contact. As already seen, in the very
vesicular lavas the pore space may vary from a small per cent to a very
large amount, even to 75 per cent or more. The openings are more likely
to be continuous where the pore space is large than where it is small. But
even where the pore space is very large the openings of lavas are not nearly
so continuous nor the minima of the tubes so large as in sands. In the
igneous rocks and in the rocks metamorphosed under deep-seated conditions
the openings are minute; they are controlled by the form of the grains.
They are, therefore, very irregular and discontinuous.
SIZE OK ol'KXIXfiS.
The size of the openings is very important in the circulation of ground
water. The size of openings must be discriminated from the amount of
pore space. The amount of pore space may be the same in two cases, but
in one the openings may be very few and large, and in the other very
numerous and small. The flowage in the two cases, other conditions being
equal, is very different, For a given mass of water the internal friction,
both within the moving water and between the moving and fixed water
increases very greatly as the openings decrease in size. It is, therefore,
necessary to consider the various classes of openings in reference to size.
Upon the basis of size openings in rocks may be divided into (a) open-
ings larger than those of capillary size, or supercapillary openings; (b)
capillary openings, and (c) openings smaller than those of capillary size, or
subcapillary openings.
For water, openings larger than capillary openings, according to
Daniell," may be circular tubes which exceed 0.508 mm. in diameter, or may
be sheet openings, such as bedding partings, faults, joints, etc., the widths of
which exceed one-half of this, or 0.254 mm. To movement of water in such
"Paniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, pp. 315-317.
SIZE OF OPENINGS IN ROCKS. 135
openings the ordinary laws of hydrostatics apply. Capillary openings for
water solutions include those which, if circular tubes, are smaller than
0.508 mm. in diameter, and those which, if sheet spaces, are narrower than
0.254 mm., and which in either case are larger than the openings in which
the molecular attractions of the solid material extend across the space.
Such openings in the case of circular tubes are those smaller than 0.0002 mm.
in diameter, or, if sheet passages, are below 0.0001 mm. in width. Capil-
lary openings, therefore, include circular tubes from 0.508 to 0.0002 mm.
in diameter, and sheet passages from 0.254 to 0.0001 mm. in width. Capil-
lary openings of other forms have a range limited between 0.508 and
0.0001 mm., but no one form has so wide a range as this. To movement
of water in openings such as these the laws of capillary flow apply. By
subcapillary openings are meant those in which the attraction of the solid
molecules extends from wall to wall. These include all tubes smaller than.
0.0002 mm. in diameter, and sheet openings smaller than 0.0001 mm. in
width. For intermediate forms the subcapillary openings have as their
maximum limit a range from 0.0002 to 0.0001 mm.
It is not supposed that supercapillary openings, capillary openings,
and subcapillary openings are sharply separated from one another. They
grade into one another, and the laws below given which control the flowage
in one class of openings are gradually modified until they pass into the
laws which control the flowage in another class of openings. For instance,
water in circular tubes slightly larger than 0.508 mm. in diameter would
to some extent obey the laws of flowage of capillary openings, and water
in tubes slightly less than 0.508 mm. in diameter would to some extent
obey the laws of supercapillary flow. In short, flowage in openings near
the dividing line between two classes obeys laws intermediate between
those controlling flowage in the typical cases of each class.
The areas of openings of variable size and similar form vary as the
squares of their respective diameters. The circumferences of openings of
variable size and similar form vary as their respective diameters. It follows,
for a given volume of water, that the larger the openings in which it is
contained the less is the surface of contact. For instance, if for an opening
of any form, of given diameter, the surface of contact for 1 cm. of length
be 1 sq. cm., if the cross diameter be doubled, the length remaining the
same, the volume of the water is four times as great, but the surface of
136 A TREATISE ON METAMORPHISM.
contact is only twice as great. If the diameters be decreased to one-third,
the volume of the water is decreased to one-ninth, but the surface of contact
to one-third only.
As a consequence of the relation between size of openings and area of
contact, it follows that in small openings a given volume of water is capable
of performing much more work upon the rocks than in openings of larger
size, for the surfaces of contact are the places where chemical interaction
between the water and rock takes place. How important is the factor of
small size in the amount of work which may be accomplished by ground
water can be adequately comprehended only when the surface of action for
a given volume of water for small openings is calculated. To illustrate, if
the openings are circular tubes of a size at the border line between those of
Bupercapillary and capillary size that is, tubes 0.508 mm. in diameter 1
cu. cm. of water would have a surface contact with the rocks of about -78.74
sq cm. If the openings be sheet openings at the boundary between super-
capillary and capillary that is, 0.254 mm. in width 1 cu. cm. of water
would have a surface contact of about 78.74 sq. cm. If the openings be
circular tubes at the border line between those of capillary and subcapillarv
openings that is, 0.0002 mm. in diameter 1 cu. cm. of water would have
a surface contact of about 200,000 sq. cm. If the openings be sheet open-
ings at the border line between those of capillary and subcapillary size-
that is, have a width of 0.0001 mm. 1 cu. cm. of water would have a
surface contact of 200,000 sq cm. Therefore 1 cu. cm., or 1 gram of water,
has a surface contact varying from 0.007874 to 20 square meters in circular
capillary tubes; and in sheet passages has a surface contact varying from
O.OU7874 to 20 square meters. It has been calculated by Whitney that
"the grains in a cubic foot of soil have, on the average, no less than 50,000
square feet of surface area." The magnitude of these numbers shows how
important a factor in the work of a given volume of ground water is the
size of the openings in which the water is contained.
It follows from the above relations that the area of contact, and
therefore the friction between moving water and the fixed film of water
adherent to the walls, is inversely as the size of the openings. As will be
" Whitney, Milton, The physical principles of soils in their relations to moisture and crop distri-
bution: Bull. Weather Bureau No. 4, U. S. Dept. of Agric., 1892, p. 14.
FLO WAGE IN SUPERCAP1LLARY OPENINGS. 137
seen, this is a matter of controlling consequence in flowage in small and
especially in very small openings.
supercapuiary openings r pi ie flowage of water through supercapillaiy tubes
is controlled by the ordinary laws of hydrokinetics. Ignoring friction,
the flowage of water is as the square root of the pressure or head. If
Vrrvelocity, H=head, and G=force of gravity, then V per second
= \/2GH. For instance, the velocity resulting from a pressure of 1 atmos-
phere or a head of 1033.3 cm. would be the square root of 2 X 981 X
1033.3 1423.8 cm. per second."
This formula is only approximately correct, for the internal friction in
supercapillary tubes is dependent upon the viscosity of the solutions, upon
the regularity of the openings, upon their length and size, and upon the
velocity of flowage. If the openings be not straight, or vary in size, or
both, eddies form, which increase the internal friction and decrease the
speed of movement. The friction between the moving liquid and that fixed
to the walls increases with increase of length, with decrease of size, with
roughness of surface, and with increase in velocity. If the available area
be great and the movement consequently very slow, the resistance per unit
of length due to friction becomes so small as to be almost inappreciable.
But even if the openings be large and continuous the formula gives some-
what too high results. If the flow be rapid in long, rough, irregular
underground passages, the resistance is so great as to make the formula
above given inapplicable.
Supercapillary openings include the greater number of bedding part-
ings, fault openings, joint openings, some of the openings of fissility, and
the openings in the coarser mechanical sediments, such as coarse sandstones
and conglomerates. The distance from an angle to the opposite side of the
roughly triangular tubes (fig. 3, p. 132) in sandstones composed of spherical
grains of equal size, which average 3 nun. in diameter, somewhat exceeds
0.508 mm.* The average diameter of the pores in the system of closest
packing is 43 per cent greater than the minimum section of the triangular
pores. 5 It therefore follows that a sediment composed of grains just large
f'Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York, 1895,
p. 303.
fcSlichter, C. 8., Theoretical investigation of the motion of ground water: Nineteenth Ann.
Kept. U. S. Geol. Survey, pt. 2, 1899, p. 316.
< Slighter, cit., p. 317.
138 A TREATISE ON METAMOKPHISM.
enough to make the pores capillary at the smallest section have super-
capillarv p< res in other parts of the section. Hence, it may be said that
sandstones and conglomerates the grains of which exceed 3 mm. in diameter
have tubes which are greater than those of capillary size. But the grains
in the great majority of sandstones average less than 3 mm. in diameter,
and hence the pore openings in sandstones are for the most part capillary,
and are considered under the next heading.
It is through openings exceeding those of capillary size that is, cir-
cular tubes larger than 0.508 mm. in diameter and sheet openings greater
than 0.254 mm. in diameter that the rapid circulation of underground
water is accomplished. For instance, the openings through which springs
of large size issue mainly exceed those of capillary dimensions.
capniary openings. Capillary openings include the great majority of the
openings of sands and sandstones, many of the openings of fine conglom-
erates, many of the openings of porous lavas, and many of the openings
produced by fracture. As already noted, the superior limit of size of
grains of sands and sandstones composed of grains of uniform size, the
smallest openings of which are capillary, is 3 mm. in diameter. The inferior
limit of size are grains, the diameters of which are six times the maximum
diameter of subcapillary tubes, or 0.0012 mm. The majority of the par-
ticles of most clays, shales, and slates are much smaller than this, and
therefore the openings of these rocks are subcapillary. Hence capillary
openings in mechanical sediments range from very fine sands to very
coarse sands. Many of the openings of fissility are capillary; but the
majority of bedding partings, fault openings, and joint openings are partly
supercapillary, although often the walls of such fractures are so close
together as to make even these openings capillary in part.
In capillary openings the resistance to flow increases very rapidly as a
tube diminishes in size. This is due to the fact, already explained, that
the area of contact between the moving liquid and that fixed to the wall
increases inversely as the size of the openings. Indeed, the friction between
the moving and the fixed liquid becomes the dominant factor in the resist-
ance to flowage in capillary tubes. As openings decrease in size, at the
diameter at which this factor controls for a given liquid the openings
become of capillary size for that liquid.
FLO WAGE IN CAPILLARY OPENINGS. 139
According to Poiseuille, the general formula for the flow through a
tube of circular section is
J ~
in which /is the discharge in cubic centimeters per second, a is the radius
of the tube, I its length, p is the difference in pressure at its ends in dynes
per square centimeter, and fi is the coefficient of viscosity of the liquid. 01
According to Slichter, "if A is the area of cross section, this formula may
be written
f - A'P
>- Hx /if
and the mean velocity of the fluid in the tube is given by
^^ = (0-03979)^""
In a triangular tube the flow per second is represented by the formula
and the velocity by the formula
v= (0.02887)^
t** .
"The mean velocity for a circular tube of equivalent area of cross section
was found to be about 38 per cent more." 6 Slichter finds the volume and
velocity of flow in an elliptical cylinder to vary but slightly from that of
a circular tube. "Even an eccentricity of 0.866 will change the flow by
but 10 per cent, and an eccentricity of one-half will reduce the flow by
about one-half of 1 per cent. Thus it is clear that a slight change in the
shape of the cross section of a tube will change but slightly the flow
through it. Analogy wan-ants us in extending this truth to tubes having
other than elliptical sections. For example, we may conclude that the flow
through a tube whose section is an oblique triangle is given approximately
by the formula for a tube whose section is an equilateral triangle of the
same area, even though the shape of the section of the given tube differs
slightly, or even materially, from that of an equilateral triangle." 4 Further-
" Slichter, C. S., Theoretical investigation of the motion of ground water: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, p. 317.
* Slichter, cit., p. 319.
140 A TREATISE ON METAMORPHISM.
more, in capillary tubes "the velocity of flow through a tube of variable
section will be less than the velocity of flow through a tube having a
uniform section equal to the mean section of the first tube, because of
the viscosity or internal friction of the expanding or contracting stream.""
Daniell expresses a part of the laws of capillary flow in words, instead
of in a formula, as follows: "The flow in capillary tubes is proportional not
to the square, but to the fourth power of the radius; the velocity is propor-
tional not to the square root of the pressure, but to the pressure itself.
The resistance in capillary tubes varies directly as the velocity; in wide
tubes approximately as the square of the velocity. This seems discrepant;
but it is due to the formation of eddies in the wider tubes; in a capillary
tube the flow is steady.'" 1
From the foregoing it follows that the flow in a tube with a radius
one-fifth millimeter in diameter is sixteen times as great as in a .tube
one-tenth millimeter in diameter. Furthermore, in a tube of any definite
length, if the pressure be doubled the flow is doubled; if trebled the flow is
trebled, etc. However, experimental work by King upon the flowage of
water through capillary openings of sandstones and sands gave results
showing that under the conditions in which he performed his experiments
the flowage increased faster than the pressure. The pressure in the experi-
ments varied from a small fraction of an atmosphere to somewhat more than
an atmosphere. The departure from I'oiseuille's law varied from less than
1 per cent to more than 50 per cent.' In the experiments the departures
seemed to be greater, on the average, when very low pressures were used
than when moderate pressures were used. The very variable results nmv
be partly explained by the conditions under which the experiments were
performed, but it is entirely possible that the departures are partly to be
explained by the relative importance of internal friction due to viscosity
when the rates of movements are slow. (See pp. 141-143.)
Also, according to Poiseuille's law, the flowage is inversely as the
viscosity. When it is remembered that the viscosity of water decreases
rapidly with increase of temperature, it is seen that this is a very important
"Slighter, C. S., Theoretical investigation of the motion of ground water: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, p. 320.
f> Daniell, Alfred, A text-)x>ok of the principles of physics, 3d ed., Macmillan Co., New York,
1895, p. 316.
'King, F. H., Principles and conditions of movements of ground water: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, pp. 135-157.
FLOWAGE IN CAPILLARY OPENINGS. 141
factor. The relative viscosity of water at various temperatures below
100 C. is as follows:"
R<lti1i r, I'lKi'uxifij of water at rfffi-rent temperatures.
............................................................. 100.00
15 ........................................ ...................... 63.60
30 ............................................................. 44. 90
4o ............................................................. 33.89
60 ......................................................... .... 26. 94
75 ............................................................. 21.75
90 ...... . ...................................................... 18.16
From this table it appears that the viscosity of water at 45 C. is about
one-third its viscosity at C.; at 90 C., less than one-fifth as great as
at C. It therefore follows that temperature is a factor of the greatest
importance in the flowage of water through capillary openings in the litho-
sphere. It is shown (pp. 138, 145-146) that the openings in the lithosphere
are largely those of capillary or subcapillary size ; hence the importance of
the temperature element.
Another factor entering into the flowage of ground water is the
influence of the meniscus where the openings are not fully occupied by
water. Wolff 6 has shown that if water be introduced into an empty
capillary tube, the meniscus in advance of the column is an important
retarding influence, and consequently that the movement is slower than
under circumstances where there is no meniscus. This influence is likely
to be important in many cases in the belt of weathering, where partial
filling is the rule, but is probably of little consequence in the belt of
cementation below the level of ground water, where saturation is the rule.
In conclusion, it should be fully understood that the laws of capillary
flow, as developed by Poiseuille and others, involve rather rapid movement
through the capillary openings. It has already been stated that viscosity
of the solutions and friction between the moving and the fixed water are
the determinative factors in reference to capillary flow. It is highly
probable that where the movements are very slow the friction is minute or
inappreciable and that the consequent departures from Poiseuille's laws are
very great. Apparently in the exceedingly slow movements of many of
Landolt and Bernstein Tabellen, 1894, p. 288; supplemented by experimental data furnished by
Mr. C. F. Bo wen.
6 Wolff, H. C., The unsteady motion of viscous liquids: Trans. Wisconsin Acad. Sci., Arts, and
Letters, vol. 12, pt. 2, 1900, pp. 552-553.
142 A TREATISE ON METAMORPH1SM.
the larger masses of ground water the viscosity of water and the friction
becomes almost zero per unit area. Evidence of this is furnished by the
fact that artesian water flowing through rocks for hundreds of kilometers,
the openings <>f which are capillary, may have nearly the full pressure due
to head. For instance, the artesian water adjacent to Lake Michigan at
Chicago at the early wells, before they became so numerous as to interfere
when allowed to flow, had a head of 30 meters above the surface, and the
feeding area is only about 80 meters above Chicago; yet the water has
traveled underground from 150 to 250 kilometers. The resistance causing
the loss of head of 50 meters is to be distributed through this distance;
therefore the friction per meter must have approached an infinitesimal
amount. The same thing is again finely illustrated by the artesian wells
of the James River Valley of South Dakota. The water of these wells
must have traveled at least from the eastern border of the Black Hills, 400
kilometers. The elevation at the source is 1,500 meters and at the James
River 500 meters. The consequent loss of head of considerably less than
1,000 meters is due to resistance through the entire distance, and again must
be almost immeasurably small per meter. 6 In all such instances the average
movement is exceedingly slow, for it will be shown that to accomplish the
first of the above journeys more than a century was perhaps required, and
for the second possibly centuries were necessary. (See pp. 585-586.)
But the moment the speed of movement becomes appreciable the resist-
ance promptly runs up. This is shown by the very slow fall of a slanting
water table in sands as the result of lateral flowage. The best illustration of
this of which I know is that kindly furnished me by J. B. Lippincott, city
engineer, of Los Angeles, Gal. The Los Angeles River is mainly fed by
ground waters derived from granitic and other sands which are of moderate
coarseness, but the openings of which are capillary. The water table
rises from the headwaters of the river to a point north of Fernando about
16.1 kilometers from a little more than 180 meters to a little more than
330 meters, or 9.3 meters per kilometer. Mr. Lippincott says that from
1896 to 1900, inclusive, five years, there was practically no rainfall, and
"Leverett, Frank, The water resources of Illinois: Seventeenth Ann. Kept. U. S. Geol. Survey,
pt. 2, 1896, pp. 805-806, 811.
ft Darton, N. H., Artesian waters of tlie Dakota*: Seventeenth Ann. Kept, U. S. Geol. Survey,
pt. 2, 1896, pp. 665-670, pi. Ixx.
FLOW AGE IN SUBCAPILLAKY OPENINGS. 143
therefore no addition to the- ground waters During- that time the water
table fell in the granitic sand, on an average, at the rate of 0.38 meter per
kilometer per annum. This fall of water during these years in the granitic
sands alone, Mr. Lippincott says, i.s sufficient to account for the entire dis-
charge of the Los Angeles River. A head of 9.4 meters per kilometer in
large channels where friction is small would result in the outpouring of the
great quantity of water held in the gravels into the Los Angeles River in a
very short time. But the openings in the sands are capillary, and the resis-
tance due to friction and to viscosity is such that the water was very slowly
delivered to the river under a head of 9.4 meters per kilometer, the average
fall being, as explained, 0.38 meters per kilometer per annum.
Movement as slow as this must be rapid as compared with the exceed-
ingly slow movement of the ground water in the artesian basins referred
to. It follows from these illustrations that the ordinary rates of movement
in the belt of cementation are very much slower than were the move-
ments under the conditions in which Poiseuille, King, and others earned
on their experiments. It is plain that the laws derived from experiments
as given by Poiseuille and King in reference to capillary flow are only
very partially applicable to movements of ground water; indeed, their
application is probably limited to the somewhat rapid movements of the
water in the capillary tubes above the level of ground water in the belt of
weathering where gravity has its full effectiveness, and adjacent to large
openings, either natural or artificial.
subcapiiiary openings. By subcapillaiy openings, as already explained, are
meant openings smaller than capillary openings. In subcapiiiary openings
the attraction of the solid molecules extends from wall to wall, and there-
fore in these openings the water is wholly that of the films attached to the
walls by molecular attraction. There is no free water, in the sense that
the molecules are free to move among themselves, resisted only by the vis-
cosity of the fluid. The ratio of the resistance to movement of water thus
attached as films to solids is almost infinitely great as compared with that
of free molecules. Water thus attached is as if glued to the walls.
Quiucke has determined that the attractive influence of glass upon a
fluid extends through a silver film 0.00005 mm. thick; or, stated in another
way, he finds that the distance through which molecular attraction acts is
144 A TREATISE OM METAMORPHISM.
in general 0.00005 mm." Plateau made the distance through which mole-
cular attraction acts p,^ mm., 6 which amount is slightly greater than
Quincke's determination. Since each wall holds a film of water, sheet pas-
sages below 0.0001 mm. in diameter are subcapillary. The maximum size
for the subcapillary circular openings is twice as great, or 0.0002 mm. in
diameter.
The laws of flowage of water through tubes of such small size have
not been investigated, so far a's I am aware. However, upon theoretical
grounds one would expect that the flow would be exceedingly, indeed
indefinitely, slow even as compared with flow in capillary tubes. -This
Anticipation is fully justified by the observed facts of geology. It is well
known that natural oil and gas may be held in anticlinal arches and domes
for long periods of time, even when under great pressure. It is certain in
these cases that the escape of oil, or even gas, through the subcapillary
openings of the shales is slower than the manufacture of these products in
nature's laboratory. The facts as to the retention of oil and gas under
shale roofs render it highly probable that flow in subcapillary openings
is so slow as to be inappreciable during the time through which an experi-
ment is ordinarily continued; but the flow in subcapillary openings during
geological periods is probably of great consequence. (See pp. 892-904.)
It may be anticipated that the slow movement of water in subcapillary
openings is greatly influenced by change of temperature. At high temper-
atures the viscosity of water is an important element in flow, and this
rapidly decreases with increasing temperature. That water gas does not
obey the law of flow of liquids in subcapillary tubes is shown by the
experiment of Daubri-e," in which the vapor of water at a temperature of
160 C., nnd consequently at a pressure of 6 atmospheres, passed through
a layer of apparently solid rock 2 cm. in thickness, and gave a pressure on
the other side of 1.9 atmospheres. This experiment shows beyond all
question that water gas under high pressure and temperature does not
adhere to the walls sti-ongly, and has such a small viscosity that it slowly
but surely passes through subcapillary openings. However, ground water
at all temperatures below the critical temperature under ordinary conditions
Quincke, M., Ueber die Entfernung in welcher die Molecularkrafte der Capillaritat noch wirk-
sind: Poggendorff, Annalen, vol. 138, p. 402.
& Plateau, J., Statique des liquids, vol. 1, 1873, p. 210.
Daubrt'-e, A., Geologic exp^rimentale, Paris, 1879, vol. 1, pp. 236-238.
FLOW AGE IN SUBCAPILLARY OPENINGS. 145
is held by the pressure in the form of a liquid. But at temperatures
higher than 365 C., or the critical temperature of water, whatever the
pressure, the water is in the form of water gas. In this case it may be
supposed to have a much greater penetrating power than in the form of
liquid, since it can not be considered as adhering to the walls of the
openings.
Even if subcapillary openings be very small and the flow very slow,
it does not follow that the water within these minute openings is not an
agent through which important geological work is accomplished. The
water in such spaces is capable of taking into solution the substances with
which it is in contact, of depositing material from solution, of reacting upon
the substances by hydration; in short, is capable of performing all the
transformations which freely moving water is able to accomplish. Indeed,
it has already been seen that all transfers of material between water and
rock must take place through the fixed films of water. (See p. 64.)
The transfer of material in subcapillary openings is confined to short
distances because there is no free circulating water. The interchanges of
material are probably slow, except between adjacent or nearly adjacent
mineral particles; therefore it seems highly probable that a given volume
of water in the subcapillary openings is far more effective in transforming
rocks than an equivalent volume in larger openings. The same reasoning
applies here as in the case of the capillary openings as compared with
supercapillary openings. The surface of action per unit volume in the
subcapillary tubes is vastly greater than in larger openings. As shown
on pages 686-698, the above conclusion as to the efficacy of water in
subcapillary openings is fully justified by the facts. It is there seen
that the minute amount of water contained in the. subcapillary openings is
the medium through which the complete transformation of rocks to schists
and gneisses has been accomplished. I therefore conclude that, while it
is probable that the actual flow of water and transfer of material in
subcapillary openings is comparatively slow, it is certain that most
profound alterations of rocks take place through this water as the agent of
transformation.
Subcapillary openings include the openings of mechanical sediments
the particles of which, if spherical and of uniform size, are not greater than
00012 mm. in diameter. As a matter of fact, many of the openings in
MON XLVII 04 10
146 A TREATISE ON METAMORPHISM.
which a portion of the particles are larger than this have subcapillary
openings, since the larger openings are occupied by grains as small as or
smaller than the above dimensions. The great majority of the clays,
shales, and slates are largely composed of particles smaller than 0.0012
mm. in diameter and their openings are subcapillary. Minute openings
between the grains of the igneous rocks and of the rocks metamorphosed
to schists and gneisses are also usually subcapillary. Where practically
all of the openings are subcapillary, whether they be the openings of
sedimentary, igneous, or metamorphic rocks, such rocks constitute practi-
cally impervious strata; for the contained water is in fixed films held by
molecular attraction, and the circulation, as already explained, is so slow as
to be negligible during short time intervals.
PERCENTAGE OF OPENINGS, OB PORE SPACE.
The percentage of openings in the rocks, or the pore space, is a func-
tion of the number and the size of the openings. In so far as the openings
in rocks are large and numerous, there is a large pore space. It has
already been seen (pp. 124-129) that the absolute amount of openings in
rocks, as shown by observation, varies from a small fraction of 1 per cent to
over 50 per cent. The larger the pore space the more favorable the condi-
tions for circulation, but since the variation in pore space is so great it is
evident that the flowage of water dependent upon porosity is very variable.
Water passes readily through rocks which contain much pore space ; water
does not flow to an appreciable extent through rocks which have a small
fraction of 1 per cent of pore space. Other factors being the same, and the
pore space of the same character, the flowage is in direct ratio to the amount of
pore space.
FORCES PRODUCIX WATEB CIRCULATION.
The forces producing circulation of ground water are gravity, heat,
mechanical action, molecular attraction, and vegetation. The dominant
force, upon which the movement of ground water mainly depends, is gravi-
tative stress.
GraV i t y. Gravity ever tends to pull the water downward. And this
never-ceasing force at work throughout the zone of water circulation, on
the average continuously carries the circulating water to lower levels. This
condition of affairs is analogous to the work of gravity in earth movements.
o Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 11., 1898,
pp. 465-516.
GRAVITY PROMOTES UNDERGROUND CIRCULATION. 147
But in earth movements and water circulation alike, all the elements of the
movement must be taken into account. The downward movement of a
greater mass of earth or water may result in the upward movement of a
lesser mass. The upward movements of water dependent upon downward
movements of other water are of relatively greater importance in the water
circulation than are the upward movements of rocks consequent upon
downward movements of larger masses of material in earth movements^
Indeed, it will be seen that commonly the circulation of a system of
ground water in the belt of cementation involves both downward-moving
and upward-moving masses. In such systems of ground-water circulation
gravity is effective in the movement in proportion to the head. Head is
due to the fact that the water entering the ground at a certain level, after a
short or long underground journey, issues at a lower level.
Also where there is a difference in the density of the two columns due
to difference in the amount of material held in solution, gravity promotes
circulation independently of head, the column holding more salts being
pulled down and the lighter column driven upward. Probably the amount
of material in solution is usually not so great as to make this an important
factor in the process, but in salt regions it may be important. The density of
the water of the sea as compared with fresh water is 1.02765 to 1.02795," and
the density of a saturated solution of sodium chloride at 4 C., as experi-
mentally determined by Mr. S. H. Ball, is 1.2063. Of course in actual cases
such differences as these are not found, for both columns are sure to have
salts in solution ; but where springs empty under the sea the first case iS
approached. In such instances, the increased density of the sea water
opposes the head of the lighter stream of relatively pure water.
Heat. Change in temperature may result in the expansion and contrac-
tion of water, and such changes in volume necessarily involve some move-
ment. The volume of water varies as the temperature. Taking the
volume of water at 4 C. as 1, its volume at 50 C. is 1.0120, at 75 C. is
1.0258, and at 100 C. is 1.0432. 6 Therefore the increase in the
temperature of underground water may increase its volume and lessen its
density as much as 4 per cent without exceeding its boiling point at atmos-
pheric pressure, and a difference in the density of two columns by 1 per
Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison <fe Sons, London, vol. 1, 1854, p. 97.
6 Austin, L. W., and Thwing, C. B., Exercises in physical measurements, Allyn & Bacon, Boston,
1895, p. 151.
148 A TREATISE ON METAMORPHISM.
cent or more is probably not uncommon. Decrease in temperature may
correspondingly increase the density of water.
Gravity and heat While change of temperature necessarily involves some
movement, its chief effect in water circulation is as a force subordinate to
gravity. In so far as water in a connected descending and ascending
system is warmer at its point of issuance than it was when it joined the sea
of underground water, this gives gravity an effect in circulation in the
same direction as head. This is consequent upon the fact, noted above,
that the density of water varies inversely with the temperature.
It is therefore evident that in columns of water of equal length the
stress of gravity is greater upon the column having the lower temperature.
That the diffence in gravitative stress due to difference in temperature may
be sufficient to produce rapid circulation in pipes that are supercapillary is
shown by the use of the principle in the hot-water system of heating
buildings. Underground, as in the hot-water system of heating, heat is the
energy which causes the water to expand, and gives a difference in density.
When heat has produced a difference in density of the two columns,
gravity is the force which inaugurates and maintains the circulation.
It is believed that underground circulation may be promoted to an
important extent by difference in temperature of the descending and
ascending columns of water, resulting from heat abstracted from the rocks
due wholly to their normal increment of temperature with depth. Later it
will be shown that the downward-moving water is ordinarily dispersed in
many small openings and moves relatively slowly; therefore it may be
supposed at any given place to have approximately the temperature of
the rocks. The upward movement of water, on the contrary, is shown to
be usually in the larger openings and relatively rapid; therefore at any
given place its temperature is probably higher than is normal for the rocks
at that depth. The result is a difference in temperature between the des-
cending and ascending columns, the ascending column being the warmer.
In regions where volcanism, or mechanical action, or both, have
recently occurred, the difference in density resulting from difference in
temperature between the descending and ascending columns is likely to be
a much more important influence in the circulation of the ground waters
than in regions where the difference in temperature is due to the normal
heat of the rocks. Such a region is the Yellowstone Park.
HEAT INFLUENCES IN UNDERGROUND CIRCULATION. 149
In some countries the issuing waters throughout great regions are very
clearly at a higher temperature than the entering waters, and in such
regions the difference in temperature must be a very important factor in
the underground circulation. In such cases the difference in temperature
between descending and ascending waters generally results from a combina-
tion of the normal increase of temperature due to depth, from regional
volcanism, and from the rocks having a higher temperature than normal
because of recent orogenic movements. An excellent illustration of such
regions is the Cordilleran region of western United States. (See pp. 591-
592.)
As already noted, the expansion of water with increase of temperature
is considerable, amounting to over 4 per cent betv/een and 100 C.;
ihat is, a given mass of water occupies a volume 4 per cent greater at the
latter than at the former temperature. In other words, if there be an
average difference of 100 C. between the ascending and descending
columns, 100 meters of the downward-moving water balances 104 meters oi 1
the upward-moving water. If we suppose the descending and ascending
columns to be connected, of equal height, and having an average difference
in temperature of 100 C., this would be equivalent to a head of 4 meters per
100 meters for the entire height of the column. Probably the difference in
temperature between two columns is not often so great as 100 C., but if it
be sufficient to give a difference in density of 1 per cent, and the ascending
and descending columns be the same length, this is ample to give a stress
sufficient to overcome friction and viscosity, and give a decided movement
to ground water. As an illustration of the principle may be mentioned the
water power of the sea mills of Cephalonia, which, according to the Crosbys,
is wholly due to difference in temperature between the descending and
ascending waters.
Mechanical action. A third force influencing ground-water circulation is
mechanical action. Earth movements may close or partly close the
openings in the rocks, and in this process squeeze out the water, as in the
production of the schists and gneisses from the sedimentary rocks. If the
deformation of the rocks be referred to their ultimate cause, gravity, even
the circulation of the water resulting from deformation is indirectly due to
the stress of gravity.
"Crosby, W. F., and Crosby, W. O., The sea mills of Cephalonia: Tech. Quar., vol. 9, 1896,
pp. 6-23.
150 A TREATISE ON METAMORPHIS.M.
Moiuiar attraction. The fourth force affecting the movement of ground
water is molecular attraction. This attractive force works between the
particles of water themselves (cohesion) and between the particles of water
and rock (adhesion).
As a result of molecular attraction water may rise against gravity in
capillary or hair-like openings, thus saturating the rocks at higher altitudes
than it would were it not for this cause; it may creep aloiig the walls of
the openings of rocks without extending from wall to wall, and therefore
without saturating the rocks.
The rise of water when it fills capillary openings raises the free surface
of water above the normal level. This rise of the free surface is explained
by the attraction between the water and the walls, and the attraction of the
molecules of water for one another. The strong attraction between the
surfaces of mineral grains and water has already been alluded to. As a
result of this, water tends to rise along a wall or tube. This is dependent
upon the fact that there is greater attraction between the molecules of rock
and water (adhesion) than between the molecules of water themselves
(cohesion). However, the molecular attraction between the particles of
water is very great. The strength of the surface tension of a film of pure
water is dependent upon cohesion, and is 81.96173 dynes per square centi-
meter." When a molecule is surrounded on all sides by free water the
attractions in the various directions equalize one another, and so particles
are comparatively free to move However, at the surface the upward com-
ponent of the attraction is zero; hence there is effective tangential and
downward attraction. The rise of the water along the walls is due to
adhesion. As a result of this attraction a film of water is drawn along the
walls. Because of the attraction of cohesion the film of adherent water
draws up the next row of molecules away from the walls; these molecules
in turn exert an attractive force on the next adjacent molecules, and so on.
The attractive force of the surface film of water for the water below draws
up the molecules constituting it; this in turn acts upon the film below, and
so on. The total effect of the molecular attraction between the walls and
Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macmillan Co., New York,
1895, pp. 271-279. Ostwalrt, W., Outlines of general chemistry, translated by James Walker, 3d ed.,
Macmillan Co., New York, 1895, pp. 107-111. Barker, Geo. F., Physics, Holt & Co., New York, 1892,
pp. 200-211.
MOLECULAR ATTRACTION AND GROUND CIRCULATION. 151
the water and the molecular attraction between the particles of water is to
produce an elevation above the normal surface of the water, the upper
surface of which is of a shape as though it were an elastic membrane
adhering to the walls and being stretched by the weight of the water above
the ordinary level below.
The height to which water rises above this natural level is indirectly
as the diameter of the capillary openings. In circular glass tubes 1 mm. in
diameter, at 20 C., pure water rises 3.32 cm." Between plates 1 mm. apart
it rises half of this amount. Since the height is inversely as the diameters
of the openings, in circular tubes 0.01 mm. in diameter, the height in tubes
would be 3.32 m. and in sheet openings 1.66 m.
In circular tubes 0.001 mm. in diameter the height in tubes would be
33.2 m., and in sheet openings 16.6 .m.; and in circular 1 tubes 0.0002 mm. in
diameter that is, openings of a size intermediate between subcapillary and
capillary the water would rise to a height of 166 in., and in sheet open-
ings 83 m. Since many rocks have openings as small as or even smaller
than this, capillary attraction may be very important in the position of the
ground-water level. (See pp. 411-412.) If the openings are inclined the
lengths of the openings thus filled are correspondingly great.
The height to which the water rises is independent of the character of
the walls, provided the walls are wetted, 6 and hence the above numbers are
applicable to rocks. However, the height to which the water rises dimin-
ishes as the temperature increases; hence, the above numbers should be
modified somewhat as the top of the sea of ground water has a temperature
below or above a temperature of 20 C. Ordinarily this modification is of
minor importance.
Above the level to which the water may be raised as a continuous sheet
in the capillary openings, the water may still creep along the walls of the
openings without filling them. The obstinacy with which a film of water
holds to the rock surface has already been explained. This water is that of
imbibition (p. 124). In proportion as the water of imbibition varies in
amount the water under molecular attraction creeps from areas of greater
humiditv to areas of less humidity. To the rise of the free surface due to
capillarity there is a definite limit; there is no limit to the creep of water
along the walls. It is presumable, however, that such movement is rela-
a Barker, cit., pp. 209-210. & Barker, cit., p. 210.
152 A TREATISE ON METAMORPHISM.
tively slow, and that the amount of water which is thus transferred for
a given surface is small. But in the soils very large surfaces are available
for creep, and therefore this process is a very important One, especially
in connection with plant growth. The process is one which especially
pertains to the belt of weathering and is therefore later considered. (See
pp. 412-423.)
The rise of the free surface of ground water above the normal level
saturating the rocks, the creep of water along the walls without saturation,
and the flowage of water through small tubes where there is no free sur-
face are generally described under the term capillarity. However, it is
evident that under the term thus used are included three very different
things. The principles involved in the flow of water through capillary
tubes are very different from those which control the free surface of ground
water in capillary tubes, and these laws again are different from those
which control the creep of water along the walls of openings.
vegetation The roots of plants absorb ground water and transport it to
the surface. 'The absorption of water by plant roots causes a relative
deficiency of water. This deficiency is remedied by the movement of
water from other places toward the roots by the forces already considered.
But the influence of roots upon the flow of ground water mainly concerns
the belt of weathering. The subject is therefore later considered. (See
pp. 417, 422-423.)
General statements. In conclusion, it may be said that the immediate cause
of movements of ground water are five gravity, heat, mechanical action,
molecular attraction, and vegetation.
So far as the forces are concerned, the vertical component of the move-
ments of ground water is of far the greatest importance.
But whatever the cause of the flow of ground water, the direction of
movement is from places of greater pressure to places of less pressure. A
current going in any direction is evidence of an excess of pressure in the
rear of the current. Thus water which enters by seepage or through capil-
lary tubes into a larger opening, such as a fissure, must be under greater
pressure than the column of water into which it makes its way. Whether
the motive force in the movement of the water be difference in gravitative
stress or temperature, or any other cause, the excess of pressure resulting
in movement is behind the current.
VISCOSITY RETARDS UNDERGROUND CIRCULATION. 153
THE FACTOR OPPOSIXii WATER CIRCULATION.
The factor opposing water circulation is internal friction of the water.
The internal friction is dependent upon the viscosity of the solutions. The
elements entering into viscosity are the concentration of the solutions and
the temperature. The more concentrated the solutions the greater the
viscosity ; but as the underground solutions of water are commonly not
strong, this is ordinarily not an important element. The viscosity of water
decreases very rapidly with increase of temperature. The relative viscosity
of pure water at C., 45 C., and 90 C. is respectively 100.00, 33.89, and
18.16. (See p. 141.) From these ratios it is apparent that the viscosity of
water at 45 C. is about one-third of that at C., and at 90 C. only about
one-fifth of that at C.
It is therefore clear that the higher the temperature the less the
viscosity and the less the internal friction. Internal friction due to viscosity
results from the variable speeds of different parts of moving water columns
and from the friction between the moving and fixed portions. The greater
the variations in speed of the moving parts the greater the internal friction
due to this cause.
Water usually wets the surface of the rocks. In other words, there is
molecular attraction between the water solutions and the minerals com-
posing the rocks. This attraction is so strong that a thin film of water is
firmly held by the walls of the openings so firmly that it may be consid-
ered as fixed; at least the only interchange which occurs between it and
the passing water currents is that of diffusion, not that of flow. Daniell
says the friction between the layer of adherent water and the rock is
infinite as compared with the friction within the liquid." That the friction
is between the moving liquid and the fixed film of liquid is shown by the
fact that for any liquid the composition of the walls has no effect upon the
flowage. 6 This being the case, it is clear that in the flowage of water
through tubes there is no friction of the water against the rock walls. The
adherent films of water are the walls of the moving columns, and the internal
friction between the water and the walls is that between the fixed films and
moving water.
"Daniell, Alfred, A text-book of the principles of physics, 3d ed., Macuiillan Co., New York,
1895, p. 306.
t> Daniell, cit, p. 316.
154 A TREATISE ON METAMORPHISM.
The greater the speed of the moving water the greater the internal
friction, because of the differential movements both in the moving water
and between the moving water and the films fixed to the walls. Where
the rate of movement is sufficiently slow the internal friction due to viscosity
drops to an almost inappreciable factor. Therefore where the movement
is very slow, even if the passages be long and small, the pressure due to
head may dimmish very slowly. Indeed, nearly the full pressure may be
maintained for long distances many or even hundreds of kilometers.
This principle is of the utmost importance in the flowage of ground water,
and its applications are later developed. (See pp. 585-588.)
(JKXEKAIi STATKMKXTS.
Ill general it may be said that in proportion as the driving forces,
gravity, mechanical action, etc., are great, circulation is likely to be rapid.
In proportion as the opposing force, internal friction, is great, circulation is
likely to be slow. In proportion as the openings approach the circular
form, circulation is likely to be rapid. In proportion as the openings are
continuous, the circulation is likely to be rapid. In proportion as the pore
space is great, circulation is likely to be rapid.
However, of all these various factors dependent upon the character of
the openings, that of size is probably the most important; for rocks which
do or do not readily transmit water may have the same proportion of pore
space. For instance, if the grains be supposed to be spherical, of the same
size, and arranged in the most compact fashion possible, the unoccupied
space is 0.26 of the entire space, without reference to the size of the grains.
Thus the relative proportion of the openings in a great bowlder conglom-
erate and a fine-grained clay may be the same. But the capacity for the
transmission of water by the former will be indefinitely greater than by the
latter. As illustrating this, an experiment showed that a quartz sand, the
water of saturation of which was the same as that of a certain chalk, trans-
mitted water under a certain pressure six hundred times as fast as the chalk."
In the compact soils, the particles of which are exceedingly small (see
pp. 138-146), the openings between the particles are of capillary or sub-
a Prestwich, Joseph, Geology, chemical, physical, and s,tratigraphical, Clarendon Press, Oxford,
vol. 1, 1886, p. 159.
PORE SPACE AND UNDERGROUND CIRCULATION. 155
capillary size. In the case of the fine soils and clays the pores may be almost
wholly subcapillary, or the water is that of imbibition. In this fact we
have the explanation of the retention of soil moisture in fine clays. The
moisture is glued to the grains. There is practically no circulation, and
the water is removed only by high temperature or high pressure, or the
two combined. It follows from the foregoing that, under given conditions
with a given pore space, the coarse conglomerates, furnish a much larger
flow than fine conglomerates, the fine conglomerates a larger flow than the
sandstones, and these a vastly greater flow than the soils, clays, and shales.
Bedding, fault, joint, and fissility openings may be so close together
that the pore space is very large. Ordinarily fault openings are wider
spaced but larger than the joint openings, and joint openings are wider
spaced and larger than the openings of fissility. It can not be said which
kind of opening gives, on the average, the larger pore space. Since,
however, large openings are favorable to rapid flow, for a given pore space
the fault openings are likely to give a greater flow than joint openings, and
joint openings a greater flow than those of fissility. This follows from the
greater size of the fewer openings. To this is to be added the element of
greater continuity of the larger openings, as explained on pages 130-
131. Therefore, with a given pore space the flow may be vastly greater
in the case of faults than in the case of joints, and much greater in the
case of joints than in the case of fissility.
In this connection it may be said that the capacity of a rock for
imbibition gives a very good idea as to its power of transmission. The
water of imbibition, it may be recalled (see p. 124), is the amount which
adheres to the walls of the openings. It is evident that in rocks containing
the same percentage of water when saturated the power of transmission
varies inversely as their capacity for imbibition. If the openings of a rock
be very small, but numerous, there is in a cubic centimeter a large surface
to which the water can adhere. If the openings be subcapillary, the water
of imbibition and saturation are the same and the powers of transmission
practically nil. If the spaces be capillary, the water of imbibition is much
less and the power of transmission greatly increased. If the spaces be
supercapillary, the water of imbibition is slight in amount and the power
of transmission very great.
156 A TREATISE ON METAMORPHISM.
As already noted, there are two zones of metamorphism, that of kata-
morphism and that of anamorphism, and the former consists of a belt of
weathering and a belt of cementation.
The major part of the water entering the ground must finally reach the
surface. A small part may be combined with the rocks in the underground
course of the water. A small part may possibly penetrate deep within the
zone of anamorphism, but it is safe to say that at least 99 per cent of the
water entering the ground reappears at the surface in some manner. A
very large part of the water penetrating the soil is drawn to the surface
after having taken a longer or shorter journey in the belt of weathering. A
lesser part of the water joins the sea of ground water and takes a journey
of greater or less distance in the belt of cementation before it reaches the
surface. This journey may be merely from the top of a small hill to its
base, or it may be hundreds of kilometers. An exceedingly small fraction
of the water doubtless penetrates the zone of anamorphism, although, as
explained (pp. 665-668), the general movement is from rather than to this
zone. The underground journeys of water, whether the exceedingly
short ones within the belt of weathering or the longer journeys in the belt
of cementation or the zone of anamorphism, may be resolved into two
components, one parallel to the surface of the earth and one at right
angles to this surface. The first may be called the horizontal component,
the second the vertical component. On the average, the horizontal com-
ponent of the journey is many times longer than the vertical component.
GEOLOGICAL WORK OF GROUND WATER.
From the foregoing it follows that the geological work of ground
water is favored by smallness of openings, by length of time, by pressure,
and by high temperature. Water enters the rocks mainly through the
smaller openings. A very large surface of the rock material is exposed to
water action. In so far as the water passes from the smaller openings to
the larger openings its geological work is lessened. The geological work
may be considered as directly proportional to the time. The smaller the
openings the greater the resistance, and therefore the greater the time for a
given journey. That the resistance runs up very rapidly as the openings
become small, and especially as they become capillary or subcapillary, has
GEOLOGICAL WORK OF GROUND WATER. 157
already been shown. Since the horizontal journey is, on the average, long
as compared with the vertical journey, the element of time is of much
greater importance in the horizontal component of the journey than in the
vertical component. The capacity for geological work is increased by
pressure and by temperature. These forces, under ordinary conditions, are
a function of depth, and these factors in the work mainly concern the ver-
tical component of movement. During the downward journey the pressure
and temperature steadily increase, and the amount of material in solution
increases. During the upward journey the pressure and temperature
diminish and the tendency for material to pass from solution or to be
precipitated increases, and the amount held in solution diminishes.
Pressure and temperature are ever working together according to
definite laws. Both increase in efficiency with depth, and they greatly
promote the activity of deep ground waters. However, of all the vary-
ing factors, varying temperature is the one which is of incomparably
the greatest importance. High temperature ordinarily results from depth
of penetration; but it has been pointed out that it may result from various
other causes, of which chemical action, mechanical action, and the pres-
ence of intrusive igneous rocks are the more important. The capacity
which water has for taking and holding in solution various relatively
insoluble compounds, and the velocity of chemical reactions, increase
enormously with increase of temperature. Not only is high temperature
favorable to geological work, because of the chemical activity of the water,
but, as already pointed out, high temperature greatly decreases its viscosity,
and this, as already explained, is favorable to depth of penetration and
flow through minute openings. Since the temperature changes of ground
water are commonly dependent upon depth, the vertical component of the
movement of underground water is ordinarily far more important than the
longer horizontal component.
The underground journey of water may occupy hundreds of years.
(See pp. 585-586.) The surface of contact in very small openings is
very great. Under these conditions of slow movement and small openings
there is sufficient time nearly to establish complete equilibrium between
the solutions and the solids with which they are in contact; but it has been
seen (pp. 34-35) that rarely or never is the adjustment of a rock to its
158 A TREATISE ON METAMORPHISM.
environment complete. In so far as the adjustment is not complete, changes
are going on, and the conditions are everywhere those of chemical dynamics,
although the chemical action may be so slow that if the operations were
conducted in a laboratory it might be concluded that the conditions were
those of chemical statics. This, however, but illustrates the importance of
time in geological operations.
Thus far the treatment of the circulation and work of ground water
has been general. There are many other factors concerned in the circula-
tion which have not yet been considered, but these are factors special to
the different belts and zones. They will therefore be treated in Chapters
VI, VII, and VIII, on the belt of weathering, the belt of cementation, and
the zone of anamorphism, respectively.
CHAPTER IV.
THE ZONES AND BELTS OF METAMORPHISM.
GENERAL CONSIDERATIONS.
The various geological factors which bear upon metamorphism have
been briefly discussed in the introductory chapter. It is there held that
the geological factor of dominating importance is depth. Upon the basis
of depth it is stated that the known crust of the earth is divisible into upper
and lower zones of metamorphism; the first is called the zone of katamor-
phism, and the second the zone of anamorphism. It is further stated that
the zone of katamorphism is divisible into two belts, an upper belt of
weathering and a lower belt of cementation.
While in the introductory chapter these general statements were made,
there was no attempt to show that they are correct. It is one of the pur-
poses of this and the following chapters to furnish evidence of the utility
of this classification, and to show that very different metamorphic effects
follow from the work of the same forces and agents in the different belts and
zones. In the present chapter a brief general statement will be made as to
the characteristic reactions of the different zones and belts. This statement
is primarily from physical and chemical points of view. The next chapter
will treat of the alterations of minerals with reference to the different zones
and belts. In succeeding chapters the alterations of the rocks in the belt
of weathering, the belt of cementation, and the zone of anamorphism will
be taken up in detail. The treatment will be primarily from the geological
point of view, but with reference to physical and chemical principles.
Finally, the alterations of the individual rocks will be considered. This
and the following chapters might be regarded as a consideration of the
metamorphism of the crust of the earth from the point of view of the
physical-chemical principles developed in Chapters II and III.
159
160 A TREATISE ON METAMORPHISM.
It has just been stated that the nature of the metamorphism varies
greatly with depth. The physical reasons for this are that, as depth
increases, temperature and pressure increase. It has been seen in Chapters
II and III that where the pressure is moderate chemical reactions are likely
to be such that heat is liberated, and this is a fact whether the reactions
decrease or increase the volume. It has also been seen that where the
pressure is great this is likely to be the controlling factor, and that under
such circumstances reactions take place which lessen the volume of the
materials. Whether the reactions take place with liberation of heat or
with absorption of heat is a subordinate matter; but very commonly the
reactions are of a kind that absorb heat.
When the law of chemical affinity controls, and the reactions take
place with liberation of heat irrespective of the volume change, the reac-
tions may be said to be chemical -physical reactions. Where pressure is a
dominant factor and reactions take place with diminution of volume
irrespective of the heat change, the reactions may be said to be physical-
chemical. It is because variations in the geological factor of depth result
in these contrasting reactions that the lithosphere is divisible into a zone of
katamorphism and a zone of anamorphism.
ZONE OF KATAMORPHISM.
From the surface of the earth to a very considerable depth below the
surface (for strong rocks possibly 10,000 or 12,000 meters under quiescent
geological conditions) the rocks as originally formed may contain many
openings, as, for instance, those of sandstones, vesicular lavas, etc. Even if
not originally porous deformation may fracture the rocks and thus produce
many openings. Where the rocks contain openings chemical reactions may
take place, increasing the volume of the material without rupturing the
rocks and without raising them to a higher position. In the outer litho-
sphere the pressures and temperatures are moderate. Under such circum-
stances the reactions which take place are controlled mainly by the laws
of chemical affinity, not by the influence of pressure. At low temperatures
the fundamental chemical law is that, on the whole, the preponderating
chemical reactions are those which take place with the liberation of heat in
accordance with the first part of van't Hoffs law. Therefore in this zone the
occurrence of a reaction in the alteration of a rock is favorable to further
REACTIONS IN ZONE OF KATAMORPHISM. 161
alteration; for the heat developed by the first reaction is retained by the
adjacent material, at least for a time, and this promotes further reaction, etc.
But this tendency, as has been seen, may be reversed if the temperature
becomes too high. (See p. 79.)
Since the law of chemical reactions with the liberation of heat is the
dominant factor in this upper zone, alterations may take place which
work with or against pressure. In the first case both the chemical reaction
and the compression in volume result in the liberation of heat. In the
second case the heat liberated is that developed by the chemical reaction
minus that absorbed as a result of the work done in expanding the volume.
As a matter of fact, near the surface of the earth the very important
reactions from the point of view of the nonmetallic elements, aside from
solution, are those of oxidation, hydration, and carbonation. Oxidation
and hydration commonly involve the addition of material, although the
former frequently occurs by substitution of oxygen for sulphur, and
therefore by desulphidation. Carbonation frequently involves the addition
of material, but more commonly occurs by the substitution of CO 2 for SiO 2
and the decomposition of silicates. Often the freed silica, or a part of it,
remains in situ. All of these reactions are well known to liberate heat.
Commonly they decrease rather than increase the specific gravity of the
minerals. Since they usually involve addition of material, it is clear that
where all the residual material, or a large part of it, remains in situ the
volume of the rocks is considerably increased. However, it will be seen
that solution is also a very important reaction in parts of the zone of
katamorphism, and where this takes place to a sufficiently great extent
the volume of material may be decreased.
The main part of the oxygen and much of the carbon dioxide for
oxidation and carbonation is directly or indirectly derived from the atmos-
phere. The water is chiefly that of the ground circulation. It is there-
fore clear that in the upper zone oxygen and carbon dioxide are being
steadily abstracted from the atmosphere and fixed in the rocks, and ground
water is steadily becoming fixed by hydration. The amount of oxygen
and carbon dioxide thus fixed is great. If it were not for replenishment,
it is little short of certain that the carbon dioxide of the atmosphere
would have long since become exhausted. But probably the amount of
water fixed by hydration is even greater than that of the gases, oxygen
MON XLVII 04 11
162 A TREATISE ON METAMOKPHISM.
and carbon dioxide. Analyses of rocks in the upper zone of metamorphism
show that the amount of combined water runs as high as 4.42 per cent in
shales (see p. 744), and it probably averages as high as 1.64" per cent.
When it is remembered that the zone of katamorphism extends to a depth
of thousands of meters, it is apparent that the amount of water which is
thus fixed in the rocks by the process of hydration is enormous. However,
it will be seen that the process of hydration, like that of carbonation, is
reversed in the zone of anamorphism.
By the statement that oxidation, carbonation, and hydration are the
very important characteristic reactions of the zone of katamorphism it is
not meant to imply that the reverse reactions do not take place to some
extent. In fact, deoxidation, decarbonation, and dehydration all occur;
but oxidation, carbonation, and hydration are greatly preponderant, and
indeed dominant over the reverse reactions.
Summarizing so far as the energy factors are concerned, the changes in
volume commonly absorb heat, the chemical reactions dominantly liberate
heat and only exceptionally absorb heat. The heat liberated by the chem-
ical reactions is certainly very much greater than the sum of that absorbed
by the volume changes and that absorbed by the exceptional chemical
reactions. Therefore, so far as the rocks of the zone of katamorphism are
concerned, the total of the volume and chemical changes results in the
liberation of heat and the dissipation of energy.
The minerals formed in the zone of katamorphism are comparatively
few in number, with low specific gravities and probably for the most part
comparatively simple molecules; hence the propriety of calling this zone
the zone of katamorphism, or katamorphic zone. This use of the term
katamorphism is parallel to the use of the term katabolism in biology to
designate those chemical changes within a living body which result in the
production of simple compounds from more complex ones. The zone of
katamorphism may therefore be defined as the zone in which alterations of
rocks result in the production of simple compounds from more complex ones.
The zone of katamorphism is divisible into two belts, (1) an upper
belt of weathering, and (2) a lower belt of cementation. The belts are
"This is the average taken from analyses of shales, sandstones, limestones, and volcanic and
crystalline rocks, given by F. W. Clarke in Bulls. U. S. Geol. Survey No. 78, pp. 36-37, and No. 168,
pp. 16-17.
EEACTIONS IX BELT OF WEATHERING. 163
delimited by the level of ground water. The separation of the belt of
weathering from the belt of cementation is therefore based upon the posi-
tion of an agent of metamorphism. It has been seen that the zone of kata-
morphism is separated from the zone of anamorphism by a reversal of the
physical-chemical factors. As one would suppose, the latter distinction is
of much more fundamental importance than the former.
BELT OF WEATHERING.
By some it has been proposed to call the belt of weathering that of
demorphism; and to call the alterations of all rocks below this belt
metamorphism. The fact that the alterations in the belt of weathering are
very different from the belts below has been well known for many years.
But it has not been generally recognized that the belts of weathering and
cementation are delimited by the level of ground water. This is doubtless
due to the fluctuations of that level and to a considerable transition band
between the two belts (see pp. 423-429, 560-561); but in many places the
change in the character of the alterations in passing from the belt of
weathering to the belt of cementation is very sudden, and at such places
is very clearly connected with the level of ground water.
The belt of weathering is therefore denned to extend from the surface
to the level of ground water. In this belt all of the very important reac-
tions characteristic of the zone of katamorphism viz, oxidation, carbona-
tion, hydration, and solution are at their maximum activity; but on the
whole, of these three reactions the most characteristic, but not the dom-
inant one, is that of the carbonation of the silicates. This reaction takes
place on a vast scale, producing carbonates from the silicates, and at the
same time setting free silica or colloidal silicic acid. Hydration is the most
extensive simple reaction in the belt of weathering. Oxidation is also very
important. As will be seen, this reaction is very general in this belt,
because not being saturated with water the oxygen of the atmosphere very
rapidly makes its way through the porous rocks and continually supplies
oxygen to replace that element used in the process of oxidation. The total
effect of these chemical reactions is decomposition. While hydration and
oxidation are usual for this belt, under special conditions these reactions may
be reversed. In places of luxuriant vegetation and very high humidity
deoxidation may 'take place. In regions of great heat and temporary or
164 A TREATISE ON METAMORPHISM.
permanent aridity dehydration may locally occur. As already noted, as a
result of oxidation, carbonation, and hydration, the volume of the rocks
would be greatly increased if all the compounds formed remained in situ;
but the complex process of solution is dominant. Many of the compounds
formed are dissolved in large quantities and transferred by the overground
water circulation to the sea, or by the underground water circulation to the
belt of cementation below. Consequently the volume of the rocks contin-
uously decreases in the belt of weathering; and finally the resultant material
may occupy but a small fraction of the original volume.
In the belt of weathering, in addition to the characteristic chemical
reactions, mechanical disintegration is the rule. Thus the complex results
of weathering may be classified into disintegration, decomposition, and
solution. As a final result of the various mechanical and chemical changes,
rocks soften and degenerate. As coherent solids they are destroyed. The
processes of the belt of weathering are therefore destructive. The minerals
which remain are usually few and simple, and ordinarily are not well
crystallized. In the destructive processes all of the agents of meta-
morphism, both inorganic and organic, are actively at work. The details
of these processes are fully developed in Chapter VI, on " The belt of
weathering."
BELT OF CEMENTATION.
The belt of cementation extends from the bottom of the belt of
weathering to the bottom of the .zone of katamorphism. On the average
this belt is therefore much thicker than the belt of weathering. All of the
very important reactions characteristic of the zone to which the belt
belongs viz, oxidation, carbonation, and hydration take place. Water
is everywhere abundantly present in the belt, and hence hydration is the
most important of the three reactions. The minerals produced by meta-
somatic change from the original minerals and those deposited from the
solutions are likely to be strongly hydrated. The processes of carbonation
and oxidation in the belt of cementation are largely limited by the amount
of carbon dioxide and oxygen there contained.
It will be seen (pp. 608-610) that carbon dioxide is derived from several
sources and that carbonation is usual throughout the belt, but that the
oxygen is limited to that derived from above, and consequently that oxidation
REACTIONS IN BELT OF CEMENTATION. 165
is usual iu only a very limited part of the belt. Not only are the processes
of earbonation and oxidation subordinate to hydration, but the process of
oxidation not infrequently is stopped or reversed in all but the upper part
of the belt of cementation. This anomaly is due to the fact that many of
the rocks contain organic materials or sulphides or both which have a
strong affinity for oxygen. When the oxygen is exhausted from the water
derived from the belt of weathering the reducing compounds may act
directly as reducing agents or may produce reducing solutions. The
demands of these reducing agents for oxygen may abstract this material
from highly oxidized compounds, such as ferric oxide, basic ferric sulphate,
etc. Deoxidation in the belt of cementation is most commonly the result of
the burial of the higher oxide of iron and sulphates with a considerable
amount of organic material in the presence of abundant water. Under
these circumstances the ferric compounds may be reduced to ferrous
compounds and the sulphates to sulphides.
But it is to be noted that the reduction of these compounds involves
simultaneous oxidation of the organic compounds, the resultant products
being C0 2 and water. The carbon dioxide may escape from the belt or
enter iiito other combinations. For instance, as explained fully in another
place, the ferrous compounds largely unite with the carbon dioxide, pro-
ducing carbonates. Similar reactions may take place with reference to
other less abundant metals, as, for instance, manganese, and some metals
may even be reduced to the metallic condition, for instance, copper, silver,
and gold. These reducing reactions in the belt of cementation, except in
the case of iron, are of small consequence from a geological point of view,
but they have a most important bearing upon the deposition of ores. (See
Chapter XII.) It thus appears that oxidation and deoxidation are both
rather important in the belt of cementation.
The changes in the belt of cementation ordinarily produce crystalline
minerals Minerals which were partly altered by processes in the belt of
weathering may be regenerated. This applies only to those minerals which
are adapted to the belt of cementation. The average specific gravity of the
rocks is usually lessened.
It has been noted that the most characteristic reaction of the belt
of weathering is solution. In contrast with this the most characteristic
reaction of the belt of cementation is deposition in the openings of the
166 A TREATISE ON METAMORPHISM.
rocks. The material deposited is derived from the belt of weathering or
from the alterations within the belt of cementation itself. Much of the
material dissolved in the belt of weathering is continuously transferred to
the belt of cementation by the downward movement of water. The total
amount of material which is thus derived from the belt of weathering is not
limited to the thin belt which exists at any given time; for, as a result of
denudation, the belt of weathering is constantly migrating downward and
encroaching upon the upper part of the belt of cementation; and thus
there is never a lack of material for solution in the belt of weathering which
may be dissolved and transferred to the belt of cementation. Within
the belt of cementation itself the reactions of oxidation, carbonation, and
hvdratiou all increase the volume, provided all the compounds formed, or
a large part of them, remain as solids. The material added to the belt of
cementation from the belt of weathering, and the reactions within the belt
of cementation, furnish an abundant supply of material for deposition in
the openings of the rocks, whether these openings be those originally
present or produced by erogenic forces. And, as a matter of fact, in the
belt of cementation the openings are continuously filled by mineral
matter and finally closed; but this does not show that solution may not
preponderate over deposition in this belt if the effect upon the original rocks
and the openings both be considered. (See pp. 612-617.) The mechanical
result of the various processes is to indurate the rocks. The processes of
the belt of cementation are constructive. The belt of cementation, from a
geological point of view, is fully considered in Chapter VII.
BELTS OF WEATHERING AND CEMENTATION CONTRASTED.
The alterations in the belts of weathering and cementation, while not
so fundamentally different as those in 'the zones of katamorphism and
anamorphism, contrast strongly. In the belt of weathering, of the great
reactions characteristic of the zone of katamorphism oxidation, carbonation,
and hydration all are important, but carbonation is most characteristic. In
the belt of cementation, of these reactions hydration is most important. In
the belt of weathering, solution greatly dominates over deposition. In the
belt of cementation solution and deposition are more nearly balanced, but
because of reactions which increase the volume of the rocks the openings are
REACTIONS OF ZONE OF ANAMORPHISM. 167
filled. In the belt of weathering, the material continuously decreases in
volume due to solution; in the belt of cementation it continually increases in
volume due to deposition of material through reactions involving expansion
of volume. These changes of volume due to addition or subtraction of
material commonly involve decrease in specific gravity. In the belt of
weathering the mechanical results are disintegration and softening; in the
belt of cementation, cementation and induration. The belt of weathering
is therefore especially characterized by solution, decrease of volume, and
softening, resulting in physical degeneration. The belt of cementation is
especially characterized by deposition, increase of volume, and induration,
resulting in physical coherence.
ZONE OF ANAMORPHISM.
At a variable depth below the surface of the earth the pressure is so
great that it can not be supposed that considerable openings permanently
exist. The depth at which this condition of affairs is reached depends
largely upon the character of the rocks. For the strong rocks, as already
noted (p. 160), this depth,, under quiescent geological conditions, may be as
great as 10,000 or 12,000 meters. If openings be originally present in
the rocks of the zone of anamorphism, as, for instance, sandstones, vesicular
lavas, etc., or be due to fracture while the rocks are not deeply buried,
when such rocks become sufficiently deeply buried to be in the zone of
anamorphism, it is certain that rock flowage will take place and the
openings will be closed, except possibly those of subcapillary size and
other minute openings in which water, carbon dioxide, or other liquids and
gases are occluded. In the zone of anamorphism there is great pressure in
all directions, and mechanical energy becomes the dominant factor which
controls the reactions. Changes consequently take place which diminish
the volume of the rocks. This volume change increases the specific grav-
ity, and contrasts with the volume changes of the zone of katamorphism.
The fundamental chemical law of energy in reference to heat is subordinate.
Reactions take place with the liberation or absorption of heat, depending
upon what is demanded by the pressure. Commonly, the preponderant
chemical reactions are those which take place with absorption of heat
The depth at which pressure becomes dominant is variable, depending
168 A TREATISE ON METAMORPHISM.
upon the character of the rock and upon whether the conditions are mass-
static or mass-mechanical.
It has been seen that at the moderate temperatures of the zone of kata-
morphism the preponderant chemical reactions are those which take place
with the liberation of heat. As the depth below the surface increases, the
temperature ever becomes higher; and consequently the temperature may
become so high that the tendency for chemical reactions to take place
with the liberation of heat is less dominant, and at sufficiently great depths
the heat may be so great that this tendency ceases, or is even reversed.
Or, using the words of vaii't Hoff, at high temperatures the preponderating
chemical reactions, or associations, which take place at lower temperatures
with the development of heat are replaced by preponderating chemical
reactions^ or dissociations, which take place with the absorption of heat."
However, at moderate depths in the zone of anamorphism under ordinary
conditions the temperatures are not very high. For instance, at a depth of
9,000 meters the temperature is probably in the neighborhood of 300 C.
Therefore, so far as the temperature is concerned, for that part of the crust
of the earth within observation the preponderant chemical reactions would
probably take place under the first part of van't Hoff 's law, rather than under
the second part, if it were not for the pressure. But the pressure is the
dominant factor which controls the reactions. The rocks in this zone are
under so great pressure in all directions that this fact demands chemical
reactions which produce diminished volumes irrespective of whether heat is
liberated or absorbed by them.
The very important reactions in the zone of anamorphism are silica-
tion, or union of silicic acids with bases producing silicates, and dehydra-
tion. Deoxidation is subordinate The process of silication commonly
takes place upon carbonates, and consequently involves decarbonation and
the liberation of the carbon dioxide, which may escape and thus the volume
be decreased. To what extent the pressure is the controlling factor in the
production of this reaction is difficult to say. Probably it is the dominant
cause, but it is possible that at the temperatures which prevail in this zone
silicic acid may be relatively more active than at the lower temperatures of
the zone of katamorphism, where carbonic is the stronger acid.
"NernBt, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895,
p. 583.
REACTIONS OF ZONE OF ANAMORPHISM. 169
As illustrations of the process of silication may be mentioned the
formation of wollastonite from pure limestone, of tremolite from dolomitic
limestone, of actinolite from ankerite, and of grunerite from siderite. (See
pp. 239, 241, 243, 244.) In the impure limestones under deep-seated con-
ditions, where numerous bases are present, various complicated silicates
form, such as other pyroxenes and amphiboles, tourmaline, choudrodite, etc.
The process of dehydration involves the liberation of water. This
reaction, it is safe to say, is one which is controlled by pressure. The
combined water is actually squeezed out of the hydrated mineral particles,
transforming them to less hydrous and to anhydrous forms in a manner
similar to that in which free water is pressed from a sponge.
Whether or not pressure in the zone of anamorphism is sufficient to
deoxidize compounds is uncertain. Certainly it can not be asserted that
the pressure is sufficient to squeeze out a part of the oxygen of hematite,
thus transforming it to magnetite. So far as deoxidation occurs, probably
the oxygen abstracted from the rocks usually unites with the elements of
organic compounds, thus producing carbon dioxide and water. Thus the
chief products liberated by silication, dehydration, and deoxidation are car-
bon dioxide and water. These join the interstitial water in the subcapillary
spaces and probably slowly escape into the zone of katamorphism above.
(See pp. 665667.) This results in loss of material, and since the specific
gravity of the minerals is increased on the average, the volume of the
rocks is decreased.
Besides the above processes, condensation may also be accomplished
by recrystallization, although this process generally takes place in connec-
tion with them. The process of recrystallization produces a rearrangement
of the elements in such a way as to form compounds of higher specific
gravity. This is well illustrated by the devitrification of glass.
The minerals produced in the zone of anamorphism are numerous,
definite, stable, crystalline, of high specific gravities, and probably have
complex molecules. The rocks formed are compact and strong. The
lower zone may therefore properly be called the zone of anamorphism, or
anamorphic zone. This use of the term anamorphism is parallel to the use
of the term anabolism in biblogy to designate those chemical changes in a
living body which result in the production of complex compounds from more
170 A TREATISE ON METAMORPHISM.
simple ones. The zone of anamorphism may be defined as the zone in
which alterations of rocks result in the production of complex compounds
from more simple ones.
Summarizing the energy factors in the zone of anamorphism, so far
as the volume change is concerned, the result is to liberate heat; so far
as the chemical reactions are concerned, heat may be liberated or absoi'bed,
but the latter reaction is more common. In the latter case the heat
absorbed is almost certainly much greater than that liberated by decrease of
volume. If it were not that a considerable number of the chemical reactions
liberate heat, it would be certain that heat is absorbed in the zone of
anamorphism But the heat liberated by some chemical reactions must
be added to that liberated by decrease of volume. Whether this sum is as
great as the heat absorbed by the preponderating chemical reactions is
somewhat uncertain; but it is thought to be rather probable, for the com-
pounds immediately concerned in the reactions, that the total effect is to
absorb heat and store energy. However, in order to accomplish this, energy
must be derived from an outside source, and when all the factors which in
any way affect the reactions are taken into account, including the movement
of the superincumbent material, heat is dissipated and energy lost. (See
p. 182.)
RELATIONS OF ZONES OF KATAMOKPHISM ATSTD ANAMORPHISM.
We shall now consider the zones of metamorphism together with
reference to the energy factors. So far as the chemical reactions are
concerned, it has been seen that they may take place with liberation or
absorption of heat. So far as heat is liberated energy is dissipated. So
far as heat is absorbed energy is stored. The change in volume may also
result in the dissipation or storage of energy. Where increase of volume
is preponderant energy may be stored (1) by increasing the volume of the
rocks affected by the reaction or (2) by elevating the overlying rocks in
order that the space shall be available for the expenditure. In a given
case the energy may be stored by (1) or (2) or a combination of them.
Where decrease of volume is preponderant energy is dissipated (1) by the
decrease of volume of the rock affected by the reaction or (2) by subsid-
ence of the overlying material, or by both. Below the extreme outer film
of the earth the factor of elevation or subsidence of the overlying rocks is
of vastly greater importance than the volume change, and the relative
RELATIONS OF ZONES. 171
importance of this factor steadily increases with depth. This is more
broadly true in the case of increase of volume than in that of decrease
of volume; for in the latter case in the zone of katamorphism the strength
of the rocks near the surface may prevent subsidence, and the decrease of
volume simply produce porosity. A common illustration of that is vesicu-
lar dolomite. However, in the zone of anamorphism, when the reactions
result in decrease of volume, subsidence occurs and energy is dissipated.
The importance of the necessity of lifting the overlying material in order
to find more room in the case of increase of volume is well illustrated by
the frequent rapid hydration or slacking, with great expansion and rapid
disintegration, which follows when a partly hydrated rock, buried but a
few feet, is brought to the surface." Apparently when buried the tendency
for hydration and necessary expansion with liberation of heat was not
sufficient to lift the superjacent material. When the necessity of elevat-
ing the superjacent material was removed by transfer to the surface the
process of hydration and expansion went on to completion with great
rapidity.
I conclude from the foregoing that in so far as energy is concerned
there are four cases. Chemical reaction may (1) release energy and result
in the liberation of heat, or (2) may consume energy and result in the
absorption of heat. The change of volume may be (3) by decrease of
volume, and result in the release of energy and the liberation of heat, or
(4) by increase of volume, and result in the consumption of energy and
in the absorption of heat. (1) and (3) will be called plus, and when they
are combined the heat developed is equal to their sum; (2) and (4) will
be called minus, and when they are combined the heat absorbed is equal
to their sum. When (1) and (4) or (2) and (3) are combined heat may
be liberated or absorbed, and consequently energy dissipated or stored,
depending upon the relative values of the opposing factors.
It has been noted that the three important reactions in the zone of
katamorphism are oxidation, carbonation, and hydration; and in the zone
of anamorphism are deoxidation, silication, and dehydration.
Since all of the abundant metallic elements except iron are completely
oxidized as they occur in the original rocks, the important inorganic com-
pounds which are oxidized in the zone of katamorphism are mainly those
"Merrill, G. P., Disintegration of the granitic rocks of the District of Columbia: Bull. Geol. Soc.
America, vol. 6, 1895, p. 332.
172 A TREATISE ON METAMORPHISM.
of iron. Iron occurs extensively in the ferrous form, in magnetite, in car-
bonates, and in silicates. To a considerable extent it occurs as a sulphide.
To a small extent it occurs as metallic iron. In all of these forms it is
capable of oxidation. The main result of the oxidation of these com-
pounds, so far as the iron is concerned, is to change the monoxide to
ferric oxide. But where it is present as a sulphide it may be changed to a
sulphate, and then be thrown down as a basic ferric sulphate. Ferric oxide,
hydrous or anhydrous, is an important constituent in the sedimentary
rocks, and its presence is, without doubt, largely due to oxidation in
the zone of katamorphism. To a far less extent other metals, such as
copper, lead, zinc, etc., occur in the native form, in partially oxidized
forms, or as sulphides. All these substances may be oxidized. These
substances have little importance in general geology, but are of great
importance in the production of ores. All of the reactions of oxidation
take place with great liberation of heat and with increase of volume. In
the zone of anamorphism partial or complete deoxidation of the highly
oxidized compounds may occur. The ferric iron may be reduced to the
ferrous form. The sulphates of iron and the other metals may be reduced
to sulphides. In most cases the reducing agent is organic matter. The
reduction of the metals by organic compounds results in the oxidation of
the carbon and hydrogen, thus producing carbon dioxide and water. The
carbon dioxide and water largely escape. Where reducing agents, are not
present the highly oxidized materials produced in the zone of katamorphism
commonly remain in this condition even if the material passes into the
zone of anamorphism. Deoxidation can not, therefore, be said to be char-
acteristic of the zone of anamorphism to the degree that oxidation is
characteristic of the zone of katamorphism. The reducing reactions all
take place with great absorption of heat, so far as the metals are concerned,
and with decrease of volume. However, since heat is liberated by the
oxidation of the carbon and hydrogen, it is probable that the sum total
of the heat reaction in deoxidation in the zone of anamorphism is to
liberate heat.
In the matter of oxidation and deoxidation, the zone of katamorphism
presents a case in which the chemical law of the liberation of heat controls,
without reference to change in volume, while in the zone of anamorphism
the pressure tending to produce decrease of volume and chemical reactions
with the liberation of heat probably work together.
CHEMICAL RELATIONS OF SILICON AND CARBON. 173
Another set of reactions, of the most fundamental importance and
widespread character, which occur in an opposite sense in the two zones of
metamorphism are the mutual replacements of carbon dioxide and silicon
dioxide. It has already been noted that near the surface, or in the zone
of katamorphism, carbonic replaces silicic acid. Deep below the surface,
or in the zone of anamorphism, silicic replaces carbonic acid. Under the
conditions near the surface, where the pressure is small and the tempera-
ture is low, carbonic is the stronger acid; and under the conditions deep
below the surface, where the pressure is great and the temperature is
high, silicic is the stronger acid. The importance of the mutual replace-
ment of these compounds under different conditions makes it advisable to
summarize the chemical analogies of silicon and carbon. Silicon is the
characteristic element of inorganic compounds; carbon is the characteristic
element of organic compounds. How closely analogous are these two
elements is shown by the following comparative table:
Chemical relations of silicon and carbon.
SiO 2 silica, anhydride, solid ...................... C0 2 carbon dioxide, gas.
SiH 4 silicon hydride, gas ....... .................. CH 4 methane, gas.
SiCl 4 silicon chloride, liquid ...................... CC1 4 carbon tetrachloride, liquid.
Boils at 57 ............................... Boils at 76.
SiHClj silicon chloroform, liquid ................. OHC1 S chloroform, liquid.
Boils at 34 ............................... Boils at 60.
Si (C 2 H 5 ) 4 silicon ethyl, liquid .......... ......... C(C 2 H 5 ) 4 tetraethylmethane, liquid.
Boils at 150." ............................ Boils at 120.
Si(OC 2 H 5 ) 4 ethyl orthosilicate, liquid. - .. .......... C(OC 2 H 5 ) 4 ethyl orthocarbonate, liquid.
Boils at 160 .............................. Boils at 158. (See Mendeleeff, Vol. II,
Chap. XVIII, pp. 99-100.)
H 4 SiO 4 orthosilicic acid .......................... H 4 CO 4 orthocarbonic acid.
OH OH
\OH
OH OH
SiO 4 (C 2 H 5 ) 4 ethyl orthosilicate. x
(MgFe) 2 SiO 4 olivine Exists only in certain artificial organic compounds,
CaAl 2 (SiO 4 ) 2 anorthite [Natural orthosilicates. as ethyl orthocarbonate, CO 4 (C 2 H 5 ) 4 .
K" 3 R'" 2 (SiO 4 ) 3 garnet, etc.)
H 2 SiO 3 metasilicic acid ......................... H 2 CO 3 carbonic acid.
.OH /OH
0-Si^ 0=C(
X OH X OH
Exists in salts and in solution. Forms normal
(neutral) and acid salts ("bicarbonates").
174 A TREATISE ON METAMORPHISM.
Chemical relations of silicon and carbon Continued.
SILICON.
KAl(SiO s ) 2 leucite (normal salts)
AlfO-Si=O
\
O.
K-0'
o=c
0-Na
Natural metasilicates. O Na
O=C\ /Ca
etc. (acid salts)
/OH
o=c(
x ONa
or
/OH
0=CC
N_
0=C
O'
"OH
H,SijO 5 disilicic or dimetasilicicacid..... ......... H 2 C 2 O 5 dicarbonic or pyrocarbonic acid.
/OH ,OH
0=Si/ O=C^
> )0
0=Si( O=C(
^OH N OH
Si :0 ::2 :7 for basic salts. Known only in salts, as C 2 O 3 (NaO) 2 , produced by
Si : O : : 2 : 5 for acid salts. heating the acid salt.
Not known in free state.
LiAl (Si 2 O 5 )j petalite.
H 6 Si 2 O, diorthosilicic acid ....................... The corresponding carbon acid does not exist.
\OH),
H 4 Mg 3 Si 2 O 9 serpentine (normal salt).
0-Mg-OH
/0>Mg
Si-0 X
0-Mg-OH
H 2 CaSi 2 O 6 +H.jO okenite (acid salt).
H 4 Si s O 8 polysilicic acid or trisilicic acid ........... The corresponding carbon acid does not exist.
(May be considered as metasilicic acid plus
disilicic acid. )
/OH
Si=O
8i=O
>
Si=(OH),
(neutral salts of trisilicic acid)
KAlSi,O 8 orthoclase.
NaAlSi,O 8 albite.
VOLUMES OF SILICON AND CARBON COMPOUNDS. 175
Another close analogy which exists between the carbonates and the
silicates is the fact that many salts of both give alkaline reactions, or under
the theory of dissociation are hydrolized as explained (pp. 86-87), and
that alkalinity increases with the temperature.
The specific volumes of the silicates and carbonates also have very
close relations. In general the specific volumes (the molecular weights
divided by the specific gravities) of the silicon compound are slightly the
greater. The comparative specific volumes of a number of the correlative
silicon and carbon compounds are as follows:"
Specific volumes of silicon and carbon compounds.
CC1 4 94
CHC1 S 81
C(OC 2 H 5 ), 186
CaCOj 37
SiCl 4 112
SiHCL, 82
Si(OC 2 H 6 ) 4 201
CaSiO 3 . . 41
The specific volumes of Si0 2 and C0 2 are wholly different, but this is
explained by the fact that one is a solid and the other a gas.
Since the specific volumes of the carbon compounds are less than those
of the silicon compounds, if there be a simple substitution of carbon for
silicon the volume is decreased; if silicon for carbon, the volume is
increased. However, as a matter of fact, the changes in the rocks are
never so simple as this. The volume changes in carbonation with desilica-
tion, and in silication with decarbonation in the rocks largely depend upon
whether the reacting and resultant compounds are gaseous, liquid, or solid,
and whether the products remain as solids or are dissolved and transported
elsewhere.
In the zone of katamorphism carbon dioxide replaces silicon dioxide
ordinarily with liberation of heat
The fact of the carbonation of the silicates is well known. So far as I
know, the importance of this process was first realized by Bischof. He
attributes the general decomposition of the rocks near the surface mainly
to the action of carbonic acid, thus producing the carbonates which are
found in spring water. He shows by experiment that "the silicates of
alkalies, alkaline earths, protoxides of iron and manganese are decomposed
" Mendeleff , D., The principles of chemistry, translated by Geo. Kamensky, Longmans, Green
& Co., London, 189", vol. 2, pp. 99-100.
176 A TREATISE ON METAMORPHISM.
by carbonic acid at ordinary temperatures."* But he says that, since
carbonic acid does not combine with alumina or peroxide of iron, the
silicates of these compounds are not decomposed by carbonic acid." How-
ever, we now know that the process of carbonation takes place with all the
natural silicates. It will be shown in Chapter VII that this process of
carbonation goes on throughout the entire zone of katamorphism, but it is
in the upper of the two belts of the zone of katamorphism, that of weath-
ering, in which the process of carbonation goes oil with greatest rapidity
and is especially characteristic. Simultaneously with the substitution of
the carbon dioxide for the silica much of the silica separates as colloidal
silicic acid, is taken into solution, and is carried downward to the belt of
cementation by the percolating waters. In this belt the silica is deposited
on an enormous scale. The carbon dioxide is furnished in solution, being
mainly derived directly or indirectly from the atmosphere. When carbon
dioxide replaces silicon dioxide the volume would be decreased, provided
all of the silicic acid were abstracted in solution. But it is probable that
the larger portion of the silica set free in the zone of katamorphism by
carbonation is deposited in the belt of cementation, and therefore the
volume of the zone of katamorphism as a whole, so far as this reaction is
concerned, is increased. The deposition of silica in the belt of cementation
is probably accompanied by a considerable absorption of heat, under the
law that the negative value of the heat of solution is greater the more
insoluble the substance.
Carbonation in the zone of katamorphism may take place without
replacing silica, as in the case of the union of carbon dioxide with iron
oxide in magnetite, thus producing iron carbonate. In this case the
liberation of heat and the increase in volume are both great.
In the zone of anamorphism, and especially under mass-mechanical
conditions, silica replaces carbon dioxide in the carbonates on the most
extensive scale. So far as I am aware, Bischof was the first to realize that
under proper conditions the process of carbonation of the silicates could be
reversed. He shows by experiment that carbonates of calcium, magnesium,
and iron are decomposed by silica at a boiling temperature, and cor-
" Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond,
Harrison & Sons, London, vol. 1, 1854, p. 2.
6 Bischof, cit., vol. 1, pp. 4-5.
VOLUMES OF SILICON AND CARBON COMPOUNDS. 177
rectly infers that when any of these carbonates occur with quartz at a
sufficient depth within the earth, where a temperature of 100 C. is reached,
this reaction may take place. He calculates that this depth will be 2,440
meters. He correctly infers that the presence of abundant carbon dioxide
in deep-seated waters is probably due to this process of silication." We
now understand that under conditions of moderate pressure and temperature
not only are the carbonates which Bischof mentioned decomposed, but other
carbonates may be altered in a similar manner. However, it is noteworthy
that the carbonates which Bischof mentioned are those of predominant
importance.
The substitution of silicon for carbon would result in increase of
volume provided silica were derived from the solutions and the carbon
dioxide passed into the solutions. But in the process of silication in the
belt of anamorphism little material is available from outside sources.
Therefore the most of the silica which replaces carbon dioxide in carbonates
must be considered as a solid. It is probable that a large part of the freed
carbon dioxide slowly escapes; for at temperatures prevailing in the zone
of anamorphism the carbon dioxide is above its critical temperature, and
therefore a gas, and probably slowly makes its way through the subcapil-
lary spaces to the zone of katamorphism (see p. 667.) Hence the volume
comparison must be made between the carbonate and replacing silica
combined and the resultant silicate. On this basis there is a marked
diminution of volume. One of the simplest illustrations of the formation
of the silicates with condensation of volume is the development of wollasto-
nite from calcium carbonate and quartz. In this change the volume of the
solid remainder is decreased 31.48 per cent. However, this calculated
decrease is somewhat too great; for it will be seen (p. 667) that some
of the carbon dioxide does not escape, but is retained in the rocks in the
form of numerous inclusions.
It appears from the foregoing that in the replacement of silicon dioxide
by carbon dioxide in the zone of katamorphism, the chemical law of reac-
tions with liberation of heat dominates over that of pressure; and that in
the substitution of silicon dioxide for carbon dioxide in the zone of
anamorphism the physical law that pressure demands decrease of volume
dominates over the chemical law of reactions with liberation of heat.
a Bischof, cit., vol. 1, pp. 237-241.
MON XLVII 04 12
178 A TREATISE ON METAMORPHISM.
The third important case in which the reactions occur in the opposite
sense in the zones of katamorphism and anamorphism are hydration and
dehydration.
Hydration is a characteristic reaction of the zone of katamorphism,
only less important than that of carbonation ; moreover, hydratioii occurs
on a great scale both in the belt of weathering and in that of cementation.
That hydration occurs extensively deep in the belt of cementation is
evidenced by the hydrated minerals which develop in the cavities of the
rather deeply buried rocks, such as the amygdules of amygdaloids.
Hydration represents, in the words of the first part of van't Hoff's law, "an
association which takes place with great liberation of heat." This process
also results in very considerable increase of volume, provided all or nearly
all of the products formed remain in situ.
Dehydration is a characteristic reaction of the zone of anamorphism,
only less important than that of silication. When the hydrated minerals
formed in the belt of katamorphism pass into the zone of anamorphism by
deep burial they are dehydrated. The pressure, or the high temperature,
or the two combined, unite to drive off a large part of the water. Dehy-
dration, in the words of the second part of van't Hoff's law, represents " a
dissociation which takes place with great absorption of heat" and it takes
place with decrease of volume.
Therefore, so far as hydration and dehydration are concerned, in the
upper zone the first part of van't Hoff's law, that of chemical reactions with
the liberation of heat obtains, but in the lower zone the law of diminution
of volume controls, regardless of the heat effect. The first part of this
statement is sufficiently evident; the second possibly needs further expla-
nation. To drive off the combined water of rocks at ordinary pressure
usually requires a temperature above 110 C. This temperature under
mass-static conditions would not be found until a depth of 3,300 meters had
been reached. It is certain that at depths much less than this, and at
temperatures lower than this, dehydration takes place on an important scale ;
for it will be shown (p. 744) that in the transformation of mudstones to
shales there is a loss of about one-half of the combined water. I conclude
that under many circumstances the increase in temperature is not suffi-
cient to reverse the reaction of hydration, and therefore the reversal must
VOLUME EFFECTS OE HYDRATION AND DEHYDRATION. 179
be due to the pressure. However, in the lower part of the zone of ana-
morphism the temperature is frequently higher than 110 C., and under
such circumstances both the pressure and the temperature may work together
to produce dehydration.
The statement that the volume is decreased by dehydration is only
true provided the separated water, or a large part of it, escapes; for the
volume of the hydrated solid is less than that of the residual solid plus the
separated water; therefore, if the water could not escape, pressure would
tend to preserve the combination. Hence, the fact that the reaction does
take place in the zone of anamorphism shows that there is sufficient
pressure not only to separate the combined water from the rocks, making
it free water, but to squeeze the free water from the rocks as one can
squeeze the water from a sponge. The effective pressure doing the work
is equal to the pressure of the adjacent rocks less the weight of an equal
column of water extending to the surface. Thus, under mass-static condi-
tions, if the rocks have a specific gravity of 2.7, the effective weight in
producing dehydration and driving out the free water at a depth of 3,300
meters is that of a column of material of this height with specific gravity
of 1.7. Under mass-mechanical conditions, where the pressure as a result
of thrust may be much greater than that due to weight, the effective
pressure tending to separate the combined water is much greater. Conse-
quently, under such conditions dehydration may occur at much less depth
than under mass-static conditions. (See pp. 766-768.)
One or two minerals may be mentioned which illustrate the processes
of hydration and dehydration in the two physical-chemical zones. Near
the surface and to a considerable depth, under mass-static conditions,
limonite and other hydrated oxides of iron develop. Deeper down, and
especially in connection with mass-mechanical action, limonite is dehy-
drated, and hematite is produced. As another illustration may be men-
tioned the somewhat similar compounds, chlorite and biotite. Near the
surface and under quiescent geological conditions chlorite forms. Deep
below the surface, and especially under mass-mechanical conditions, biotite
ordinarily develops. This is nowhere better illustrated than in the Michi-
gamme formation in the Marquette district of the Lake Superior region,
where these two minerals directly replace each other under the law just
180 A TREATISE ON METAMOKPHISM.
stilted." In the zone of katamorphism the complex hydrous silicates, such
as the kaolins, serpentines, and zeolites form. In the zone of anamorphism
these minerals are largely dehydrated, and such minerals as muscovite,
andalusite, gajnet, staurolite, etc., are produced.
The physical-chemical principles cited (pp. 45-123) give reasons for
the existence of the above reverse sets of reactions in the two zones. We
can now give chemical or physical causes why oxidation, carbonation, and
hvdratioii take place in the zone of katamorphism, and deoxidation,
silication, and dehydration in the zone of anamorphism, and so on for other
reactions.
While each of these sets of processes is particularly characteristic of
one zone, it is not meant to imply that each reaction may not occur in
both zones. But in the zone of katamorphism, oxidation, carbonation,
and hydration greatly predominate over the reverse processes. On the
other hand, in the zone of anamorphism, deoxidation, silication, and dehy-
dration predominate over the reverse processes.
If all of these sets of processes reversed as preponderant reactions at
the same depth, it would be possible to sharply separate the zones of
katamorphism and anamorphism. If, for instance, for a given region above
a depth of 10,000 meters the sum totals of the oxidation, carbonation, and
hydration were greater than the sum totals of reverse processes, the zone
of katamorphism would be sharply separated from the zone of anamor-
phism at this depth. But this is not the case. The reversal of each pair
of processes occurs at different depths; and, further, the reversal for a
given pair of processes is at different depths under different conditions.
One of the most important of these is as to whether the conditions are
mass-static or mass-mechanical.
Of the tliree sets of reversing reactions, oxidation and deoxidation,
carbonation and silication, hydration and dehydration, the first reverses
with the least depth and pressure, the second requires the greatest depth
and pressure, and the last a mean depth and pressure. It has already been
noted that oxidation very frequently is replaced by deoxidation in the
lower part of the zone of katamorphism. It is certain that the process of
hydration is very greatly stayed, if it does not altogether cease, and may
Van Hise, C. R., and Bayley, W. 8., The Marquette iron-bearing district of Michigan: Mon. U. S.
Geol. Survey, vol. 28, 1897, pp. 444-459.
CONTRASTING REACTIONS OF DIFFERENT ZONES. 181
even be reversed in the lower part of the zone of katamorphism. It is
therefore apparent that the two zones are not sharply delimited. In
general, however, it may be said that the outer zone to a depth in which
oxidation, carbonation, and hydration preponderate is that of katamor-
phism, and that the deeper-lying- zone, in which the reverse of these
processes preponderate, is that of anamorphism. But carbonation and its
opposite, desilication, are the most fundamental reactions of the zone of
katamorphism. Silication and decarbonation are the most fundamental
reactions of the zone of anamorphism. By these reactions more than by
any others, these zones are delimited. The three sets of reversing reac-
tions, oxidation and deoxidation, carbonation and silication, hydration and
dehydration, constitute three cycles in metamorphism. The second of
these cycles was recognized many years ago by Bischof (see pp. 176-177),
and was called the carbono-silicic cycle.
From the foregoing statement it is clear that the work of the zones of
katamorphism and anamorphism are opposed to each other. What the one
is doing the other is undoing. At the present time it is therefore possible
that in the case of any one of the pairs of opposed reactions, consider-
ing both the zones, either one of them preponderates, or that they are
approximately balanced. For instance, the amount of water being fixed
in the zone of katamorphism may be greater or less than the amount of
water being freed by dehydration in the zone of anamorphism, or the
t\vo may be nearly balanced. The same statement may be made in refer-
ence to the other reversing reactions. Upon the preponderance of these
opposing sets of reactions in the opposite zones depends the answer to
the question whether, on the whole, oxygen, carbon dioxide, and water
from the atmosphere and hydrosphere are being .fixed or freed by meta-
morphism. This question is considered in Chapter XI.
\Vliile the zones of katamorphism and anamorphism are separated
from each other by contrasting reactions, all reactions do not reverse in the
two physical-chemical zones. The first part of van't Hoff's law of heat
and the law of pressure may work together that is, in both zones reactions
may occur which, simultaneously with the liberation of heat by chemical
action, also result in liberation of heat by condensation. In so far as
there are cases of this kind it is to be presumed that such reactions are
common to both zones. As an instance in which heat is probably evolved
182 A TREATISE ON METAMORPHISM.
both by the chemical reactions and by the volume change in both zones may
be mentioned the devitrification of glass. (See Chapter V, pp. 251-252.)
The chemical reaction is presumably under the first part of van 't Hoff's
la-w, and the volume is decreased. Another instance of chemical reaction
with the liberation of heat and condensation of volume is the replacement
of calcium by magnesium in limestone, thus transforming the rock into
dolomite."
It is thought to be certain that the total of all the changes taking place
in the whole of the mass of rocks concerned in any given modification of
the lithosphere results in the dissipation of energy, and it is believed that
such is the fact for each of the physical-chemical /ones separately. In the
zone of katamorphism the chemical reactions result in liberation of heat;
the average volume reaction results in absorption of heat. It is, however,
thought certain that the residual is in favor of the former. In the zone
of anamorphism the average of the chemical reactions results in absorption
of heat; the average of the volume reactions results in the liberation of
heat. It has already been seen (pp. 170-171) that the amount of energy
required for the volume change rapidly increases with depth, and in the
lower zone it is thought that the heat liberated from the volume changes is
greater than the heat absorbed by the chemical reactions, and therefore
that the residual is in favor of the liberation of heat.
Hence, it is concluded that the changes which take place in each of
the zones are under the general law of the running down of energy into
the form of heat which is dissipated, and this accords with the apparent
order of the universe.
A corollary to the foregoing pages is the conclusion that in the upper
zone, where pressure is relatively unimportant, on the average, alterations
result in the expansion of the volume of the rocks; and that in the
deeper-seated zone, where pressure is important or dominant, on the average
the alterations result in the contraction of the volume of the rocks. It
follows as a further conclusion from this that the tendency of the alterations
"The verification from authorities of the heat of the chemical reactions and the volume relations
for the majority of the changes above mentioned have been very kindly made for me by Mr. A. T.
Lincoln. Mr. Lincoln either has found the results used in the works of Thomsen, Ostwald, Mendeleeff,
or other standard authorities, or from the data there found has been able to calculate results which
answer the specific questions I gave to him.
SPECIFIC GRAVITIES OF MINERALS IN THE ZONES. 183
in the first zone is, on the average, to produce minerals of lower specific
gravity than the original minerals, while in the deeper-seated zone the
tendency, on the average, is to produce minerals of higher specific gravity.
Illustrations of the first rule are the minerals produced by the disinte-
gration and decomposition of rocks near the surface, out of which the sedi-
mentary rocks are built. Some of these are kaolinite (sp. gr. 2.6-2.63),
quartz (sp. gr. 2.65), calcite (sp. gr. 2.72), chlorite (sp. gr. 2.60-2.96),
serpentine (sp. gr. 2.5-2.65), talc (sp. gr. 2.7-2.8), zeolite (sp. gr. 2-2.4),
limonite (sp. gr. 3.5-3.96), etc. All of these minerals and most of the other
abundant undecomposed minerals, such as feldspar (sp. gr. 2.55-2.75),
which make up great masses of sedimentary rocks, have comparatively
low specific gravities.
The second rule is illustrated by the change from low to high specific
gravity of the minerals where the sedimentary rocks are metamorphosed.
As just seen, the minerals which compose the unaltered sedimentary rocks
are originally those of low specific gravity. Some of the abundant result-
ant minerals in the equivalent metamorphosed rocks have considerably
higher specific gravities, as, for instance, muscovite (sp. gr. 2.76-3), biotite
(sp. gr. 2.7-3.1), pyroxene (sp. gr. 3.2-3.6), and amphibole (sp. gr. 2.9-3.4),
and the still heavier minerals, garnet (sp. gr. 3.15-4.3), staurolite (sp. gr.
3.65-3.75), chloritoid (sp. gr. 3.52-3.57), hematite (sp. gr. 4.9-5.3), and
magnetite (sp. gr. 5.168-5.180). Less common heavy minerals are andalusite
(sp. gr. 3.16-3.2), fibrolite (sp. gr. 3.23-3.24), and chondrodite (sp. gr. 3.118-
3.24). With the above are the lighter minerals, quartz and feldspar; but
even these are quite as heavy as the average of the original minerals.
It is noticeable in the altered rocks that in proportion as deep-seated
metamorphism is advanced the heavier of the above minerals appear. In
the early stages of the metamorphism of shales, mica develops plentifully,
and the rocks become slates. Where the metamorphism is more intense
the heavier minerals, garnet and staurolite, appear, the material of the pre-
viously developed micas being absorbed at the places occupied by the
garnet and staurolite.
The garnet-, staurolite-, chloritoid-, andalusite-, and tourmaline- bearing
schists and gneisses of the Penokee and Marquette districts of Michigan
and Wisconsin and the Black Hills of South Dakota, produced by the
184 A TREATISE ON METAMORPHISM.
alteration of clastic rocks, are perfect illustrations of the above changes."
In these rocks the acid feldspars (sp. gr. 2.55-2.67) have extensively
altered into quartz (sp. gr. 2.65) and mica (sp. gr. 2.76-3.01), and therefore
have passed into minerals denser on the average than those from which
they were derived. Also the heavier minerals, garnet, etc., have developed
on an extensive scale in the more metamorphosed varieties.
When all the minerals formed are taken into account the average-
is as given. But it is not supposed that there are not exceptions to each of
the rules that in the upper physical-chemical zone lighter minerals form
and in the lower zone heavier minerals develop. Indeed exceptions are
known to both. An illustration of such exceptions in the upper zone is the
case already mentioned (see pp. 181182), the replacement of calcium by
magnesium. A case of the change from higher to lower specific gravity
in the lower zone is the alteration of pyroxene into amphibole. On the
average the former is slightly heavier, and yet in the lower zone, under
both mass-static and mass-mechanical conditions, pyroxene very generally
alters to amphibole. Of course in this transformation a change simultane-
ously takes place in the chemical composition (and this may have an effect
upon the volume of the minerals); for, in general, pyroxene contains a
greater proportion of calcium and less proportions of magnesium and iron
than the amphiboles. If all of the compounds concerned in the change
were taken into account this apparent exception to the rule of the production
of compounds of high specific gravity in the lower zone would probably
disappear. In some of the deepest-seated schists, pyroxene and not
amphibole has developed, and it is suspected that sufficiently deep this is
the rule. If this be the case the real meaning of the change of pyroxene
to amphibole is, in order that pressure shall become the dominant factor for
each of the minerals as well as for the average, that the pressure must be
very great.
But whatever exceptions may be discovered in the cases of individual
minerals, the rules that in the upper physical-chemical zone the alterations,
on the average, result in decrease of specific gravity, and that in the lower
a Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 302-331; also vol. 28, 1895, pp. 448-450, 452-454, 456-459.
Van Hise, C. R., The pre-Cainbrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 1890,
pp. 222-229.
SPECIFIC GRAVITIES OF MINERALS IN THE ZONES. 185
zone the alterations result in increase in specific gravity, are believed to
hold and to be of fundamental importance in the metamorphism of rocks."
This principle of the development of minerals of low specific gravity
near the surface and of high specific gravity at depth has a direct applica-
tion to the crystallization of magmas. From a magma of a given chemical
composition there can be little doubt that the greater the depth, and there-
fore the greater the pressure at which crystallization occurs, the higher the
average specific gravity of the rocks. In this treatise no attempt will be
made to work out the applications of the rule to individual minerals and
rocks, but one illustration in reference to minerals and one illustration in
reference to rocks may be cited. It is well known that in the lavas silica
frequently crystallizes in the form of tridymite (sp. gr. 2.28-2.33), but that
in the deep-seated igneous rocks quartz (sp. gr. 2.65) only is found. It is
believed that the explanation of this fact is that near the surface other
factors than pressure control the crystallization, and therefore that the less
heavy form of crystallized silica tridymite may be produced; and that in
the lower zone pressure is the determinative factor in the crystallization,
and therefore that the heavy form of crystallized silica quartz invariably
results. An illustration in reference to rocks is the presence or absence of
glass. Glass has a lower specific gravity than the equivalent crystallized
substance. It is well known that where magmas crystallize near the surface
glass is a frequent product, and that where magmas crystallize deep below
the surface glass is either very subordinate or absent altogether. While
other factors besides pressure enter into this result, it is believed that the
frequent presence of glass near the surface and the presence of dense
crystallized minerals in the equivalent deeper-seated rocks crystallizing
from magmas is a very striking illustration. of the truth of the principle of
the development of minerals and rocks of low density where the pressure
is small and of great density where the pressure is great.
<* The above conclusions as to the condensation of material at considerable depths has an important
bearing upon Reade's theory of mountain making. (Reade, T. Mellard, The origin of mountain
making, London, 1886.) His explanation of the rise of mountains is that the volume of the thick
deposits of sediments increases as a consequence of the rise of the isogeotherms. I believe that possible
expansion due to this cause is more than compensated in the case of the sediments by the mechanical
bringing of the particles closer together as the result of pressure, in many instances to the practical
disappearance of the interspaces, and by the condensation of the material itself by the physical-
chemical changes above explained. The condensation also has a bearing upon estimates of crustal
shortening. In so far as condensation occurs, shortening of the outer crust of the earth may allow
accommodation to a nucleus of decreasing size without crustal corrugation.
186 A TREATISE ON METAMORPHISM.
la conclusion of this part of the subject it may be said that in the
zone of katamorphism the alterations are mainly controlled by the chemical
law that reactions take place with liberation of heat, and this ordinarily
results in increase of volume, provided the compounds which form remain
as solids. In the zone of anamorphism the reactions are mainly controlled
by the physical law that reactions take place with decrease of volume, and
this commonly results in chemical reactions with absorption of heat. In
the upper zone chemical law is the determinative factor in the reactions;
in the lower, physical law. In the upper zone the important chemical
reactions are those of oxidation, carbonation (involving desilication), and
hydration; in the lower zone the important reactions are those of deoxida-
tion, silication (involving decarbonation), and dehydration. In the upper
zone the minerals are few in number, of low specific gravity, and probably
of simple molecular structure; in the lower zone the minerals are
numerous, of high specific gravity, and probably of complex molecular
structure.
If one were to select three words which roughly represent the charac-
teristics of the alterations in the belt of weathering, the belt of cementation,
and the zone of anamorphism, these three words would be, respectively,
destruction, construction, and reconstruction.
GENERAL CONSIDERATIONS.
It is now apparent that a geological classification of metamorphism
dependent upon depth carries with it profound chemical and physical
significance. Not only is the classification applicable to all parts of the
earth, but the alterations of the zones of katamorphism and anamorphism
are more fundamentally different than any distinction heretofore made with
reference to metamorphism. Also, the alterations of the belt of weathering
and the belt of cementation of the zone of katamorphism, again dependent
upon depth, are very different so far as the geological facts are concerned,
but are closely allied from the chemical and physical point of view, and
therefore belong together in a single zone.
It is further to be noted that the classification is based upon one idea
throughout. There is no overlapping, although the different belts and
zones are side by side and grade into one another.
After the universal geological applicability of the classification proposed
ZONES OF METAMORPHISM, FRACTURE, AND FLOW AGE. 187
is understood and its chemical and physical significance appreciated, one
sees how partial, how overlapping-, and how variable are the criteria upon
which are based such classifications as thermo-metamorphism, hydro-
metamorphism, dynamic metamorphism, contact metamorphism, regional
metamorphism, etc.
RELATIONS OF ZONES OF KATAMORPIIISM AND ANAMORPHISM TO
/ONES OF FRACTURE AND FLOWAGE.
In connection with metamorphism in the zones of katamorphism and
anamorphism it may be recalled that the outer part of the surface of the
earth may be divided into two zones upon a different basis." It has been
shown from the structural point of view that we may divide the rocks of
which we have knowledge into an upper zone of rock fracture and a lower
zone of rock flowage. Where the rocks are subjected to deformation while
in the upper zone they mainly undergo mass fractures called bedding part-
ings, faults, joints, fissility, etc. The deformation is accomplished .but to a
slight extent by fracturing of the individual particles and by differential
movement between them. In the zone of fracture openings other than
those formed by deformation may exist. In the sedimentary rocks are
openings between the grains. In the lavas there may be gas openings.
So far as the mass-mechai deal forces are concerned openings of all of these
different classes may persist indefinitely. In the lower zone that of rock
flowage if openings could be supposed to be produced in any way, the
pressure is so great that the rock flows and fills them nearly completely,
except that water solutions and gases are to a small extent included in
minute cavities. Rdck flowage will be subsequently shown to be mainly
accomplished by innumerable fractures of the mineral particles, by the
recrystallizatioii of the mineral particles, or by a combination of the two
processes in any proportion. Mass fractures play but a subordinate part.
UPPER LIMIT OF ZONE OF FLOWAGE.
The depth at which deformation by flowage occurs depends upon many
factors, of which the character of the rocks, the temperature, the water
content, and the speed of deformation are of consequence. The more
o Van Hise, C. R., Principles of North American pre-Cambrian geology; with an appendix on
flow and fracture of rocks as related to structure, by L. M. Hoskins: Sixteenth Ann. Kept. TJ. S. Geol.
Survey, pt. 1, 1896, p. 589.
188 A TREATISE ON METAMORPHISM.
important factors which enter into the character of the rocks are the
strength and the mineral composition. The stronger the rock the greater
is the depth at which flowage begins. The rocks the materials of which
are refractory, as, for instance, those composed of quartz and feldspar,
require a greater depth in order that deformation ma}- take place by flow-
age than those rocks the materials of which are readily acted upon
chemically, as, for instance, calcite.
The higher the temperature the less the depth in order that deforma-
tion may take place by flowage. Since the temperature increases normally
at the rate of 1 C. for 30 meters, and since in consequence of orogenic
movements and igneous intrusions the increase in temperature with depth
is often much more rapid than this, heat is a very important factor in the
depth at which rock flowage occurs.
Since rock flowage may be in large measure by recrystallization, and
recrystallization is dependent to a large extent upon the amount of water
present, the greater the amount of water the more readily does deformation
take place by flowage and therefore the less is the depth at which flowage
begins.
The speed of deformation is also of very great consequence in limiting
the upper part of the zone of flowage. The more rapid the deformation the
greater the depth of the zone of flowage; the slower the deformation the
more moderate its depth. Speed of deformation, and therefore the time
consumed in a given deformation, is of very great importance. It is well
known that a stress not sufficient to rupture a material or to appreciably
deform it within a short time, if applied for a long time may produce
important flowage deformation. The geologist must give this factor of
time greater weight than scientists in any other subject. How important
it is may be illustrated by the deformation of rock" as a result of placing it
in an unusual position. In cemeteries marble slabs have been placed
horizontally and supported at the ends; in the course of a score or more of
years such slabs are found to have sagged in the middle a very considerable
amount. This is illustrated in the cemetery of Jefferson City, Mo., where
a slab about 1.8 meters long, .9 meter wide, and 5.08 centimeters thick,
Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept.
U. 8. Geol. Survey, pt. 1, 1896, p. 594.
DEPTH OF ZONE OF FLO WAGE. 189
suspended at the ends, has sagged 3.8 centimeters in the middle. If it had
been attempted to bend the slab at the outset to this extent, undoubtedly
it would have been raptured. The change in form without rupture is pos-
sible only by rock flowage, through a rupturing and differential movement
of the solid particles with reference to one another, or by solution and
redeposition i. e., by granulation or recry stall ization, or by the two
combined.
On the assumptions (a) that the strength of the rocks is the same as at the
surface, (b) that the rocks are all of the same kind, (c) that the temperature
is the same as at the surface, (d) that the water present does not make any
difference in the character of deformation, (e) that the rocks yield as readily
by fracture as by flowage, (f) that the rocks break as readily by fracture
when the deformation is slow as when it is rapid, and (g) that the rocks are
among the strongest, I have calculated that the maximum depth of the
upper part of the zone of flowage under mass-static conditions can not be
greater than 12,000 meters. All of these assumptions, except the first, are
in favor of great depth for the zone of flowage. It is explained (Chapter
VIII, p. 672) that where rocks are under pressure in all directions the
rigidity is probably greater than at the surface. Therefore the assumption
that the rocks are no stronger below than at the surface might lead to too
small a depth. However, the other assumptions would give too great a
depth, because the great majority of rocks are not nearly so strong as the
strongest, and many of them have only a small fraction of this strength;
because the temperature increases with increase of depth, with orogenic
movements, and with intrusives; because water is present in considerable
quantity, and where this agent is available with higher temperatures the
rocks are deformed by flowage rather than by fracture (see p. 188); and,
finally, because the rocks ai - e ordinarily deformed so very slowly that with
a rather moderate pressure the deformation takes place by flowage rather
than by fracture. I can see no way to determine to what extent these
factors render the .maximum depth calculated too great; nor can any
estimate be made as to how far the factor (a) renders the maximum depth
calculated too small ; but I suspect that the various factors giving too great
a depth are of far greater consequence than the one factor giving too small
a Winslow, A., An illustration of the flexibility of limestone: Am. Jour. Sci., 3d ser., vol. 43,
1892, pp. 133-134.
190 A TKEATISE ON METAMORPAISM.
a depth. Since there is no theoretical way accurately to evaluate these
factors and thus to calculate the maximum depth of the upper part of the
zone of flowage, one can judge of its real depth only by observation in
mountain areas where deep-seated rocks have been deformed when buried
under an approximately determinable thickness of rocks and afterwards
have been brought to the surface by denudation. From observation data
I suspect the maximum depth calculated is much too great, perhaps twice
too great even for the strongest rocks; and for the weaker rocks it is certain
that the alterations characteristic of the zone of flowage occur at depths but
a fraction of 10,000 or 12,000 meters.
The boundary between the zone of fracture and the zone of flowage is
approximately the same as the boundary between the upper and lower
physical-chemical zones if indeed it is not identical with it. We may
therefore say that the upper physical-chemical zone, the zone of katamor-
phism, and the zone of fracture are synonymous terms, as are also the
lower physical-chemical zone, the zone of anamorphism, and the zone of
flowage. The reasons for the correspondence of the zone of fracture with
the zone of katamorphism, and of the zone of flowage with the zone of
anamorphism, are clear.
To the bottom of the zone of fracture the rocks are strong enough to
support themselves, hence there is not pressure in all directions greater than
the strength of the rocks, and openings may exist. The reactions may
therefore take advantage of these spaces and nil them, thus expanding the
volume of the rocks without lifting them or doing the mechanical work of
rupturing them. The openings which may thus be utilized vary from those
of supercapillary size, such as bedding partings, fault and joint openings, to
subcapillary openings between the individual grains. In order to thus fill
these spaces, no large amount of work must be done against pressure by the
chemical agents, but in proportion as the spaces are filled it is more and
more difficult for the reactions to occur requiring expansion of volume, as
an increased amount of work must be done against pressure.
However, below the bottom of the zone of fracture, in the zone of
flowage, the pressure in all directions is greater than the strength of the
rocks. If supercapillary spaces be supposed to be present they would be
closed by flow, unless this were prevented by occluded water or some other
liquid or a gas which could not escape. If a reaction here occurs which
TRANSITIONS BETWEEN ZONKS. 191
demands expansion of volume, it would be necessary to lift the entire
superincumbent mass of rock, and this would require a vast amount of
work. This work chemical affinity is usually not sufficiently strong to
accomplish, therefore reactions do not take place which give increased
volume; but on the contrary, the pressure forces reactions in the opposite
sense from those in the upper zone, as a result of which the volume of a
material is diminished. If the reactions diminishing volume can be of such
a character as to liberate heat, this will occur; but frequently, in order to
produce a decreased volume, chemical reactions must take place which
absorb heat. In this paper on metamorphism the terms zone of katamor-
phism and zone of anamorphism are used, as being most serviceable. In
structural work, however, the equivalent terms zone of fracturing and zone
of flowage are morfe serviceable and therefore will there hold their place.
It may be seen on page 167, also on pages 766-768, that the passage
from the zone of katamorphism to the zone of anamorphism is a gradation
and not an abrupt change. The same is true of the change from the
zone of fracture to the zone of flowage. (See pp. 187-189.) Therefore,
whether the division of the outer crust of the earth into two zones be
considered from the metamorphic point of view or from the structural
point of view, there is a transition between the two.
CHAPTER V.
MINERALS.
In the previous chapters I have discussed the forces and agents of
alteration, and the general nature of the alterations in the zones of
anamorphism and katamorphism, including the two belts of the latter zone.
We are now prepared to consider the particular alterations which affect the
individual minerals in reference to these forces, agents, zones, and belts.
SECTION 1. CHEMICAL AND MINERAL COMPOSITION OF THE KNOWN
CRUST OF THE EARTH.
For convenience the outer part of the crust of the earth of which we
have positive knowledge will be called the crust. Clarke," for the purpose of
considering the chemical composition of the outer part of the earth, confines
this term "crust" to the part of the earth which extends from the tops
of the mountains to 10 miles below sea level. He thinks it fair to assume
that we may infer the approximate composition of this small part of the
earth by the parts of it which may be observed at or near the surface. 6 The
term "crust" in this treatise will be used in the restricted sense of Clarke-
But in so using the term there is no intention to imply that there is any
sharp division between the crust and the deeper part of the earth, to which
the term "centrosphere" is applied.
Below is a table which gives the relative proportions of the twenty-one
elements composing as much as 0.01 per cent of the crust of the earth as
above denned, including the lithosphere, hydrosphere, and atmosphere, and
also their atomic weights."
"Clarke, F. W., Kelative abundance of the chemical elements: Bull. U. S. Geol. Survey No. 78,
1891, p. 34.
Clarke, cit., pp. 34-37.
"Clarke, F. W., Analyses of rocks, laboratory of the United States Geological Survey, 1880-1899:
Bull. U. S. Geol. Survey No. 168, 1900, p. 15.
192
CLASSIFICATION OF MINERALS.
Elements of the eartKs crust.
193
Element.
Proportion.
Atomic
weight.
Element.
Proportion.
Atomic
weight.
Oxveen
Per cent.
47.02
15. 88
Manganese
Per cent.
.07
54.57
Silicon
28.06
28.18
Sulphur ....
.07
31.83
Yluminum
8. 16
26.91
Barium
05
136 39
Iron
4.64
55.60
Strontium
02
86.95
Calcium .........
3.50
39.76
Chromium . ......
.01
51.74
Magnesium
2.62
24.10
Nickel
01
58 24
Sodium
2.63
22.88
Tjithium
01
6.97
Potassium
2.32
38.82
Chlorine
01
35 18
Titanium
.41
47.79
Fluorine
01
18 91
Hydrogen
. 17
1.00
Carbon
.12
11.91
100.00
Phosphorus
.09
30. 79
From this table it is seen that of the metallic elements aluminum, iron,
magnesium, calcium, sodium, and potassium are the only ones which may
be called abundant, and that of the nonmetallic elements oxygen and
silicon are the only two which are abundant, although carbon, sulphur, and
chlorine are very important, and still others of the nonmetallic elements,
such as fluorine and phosphorus, are of considerable consequence. These
elements combined, or rarely alone, as they occur in the natural state, are
called minerals.
The more important minerals are classified into (1) elements, (2) oxides,
(3) salts of the binary acids, and (4) salts of the ternary acids. Of the
twenty-one elements above given, only oxygen, iron, nickel, sulphur, and
carbon occur in the elemental form, and with the exception of oxygen the
amounts thus occurring are insignificant. The free oxygen is mainly con-
tained in the atmosphere, but large quantities are also included in the
hydrosphere and lithosphere. The oxides comprise both hydrous and
anhydrous minerals. Of the salts of the binary acids, the sulphides are
of predominant importance, but the chlorides and fluorides are of some
consequence. The salts of the ternary acids are the predominant minerals
of the earth's crust. They include silicates, carbonates, titauates, phos-
phates, and sulphates. These compounds are mentioned in the order of
their importance; indeed, the silicates are of dominating importance, but
MON XLVII 04 13
194 A TREATISE ON METAMORPHISM.
next to them stand the carbonates, and the titanates and phosphates are
subordinate. Therefore the acids of the silicates and carbonates i. e.,
silicic and carbonic acids are the great rock-forming acids.
The natural combinations of the elements, so far as they occur as
important rock-making constituents, their systems of crystallization, chem-
ical formulae, molecular weights, logarithms of molecular weights, specific
gravities, logarithms of specific gravities, molecular volumes (i. e., molecular
weights divided by the specific gravities), and logarithms of molecular
volumes, are given in tables below alphabetically arranged.
In the tables the chemical formulae are usually those of the smallest
possible molecules. There is no attempt to make the formulae correspond
with the real molecular structure of the minerals, since in the present state
of knowledge it is. quite impossible to do this. It therefore follows that
the molecular weights, molecular volumes, and logarithms of the same in a
given case are relative. The table should not be used to compare the
absolute molecular weights and molecular volumes of the different minerals,
as this would give wholly misleading results. For instance, actinolite, the
second mineral in the table, varies in the amount of magnesium and iron
so that it may have three special formula}. These formulae as written
make the molecular weight of the mean molecule twice that of the
smallest and that of the largest molecule four times that of the smallest.
This results in similar variations in the molecular volumes and also consid-
erable variations in the logarithms Manifestly there are no such differ-
ences as these. Probably the true molecular weights and molecular
volumes, and consequently the logarithms, are very close to one another.
However, the numbers given in the table serve the purpose, as explained
011 pages 208-210, of calculating the changes of volumes when the particulai
varieties of the mineral actinolite represented by the formula? are trans-
formed to other minerals. These remarks as between the different varieties
of actinolite apply equally well as between other minerals and their different
varieties.
The letters D, Gr, H, and C, following formulae or specific gravities,
signify that the authorities from which the same are taken are, Dana,
Groth, Hintze, and Clarke, respectively.
ROCK-MAKING MINERALS.
195
III
a'
I-H * ^5
1
O CO <D
111! i
^ S? 2? *~
s a g
". 3. s .
gi Q >C
& s s
i^ CO i-t ^ -^
ft?
: Ml
U;
: S"
3 6
o $ i o
1 1 ; ^
i
-
B
<S f
31
am 3
(AlFeJjSiOe (H).
(D,G).
,o rn. m...
'
.5
vj> "in
- - * O 9 !g E 8 j08 H 9i H ( i:g: :s
3
*S
'
, **
;
I 1
" i
Na 2 CaFe),
CaMg),(Al
a(MgFe)Si
(MgFe)(
aMgSl s 6
Mg : F
gsAlj
2-
1
K
.-
llfi
1 1
'l
1 1 1 1 1
E
si
C e
a-s.
s a
' ?
PQ 2
196
A TREATISE ON METAMORPHISM.
8 8 s a g
co" co' cd oi oi
2 So - r- 3
oi ci c*i oi CN oi oi co
co'-i-oi -<rco"soco'co
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81
ts ta
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=
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a i g s
< O C u
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ROCK-MAKING MINERALS.
197
ll!
n o S S o
S g 5 S
3
?p
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s i s s -
CJ CC CO C^l C* C**
, 0.2-
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if !8 S S
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s 2 ^
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c
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r
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198
A TREATISE ON METAMORPHISM.
I
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ul
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If
til!
-c
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f*"l
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so a
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S S =!3f:
tB 3 B B
g .i
o f
II
9 o
III
M *S 5
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1 1. 1 !.
53 K
siifi
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15
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ROCK-MAKING MINERALS.
199
ilii
sill li lull
o cyo O'H?i^*'^Odr~^H
ill
~-c5>
o
Molecular
volume-.
^^KJCC t^ o oioTpco"
eo C-) co o cot'- cor ift^jc'
i ss gsssgsgacS
r- t> 50
^H f Oi
s S
&!!
llil ii mil
1 i iiiiiiMi
111
^^ c. 5 -
u5 ^n & * CO* 3 * 9 5* i
cc *S inS?5-J i o : i ^' i f i a
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c '6
o
|JS
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CC 00*f CQiCrHiCiOOOOt^O
s s i
li
cc be
eo co w c*i c-ic^i ei c4 ci ci o
es coco cojorNicieoMMM-^ 1 "
B
*
ilii
2221 IS
| | II
232
Sg*
o
"3
Ij
CO I-H ^i ,H lOQO ** ^" O O *
E S ScS co^ScocoScSSS
^
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ins is aass;
; ift Oi>-^ '"'SSK'^SSSS
| S | "S
; ; . as
: : i : : : : : : :
: : o : :
: ; o :::::::
B
B--i 2 = :
i 1 i i $3 1 i ;~ = ;
: : :
~. SB : :
j : '2^05-: :
o " - o : :
i Q ?2I &ft "ls
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3 ^o
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e r- - , ,-S -r* CH
;o-< o o " < 9i o 5"
[ i i Q % ^ S ff a 5 ? % ^
M| i| *
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. AloCasKiiSi^O
.. KH 2 Mg 3 A18i 3 (
. H 3 Mg AlSi 3 0,
Pure.
: : 3 : : :
2 : : : a : : : ?
cc
: -c : : :
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do
do
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: : S
Minera!
: : : g a Q
S^^
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S S S S S S5 S5 !
3 K :::::::
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I siui 1 i 1 i i i
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5 & O C OOOCUCL,P-"&H
Phillipsite
Phlogopite ...
Phlogopite-chl
03
3
C
w
I
I
200
A TREATISE ON METAMOBPHISM.
ill!
01 -r B H 9 n 1
1111 |i|
if? W iC M CO CO t
* ^ fe ^H 2J C
r* r- os ec t t-
O * -X> CO O if
to ;c oo co eo
r~ i i o 01 ^
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P =
g glgl;
M Oi CO SC M Si
?5 X eft t- 3 !^ OS
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IN co cc 55 r- J
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8 33 a l
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Continued.
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o C Co
so so
1 i
: : : : : :
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: : :
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D
a
S
S S :
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s g g 2 t _
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O3 00 OD H
ROCK-MAKING MINERALS.
201
3
a
ft?
Molecula
volume.
Loga-
rithm of
specific
gravity.
Molecul
weigh
1C
30
O> i 1 I 1
to CO
TiSiO 6 (D,G)
AloF 2 SiO 4 (C,H
Ca
9 3
3. f
.-
"0
. 5 3 ~
. .* 5
' L i a
' 2 B! "
5 1
s 5
1
I
Is
f
So
i
bo
cli
rSiO, (C,D,G)
aj(AlOH)Al 2 (SiO 4 ) 3 (C, D, G
HjCa^AleSi.Oj, (H)
a
si! \
E g i4
" g '
1 1 1
S H O
2 2
1 -
i?
H H
c
ite
nite
S > .s a
5
*c > S
H P >
% S
Z
8 . 1
N X X
202 A TREATISE ON METAMORPHISM.
SECTION 2. GENERAL NATURE OF ALTERATIONS.
Minerals may be altered (1) without chemical change and (2) with
change of chemical composition.
ALTERATION WITHOUT CHANGE IN CHEMICAL COMPOSITION.
The alterations which occur without changes in chemical composition
are (a) molecular rearrangement and (b) simple recrystallization.
MOLECULAK REARRANGEMENT.
Molecular rearrangement alone means passage from one crystalline
form to another crystalline form. Such change of form may result from
changed physical conditions, as, for instance, change in temperature or
pressure or movement. As an example of molecular rearrangement due
to change of temperature may be mentioned leucite, which crystallizes
from a hot magma in the regular system, but which changes upon cooling
to ordinary temperatures to a complex twinned anisometric form. An
example of a change due to pressure is furnished by orthoclase, which is
said for this reason to alter to microcline. Molecular readjustments such
as above are simply changes of form, and are therefore called paramorphism.
SIMPLE RECRYSTALLIZATION.
Simple recrystallization usually but probably not always occurs through
the medium of a certain amount of water, which is able to take material into
solution and deposit it from solution. Changing pressure and comparatively
high temperatures are favorable conditions for such recrystallization. Per-
haps the most common example of recrystallization without chemical
change is that of the transformation of amorphous or finely crystalline
calcium carbonate to crystalline or more coarsely crystalline calcium
carbonate, such as occurs in limestones and marble. This process has
been called marmorosis. Another instance of recrystallization without
change in chemical composition which takes place, on an extensive scale, is
alteration of flinty or finely crystalline quartz to coarsely crystalline
quartz.
ALTERATION WITH CHANGE IN CHEMICAL COMPOSITION.
Alterations with chemical change may take place (1) witi.out the
addition or subtraction of material or (2) with the addition or subtraction
Dana, J. D., A system of mineralogy, Descriptive mineralogy by E. S. Dana, Wiley & Sons, New-
Tone, 6th ed., 1892, p. 318.
ALTERATIONS OF MINERALS. 203
of material. For either of these changes the presence of water is required
in most instances, the alterations taking place through solution and redepo-
sition, although it is not impossible that solids may act upon one another
to an important extent without the help of water.
ALTERATION WITHOUT ADDITION OR SUBTRACTION OF MATERIAL,.
Iii the changes which occur under this case the material moves only
short distances. Such changes may be (a) a crystallization of an amor-
phous substance or (b) interior alteration of mineral particles.
An instance of the crystallization of an amorphous substance is
furnished by the devitrification of glass. In this alteration the uniform
homogeneous solid glass changes into a heterogeneous crystalline solid, the
different mineral particles of which have differing compositions. This
involves segregation of the different elements in various proportions into
the different minerals. It is therefore clear that the materials have moved
very short distances.
Interior alteration of mineral particles is effected by the change of
one mineral into two or more minerals. This is illustrated by the change
of pyrope into enstatite, spinel, and quartz; the change of pyrope into
hypersthene, spinel, and quartz; the change of spodumene into eucryptite
and albite; the change of almandite into hypersthene, spinel, and quartz;
and the change of titanite into perovskite and quartz.
ALTERATION WITH ADDITION OR SUBTRACTION OF MATERIAL.
The changes which take place with the addition or subtraction of
material may vary from those which involve the slightest addition or
subtraction to complete substitution. The added material may come from
afar or from the adjacent mineral particles. The subtracted material may
enter into an adjacent mineral particle or may be transported great
distances before entering into a new mineral. Reactions between adjacent
minerals may produce new minerals. Two or more minerals may unite to
produce a single mineral. For example, olivine and quartz may pass into
anthophyllite; nephelite and halite into sodalite; albite and halite into
rnarialite. Or two or more minerals may unite to produce two or more
new minerals. For example, rutile and magnetite may pass into ilmenite
and hematite; diopside and magnetite into tremolite and calcite; sahlite,
siderite, and magnesite into actinolite and calcite; augite, siderite, and
204 A TREATISE ON METAMORPHISM.
magnesite into hornblende and calcite; or the reverse of this, hornblende
and calcite into augite, siderite, and magnesite.
The changes here belonging are by far the most numerous and impor-
tant of the various classes; indeed, are vastly more important than all of
the other classes together. By far the greater number of reactions written
out on the succeeding pages for the alterations of the various minerals fall
under this heading.
The more important of these alterations, considered from the point of
view of the nonmetallic elements, may be classified into:
(I) Oxidation. (2) Deoxidation.
(3) Hydration. (4) Dehydration.
(5) Carbonation. (6) Decarbonation.
(7) Silication. (8) Desilication.
(9) Silicification. (10) Desilicification.
Less important reactions are:
(II) Sulphidation. (12) Desulphidation.
(13) Sulphation. (14) Desulphation.
(15) Titanation. (16) Detitanation.
(17) Phosphation. (18) Dephosphation.
(19) Chloridation. (20) Dechloridation.
(21) Fluoridation. (22) Deflnoridation.
(23) Boration. (24) Debor?tion.
A number of these reactions are of small consequence so far as the
alterations of rocks are concerned; but all are important with reference to
the development of minerals, and especially in reference to economic
products. This phase of the subject in reference to the metallic products
is treated in Chapter XII.
(1) Oxidation is the addition of oxygen. Frequently the added oxygen
is substituted for another element, often sulphur.
(2) Deoxidation is the subtraction of oxygen. Often the subtracted
oxygen is replaced by another element for instance, sulphur.
(3) Hydration is the addition of water, producing hydroxides.
(4) Dehydration is the subtraction of water from hydroxides. When
carried to completion, anhydrous compounds are formed.
(5) Carbonation is the union of carbonic acid and base, or the substi-
tution of carbonic acid for another combined acid, in either case producing
carbonates. The oxide with which carbonic acid most frequently unites is
CLASSIFICATION OF ALTERATIONS. 205
iron oxide. Carbonic acid may replace any of the other ternary rock-
forming acids, including silicic, titanic, and phosphoric, and thus become
united with any of the important bases. The carbonation of the silicates
is of fundamental importance. The carbonation of the titanates and phos-
phates is unimportant.
(6) Decarbonation is the separation of carbonic acid from a base with-
out the addition of other compounds, or with the substitution of another
acid for the carbonic. The most frequent substituted acid is silicic.
(7) Silication is the union of silicic acid and base, or the substitution
of silicic acid for a combined acid, in either case producing silicates. The
only important oxide with which silicic acid unites as a rock-forming con-
stituent is iron oxide Silicic acid may replace carbonic, titanic, or phos-
phoric acid, thus becoming united with any of the bases with which it can
combine. The silication of the carbonates is of fundamental importance.
The silication of the titanates and phosphates is unimportant.
(8) Desilication is the separation of silicic acid and bases without the
addition of other compounds, or with the substitution of another acid for
the silicic acid. The most frequent acid substituted is carbonic.
(9) Silicification involves the addition of silica without union with
bases. The added silica may or may not replace other compounds.
(10) Desilicification involves the subtraction of free silica. The sub-
tracted silica may or may not be replaced by other compounds.
(11) Sulphidation is the union of sulphur with a metal forming
sulphides. Added sulphur may be substituted for another element, usually
oxygen.
(12) Desulphidation involves the subtraction of sulphur. Generally
the subtracted sulphur is replaced by another element, usually oxygen.
(13) Sulphation is the union of sulphuric acid with base or the
substitution of sulphuric acid for another combined acid, in either case
producing sulphates.
(14) Desulphation is the separation of sulphuric acid and base, or the
substitution of another acid for the sulphuric.
(15) Titanation is the union of titanic acid with base, or the substitution
of titanic acid for another combined acid, in either case producing titauates.
(16) Detitanation is the separation of titanic acid and base, or the
substitution of another acid for the titanic.
206 A TREATISE ON METAMORPHISM.
(17) Phosphation is the union of phosphoric acid with base, or the
substitution of phosphoric acid for another combined acid, in either case
producing phosphates.
(18) Dephosphation is the separation of phosphoric acid and base, or
the substitution of another acid for the phosphoric acid.
(19) Chloridation is the addition of chlorine, forming chlorides.
(20) Dechloridation is the subtraction of chlorine, destroying chlorides.
(21) Fluoridation is the addition of fluorine, forming fluorides.
(22) Defluoridation is the subtraction of fluorine, destroying fluorides.
(23) Boration is the union of boric acid with base, or the substitution
of boric acid for another combined acid, in either case producing borates.
(24) Deboration is the separation of boric acid and base, or the
substitution of another acid for the boric.
GENEKAL STATEMENTS.
The foregoing processes are seen to be in pairs, in each case one of a
pair being the reverse of the other. That is, deoxidation is the reverse of
oxidation, dehydration is the reverse of hydration, etc. Moreover, one of
the processes of a pair is in several of the cases frequently the complement
of that of another pair. To illustrate, the processes of the following pairs
are often complementary of each other, viz, oxidation and desulphidation,
sulphidation and deoxidation, carbonation and desilication, silication and
decarbonation. By complement is meant that one takes place simultaneously
with the other, and that the two may really be one chemical reaction. In
such a case the change may be considered from either of two points of view.
To illustrate, the process of carbonation may be also a process of desilication,
and the process of silication may be also a process of decarbonation.
In general the process is named on the basis of the substance added rather
than that subtracted, for such substance is the active agent which drives
off" the other and takes it place. It has been shown (pp. 168, 170-181)
that for several reactions one of a pair is particularly characteristic for one
of the zones of metamorphism. To illustrate, oxidation and its complement
desulphidation, carbonation and its complement desilication, and hydration
are particularly characteristic of the zone of katamorphism ; sulphida-
tion and its complement deoxidation, silication and its complement decar-
bonation, and dehydration are particularly characteristic of the zone of
anamorphism.
MINERALS. 207
SECTION 3. ROCK-MAKING MINERALS.
MANNER OF TREATMENT.
GENERAL STATEMENTS.
In this treatise only the principal rock-forming minerals will be consid-
ered. The point of view is not that of mineralogy but that of metamorphism.
So far as it seems advisable without too much repetition, I shall consider
each mineral in reference to the following:
(1) Its composition, crystallization, specific gravity, and source, so far
as it is a rock-making mineral ; but its occurrence in veins will not be
considered.
(2) The minerals into which it may pass, giving their crystallizations,
specific gravities, and compositions.
(3) The chemistry and physics of the processes of change, including
the volume relations.
(4) The natural conditions under which the changes occur, and the
causes of the changes.
With many minerals this outline can be carried out nearly to com-
pletion. With others the present state of knowledge is such that it can
be only very incompletely done. Consequently there is great variation
in the satisfactoriuess of the discussion of the different minerals. When
the treatment of each of the minerals from these various points of view
can be carried out we shall have an interlocking system by which each
mineral is considered in its most important metamorphic connections.
To a certain extent the plan involves repetitions, but in each case the
important facts which concern an individual mineral are brought together.
The method of treatment proposed seems advisable, for many minerals
are both primary and secondary, and only by considering each mineral
from both points of view is it possible to understand the causes of the
changes as well as the changes themselves. Ordinarily the latter only are
considered. When one of the sources of a mineral is the alteration of
another the exact reactions concerned in the change are not given under
the former, but may be found by referring to the latter mineral which is
mentioned as its source. Ordinarily, however, qualitative statements are
made. To illustrate, a source of limonite is siderite. The reactions involved
in this change are to be found under siderite, not under limonite; but under
208 A TREATISE ON METAMOKPHISM.
the latter mineral the statement is made that the change generally involves
liberation of heat and decrease of volume. But when a mineral is derived
by precipitation from a solution, or results by the combination of several
minerals, it is necessary to consider the chemistry and physics of the change
in connection with the sources, for otherwise this important part of the
history of metamorphism of minerals would be omitted.
In discussing the sources of a mineral when it is derived from other
minerals the natural conditions of the alterations are not given, but may be
found by referring to the minerals from which the one under discussion
is derived. But where a mineral is derived by the interaction or union of
several other minerals the natural conditions are discussed under the source
of the mineral, for otherwise this part of the subject would be omitted.
As this treatise was originally planned it was designed to include the
heat and volume changes with the chemical reactions. But with the
present state of knowledge of the heat relations in chemical transformations
the first has been found impracticable. While very few quantitative
results can be given, in many cases it is possible to make a qualitative
expression of the heat reaction. To illustrate, the heat of combination of
calcium is far greater than that of iron in all analogous compounds in
which determinations have been made; but such determinations have not
been made with reference to the silicates. Where calcium is replaced by
iron in the alteration of the silicates it is inferred that a considerable amount
of heat is absorbed, though the exact amount can not be specified. Vice
versa, where iron is replaced by calcium, a considerable amount of heat is
liberated. Of course, in each reaction the other chemical combinations
which occur simultaneously should be considered, for they constitute a part
of the chain, and in obtaining a correct end result their effects are vital. If,
for instance, a salt of iron and a salt of calcium interchange acids, no general
statement can be made as to the heat reaction. Therefore, if at the same
time the iron replaces the calcium the calcium unites with an acid which
was before in combination with the iron, the inference above given as to
absorption of heat can not be made.
For the calculation of the volume changes, the equations of the chem-
ical reactions written out by me and the specific gravities of the minerals,
taken from the standard Mineralogies, were turned over to Mr. A. T. Lincoln,
who made the numerical computations. Subsequently Mr. R. M. Chapman
CALCULATION OF VOLUME RELATIONS. 209
repeated the work in order to verify it. The following well-known principle
was employed:
The volume of the original compound is to the volume of the compound
produced directly as their molecular iveights and indirectly as their specific
gravities.
Under this general principle are two cases :
Case 1. Where one solid compound alters into another solid compound.
This case is illustrated by the well-known changes of limestone to dolomite.
In this change we have 2CaCO 3 replaced by MgCa(CO 3 ) 2 . The molecular
weight of 2CaCO 3 is 198.62. The molecular weight of MgCa(CO 3 ) 2 is
182.96. The specific gravity of calcite may betaken as 2.7135; of dolomite,
as 2.85. The compound proportion is therefore as follows:
v Vl 198.62:182.96
2. 85 : 2. 7135
or the volume of the dolomite is 87.70 per cent of that of the calcite;
or, therefore, there is a decrease in volume of 12.30 per cent.
Case 2. This has three phases: (a) where two or more solid compounds
unite to produce a single solid compound ; (b) where a single solid
compound breaks up, producing two or more compounds, and (c)
1 where two or more solid compounds unite to produce two or more solid
compounds. In this case the method of calculation is slightly different
from case 1. The molecular weights of each of the compounds represented
in the equations are divided by the specific gravities of the respective
compounds. This gives their relative volumes. In phase (a) the volume
of the resultant single compound is divided by the sum of the volumes of
the producing compounds, and this gives the percentage of change. In
phase (b) the sum of the volumes of the resultant compounds is divided by
the volume of the original compound. In phase (c) the sum of the volumes
of the resultant compounds is divided by the sum of the volumes of the
original compounds. These different phases are so similar in method that
it is necessary only to illustrate one of them. The first phase is illustrated
by the formation of wollastonite by the union of calcite and quartz, the
reactipn being:
CaCO 3 +SiO 2 =CaSiO s +CO 2 .
The molecular weights of the three solid compounds are, respectively,
MON XLVII 04 14
210 A TREATISE ON METAMORPHISM.
99.31, 59.94, and 115.58. Their specific gravities are 2.7135, 2.6535, and
2.85, respectively. The volume of the wollastouite is, therefore:
115.58 . /99.31 . 59.94 \_
^85 ^ ^7135+ "276535 J~
That is, the decrease in volume in this case of silication of calcite is 31.5
per cent.
In order to expedite the laborious numerical calculations of the volume
relations for the very numerous alterations, Mr. Lincoln completed the table
on pp. 195-201 by adding the molecular weights, the logarithms of the
molecular weights, the logarithms of the specific gravities, the molecular
volumes, and the logarithms of the molecular volumes of each of the min-
erals. These determinations have been carefully verified by Mr. Chapman,
and may be used to check the volume changes given in the succeeding
pages, and also to make additional volume calculations.
In calculating the volume relations, unless otherwise specified, the
compounds on both sides of the equations are regarded as solid except those
which by themselves independent of the solvents are liquids or gases, such
as H 2 O and C0 2 . All such compounds are supposed to be added in the
solutions or to be taken away by the solutions, and therefore are not taken
into account in the volume calculations. In general these liquid and
gaseous compounds do undoubtedly escape in large measure, although in
some cases they are confined as inclusions within the minerals formed.
(See p. 678.)
Where -j-k is added to the equation, this signifies that heat is liberated;
where k appears, this means that heat is absorbed by the reaction.
No claim is made that the equations which ^re written in the following
pages exactly represent the changes that take place in the alterations of
the various minerals into other minerals Indeed, the probability is that
not half exactly represent the facts; for the great majority of the reactions
are more complicated than written, and in many cases substances in the
solutions or as solids not taken into account are concerned. Since these
are the facts, the question may be asked why the equations are written.
The answer is, first, that at some time the attempt must be made to givr
a first approximation to quantitative exactness in the alteration of minerals.
The equations found on the following pa^es represent, such an attempt.
Before the appearance of this treatise scarcely more than a score of mineral
CALCULATION OF VOLUME RELATIONS. 211
alterations have been expressed by chemical equations, and in fewer still
have the volume relations been calculated. Second, the imperfect
equations herein contained will be sure to lead to closer investigations of
the nature of the alterations, and to improved equations representing them.
Thus the progress of science will be promoted by the set of equations here
given, even if the great majority of them are defective. Third, it is
believed that when a more nearly correct set of equations is written it will
be found that the large majority of the equations herein contained substan-
tially represent the facts, and consequently that the volume changes are in
most cases roughly approximate. Many of them may be changed by a
few per cent one way or the other; but the sign of few will be changed,
and this is the fundamental point in reference to the zones in which the
alterations occur.
The weakest point in the accuracy of the volume reactions is not tound
in the chemical equations, but in the inexactness of the specific gravities of
the minerals as given in the text-books. For most minerals there is a con-
siderable range of specific gravity given; and with the exception of one or
two minerals, such as calcite and quartz, it is impossible to ascertain the
exact specific gravity of the pure minerals. In the table the mean between
the two best determined extremes is given as the best approximation
available of the specific gravities of the pure minerals. For most minerals
these extremes are taken from Dana's System of Mineralogy.
The facts as to the occurrences and alterations of the various minerals
given in the following pages are largely taken from the standard text-
books . of mineralogy and petrology, and especially from Dana's great
System of Mineralogy. The information available is especially imperfect
as to the manner in which the complex minerals,- and particularly the
complex silicates, break up into simpler compounds in the belt of weather-
ing. As explained fully in the following chapter, this is a general process.
For the better known of these changes equations are written, but no
attempt is made to express by equations the manner in which many of the
minerals decompose and degenerate, because so little exact information is
available upon which to base such equations.
As already stated, only those minerals will be considered which are
important rock-making constituents. It is impracticable at the present time
to consider the physical-chemistry of the rarer minerals.
212 A TREATISE ON METAMORPHISM.
l
Following the ordinary classification, the abundant rock-making
constituents may be considered under the headings: Native Elements,
Sulphides, Fluorides, Oxides, Carbonates, Silicates, Titanates, Phosphates,
and Sulphates.
NATIVE ELEMENTS.
GRAPHITE.
Graphite:
Crystallized carbon (C).
Rhombohedral.
Sp. gr. 2.09-2.23; av. 2.16.
occurrence Graphite occurs as a very widely disseminated constituent
in the extremely metamorphosed sedimentary rocks, which in their original
condition contained carbonaceous material. It is especially prevalent in
scales in the marbles, schists, and gneisses. In some instances the original
beds were so heavily carbonaceous as to give considerable layers a large
percentage of which is graphite. Such layers are illustrated by the
graphitic shales of Worcester, Mass." Graphite occurs to some extent with
the very hard anthracite coals, a part of the carbon having passed over to
the graphitic condition. Such graphitic coals occur in the Rhode Island
coal field. 6 The reaction producing graphite as a metamorphic mineral
requires great pressure and takes place with decrease in volume. This
mineral in the sedimentary rocks is therefore a product of the zone of
anamorphism.
Graphite is said to occur as an original constituent in some basaltic
rocks. During the alterations of carbonaceous rocks the hydrocarbon com-
pounds, as gases, oils, and bitumen, wander widely in the solutions. In
some cases such compounds are deposited in the openings of original rocks
Later these compounds may be altered to graphite, and yet the carbon not
be an original constituent of the magma from which the rocks crystallized.
Alterations Alterations of graphite are not recorded, but it is by no
means certain that this mineral is not very slowly oxidized under favorable
conditions in the belt of weathering.
THE SULPHIDES.
The sulphides which are important as rock-making minerals are
pyrrhotite, pyrite, and marcasite. Many other sulphides are important in
a Perry, J. H., Note on a fossil coal plant found at the graphite deposit in mica-schists at Worcester,
Mass.: Am. Jour. Sci., 3d ser., vol. 29, 1885, pp. 157-158.
&Shaler, N. S., Woodworth, J. B., and Foerste, A. F., Geology of the Narragansett Basin: Mon.
U. 8. Geol. Survey, vol. 33, 1899, p. 82.
OCCURRENCE OF PYRRHOTITE, PYRITE, AND MARCASITE. 213
the genesis of ore deposits. These, however, will be considered only in the
chapter on that subject.
PYRRHOTITE, PYRITE, AND MARCASITE.
Pyrrhotite:
Fe 5 S 6 to Fe 15 S 16 ; chiefly FeuS,,.
Hexagonal.
Sp. gr. 4.58-4.64.
Pyrite:
FeS 2 .
Isometric.
Sp. gr. 4.95-5.10.
Marcasite:
Fe8 2 .
Orthorhombic.
Sp. gr. 4.85-4.90.
occurrence. Pyirliotite, pyrite, and marcasite are very widespread acces-
sory minerals, occurring in rocks of all ages and all kinds. So far as known,
these minerals arc not abundant original pyrogenic constituents, although
they frequently are found along the contact between intrusive and other
rocks, occurring in both the intrusive and the intruded rocks. Pyrrhotite
is an original mineral in meteorites. These minerals extensively form in
rocks in volcanic districts through the action of solutions of hydrogen
sulphide and other sulphide solutions upon iron salts. As secondary minerals
in the sedimentary rocks, and to a less extent in the igneous rocks, the
sulphides are extensively formed through the reducing action of organic
compounds upon the sulphites and sulphates, especially the latter, and par-
ticularly iron sulphate. Such reduction is characteristic of the belt of
cementation and the zone of anamorphism ; but in the latter zone pyrrhotite
or pyrite, rather than marcasite, probably forms.
The reducing agent of the sulphites and sulphates may be either a
solid organic compound or one of its gaseous products of decomposition,
such as carbon monoxide (CO) and carburetted hydrogen (CH 4 ). If the
reducing agent be taken as CO, the reaction for pyrite and marcasite
may be:
2FeSO 4 +7CO=FeS 2 +FeCO 5 +6CO 2 +ka
and for pyrrhotite:
12FeSO 4 +45CO=Fe 11 S, 2 +FeCO. ) +45COj-|-k.
If the reducing agent were taken as carbon, similar results would be
obtained, except that the amount of CO 2 would be less. This action, while
"See page 210.
214 A TREATISE ON METAMORPH1SM.
ordinarily called a reduction, is reduction so far as the iron sulphate is
concerned, but is oxidation so far as the carbon compound is concerned,
and hence the explanation of the liberation of heat.
Pyrite, raarcasite, and pyrrhotite are also doubtless produced by the
action of soluble sulphides upon the iron oxides or iron salts. In the
change from crystallized Fe 2 3 (hematite) to FeS 2 (in the form of pyrite),
the volume increases 56.14 per cent.
Alterations. The first alteration to be considered is that of marcasite into
pyrite. In this alteration there is recrystallization, an increase of symmetry,
a decrease of 2.98 per cent in volume, but no change in chemical compo-
sition. The heat effect is undetermined, but probably heat is liberated.
The mineral pyrrhotite by recrystallizatiou passes into pyrite. This
change may occur in volcanic districts by the action of hydrogen sulphide
upon the pyrrhotite, the reaction perhaps being:
Fe u S l2 +10H 2 S=llFeS 2 +10H 2 .
Ill this change the volume is increased 21.13 per cent.
The minerals pyrite and marcasite may by oxidation pass directly
into (1) hydrated sesquioxide of iron, of which, ordinarily, limonite (not
crystallized; sp. gr. 3.80) is the most common kind; (2) magnetite (isomet-
ric; sp. gr. 5.174); (3) ferrous sulphate, which may be removed in solution,
or (4) may be decomposed by further oxidation, either at the place of
formation or elsewhere, after a longer or shorter time, into hydrated sesqui-
oxide of iron, ordinarily limonite. The reactions for marcasite and pyrite
may be as follows, assuming in each case that the sulphur, or a part of it,
is also oxidized:
(1 ) 4FeS 2 +22O+3H 2 O=2Fe 2 O,.3H 2 0+8SO 2 +k.
(2) 3FeS,+16O=Fe s O 4 +6SO,+k, or
3FeS !1 +4H 2 O+4O=Fe,O 4 +4H. 1 S+2SO 2 +k.
(3) FeS 2 +6O=FeSO 4 +SO 2 +k, or
FeSj+3O+H,O=FeSO 4 +H 2 S+k.
(4) 4FeSO 4 +2O+7H 2 O=2Fe,O 3 .3H s O+4H a SO 4 +k.
As shown in Chapter XI, on "Ore deposits," pyrite and marcasite also
alter to hematite without oxidation by the reaction of an alkaline carbonate.
The alteration of common pyrrhotite into magnetite aud limoiiite may
be written as follows:
(5) 3Fe 11 S 1 ,+1160=llFe,0 4 +36SO.,+k, or
3Fe u S 12 +36H 2 O+8O=llFe3O 4 +36H 2 S+k.
(6) 4Fe n S I2 +33H J 0+1620=ll(2Fe 2 O s .3H 2 0)+48S0 2 +k. A
ALTERATIONS OF PYRKHOTITE, PYRITE, AND MARCASITE. 215
If iu the production of the limonite the pyrrhotite passes through the
stage of ferrous sulphate the reaction producing the sulphate may be:
(7) Fe,,Si,+46O=llFeSO 4 +SO 2 +k, or
Fe u S 12 +H 2 O+43O=llFeSO 4 +H.,S+k.
The change from the ferrous sulphate to the limonite is the same as in
the case of pyrite and marcasite. Where water is present the SO 2 produced
in the above reactions would unite with water and form H 2 SO 3 , or if further
oxidized H 2 S0 4 .
As the end results of alteration are usually limonite or magnetite, the
volume relations for these two compounds will be given. In the change
of pyrite to limonite the volume is increased 2.93 per cent; to magnetite,
is decreased 37.48 per cent. In the change from marcasite to limonite the
volume is decreased 0.14 per cent; to magnetite, is decreased 39.34 per
per cent. In the change of pyrrhotite to magnetite the volume is decreased
24.27 per cent; to limonite, is increased 24.68 per cent.
When pyrite and marcasite pass into limouite there is a change from
a crystalline to an amorphous form. In the alteration of pyrite to magne-
tite the system does not change. In the alterations of pyrrhotite and mar-
casite to pyrite there are changes from lower degrees of symmetry to the
highest degree of symmetry, that of the isometric system. The change
from marcasite to pyrite occurs especially in the zone of anamorphism,
subject to the principle there obtaining that the changes take place with
decrease in volume. The change of marcasite to pyrite is an excellent
illustration of the principle that where the pressure is great minerals tend
to pass into other minerals having a higher degree of symmetry and a higher
specific gravity (see pp. 360-365). The abundance of marcasite as an
autogenic constituent in rocks not deeply buried, its absence in the rocks
which have been in the lower zone, and the presence of pyrite in these
rocks, are thus all explained. Where the pressure is small near the surface
marcasite with lower symmetry and lower specific gravity than pyrite may
abundantly form. At depth where the pressure is great pyrite of higher
specific gravity and higher symmetry forms. If rocks near the surface in
which marcasite has formed are buried to a great depth by superimposed
strata the marcasite previously formed changes to pyrite.
Similar statements can not be made concerning pyrrhotite and pyrite,
for these minerals have unlike compositions. Doubtless where the necessary
216 A TREATISE ON METAMORPHISM.
chemical reactions can take place there is a tendency in the lower zone for
pyrrhotite to alter to pyrite.
The natural conditions for the transformation of pyrite, marcasite, and
pyrrhotite to limonite are those of abundance of oxygen and moisture.
These conditions are found in the zone of katamorphism, and especially in
the belt of weathering. In this belt the process goes on with such rapidity
that pyrite, marcasite, and pyrrhotite have generally been completely
oxidized where the rocks have been long exposed to the reactions of the
belt. The reactions are oxidation and hydration. They take place with
great liberation of heat and, for pyrite and pyrrhotite, with some expansion
of volume, and these changes may therefore be taken as typical illustrations
of alterations of the belt of weathering.
The conditions for the formation of magnetite from pyrite, marcasite,
and pyrrhotite are the presence of some oxygen, but not a sufficient amount
to fully oxidize the iron, and considerable pressure. Where iron carbonate
is present, which also alters to magnetite, oxygen is not necessary. This
reaction is of great consequence. (See p. 244.) The alterations of the
sulphides to magnetite involve a decrease of volume of 24 to 39 per cent
and liberation of heat. Corresponding with this fact, the changes take
place in the belt of cementation or in the zone of anamorphism.
THE FLUORIDES.
Among the fluorides the only important rock-making mineral is fluorite.
FLUORITE.
Fluorite:
CaF 2 .
Isometric.
Sp. gr. 3.01-3.25.
occurrence Fluorite occurs as an accessory constituent, especially in
granitic and syenitic rocks. It is also found in other eruptive rocks, and
in metamorphic rocks, such as the schists and marbles. It therefore has a
somewhat widespread occurrence, but is of very subordinate importance.
Alteration. By the action of alkaline waters fluorite alters into calcite
(rhornbohedral; sp. gr. 2.7135). Supposing the alkaline compound to be
sodium carbonate, the reaction is :
CaF,+Na,CO,=CaCO s +2NaF+k.
The increase in volume of the calcite as compared with the fluorite is 47.66
per cent.
OCCURRENCE OF QUARTZ. 217
THE OXIDES.
The more important oxides occurring as rock-building constituents are
those of silicon, iron, and titanium. The oxides of silicon are quartz,
tridymite, and opal. The important oxides of iron are hematite, magnetite,
and limonite. The important oxides of titanium are rutile, octahedrite,
and brookite. One oxide of iron and titanium, or else a ferrous titanate,
has a widespread occurrence; this is ilmenite.
QUARTZ.
Quartz:
SiO,.
Rhombohedral.
Sp. gr. 2.653-2.654.
occurrence. Quartz is second in abundance only to the minerals of the
feldspar group. According to Clarke, quartz comprises 12 per cent of
the lithosphere. It is very abundant as an original pyrogenic constituent
of the igneous rocks, as an allogenic constituent of the clastic rocks, and
as an autogenif, mineral in all classes of metamorphosed rocks. The
material for secondary quartz may be derived from the alterations of many
minerals in situ, or from the decomposition of minerals at some distance.
The most widespread of all the alterations which furnish silica to the
solutions is that of the decomposition of the silicates by carbonic acid iu
the belt of weathering, with the simultaneous production of carbonates and
quartz, or a solution of colloidal silicic acid from which opal, chert, or
quartz may later separate. Such quartz may be extensively deposited from
the solutions in the porous rocks of the belt of cementation. It there fills
the minute spaces between the individual grains of sedimentary rocks.
It occupies spaces iu porous tuffs or in vesicular igneous rocks. It fills
openings between laminae, and joint, fault, and breccia openings. The
quantity of quartz thus deposited is far greater than that of any other
mineral, and not improbably greater than that of all other minerals com-
bined. By this process the rocks are cemented. (See pp. 617-621.) Not
only may the openings be occupied by quartz, but at the time of the
deposition of the quartz other minerals may dissolve and their places be
taken by the quartz. This process of deposition of silica as quartz is called
silicification. (See p. 205.)
"Clarke, F. W., Analyses of rocks from the laboratory of the United States Geological Survey,
1880-1899: Bull. U. S. Geol. Survey No. 168, 1900, p. 16.
218 A TREATISE ON METAMORPHISM.
As a metamorphic mineral, quartz is derived from actinolite, anorthite,
anorthoclase, anthophyllite, augite, biotite, bronzite, chalcedony, cumming-
tonite, diopside, enstatite, epidote, garnet, grossularite, hornblende, hypers-
thene, microcline, olivine, opal, orthoclase, plagioclase, prehnite, pyrope,
sahlite, scapolites, serpentine, tridymite, and zoisite.
Modifications. The most frequent and important modification of quartz is
by recrystallization. Crystallized quartz is dissolved under conditions of
weathering, as are all other minerals. This process is, however, exceed-
ingly slow. As a result of solution the quartz crystals may be corroded.
Such corrosion has been described by Hayes." In the belt of cementation,
and especially adjacent to trunk channels of circulation, quartz may be ex-
tensively dissolved from veins and from the wall rocks. (See pp. 848-849.)
Granulation and recrystallization of quartz occur on a most extensive
scale in all quartzose rocks which are subjected to mass-mechanical action
or other favorable conditions in the zone of anamorphism. These changes
involve no heat and volume reactions so fat 1 as the quartz itself is concerned,
except that as the original minerals may be strained, or the new grains are
imperfectly adjusted, the change may involve a slight expansion. But
such expansion is followed by an equal contraction when the material is
recrystallized into quartz free from strain. In the recrystallization many
small individuals may be merged into one large individual. In some
instances of recrystallization, where large grains are produced from smaller
ones, the large individuals may average more than a million times as great
as the small individuals from which they are derived. (See p. 695.) In
the production of a comparatively few large individuals from a multi-
tude of small individuals there is probably a release of energy. (See
p. 771.) During recrystallization the material taken into solution may
be deposited practically in situ or may travel far and be extensively
deposited elsewhere. Often quartz deposited in situ, or nearly so, can not
be discriminated from quartz deposited from solutions coming from distant
sources, as above described.
A second modification of quartz only less important than that of
recrystallization is silication by the union with bases united with other
acids, thus forming silicates. Of such acids carbonic is by far of
a Hayes, C. W., Solution of silica under atmospheric conditions: Bull. Geol. Soc. America, vol.
8, 1897, pp. 213-220.
MODIFICATIONS OF QUARTZ. 219
the greatest consequence;. Some of the more common minerals in which
silication occurs on an extensive scale are calcite, dolomite, ankerite, and
siderite, thus producing wollastonite, diopside, tremolite, sahlite, actinolite,
and griinerite. The silica ma}' unite with the bases of various carbonates
producing various complex silicates, such as chondrodite, augite, horn-
blende, garnet, etc. At the same time the material of previous silicates
may be absorbed. The heat and volume reactions in many of these
changes may be found under the carbonates mentioned.
In this process of silication of carbonates it is not often possible to
identify the remnants of the quartz individuals which furnished the silica
for the reactions. But apparently the quartz particles which furnished the
silica for the process of silication may be identified in some instances.
This is best seen for such fibrous minerals as serpentine, talc, and actinolife,
the needles or fibers of which appear to grow into the quartz, in some
instances deeply. In such cases it seems clear that the silica of the quartz
furnished at least a part of the silica for the silicate, the bases being
furnished by the solutions.
One of the best instances of the extensive union of quartz with bases,
producing serpentine pseudomorphous after quartz, is that described by
Becker." He describes the exteriors of original clastic grains of quartz to
be "entirely occupied by felted fibers of serpentine, and long, slender
microlites pierce the quartz grain toward its center, like pins in a cushion.'" 1
This is but one illustration of a very widespread replacement of quartz
by serpentine in the Coast Ranges. The growth of actinolite into quartz
is illustrated in the Tyler slate of the Penokee district of Wisconsin."
In instances where the quartz furnishes the silica for the penetrating
silicates the migration of the silica is microscopical, and it might be sup-
posed that the reactions occur without the solution of the silica of the
quartz; but it seems probable, even in such cases as these, that there is
solution of the silica before combination with the bases. In such reactions
it is presumed that the bases which unite with the silica were before united
with some other acid, and it is only when the previous combination is known
that the heat and volume relations of the reactions can be ascertained.
"Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. U. 8. Geol.
Survey, vol. 13, 1888, pp. 120-127.
6 Becker, cit., p. 124.
Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin:
Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 210-215.
220 A TREATISE ON METAMORFHISM.
In a third class of changes quartz may be wholly replaced by other
minerals, as by magnetite and hematite. Very frequently the deposition of
the new minerals seems to be conditioned upon the solution of the quart/.
The replacement of quartz by iron oxide is illustrated in the Lake Superior
region in both the iron-bearing and the slate formations."
The most favorable conditions for the solution of silica, especially of that
formed by the decomposition of the silicates by carbonation, are furnished
by the belt of weathering. The most favorable conditions for the deposi-
tion of silica as quartz are those of the belt of cementation. The solution of
silica in the belt of weathering of the zone of katamorphism and its deposi-
tion in the belt of cementation of this zone is perhaps the best illustration
of the principle explained on pages 634-636, that material dissolved in
the belt of weathering may be extensively deposited in the belt of
cementation. Recrystallization of quartz mainly takes place in the zone
of anamorphism, although it undoubtedly occurs to some extent in the
zone of katamorphism, and especially in the belt of cementation. The
process of silication takes place almost invariably with decrease in volume,
provided all the compounds concerned are solids. Where the carbonates
are silicated the decrease in volume ranges from 20 to 40 per cent. Silica-
tion occurs upon a great scale in the zone of anamorphism is, indeed, one
of the most distinctive chemical reactions of that zone.
TRIDYMITE.
Tridymite:
SiO 2
Hexagonal, or pseudo-hexagonal.
Sp.gr. 2.28-2.33.
occurrence. Tridymite usually occurs as an autogenic mineral in cavities
in lavas, such as rhyolite, andesite, trachyte, etc.
Modifications. Tridymite is dissolved more readily than quartz. The
material of tridymite may go through any of the changes which silica of
quartz may pass through, with the difference that its recrystallization would
result in the production of quartz (rhombohedral ; sp. gr. 2.652-2.654)
rather than the original mineral, tridymite. The changes of tridymite into
other minerals than quartz need not be discussed in detail, since the reac-
tions are the same as with quartz, except that the volume decrease is greater
Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon. U. S.
Geol. Survey,, vol. 28, 1897, pp. 370, 400-405.
OPAL. 221
in the changes of tridyrnite than with quartz. In the change of tridymite
to quartz there is a diminution of volume, amounting to 14.24 per cent, and
there is also probably liberation of heat. Energy is therefore poteutialized
in tridymite as compared with quartz. The change is one which is particu-
larly likely to occur in the zone of anamorphism, where pressure is the
dominant factor. In the fact that quartz is a denser mineral than
tridymite we probably have a reason not only for the passage of tridymite
into quartz in the lower zone, but for the absence of tridymite as an
original pyrogenic constituent in the plutonic igneous rocks which crystal-
lized originally in this zone. Under its conditions the denser mineral,
quartz, formed.
OPAL.
Opal:
SiO 2 .nH 2 O ( H 2 O 2 to 13 per cent; but mostly 3 to 9 per cent. )
Amorphous.
Sp.gr. 2.1-2.2.
occurrence. Opal, like most other hydrous minerals, is a product of the
zone of katamorphism. Opal is a direct deposit from hot springs. In the
sedimentary rocks it is abundantly formed from the siliceous skeletons of
certain animals and plants, such as radiolaria, sponges, and diatoms. Opal
is plentifully deposited in cavities in rocks by subterranean waters. Its
most common places of occurrence are the limestones, where it is largely
of organic origin, and the porous igneous rocks, especially as amygdules of
the amygdaloids, where it is a chemical precipitate.
In general, as a metamorphic product opal may be derived from the
same minerals as quartz.
Modifications. The most frequent change of opal is to quartz (rhombohe-
dral; sp. gr. 2.652-2.654). Frequent intermediate products are chalcedony
and chert, which appear to be partly crystalline substances. (See p. 222.)
In the passage of opal into quartz, the changes are three: dehydration,
reduction of volume, and recrystallization. Supposing the composition of
the opal is SiO 2 .|H 2 O, which would be about 6 per cent of water, the
decrease of volume would be 22.81 per cent. The change from opal to
quartz above given is commonly accomplished by solution and redeposition
or recrystallization. When the material is taken into solution this silica may
be deposited near by or transported elsewhere. It may unite with free
bases, producing silicates; it may displace other acids combined with bases,
222 ' A TREATISE ON METAMORPHISM.
as, for instance, carbonic acid, thus also producing silicates. The heat and
volume relations of these reactions are discussed under "Quartz."
The reactions of dehydration, crystallization, and lessening of volume,
as seen on pages 167-170, are particularly characteristic of the zone of
anamorphism, and it is in this zone that the change from opal to quartz
probably most extensively occurs. As evidence of this is the frequent
occurrence of opal in the zone of katamorphism, and the general absence
of opal in the rocks which have been metamorphosed in the lower zone.
CHERT, CHALCEDONY, ETC.
Standing between opal and quartz are numerous varieties of partly
crystallized or very finely crystallized silica, of which chert and chalced-
ony may be taken as the more important kinds. With these substances
are frequently small but variable amounts of opal containing combined
water. The specific gravities of chert and chalcedony are intermediate
between those of opal and quartz, i. e., between 2.15 and 2.65. Their most
frequent occurrence is as veins, nodules, belts, and members in carbonate
formations. Ordinarily they are derived from organic forms, such as
radiolaria, diatoms, and sponges, which lived under conditions similar to
those under which the limestone-building animals lived. (See p. 817.)
Chert and chalcedony are derived from opal. The material here
included varies from that which is close to opal, having only a few minute
crystallized spots, through material which shows more and more evidence of
crystallization, to material which contains comparatively little amorphous
silica, and thence into fully crystallized silica or quartz. The transition
varieties may have the peculiar spotty appearance in polarized light char-
acteristic of ordinary chert or the peculiar radial fibrous polarization of
chalcedony or any combination of the two.
The alterations of chert and chalcedony are into quartz, or by combi-
nation with bases producing silicates, the same as opal. The chemistry and
physics of the change are the same as for opal except that the decrease in
volume is less, and therefore they need not be repeated.
OCCURRENCE OF CORUNDUM. 223
, HEMATITE GROUP.
CORUNDUM, HEMATITE, AND ILHEMTE.
Corundum:
A1 2 O 3 .
Rhonibohedral.
Sp. gr. 3 95-1.10.
Hematite:
Fe.A-
Rhombohedral.
Sp. gr. 5.20-5.25.
Ilmenite:
FeTiO s ; varies to mFeTiO s .nFe.;O,.
Rhombohedral.
Sp. gr. 4.50-5.02.
CCRUNDUM.
occurrence. In Canada at one locality corundum occurs as an original
constituent of a syenite." Also, corundum as an accessory mineral has been
noted in granite, andesite, and other rocks. Corundum is, therefore, an
original pyrogenic constituent of igneous rocks. Corundum occurs along
the contact of intrusive basic rocks rich in alumina, especially those con-
taining more than 30 per cent, such as peridotites and pyroxenites. The
intruded rocks may be either igneous rocks or gneisses and schists. But
where corundum occurs in veins along contacts it is in many cases an
aqueo-igneous product (see pp. 720-728) or an aqueous deposit. Corundum
is a widespread accessory constituent in various micaceous, chloritic, and
hornblendic schists and gneisses, and in marble. Corundum, as a meta-
morphic mineral, is associated with chlorite and corundophilite. It is often
associated with other heavy metamorphic minerals, such as andalusite,
sillimanite, cyanite, spinel, rutile, etc. As a metamorphic mineral it is
derived from andalusite, cyanite, diaspore, gibbsite, sillimanite, staurolite,
and topaz.
Alterations. Corundum alters into diaspore (orthorhombic; sp. gr. 3.40),
gibbsite (monoclinic ; sp. gr. 2.35), spinel (isometric; sp. gr. 3 8), sillimanite
(orthorhombic; sp. gr. 3.235), cyanite (triclinic; sp. gr. 3.615), muscovite
(damourite), (monoclinic; sp. gr. 2.88), margarite (monoclinic; sp. gr. 3.035),
and zoisite (orthorhombic; sp. gr. 3.31). The reactions for the formation of
diaspore and gibbsite are simple reactions of hydration. The reactions for
"Miller, VV. G., Economic geology of eastern Ontario; corundum and other minerals: Seventh
Kept. Ontario Bureau of Mines, 1897, Toronto, 1898, p. 213.
224 A TREATISE ON METAMORPHISM.
the production of the other minerals require the addition of various other
constituents in the case of spinel, magnesia ; in the case of sillimanite and
cyanite, silica; in the case of the complicated silicates, muscovite, margarite,
and zoisite, various bases and a large amount of silica. Therefore in these
cases it is clear that the common statement that corundum alters to the
minerals muscovite, margarite, and zoisite can have only the meaning that
the relations are such that corundum furnishes the alumina for the resultant
compound, and that the additional compounds are derived from another
source. It will be assumed in the alterations that the magnesia, lime,
and potash are derived from the solid carbonates and that the siliga is
added as quartz. The equations for the reactions are as follows:
(1) Al 2 O,+H s O=2[AlO.(OH)]+k.
(2) Al,O s -i-3H 2 0=2Al(OH) s +k.
(3) Al 2 0,+MgCO s =MgAlA+C0 2 +k.
(4) AlA+SiO 2 =Al 2 SiO 6 +k.
(5) 3Al 2 3 +6SiO 2 + K 2 CO3+2H 2 O=2H s KAl 3 SisO,2+CO 2 +k.
(6) 2AlA+2Si0 2 +CaC0 3 +HA=H 2 CaAl 4 Si 2 12 +CO,+k.
(7) 3Al 2 O s +6SiO 2 +4CaCO,+H 2 O=H 2 Ca,Al 6 Si 6 O 26 +4CO 2 +k.
The increase in volume as compared with corundum is, for diaspore
(equation 1), 39.25 per cent; for gibbsite (equation 2), 161.83 per cent.
The volume of the corundum and the magnesite in passing to the spinel
(equation 3) is decreased 29.17 per cent. The volume of the corundum
and quartz in passing into sillimanite (equation 4) is increased 4.38 per
cent; into cyanite (equation 4) is decreased 6.59 per cent. If the volume
of the corundum be compared with that of the muscovite (equation 5),
with that of the margarite (equation 6), and with that of the zoisite
(equation 7), there will be great volume increases. If, on the other
hand, all the products which unite with the corundum in each case, with
the exception of the water, be counted as solid, there would be small
inci'ease in the volume for muscovite, a considerable decrease for zoisite,
and a small decrease for margarite. On the first hypothesis the increase in
the volume in the production of muscovite is 264.25 per cent; in margarite,
159.02 per cent; in zoisite, 261.34 per cent. On the second hypothesis the
increase in volume in the production of muscovite is 1.62 per cent; to form
margarite the decrease is 1.22 per cent; to form zoisite the decrease is 23.58
per cent.
It is reasonably certain that the passage of corundum to diaspore and
gibbsite is a reaction characteristic of the zone of katamorphis-m, and
OCCURRENCE OF HEMATITE. 225
especially the belt of weathering. It is almost equally certain that the
passage of corundum into spinel, sillimanite, and cyanite is characteristic
of the zone of anamorphism.
The case, however, is not clear in reference to the muscovite, margarite,
aiid zoisite. The equations as written are those of silicifiation and slight
hydration. If these equations be correct, they should occur in the lower
part of the belt of cementation or in the zone of anamorphism. It is
tolerably certain that margarite, zoisite, and muscovite form in the lower
part of the belt of cementation; but the zone in which muscovite charac-
teristically develops is that of anamorphism. It is not at all impossible
that the potassium carbonate, and perhaps the calcium carbonate, or even
the silica, are added in solution for the margarite and zoisite. In this case
there would be a considerable volume increase. Whether the same may
be assumed for the muscovite is uncertain. Very likely the materials
added to the corundum are in some cases carried in by the solutions, in
others are derived from adjacent minerals, and in still others partly from
both. Where the lime and potash are derived from minerals adjacent, they
may come from other compounds than carbonates, and the silica may have
been previously united with other bases. So far as this is so, in considering
the variations in volume the minerals from which the elements added to the
corundum to produce the muscovite, margarite, and zoisite were derived
must be taken into account. It is clearly impracticable in the present state
of knowledge to give definite statements as to the volume changes for these
minerals.
HEMATITE.
occurrence. Hematite is a pyrogenic constituent in igneous rocks and is
an abundant metamorphic mineral. Its most abundant source in the
metamorphic rocks is by the dehydration of limonite, a reaction occurring
with the absorption of heat and reduction of volume. A second important
source of hematite is from iron carbonate by loss of carbon dioxide and by
oxidation, a reaction occurring with the liberation of heat and reduction of
volume. Hematite may also be produced by the oxidation of magnetite, a
reaction resulting in liberation of heat and expansion of volume. Fre-
quently after this change the hematite has the isometric form of the original
magnetite and is called martite. A fourth source of hematite is by the
oxidation of the ferrous iron of silicates at the time of their decomposition.
MON XLVII 04 15
926 A TREATISE ON METAMORPHISM.
A fifth source is by oxidation of ferrous iron solutions, which may result in
the precipitation of hematite. The first reaction occurs most extensively
in the zone of anamorphism; the other four occur in the zone of katamor-
phism, and to these positions the heat and volume reactions correspond.
Finally, as shown in Chapter XII, on "Ore deposits," hematite may be
formed from pyrite by the action of alkaline carbonate solutions.
In summary, hematite is derived from actinolite, ankerite, anthophyl-
lite, biotite, bronzite, garnet, greenalite, grunerite, hornblende, hypers-
thene, ilmenite, limonite, magnetite, olivine, parankerite, pyrite, serpentine,
and siderite.
Alteration.. The most frequent alteration of hematite is into limonite
(amorphous; sp. gr. 3.6-4). The reaction is as follows:
2Fe,0 3 +3H 2 0=2FeA.3H ir O+k.
Iii the change the volume is increased 60.72 per cent. A second altera-
tion of hematite is into magnetite (isometric; sp. gr. 5.168-5.18). This
may be accomplished by any of the reducing agents furnished by organic
compounds. Supposing the reducing agent to be the partially oxidized
carbon compound CO, the reaction is:
3Fe 2 O s +CO=2Fe,O 4 +CO 2 +k.
While a reduction of the oxide of iron occurs a simultaneous oxidation of
the organic compound occurs, and the end result is the liberation of heat.
In the change the volume is decreased 2.38 per cent. A third alteration
of hematite is to pyrite (isometric; sp. gr. 5.025) or marcasite (orthorhom-
bic; sp. gr. 4.875). In the best-known instances siderite (rhombohedral ;
sp. gr. 3.855) or some other iron-bearing carbonate is simultaneously
produced. The reaction may be:
Fe,0,+2H 2 S+CO 2 =FeS 2 +FeCO s +2H,0+k.
In the change to pyrite and siderite the volume is increased 76.12 per cent,
and to marcasite and siderite 78.73 per cent.
The alterations of hematite to limonite occur in the zone of katamor-
phism, and especially in the belt of weathering. Corresponding with this
position the reaction is with liberation of heat and expansion of volume.
The alteration of hematite into magnetite occurs in the belt of cementation
and the zone of anamorphism. This agrees with the fact that the reaction
ALTERATION OF ILMENITE. 227
liberates heat and diminishes the volume. The alteration of hematite to
pyrite and marcasite is best known where organic compounds are present
to reduce sulphuric acid to hydrosulphuric acid and to furnish carbonic acid
to form the carbonates. The reaction is especially characteristic of the belt
of cementation, and to this position the expansion of volume and the
liberation of heat correspond.
ILMENITE.
occurrence. Ilmenite is an abundant pyrogenic constituent of the igneous
rocks. It is found both as an allogenic and as an autogenic constituent in
metamorphic rocks. As an autogenic constituent the compounds which
unite to produce it have not been worked out. As a metamorphic mineral
ilmeuite is derived from perovskite and rutile.
Alterations. Ilmeiiite alters to titanite (monoclinic; sp. gr. 3.48), to rutile
(tetragonal; sp. gr. 4.18-4.25), and to octahedrite, or anatase (tetragonal;
sp. gr. 3.82-3.95). With these minerals magnetite (isometric; sp. gr. 5.174)
or hematite (rhombohedral; sp. gr. 5.225) or limonite (amorphous; sp.
gr. 3. HO) is simultaneously produced. One of the most frequent reactions
in the production of titanite is probably along the following lines :
3FeTiO s +3CaCO,+3SiO 2 +O=3CaTiSiO !i +Fe 8 O 4 +3COj+k.
The decrease in volume of the ilmenite, calcite, and quartz in passing into
titanite and magnetite, supposing the CO 2 to escape, is 22.35 per cent; but
the increase in volume of the titanite as compared with the ilmenite alone
is 76.35 per cent. The alteration of ilraenite to rutile and octahedrite, with
combined magnetite, is as follows:
3FeTiO 3 +O=3TiO 2 +Fe s 4 +k.
In case hematite is produced instead of magnetite the reaction is:
2FeTiO s +O=2TiOj+Fe 2 O,,+k.
In case limonite is produced, one and one-half molecules of water are added
to both sides of the equation.
The increase in volume of the ilmenite in passing into rutile and mag-
netite is 6.02 per cent; into octahedrite and magnetite, 11.07 per cent. In
case hematite or limonite be produced, the increase in volume is corre-
spondingly greater.
It is certain that titanite forms from ilmenite in the lower zone. In this
zone, as explained on pp. 764-765, the CaCO 3 and SiO 2 can not be supposed
228 A TREATISE ON METAMORPHISM.
to have been brought in from the outside, and therefore the change takes
place with decrease in volume. It is also certain that titanite forms exten-
sively in connection with chlorite, which commonly develops in the belt of
cementation. In this case the calcium carbonate and silica may be intro-
duced in solution from an outside source, under which circumstances the
volume is increased.
The alterations of ilmenite to rutile and octahedrite, or any combina-
tion of them, certainly occur in the zone of katamorphism, and to this
position the heat and volume reactions correspond. However, I have not
found sufficient information on the subject to assert that these reactions do
not also occur in the zone of anamorphism.
SPINEL GROUP.
SPINEL, >U(.M, I ; I I . AXD i II IJOMI 1 1 .
Spinel:
MgALA-
(Hercynite, FeAl 3 O 4 -)
(Pleonaste, [MgFe] [AlFe] 2 4 .)
(Picotite, [MgFe] [AlCr] 2 O 4 -)
Isometric.
Sp. gr. 3.5-4.1.
Magnetite:
Fe,0 4 .
Isometric.
Sp. gr. 5.168-5.180.
Chromite:
FeCr 2 4 -
Isometric.
Sp. gr. 4.32-4.57.
SPINEL.
occurrence. Spinel occurs as an original constituent in the igneous rocks,
but is much more abundantly present as a secondary constituent in the
metamorphic rocks, especially those which are rich in magnesium. In
many cases it is secondary to olivine and other minerals rich in magnesium.
The more important minerals from which spinel is derived are almaiidite,
biotite, chlorite, corundum, diaspore, garnet, gibbsite, olivine, and pyrope.
Alterations. According to Dana, spinel has been observed as altering to
talc (orthorhombic or monoclinic; sp. gr. 2.75), serpentine (monoclinic;
sp. gr. 2.575), and mica (mouoclinic; sp. gr. 2.88-2.90). However, the
character of the alterations and the conditions under which they occur are
so little known that I shall not attempt to treat them from the physical-
chemical point of view.
SPINEL GROUP. 229
MAGNETITE.
occurrence. Magnetite is a very abundant pyrogenic constituent in
igneous rocks. It is abundantly deposited from solutions, and especially
from solutions bearing iron carbonate, according to the reaction:
3FeC0 3 +O =FeA+ 3CO 2 + k .
Magnetite also extensively forms from siderite in situ. These changes
liberate heat and decrease the volume. A third source of magnetite is by
incomplete oxidation of pyrite and marcasite, reactions occurring with
liberation of heat and diminution of volume. Fourth, frequently siderite
and iron sulphide together pass into magnetite with decrease in volume.
(See pp. 244, 845.) A fifth way in which magnetite may be produced is
by the reduction of hematite by organic compounds, a reaction occurring
with the liberation of heat, because of the simultaneous oxidation of the
organic compounds, and with diminution of volume. A sixth way in which
magnetite is produced is by the incomplete oxidation of ferrous iron of
silicates; for instance, olivine and garnet.
In summary, magnetite is derived from actinolite, ankerite, arfvedsonite,
augite, biotite, bronzite, diopside, garnet, greenalite, griinerite, hematite,
hornblende, hypersthene, ilmenite, marcasite, and pyrite.
Alterations. Magnetite alters into hematite (rhombohedral; sp. gr. 5.225),
lirnonite (amorphous; sp. gr. 3.80), and siderite (rhombohedral; sp. gr. 3.83-
3.88). The reactions are as follows:
(1) 2Fe,O 4 +O=3Fe,0,+k.
(2) 4Fe 3 O 4 +2O+9H 2 O=3(2Fe 2 O s -3H 2 0) +k.
(3) Fe 3 O 4 +CO+2CO 2 =3FeCO 3 +k.
i
In the change the increase in volume is, for (1), 2.44 per cent; for (2),
64.63 per cent, and for (3), 101.30 per cent. The -increase in volume in the
change from magnetite to siderite over 100 per cent is the greatest
volume change in which only two minerals are concerned which the calcu-
lations of Mr. Lincoln have given, with the exception of the alteration of
corundum into gibbsite. (See p. 224.) All of the above changes are well
known to occur in the zone of katamorphism, and corresponding with this
position they all take place with the liberation of heat, expansion of volume,
and decrease in symmetry.
CHROMITB.
occurrence. Chromite occurs in the igneous rocks, especially those rich
in magnesium. It also occurs in the metamorphic rocks, often in connec-
230 A TREATISE ON METAMORPHISM.
tion with serpentine. In these positions it is very frequently a secondary
product of olivine. The reactions occurring in its production are given
under that mineral.
Alterations. The alteration of chromite into other minerals has not been
noted.
RUTILE GROUP.
Ill I I I.I . OCTAHEDBITE, A>'D BKOOKITE.
Rutile:
TiO 2 .
Tetragonal.
Sp. gr. 4.18-4.25.
Octahedrite:
TiO v .
Tetragonal.
Sp. gr. 3.82-3.95.
Brookili:-
TiO 2 .
Orthorhombic.
Sp. gr. 3.87-1.082.
occurrence. Rutile is a pyrogenic constituent in igneous rocks, and has a
widespread occurrence in the clastic and metamorphic rocks, both as an
allogeuic and as an autogenic constituent, in the latter case generally being
derived from ilmenite. Rutile is also derived from brookite, ilmenite, octa-
hedrite, and titanite.
Octahedrite in the metamorphic rocks is a secondary alteration of
other titanium-bearing minerals, especially of titanite. It is also derived
from ilmenite.
Brookite occurs sparingly, both in altered igneous rocks and in sedi-
mentary rocks. In some cases it is secondary to titanite.
Alteration.. Both Octahedrite and brookite alter to rutile (tetragonal; sp.
gr. 4.18-4.25). In the case of octahedrite the decrease in volume is 7.83 per
cent; in the case of brookite the decrease is 5.69 per cent. The heat change
is undetermined, but probably the alterations occur with the liberation of
heat. If this be the case the alterations involve recrystallizatiou, diminu-
tion of volume, and liberation of heat. In the case of octahedrite the sym-
metry remains the same; in the case of brookite the symmetry is increased.
It may be inferred that such changes as these occur in both zones,
being in all respects analogous to the changes which take place in dolomiti-
zation. (See pp. 238-240.) However, the geological occurrences of these
alterations have not been given with such definiteness as to enable one to
make definite statements as to the actual facts.
ALTERATIONS OF RUTILE. 231
In this connection the experiments of Hautefeuille" are very interesting.
He produced rutile, brookite, and octahedrite from the same compounds,
but at different temperatures, rutile forming when red heat was used,
brookite when the temperature was between that required for the volatiliza-
tion of cadmium and zinc, and octahedrite when the temperature was a little
below that for the volatilization ofi cadmium. Rutile, the mineral with the
highest specific gravity, forms at the highest temperature, and high tempera-
ture is especially characteristic of the zone of anamorphism.
Rutile may alter into ilmenite (rhombohedral ; sp. gr. 4.75) and into
titanite (moiiocliuic; sp. gr. 3.48). In the change to ilmenite the reactions
may be:
(1) TiO 2 +Fe s 4 =FeTiO 3 +Fe,0 8 +k, or
(2) TiO 2 +FeCO 3 =FeTiO 8 +CO 2 +k.
lu (1) the decrease in volume of the ilmenite and hematite as compared
with the rutile and magnetite is 1.88 per cent. In (2) the decrease in
volume is 34.77 per cent, provided the iron carbonate is present as solid
siderite and the C0 2 escapes.
In the change to titanite the most probable reaction is:
(3) TiO 2 +CaCO 3 +Si0 2 =CaTiSiO 5 +CO 2 +k.
The decrease in volume is 28.17 per cent, provided the compounds which
unite with the rutile are solids and the liberated C0 2 escapes.
Those who have described the changes of rutile to ilmenite and titanite
have not indicated whether or not they occur as deep-seated alterations. It
may, however, be anticipated that such is the case, for they are changes
which involve liberation of heat and condensation of volume, and therefore
the kind which normally occur in the zone of anamorphism.
DIASPORE GROUP.
DUSPOKE AND 1,1 HUM IK.
Diaspore:
AIO(OH).
Orthorhombic.
Sp. gr. 3.3-3.5.
lAmonite:
2Fe 2 O 3 .3H 2 O.
Amorphous.
Sp. gr. 3.6-4.00.
"Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana; Wiley & Sons,
New York, 6th ed., 1892, p. 239.
232 A TREATISE ON METAMORPHISM.
DIASPOKE.
occurrence. Diaspore is especially found in the serpentine- or chlorite-
bearing schists and gneisses and in dolomites. In these rocks it is frequently
associated with corundum. Diaspore has been recorded as a constituent
of granite, nepheline-syenite, and basaltic rocks. As noted on other pages,
for the alterations of various minerals ill the zone of katamorphism, espe-
cially the belt of weathering, it may be produced as one of the alteration
products of the following minerals: Biotite, corundum, gibbsite, haiiynite,
muscovite, nephelite, noselite, phlogopite, scapolites, sodalite.
Alterations. No alterations of diaspore are recorded. However, it is
probable that where diaspore is deposited in sedimentary rocks and is deeply
buried, so as to undergo alteration in the zone of anamorphism, it passes
into corundum (rhombohedral, sp. gr. 4.025); or, like corundum, unites
with other bases to produce such minerals as spinel (isometric, sp. gr.
sillimanite (orthorhombic, sp. gr. 3.235), and cyanite (triclinic, sp. gr. 3.615);
muscovite (monoclinic, sp. gr. 2.88); margarite (monoclinic, sp. gr. 3.035);
and zoisite (orthorhombic, sp. gr. 331). So far as the hydrous minerals
muscovite, margarite, and zoisite are formed, the water may have been
derived from the diaspore, which contains more water, and thus their forma-
tion be really a process of dehydration and silication. For these supposed
reactions the equations may be:
(1) 2AlO(OH)=Al 2 0,+H 2 0-k.
(2) 2A1O(OH) +MgC0 3 =MgAl 2 O 4 +CO 2 +H 2 O-k.
(3) 2AlO(OH)+SiO 2 =AL ! SiO 5 +H 2 0-k.
(4) 6AlO(OH)+6SiO 2 +K 2 CO 3 =2H 2 KAl 3 Si s 12 +CO 2 +H 2 O-k.
(5) 4A1O(OH) +2SiO 2 +CaCO 3 ==H 2 CaAl 4 Si 2 O 12 +CO 2 +H 2 O-k.
( 6) 6A1O (OH ) +6SiO 2 +4CaCO 3 = H 2 Ca ) Al 6 Si 6 O 26 +4C0 2 +2H. i O-k.
Supposing that all the compounds in the first member of the equation are
solids, and that the liberated C0 2 and H 2 escape, the decrease in volume
for corundum (1) would be 28.18 per cent; for spinel (2), 40.39 per cent;
for sillimanite (3), 13.52 per cent; for cyanite (3), 22.61 per cent; for
muscovite (4), 54.21 per cent; for margarite (5), 14.08 per cent; for zoisite
(6), 2i).44 per cent. All take place with absorption of heat. Therefore
all of these reactions are characteristic of the zone of anamorphism.
occurrence. Limonite as a mineral is produced either as an original
chemical precipitate or by the alteration of other minerals. It is not,
OCCURRENCE AND ALTERATIONS OF LIMONITE. 233
i as known, an original pyrogenic constituent of the igneous rocks.
The uost important source of bodies of limonite is precipitation from
ir< Hi- Baring- solutions, especially iron carbonate. For iron carbonate the
in is
4FeC0 3 +2O+3H 2 0=2Fe 2 3 .3H 2 O+4CO 2 +k.
e >nd important source for limouite is the oxidation and hydration of
ron carbonate in rocks, especially siderite, ankerite, parankerite, and
in >n earing limestone or dolomite. The source next in importance is the
oxid ion and hydration of pyrite, marcasite, and other sulphides. A fourth
ant source of limonite is the oxidation arid hydration of the ferrous
iron if silicates. A fifth source is the hydration of hematite. A sixth
In it [important source is the oxidation and hydration of magnetite. All the
>ns involve oxidation or hydration, or both, and therefore take place
Avitl the liberation of heat. In the production of limonite from iron
late there is an important contraction of volume; in the other cases
the )lume of the limonite is greater than that of the compounds from
whi( it is derived. All the above reactions producing limonite occur in
the >ne of katamorphism, and the controlling factor is the first part of
Hoff's law, that of chemical reactions with the liberation of heat.
Limnite does not develop in the zone of anamorphism.
n summary, limonite is derived from the following minerals: Actino-
lite, ukerite, anthophyllite, arfvedsonite, biotite, bronzite, epidote, garnet,
gri-i ilite, griinerite, hematite, hornblende, hypersthene, ilmenite, mag-
ncti , marcasite, olivine, parankerite, pyrite, pyrrhotite, serpentine, and
side r,e.
^iterations. The important alterations of limonite are into hematite
(rlu ibohedral; sp. gr. 5.20-5.25) and siderite (rhombohedral ; sp. gr.
3.833.88). Hematite produced from limonite may be earthy or crystalline.
Tlu eaction is
2Fe 2 O 3 .3H 2 O=2FeA+3H 2 O-k.
Tlu lecrease of volume is 37.78 per cent. The change is therefore one
of ( hydration, reduction of volume, and crystallization. The transforma-
tion akes place on a great scale in the zone of anamorphism, that in which
pres-ure controls whether heat is absorbed or liberated.
The second important change of limonite is into iron carbonate.
Whre this change occurs organic compounds are commonly present and
234 A TREATISE ON METAMORPHiSM.
decomposing to serve as reducing agents and to furnish abundant CO 2 to
unite with the iron. The reduction may be by the passage of CO into
C0 2 , of C into C0 2 , or of C into CO, as follows:
(1) 2Fe 2 O,.3H,O+2CO+2CO 2 =4FeC0 3 +3H 2 O+k.
(2) 2Fe 2 O 8 .3H 2 0+C+3CO 2 =4FeCO s +3H. 1 O+k.
(3) 2FeA-3H 2 O+2C+4C0 2 =4FeCO 3 +3H 2 O+2CO+k.
So far as the iron is concerned, its reduction and dehydration absorb heat,
but the oxidation of the C or CO and the union of the CO 2 and FeO both
liberate heat, the amount of which is greater than that absorbed, so that in
each of these reactions heat is liberated. In all of the reactions the volume
is increased 22.27 per cent.
The reduction of the iron of limonite so as to produce protoxide for
the formation of iron carbonate may of course be accomplished by carbu-
reted hydrogen, especially methane (CH 4 ), rather than by the compounds
suggested; but the carbureted hydrogen compounds are so numerous and
the resultant compounds so uncertain that no attempt will be made to
formulate equations for possible changes with these substances as reducing
agents.
The change of limonite to siderite is one which occurs extensively in
rocks bearing organic compounds in the zone of katamorphism. The
formation of the abundant siderites which are used as iron ores of Carbon-
iferous and later age are believed for the most part to be thus derived from
limonite in the upper zone. The reactions correspond perfectly to this
position, being those which occur with liberation of heat and very consid-
erable expansion of volume. The siderite thus formed may later be
decomposed into various other compounds, or even reproduce limonite, but
the consideration of such changes belongs under "Siderite."
Brucite:
Mg(OH) 2 .
Rhombohedral.
Sp. gr. 2.38-2.40.
Gibbgite (hydrargillite):
A1(OH),
Monoclinie.
Sp. gr. 2.28-2.42.
BEUCITE GROUP.
BBUCITE AJiD (JIBBS1TE.
BRUCITE AND GIBBSITE. 235
BKUCITE.
occurrence. Brucite is one of the minerals which is produced in the
upper physical-chemical zone, especially in the belt of weathering. Brucite
is produced by the alterations of minerals rich in magnesia, being recorded
as secondary to chondrodite, clinohumite, humite, and serpentine. It is
especially prevalent in serpentinous rocks and veins. Doubtless in many
instances it forms simultaneously with the serpentine and perhaps other
minerals, rather than secondary to them.
Alterations. The one alteration of brucite noted is that of carbonation,
into hydromagnesite (monoclinic; sp. gr. 2.145-2.180). The reaction
representing the change is
4Mg(OH) 2 +3CO 2 = Mg 2 (CO 3 ),.2Mg(OH).3H.,O+k.
The increase in volume is 73.08 per cent. The alteration is therefore
one of simple carbonation, and takes place in the zone of katamorphism,
especially in the belt of weathering, with expansion of volume and
liberation of heat.
GIBBSITE.
occurrence. Gibbsite occurs as an accessory constituent in many of
the schists and gneisses, especially those which have been subjected to the
forces of the upper physical-chemical zone, and particularly in the belt of
weathering. As noted on subsequent pages, it may be a result of the
alteration of many minerals, the more important of which are as follows:
Anorthoclase, andalusite, biotite, cancrinite, corundum, cyanite, epidote,
haiiyiiite, microcline, muscovite, nephelite, noselite, orthoclase, phlogopite,
plagioclases, pyrope, the scapolites, sillimanite, sodalite, topaz, tourmaline,
and zoisite. By reference to the discussion of these minerals and the
minerals which simultaneously form, the conditions of its formation may
be ascertained.
Alterations. No alterations of gibbsite are recorded in the standard text-
books, but where sedimentary rocks containing gibbsite are so deeply
buried as to pass into the zone of anamorphism it may become partly
dehydrated, producing diaspore (orthorhombic ; sp. gr. 3.40), or wholly
dehydrated, producing corundum (rhombohedral ; sp. gr. 4.025); or the
aluminum may unite with other compounds, producing the same minerals
that are produced by corundum or diaspore. It is believed that these
236 A TREATISE ON METAMORPHISM.
alterations from diaspore and gibbsite have taken place on an extensive
scale, even if they have not been recorded. There is no doubt about the
formation of gibbsite abundantly in the zone of katamorphism, especially
in the belt of weathering. To my mind there is as little doubt that the
widespread corundum of the schists, gneisses, and marbles is derived in
large measure from gibbsite. I am confident that the hydrous aluminum
oxides furnish the bases for much of the spinel (isometric; sp. gr. 3.80),
sillimanite (orthorhombic; sp. gr. 3.235), and cyanite (triclmic; sp. gr-
3.615) which occur in these rocks. And it is little short of a certainty that
gibbsite furnishes alumina for the silicates, muscovite (monoclinic; sp. gr.
2.88), margarite (monoclinic; sp. gi\ 3.035), and zoisite (orthorhombic;
sp. gr. 3.31). As with diaspore, the reactions producing all the above-
mentioned silicates are those of dehydration and silicifiation. The following
equations may be written for the above supposed reactions:
(1) Al(OH) = =AlO(OH)+H 2 0-k.
(2) 2A1(OH) 3 =A1 2 O S +3H 2 O- k.
(3) 2Al(OH) 3 +MgC0 3 =MgAl 2 O 4 +CO 2 +3H,O-k.
(4) 2A1 (OH),+Si0 2 =Al.,SiO 6 +3H 2 O-k.
(5) 6Al(OH) 8 -fGSiO,+K 2 C0 3 =2II 2 KAl 3 Si s O 12 T-CO 2 +7H 2 O-k.
(6) 4Al(OH) 3 +2SiO 2 +CaCO 3 =H.,CaAl 4 Si 2 O 12 +CO 2 +5H.,0-k.
(7) 6Al(OH) s +6Si0 2 +4CaC0 3 =H 2 Ca ( Al 6 Si 6 26 +4C0 2 +8H 2 0-k.
Regarding all the minerals as solid, the decrease of volume for diaspore (1)
is 46.82 per cent; for corundum (2), 61.81 per cent; for spinel (3), 60.12
per cent; for sillimanite (4), 43.68 per cent; for cyanite (4), 49.61 per cent;
for muscovite (5), 64.99 per cent; for margarite (6), 38.92 per cent; for
zoisite (7), 43.06 per cent. The decreases of volume are greater for the
corresponding minerals than for diaspore because of the greater amount of
water in the gibbsite. In all the reactions heat is absorbed. The reactions
are therefore typical of the zone of anamorphism.
THE CARBONATES.
The important carbonates which occur as rock-making constituents
are the calcite group, including calcite, dolomite, ankerite and parankerite,
magnesite, and siderite, and the aragonite group, of which aragonite is the
only important rock-making member.
OCCURRENCE OF CALCITE. 237
CALCITE GROUP.
CALCITE, DOLOMITE, ANKERITE, PABAXKERITE, MAGNESITE, AND M Ml I; I I I .
Calcite:
CaCO 3 .
Rhombohedral.
Sp. gr. 2.713-2.714.
Dolomite:
CaMgC 2 O 6 .
Rhombohedral.
Sp. gr. 2.8-2.9.
AnkerUe:
CaFeC 3 O 6 .CaMgC 2 O 6 ;(CaMgC 2 O 6 :CaFeC,O 6 ::i:l to 2:1).
Rhombohedral.
Sp. gr. 2.95-3.1.
Parankerite:
CaFeC 2 O 6 .2CaMgC.,O 6 ;(CaMgC 2 O 6 :CaFeC 2 O 6 ::2:l to 10:1 ^
Rhombohedral.
Sp. gr. 2.95-3.1.
Magnesile:
MgC0 3 .
Rhombohedral.
Sp. gr. 3.00-3.12.
Siderite:
FeCO 3 .
Rhombohedral.
Sp. gr. 3.83-3.88.
CALCITK.
occurrence. The chief sources of calcite are (1) organic precipitates, (2)
chemical precipitates, (3) by alteration of aragouite, and (4) by carbonation
of silicates.
The chief direct source of calcite is organic. Corals and innumerable
other kinds of shell animals, especially in the sea, abstract calcium carbonate
from the water and build it into their external or internal structures. Calcite
as a chemical precipitate may be deposited from the waters of the sea,
especially in inclosed lagoons; by the waters of inland lakes, especially
those having no outlet; by springs and streams, especially hot springs and
desert streams; and by underground waters in the openings of rocks, such
as the interstices between grains, the cavities of porous igneous rocks,
especially amygdules, and in cave, fault, joint, and fissility openings. The
deposited calcite may replace a considerable number of other minerals. As
a deposit in the openings of rocks calcite is second in abundance only to
quartz. Calcite is an alteration product of a large number of minerals, of
which the following are the more common: Actinolite, ankerite, antho-
238 A TREATISE ON METAMORPHISM.
phyllite, aragonite, augite, diopside, dolomite, epidote, fluorite, garnet,
grossularite, gypsum, haiiynite, hornblende, noselite, parankerite, pyrope,
sahlite, scapolites, tremolite, and zoisite.
While the abundant direct sources of calcite are (1), (2), and (3)
above, the indirect and ultimate source which has probably furnished the
great quantity of calcium carbonate is the carboiiation of the silicates. (See
pp. 473-480.) This process occurs on a great scale in the zone of kata-
morphism, especially in the belt of weathering. It is a reaction which
takes place with liberation of heat and increase of volume in case the
replaced silica separates as quartz in situ. Many of the individual carbo-
nation reactions of the silicates, as, for instance, wollastouite, diopside, etc.,
are given under that class of minerals.
Alterations. The first of the alterations of calcite is recrystallization. Cal-
cite is the most mobile of the abundant rock-making minerals. It responds
readily to changes of physical conditions, and is very susceptible to weak
chemical agents. A slight stress may produce in it twinning structure. A
state of unequal strain favors its solubility. Where the pressure increases,
solution increases; where pressure is lessened, deposition takes place.
Increase of temperature greatly increases its solubility, and vice versa.
The increase of carbon dioxide in water greatly increases its solubility,
and vice versa. Thus it happens that in rocks where the calcite is almost
constantly subjected to changing pressure, temperature, and varying
amounts of carbon dioxide it is constantly being taken into solution
and, after a greater or less journey, being deposited from solution or
carried to the sea to be ultimately precipitated by organic agents. The
recrystallization of great masses of calcite, the solution of calcite in the
belt of weathering and its partial deposition in the belt of cementation, the
formation of caves, cave deposits, etc., are considered later.
The second important change of calcite is partial replacement of cal-
cium by magnesium, often producing dolomite (rhombohedral ; sp. gr.
2.8-2.9). The generalized reaction is:
(1) 2CaCO s -fMg=CaMg(CO,) 2 +Ca+k.
Supposing the calcium to be present as a carbonate, and supposing the
added magnesium to be a chloride and this is believed to be a very
common case the reaction would be:
(2) 2CaCO s +MgCl 2 =CaMgC.A+CaCl., ! +k.
ALTERATIONS OF CALCITE. 239
Or supposing that the magnesium salt is a carbonate, and that this is depos-
ited and an equivalent amount of calcium carbonate is taken into solution,
the reaction would be:
(3) 2CaCO 8 +MgCO 3 =CaMgO 2 O 6 +CaCO s +k.
Either of these changes is accompanied by the decrease in volume of 12.30
per cent if the original calcite be compared with the produced dolomite.
There might be no diminution in volume, or even an increase in volume,
in case less than the molecular weight of calcium salt equivalent to the
introduced magnesium was dissolved. For instance, in an extreme case
the reaction might be:
(4) CaCO s +MgCO s =CaMg(CO,) 2 +k,
the MgC0 3 being added through solutions, and no calcium carbonate
dissolved. In this case the expansion in volume over the original calcite
would be very great 75.41 per cent. However, the normal case in
dolomitization, as noted below, appears to be the molecular replacement
represented by the specific equations (2) and (3). The compounds
concerned in these reactions are so important that the heat relations have
been determined as above given; so it can be asserted positively, from
chemical studies, that heat is liberated by them.
The calcium of calcium carbonate may be replaced by other metals
besides magnesium, or calcite may be replaced by an oxide. The most
important of the elements which enter into such combinations, and the
only one which need be mentioned, is iron. At many localities, partly
or wholly occupying the place once held by calcite, iron carbonate is
found. For any definite proportion of iron replacing the calcium, equations
may be written paralleling those for the replacement of calcium by
magnesium.
The third important alteration of calcite is to wollastonite (monoclinic ;
sp. gr. 2.8-2.9). This alteration is, indeed, the chief source of wollastonite.
The equation is:
(5) CaCO 3 +SiO 2 =CaSiO 3 +CO 2 -k.
In the change the volume is decreased 31.48 per cent, provided the silica
used is a solid and the carbon dioxide escapes. In case the silicic acid be
brought in solution from an outside source, the volume of the solid is
increased 10.81 per cent. Between these extremes there are theoretically
240 A TREATISE ON METAMORPHISM.
all gradations, but, as noted below, an approach to the former extreme
probably is the common case.
Recrystallizatiou of calcite and dolomitizatioii take place on the most
extensive scale at all depths and under both mass-static and mass-dynamic
conditions; they are therefore alterations which are common to both
physical-chemical zones. By dolomitization it is believed that great masses
of calcite have been transformed to dolomite. The evidence of this trans-
formation and the detailed facts in connection with the change are given
under dolomite. (See pp. 798-808.) The fact that dolomite forms in both
zones would be sufficient evidence that the reactions producing this com-
pound liberate heat, even if this had not been experimentally determined
to be the fact, It has been pointed out, before (pp. 181-182) that the
formation of dolomite is a typical illustration of an alteration in which both
the volume and the chemical changes liberate heat, and which therefore
may occur in all zones and belts of the lithosphere.
The change from calcite to wollastonite occurs chiefly or wholly in
the very deep-seated rocks, especially in the zone of anamorphism. In this
zone, as noted (pp. 764-766), it can not be assumed that material is added
in considerable quantity from an outside source by circulating water; hence
in this zone silica for the change is believed to have been a solid. The
reaction is therefore one taking place with the absorption of heat and
condensation of volume. The silication of calcite to wollastonite in the
zone of anamorphism may be taken as a typical example of the heat and
volume change of silication of carbonates in that zone.
DOLOMITE.
occurrence. The chief source of dolomite is believed to be the dolomiti-
zation of calcite (see pp. 238-239), but dolomite is also a direct chemical
precipitate. Dolomite also forms in subordinate amount by the alteration
of ankerite. The ultimate source of the magnesium carbonate for the
dolomitization of the calcite is the magnesium liberated by the carboua-
tion of the silicates in the zone of katamorphism, especially in the belt
of weathering. The reactions for the decomposition of some of the
simple silicates, such as diopside and tremolite, are given under those
minerals. The magnesium for the dolomitization need not be directly
derived from a silicate, but may be from the solutions of the sea or from
ALTERATIONS OF DOLOMITE. 241
a previously formed magnesium limestone or dolomite which is in the
belt of weathering. Dolomite produced by the carbonation of the silicates
or by solution of dolomitio formations is an important chemical precipitate
in caves and small crevices in the rocks, the same as calcite.
In summary, dolomite is chiefly derived as a secondary mineral from
ankerite, calcite, and parankerite.
Alterations. An important alteration of dolomite is to diopside (mono-
clinic; sp. gr. 3.2-3.38). This alteration is a typical example of silication.
(See p. 205.) The most probable reaction is:
( 1 ) MgCaC 2 O 6 +2Si0 2 = MgCaSi 2 O 6 +2CO 2 - k.
The decrease in volume is 40.11 per cent, provided all of the silica entering
into the combination was a solid. In case all of the silica were introduced
through water solutions there would be an increase in volume of 2.03 per
cent. More important alterations of dolomite are into trernolite (mono-
clinic; sp. gr. 2.9-3.1) and calcite (rhombohedral ; sp. gr. 2.713-2.714),
or into tremolite and wollastonite (monoclinic; sp. gr. 2.8-2.9). In the first
case the reaction is:
(2) 3CaMgC 2 O 6 +4SiO 2 =Mg 3 CaSi 4 O 1 2+2CaCO 3 +4CO.,-k.
The decrease in volume, provided the silica is present as a solid, the
calcite remains as a solid, and the carbon dioxide escapes, is 25.20 per cent.
However, the excess of calcium carbonate may simultaneously change to
wollastonite. In this case the reaction would be:
(3) 3CaMgC 2 O 6 +6SiO 2 =Mg 3 CaSi 4 O 12 +2CaSiO 3 +6CO 2 --k.
The decrease in volume as compared with the dolomite and quartz of the
tremolite and wollastonite is 33.09 per cent. In both of the changes, if a
portion of the silica be supposed to be introduced from an outside source
the decrease in volume would be lessened, and if all of it were thus sup-
posed to be introduced there would be an increase in volume from the
solid dolomite of 9.89 per cent in the case of tremolite and calcite, and 14
per cent in the case of tremolite and wollastonite.
The space once occupied by dolomite, like that occupied by calcite,
may be taken by other carbonates or by various oxides. The most impor-
tant of these are carbonate of iron and oxide of iron. The carbonate may
be a replacement, or possibly a substitution, of the iron of some other iron
MON XLVII 04 16
242 A TREATISE ON METAMORPHISM.
salt for that of the calcium and magnesium. The oxide of iron is an illus-
tration of a pure replacement, not of an alteration.
The formation of diopside, tremolite, and wollastonite is known to
occur in deep-seated rocks, and especially in connection with mass-
mechanical action where the rocks are deformed by flowage. As
repeatedly noted, in the zone of anamorphism the circulation of water is
reduced to a minimum; and it can not be supposed that important addi-
tions are made from the outside, and therefore the silica must be supposed
to have been previously present in the rocks. Indeed, we know that silica
usually accompanies deposits of calcite and dolomite ; hence I conclude that
the reactions take place with substantially the decrease in volume above
assigned to the changes. In the reactions heat is absorbed. The changes
are therefore again typical illustrations of silication in the lower physical-
chemical zone.
ANKERITE AND PARANKERITE.
occurrence. All the compounds from normal ankerite and parankerite to
the extremes of composition given above (p. 237) are included under the
general term ferro-dolomite, which I have elsewhere used as covering all
the ferriferous compounds standing between dolomite on the one side and
siderite on the other. (See p. 823.)
The sources of ankerite and parankerite are the same as siderite,
with the difference that at the time of the formation of the iron carbonate,
calcium and magnesium carbonate are present, or formed, and unite with it.
Alterations. The more common alterations of ankerite and parankerite
are to limonite (amorphous; sp. gr. 3.80), hematite (rhombohedral ; sp.
gr. 5.225), and magnetite (isometric; sp. gr. 5.174), the calcium and
magnesium carbonates either separating or simultaneously undergoing the
alterations given under "Calcite" and "Dolomite." Equations may easily
be written for any definite compound by which the iron carbonate passes
into the minerals mentioned in the same way that siderite does and the
calcium-magnesium carbonates separate. The volume changes are in the
same direction, and the physical conditions under which aukerite and par-
ankerite alter to limonite, hematite, and magnetite are the same as those
for the alteration of siderite to the like compounds. Therefore the equa-
tions and summary of physical conditions will not be here repeated.
Other important alterations of the ferro-dolomites are to sahlite (mono-
ALTERATIONS OF ANKERITE AND PARANKERITE. 243
clinic; sp. gr. 3.253.4) and to actinolite (rnonoclinic; sp. gr. 3.00-3.20).
Supposing that the magnesium and iron are present in equal quantity in
the sahlite, the reaction in the case of normal ankerite is :
( 1 ) CaFeCA.CaMgCA+4SiO 2 =AIgFeCa 2 Si 4 O 12 +4CO 2 -k.
Supposing the silica to be present as a solid, the decrease in volume is
37.27 per cent. In the formation of actinolite from normal aukerite, on
the supposition that the iron and magnesium are present in equal quantity
in the actinolite, the reaction is:
(2) SCaFeCA.CaMgCA+SSiO^MgsFesCa^SiAi+^CaCOs+SCOj-k.
The decrease in volume, supposing the silica to be present as a solid and
the CaC0 3 as a solid, is '22.Q2 per cent. Of course, if the ferro-dolomite
were one in which the calcium carbonate is not so plentiful, being replaced
in equal molecular parts by magnesium and iron, it would not be necessary
for any calcium carbonate to form as a result of the reaction. For instance,
if the ferro-dolomite were CaFeaC^D^.CaMgsC^a the reaction would be as
follows :
( 3) CaFe s C A,. CaMgAO,., + 8SiO 2 = Mg 3 Fe s Ca,Si 8 O 24 + 8CO 2 - k .
Using the specific gravity of normal ankerite, the decrease of volume of
the actinolite as compared with the ankerite and quartz is 32.72.
Sahlite and actinolite are both known to form abundantly in the zone
of anamorphism. Sahlite is found in the marbles of eastern United States.
Actinolite is very abundant iir the iron-bearing formations of the Lake
Superior region. The development of these silicates may be taken as
typical illustrations of the reaction of silication in the lower physical-
chemical zone, with condensation of volume and absorption of heat.
MAONESITE.
occurrence. Magiiesite may be a product of the alteration of any of the
heavily magnesian rocks. It is especially prevalent in the olivinitic
rocks and the chloritic, serpentinous, and talcose schists and gneisses,
being a product which is produced .by the alteration of original minerals
simultaneously with the formation of chlorite, serpentine, and talc. It
is also found in dolomite. The more important minerals from which it is
recorded as forming are common garnet, olivine, pyrope, and serpentine.
Alterations. No alterations are recorded for magnesite. There is, how-
ever, no doubt that this compound does break up in the zone of aiiamor-
244 A TREATISE ON METAMORPHISM.
ism, the carbon dioxide being liberated and the magnesia being furnished
for the formation of various dense magnesian minerals, such as enstatite,
tremolite, olivine, pyrope, etc. These changes would involve a diminution
of volume and an absorption of heat.
occurrence. The chief source of siderite is believed to be the reduction,
dehydration, and carbonation of limonite. (See pp. 233-234.) This change
is one occurring with the liberation of heat if the reaction upon the organic
compound be taken into account, and increase of volume. A subordinate
amount of siderite is also derived from magnetite. This change takes
place with liberation of heat and increase of volume. Siderite also forms
from ankerite and parankerite, arfvedsouite, garnet, hematite, hornblende,
hydrous ferrous silicate, limonite, magnetite, and olivine, and replaces
calcite and dolomite.
Alterations. The important alterations of siderite are into limonite
(amorphous; sp, gr. 3.6-4.0), hematite (hexagonal-rhombohedral ; sp. gr.
5.225), magnetite (isometric; sp. gr. 5.168-5.18), and griinerite (monoclinic;
sp. gr. 3.713). The reactions are as follows:
(1) 4FeCO s +2O+3H J 0=2Fe,O s .3H 2 0+4CO. ! +k.
The decrease in volume is 18.22 per cent.
(2) 2FeCO,+O=Fe 2 O s +2CO a rk.
The decrease in volume is 49.11 per cent.
(3) 3FeCO s +0=Fe,O 4 +3C0 2 +k.
Very often iron sulphide, as pyrite (isometric; sp. gr. 5.025) or
marcasite (orthorhombic ; sp. gr. 4.875), unites with the siderite to form
magnetite. This reaction is probably of great consequence in forming the
heavy beds of magnetite. (See p. 845.) It may be written:
(4) 2FeCO s +FeS 2 +2H a O=Fe,0 4 +2H 2 S42C0 2 -k.
The decrease in volume for the siderite alone to the magnetite, equation
(3), is 50 32 per cent; for siderite and pyrite, 46.67 per cent; for siderite
and marcasite, 47.135 per cent.
(5) FeCO s +SiO,+nH 2 0=FeSi0 8 +C0 2 +nH 2 0-k.
The decrease in volume, regarding the silica as a solid, is 32 53 per cent.
The alteration to limouite occurs in the zone of katamorphism, especially
OCCURRENCE AND ALTERATIONS OF ARAGONITE. 245
in the belt of weathering. The alteration to hematite occurs as a somewhat
deeper seated change, usually in the belt of cementation of the zone of
katamorphism. The alteration to magnetite is especially characteristic of
the zone of anamorphism, but it can not be asserted not to take place in
the belt of cementation. The alteration to griinerite occurs under deep-
seated conditions, and is in its heat and volume relations a characteristic
reaction of the lower zone. Magnetite and griinerite often form simul-
taneously. (See p. 284 ) The series of changes from siderite are very
interesting, in that the volume changes are all diminutions, and therefore,
so far as this factor is concerned, might take place in either zone. The first
three reactions (equations 1, 2, and 3) liberate heat, and hence these reac-
tions in their physical -chemical relations are similar to those of dolomite,
discussed on pages 182, 240, and may take place in both zones. But the
reaction of equation (4) probably absorbs heat, and that of (5) certainly does.
Magnetite having the origin represented by equation (4) is probably, and
griinerite is certainly, confined to the zone of anamorphism, where pressure
is a controlling factor.
ARAGONITE GROUP.
The only important rock-making member of this group is aragonite.
AIUtiOMTE.
Aragonite:
CaCO 3 .
Orthorhombic.
Sp. gr. 2.93-2.95.
occurrence. A chief source of aragonite is as an organic precipitate.
It occurs intimately associated with calcite in numerous marine shells.
While abundant, it is very subordinate to calcite as an organic deposit. A
second abundant source of aragonite is as a chemical precipitate, frequently
in association with beds of iron carbonate and gypsum. It also occurs as
a chemical precipitate from ground-water solutions, in openings in rocks,
especially at places where the temperature of the solutions is from 30 to
100 C. or more. Aragonite is not mentioned as an alteration product
of other minerals.
Alterations. The chief change of aragonite is to calcite (rhombohedral ;
sp. gr. 2.7132.714). This is a change involving recrystallization, increase
of symmetry, and lowering of specific gravity. The increase in volume is
8.35. per cent. The heat effect of the change has not been found; but it
246 A TREATISE ON METAMOKPHISM.
seems probable that heat is liberated, for the transformation of aragonite to
calcite occurs in both the physical-chemical zones, and I know of no excep-
tion to the principle that such reactions take place under the first part of
van't Hoff's law (see pp. 107, 181).
The change from aragonite to calcite is so complete in rocks of mod-
erate age that the presence of aragonite in the metamorphosed rocks is
almost unknown. The alteration of aragonite to calcite in both zones is of
considerable interest, as it presents a somewhat exceptional case. As
explained on pages 182-186, the common rule of change in the zone of
anamorphism is increase in specific gravity and increase of symmetry,
provided the volume change demanded will allow this. However, the
change of specific gravity in this case is a decrease rather than an increase,
and hence aragonite conforms only to the second of these rules the first,
and usually the controlling rule, for the zone of anamorphism being
violated. These facts suggest the conclusion that in this instance sym-
metry is a more important factor than density a very exceptional thing.
If this be so, the conclusion would follow that the symmetrical arrange-
ment of the molecules in calcite are those which best resist the changing
conditions of mass-static and mass-mechanical action in the lower zone
The suggestion occurs to one that, if rocks were very deeply buried, so as
to be extraordinarily deep in the lithosphere, pressure might control the
form, and calcite alter to aragonite. This, however, is a speculation which
has no verification.
THE SILICATES.
The silicates are the most important of rock-making constituents.
They include natural glass and many mineral groups. The groups of
rock-making silicates are as follows: Feldspar, leucite, pyroxene, amphi-
bole, nephelite, sodalite, garnet, chrysolite, scapolite, zircon, aluminum-
silicate, epidote, humite, zeolite, mica, clintonite, chlorite, serpentine-talc,
and kaolin. Besides the members of the above groups are a number of
important rock-making silicates not so included.
GLASS.
Glass, while not a definite silicate or ordinarily included among the
specific minerals, is an important rock- making constituent, and therefore must
be treated in connection with the silicates in a treatise on metamorphism.
DEVITRIFICATION OF GLASS. 247
occurrence. Natural glass is an abundant constituent of the effusive rocks.
It is especially prevalent in the more acid ones, but is not confined to them,
being not infrequently abundant in the intermediate rocks, such as basalts.
A lava or tuff may be almost wholly composed ot glass, or glass may con-
stitute but a small part of the background. There are thus all gradations
between completely crystalline rocks and glassy rocks. Of the more recent
effusive rocks glass not infrequently composes a large part of the flows.
An instance is Obsidian Cliff, in the Yellowstone National Park. But in
proportion as lavas are old, glass is less and less likely to be found, and in
the more ancient lavas is ordinarily absent. The explanation of this
absence is devitrification after solidification.
Evidence that devitrification takes place. That Certain 1'Ocks HOW wholly COmpOSed
of minerals were once glasses is shown by the preservation in perfection of
the flow structures and very delicate trichitic, perlitic, spherulitic, and other
textures characteristic of glass.
scale of devitrification. It is also certain that the process of devitrification
has taken place in nature on a great scale. As evidence of this may be
cited the well-known American instances of devitrified glass in the original
Huronian district, described by Williams," the aporhyolite of South Moun-
tain, Pennsylvania, described by Williams and Bascom,* the metarhyolites
of the Fox River Valley of Wisconsin, described by Weidman," and the
devitrified glasses of the Crystal Falls district of Michigan, described by
Clements. rf In the papers of these authors many other instances of
devitrification are cited, including European instances.
Not only does devitrification of natural glass take place, but under
proper conditions artifical glass devitrifies in a similar manner. Well-
known cases of the devitrification of artificial glass under conditions of
weathering are those of the buried ancient glasses of Nineveh and of Rome.
"Williams, G. H., Notes on the microscopical characters of rocks from the Sudbury mining
district, Canada: Ann. Kept. Geol. and Nat. Hist. Survey of Canada, vol. 5, Pt. F, Appendix 1, 1890-
1891, pp. 74-82.
* Williams, G. H., The volcanic rocks of South Mountain, in Pennsylvania and Maryland: Am.
Jour. Sci., 3d ser., vol. 44, 1892, pp. 486-490.
.Bascom, Miss Florence, The ancient volcanic rocks of South Mountain, Pennsylvania: Bull. U. S.
Geol. Survey No. 136, 1896, pp. 42-61.
Weidman, Samuel, A contribution to the geology of the pre-Cambrian igneous rocks of the Fox
River Valley, Wisconsin: Bull. Wisconsin Geol. and Nat. Hist. Survey No. 3, 1898, pp. 4-31.
^Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan:
Mon. U. S. Geol. Surv., vol. 36, 1899, pp. 87, 101-103, 138.
248 A TREATISE ON METAMORPHISM.
In glass found in the lake at Walton Hall, near Wakefield, Bingley found
that the alkalies had been wholly removed by decay. Another case of
devitrification largely clue to original state of strain is the glass of certain
old buildings, such as cathedrals. A well-known instance is that of St.
Andrew's Chapter House. 6
Rate of devitrification. The rate of devitrification of glass depends, among
other things, upon (1) composition, (2) strain or lack of strain, (3) pressure,
(4) mass-mechanical action, (5) temperature, (6) moisture.
In any given case of devitrification several and sometimes all of these
factors enter, and hence it is impossible to discriminate the effect of each.
Very often devitrification has been described as hydro-metamorphism, but
by this no more can be meant than that water is usually an important
factor in the process.
(1) The rate of devitrification of glass increases with its basicity. This
follows from the ready solubility of basic glasses. It has also been deter-
mined that glasses rich in soda devitrify faster than those rich in potash.
This corresponds with the fact emphasized in another place (see p. 516)
that minerals rich in soda are more readily decomposed than those rich
in potash.
(2) It is shown in another place that a state of strain in minerals
promotes alteration. (See pp. 95-98.) The same is true of glass. It is
definitely known that unannealed glass, which therefore cooled irregularly
and is in a state of strain, independently of pressure or movement may
partly devitrify in a few years. For instance, drawn-glass tubing, such as
is used in the chemical laboratory, if kept for a few years may devitrify so
as to become useless. Another well-known case of devitrification probably
due to strain is the glass of certain cathedral windows. As large masses
of glass cool under natural conditions, they must often be almost at the
extreme of the unannealed condition, and therefore in a high state of
strain. So far as glass is in this condition, even without reference to any
extraneous pressure or movement, there is a marked tendency toward
devitrification. The stage of the process due to this cause is dependent
upon the amount of strain and the time.
Bingley, C. W., On the peculiar action of mud and water on glass, as more especially illustrated
by some specimens of glass found in the lake at Walton Hall, near Wakefield: Rept. Twenty-eighth
Meeting British Assoc. Adv. Sci., London, 1859, pp. 45-46.
^Brewster, Sir David, On the decomposition of glass: Rept. Tenth Meeting British Assoc. Adv.
Sci., London, 1841, pp. 5-7.
DEVITRIFICATION OF GLASS. 249
(3) Pressure produces a state of unequal strain, and hence is favorable
to devitrification.
(4) Mass-mechanical action not only produces a state of unequal
strain in minerals, but fractures the material, and this gives a large surface
of action for the solutions. It is therefore clear that mass-mechanical action
is very favorable to devitrification.
(5) Experiments in the laboratory show that if glass be raised to a
temperature short of fusion the tendency to devitrification is greatly
promoted. It is therefore certain that conditions of dry heat after solidifi-
cation are favorable to devitrification. As glass occurs in considerable
bodies in a state of nature, it must for a long time, perhaps hundreds ot
thousands of years, have a high temperature due to the residual heat of
the mag-ma, and only very gradually assumes the normal temperature
corresponding with its depth of burial. It is rather probable that micro-
lites and crystallites, which so frequently occur in glass, largely form
during this process of cooling after solidification.
(6) While devitrification of glass may occur without the presence of
abundant water, it is probably rare indeed that in nature the process occurs
without the presence of some moisture, and in general moisture is a very
important factor favorable to devitrification.
It is therefore clear that each of the above factors may give a condi-
tion favorable to devitrification, but in general actual devitrification is
due to a combination of two or more of them.
Devitrification in the two zones. In the zone of katamorphisiii under ordinary
conditions it is probable that the devitrification occurs somewhat slowly.
But in areas of regional volcanism, and often in those of local volcanism,
the lava flows follow one another in such rapid 'succession that beds are
piled up so deep that the water is held at a high temperature. By complex
intrusion the entire mass of a cooled glass may again be raised to a high
temperature. Orogenic movement if severe may produce a high tempera-
ture. Under any of these circumstances the conditions are furnished for
the complete and rapid devitrification of the glass.
The nature of the devitrification is certainly different in the belt of
weathering and the belt of cementation, although available descriptions do
not furnish data for accurate statements as to the differences. But it is
certain that in the belt of weathering the sevei-al changes are along the
250 A TREATISE ON METAMORPHISM.
lines given on pages 506-527 for that belt, finally resulting in the oblitera-
tion of textures and structures and producing an incoherent rock.
In the belt of cementation ordinarily the alterations do not result in
the obliteration of the original textures and structures of the glasses. This
is sufficiently evident where the alterations occur under mass-static condi-
tions, and even where mass-mechanical conditions prevail. The glass is
simply fractured, as explained on pages 601-602, and the individual blocks
are altered by metasomatism under mass-static conditions.
So far as we know, glasses originally form only in the zone ot
katamorphism, and mainly at or near the surface. Therefore a glass can
get into the zone of anamorphism only by being buried under succeeding
lava flows or tuffs or under sedimentar3 T rocks. Hence, before glass
reaches the lower zone, it must have been subjected for a long time to
devitrification in the belt of cementation, and the question arises whether
or not a glass would not be completely devitrified before it becomes
sufficiently deeply buried to reach the zone of anamorphism. However, if
glass ever does reach the lower zone, it is certain that its devitrification will
take place rapidly under either mass-static or mass-mechanical conditions.
The rocks in this lower zone are everywhere at temperatures exceeding
100 C; they contain water; hence, even under conditions of absolute
quiescence, it is certain that glass could not long exist. The crystallization
would be even more rapid under mass-mechanical conditions.
In so far as the glass had devitrified in the zone of katamorphism, and
had produced minerals characteristic of that zone, in the lower zone these
minerals would be recrystallized and minerals formed characteristic of the
latter zone. If mass-static conditions prevail this recrystallization may take
place without obliterating previous textures and structures. However, if
recrystallization takes place under conditions of mashing, the original
textures and structures are lost, and minerals are produced of such kinds
and proportions as correspond with the composition of the glass. More-
over, when the glass passes into the zone of anamorphism, textures and
structures may be formed characteristic of the slates, schists, and gneisses.
When such alteration is complete it is often impracticable to determine
whether the rock was originally glass or not. There can be little doubt
that many of the finer-grained schists are derived from rocks which were
DEVITRIFICATION OF GLASS. 251
originally partly or wholly glassy. For instance, the Berlin gneiss of
central Wisconsin is in chemical composition the same as that of various
associated aporhyolites. The aporhyolites show that they were originally
glasses by retaining the characteristic textures of glass. The Berlin gneiss
which was altered under conditions of mashing in the deep-seated zone is
entirely devoid of any .structure which can be attributed to glass, and one
can not be certain that it did originally have a glassy base, although this
seems probable.
Minerals produced. The minerals which are produced by the alterations of
glass are very numerous. It has already been noted that glasses form
from the most acid magmas, and also from those which are intermediate or
basic in character. Furthermore, it has been seen that glass is devitrified
in both the upper and the lower physical-chemical zones, and in the upper
zone both in the belt of weathering and in that of cementation. In each of
these zones and belts minerals form from the glass which are characteristic
of them. It is plain from the foregoing that every mineral which may be
a metamorphic product of an igneous rock of any kind may result from the
devitrification of glasses of different kinds under the different conditions
which obtain in the zones and belts of alteration.
Heat and volume relations. The devitrification of glass is a process which
probably results in the liberation of heat. This is certainly true for the
zone of katamorphism, where oxidation, hydration, and carbonation take
place. As to the volume relations of the change, the devitrification itself by
means of which the substance passes from an amorphous to a crystalline
condition would decrease the volume, provided there were no additions of
other compounds. But where devitrification is accompanied by oxidation,
carbonation, and hydration there are considerable additions of material.
Therefore, in the belt of cementation there can be little doubt that
expansion of volume is the rule where glasses are devitrified; but in the
belt of weathering, where solution is prominent, doubtless there is diminution
in volume with glass as with other compounds. In the zone of anamorphism
devitrification takes place with decrease of volume, the reactions being
controlled by pressure. Whether heat be liberated or absorbed in the zone
of anamorphism doubtless depends in large measure upon how far the
reactions of the zone of katamorphism have taken place during the time
252 A TREATISE ON METAMORPHISM.
the glass was passing through that zone. If these had gone far, the undoing
of the oxidation, hydration, and carbonation would probably absorb heat.
But if the glass reached the zone of anamorphism in an anhydrous condition,
the crystallization, producing a decrease in volume, would liberate heat.
Thus no general statement can be made as to the heat reaction in the zone
of anamorphism.
FELDSPAR OROUP.
The minerals of the feldspar group are the most abundant of the
silicates. According to Clarke," the feldspars comprise 60 per cent of the
minerals of the lithosphere. The feldspars include minerals of two classes
of symmetry, monoclinic or pseudomonoclinic, and triclinic. Those of the
first class comprise orthoclase, microcline, and anorthoclase ; those of the
second class include albite, oligoclase, andesine, labradorite, bytownite,
and anorthite.
In chemical composition the feldspars vary from orthosilicates, through
metasilicates, to polysilicates. The readiness of decomposition is indirectly
proportional to the acidity, the orthosilicates being the most easily decom-
posed, and the polysilicates being the most difficult to decompose.
The more frequent alterations of the monoclinic feldspars and of the
polysilicate plagioclase feldspars are to mica, especially muscovite, and to
hydrated silicate of aluminum, especially kaolin. In this alteration there
is simultaneous liberation of silica, which may separate as quartz. Very
frequently also gibbsite is formed at the same time. Where the mica biotite
is produced it is necessary that iron and magnesium shall be added. The
most common alterations of the orthosilicate plagioclase feldspars are to
zeolites, epidote, and zoisite, frequently with the simultaneous formation of
another plagioclase and chlorite. Where epidote is produced it is necessary
that iron be added from some other source; where chlorite is produced it is
necessary that magnesium and iron be added from some other source.
All the important minerals produced by the alterations of the feldspars,
with the exception of quartz and plagioclase, are hydrated, though in
varying degrees; hence, in general, water is added during the alteration of
the feldspars. From the intermediate plagioclases there may be produced
any of the foregoing minerals
"Clarke, F. W., Analyses of rocks, laboratory of the TJ. S. Geol. Survey, 1880-1899: Bull. U. S.
i ;..,,!. Survey No. 168, 1900, p. 16.
ORTHOCLASE AND MICROCLINE. 253
MOXOCL1SIC OK I'SKI'DOMONOCLIXK'.
HIM HIM I. isl.. 1111 1:01 I.I \l. AND AXOBTHOCLASE.
Orthoclase :
KA18i,O 8 .
Monoclinic.
Sp. gr. 2.57.
Microcline:
KAlSi 3 8 .
Triclinic.
Sp. gr. 2.54-2.57.
Anorihodase:
NaA],Si 3 O 8 .nKAl,,Si 3 O 8 . (Na-silicate:K-si1ioate:: 2:1 or 3:1, usually.)
Pseudornonoclinic or triclinic.
Sp. gr. 2.57-2.60.
ORTHOCLASE ANJ) MICROCLINK.
occurrence. Orthoclase and microcline have a very widespread occur-
rence as chief pyrogenic constituents. The minerals also are allogenic
constituents of the clastic rocks. They further have a very widespread
occurrence in the metamorphic rocks, being chief constituents both as
allogenic and as autogenic constituents of the schists and gneisses of both
aqueous and igneous origin. In the development of the feldspars as
autogenic constituents it is usually necessary that two or more minerals
unite, except in the case of the derivation of the acid feldspars from the
more basic ones or from leucite. As a metamorphic mineral orthoclase is
derived from analcite, heulandite, leucite, laumontite, and stilbite. Micro-
cline is recorded as derived from spodumene.
Alterations. One of the most important alterations of orthoclase and
microcline is to kaolinite (monoclinic; sp. gr. 2.60-2.63). The most prob-
able reaction, for reasons given below, is believed to be:
(1) 2KAlSisO 8 +2H 2 O+CO 2 =H 4 Al 2 Si 2 O 9 +4SiO 2 +K 2 CO 3 +k.
The decrease in volume, supposing the freed silica to separate as quartz,
and K 2 CO 3 dissolved, is 12.57 per ce.nt. If all of the freed silica be dis-
solved, the decrease in volume would be 54 44 per cent. In calculating
these volume changes and those which follow, the specific gravity of
orthoclase is used.
While the ordinary alteration of the potash feldspars to the kaolin
group is to kaolinite as indicated, the alteration may be to other minerals
of this group; for instance, to pyrophyllite (monoclinic (?); sp. gr. 2.8-2.9),
halloysite (massive; sp. gr. 2.1), newtonite (rhombohedral ; sp. gr. 2.37),
L>;>4 A TREATISE ON METAMORPHISM.
cimolite (amorphous; sp. gr. 2.24), nllophane (amorphous; sp. gr. 1.87), and
perhaps others. The chief differences are in the amounts of water added
and the amount of silica which separates. Pyrophvllite (H 2 Al 2 (SiO a ) 4 )
differs from kaolinite in that less silica is removed and less basic water is
added; it therefore might be considered as an intermediate stage in the
alteration. Halloysite (H 4 Al 2 Si 2 O 9 Aq.) differs from kaolinite only in having
water of hydration. Newtonite (H g Al 2 Si 2 O n .Aq.) differs from kaolinite in
containing twice as much basic water as that mineral, and in being hydrated.
Cimolite (H 6 Al 4 (SiO 3 ) 9 .3H 2 O) differs from kaolinite in containing more
silica, more basic water, and water of hydration. Allophane (Al 2 Si0 5 .5H 2 O)
differs from kaolinite in containing less silica and much water. It would
be easily possible to formulate equations along the line of that given for
kaolinite for each of these minerals and to calculate the volume relations.
However, this hardly seems necessary since these minerals as secondary
products to orthoclase and microcline appear to be Very subordinate in
amount.
Another alteration of orthoclase and microcline of some little impor-
tance is into gibbsite (monoclinic; sp. gr. 2.3-2.4). The reaction is:
(2) 2KAlSi s O 8 4-3H 2 O+CO 2 =2Al(OH) 8 +6Si0 2 +K 2 CO 3 +k.
The decrease in volume of the gibbsite and quartz as compared with the
orthoclase is 6.61 per cent.
Another of the very important alterations of orthoclase and microcline
is to muscovite (monoclinic; sp. gr. 2.76-3.0) and quartz (rhomboheclral;
sp. gr. 2.653-2.654). The reaction is:
(3) SKAlSijOj+HjO+CO^KHjAlsSiAi+SSiOj+KjCOs+k.
Provided the silica separates as quartz and the potassium unites with
carbonic acid and the potassium carbonate be removed in solution, the
decrease in volume is 15.58 per cent.
While this reaction may take place under exceptional conditions, it is
believed, as explained below, that where muscovite forms from orthoclase
one of the rich aluminous minerals often unites with the orthoclase to
produce the mica. Supposing the aluminous mineral to be gibbsite, the
reaction is :
(4) KAlSi,0 8 +2Al(OH) s =KH 2 A1 s Si,0 1 . ! +2H 2 0-k.
The decrease in volume of the muscovite as compared with the orthoclase
and gibbsite is 20.81 per cent.
'ALTERATIONS OF ORTHOCLASE AND MICROCLINE. 255
The alteration may be to hydro-muscovite or damourite (monoclinic;
sp. gr. 2.76-3.00). This is believed by most mineralogists to differ from
muscovite only in containing more water, but Dana states that a greater
content of water in damourite than that contained by ordinary muscovite
is not necessary.
From orthoclase and microcline, with the addition of magnesium and
iron compounds, biotite (monoclinic; sp. gr. 2.7-3.1) may be formed. If
the hydrogen and potassium be supposed to be present in equal proportions
and the same supposition be made with reference to magnesium and iron,
and the latter elements are supposed to be present as carbonates, the
reaction may be as follows :
(5) 4KAlSi 3 O 8 +2MgCO 3 +2FeCO 3 +H 2 O==
2H KMgFe A ] 2 Si 3 O 12 + 5SiO 2 + K 2 SiO 3 +4CO 2 +k.
The decrease in volume of the feldspar, magnesium carbonate, and iron
carbonate in passing into the biotite and quartz is 22.64 per cent. But, as
with muscovite, the more frequent reaction probably involves gibbsite,
thus:
(6) KAlSi 3 8 fMgCO 3 +FeCO 3 +Al(OH) 3 =HKMgFeAl 2 Si 3 O 12 +H 2 O+2CO 2 +k.
This greatly simplifies the equation. The decrease in volume of the bio-
tite as compared with the compounds from which it is derived is 22.33
per cent.
Orthoclase and microcline are said also to alter to epidote (monoclinic;
sp. gr. 3.25-3.50); but if this be so calcium and iron must be introduced.
The forms in which these compounds are present during the alteration are
doubtless variable. If they be assumed to be present as calcium carbonate
and iron sesquioxide, the reaction might be as follows:
(7) 4KAlSi 3 O 8 +Fe 2 3 +4CaCO 3 +H. ( O=2HCa 2 Al. ! FeSi 3 0, 3 +6SiO 2 +2K.,CO 3 +2CO 2 +k.
Supposing the Al is to the Fe as 2 is to 1, the decrease in volume of the
epidote and quartz as compared with the feldspar, calcite, and iron oxide
together is 33.73 per cent. However it is so uncertain as to the forms of
the accessory compounds, both before and after reaction, that it is impos-
sible to make a definite statement as to the volume relations.
The alteration of orthoclase and microcline to minerals of the kaolin
group and to gibbsite occurs in the zone of katamorphism. The process
takes place on the most extensive scale in the belt of weathering, especially
256 A TREATISE ON METAMORPHISM.
in the soil horizon. Wherever the feldspathic rocks are exposed to atmos-
pheric agencies this change steadily goes on, though not so rapidly as with
the orthosilicate feldspars. (See p. 519.) But wherever the potash feld-
spars have been very long exposed to the weathering agencies they have
been partly or wholly decomposed, and in some places to a depth of several
hundred feet. The change is one of the most important of all those which
affect rocks. It is partly because the alterations take place near the
surface, where carbon dioxide is abundant, that it is believed that the freed
alkali largely unites with carbon dioxide, as given in the reaction. The
silica freed in the belt of weathering is in part undoubtedly taken into
solution as colloidal silicic acid and carried downward to the belt of
cementation. Indeed, the silica for the process of silicification in this belt,
which, as explained on page 480, is derived from the decomposition of the
silicates, probably in good part comes from the alteration of the feldspars.
Under the same conditions in which a part of the feldspar breaks up into
kaolinite another part of the feldspar may produce gibbsite, quartz, and
potassium carbonate. The potassium carbonate liberated at the time of
the formation of the kaolinite and gibbsite is largely dissolved and trans-
ported elsewhere, although the soluble potassium compounds are often held
in the soil to a considerable extent. (See pp. 498, 541543.)
The alteration of orthoclase and microcline to minerals of the kaolin
group and to gibbsite is not, however, confined to the belt of weathering.
It takes place on an important scale in the belt of cementation, though not
on a scale comparable to that in the belt above. So far as known, kaolin-
ization is not a reaction which occurs in the zone of anamorphism; at least,
if it does there take place it is a very subordinate phenomenon. As seen
above, the reaction is one taking place with liberation of heat and fre-
quently with decrease of volume, since much and perhaps the most of the
freed silica is taken away in solution. The heat reaction controls, and
hence the change is under the rules of the upper physical-chemical zone.
The alteration of orthoclase and microcliue to mica occurs in rocks
which have been somewhat deeply buried, and the change has been noted
in connection with both mass-static and mass-mechanical action. Under
either of these conditions the alteration may be nearly or quite complete.
But it has taken place on the most extensive, scale in connection with mass-
mechanical action, where the secondary structures, such as cleavage, are
OCCURRENCE OF ANORTHOCLASE. 257
produced, and therefore in the belt of rock flowage. In the formation of
muscovite and quartz from feldspar by equation (3), as the specific gravity
of the separated quartz is somewhat greater than that of the original
feldspar, and that of the muscovite is considerably greater than that of the
feldspar, the condensation in volume above calculated is accounted for,
although the alteration is one involving hydration and possibly carbonation.
In the zone of anamorphism the wateradded is doubtless largely derived from
other minerals, as this is a belt of dehydration, and destruction of previous
minerals containing hydroxides. This passage from one mineral to another
would involve no increase in the total volume, the controlling consideration.
The most doubtful point concerning equation (3) is the carbonation of the
potassium. It might be supposed that the potassium unites with a part of
the freed silica and with other elements to form potassium minerals. But it
is not easy to suggest such minerals, as leucite is not recorded as a meta-
morphic mineral. The more probable solution of the problem is that
potassium and a portion of the silica unite with the alumina of the gibbsite
or some other minerals and produce one molecule of mica from one of
orthoclase, as suggested in equation (4). This suggestion is rendered
especially plausible for the slates, schists, and gneisses derived from sedi-
ments, for such rocks usually contain residual orthoclase and also aluminum
hydroxide. (See pp. 232, 235, 898-900.) The reaction of equation (4) pro-
duces great decrease in volume, is one of dehydration, and thus absorbs heat;
it is therefore a perfect example of the rules of the zone of anamorphism.
The same remarks are applicable to equations (5) and (6), respectively, for
the production of biotite, as to (3) and (4) for the formation of muscovite,
with the addition that the development of biotite involves silication and
decarbonation, and therefore still better than muscovite illustrates the
reactions of the zone of anamorphism.
The physical-chemical principles for the alteration of orthoclase and
microcline to epidote are the same as for the alterations of the more basic
feldspars to epidote. As the process occurs much more extensively in con-
nection with the latter minerals, it is discussed under the basic plagioclases.
(See pp. 263-264.)
ANORTHOCLASE.
occurrence This mineral is subordinate in quantity to orthoclase and
microcline. It occurs in both deep-seated and effusive igneous rocks; in
MON XLVII 04 17
258 A TREATISE ON METAMORPHISM.
the latter, chiefly in the andesitic lavas. As an allogenic mineral it also
is found in the sedimentary rocks. Whether it occurs as an autogenic
mineral in the inetamorphic rocks has not been determined.
Alterations. Both orthoclase and microclme contain some sodium. When
the sodium becomes important the mineral is anorthoclase. It naturally
follows from this fact that the alterations of anorthoclase are in all respects
like those of orthoclase and microclme, with the exception that the freed
alkalies are in good part sodium. The reactions are analogous to those
already given for orthoclase, but with the muscovite or biotite the soda-mica
paragonite (monoclinic; sp. gr. 2 84) is formed. Supposing the sodium
silicate is to the potassium silicate as 2 to 1, the more important reactions
may be written as follows:
(1) 2(2NaAlSi s 8 .KAlSi 3 8 )+6H/J+3C0 2 =3H 4 Al 2 SiA+12SiO. ! +K 2 OO s +2Na i C0 8 +k.
(2) 2(2NaAlSi 3 O 8 .KAlSi 3 O 8 )+9H 2 O+3CO 8 =6Al(OH) 3 +18SiO 2 +K 2 CO s +2]S T a. 1 CO 8 +k.
(3) 2NaAlSi 3 O 8 .KAlSi s O 8 +6Al(OH) 3 =KH 2 Al 3 Si 3 O 12 +2NaH 2 Al 3 Si s O 12 +6H 2 0--k.
(4) 2NaAlSi 3 O 8 .KAlSi 3 O 8 +MgCO 3 +FeCO,+5Al(OH) 3 =
HKMgFeAl 2 Si 3 O, 2 +2NaH 2 Al 3 Si 3 O 12 +5H 2 O+2CO 2 k.
Supposing the sodium silicate is to the potassium silicate as 3 to 1, we have:
(5) 2(3NaAlSi 3 8 .KAlSi 3 O B )+2F 6 ,0 3 +8CaCO 3 +2H 2 O=
4HCa 2 Al 2 FeSi 3 O 13 412SiO 2 +K 2 C0 3 +3Na 2 CO 3 +4COj+k.
The equations corresponding to (3) and (5) under orthoclase and
microcline are not written, since their occurrence is very doubtful. The
decrease in volume of the kaolinite and quartz as compared with the
anorthoclase, equation (1), is 9.56 per cent, or of the kaolinite alone is 52.19
per cent. The decrease in volume of the gibbsite and quartz as compared
with the anorthoclase, equation (2), is 3.30 per cent, or of the gibbsite alone
is 68.02 per cent. The decrease in volume of the muscovite and paragonite,
as compared with the anorthoclase and gibbsite, equation (3), is 20.04 per
cent, The decrease in volume of the biotite and paragonite, as compared
with the anorthoclase and gibbsite, equation (4), is 10.91 per cent. The
decrease in volume of the epidote and quartz, as compared with the anortho-
clase, hematite, and calcite, equation (5), is 28.30 per cent. Equations cor-
responding with the above and the volume relations can be easily worked
out along analogous lines for other ratios of the sodium-bearing and potas-
sium-bearing silicates, but the general results would be the same, so this is
hardly worth the while.
The geological positions and physical conditions under which the
THE PLAGIOCLASE FELDSPARS. 259
changes take place are identical with the corresponding changes of ortho-
clase and microcline i. e., alterations represented by equations (1) and (2)
take place in the zone of katamorphism, and especially the belt of weathering.
Alterations (3) and (4) occur in the zone of anamorphism, and that of
equation (5) is known for the belt of cementation. One general point is
clear from the above, that in the anorthoclase rocks we have a source for
paragonite in the paragonite-schists and paragonite-gneisses.
TRICLINIC.
The plagioclase feldspars are a group of triclinic feldspars which range
from sodium-aluminum silicate to calcium-aluminum silicate. The former
is a polysilicate and the latter an orthosilicate, hence there is. great variation
both as to composition and as to acidity. The names, compositions, and
specific gravities of the species, as given by Tschermak and Dana, are as
follows:
U.ltn I,. OI.K.OI HM . AXDESIXE, LABBADOBITE, BYTOWXITE, AXD AXOBTHITE.
Attnte:
NaAlSi,0 8 .
Triclinic.
Sp. gr. 2.62-2.65.
Oligoclase:
Ab to AbsAiij.
Triclinic.
Sp. gr. 2.65-2.67.
Andesine:
AbsAn, to A^An,.
Triclinic.
Sp. gr. 2.68-2.69.
Labradorite:
Ab,Ani to Ab,An 3 .
. Triclinic.
Sp. gr. 2.70-2.72.
Bylcnvnite:
AbjAnj to An.
Triclinic.
Sp. gr. 2.72-2.74.
Anorthite:
CaAl 2 SiA-
Triclinic.
Sp. gr. 2.74-2.76.
occurrence. The plagioclases are probably the most important rock-
making constituents, being approached in abundance only by the orthoclase
feldspars and by quartz. The plagioclases are present as pyrogenic con-
stituents in the great majority of igneous rocks. They also occur very
260 A TREATISE ON METAMOEPHISM.
abundantly as allogenic constituents in the sedimentary rocks. In such
rocks the more siliceous plagioclases are more plentiful than the less siliceous
plagioclases, because of the more ready decomposition of the latter. The
plagioclases develop abundantly as autogenic constituents in the metamor-
phic rocks of both sedimentary and igneous origin. The plagioclase albite
is recorded as being derived from analcite, heulandite, laumontite, plagio
clases (with orthoclase), sodalite, spodumene, and stilbite. The plagioclase
anorthite is not recorded as being derived from other minerals. But it is
seen in the zone of katamorphism that anorthite passes into various zeolites
by simple hydration. It can hardly be doubted that when such zeolites
pass into the zone of anamorphism by dehydration they are sources of anor-
thite. Doubtless also anorthite is produced in different ways from the com-
binations of various minerals, just as it passes into different combinations of
minerals. The intermediate feldspars, which are intermolecular mixtures of
albite and anorthite, may be derived from any of the minerals from which
albite and anorthite are formed.
Alterations. In treating the alterations of the plagioclases the only prac-
ticable plan is to calculate equations and volume reactions separately for
albite and for anorthite. For any of the intermediate feldspars the corre-
sponding equations may be written by multiplying the albite and anorthite
equations by the number of molecules of these compounds, respectively,
and adding the products. However, the alterations of the plagioclases are
so complicated that I have not been able to make the treatment more than
very partial.
The species belonging to the more siliceous half of the plagioclase feld-
spars i. e., albite, oligoclase, and audesine frequently undergo. alterations
similar to those of the monoclinic feldspars, producing kaolin (monoclinic;
sp. gr. 2.615), gibbsite (monoclinic; sp. gr. 2.35), and quartz (rhombohe-
dral; sp. gr. 2.6535). These alterations may be considered as coming
from the albite molecule.
But the more common alterations of the plagioclases are into the zeo-
lites, epidote (monoclinic; sp.gr.3.38), quartz, the scapolites, and paragonite
(monoclinic; sp. gr. 2.84), and the less siliceous feldspars into more siliceous
plagioclase feldspars. The plagioclases are also recorded as altering into
prehnite (orthorhombic; sp. gr. 2.875) and albite. By pyrochemical
methods plagioclase and sodium carbonate at 220 C. produce the zeolite
analcite (isometric; sp. gr. 2.255), and this process is more rapid in propor-
ALTERATIONS OF PLAGIOCLASE FELDSPARS. 261
tion as the feldspars are less siliceous. The alteration of a given feldspar
may be into two or more of the above minerals. Doubtless often orthoclase
and plagioclase together pass into other minerals. One such reaction has
been considered by Becke, and is given below.
The less siliceous plagioclases, labradorite, bytownite, and anorthite,
alter rarely to kaolin alone, but this mineral may separate simultaneously
with zoisite (orthorhombic; sp. gr. 3.31) or epidote.
The alteration of albite to kaolin and quartz, to gibbsite and quartz,
and of albite and gibbsite to paragonite, respectively, may be written as
follows :
(1) 2NaAlSi 3 O 8 +2H 2 OrCO,=H 4 Al 2 Si 2 O 9 ^4SiO 2 +Na 2 CO 3 +k.
(2) 2NaAlSi 3 O 8 ^3H 2 O+C0 2 =2[Al(OH) 3 ]+6Si0 2 4-Na 2 CO 3 +k.
(3) NaAlSi 3 8 +2Al(OH) 3 =NaH 2 Al 3 Si 3 O 12 +2H 2 O+k.
The decrease in volume in equation (1) of the kaolin and quartz is 4.89
per cent; in equation (2) the increase for the gibbsite and quartz is 1.58 per
cent; in equation (3) the decrease for the paragonite, as compared with the
albite and gibbsite, is 18.85 per cent.
Analcite (isometric; sp. gr. 2722-2.29) may be derived from albite
according to the following reaction :
(4) 2NaAlSi 3 O 8 +2H 2 O=Xa 2 Al 2 Si 4 O 12 .2H 2 O+2SiO 2 +k.
The increase in volume is 20.82 per cent, supposing the silica separates as
a solid.
Natrolite (orthorhombic; sp. gr. 2.20-2.25) may also be derived from
albite according to the reaction:
(5) 2NaAlSi 3 O 8 +2H 2 O = H 4 Na 2 Al 2 Si 3 Oi 2 +3SiO 2 +k.
The increase in volume is 19.95 per cent, supposing the silica separates as
a solid.
From anorthite a number of zeolites are derived. Clarke is one of the
latest authors who has discussed the relations of the zeolites to the feldspars,
;ind the chemical alterations given are obtained mainly from his paper."
The equations for the more commpn varieties may be written as follows:
Thomsonite (orthorhombic; sp. gr. 2.3-2.4) is derived from anorthite
according to the following reaction :
(6) 3CaAl 2 Si 2 O 8 +7H 2 O=Ca 3 Al 6 Si 6 O 2 4- 1 7H 2 O+k.
The increase in volume is 34.65 per cent.
o Clarke, F. W., The constitution of the silicates: Bull. TJ. S. Geol. Survey No. 125, 1895, pp. 32-45.
262 A TREATISE ON METAMORPHISM.
Gismondite (monoclinic; sp. gr. 2.265) is derived from anorthite
according to the following reaction:
(7) 3CaAl : ,SiA+12H ;! 0=Ca 3 Al 6 Si 6 21 .12H J 0+k.
The increase in volume is 52.76 per cent.
For laumontite (monoclinic; sp. gr. 2.305) the change does not appear
to have been determined with reasonable certainty. It may be supposed
to be derived from anorthite by the simultaneous union of freed calcium
and aluminum with other compounds, the calcium perhaps passing into the
carbonate and the aluminum into the hydrate. On this hypothesis the
reaction is:
(8) 2CaAl s Si 2 8 +7H 2 0+C0 2 =H 1 CaAl. 1 Si 4 O u -2H 2 0+CaCO,,+2[A1(OH)3]+k.
The increase in volume is 33.65 per cent, supposing the calcium carbonate
to be dissolved and the aluminum hydroxide to remain as gibbsite. How-
ever, Clarke regards laumontite as derived from equal quantities of anorthite
and the hypothetical compound trisilicic anorthite a (Ca 3 Al 6 (Si 3 O 8 ) 6 ).
The zeolite phillipsite (monoclinic; sp. gr. 2.20) may be regarded as
derived from albite, anorthite, and leucite, as follows:
(9) 6CaAl 2 Si 2 8 4-4NaAlSi 5 O 8 +6KAlSi 2 O 6 +48H 2 O+2CO 2 =
3(K 2 Ca 2 Al 6 Si 12 O 36 .14H 2 O)+2Na 2 CO 3 +4Al(OH) s +k.
The leucite is added as a source of the potassium. The increase in volume
of the three compounds in passing into phillipsite is 31.98 per cent.
Heulandite (epistilbite) (monoclinic; sp. gr. 2.20) and stilbite (mono-
clinic; sp. gr. 2.1495) are regarded by Clarke as derived from the
hypothetical compound trisilicic anorthite. Chabazite (rhombohedral ; gp.
gr. 2.12) is regarded by him as derived from this compound and from
normal anorthite. All four, however, may be equally well considered
as derived from intermediate plagioclases with carbonation of the excess
of calcium and hydration of the excess of aluminum. On these hypotheses
the reactions for the four may be written as follows:
(10) 4NaAlSi s O 8 +3CaAl 2 Si 2 O 8 +21H 2 O+2CO 2 =
3(H 4 CaAl 2 Si 6 Oi e .3H 2 O)+2Na 2 C0 3 44[Al(OH) s ]+k.
(11) 4NaAlSi 3 8 +3CaAl 2 Si 2 O 8 +24H 2 O+2CO 2 =
Oa 3 Al 6 (Si 3 O 8 ) 6 .18H 2 O+2Na 2 CO 3 +4Al(OH) s +k.
(12) 6NaAlSi 8 O 8 +6CaAl,Si 2 O 8 +3C0 2 +45H 2 O=
2[Ca,Al 6 (Si0 4 ) s (Si s 8 ) 8 .18H ! 0]+3Na J Cp 5 +6[Al(OH) s ]+6SiO I -l-k.
"Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 42.
ALTERATIONS OF PLAGIOCLASE FELDSPARS. 263
I
Supposing the sodium carbonate is dissolved and the other compounds
are solids, the increase in volume for (10) is 37.14 per cent, for (11) is
43.50 per cent, and for (12) is 46.76 per cent.
Scolecite (monoclinic; sp. gr. 2.16-2.40) may be derived from anorthite,
according to the following reaction:
(13) 3CaAl 2 Si 2 O 8 +9H 2 O+CO 2 =2CaAl 2 Si 3 O I0 .3H 2 O+2Al(OH) 3 +CaCO s +k.
The increase in volume is 35.23 per cent, provided the gibbsite separates
as a solid and the CaC0 3 is dissolved.
Mesolite (monoclinic or triclinic; sp. gr. 2.29), according to Clarke,
is an isomorphous mixture of equal quantities of natrolite and scolecite;
therefore the reaction for this compound may be expressed by the following:
(14) 4NaAlSi 3 O 8 +3CaAl 2 Si 2 O 8 -!-13H 2 0+CO 2 =
2(H 8 Na 2 CaAl 4 Si 6 O 24 .H 2 O)+6Si0 2 +2Al(OH) 3 +CaC0 3 +k.
The expansion in volume is 24.96 per cent, provided the silica and gibbsite
separate as solids and the CaCO 3 is carried away in solution. If all products
are solid the increase in the volume is 30.19 per cent.
Turning now from the zeolites to other minerals, the plagioclases are
recorded as altering into prehnite (orthorhombic; sp. gr. 2.875). Since
albite is recorded as simultaneously separating, the most probable reaction
is by hydratiou of the anorthite molecule.
(15) 4CaAl 2 Si 2 O 8 +8H 2 O=2H 2 Ca 2 Al 2 Si 3 O I2 +4Al(OH) 3 +2SiO 2 +k.
Supposing the compounds formed to be solids, the increase in volume is
14.85 per cent.
Another important alteration of the plagioclases is into zoisite
(orthorhombic; sp.gr. 3.25-3.37) or epidote (monoclinic; sp. gr. 3.25-3.5),
with the simultaneous formation of kaolinite or gibbsite. The reaction for
anorthite in the case of zoisite is probably:
(16) 4CaAl 2 Si 2 O 8 +3H 2 0=H 2 Ca 4 Al 6 Si 6 O 26 +H 4 Al 2 Si 2 O 9 +k,
or
(17) 4CaAl 2 Si 2 O 8 +4H 2 O=H 2 Ca t Al 6 Si 6 O 26 +2Al(OH) 3 +2SiO 2 +k.
The decrease in volume of the solids in (16) is 7.77 per cent, and in (17)
is 4.58 per cent.
In the formation of epidote the reactions are of a similar kind, but
Fe 2 O 3 replaces some A1 2 O 3 of the feldspar molecule. Supposing the
a Clarke, cit.. Bull. 125, pp. 35-36.
264 A TREATISE ON METAMORPHISM.
aluminum is to the iron as 2 to 1, that the iron is derived from hematite, and
that excess of aluminum separates as gibbsite, the reaction for anorthite is:
(18) 4CaAl 2 Si 2 8 +Fe 2 3 +6H 2 0=H 2 Ca 4 Al 4 Fe 2 SiA6+H 4 Al 2 Si 2 9 +2Al(OH) 3 +k,
or
(19) 4CaAl 2 SiA+7H 2 0+F&A=H 2 Ca 4 Al 4 Fe 2 Si 6 26 +4Al(OH) s +2Si0 2 +k.
The increase of the volume of the epidote, kaolin, and gibbsite as compared
with the anorthite and hematite, equation (18), is 3.6 per cent. The
increase in volume of the epidote, gibbsite, and quartz as compared with
the anorthite and hematite, equation (19), is 6.57 per cent.
The scapolites include marialite (tetragonal; sp. gr. 2.566), meionite
(tetragonal; sp. gr. 2.70-2.74), and various isomorphous mixtures. (See
pp. 311-312.) The alterations of the plagioclases into these two minerals
are given. From these equations those for any definite isomorphous mix-
ture of marialite and meionite can easily be formulated. According to
Clarke," albite changes into marialite, and anorthite into meionite. The
reactions may be written as follows :
(20) 3NaAlSi 3 8 +NaCl=Na 4 Al,Si 9 O,4Cl+k.
Supposing the NaCl to be in solution, the increase in volume of the solid
compound is 10.29 per cent. If the NaCl be supposed to be a solid, the
increase in volume of the solid compound is 1.84 per cent. The change
from anorthite to meionite is represented by the following reaction:
(21) 3CaAl 2 Si,O 8 +CaCO a =Ca 4 Al 6 Si 6 O 26 +CO 2 +k.
If the calcium carbonate be supposed to be present as a solid, the decrease
in volume is 3.78 per cent; if it be supposed to be added in solution, the
increase in volume is 7.87 per cent.
Becke records the alteration of orthoclase and plagioclase into albite
(tri clinic; sp. gr. 2.635), zoisite (orthorhombic; sp. gr. 3.31), muscovite
(monoclinic; sp. gr. 2.88), and quartz (rhombohedral; sp. gr. 2.6535).
His equation for this reaction is as follows: b
(22) x(NaAlSi 3 8 )+4(CaAl 2 SiA)+KA18i 3 O H +2H 2 O=
x(NaAlSi s O 8 )+2(HCa 2 Al 3 Si s O 13 )+H 2 KAl 3 Si 3 O 12 +2SiO 2 .
It is impracticable at the present state of knowledge to write reactions
representing the changes of the less siliceous plagioclases into more sili-
ceous plagioclases and other minerals, because the exact compositions of
the original and resultant minerals are not known. The alterations of the
a Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 29.
Becke, F., Ueber Beziehungen zwischen Dynainometamorphose und Molecularvolumen: Neues
Jahrbuch fur Mineralogie, etc., vol. 2, 1896, p. 182.
RELATIONS OF ALTERATIONS OF FELDSPARS. 265
plagioclases to kaolinite, equation (1), and to gibbsite, equation (2), are
by reactions of carbonation and hydration; the. alterations to the zeolites,
equations (4) to (14), inclusive, are by reactions of hydration, and (8)
to (14), inclusive, also involve carbonation. The alteration to prehnite,
equation (15), is a reaction of hydration and desilication. The alterations
to zoisite and epidote, equations (16), (17), (18), and (19), are by reactions
of hydration. All but (1), (3), (16), and (17) take place with ncrease of
volume ranging from 1.58 to 52.76 per cent, provided all the compounds
formed remain in situ. All take place with liberation of heat. The altera-
tions in all particulars are characteristic of the zone of katamorphism.
The development of kaolin is probably more characteristic of the belt
of weathering than of the belt of cementation. The development of the
zeolites and cpidotes is known to occur on an extensive scale in the belt of
cementation, both within the bodies of other minerals and within the open-
ings in rocks. Amygdules and veins of these minerals, with quartz, are of
great importance in cementing rocks. The material for this work is doubt-
less in large part, though not altogether, derived from feldspathic minerals.
For the intermediate scapolites, equations (20) and (21), the alterations
take place with a slight increase in volume for marialite and a slight
decrease for meionite, provided all the compounds which enter into them
are solids. The reactions are those of silication and decarbonation to some
extent, and this involves absorption of heat. The geological occurrences
correspond with the physical-chemical facts. The most common of the
scapolites which occurs as a secondary product in the altered rocks is
wernerite, an isomorphous mixture of meionite and marialite molecules.
As stated by Dana, wernerite "occurs in metamorphic rocks, and most
abundantly in granular limestone near its junction with the associated
granitic or allied rock."" Wernerite is associated with such minerals as
pyroxene, amphibole, and garnet, which occur as deep-seated alterations.
The formation of wernerite from feldspar is probably, therefore, a deep-
seated change which occurs in the zone of anamorphism.
The alteration of orthoclase and plagioclase together to albite, zoisite,
muscovite, and quartz, equation (22), is a reaction of hydration and desili-
cation. It involves an increase in volume. One would therefore expect
the reaction to take place in the zone of katamorphism.
"Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana; Wiley & Sons,
New York, 6th ed., 1892, p. 470.
266 A TREATISE ON METAMORPHISM.
LEUCITE GROUP.
Leucite is the only rock-making mineral belonging to this group.
LEl'CITE.
Leucite:
KAlSijO,.
Isometric.
Sp. gr. 2.45-2.5.
occurrence. Leucite is a common constituent of volcanic rocks, especially
the more recent ones. The fact that it is not abundant in the older volcanic
rocks is probably due to its ready alteration. It may have been present
originally.
Leucite is not known as a constituent of the schists and gneisses
derived from the sediments. Leucite has been produced by pyro-chemical
methods from analcite and potassium chloride. This is a reversal of the
reaction in the case of change of leucite to analcite.
Alterations. Leucite frequently alters to analcite (isometric; sp. gr. 2.22-
2.29); to a mixture of orthoclase (monoclinic; sp. gr. 2.53-2.6) and kaolinite
(monoclinic; sp. gr. 2.6-2.63); to a mixture of orthoclase and muscovite
(mouoclinic; sp. gr. 2.76-3.0); and to a mixture of orthoclase and nephe-
lite (hexagonal; sp. gr. 2.55-2.65).
The change of leucite to analcite requires a substitution of sodium for
potassium; hence sodium carbonate or some other sodium compound must
be supposed to be present. Supposing sodium carbonate to be the com-
pound, the reaction is:
(1) 2KAlSi 2 O 6 +NXCO s +2H,O=Na 2 Al 2 Si 4 O 12 .2H 2 O+K 2 CO s +k.
Ignoring the carbonates, the increase in volume is 10.74 per cent.
The passage of leucite into orthoclase and kaolin is as follows, sup-
posing the freed potassium to unite with carbon dioxide:
(2) 4KAlSi 2 O 6 +CO 2 i-2H 2 O=2KAlSi 3 O 8 +H 4 Al 2 Si 2 O 9 +K 2 CO,+k.
Supposing the potassium carbonate to be taken into solution, the decrease
in volume of the orthoclase as compared with the leucite is 38.57 per cent,
and the decrease of the orthoclase and kaolinite together as compared with
the leucite is 10.58 per cent.
In a similar way the passage of leucite into orthoclase and muscovite
is as follows:
(3) 6KAlSi 3 O,+CO 2 +H 2 O=3KAlSi,O 8 +KH 2 Al 3 Si 3 O 12 +K 2 CO s +k.
THE ORTHOKHOMBIC PYROXENES. 267
As before, supposing the potassium carbonate to be taken into solution, the
decrease in volume of the orthoclase and muscovite as compared with the
leucite is 12.43 per cent.
In the passage of leucite into orthoclase and nephelite it is necessary
to suppose that a part of the potassium of the leucite is replaced by sodium.
Supposing the nephelite formed to be a pure soda-uephelite, the reaction
would be:
(4) 4KAlSi 2 O 6 +Na 2 CO3=2KAlSi s O 8 +2NaAlSiO 4 +K 2 CO 3 +k.
Ignoring the carbonates, the decrease in volume of the orthoclase and
nephelite as compared with the leucite is 7.59 per cent.
The reactions above given, except the last, are those of hydration,
and the second and third are those of carbonation also. They are,
therefore, reactions which are to be expected in the zone of katamorphism.
The change of leucite into orthoclase and nephelite gives decrease in
volume, with neither hydration nor dehydration, carbonation nor silication.
It is, therefore, to be expected that the change is one which takes place in
the zone of anamorphism.
The artificial transformation of leucite into analcite by treatment
with soda solutions, and the reverse alteration of analcite into leucite
by treatment with potassium solutions, as shown by Lemberg, is an
excellent illustration of the law of mass action and proves the importance
of this principle under natural conditions.
PYROXENE GROUP.
ORTHOKHOMBIC PYROXENES.
EN8TATITE, BHON/ITK, AXD HYPERSTHENE.
Enstatite:
MgSiO 2 .
Orthorhombic.
Sp. gr. 3.1-3.3.
Bronzite:
(MgFe)SiO 3 where Mg:Fe : : 8:1, 6:1, and 3:1.
Orthorhombic.
Sp. gr. 3.2-3.3.
Hypersthene:
(MgFe)SiO 3 where Mg:Fe : : 3:1, nearly to 1:1.
Orthorhombic.
Sp. gr. 3.4-3.5.
Lemberg, J., Ueber Silicatumwandlungen: Zeitschr. Deutsch. geol. Gesell., vol. 28, 1876, pp.
536-545.
268 A TREATISE ON METAMORPHISM.
occurrence. The rhombic pyroxenes are common pyrogenic constituents
of igneous rocks rich in magnesium. They are common in the normal
diabases, gabbros, and basalts, and are abundant in the norites, peridotites,
etc. They also occur in the intermediate, basic, and ultrabasic volcanic
rocks, including both lavas and tufts. A very common associate of the
rhombic pyroxenes is oliviue. The rhombic pyroxenes also occur in the
schists and gneisses, especially those derived from eruptives. In such
rocks they are frequently associated with the monoclinic pyroxenes. They
further occur as vein materials and are found in meteorites.
As metamorphic minerals, enstatite is derived from pyrope and
hypersthene from almandite, biotite, and common garnet.
Alterations. The most frequent alteration of the rhombic pyroxenes is
to talc (orthorhombic or monoclinic; sp. gr. 2.7-2.8). The less frequent
alterations are to serpentine (monoclinic; sp. gr. 2.50-2.65), bastite (ortho-
rhombic; sp. gr. 2.50-2.75), actinolite (monoclinic; sp. gr. 3-3.2), and
anthophyllite (orthorhombic; sp. gr. 3.1-3.2).
For the sake of simplicity it is assumed that where pure talc or
serpentine is produced these materials are derived from enstatite; and that
where bastite, actinolite, and anthophyllite are produced these minerals are
derived from bronzite or hypersthene. Of course, serpentine or talc may
be produced from bronzite or hypersthene, the iron separating as oxide or
carbonate. One such possible altei'ation is written. However, the ordinary
alterations of the ferriferous pyroxenes are to bastite, which is iron-bearing.
The change of enstatite to talc is as follows :
(1) 4MgSiO s +CO 2 +H 2 0=H 2 Mg 8 Si 4 O, 2 +MgCO s -}-k.
Supposing the magnesium carbonate to be dissolved, the increase in
volume is 9.93 per cent. If a ferriferous pyroxene be supposed to alter
to talc, iron oxide must separate. Supposing this to be in the form of
magnetite (isometric; sp. gr. 5.174), and supposing that the magnesium
is to the iron as 3:1, or that the mineral is intei'inediate between bronzite
and hypersthene, the reaction may be written:
(2) 3Mg 3 FeSi 4 I2 +3H 2 0+0=3H 2 Mg 3 Si 4 I2 +Fe s 4 -:-k.
Similar equations may be written by which, instead of magnetite,
hematite (rhornbohedral; sp. gr. 5.225) or limonite (amorphous; sp. gr.
3.80) is produced, in which case the expansion of volume would be
ALTERATIONS OF ORTHOKHOMBIC PYROXENES. 269
greater. The calculated increase in volume of the talc and magnetite, as
compared with the pyroxene, is 14.68 per cent, provided the average
specific gravity of bronzite be used, and 21.73 per cent provided the
average specific gravity of hypersthene be used. Probably the real
increase in volume is the average of the above, or about 18.20 per cent.
In the case of a hypersthene in which the iron is to the magnesium as
1:1 the alteration to talc may be as follows, provided the iron separate as
magnetite : ,
(3) 3MgFeSi 2 O 6 +H 2 0+0=H 2 Mg,Si 4 O 12 +Fe 3 O 4 +2SiO, ! +k.
The increase of volume of the talc, magnetite, and quartz as compared
with the hypersthene is 12.84 per cent.
Serpentine is produced from enstatite by the following reaction :
(4) 3MgSiO s +2H,0=H 4 Mg 8 SiA+8iO,+k.
In case the Si0 2 is dissolved, the increase in volume is 14.25 per cent ;
if it separates as quartz (rhombohedral ; sp. gr. 2.6535) the increase in
volume is 38.36 per cent.
If a rhombic pyroxene be taken in which the magnesium is to the iron
as 3 : 1 i. e., stands on the border line between hypersthene and bronzite
serpentine might be produced by the following reaction, with the simul-
taneous separation of hematite and quartz :
(5) 2Mg 3 FeSiA2+4H 2 O+O=2H,Mg s SiA+FeA+4SiO 2 +k.
Using the specific gravity of hypersthene, in case only serpentine and
hematite separate as solids, the decrease in volume is 2 21 per cent, and if
the silica separates as quartz the increase of volume is 33.94 per cent.
Supposing the calcium is to the iron as 1:1 and the excess of iron
separates as magnetite, the reaction is :
(6) 3MgFeSi 2 O 6 +2H 2 O+O=H 4 Mg s SiA+Fe 3 O 4 +4SiO 2 +k.
The increase in volume of the serpentine, magnetite, and quartz as
compared with the hypersthene is 20.24 per cent.
Other reactions may be written which represent the alterations of
bronzites and hypersthenes, in which the proportions of magnesium and
iron are different. Also reactions may be written in which the oxide of
iron forms as magnetite or limonite. Where magnetite forms, the increase
in volume would be less than for hematite, and where limonite forms the
increase in volume would be considerably greater.
270 A TREATISE ON METAMORFHISM.
If in the formation of bastite, a pyroxene be taken which stands
intermediate between bronzite and hypersthene i. e., in which the magne-
sium and iron are as 3:1 and if the same proportions of these constituents
be supposed to hold in the bastite, the reaction is as follows:
(7) 3Mg 3 FeSi < O 12 ^8H 2 O=H I6 Mg 9 Fe,Si 8 36 +4SiO 2 ^k.
Using the specific gravity of hypersthene, if the silica be dissolved the
increase of volume is 22 77 per cent (if the specific gravity of bronzite be
employed, 15.65 per cent); if the silica separates as quartz, 46.87 per cent.
Similar reactions may be written which represent the formation of bastites
which are richer and poorer in iron, in which cases the volume changes are
slightly different.
The passage of ferriferous rhombic pyroxene into anthophyllite may
be one of pure paramorphism, since in anthophyllite the proportions of
magnesium to iron have ranges paralleled by bronzite and hypersthene.
Therefore, the only necessary change is a molecular one, a mineral being
produced of lower symmetry and lower specific gravity as a result of the
alteration. If the specific gravity of hypersthene be used, the calculated
increase in volume due to the lower specific gravity of the resultant
mineral is 8.70 per cent.
In the formation of actinolite from a rhombic pyroxene, it is necessary
that lime and silica be added. Supposing the magnesium is to the iron as
3:1 in both the rhombic pyroxene and actinolite, the equation is as follows:
(8) 3Mg,FeSi 4 Oi 2 +4CaCO 3 +4Si0 2 =Mg 9 Fe 3 Ca 4 Sii 6 O 48 +4CO,-f k.
The decrease in volume of the actinolite as compared with pyroxene,
calcite, and quartz is 7.40 per cent if the specific gravity of hypersthene
be used, and if that of bronzite is 10.77 per cent. Similar equations may
be written in which the proportions of magnesium and iron are different.
The changes of the rhombic pyroxenes to talc involve reactions of
carbonation and hydration, or of hydration and oxidation, or of all three
together. The changes of the rhombic pyroxenes to serpentine and bastite
involve hydration alone, or hydration and oxidation. All take place with
increase of volume and liberation of heat,
Corresponding with these facts, as a matter of observation the devel-
opment of serpentine, bastite, and talc from the rhombic pyroxenes takes
place in the zone of katamorphism. The development of talc is especially
characteristic of the belt of weathering, and serpentine and bastite of the
MONOCLINIC PYROXENES. 271
belt of cementation, although it can not be asserted that the formation of
any of these minerals is confined to either belt.
The paramorphic change of rhombic pyroxene into anthophyllite
being one involving lessening of specific gravity and decrease of symmetry,
one would expect the change to take place in the upper physical-chemical
zone, but I have been unable to ascertain from the literature the facts in
this case.
The formation of actinolite from a rhombic pyroxene requires the
assistance of calcite and silica. This reaction is one of silication and
decarbonation. It occurs with diminution of volume and absorption of
heat. As a matter of observation, corresponding with these facts it is
well known that the change is a deep-seated one.
MONOCLINIC PYROXEXEK.
DIOPSIDE, SAHLITE, HEDEXBERUITK, Al'UITE, ACMITE, SPODUMEJfE, WOLLASTOXITE, AXD PECTOLITE.
Diopside:
CaMgSi 2 O 6 -
Monoclinic.
Sp. gr. 3.2-3.38.
Sahlite.:
Ca(MgFe)Si 2 6 .
Monoclinic.
Sp. gr. 3.25-3.4.
Hedenberyite:
CaFeSi 2 O 6 -
Monoclinic.
Sp. gr. 3.5-3.58.
Augite:
Ca(MgFe)Si 2 O 6 with (MgFe) (AlFe) 2 SiO 6 .
Monoclinic.
Sp. gr. 3.3-3.5.
Acmite:
NaFeSi 2 6 .
Monoclinic.
Sp. gr. 3.50-3.55.
Spodumene:
LiAlSi 2 Og-
Monoclinic.
Sp. gr. 3.13-3.20.
Wollaslonite:
CaSiO s .
Monoclinic.
Sp. gr. 2.8-2.9.
Pectolite:
Monoclinic.
Sp. gr. 2.68-2.78.
272 A TREATISE ON METAMORPBISM.
The minerals diopside, sahlite, and augite constitute the so-called
diopside-augite series.
occurrence. The pyroxene group is one of the most widespread and
important. One or another variety of pyroxene may occur in almost any
rock; but the pyroxenes are much more abundant in the intermediate and
basic than in the acidic rocks. Pyroxene is found in the plutonic and
volcanic rocks, as an original constituent of the clastic rocks, and as an
original and secondary constituent of the metamorphosed rocks, both of
igneous and of aqueous origin. The minerals of the pyroxene group occur
extensively in veins.
Diopside occurs in marbles, especially magnesian marbles. Indeed,
this is the common form of pyroxene which develops as a secondary
constituent during the metamorphism of the magnesian limestones. It also
occurs in veins. As a metamorphic mineral diopside is derived from
dolomite.
Sahlite occurs in ferriferous magnesian marbles. Like diopside, it is
also found in veins. Unlike diopside, it is a common product in many
horublendic schists and gneisses, such rocks probably having been in their
original condition calcareous, magnesian, and ferriferous. Sahlite is derived
from ankerite and parankerite.
Hedenbergite occurs as a rather common constituent of some nepheline
syenites and other basic syenites.
Augite is a common form of pyroxene in the eruptive rocks, both
plutonic and volcanic. It occurs in many mechanical sediments. It also
is found in metamorphic rocks of both igneous and sedimentary origin,
though in the sedimentary metamorphosed rocks it is less common than
diopside and sahlite. But augite develops to a considerable extent in the
sedimentary rocks which are intermediate between the chemical and
mechanical rocks that is, those which contain abundant calcium carbonate
and also are rich in aluminum. Augite is recorded as a metamorphic
mineral derived from hornblende.
Wollastonite occurs especially in the metamorphosed calcareous and
sedimentary rocks, it being a secondary product produced by metamorphism.
It is found abundantly in marbles, and in schists and gneisses which were
originally calcareous, especially the calcareous feldspathic schists. It also
develops in calcareous inclusions in eruptive rocks, and is found as a contact
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 273
product of igneous and calcareous rocks. The schists and gneisses contain-
ing wollastonite are often garnetiferous and epidotic.
The very frequent development of the above pyroxenes in the sedi-
mentary rocks which are calcareous, rather than amphiboles, is due to the
fact that the pyroxenes are richer in calcium than are the amphiboles. Where
sedimentary rocks contain magnesium abundantly with the calcium, the
amphiboles are likely to form rather than the pyroxenes.
Acmite occurs mainly in the eruptive rocks, and especially in those
which are rich in alkalies. According to Rosenbusch, it occurs especially
in granites and syenites rich in sodium, in the ela3olite- syenites, phonolites,
and leucitophyres. As a metamorphic mineral acmite is derived from
arfvedsonite.
Spodumene sometimes occurs as an accessory constituent in the granites,
schists, and gneisses, and in some cases as considerable masses.
Pectolite, while not an abundant mineral, is present as a secondary
constituent in many basic eruptive rocks, both plutonic and volcanic. It is,
however, especially prevalent in the volcanic rocks, since these are more
porous, and pectolite is especially likely to occur in cavities or seams.
Occasionally pectolite is found in the metamorphic rocks as a product of
apophyllite.
Alterations of the diopside-augite series. The most common alteration of the non-
aluminous diopside and sahlite is into talc (orthorhombic or monoclinic;
sp. gr. 2.7-2.8). They also often alter into serpentine (monoclinic; sp. gr.
2.5-2.65). These changes are accompanied by the formation of calcium
carbonate, and frequently by the separation of a part of this carbonate as
calcite (rhombohedral ; sp. gr. 2.7135).
The aluminous pyroxenes, augite, and diallage, under the conditions of
the zone of katamorphism, change into chlorite (monoclinic; sp. gr. 2.08-
2.16'), with which are usually associated epidote (monoclinic; sp. gr. 3.25-
3.5), this mineral often being embedded in the chlorite and calcite. Under
conditions of weathering, any of the minerals of the diopside-augite series
may be partly or entirely replaced by quartz (rhombohedral; sp. gr.
2.6535), chalcedony (cryptocrystalline; sp. gr. 2.6-2.64), or calcite. Such
replacements are particularly common in the case of the porous andesites
and trachytes, and also in tuffs. Not infrequently this replacement of
the pyroxene occurs without the feldspar being greatly affected.
MON XLVII 04 18
274 A TREATISE ON METAMORPHISM.
However, perhaps the most frequent and characteristic of the altera-
tions of the diopside-augite series is uralitization or change to amphibole
(monoclinic; sp. gr. 2.9-3.4). This process is particularly characteristic of
the ancient igneous rocks, and especially those which are under compara-
tively deep-seated conditions, although the alteration is by no means con
fined to deep-seated rocks. It occurs on a great scale under the conditions
of the transformation of the igneous rocks into schists and gneisses. During
the process of uralitization epidote also very frequently forms. Not infre-
quently also magnetite (isometric; sp. gr. 5.165.18) and calcite separate.
In some cases the change is accompanied by the development of a feldspar,
such as albite (triclinic; sp. gr. 2.62-2.65). The kind of amphibole which
forms depends upon the variety of the pyroxene. From diopside, tremolite
(monoclinic; sp. gr. 29-3.1) is the ordinary product; from sahlite, actino-
lite (monoclinic; sp. gr. 3.0-3.2) is normally to be expected; from diallage
and omphacite (according to Zirkel," varieties of augite), smaragdite (a
variety of hornblende) is ordinarily produced; and from ordinary augite,
hornblende (monoclinic; sp. gr. 3.053.47) is usually developed. Finally,
it not infrequently occurs that augite changes directly into biotite (mono-
clinic; sp. gr. 2.90).
The change of diopside to talc may be written as follows :
(1 ) 3CaMgSi 2 O 6 4-3CO 2 f H 2 O=H 2 Mg s Si 4 Oi 2 +3CaCO s +2SiO 2 +k.
The increase in volume, supposing all compounds remain as solids, is 48.74
per cent. If only the talc remains, the decrease is 30.13 per cent. Sup-
posing the diopside were one in which a part of the calcium and magnesium
were replaceable by iron, so that the calcium and magnesium and iron are
present in equal proportion, thus approaching sahlite in composition, and
supposing the iron to pass into magnetite, the reaction is
(2) 3CaMgFeSi,O 9 +3C0 2 +H 2 O+O=H 2 Mg,SiA 2 +3CaCO,+Fe 3 O 4 +5SiO 2 +k.
The increase in volume .of the talc, calcite, magnetite, and quartz, as com-
pared with the diopside, is 27.88 per cent.
The change of diopside to serpentine may be represented by the
following equation:
(3) 3CaMgSiA+3C0 2 +2H a O=H 4 Mi ?3 SiA+4'Si0 2 +3CaC0 3 +k.
"Xaumann, C. F., and Zirkel, F., Elementeder Mineralogie, Leipzig, 1898, p. 696.
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 275
Supposing all the compounds separated as solids, the increase in volume is
f>6.32 per cent. If only the serpentine and quartz remain as solids, the
increase in volume is U.44 per cent.
The change from sahlite to ferriferous bastite, provided that in both
compounds the magnesium is to the iron as 3 : 1, is
(4) 3Ca 4 Mg 3 FeSiA4+12CO 2 +8H 2 O = H 16 Mg 9 Fe s Si 8 O S6 +16SiO 2 +12CaCO 3 +k.
If all the compounds separate as solids, the increase in volume is 56.41 per
cent; if the bastite and quartz remain as solids, 1.93 per cent. Supposing
the calcium, magnesium, and iron were in equal proportions in the sahlite,
and that in the bastite the magnesium were to the iron as 3 : 1, the equation
may be written
(5) 9CaMgFeSi 3 O 9 +9CO. ! +8H. 1 O+2O=H 16 Mg 9 Fe s Si 8 O3 6 +2Fe s O 4 +9CaCO 3 +19SiO.,+k.
The increase in volume of the serpentine, magnetite, calcite, and quartz,
as compared with the sahlite, is 37.50 per cent.
It is, of course, not impossible that serpentine shall develop as one of
the products from augite. In this case it doubtless forms from the sahlite
molecule of the augite compound, the sesquioxide compounds passing into
some other mineral. It hardly seems advisable to attempt to write equa-
tions representing such an alteration.
In writing equations for the alterations of the aluminous pyroxenes into
chlorite and epidote it is necessary that certain assumptions shall be made
in reference to the relative proportions of the various elements. Moreover,
if equations are written which produce chlorite alone, a large amount of the
sesquioxide bases must be left over. If an equation be written for the forma-
tion of epidote, a large amount of magnesium is unaccounted for. Since it
is very common for the minerals chlorite and epidote to form simultaneously,
an equation is written on this supposition. In order to give definiteness to
the compound, it is supposed that there are twice as many molecules of the
diopside part of the augite molecule as of the other part. Furthermore, it
is supposed for the diopside molecule that the magnesium is to the iron as
2:1; and for the other molecule that the aluminum is to the iron sesquioxide
as 3:1. An epidote is taken in which the aluminum is to the iron as 2:1.
A chlorite between clinochlore and prochlorite is taken, as such a chlorite
is at about the middle of the series. As may be seen by reference to the
analyses of augites and epidotes, the proportions taken represent about their
276 A TREATISE ON METAMORPHISM.
average compositions. With all these hypotheses, and supposing the extra
silica to separate as quartz, the magnesia to separate as magnesium carbonate,
and the iron as sesquioxide of iron, the equation may be written as follows:
(6) 6[2(Ca,Mg 2 FeSi 6 18 ).Mg 4 Fe 2 Al 9 Fe s Si 6 S6 ]-H2C0 2 +39H 2 0+120=
If all the compounds remain as solids the increase in volume is 15.43 per
cent. If the magnesium carbonate be dissolved the increase in volume is
8.58 per cent.
It is evident that many other equations could be written if other sup-
positions be made as to the relative proportions of the magnesium to the
iron and the aluminum to the iron in the respective compounds, and if other
chlorite? than the particular one chosen be produced. For the complex
silicates, present knowledge is not sufficient to determine whether or not
particular equations written accurately represent the alterations which take
place, although closer study in the future may possibly determine this.
But there is little doubt that substantially the change represented by equa-
tion (6) has occurred in many instances, whether it can be verified in an
individual case or not, as doubtless have also a multitude of alterations
which might be represented by other possible equations. The difficulty is
to ascertain in a given instance which of the equations represents a given
alteration. It is hoped that the quantitative statement of the problem given
by equation (6) and following equations will lead to closer study of the
compounds which enter into new compounds and the compounds which are
produced, and thus to more exact knowledge of the various alterations of
augite.
According to the above reactions, as would be expected from the nature
of the compounds, the alteration of diopside and sahlite more frequently
produces talc, serpentine, and bastite, while the alteration of augite more
frequently produces chlorite and epidote.
As already noted, perhaps the most characteristic of the alterations of
the pyroxenes is to the amphiboles. This alteration involves the substitu-
tion of magnesium, or magnesium and iron, for calcium. It is supposed
that the iron and magnesium are added in the form of carbonate, and that
the liberated calcium separates in the form of carbonate. Parallel equa-
tions can, however, readily be written on the basis of any other magnesium
compound being added and similar iron and calcium compounds being
ALTERATIONS OF DIOPSIDE- AUGITE SERIES. 277
produced. It is assumed, further, that the alteration of a pyroxene results
in the production of the most closely allied amphibole. Of course this is
not always the fact, but it is believed to be usual. Following this assump-
tion, the alteration of diopside is to tremolite, of sahlite is to actinolite, of
augite is to hornblende.
The change from diopside to tremolite may be written as follows:
(7) 2CaMgSiA+MgCO,=CaMg 3 Si 4 O, 2 +CaCO 3 +k.
Regarding the magnesium carbonates as added in solution and the calcium
carbonate as subtracted in solution, the increase in volume is 5.68 per cent.
If the magnesium carbonate be considered as present as magnesite, and the
calcium carbonate be considered as present as calcite, the increase in volume
is 10.55 per cent.
The change from sahlite to actinolite, supposing the magnesium and
iron to be present in the sahlite in equal proportions, is as follows :
(8) 2Ca 2 MgFeSi 4 12 +FeCO 3 -t- MgC0 3 =Ca 2 Mg 3 Fe 3 Si 8 21 +2CaCO 3 +k.
Supposing the sahlite and actinolite only to be solids, the increase in vol-
ume is 7.28 per cent. If all the compounds are regarded as solids on both
sides of the equation, the increase in volume is 10.81 per cent.
Supposing that in the augite compound there are two of the sahlite
molecules to one of the sesquioxide molecule, and supposing that the mag-
nesium and iron are in equal proportions in both the augite and hornblende,
the general alteration may be written as follows:
(9) 2[Ca !! MgFeSi 4 1 2. (MgFe) (AlFe) 2 Si0 6 ]+FeCO 3 rM g CO 3 =
.- (MgFe) 2 (AlFe) 4 Si 2 O 12 +2CaCO 3 +k.
But before the volume relations can be calculated it is necessary to
assume definite proportions between Mg and Fe, and Al and Fe, in the
second members of the augite and hornblende molecules. If the magne-
sium be taken to the iron as 2:1 and the aluminum to the iron as 2:1,
an average case, and only the augite and hornblende be considered as
solids, the increase in volume is 4.30 per cent. If all the compounds in
both equations are solids, the increase in volume is 6.14 per cent.
An inspection of the above equations giving the alterations of the
diopside-augite series to amphibole shows that the chemical change in the
alteration of diopside and sahlite to tremolite and actinolite is relatively
greater than in the alteration of augite to hornblende. Moreover, if it be
278 A TREATISE ON METAMOKPHISM.
supposed that the last half of the augite and hornblende molecules are
present in greater proportion than given in the equations, the chemical
change would be of still less relative importance. This is of interest because
the alteration of augite to hornblende is a far more common phenomenon
than the alterations of diopside and sahlite to tremolite and actinolite The
equations also give reasons for the very frequent occurrence of calcite with
uralite. The nature of the alterations is such that calcium carbonate must
be produced, and very naturally a portion of this substance frequently
separates as calcite.
In the change of augite to biotite it is necessary that potassium be
derived from some source. Supposing it to be furnished in the form of
potassium carbonate, as a result of the decomposition of some of the potas-
sium-bearing silicates, the simplest form of reaction may be written as
follows :
(10) 2[Ca(MgFe)Si 2 O 6 .(MgFe)(AlFe) 2 Si0 6 ]
2HK(MgFe),(AlFe),Si,O, a +2CaCO,+k.
Supposing the MgO: FeO: : 2: 1, and the A1 2 O 3 : Fe 2 O 3 : : 3: 1 these ratios
being chosen because they represent about an average of the analyses
and multiplying the above equation by 6, we have:
( 11 ) 2[Ca fl Mg 4 Fe 2 Si, 2 S6 .Mg 4 Fe 2 Al i( Fe s Si 6 S6 ] +6K 2 CO,+6H 2 O+6CO ! =
2( H 6 K 6 Mg 8 Fe 4 Al 9 Fe s Si l8 O T2 ) -f 12CaCO s +k.
Disregarding all other compounds, the increase in volume of the biotite as
compared with the augite is 17.26 per cent.
The alteration of diopside and sahlite to talc, serpentine, and bastite,
equations (1), (3), and (4), all involve increase in volume and liberation of
heat; also they are alterations involving carbonation and hydration. Equa-
tions (2) and (5) involve carbonation, hydration, and oxidation. In all
except equation (1), even if all of the separated quartz and calcite is
dissolved, there is still an increase in volume. They therefore stand as
alterations that are typical of all the principles of metamorphism in the
zone of katamorphism.
The changes of the pyroxenes, especially augite, to chlorite and epidote,
equation (6), involve hydration, carbonation, and oxidation. The change
occurs with increase in volume and liberation of heat, even if the resultant
oxide of iron and magnesium carbonate be ignored. If these separate as
solids, the increase in volume is considerable. The alteration is, therefore,
ALTERATIONS OF DIOPSIDE-AUGITE SERIES. 279
like the change to talc, serpentine, and bastite, one characteristic of the
upper physical-chemical zone. The change of pyroxene to the fibrous
aniphibole known as uralite occurs in the belt of cementation on an exten-
sive scale, and to this position the volume change corresponds.
But the passage of pyroxene into definite amphibole individuals is one
of the most common alterations in the zone of anamorphism, and especially
under conditions of mashing. The rule for this zone is for alterations to
occur which result in minerals of higher specific gravity. The alteration
of pyroxene to amphibole seems to be an exception to this rule; for the
specific gravity of the pyroxenes ranges between 3.2 and 3.6, while that of
the amphiboles varies from 2.9 to 3.4.
Mainly in consequence of this decrease in specific gravity the increase
in volume, as already seen, of all compounds entering into the reactions in
the change from diopside to tremolite, equation (7), is 10.55 per cent; of
sahlite to actinolite, equation (8), 10.81 per cent; of augite to hornblende,
equation (9), 6.14 per cent, supposing that the necessary chemical constitu-
ents added to the pyroxene are solid carbonates and the other compounds
produced are solid carbonates.
Unlike the previous alterations, these changes do not involve oxidation,
hydration, or carbonation ; nor, on the other hand, do they involve deoxi-
dation, dehydration, or silication They are substitution reactions, by which
magnesium, or iron, or both take the place of calcium. They are, there-
fore, analogous to the dolomitization or ferritization of the limestones; but
the volume change is in an opposite sense from those alterations.
But another factor may enter into the problem, the effect of which is
hard to estimate. The exchange of the magnesium and iron for calcium is
supposed to take place with the separation of a carbonate. If such carbon-
ate were simultaneously silicated, the entire volume change for all the fac-
tors concerned would be decrease. It is necessary to consider the volume
relations of all the resultant minerals rather than those of the pyroxene and
amphibole alone, and hence it may be that in the change of pyroxene to
amphibole in the lower physical-chemical zone, if one could ascertain the
entire effect of this alteration in connection with other alterations, the
volume would not be expanded but contracted, and thus there be no real
exception to the law that the reactions here take place with condensation
of volume.
280 A TREATISE ON METAMORPHISM.
But, even if this be true, it is freely admitted that the case is not fully
covered, for it is very uncommon indeed for the chief resultant mineral of
an alteration in the zone of anamorphism to have a lower specific gravity
than the minerals from which it is derived with comparatively small chem-
ical change. Apparently, for some reason the amphiboles are more stable
under conditions of moderately deep-seated metamorphism than the pyrox-
enes. This view is confirmed by the fact that, while the majority of the
schists and gneisses are amphibolitic rather than pyroxeuitic, in some of the
gneisses and schists which have been altered under very deep-seated condi-
tions the pyroxenes are present instead of the amphiboles. The significance
of this fact is probably that an unusually high pressure is required in order
to produce the mineral of the highest specific gravity in the case of the
pyroxene-amphibole group.
The change from augite to biotite, equations (10) and (11), is one which
takes place in the zone of anamorphism especially under conditions of
mashing. In this change the volume of the biotite produced is greater
than that of the pyroxene; in the case of the equation (11) 17.26 per cent.
However, this case is similar to that of hornblende. Potassium salt must
be added from some other mineral and a calcium salt is produced. In
order to get the real volume relation of the reaction it would be necessary
to know the source of the potassium and the place to which the calcium
goes; and as present information does not enable us to determine this, no
definite statement can be made as to the total effect of all the changes
involved in the alteration of augite to biotite.
Alterations of pyroxenes other than the diopside-augite series. No 6qUatioilS ai'6 Written
for the alterations of wollastonite, hedenbergite, acmite, and pectolite,
because the character of the alterations of these compounds has not been
described in the standard authorities, although there is no doubt that these
minerals, like all others, do undergo various alterations. All these minerals
form under deep-seated conditions; and it is to be expected that under the
conditions of the zone of katamorphism, especially in the belt of weathering,
they would be decomposed; but, if so, the minerals into which they change
are unknown.
Alterations of spodumeue are recorded. According to Dana, the first
stage in the alteration of spodumene is to beta-spodumene (crystallization
not determined; sp. gr. 2.644-2.649), in which one-half of the lithium is
ORTHORHOMBIC AMPHIBOLES. 281
replaced by sodium. The second stage in the process of alteration is the
beta-spodumene passing into eucryptite (hexagonal; sp. gr. 2.667) and albite
(triclinic; sp. gr. 2.62-2.65), or into muscovite (monoclinic; sp. gr. 2.76-3)
and albite, the uniform mixture of which has been known as cymatolite
(sp. gr. 2.692.70); or spodumeiie may pass into muscovite and microcline
(triclinic; sp. gr. 2.54-2.57). The reactions representing the above changes
may be expressed in the manner shown by the equations given below.
The change from spodumene to beta-spodumene may be written:
(12) 4IJAlSi 2 O 6 + N %CO 3 =2LiNaAl 2 Si i O 12 +Li 2 CO,,+k.
The increase in volume is 24.72 per cent. Where the beta-spodumene
breaks up into eucryptite and albite the reaction is:
(13) LiXaAl 2 Si 4 O,2= LiA1 SiO,+NaAlSi 3 O 8 +k.
The increase in volume is 0.05 per cent. Where the beta-spodumene
passes into muscovite and albite the reaction is:
(14) 6LiNaAl 2 Si 4 O,2+K 2 CO 3 +2H.,O+2CO 2 =2H 2 KAl 3 Si3O I2 +6NaAlSi 3 O 8 +3Li 2 CO3+k.
The decrease in volume is 0.76 per cent. In case the spodumene changes
into muscovite and microcline the reaction is:
(15) 12LiAlSi 2 O 6 +4K 2 CO i ,+2C0 2 +2H 2 O=2H !! KAl 3 Si 3 O 1;i +6KA]Si 3 8 +6Li 2 CO s +k.
The increase in volume is 31.74 per cent.
One would expect reactions (12) and (15) to take place in the zone of
katamorphism, but I know of no observations on this point, nor as to the
conditions under which reactions (13) and (14) occur.
AMPHIBOLE GROUP.
ORTHORHOMBIC AMPHIBOLES. .
ANTHOPHYLLITE AND UEDBITE.
Anthophyllite:
(MgFe) SiO 3 Mg : Fe : : 4: 1, 3 : 1, etc.
Orthorhombic.
Sp. gr. 3.1-3.2.
Oedrite:
( MgFe ) jSij0 6 . MgAl 2 SiO 6 .
Orthorhombic.
Sp. gr. 3.1-3.2.
occurrence. Aiithopliyllite and gedrite occur in the schists and gneisses,
both those derived from sedimentary and those derived from igneous rocks.
282 A TREATISE ON METAMORPHISM.
They are frequently associated with hornblende and mica. Anthophyllite
occupies the same position in the rhombic amphiboles that bronzite does in
the rhombic pyroxenes, and gedrite the same position as hypersthene. As
already described (p. 270), the bronzites and hypersthenes alter into
anthophyllite. It is to be expected that gedrite in a similar manner forms
from hypersthene, but this particular alteration is not mentioned in the
standard books of reference. Also, as described (p. 310), anthophyllite
forms as a secondary product from olivine.
Alterations. Anthophyllite by hydration passes into talc (orthorhombic
or monoclinic; sp. gr. 2.75) or bastite (orthorhombic; sp. gr. 2.6). Also,
Lacroix states" that rarely it alters into calcite (rhombohedral ; sp. gr.
2.7135). Supposing the magnesium is to the iron as 3:1, and that the
freed iron separates as hematite (rhombohedral; sp. gr. 5.225), the alteration
to talc may be written as follows:
(1) 2Mg 9 FeSi 4 O 12 +2H 2 O+O=2H 2 Mg s Si ) O 12 +Fe 2 O 3 f k.
The increase in volume of the talc and hematite, as compared with the
anthophyllite, is 11.41 per cent. If the iron oxide be supposed to be
hydrated into limonite (not crystallized; sp. gr. 3.80), the increase in
volume would be still greater. If bastite be produced, and it be supposed
that the magnesium is to the iron as 3:1, the same as in the anthophyllite,
the equation may be written:
(2) 3Mg s FeSi 4 O 1 . 2 +8H 2 O=H 16 Mg 9 Fe s Si 8 O S6 +4Si0 2 +k.
The increase in volume of the bastite and quartz (rhombohedral; sp. gr.
2.6535) as compared with the anthophyllite is 34.09 per cent. If the silica
be supposed to be dissolved the increase in volume is 12.09 per cent.
The particular alterations which gedrite undergoes are not described
in the standard text-books; therefore no attempt is made to write equations
for changes of this mineral.
The alteration of anthophyllite to talc and iron oxide involves hydra-
tion and oxidation. The alteration of anthophyllite to bastite involves
hydration and desilication. Both sets of reactions are, therefore, char-
acteristic of the zone of katamorphism; and it is in this zone, especially in
the belt of weathering, that the changes occur.
" Lacroix, A., Minralogie de la France, Paris, 1893-95, vol. 1, p. 637.
OCCURRENCE OF MONOCLINIC AMPHIBOLES. 283
HONOCLIK1C AMPIIIKOLES.
The monoclinic amphiboles include the following rock-making minerals:
TKKMOUTE, ACTIXOIJTK, (TMXIM1TOMTK, (JRUXERITE, HORNBLENDE, ULAl'COPHANj;. K1EBECKITE, AND
ARPVEDSOMTE.
Tremolite:
CaMaSi.0,,
Monoclinic.
Sp. gr. 2.9-3.1.
Actiiiolite:
Ca(MgFe) 3 SiA 2 .
Monoclinic.
Sp. gr. 3-3.2.
Cummingtonite:
(MgFe)Si0 3 .
Monoclinic.
Sp. gri 3.1-3.32.
Grunerite:
FeSi0 3 .
Monoclinic.
Sp. gr. 3.713.
Hornblende:
Chiefly Ca(MgFe) 3 Si 4 O 12 with (MgFe). 2 (AlFe) 4 Si 2 O 12 , anti Na,AljSiO,,.
Monoclinic.
Sp.gr. 3.05-3.47.
Glcmcophane:
NaAlSi 2 O 6 -(FeMg)SiO s .
Monoclinic.
Sp. gr. 3.103-3.113.
Siebeckite:
Na 2 Fe 2 Si 4 O 12 .FeSiO 3 .
Monoclinic.
Sp. gr. 3.3.
Arfoedsonite:
(Xa 2 CaFe) 4 Si 4 12 -(CaMg) 2 (AlFe) 4 Si 2 O 12 .
Monoclinic.
Sp. gr. 3.44-3.45.
occurrence. The monocliuic ampliibole group of minerals is one of the
most important of the rock-making minerals. Like the pyroxenes, one
form or another of ampliibole may occur in almost any kind of rock,
running from the most basic to the most acid, including both plutonic and
volcanic rocks, the unmodified sedimentary rocks, and metamorphosed,
igneous, and sedimentary rocks. The amphiboles develop extensively as
secondary minerals, and especially is this true for the variety of ampliibole
known as uralite, which, as seen on pp. 274-275, 276-278, is derived from
corresponding pyroxenes.
Tremolite and actinolite are very abundant in the schists metamor-
phosed from carbonate rocks, especially those rich in magnesium and iron.
284 A TREATISE ON METAMORPHISM.
They also occur in the metamorphosed calcareous fragmental sediments.
Where iron is not abundant, as in the marbles, tremolite is the mineral
which ordinarily develops. Where ferrous iron is plentiful actinolite
normally forms. Where iron is the chief or only carbonate, griineritc
ordinarily develops. Tremolite and actinolite also occur as alteration
products in igneous rocks, being noted in diabases, gabbros, and more basic
rocks. The secondary products frequently take the form of asbestos and
jade'. They are frequently associated with talc and serpentine hi steatite-
schists or serpentine-schists. Often, also, tremolite and actinolite are asso-
ciated with pyroxene, epidote, and chlorite. These amphiboles also occur
in veins. Summarizing, as metamorphic minerals, tremolite is derived from
diopside, dolomite, and olivine; actinolite from aukerite, bronzite, hyper-
sthene, olivine, parankerite, and sahlite.
Cummingtonite, the monoclinic amphibole corresponding in composi-
tion with the orthorhornbic amphibole anthophyllite, occurs in various
schists of metamorphic origin. It is not known as an original constituent
of the igneous rocks.
Griinerite occurs most extensively in connection with magnetite and
quartz, or with quartz alone, thus constituting griinerite-magnetite-quartz-
schists, or griinerite-quartz-schists. The grtinerite in such cases often
develops as a secondary product from the alteration of siderite, as explained
on page 245. Greenalite, probably having the formula FeSi0 3 .nH 2 O, occurs
extensively, as in the Biwabik formation of the Mesabi series of Minnesota."
If such material were so deeply buried as to be altered under the conditions
of the zone of anamorphism, dehydration would take place and griinerite
would be formed. The mineral also occurs in the garnetiferous micaceous .
schists; but in some of these rocks the griinerite itself develops from the
siderite, as in the case of the pure griinerite-quartz-schists and griinerite-
magnetite-quartz-schists.
Hornblende is the most abundant of the amphiboles, and has a very
widespread occurrence, being found as a principal constituent in various
igneous rocks, including plutonic and volcanic rocks, and among the latter
both in lavas and in tuffs. It also is a constituent of some of the sedi-
mentary rocks. It is a chief constituent of many of the metamorphosed
"Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. U. S. Geol. Survey, vol. 43,
1903, pp. 101-115.
ALTERATION OF MONOCLINIC AMPHIBOLES. 285
rocks, especially those of igneous origin. Not infrequently it is also an
abundant constituent in the metamorphosed sedimentary rocks. The schists
in which the hornblende is the chief constituent, whether of aqueous or
igneous origin, are generally known as amphibolites. In many other schists
and gneisses which are chloritic and micaceous it is an important constitu-
ent. As a metamorphic mineral hornblende has been noted as derived from
almandite, augite, melanite, and pyrope.
Glaucophane occurs abundantly in certain of the amphibole-schists,
especially those which are derived from the debris of basic rocks which were
originally rich in sodium. Naturally, such rocks have a somewhat limited
occurrence; but where they do occur abundantly, as in the Coast Ranges
.of California, glaucophane is also very abundant in fact, is the chief con-
stituent of some of the schists, so that they may properly be called
glaucophane-schists.
Riebeckite occurs in some eruptive rocks which are rich in sodium and
iron, and also in metamorphosed rocks of both sedimentary and igneous
origin. Like glaucophane, it may locally occur abundantly, but is not a
widespread mineral.
Arfvedsonite, a soda-amphibole, very naturally occurs in the soda-
bearing igneous rocks, especially in elseolite-syenites and nepheline-syenites.
Alterations. The minerals of the monoclinic amphibole group, ot such- a
wide variety of composition and extensive occurrence, have naturally a
large number of alteration products. The more common of these are talc,
serpentine, bastite, chlorite, epidote, and biotite. These are frequently
accompanied by more or less magnetite, hematite, and limonite. In some
cases the amphiboles alter into the zeolites, pinite, and chabazite.
Taking up the individual minerals, tremolite is most frequently trans-
formed into talc (orthorhombic or monoclinic; sp. gr. 2.75), which may be
accompanied by calcite (rhombohedral ; sp. gr. 2.7135). Accinolite com-
monly alters into talc or serpentine (monoclinic; sp. gr. 2.575), often
with the simultaneous formation of calcite, quartz (rhombohedral; sp. gr.
2.6535), and iron oxide. Cummingtonite commonly alters to bastite (ortho-
rhombic; sp. gr. 2.6). The standard text-books do not describe the altera-
tions of griinerite, although it is believed to alter to the iron oxides.
Hornblende under weathering conditions ordinarily changes to chlorite
(monoclinic; sp. gr. 2.71-2.725), which is often accompanied by epidote
286 A TREATISE ON METAMORPHISM.
(inouoclinic; sp. gr. 3.38), calcite, quartz, iron oxides, and siderite (rhombo-
hedral; sp. gr. 3.855). Under deep-seated conditions biotite (monoclinic ;
sp. gr. 2.90) is frequently a product of the alteration of hornblende, and with
the biotite epidote may simultaneously form. Rarely serpentine is also pro-
duced. While these are the usual alterations of hornblende, in some cases,
under conditions of high temperature, hornblende alters into augite (mono-
clinic; sp. gr. 3.4), with the simultaneous separation of magnetite (isometric;
sp. gr. 5.174). Such alteration of the hornblende has been noted, according
to Lacroix, both in lavas and in bombs." In the lava the change is attributed
to the action of the magma, being analogous to resorption; but in the bombs
it is attributed to heat alone. The alterations of glaucophane and riebeckite
are not described in the standard text-books. Arfvedsonite under certain
conditions changes into an acmite (monoclinic; sp. gr. 3.525) free from
calcium. 6 With the acmite occur limonite (amorphous; sp. gr. 3.80), mag-
netite, and sometimes lepidomelane (monoclinic; sp. gr. 3.0-3.2). The
change of hornblende ta augite and the change of arfvedsonite to acmite
are the reverse of the process of uralitization.
The alteration of tremolite to talc may be written as follows :
(1 ) CaMg,Si 4 O ls + H,0+CO a =H 2 Mg 3 Si,Oi. i +CaCOs+k.
The increase in volume of the talc and calcite as compared with the
tremolite is 25.61 per cent. The decrease in volume of the talc alone as
compared with the tremolite is 0.83 per cent.
The alteration of actinolite to talc, supposing the excess of iron to sepa-
rate as hematite (rhombohedral ; sp. gr. 5.225), and supposing that the
Mg : Fe : : 2 : 1, is as follows:
(2) 6CaMg 2 FeSi 1 O 12 +4H 3 O+6CO 2 +3O=4H 2 Mg 3 Si 1 O 12 +6CaCO s +3Fe 2 3 +8SiO 2 +k.
Supposing all the compounds to remain as solids the increase in volume
is 20.33 per cent. If magnetite be formed instead of hematite the increase
in volume is somewhat less; if limonite be formed, considerably more. If
it be supposed that all the compounds except the talc are dissolved the
decrease in volume is 36.51 per cent.
The alteration of actinolite to the variety of serpentine known as
bastite may be written as follows:
(3) Ca(MgFe) s Si 4 O 12 +2H 2 O+CO 2 =H 4 (MgFe) ? 8iA+CaCO,+2SiO 2 +k.
a Lacroix, A., Mineralogie de la France, Paris, 1893-95, vol. 1, pp. 668-669.
&Br6gger, W. C., Die Mineralien der Syenitpegniatitgange der Siidnorwegischen, Augit- und
Nephelin-Syenite: Zeitschr. fur Kryst. und Min., vol. 16, 1890, pp. 406-407.
ALTERATIONS OF HORNBLENDE. 287
Supposing that the Mg : Fe : : 2 : 1, the equation is
(4) CaMg 2 FeSi 4 O I2 +2H 2 O+CO 2 =H 4 Mg 2 FeSi 2 O 9 +CaCO s -i-2SiO 2 +k.
The increase in volume, supposing all the compounds to separate as solids,
is 38.67 per cent. If only the bastite remains as a solid, the decrease in
volume is 18.06 per cent.
The alteration of cummingtonite into bastite is as follows:
(5) 3(MgFe)Si0 3 +2H 2 0=H 4 (MgFe) 3 SiA+Si0 2 +k.
Supposing that the Mg: Fe : : 3 : 1, the equation is
(6) 3Mg 3 FeSi 4 O 12 +8H 2 O=H 16 Mg 9 Fe 3 Si 8 O 36 44SiO 2 +k.
The increase in volume of the bastite and quartz as compared with the
cummingtonite is 36.76 per cent; of the bastite alone, 14.2 per cent.
In writing equations for the alteration of hornblende into chlorite and
accompanying minerals the soda-bearing part of the molecule will be
omitted, since the amount of soda present in ordinary hornblende is small.
Supposing that there are eight actinolite molecules in the hornblende to
two of the sesquioxide molecules, that the MgO : FeO : : 2 : 1, and the
AL 2 3 : Fe 2 3 : : 2 : 1, that the chlorite produced is on the border line be.tween
prochlorite and clinochlore, and that the Al : Fe in the epidote as 2 : 1, the
reaction may be written as follows:
(7) 8CaMg 2 FeSi 4 12 .2Mg 4 Fe 2 Al 8 Fe 4 Si 6 O 36 +21H 2 O+16CO 2 =
2(H 20 Mg 12 Al 6 Si 7 O 4 .,)+2HCa 2 Al 2 FeSi s O 13 +4CaCO 3 +i2FeC0 3 +24SiO 2 +3Fe 2 O 3 +k.
Provided all the compounds separate as solids the increase in volume is
25.39 per cent.
It is noticeable that the equation for the alteration of the hornblende
to chlorite as a chief resultant product demands that epidote, calcite, sid-
erite, quartz, and hematite be produced; and corresponding with this,
Lacroix noted all of these minerals as accompaniments of the chloritic
alteration with the exception of hematite." Of course all or a larger part
of the iron may pass into the form of iron oxide magnetite, hematite, or
limonite in which case some oxygen would need to be added to the equa-
tion; the amount of CO 2 required will be less; and the oxide of iron will
replace the iron carbonate partly or wholly. As these modifications can
easily be made in the equation, it hardly seems necessary to write out
formulae for them.
"Lacroix, cit., pp. 667-668.
288 A TREATISE ON METAMORPHISM.
The change of hornblende to biotite requires the addition of potassium.
The potassium can be derived from some other mineral. It is perhaps
most, frequently derived from orthoclase, although it is undoubtedly in
many cases derived from leucite. Supposing it is derived from orthoclase,
and therefore is in the form of potassium silicate, K 2 Si0 3 , and that the
actinolite molecule in the hornblende is to the sesquioxide molecule as 2 : 3,
the change may be represented as follows:
(8) 2Ca(MgFe) 3 Si 4 12 -3(MgFe) 2 (AlFe) 4 Si 2 1 2+aK 2 Si0 3 +Si0 2 +3H 2 0+2C0 2 =
6HK(MgFe) 2 (AlFe),Si,0 12 +2CaCO,+k.
In order to ascertain the volume relations it is necessary to make assump-
tions with reference to the proportions of the Mg and Fe, and of the Al and
Fe. Supposing the magnesia is to the iron protoxide as 2:1, and the
alumina is to the iron sesquioxide as 2 : 1, the equation is:
(9) 2(CaMg 2 FeSi 4 12 ).Mg 1 Fe 2 Al 8 Fe 4 Si 6 36 +3K 2 SiO,+Si0 2 +3H 2 0+2C0 2 =
H 6 K 6 Mg 8 Fe 4 Al 8 Ffe 4 Si 18 72 +2CaC0 3 +k.
The increase in volume of the biotite and calcite as compared with the
hornblende and quartz is 41 13 per cent. It has been noted, however,
that tlie alteration of hornblende to biotite is often accompanied by the
separation of epidote; and this is natural, since there is residual calcium in
the hornblende not needed by the biotite, which could pass into the
epidote. Supposing this residual calcium to pass into the epidote, the
reaction may be written as follows:
(10) 8Ca(MgFe) 3 Si 4 Oi 2 .18(MgFe) 2 (AlFe) 4 Si 2 O, 2
30HK(MgFe) 2 (AlFe) 2 Si 3 O 12 + 4HCa 2 (AlFe) 3 Si 3 O 13 +k.
In order to calculate the volume relations it may be supposed that the
MgO : FeO as 2:1, and the A1 2 O 3 : Fe 2 O 3 as 2:1, the equation being
(11) 8CaMg 2 FeSi 4 O 12 .6Mg 4 Fe.,Al 8 Fe 4 Si 6 O 36 fl5K 2 SiO 3 +19SiO 2 +17H. ! O=
The increase in volume of the biotite and epidote as contrasted with the
hornblende and quartz is 30.05 per cent. The increase in volume for
equations (9) and (11) would be much less if the K 2 SiO 3 were taken into
account,
Where serpentine also occurs as an alteration product accompanying
the chlorite, biotite, epidote, and other products, this mineral is doubtless
derived from the actinolite part of the molecule, and an equation may be
readily written which represents the simultaneous formation of the bastitic
ALTERATIONS OF HORNBLENDE. 289
form of serpentine by supposing that the number of actinolite molecules is
greater than given in the above equations and that such excess of these
molecules passes into bastite, according to equation (3).
While the more common alterations of hornblende are to chlorite,
biotite, epidote, and accompanying minerals, as above explained, the change
of hornblende into augite, just the reverse of that of augite into hornblende
described on pages 274-278, does take place, and probably on a great scale
at sufficient depth. .The equation for one case may therefore be written:
(12) Ca 2 Mg s Fe 3 SiA 4 . (MgFe) 2 (AlFe) < Si 2 O 12 +2CaCO s =
2[Ca 2 MgFeSi 4 O, 2 . ( MgFe) ( AlFe) 2 SiO 6 ] +FeC0 5 + MgCO 3 +k.
If the Mg : Fe : : 2 : 1, and the Al : Fe : : 2 : 1, the equation is
(13) Ca 2 Mg 3 Fe 3 Si 8 2 4-Mg,Fe 2 Al 8 Fe 4 Si 6 O 36 +2CaCO 3 =
2[Ca 2 MgFeSi 4 12 . Mg 2 FeAl 4 Fe 2 Si 3 18 ] +FeC0 3 +MgCO,+k.
The decrease in volume of the augite as compared with the amphibole is
4.13 per cent.
It is not supposed that the above equations for the alteration of horn-
blende necessarily represent the actual facts of specific cases. Doubtless in
most instances materials from minerals aside from those given enter into
the alterations, and the actual changes are more complex than represented.
However, the equations very clearly show why it is that the production of
chlorite from hornblende demands also the production of other minerals
which Lacroix says so generally accompany chlorite. Also, they show why
epidote so frequently accompanies biotite secondary to hornblende. The
equations may be considered as average cases, which approximate to the
alterations that actually occur in many instances. The volume relations
calculated from the equations also are probably averages, for the proportions
of the elements taken in the equations given are chosen from a considera-
tion of analyses of the various minerals. The equations at least make a
quantitative estimate of the relations of the original and secondary minerals,
and therefore will lead to closer observations as to the minerals which
result from the alteration of hornblende, and their relative proportions.
The change of arfvedsonite into acmite is so uncertain in its character
that no attempt is made to write out the equations. In order to satisfactorily
write equations for this alteration it is necessary to know the composi-
tion of the particular arfvedsonite which changes into the particular acmite,
and what other minerals aside from the acmite are produced in the change.
MON XLVII 04 19
290 A TREATISE ON METAMORPHISM.
The alteration of tremolite to talc, equation (1), is that of hydration
and carbonation. The alteration of actinolite to talc, equation (2), is that
of hydration, carbouation, desilication, and oxidation. The alteration of
actinolite to bastite, equations (3) and (4), is that of hydration, carbonation,
and desilication. The alteration of cuinmingtonite to bastite, equations (5)
and (6), is that of hydration and desilication. All these changes take place
with the liberation of heat and with expansion of volume, provided the
compounds which form mainly separate as solids. Whether or not there is
an actual increase in the volume as a result of the changes depends, of
course, upon the amounts of the secondary material which is dissolved. It
is therefore clear that all of these changes are those which are typical of the
zone of katamorphism, and especially the belt of weathering. Moreover,
some of the changes, like that of actiuolite to talc and the accompanying
compounds, illustrate all the processes normal to this position; i. e., hydration,
carbonation, oxidation, and desilication. The fact that calcite is so
frequently found associated with the talcs and serpentines secondary to
tremolite, cummingtonite, and actinolite, is rendered perfectly clear by the
equations; for there is always a residuum of calcium which evidently, under
the conditions of the upper physical-chemical zone, unites with the carbon
dioxide and produces calcium carbonate, which frequently separates as the
mineral calcite in large part, but which doubtless is frequently largely or
altogether carried away in solution.
The alteration of hornblende into chlorite and accompanying minerals
is one of liberation of heat and expansion of volume. It is an alteration
also of carbouation, and of oxidation in case some of the ferrous iron be
changed to sesquioxide. It is therefore to be expected in the upper
physical-chemical zone, and as a matter of fact it occurs there. The change
from hornblende to biotite is a much deeper seated alteration. It involves
hydration, silication, and possibly earbonation, and thus includes an
unusual combination of reactions. Corresponding with these facts the
change of hornblende to biotite is one which takes place under rather
deep-seated conditions, particularly in connection with profound mechan-
ical action. The physics of the interchanges between hornblende and
augite are elsewhere discussed (see pp. 279-280); but it may be said that
the change of the first to the second involves decrease of volume, and,
corresponding with this fact, is known to take place under very deep-
seated conditions of metamorphism.
OCCURRENCE AND ALTERATION OF IOLITE. 291
IOLITE (CORDIERITE).
lolite (cordierite) :
H 2 (MgFe) 4 Al 8 Si 10 37 .
Orthorhonibic.
Sp. gr. 2.60-2.66.
occurrence. lolite occurs in a great variety of schists and gneisses. In
some cases it is so abundant as to make the rock a cordierite-gneiss. It is
associated with the very heavy metamorphic minerals, such as tourmaline,
andalusite, sillirnanite, garnet, etc. lolite occurs, likewise, in ejected frag-
ments of volcanoes and as a contact mineral in connection with dikes ; also
rarely as an original mineral in igneous rocks.
Alterations. The most common alteration is simple hydration. Further
changes may remove some of the ferrous iron or introduce alkalies, or
both, forming pinite (massive; sp. gr. 2.775). Simultaneously with this an
isotropic substance is said to be formed. lolite sometimes passes into a
chlorite similar to talc.
By the hydration of iolite, according to Clarke, chlorophyllite (crystal-
lization not given; sp. gr. 2.77) is formed. Supposing the Mg and Fe to
be in the same proportions both in the iolite and in the chlorophyllite, the
reaction is simple:
(1) H 2 (MgFe) t Al 8 Si 10 37 +3H. 2 = H 8 (MgFe) 1 Al 8 Si 10 1 o+k.
If it be supposed that the Mg : Fe : : 3 : 1 in both compounds, the equation is
(2) H 2 Mg 3 FeAl 8 Si 10 31 -i-3H 2 0=H 8 Mg 3 FeAl 8 Si 10 10 +k.
The decrease in volume is 0.86 per cent.
The reaction being hydration, one would expect it to involve increase
of volume, but the chlorophyllite produced is enough heavier to compensate
for this. One would expect the reaction to take place in the zone of kata-
morphism, but observations on this point are not known to me.
The character of the product which forms simultaneously with pinite
being unknown, and the character of the chlorite which forms as a sec-
ondary product not being ascertained, it seems hardly worth while to
attempt to write equations for these alterations, for they would be largely
conjectural.
"Clarke, F. W., The Constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 83.
292 A TREATISE ON METAMORPHISM.
NEPHELITE GROUP.
XEPHELITE AJiD t'AXCBIMTE.
The nephelite group includes
Nephelite:
NaAlSiO 4 .
Hexagonal.
Sp. gr. 2.55-2.65.
Cancrinite :
H 6 NasCa (NaCO,),Al 8 Si,O M .
Hexagonal.
Sp. gr. 2.42-2.50.
NEPHELITE.
occurrence. Nephelite is a sodium-aluminum silicate. Commonly the
sodium is in part replaced by potassium. Nephelite occurs in both ancient
and modem igneous rocks, both surface and deep seated. It is abund-
ant in the syenite-schists and syenite-gneisses of certain localities, but
is not known in the metamorphosed secondary rocks. This is doubtless
due to its ready alteration. Nephelite has been produced artificially at
220 C. by a reaction between kaolinite and an alkaline carbonate. As a
secondary product nephelite forms from leucite, but this alteration is not
an important source of the mineral. Nephelite is al(-o probably derived
from sodalite.
Alterations. The most frequently observed alteration of nephelite is to
the zeolites, and especially to hydronephelite (hexagonal; sp. gr. 2.263),
natrolite (orthorhombic; sp. gr. 2.20-2.25), thomsonite (orthorhombic; sp.
gr. 2.32.4), and analcite (isometric; sp. gr. 2.22-2.29). Simultaneously
with the formation of some of the zeolites diaspore (orthorhombic; sp. gr.
3.3-3.5), or gibbsite (monoclinic; sp. gr. 2.35), or kaolinite (monoclinic; sp.
gr. 2.615), or some combination of these, is frequently formed.
The reaction for hydronephelite is
(1) 6NaAlSiO 4 +7H 2 O+CO,=2(HNa. ! Al s Si s O 12 .3H. ! O) -t-Na-jCOa+k.
The increase in volume is 23.49 per cent.
The alteration next in importance is to natrolite and gibbsite, or to
natrolite and diaspore. The reaction in the former case is:
(2) 6NaAlSi0 4 +7H 2 0+C0 1 =2Na 2 Al 2 H 4 Si 8 I2 +2Al (OH) s +Na,CO s +k.
ALTERATIONS OF NEPHELITE. 293
Supposing- the sodium carbonate to be earned off in solution, the increase
in volume would be 24.4G per cent. If two molecules less of water were
added, instead of two molecules of gibbsite, two molecules of diaspore
would be formed, according to the reaction:
(3) 6XaAlSi0 1 +5H 2 O+C0 2 =2Na 2 Al 2 H 1 Si s O 1 2+2AlO(OH)+Na 2 CO s +k.
In this case the increase in volume would be only 15 per cent.
In the production of thomsonite, calcium must replace the sodium. It
will be assumed that this calcium is derived from calcium carbonate. The
reaction will then be
(4) 6NaAlSiO 4 H-7H 2 O+3CaCO 3 =Ca 3 Al 6 Si 6 O 24 .7H,O+r-iNa 2 CO 3 +k.
Supposing the calcium carbonate to have been brought in solution and the
sodium carbonate carried away in solution, the increase in volume is 24.60
per cent.
The less common alteration of nephelite to the zeolite analcite, with
the simultaneous production of diaspore or gibbsite, is expressed by the
following reactions:
(5) 4NaAlSiO 4 +3H 2 O+CO 2 =Na J Al 2 Si 1 12 .2H 2 O+2[AlO(OH)]+Na 2 CO,+k
or
(6) 4NaAlSi0 4 +5H 2 O+CO 2 =Na 2 Al 2 SiA^2H 2 O+2Al(OH) s +Na 2 CO s +k.
In the first case diaspore is simultaneously formed, and in the second case
gibbsite. Supposing the sodium carbonate to be carried away in solution
the increase in volume is 5.49 per cent if diaspore be formed, and 19.68
per cent if gibbsite be formed.
Alterations of nephelite to muscovite (monoclinic; sp. gr. 2. 88), to hydro-
muscovite (pinite) (massive; sp. gr. 2.775), and to kaolinite (monoclinic; sp.
gr. 2.6-2.63) have also been noticed. Where this alteration takes place
the nephelite is probably a potassium-bearing one. Assuming that the
amount of potassium is one-third of the sodium, the reaction may be written:
(7) 2KNa s Al 4 Si 4 O I8 +4H 2 O+300 2 =2KH 2 Al s Si 8 O 12 +H 1 Al 2 Si 2 O,+ 3Na J CO 3 +k.
The decrease in volume of the muscovite and kaolin as compared with the
nephelite is 16.50, provided the sodium carbonate is carried away in solu-
tion. The decrease is 13 per cent if the products are pinite and kaolinite.
The volume of the muscovite alone is 38.46 per cent less than that of the
nephelite.
294 A TREATISE ON METAMORPHISM.
Another alteration of nephelite of some importance is to sodalite
(isometric; sp. gr. 2.14-2.30)
(8) 3NaAlSi0 4 -j-NaCl=XaC1.3NaAlSi0 4 +k.
Supposing the NaCl to be added in solution, the increase in volume is
33.14 per cent. If the sodium chloride be present as solid halite (isometric;
sp. gr. 2.1-2.6), the increase in volume would be 15.64 per cent.
While the change is not recorded, it is believed to be highly probable
that nephelite during mass deformation under deep-seated conditions may
change into feldspar, probably albite (triclinic; sp. gr. 2.62-2.65). This
reaction would require the addition of silica, as follows:
(9) NaAlSi0 4 +2SiO 2 =NaAlSi s O 8 +k.
Supposing the silica to have been present as quartz (rhombohedral ; sp. gr.
2.653-2.654), the decrease in volume would be 0.41 per cent.
The formation of the zeolites, and simultaneously the minerals gibbsite
or diaspore, equations (1) to (6), are all alterations of hydration, carbona-
tion, and expansion of volume, except that of thomsouite, equation (4),
which does not involve carbonation. It is therefore to be expected that
these are reactions which take place in the zone of katamorphism, and such
is the fact. As a result of the alteration of the nephelites to the zeolites in
this zone, a part of the sodium separates and probably goes into solution as
sodium carbonate, and thus we have one of the sources of this compound
which so frequently occurs in underground waters, especially in volcanic
regions. The formation of muscovite and kaolinite from nephelite is a
reaction involving hydration and carbonation and decrease of volume, and
therefore is characteristic of the zone of katamorphism. The formation of
sodalite from nephelite is one which might take place in either physical-
chemical zone, only in the upper zone the sodium chloride would probably
be added in solution, while in the lower zone it would pi'obably be derived
from solid halite.
CANCRIXITE.
occurrence. Cancrinite is known only in the nepheline syenites.
Alterations. By Dana it is mentioned as altering to natrolite (orthorhombic ;
sp. gr., 2.225). The reaction, supposing the excess of alumina passes into
gibbsite (monoclinic; sp. gr., 2.35), may be as follows:
) 2 Al 8 Si 9 O 36 +6H s O=
3(Na 5 Al 2 Si 3 10 .2H. i O)+2Al(OH), > 4CaCO,+Na. ( CO s +k.
OCCURRENCE AND ALTERATIONS OF SODALITE. 295
The increase in volume of the natrolite, gibbsite, and calcite (rhombo-
hedral; sp. gi-., 2.7135) as compared with cancrinite is 8.64 per cent.
The reaction is that of hydration and breaking- up of a complex com-
pound into several simpler compounds requiring greater volume, and is
therefore typical of the zone of katamorphism.
SODALITE GROUP.
KODALITK, HAl'VMTE, AND >OSELITE.
The socialite group includes
Sodalite:
XaCUXaAlSiO,.
Isometric.
Sp. gr., 2.14-2.30.
Hauyniie:
Xa.,Ca(XaSO 4 . Al) Al 2 Si 3 0, 2 .
Isometric.
Sp. gr., 2.4-2.5.
Noselite:
Na 4 (NaS0 4 .Al)Al 2 Si 3 12 .
Isometric.
Sp. gr., 2.25-2.4.
SODALITE.
occurrence. Socialite is sodium aluminum silicate with some chloride.
Socialite occurs as an original constituent in the igneous rocks, both surface
and deep seated. It is not known in the secondary rocks or their metamor-
phosed equivalents. In fact, the occurrence of sodalite is almost identical
with that of nephelite, which mineral is one of its sources.
Alterations. The alteration products of sodalite are also identical with
those of nephelite, except that nephelite passes into sodalite, and the
reverse reaction is not recorded, although, as noted below, it is believed to
occur. The alterations of sodalite into minerals similar to those into
which nephelite alters is natural, as sodalite is made up of the nephelite
molecule with the addition of sodium chloride.
Sodalite alters to the same zeolites as does nephelite, viz, to hydro-
nephelite (hexagonal; sp. gr., 2.263), natrolite (orthorhombic ; sp. gr., 2.2-
2.25), thomsonite (orthorhombic; sp. gr., 2.3-2.4), and analcite (isometric;
sp. gr., 2.22-2.29). Simultaneously with the formation of some of the
296 A TREATISE ON METAMORPHISM.
zeolites, diaspore (orthorhombic ; sp. gr., 3.3-3.5) or gibbsite (monoclinic;
sp. gr., 2.3-2.4) is frequently formed.
In the production of hydronephelite the reaction is
(1) 2(NaC1.3NaAlSiO 4 )+4H 2 O+CO 2 =2HNa,Al3Si3O 12 .3H 2 O+2NaCl+Na,CO,+k.
Supposing that the sodium chloride and sodium carbonate are dissolved,
the decrease in volume is 7.25 per cent.
In the alteration of sodalite to natrolite, gibbsite or diaspore is also
produced. The reaction, provided gibbsite be produced, is
(2) 2(NaC1.3NaAlSi0 4 )+7H 2 0+C0 2 =2Na 2 Al 9 H 4 Si 8 1J +2Al(OH) 3 +2NaCl+Na 2 CO s +k.
If two molecules less of water were added, in place of the gibbsite two
molecules of diaspore would be produced, according to the reaction:
(3) 2(NaC1.3NaAlSiO 4 )-t-5H. i O+CO. ! =2Na. ! Al 2 H 4 Si 3 O la -(-2AlO(OH)+2NaCl+Na. ! CO s +k.
Supposing the sodium chloride and sodium carbonate to be dissolved,
the decrease in volume in the first case would be 6.52 per cent, and in the
second case 13.62 per cent.
In the production of thomsonite the reaction is
(4) 2(NaC1.3NaAlSiO 4 )+7H 2 O+3paCO 3 =Ca s Al 6 Si 6 O 24 .7H 2 O+2NaCl-t-3Na 2 CO 3 +k.
Supposing the calcium carbonate to have been in solution and the sodium
chloride and sodium carbonate to be taken into solution, the decrease in
volume is 6.41 per cent.
The alteration of sodalite to analcite and to diaspore may be written
as follows:
(5) 4(NaC1.3NaAlSiO 4 ) +9H 2 O+3CO 2 =
3(Na.,Al 2 Si 4 O 12 .2H 8 O) +6A1O( OH ) +4NaCl+3( Na 2 CO s ) +k.
If six additional molecules of water were added, as in the case of the
reaction written for natrolite, gibbsite instead of diaspore would be formed.
The reaction is
(6) 4(NaC1.3NaAlSiO 4 )+15H 2 O+3CO 2 =
3(Na,Al 2 Si 4 12 .2H 2 O)+6Al(OH) 3 +4NaCl+3Na. ! C03+k.
Supposing the sodium chloride and sodium carbonate to be taken into
solution, the decrease in volume is 20.77 per cent in the case of diaspore
and 10.11 per cent in the case of gibbsite.
OCCURRENCE OF HAUYNITE AND NOSELITE. 297
The reaction for the alteration of sodalite to muscovite (monoclinic;
sp. gr., 2.76-3) and kaolinite (monoclinic; sp, gr., 2.6-2.63), supposing
potassium to replace one-fourth of the sodium of the silicate, would be
(7) 2(4NaCl.K 3 Na 9 Al 12 Si 12 O 48 )+12H 2 O+9CO 2 =
6KH 2 Al 3 Si 3 O,,+3H 1 Al 2 Si 2 O !) +8NaCl+9Na ! CO3+k.
Provided the sodium chloride and sodium carbonate are dissolved, the
decrease in volume is 37.07 per cent.
As in the case of nephelite, it is suspected that sodalite may pass into
albite or other feldspar. However, as this chang-e is conjectural, no reaction
will be written.
The various reactions above given are analogous, both from, a physical-
chemical point of view and from a geological point of view, with the corre-
sponding reactions in the case of nephelite. Hence it need only be said
that the changes written are those occurring in the zone of katamorphism,
in which rock fracture occurs and ground solutions are active. These
ground solutions by the changes become bearers of sodium chloride and
sodium carbonate.
The relations between the alterations of nephelite and sodalite illustrate
very well the law of mass action. In the laboratory, if nephelite be exposed
to the "slow action of fused sodium chloride with the addition of vaporized
Nad" it is changed into sodalite." On the contrary, however, in nature,
where water is abundant and the amount of sodium chloride is small, the
reverse reaction takes place, and sodium chloride is abstracted. Probably
at the same time the nephelite molecule is altered as above indicated.
Thus, while observation does not as yet record nephelite as an alteration
product of sodalite, it is believed to be highly probable that this mineral is
really formed as a stage in the process of alteration of sodalite.
HAttYNITE AXD NOSELITE.
occurrence. Haiiyiiite is sodium-calcium-aluminum silicate with some
sulphate. Noselite is sodium-aluminum silicate with some sulphate.
"Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 430. See also Rosenbusch, Mikroskopische Physiographic, Stuttgart, 1885, p. 284.
298 A TREATISE ON METAMORPHISM.
Haiiynite and noselite are common in certain igneous rocks, especially
those which contain nephelite and leucite. Neither of these minerals is
known in the schists and gneisses derived from the sedimentary rocks.
Alterations. The minerals alter to zeolites, especially to natrolite (ortho-
rhombic; sp. gr. 2.20-2.25), stilbite (monoclinic ; sp. gr. 2.094-2.205), and
chabazite (rhombohedral ; sp. gr. 2.08-2.16). Simultaneously with certain
of these alterations calcite (rhombohedral; sp. gr. 2.713-2.714) also forms.
Noselite passes into natrolite according to the following reaction :
(1) 2Na 4 (NaSO.Al)Al,Si,0 ls +CO,+7H,O=-
2(H 4 Na 2 Al 2 Si,0 12 )+2Al(OH) 3 +2Na 2 S0 4 +Na,C0 3 +k.
It appears that the change requires the formation of gibbsite (mono-
clinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic; sp. gr. 3.3-3.5), although
these minerals are not recorded as forming contemporaneously with the
natrolite. Supposing the gibbsite to separate as a solid, and the sodium
sulphate and sodium carbonate to be taken into solution, the decrease in
volume is 16.44 per cent.
The parallel reaction for the passage of haiiynite into natrolite and
gibbsite is as follows:
(2) 2Na,Ca(NaSO 4 . Al) Al 2 Si 3 O 12 +2CO 2 +8H 2 O=
2(H 4 Na,Al 2 Si 3 O, 2 )+2Al(OH) s +2CaCO 3 +2NaHSO 4 +k.
Supposing the gibbsite and calcite to remain as solids with the natrolite, but
the sodium acid sulphate to pass into solution, the increase in volume is 4.99
per cent.
As stilbite is a calcium-bearing silicate, it may be assumed that this
forms from haiiynite rather than noselite. The reaction is as follows:
(3) 6Na 2 Ca(NaSO 4 .Al)Al. ! Si 3 O 12 +36H 2 O+6CO 2 =
It appears that the reaction for the formation of stilbite thus requires the
formation of calcite, and also of gibbsite or diaspore. The equation is
written for the former mineral, but could readily be changed to the latter.
Supposing the calcium carbonate and the gibbsite, as well as the stilbite, to
be solids, and the other compounds to be taken into solution, the increase
in volume is 0.460 per cent.
MINERALS OF GARNET GROUP. 299
The reaction for the formation of chabazite from haiiynite is
(4) 4Na,Ca(NaS0 4 .Al)Al 2 Si 3 12 +24H 2 O+6CO 2 =
Ca 3 Al 6 (Si0 4 ) 3 (Si 3 8 )3-18H 2 0+4Al(OH) 3 +CaS0 4 +Al 2 (S0 1 ) 3 +6Na. ! C0 3 +k.
This reaction again requires the formation of gibbsite or diaspore. Sup-
posing the compound to be gibbsite, and it and the chabazite to remain as
solids, and the other compounds to be taken into solution, the decrease in
volume is 7.46 per cent.
The alterations of haiiynite and noselite to the zeolites, calcite, and
gibbsite or diaspore are all reactions of hydration and carbonation and
liberation of heat. If the readily soluble compounds are dissolved, as is
probable, the volume is decreased in most instances. The reactions are
therefore characteristic of the zone of katamorphism.
GAKNET GROUP.
(JROSSULARITE, PYROPE, ALMAXDITK, SPESSARTITE, MELAXITE, AXD UVABOTITE.
The garnet group includes the following rock-making species:
Grossularite:
Ca 3 Al. 2 Si 3 O 12 .
Isometric.
Sp. gr. 3.55-3.66.
Pyrope:
Mg 3 Al 2 Si s 12 .
Isometric.
Sp. gr. 3.70-3.75.
Almandite:
Fe 3 Al 2 Si 3 O I2 .
Isometric.
Sp. gr. 3.9-4.2.
Spessartite:
Mn 3 Al 2 Si 3 0,.,.
Isometric.
Sp. gr. 4.00-4.30.
Melanite:
Ca 3 Fe 2 Si 3 O 12 .
Isometric.
Sp. gr. 3.80-3.90.
Uvarovite:
Ca 3 O 2 Si 3 O 12 .
Isometric.
Sp. gr. 3.41-3.52.
300 A TREATISE ON METAMORPHISM.
occurrence. Some form of garnet is a very common mineral in a great
variety of the schists and gneisses, including those which are derived from
sediments and from all forms of igneous rocks, plutonic and volcanic, both
lavas and tuffs. Ordinarily the garnet is a subordinate constituent in these
rocks, although in some cases it becomes one of the chief constituents. The
mineral has its most widespread occurrence in the metamorphosed rocks
which have altered under the influence of mechanical action, or with the
assistance of igneous injections, or both. Not infrequently where garnet is
particularly abundant combined contact and mechanical action have assisted
in furnishing the conditions favorable to its formation. In many instances
the garnet develops after the mechanical action has ceased, showing that it
was not the movements themselves but the other favorable conditions result-
ing therefrom which produced the garnets. It appears, therefore, that the
conditions favorable for the extensive development of the mineral are heat,
moisture, and high pressure. The mineral garnet is the most important of
a group of heavy metamorphic minerals which form under the conditions
mentioned. Other minerals which form under similar conditions and are
frequently associated with garnet are wollastonite, cordierite, vesuvianite,
scapolite, chondrodite, staurolite, andalusite, sillimanite, cyauite, tourmaline,
zircon, etc. These minerals are all anhydrous, or nearly so, and mostly
of a high specific gravity, many of them having a high symmetry. All of
them are formed by the union of silica with bases, and are therefore
produced by processes of silication. In many instances this simultaneously
involves decarbonation, and this change, as already explained, p. 177,
absorbs heat and lessens the volume of the compounds. They are
therefore minerals which form normally in the zone of anamorphism.
Garnet thus produced can not in general be said to have been derived
from any single mineral. It is usually the result of the rearrangement of
material of two or more adjacent minerals. Dana notes that when garnet
.is fused, and the material recrystallizes, the resultant minerals are usually
pyroxene, melilite, monticellite, scapolite, anorthite, nephelite, etc.
This doubtless gives an indication as to some of the minerals which
are rearranged under the conditions above described for the development of
garnet, which are very different from those of dry fusion. Also it is
Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. 8. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 447.
MINERALS OF GARNET GROUP. 301
certain that various hydrous minerals furnish material for the formation of
garnet, and also the limestones and dolomites. As already noted, garnet
has a great variation in composition, and in a given case one of the pure
species mentioned, or a combination of the molecules of two or more of
them, will be formed which can be derived from the elements available.
For instance, from an impure limestone, calcium-aluminum garnet, grossu-
larite, is likely to form. In the magnesian rocks, magnesium-aluminum
garnet, pyrope, is likely to be produced. In the impure aluminous
carbonates of calcium, magnesium, and iron, some combination of two or
more of the species grossularite, pyrope, almandite, and melanite is likely
to be produced.
Garnet may be an original constituent of some of the igneous rocks.
If this be so, this source of garnet is comparatively insignificant, as it is
very rare indeed that garnet is found in an unaltered igneous rock. In
some of the little altered igneous rocks it is found in lithophysae, but the
garnets in this position are apparently the latest products of crystallization,
the conditions of their formation being analogous to those producing garnets
under the ordinary conditions of rock metamorphism.
Considering the garnets individually, the following statements can be
made as to their occurrence:
Grossularite is especially common in the marbles, where it is frequently
associated with vesuvianite, wollastonite, diopside, etc. It also occurs in
the calcareous schists and gneisses, especially in the calcareous siliceous
rocks, such as calcareous quartzites and calcareous novaculites. Grossularite
also is associated with common garnet in other schists and gneisses. It is
recorded as being derived from melilite and gehlenite.
Pyrope, the magnesium garnet, as would be expected, is especially
prevalent in peridotites and their derivatives, such as serpentine and talc,
since these rocks are rich in magnesium. It also occurs in some basalts.
Almandite, one of the most widespread of the pure garnets, occurs in
granites, schists, gneisses, and granulites, and thus is present in both
feldspathic and feldspar-free schists. Almandite is also known in certain
andesites. It rarely has crystalline forms.
Spessartite occurs in large and small grains in contact rocks, in
porphyritic crystals of large size in quartzites, and is abundant in certain
whetstone-schists. With topaz, it is known in lithophysse in rhyolite.
302 A TREATISE ON METAMOKPHISM.
Melanite is common in basic eruptive rocks rich in alkali. It occurs
especially with nephelite and leucite in phonolites, leucitophyres, nephe-
liuitcs, and tephrites. In connection with contact metamorphism it occurs
with wollastonite and fassaite. It is also found in many serpentines.
Uvarovite is at home in the serpentine*, particularly those which con-
tain chromite. It is also found in the marbles.
Common garnet, ordinarily a molecular mixture of two or more of the
species grossularite, pyrope, almandite, and melanite, is of course more
abundant than the pure species. It occurs in such rocks as amphibolites
and eclogites, in the metamorphosed diabases and gabbros, in the pyroxenic
rocks and their derivatives, and in the schists and gneisses both of igneous
and of sedimentary origin.
Alterations. The minerals into which garnets alter are very numerous,
chlorite (monoclinic, sp. gr. 2.71-2.725), talc (orthorhombic or monoclinic,
sp. gr. 2.75), and serpentine (monoclinic; sp. gr. 2.575), however, being the
more common products. Only the secondary products which occur on
an important scale in the rocks will be discussed, mere mineralogical
occurrences and pseudomorphs being ignored.
Alterations of grossularite are not described in the standard text-books;
but it is known that meionite (tetragonal; sp. gr. 2.72) and zoisite (saus-
surite) (massive; sp. gr. 3.-3.04) are sometimes secondary products of garnet,
and it is natural to suppose that these minerals are derived either from
grossularite or from the grossularite molacule of common garnet, since
grossularite contains the elements in about the right proportions to produce
meionite and zoisite.
Talc and serpentine are minerals which are secondary to garnet, and
from their chemical composition ought to be derived from the pyrope mole-
cule, either from the pure garnet or from the pyrope molecule in com-
bination with other garnet molecules. Pyrope is known to alter into
chlorite. As chlorite is regarded as a molecular mixture of serpentine and
amesite (crystallization not determined; sp. gr. 2.71), equations are written
for its alterations into amesite and into average chlorites. Pyrope further
alters into enstatite (orthorhombic; sp. gr. 3.2) and spinel (isometric; sp. gr.
3.8), these minerals frequently forming kelyphite rims about the garnet.
Almandite is recorded as altering into chlorite and into hypersthene
(orthorhombic; sp. gr. 3 45) and spinel, which minerals form kelyphite rims
ALTERATIONS OF MINERALS OF GARNET GROUP. 303
about the garnets. It seems probable that in such cases with the alman-
dite there is also present the pyrope molecule, and the reactions for the
formation of chlorite, .spinel, and hypersthene, after almandite, as written
include the pyrope molecule. In the case of the Spurr mine chlorite,
secondary to garnet, the species has been determined to be aphrosiderite"
(massive; sp. gr. 2.90).
Alterations of spessartite, melanite, and uvarovite, as pure species, are
not described in the standard text-books.
Common garnet most frequently alters into chlorite. Often also it
changes into epidote (monoclinic; sp. gr. 3.38) or into hornblende (mono-
clinic; sp. gr. 3.26). The mixture of almandite and pyrope altering into
aphrosiderite, and into hypersthene and spinel, may be considered as alter-
ations of common garnet. Where epidote is produced it is probable that
the molecules from which it is derived are a mixture of grossularite and
melanite. Where hornblende is produced it is probable that the molecules
are a mixture of pyrope. almandite, and melanite. In the alterations of
the common garnets any of the iron oxides, magnetite (isometric; sp. gr.
5.174), hematite (rhombohedral; sp. gr. 5.225), or limonite (amorphous;
sp. gr. 3.80), may be produced.
The change from grossularite to meionite may be written as follows:
(1) SCasA^SisOu+SCO^CatAleSisb^+SCaCOs+SSiOz+k.
The increase in volume of the meionite, calcite (rhombohedral; sp. gr.
2.7135), and quartz (rhombohedral; sp. gr. 2.6535) as compared with the
grossularite is 54.62 per cent.
The change of grossularite to zoisite may be written as follows:
(2) 3Ca 8 Al 2 Si s O I2 +5CO J +H. ! O=2HCa 2 Al 3 Si3Oi3+5CaCO s +3SiO 2 +k.
The increase in volume of the zoisite, calcite, and quartz as compared
with the grossularite is 40.49 per cent.
The alteration of pyrope to talc may be written in two ways, depend-
ing upon whether the excess of magnesium over that required for the for-
. _j i .
oPurnpelly, Raphael, On peeudomorpha of chlorite after garnet: Am. Jour. Sci., 3d ser., vol. 10,
1875, pp. 1-4. Penfield, S. L., and Sperry, F. L., Pseudomorpha of garnet from l>ake Superior and
Salida, Colo.: Am. Jour. Sci., 3d ser., vol." 32, 1886, pp. 307-311.
304 A TREATISE ON METAMORPHISM.
mation of talc is regarded as passing into magnesite (rhombohedral ; sp.
gr. 3.06) or into spinel. The first reaction is
(3) 4Mg 3 Al 2 Si 3 12 +15H 2 0+3C0 2 =3H 2 Mg 3 Si 4 12 +3MgC0 3 +8Al(OH) 3 +k.
The increase in volume of the talc, magnesite (rhombohedral; sp. gr. 3.06),
and gibbsite (monoclinic; sp. gr. 2.35) as compared with the pyrope is 75.91
per cent. The second reaction is
(4) 4Mg s Al 2 Si 3 O 12 +6H 2 O=3H 2 Mg 3 Si 4 O 12 +3MgAl 2 O 4 +2Al(OH) 3 +k.
The increase in volume of the talc, spinel, and gibbsite as compared with
the pyrope is 36.84 per cent.
The change of pyrope into serpentine is
(5) Mg 3 Al 2 Si 3 O 12 +5H 2 O=H 4 Mg 3 Si 2 O 9 +2Al(OH),+SiO 2 +k.
The increase in volume of the serpentine, gibbsite, and quartz as compared
with the pyrope is 81.61 per cent.
If amesite (hexagonal plates; sp. gr. 2.71) is produced from pyrope
the equation is
(6) Mg 3 Al 2 Si,0 I2 +2H 2 0+C0 2 =H 4 Mg 2 Al 2 Si0 9 +MgC0 3 +2Si0 2 +k.
The increase in volume of the amesite, magnesite, and quartz as compared
with the pyrope is 62.26 per cent.
In the alteration of pyrope to chlorite, supposing an intermediate
chlorite be taken, the reaction is
(7) 3Mg 3 Al 2 Si 3 12 +8H 2 O^H 16 M g9 Al 6 Si 5 36 +4SiO.,+k.
The increase in volume of the chlorite and quartz as compared with the
pyrope is 56.02 per cent. Reactions could be written which represent
other varieties of chlorite.
The change of pyrope to enstatite and spinel is
(8) Mg,Al 2 Si s O 12 =2Mg SiO 3 +MgAl 2 O 4 +SiO 2 +k.
The increase in volume of the enstatite, spinel, and quartz as compared
with the pyrope is 13.51 per cent.
The alteration of almandite and pyrope to chlorite (aphrosiderite),
ALTERATIONS OF MINERALS OF GARNET GROUP. 305
supposing the Fe:Mg:: 2:1, about the proportion shown by analysis in the
case of the Lake Superior chlorite at the Spun- mine," is
(9) 4Fe 3 Al 2 Si 3 12 .2Mg 3 Al 2 Si 3 O 12 +15H 2 O=3H 10 Fe 4 Mg 2 Al 4 SiA 5 +6SiO 2 +k.
The increase in volume of the aphrosiderite and quartz as compared with
the garnet is 50.98 per cent.
The alteration of almandite and pyrope to hypersthene and spinel,
supposing the.Mg: Fe :: 1 : 1 in the hypersthene, is as follows:
( 10) Fe 3 Al 2 Si 3 O 12 .2Mg 3 Al 2 Si 3 O 12 =3MgFeSi 2 O 6 +3Mg Al 2 O 4 +3SiO 2 +k.
The increase in volume of the hypersthene, spinel, and quartz, as compared
with the garnet, is 12.66 per cent. If a hypersthene be produced which is
less rich in iron, the amount of pyrope molecule in' the original garnet
must be increased.
The alteration of grossularite and melanite to epidote, supposing an
average epidote be produced, in which the Al: Fe:: 2: 1 is probably
(11) 2Ca 3 Al 2 Si,0 12 .Ca 3 Fe 2 8isO 1 ,+5CO 2 +H 2 0=2HCa 2 Al 2 FeSi3O ls +5CaCO,+3SiO 2 +k.
The increase in volume of the epidote, calcite (rhombohedral, sp. gr.
2.7135), and quartz, as compared with the garnet, is 40.88 per cent.
Similar equations can be written in which the pyrope molecule takes the
place of the grossularite molecule in large part, In this case magnesite,
instead of calcite, would be produced. Other reactions could be written
for the formation of epidote, in which the original molecule is a combination
of grossularite, pyrope, and melanite. The simplest case is as follows:
(12) C%Al 2 Si 3 O 12 .Mg 3 Al 2 Si 3 12 .Ca 3 Fe 2 Si 3 O I2 +H 2 O+5CO 2 =
2HCa 2 Al 2 FeSi,0 13 +2CaC0 3 +3MgCO s +3SiO 2 +k.
In this case the increase in volume of the epidote, calcite, magnesite, and
quartz, as compared with the garnet, is 39.53 per cent.
The reaction for the passage of pyrope, almandite, and melanite into
hornblende may be written in many ways, depending upon the composition
of the particular hornblende produced. Taking the case of an average
hornblende, in which there are five of the actinolite molecules to two of the
Penfield, S. L., and Sperry, F. L., On pseudomorphs of garnet from Lake Superior and
Salida, Colo.: Am. Jour. Sci., 3d ser., Vol. 32, 1886, pp. 307-311.
MON XLVII 04 20
306 A TREATISE ON METAMORPHISM.
aluminous molecules, in which the MgO: FeO:: 2:1, and A1 2 3 : Fe u O 3 :: 3: 1,
the reaction is as follows:
( 13 ) 3 [2Mg s Al 2 Si 3 O 12 . Fe s Al 2 Si 3 O 12 . Ca^SiA-,] +4CO, =
5CaMg 2 FeSi 4 12 .2[(M gl Fe 2 ) ( Al 9 Fe 3 )Si 6 S6 ] +4CaCO 3 +4SiO 2 +k.
The increase in volume of the hornblende, calcite, and quartz, as compared
with the garnet, is 24.55 per cent.
The alterations of the ferriferous garnets frequently produce iron
carbonate or iron oxides. No reactions are written to illustrate these
changes; nor would it be easy to express these alterations by reactions
without knowing what becomes of the remainder of the garnet material.
Of course, the alterations which are written above, instead of taking
place separately, may occur simultaneously. Thus the garnet may be a
complex one, which contains molecules of several of the simple garnets,
and there would be simultaneously produced a considerable number of
secondary minerals. Thus, chlorite and hornblende, chlorite and epidote,
or epidote and hornblende, might be simultaneously produced. For defi-
nite cases such as these, reactions might be written by combining the reac-
tions for the production of the individual minerals.
An examination of the equations as written shows that in almost all
cases, simultaneously with the production of the minerals which are recorded
as secondary to garnet, quartz also appears, and in some cases calcium
carbonate also must separate, which may be deposited in the form of calcite.
Less frequently siderite and iron oxide form. It is well known that with
the minerals chlorite, epidote, hornblende, etc., secondary to garnet, quartz,
and calcite are often found, and that with serpentine, talc, spinel, hyper-
sthene, and enstatite, quartz is often found. However, the quartz and calcite
are usually not regarded as derived from the garnet and called minerals
secondary to them. But the equations clearly show that these minerals
should be regarded as secondary to garnet, just as certainly as epidote,
chlorite, etc. The almost universal presence of quartz with the minerals
mentioned, and the frequent presence of calcite, are thus completely
explained. The equations also seem to demand in the alteration to serpen-
tine and talc that gibbsite or diaspore shall be produced. However, some
of the alumina may unite with silica and water and form kaolin. The
equations suggest that a search be made for gibbsite, diaspore, and kaolin
ALTERATIONS OF MINERALS OF GARNET GROUP. 307
where the serpentines and talcs are secondary to garnet. Of course, in
many cases the silica, calcium carbonate, and possibly the excess of
aluminum hydrate, may be dissolved and transported elsewhere, and thus
their absence would be no proof that the compounds were not really pro-
duced by the alteration of the garnet.
The alterations of the various kinds of garnet into different combina-
tions of the following minerals, serpentine, talc, chlorite, epidote, and zoisite,
magnesite, and gibbsite (equations 2, 3, 4, 5, 6, 7, 9, 11, 12), are all
alterations of hydration, and the majority of them of carbonation and
desilication. These reactions are notable in the amount of increase in
volume, ranging from 36 to 80 per cent. This increase in volume is a
natural consequence of the high specific gravity of the garnet. The altera-
tions of grossularite to meionite, calcite, and quartz (equation 1), and of
pyrope, almandite, and melanite to hornblende, calcite, and quartz (equa-
tion 13), are alterations of carbonation and desilication. There can be no
better illustrations of reactions characteristic of the zone of katamorphism.
It will be seen (pp. 683-685) that the development of garnet is a process
of the zone of anamorphism where the pressure is great and the tempera-
ture probably high. Naturally the extensive destruction of garnet is a
process of the upper physical-chemical zone.
The alterations of pyrope to enstatite, spinel, and quartz (equation 8),
and of almandite and pyrope together to hypersthene and spinel (equation
10), are common reactions. They do not involve hydration. They do,
however, involve desilication. The increase in volume for these changes
is comparatively small, 12 or 13 per cent. One would expect that these
reactions would take place either in the lower part of the belt of cementa-
tion or possibly in the upper part of the zone of anamorphism.
308 A TREATISE ON METAMORPHISM.
CHRYSOLITE GROUP.
FOKSTEBITE, OLIVINE, AM) FATALITE.
The chrysolite group includes
Forsterite:
Mg,SiO 4 .
Orthorhornbic.
Sp. gr. 3.21-3.33.
Olivine:
(MgFe) 2 SiO 4 where Mg:Fe::16:l, 12:1, to 2:1, in the last case the mineral being
known as hyalosiderite. (Sp. gr. 3.566.)
Orthorhombic.
Sp. gr. 3.2-3.6 according to Hintze, but ordinarily being, according to Dana, 3.27-3.37.
Fayalite:
Fe 2 SiO 4 .
Orthorhombic.
Sp. gr. 4.1.
occurrence. Tscheiinak considers olivine as an isomorphous mixture of
fayalite and forsterite. The occurrence of the three minerals is the same,
except that fayalite and forsterite are not nearly so widely known as the
intermediate common mineral, olivine. Olivine is an abundant constituent
in intermediate and basic igneous rocks, both plutonic and volcanic, in lavas
and tuffs alike. In rare cases in the volcanic rocks fayalite occurs, as, for
instance, in nodules in volcanic rocks and in lithophysse of the rhyolites of
the Yellowstone Park. Forsterite also very rarely occurs in connection
with volcanic rocks. Olivine is also an accessory constituent in the very
basic schists and gneisses, such as the amphibolites, pyroxenites, eclogites,
etc. Finally, it not infrequently occurs in marbles. In rocks of this class
forsterite also rarely occurs. It therefore appears that the chrysolite group
of minerals occurs most abundantly as original constituents, but are also
rather widely found as secondary developments in the metamorphosed rocks,
including both the carbonates and the basic schists.
Alterations. The alterations of fayalite and forsterite are exceptional;
therefore the chief alterations which are considered are those which pertain
to olivine.
The most common alteration of olivine is to serpentine (monoclinic;
sp. gr. 2.50-2.65). This is a change from an anhydrous orthosilicate to a
hydrous orthosilicate. Doubtless this explains why serpentine rather than
ALTERATIONS OF OLIVINE. 309
talc develops so generally from the olivines, because talc is a metasilicate.
Ordinarily accompanying the serpentine one or more of the following
minerals may be found: Trernolite (monoclinic; sp. gr. 3.0), actinolite
(monoclinic; sp. gr. 3.10), talc (orthorhombic or monoclinic; sp. gr. 2.75),
hydrotalcite (hexagonal; sp. gr. 2.04-2.09), magnesite (rhombohedral; sp. gr.
3.0(5), breunnerite (rhombohedral; sp. gr. 3-3.2), siderite (rhombohedral;
sp. gr. 3.83-3.88), quartz (rhombohedral; sp. gr. 2.6535), opal (amorphous;
sp. gr. 2.15), magnetite (isometric; sp. gr. 5.174), chromite (isometric; sp. gr.
4.445), hematite (rhombohedral; sp. gr. 5.225), and limonite (amorphous;
sp. gr. 3.80). One of the most frequent combinations of minerals with
serpentine is magnesite, quartz or opal, and magnetite. Frequently the
magnetite may partially or completely replace the hematite or limonite.
The formation of the serpentine is frequently accompanied by tremolite or
actinolite with iron oxide. It is much less frequently accompanied by talc.
In some instances the olivine has passed directly into magnesium carbonate
and hematite or limonite, but the former commonly being largely removed
in solution.
Other alterations of olivine are into anthophyllite (orthorhombic; sp.
gr. 3.15) into actinolite, hematite, and spinel (isometric; sp. gr. 3.8), but
these are by no means comparable in importance to the change to serpentine.
Beginning with the simplest alteration to serpentine, if an olivine be
taken in which the magnesium is to the iron as 3:1, and magnetite being
the only mineral which accompanies the serpentine, the reaction may be
written as follows:
( 1 ) 3Mg a FeSi A + 6H 2 O+0 =3H 4 Mg 3 Si 2 O 9 + Fe 3 O,+k.
The increase in volume of the serpentine and magnetite as compared with
the olivine is 29.96 per cent.
Supposing the magnesium is to the iron as 1:1 and the iron passes into
magnetite, the reaction is
(2) 3MgFeS10,+2H 2 O+O=H 4 Mg 3 SiA+FeA+SiO 2 +k.
The increase in volume of the serpentine, magnetite, and quartz as compared
with the olivine is 15.19 per cent.
If it be supposed that a third of the magnesium passes into magnesite,
and that silica also separates, the reaction may be written as follows:
(3)
310 A TREATISE ON METAMORPHISM.
The increase in volume of the serpentine, magnetite, magnesite, and quartz
as compared with the olivine is 37.13 per cent. Supposing the Mg and Fe
are present in equal proportions, the equation stands
(4) 3Mg 2 Fe 2 Si 2 8 +4H 2 0+2 O=2H 4 Mg 3 Si 2 O 9 +2FeA+2SiO 2 +k.
In this case, the olivine of which nearly corresponds to that of many rocks,
the increase in volume is 12.43 per cent.
It would be easy to write other equations for different proportions of
maglaesium and iron in the olivine, but this seems unnecessary. Also it
would be easy to write reactions by which other forms of iron compounds
than magnetite are produced, such as siderite, hematite, and limonite. If
this be done, and the volume reaction calculated, it will be found that the
increase in volume is still greater than when magnetite forms.
Olivine is described by Becke as passing into anthophyllite (where
Mg : Fe : : 4 : 1, 3:1, etc., orthorhombic ; sp. gr. 3.1-3.2). If the proportion
of the magnesium to the iron be taken as 3:1 in both the olivine and the
anthophyllite, the reaction may be written as follows:
(5) Mg,FeSi 2 O 8 +2Si0 2 =Mg s FeSi 4 O 12 - k.
The decrease in volume of the anthophyllite as compared with the original
olivine and quartz is 1.48 per cent.
Various authors have also described the alteration of olivine into
actinolite. Supposing that the magnesium is to the iron as 3 : 1 in both the
olivine and the actinolite, and supposing the calcium to be derived from
carbonate and the silica from quartz, the reaction is as follows:
(6) 3Mg s FeSi 2 8 +4CaCO s +10Si0 2 =Mg 9 Fe 8 Ca 4 Si, 6 48 +4C0 2 -k.
The decrease in volume of the actinolite as compared with the olivine,
calcite, and quartz, is 13.34 per cent,
In some instances the altei-ation into actinolite is described as taking
place in connection with feldspar as a reaction rim. In this case the calcuim
may be supposed to be derived from anorthite, as calcium silicate. The
aluminum may be supposed to pass into common spinel and hercynite
(isometric; sp. gr. 3.93), which are well known to be alteration products of
olivine. The reaction mav be:
(7) 4Mg,FeSi 2 O 8 -4CaAl 2 Si 2 8 =Mg 8 Fe,Ca,Si l6 O 48 +3MgAl J O t +FeAl a O 4 --k.
SCAPOLITE GROUP. 311
The volume decrease of the actinolite and spinels as compared with the
olivine and feldspar is 7. 18 per cent.
The reactions in the alterations of olivine into tremolite are parallel
with those for actinolite, with the exception that no iron is present, and the
mineral therefore probably forms from forsterite. The reaction may be
written:
(8) 3Mg 2 SiO,+2CaCO 3 +5SiO 2 =2Mg 3 CaSi 1 O 12 +2CO, ! -k.
The decrease in volume of the tremolite as compared with the forsterite,
calcite, and silica is 12.29 per cent.
The alteration of olivine to serpentine and the accompanying minerals
is the common one. It takes place in the zone of katamorphism on a great
scale, both in the belt of weathering and in the belt of cementation. Corre-
sponding with the position in the upper physical-chemical zone, the reactions
occur with hydration, oxidation, expansion of volume, and liberation of heat.
The developments of anthophyllite, actinolite, and tremolite from
olivine and actinolite, and of spinel from olivine and feldspar, are all deep-
seated reactions of the zone of anamorphism. Corresponding to this position
the change to anthophyllite, equation (5), is a reaction of silication; the
changes to actinolite and to tremolite, equations (6) and (8), silication
and decarbonation ; and the change of olivine and anorthite to actinolite
and spinel, equation (7), rearrangement of the silicates into denser silicates;
and all take place with diminution of volume and absorption of heat.
SCAPOLITE GROUP.
MEIONITE, WERN'EBITE, nd il\KI V1.ITK.
The scapolite group includes:
Meionite:
Ca 4 Al 6 Si 6 O 25 .
Tetragonal.
Sp. gr. 2.70-2.74.
Wernerite:
Tetragonal.
Sp. gr. 2.66-2.73.
Marialite :
Tetragonal.
Sp. gr. 2.566.
312 A TREATISE ON METAMORPH1SM.
As is well known, the scapolite group is analogous to the plagioclase
group, both consisting of sodium-aluminum-silicate molecules and calcium-
aluminum-silicate molecules in various proportions. Wernerite is a
combination of the marialite and meionite molecules in various ratios.
Generally the ratios vary between 2 : 1 to 1 : 3.
occurrence. Dana summarizes the occurrence of the scapolites as follows:
"(1) in volcanic rocks, as in ejected masses on Mte. Somma (meionite);
(2) in crystalline limestone, often as the direct result of contact meta-
morphism; (3) crystalline schists, augite-gneiss, etc.; (4) as an alteration
product of a plagioclase feldspar, sometimes on an extensive scale, as with
amphibole."
Alterations. Dana states that the scapolites are readily alterable. The
more common products of alteration are kaolin (monoclinic; sp. gr. 2.6-2.63),
talc (orthorhombic or monoclinic; sp. gr. 2.72.8), muscovite (hydromusco-
vite, pinite) (monoclinic; sp. gr. 2.76-3.0), and epidote (the Al and Fe
varying from (J:l to 3:2; monoclinic; sp. gr. 3.25-3.50). It is also recorded
that the scapolites alter into biotite (monoclinic; sp. gr. 2.7-3.1), Accom-
panying various of these alteration products quartz (rhombohedral ; sp. gr
2.653-2.654) separates. Also, it is probable that in connection with some
of them, gibbsite (monoclinic; sp. gr 2.3-2.4) or diaspore (orthorhombic;
sp. gr. 3.3-3.5) forms, and very likely also calcite (rhombohedral; sp. gr.
2.713-2.714).
In writing out equations for the alterations to the above minerals, one
is handicapped by lack of knowledge as to whether the marialite or the
meionite, or a combination of the two, produces a given mineral. In this
state of affairs the particular molecule is chosen which is most analogous to
the compound produced. It seems probable that kaolin and talc together
are produced from marialite, according to the reaction :
(1) 2Na 4 Al 3 Si 9 O 2 Cl+9MgCO 3 +9H 2 O=
3H 4 Al 2 SiA^3H 2 Mg 3 Si 4 O 12 4-3Na,CO s +2NaCl+6CO 2 +k.
The increase of volume of the kaolin and talc, as compared with the
marialite, is 7.69 per cent
It may be that kaolin and calcite are also produced from meionite, as
follows :
(2) Ca 4 Al 6 Si 6 O 25 +6H 2 0+4CO,=3H 4 Al 2 Si 2 O 9 +4CaCO s +k.
" Dana, J. I)., A system of mineralogy; Descriptive mineralogy, by E. S. Dana, Wiley & Sons, New
York, 6th ed., 1892, p. 467.
ALTERATIONS OF MEIONITE AND MAKIALITE. 313
Supposing all of the CaCO 3 to remain as calcite, the increase of volume is
35.40 per cent.
The passage of the scapolites into muscovite may be written as follows:
For marialite:
(3) 2Na ( Al 3 SiA4Cl+K 2 C0 8 +2H 2 0+2C0 2 =2KH 2 Al 3 Si 3 I2 +12SiO J +2NaCH-3Na 2 C0 8 +k.
In this reaction, as in the case of the passage of the acid feldspars into
muscovite, a large amount of the silica separates. The decrease in volume
of the muscovite and quartz as compared with the marialite is 16.74 per
cent, but if the soluble sodium salts be also taken into account the volume
is increased.
For meionite the reaction may be
(4) Ca 4 Al 6 Si 6 25 +K 2 CO 3 +3CO 2 -r2H 2 O=2KH 2 Al 3 Si 3 O 12 +4CaCO 3 +k.
The increase in volume of the muscovite and calcite as compared with the
meionite is 29.42 per cent.
As the composition of epidote is very analogous to meionite, and as it
is a calcium-bearing compound, it is thought likely, where epidote is second-
ary to a scapolite, that it is derived from a meionite molecule. Therefore,
supposing that the epidote is one in which the aluminum is to the iron as
2:1, and supposing that the iron is derived from feme oxide (Fe 2 3 ), the
reaction may be written as follows:
(5) Ca 4 Al 6 SiA 5 +Fe 2 3 +4H 2 0=2HCa 2 Al,FeSi 3 I3 +2Al(OH) 3 +k.
Supposing the hematite (hexagonal -rhombohedral; sp. gr. 5.225) to have
been present as a solid, and the gibbsite to remain as a solid, the decrease
in volume is 1.62 per cent. It is thought likely that iron for the reaction
is often derived from iron carbonate in solution,, combined with simulta-
neous oxidation. In this case the reaction would be
(6) Ca 1 Al 6 Si 6 O 2 5+2FeCO s +4H 2 O+O=2HCa,Al 2 FeSi s O, s +2Al(OH) 3 +2CO 2 +k.
The increase in volume of the epidote and gibbsite as compared with the
meionite is 7.55 per cent.
The passage of marialite into kaolinite and talc involves hydratiou,
expansion of volume, and liberation of heat. The change of meionite to
kaolinite involves hydration, carbonation, increase in volume, and libera-
tion of heat. The change of the scapolites to muscovite and accompany-
ing compounds are reactions of hydration, carbonation, increase of volume,
314 A TREATISE ON METAMORPHISM.
and liberation of heat. The change of meionite to epidote is a reaction of
hydration and possibly of oxidation.
Corresponding with these facts the alterations to kaolin and talc are
known to take place in the zone of kataraorphism, and the same is probably
true of the alterations to muscovite and epidote, although the latter reac-
tions may be more characteristic of the belt of cementation than of the belt
of weathering.
MELILITE.
Melilite:
(CaMgNa,),( Aire^SiA,. (Groth. )
Tetragonal.
Sp. gr. 2.9-3.10.
occurrence. Melilite has a widespread distribution in the leucite and
nephelite rocks. Aside from leucite and nephelite the most characteristic
associates are augite and perovskite. Some of the rocks in which melilite
occurs are leucitophyre, nepheline-syenite, and basalt.
Alterations. The alterations of this mineral are not recorded, although
from its composition there can be no doubt that in the upper physical-
chemical zone it decomposes into less complicated silicates.
GEHLENITE.
Gehlenite:
Ca,Al 2 Si 2 O 10 .
Tetragonal.
Sp. gr. 2.9-3.07.
occurrence. The only occurrence of gehlenite recorded in rocks is as a
contact product in limestone.
Alterations. According to Dana it alters to talc (orthorhombic or mono-
clinic; sp. gr. 2.75), to fassaite (monoclinic; sp. gr. 2.965-3.291), and to
grossularite (isometric; sp. gr. 3.605).
The change to grossularite involves the addition of SiO 2 , thus:
The decrease in volume of the grossularite as compared with the gehlenite
is 4.42 per cent. If the SiO 2 be added as a solid, the decrease in volume
is 18.56.
As gehlenite is so rare, and the manner of the alteration into talc and
fassaite is not clear, no attempt is made to write equations for the changes.
VESUVIANITE AND ZIRCON. 315
VESUVIANITE.
Vesuvianite:
HR" 6 Al,Si 6 O 2 i (Clarke).
Tetragonal.
Sp. gr. 3.35-3.45.
Clarke states that the R 6 in the typical mineral is replaced by calcium
and magnesium in the proportion of 5: 1, giving HCa 5 MgAl 3 Si 5 02i.
occurrence. Vesuvianite occurs in ancient ejections of Vesuvius. It is
most abundant in marbles. It is also found in various gneisses and schists,
especially those which are calcareous. It often forms in connection with
contact action. It is frequently associated with such other metamorphic
minerals as garnet, and also the micas and chlorites.
Alterations. From the literature it is impracticable to ascertain which
particular garnet, mica, or chlorite forms from a certain Vesuvianite, and
the accompanying minerals which must simultaneously form are unknown:
it therefore does not seem advisable to attempt to write equations represent-
ing the alterations, since they must be so largely speculative.
ZIRCON GROUP.
The only important rock-making mineral of the zircon group is zircon.
Zircon:
ZrSiO,.
Tetragonal.
Sp.gr. 4.66-4.70.
occurrence. Zircon is especially common in marble. It also occurs both
in massive igneous rocks, such as syenite and granite, and in the schists and
gneisses.
Alterations. According to Clarke the only alteration described is that of
hydration, producing hydrous zircon (malacoii) (tetragonal; sp. gr. 3.905),
the reaction being:
3Zr8iO 4 + H 2 G-H 2 Zr,,Si s O 1 .'.-
The increase in volume in the change is 24.05 per cent.
316 A TREATISE ON METAMORPH1SM.
ALUMINUM-SILICATE GROUP.
TOPAZ, ANDALl'Sm:, SILLIMAMTE, AJiD CYAXITE.
The aluminum-silicate group includes
Topaz:
Al a F 2 SiO 4 or Al 2 (F,OH) 2 SiO 4 .
Orthorhombic.
Sp. gr. 3.4-3.6.
Andalusite:
Al s SiO 5 .
Orthorhombic.
Sp. gr. 3.16-3.20.
Sillimaii !li:
Al 2 SiO 5 .
Orthorhoniliic.
Sp. gr. 3.23-b.24.
Cycmite (disthene):
Al 2 Si0 5 .
Triclinic.
Sp. gr. 3.56-3.67.
occurrence. Topaz is a much less common mineral than andalusite,
sillimanite, and cyanite. Like them, it occurs in the schists and gneisses
of sedimentary origin, especially those in which other fluorine minerals are
found, such as tourmaline and beryl. Unlike andalusite, sillimanite, and
cyanite, it is sometimes found in cavities in fresh volcanic rocks, as, for
instance, rhyolite.
Andalusite is a frequent constituent of the metamorphosed sedimentary
rocks, especially of the argillaceous kinds. It often occurs in crystals,
including many other minerals in the partly metamorphosed sedimentary
rocks; but is also found in large, well-formed crystals in the schists.
Frequently in the metamorphosed sedimentary rocks its development has
been promoted by the contact effect of igneous rocks, especially the
granitic rocks. Its most characteristic associates are sillimanite and cyanite.
With the former it frequently has parallel intergrowths. Also it is fre-
quently associated with garnet and staurolite, and not infrequently with
tourmaline. Andalusite is rare, if indeed not altogether absent in the
metamorphosed igneous rocks.
Sillimanite is a common mineral in the strongly metamorphosed sedi-
metary rocks, such as schists and gneisses, where it frequently replaces
ALUMINUM-SILICATE GROUP. 317
andalusite to a large extent. Like andalusite, its development may be
promoted by the presence of intrusive rocks, especially granites. In such
cases sillimanite frequently develops nearer the intrusive masses than does
the andalusite, the sillimanite therefore being the mineral which forms under
conditions of more advanced metamorphism. It is frequently associated
with garnet and with spinel and staurolite, sometimes with iolite (cordierito).
Sillimanite is derived from andalusite, biotite, corundum, cyanite, diaspore,
and gibbsite.
The occurrence and associates of cyanite are similar to those of silli-
manite; but a very frequent additional associate is corundum, and where
formed by the assistance of igneous rocks the cyanite is likely, on the
average, to be closer to the intrusive than the sillimanite, although of
course they ordinarily overlap. As a metamorphic mineral, cyanite is
derived from andalusite, corundum, diaspore, and gjbbsite.
Tremolite, actinolite, and diopside are frequent associates of andalusite,
sillimanite, and cyauite, especially of the last two.
The special homes of the aluminum-silicate group of minerals are
the metamorphosed argillaceous sedimentary rocks. As is well known,
kaolin is one of the chief constituents of such rocks, and doubtless it is
from this mineral in large part, under deep-seated conditions, that the
aluminum-silicate minerals are formed. If it be supposed that these heavy
minerals develop from kaolin, the process would be one of dehydration
and separation of silica. This silica may separate either as quartz or may
unite with other compounds, such as calcium and magnesium or other
bases, to form silicates. The breaking up of the kaolin may be repre-
sented by the following equation:
(1) H^AljSijO^A^Si
Supposing the mineral produced were andalusite, the volume of the anda-
lusite and quartz is 25.40 per cent less than that of the kaolin. If it be
supposed that calcium carbonate is present at the same time, and that the
freed silica unites with it, the equation may be written :
(2) H 4 Al 2 Si 2 O,+CaCO s =Al 8 SiO 5 +CaSiO 3 +2H 2 O+CO 2 +k.
In this case the volume of the andalusite and wollastonite is 32.32 per cent
less than that of the kaolin and calcite. If the heavier mineral silli-
manite or cyanite be produced the decrease in volume is even greater.
318 A TREATISE ON METAMORPHISM.
While for the sake of simplicity wollastonite is supposed to form, the
more frequent association of the aluminum-silicate group is with tremolite,
actinolite, and diopside. For the first and last of these minerals the freed
silica unites with the calcium and magnesium together, and for the second
with the calcium, magnesium, and iron. The equations representing the
changes are analogous to (2), and the volume changes are in the same
direction.
Alterations. The standard stated alterations of the aluminum-silicate
group are to talc (steatite) (massive; sp. gr. 2.75) and to muscovite
(damourite) (monoclinic; sp. gr. 2.88). It is recorded also that topaz and
,-nidalusite alter to kaolin (monoclinic; sp. gr. 2.615). Occasionally also
andalusite may alter into the heavier mineral cyanite (triclinic; sp. gr.
3.56-3.67).
The alterations of the minerals into talc require an entire change of
base; that is, from aluminum silicates to magnesium silicates. The reac-
tions being those of the zone of katamorphism, the most probable source
of the magnesium is doubtless the carbonate, which may be derived from
the decomposition of magnesium rocks such as the pyroxenites, olivinites,
etc. The process, however, requires the separation of aluminum either as
corundum (rhombohedral ; sp. gr. 4.025), coruudophilite (monoclinic; sp.
gr. 2.90), diaspore (orthorhombic ; sp. gr. 3.40), gibbsite (mouoclinic; sp.
gr. 2.35), or some other form. Since the reaction takes place in the upper
physical-chemical zone, gibbsite will be regarded as the product formed.
The change of the aluminum-silicate minerals to muscovite requires the
addition of potassium. This is doubtless derived from the liberation of
potassium during the decomposition of the potash feldspars, and will there-
fore be regarded as added as a carbonate. The change from audalusite to
cyanite is simply a molecular one, the result being a mineral of great
specific gravity. It has already been seen that andalusite is a product of
less intense metamorphism, and that more intense rnetamorphism produces
sillimanite and cyanite. The change of andalusite to these heavier minerals
is therefore one which requires deep-seated conditions, and is characteristic
of the zone of katamorphism.
The equations representing the change of andalusite, sillimanite, and
cyanite to talc with gibbsite may be written as follows:
(1) 4Al,SiO 6 +3MgCO 8 +13H 2 O=H 2 Mg,Si 4 O 12 +SAl(OH) 3 +3CO 2 +k.
ALUMINUM-SILICATE GROUP.. 319
The decrease in volume of the talc as compared with the andalusite is 32.37
per cent; as compared with the sillimanite, 31.20 per cent; as compared
with the cyanite, 23.12 per cent. But if the gibbsite be included as a solid
the increases in volume are 97.67 per cent, 101.09 per cent, and 124.71 per
cent, respectively.
The change of the three minerals to kaolin may be written as follows:
(2) 2Al 2 SiO 6 +5H 2 O=H 4 Al 2 SiA+2Al(OH) 3 +k.
The change in volume of the kaolin as compared with the andalusite is a
decrease of 3.15 per cent; as compared with, the sillimanite, a decrease of
1.47 per cent; and as compared with the cyanite, an increase of 10.11 per
cent. But if the gibbsite be a solid, the increases in volume are 61.87
per cent, 64.67 per cent, and 84.02 per cent, respectively.
The alterations of the same minerals to muscovite (damourite) may be
written as follows:
(3) 6Al 2 Si0 5 +K 2 CO3+llH 2 O=2H 2 KAl s Si 3 O 12 +6Al(OH) 8 + C0 2 +k.
The decrease in volume of the muscovite as compared with the andalusite
is 9.55 per cent; as compared with the sillimanite, 7.98 per cent; the
increase as compared with the cyanite is 2.83 per cent. But if the gibbsite
be regarded as a solid, the increases in volume are 55.47 per cent, 58.16
per cent, and 76.74 per cent, respectively.
The alterations of the aluminum-silicate minerals to talc, kaolin, or
muscovite, with the accompanying gibbsite, are all reactions of hydratiou.
They involve great increase of volume, from 55 to 125 per cent. To
produce the original heavy aluminum-silicate minerals in the zone of
anamorphism undoubtedly required great condensation of volume. When
the reactions are reversed in the zone of katamorphism, there is a corre-
spondingly great expansion of volume. The change of the heavy aluminum-
silicate minerals to the much lighter hydrous minerals gives one of the best
illustrative cases of typical reactions of the zone of katamorphism.
The change of andalusite to cyanite, as already explained, being a
molecular one, involves a volume relation inversely as the specific gravity,
and therefore by the change the volume is decreased 12.03 per cent. The
change of andalusite to cyanite is a reaction of the zone of anamorphism.
320 A TREATISE ON METAMORPH1SM.
EPIDOTE GROUP.
ZOISITE, EPIDOTE, PIEDMOJiTTTE, AND ALLANITE.
The epidote group includes the following minerals:
Zoisite:
C%(AlOH)Al. 2 (SiO 4 ),.
Orthorhombic.
Sp. gr. 3.25-3.37.
JUpidote:
Ca,(AlOH)(AlFe),(8iO 4 ), where Al:Fe as 6:1 to 3:2.
Monoclinic.
Sp. gr. 3.25-3.50.
Piedmontite:
Ca, ( A10H) ( Mn Al ) ., ( SiO 4 ) ,.
Monoclinic.
Sp. gr. 3.404.
Allanite (orthite):
Ca,(AlOH) (AlCeFe) a (SiO 4 ) s .
Monoclinic.
Sp. gr. 3.5-4.2.
occurrence. Zoisite is not known as an original pyrogenic constituent of
igneous rocks. It is found in the schists and gneisses, especially those
containing the amphiboles. Thus it is very common in the amphibolites,
glaucophane-schists, eclogites, etc. Zoisite frequently occurs with albite
as one of the constituents of the so-called saussurites, which develop as
an alteration of the basic feldspars, especially in gabbros. Zoisite also
occurs in the altered granites and other acid igneous rocks, although it is,
on the whole, less abundant than in the more basic rocks, but in some
localities it is plentiful even in the acid rocks. Zoisite is a very frequent
constituent in grits, graywackes, and other sediments of similar composition.
In such rocks the minerals were partly altered to zoisite during the forma-
tion of the sedimentary rocks, and this zoisite is to be classed with the
allogeiiic constituents of the mechanical sediments. Zoisite further develops
in the altered sedimentary rocks as a frequent and sometimes abundant
product of metamorphism. From the foregoing statement of occurrence it
is plain that zoisite develops in the zone of katamorphism, and especially
in the belt of cementation. As shown under the discussion of the other
minerals, it is seen that zoisite may be derived from the folio wing minerals:
Corundum, diaspore, gibbsite, grossularite, and the plagioclases.
OCCURRENCE OF EPIDOTE. 321
Epidote, like zoisite, is rarely if ever a pyrogenic constituent in igneous
rocks. It is, however, a secondary constituent in all varieties of metamor-
phosed igneous rocks, whether plutonic or volcanic, whether lavas or tuffs.
It is an allogenic constituent of the sedimentary rocks, and it extensively
develops in the sedimentary rocks as a secondary product. It is particularly
likely to form in rocks rich in calcium and iron, whether igneous or sedi-
mentary; and thus is especially abundant in those metamorphosed igneous
rocks which contain ferriferous varieties of pyroxene and amphibole, and
in metamorphic sedimentary rocks which contain a considerable amount
of calcium, as, for instance, calcareous schists and gneisses and marble.
In the metamorphosed rocks epidote occurs alike in those which have a
strongly developed schistose or gneissose structure and in those which have
merely undergone metasomatic change. It is found as one of the important
filling constituents of amygdaloids. It frequently develops at the contact
of two rocks, especially an igneous rock with other rocks, either igneous or
sedimentary. A list of different rock species which contain epidote includes
almost every variety of massive, schistose, semischistose, and little altered
igneous and sedimentary rocks. Epidote is, in fact, one of the most
important secondary constituents of all the silicates. It is an almost
constant accompaniment of the chlorites. Wherever the calcium-iron-
magnesium-silicate rocks break up, the magnesium passing into chlorite, a
part of the calcium and iron is likely to pass into epidote. The equations
for these alterations may be found under the minerals from which epidote
is derived. Where epidote becomes so abundant as to be a chief constituent
it may give a name to a rock; for instance, epidosite. From the foregoing
statements it is apparent that epidote develops abundantly under mass-
static and under mass-mechanical conditions. It forms with ease and on a
great scale in the belt of cementation of the zone of katamorphism, and it
is probable that it develops to some extent in the zone of anamorphism.
Whether it forms at all in the belt of weathering can not be stated.
Epidote is derived from the following minerals: Anorthoclase, augite,
biotite, garnet, hornblende, melanite, microcliue, orthoclase, the plagio-
clases, and the scapolites.
Piedmontite, or manganese-epidote, is apt to replace epidote in those
schists and gneisses in which manganese happens to be an important con-
stituent. Thus it is rather common in certain manganese-bearing schists of
MON XLVII-
322 A TREATISE ON METAMORPHISM.
Japan, in the manganese-chlorite-sericite-gneisses of eastern United States,
and at other localities. In some cases piedmontite occurs as nuclei sur-
rounded by ordinary epidote. Piedmontite is occasionally so abundant as
to be one of the chief constituents of rocks.
Allanite occurs as an original subordinate constituent of a great number
of eruptive rocks, such as granite, rhyolite, diorite, tonalite, andesite, dacite,
and syenite. In short, it is a common accessory in the acid and intermediate
eruptives, but is not so characteristic of the basic eruptive rocks. It also
occurs in the metamorphic rocks, such as the schists and gneisses, especially
those which are calcareous, and it may occur also in the marbles.
Alterations. Definite alterations of zoisite, epidote, and piedmontite are
not recorded. But it is certain in the belt of weathering that zoisite and
epidote break up into calcite (rhombohedral ; sp. gr. 2.7135), quartz (rhom-
bohedral; sp. gr. 2.6535), iron oxides, kaolin (monoclinic; sp. gr. 2.615),
and perhaps gibbsite (monoclinic; sp. gr. 2.35); and piedmontite and
allanite alter into other minerals in a similar fashion.
It has already been seen that in the alteration of mica, pyroxene,
amphibole, and other minerals chlorite and zoisite are frequent simultaneous
products which together use up all the material of the original minerals.
It has also been noted that the chlorite and epidote are abundantly
developed together in the sedimentary rocks. If the conditions so change
that these sedimentary rocks or other rocks in which epidote and zoisite
have formed in the zone of katamorphism become so deeply buried as to
pass into the zone of anamorphism, it is highly probable that the consti-
tuents which form epidote and zoisite and those which form chlorite reunite
to produce minerals that are on the average denser, such as mica, amphibole,
pyroxene, etc., out of which they are originally developed. This is believed
to be probable from the fact that in the most profoundly metamorphosed
sedimentary rocks, those which are true schists and gneisses, little or no
epidote and chlorite is contained, unless they have again been subjected to the
conditions of the upper physical-chemical zone. Such schists and gneisses,
having been derived from and traced into ordinary sediments, in all prob-
ability did originally contain both chlorite and epidote, which have doubt-
less united to reproduce heavy minerals similar to those from which epidote
and chlorite formed originally.
ALTERATIONS OF ZOISITE AND EPIDOTE. 323
It is not easy to approach accuracy in writing equations for the altera-
tions of the epidotes in the belt of weathering. In the equations given
below it is supposed that the calcium passes into carbonate, that the
Al (OH) goes into gibbsite, that the remainder of the aluminum goes into
kaolin, and that the excess of silica separates as quartz. In the epidote the
Al is supposed to be to the Fe as 2:1, and the iron is supposed to pass into
limonite (amorphous; sp. gr. 3.8). Upon these suppositions the alterations
stand
For zoisite
(1) Ca.,(AK^H)Al 2 (SiO t ) 3 +2CO. 1 +3H. i O=2CaCO s +Al(OH), ) +H 4 Al 2 Si 2 O i ,+SiO 2 +k
and for epidote
(2) Ca 6 (A10H) s Al 4 Fe 2 Si 9 36 +6C0 2 +8JH 2 0=
6CaCO s +3Al(OH) s +2H 4 Al 2 Si 2 O 9 +Fe 2 O 3 .lJH 2 O+5SiO 2 +k.
The increase in volume of all the compounds formed as compared with
the zoisite is 66.22 per cent, and as compared with the epidote is 69.08 per
cent.
Of course there are many other ways in which the equations could be
written. All of the aluminum might pass into gibbsite or diaspore and
more quartz form. The iron may pass into hematite in whole or in part,
etc. While all this is true, it is believed that the above equations represent
correctly the fundamental fact that by hydration and carbonatiou zoisite
and epidote in the belt of weathering pass into simpler compounds.
Similar reactions could be written for the alterations of piedmontite
and allanite, but considering the comparative rarity of these compounds
this will not be done.
AXINITE.
Axinite:
HCa s Al 2 BSi 4 Oi li . (In some casea part of the Ca is replaced by Fe and Mn. )
Triclinic.
Sp. gr. 3.271-3.294.
occurrence. Axiiiite occurs as a secondary constituent in basic eruptive
rocks, subh as the diabases and gabbros. It is found to some extent in the
schists and gneisses, and particularly in those bearing abundant pyroxene
and amphibole. It also occurs in altered sedimentary rocks as a product
324 A TREATISE ON METAMORPHISM.
formed in connection with the contact action of such rocks as granites,
granulites, diabases, and gabbros. In such positions the formation of
axinite is usually regarded as assisted by fumarole action.
Alterations. Apparently the alterations which axinite undergoes in
rocks have not been worked out, as they are not recorded in the standard
text-books.
PREHNITE.
Prehnite:
Orthorhombic.
Sp. gr. 2.8-2.95.
occurrence. Prehnite is almost identical in its occurrence with the
zeolites (see pp. 331-333). It is therefore especially prevalent in the basic
and intermediate rocks, such as anorthosite, basalt, diabase, gabbro,
andesite, diorite, and syenite; also it occurs to some extent in granites and
gneisses, where it may be associated with epidote. In the igneous rocks it
is especially prevalent in the volcanics, since these are usually more
porous. Like the zeolites, it is a very frequent occupant of amygdaloidal
cavities, and also of cracks and crevices in the rocks. As already inti-
mated, the most constant associates of prehnite are the zeolites. Prehnite
occurs to some extent in the schists and gneisses, including those derived
from igneous rocks, such as the amphibolites, and from aqueous rocks,
such as the marbles. In some cases it is found in cavities in sedimentary
rocks which have been metamorphosed by granitic or granulitic intrusions.
As a secondary mineral prehnite is often derived from aualcite, laumontite,
mesolite, iiatrolite, the plagioclases, and scolecite. Fused prehnite yields
wollastonite and ankerite.
Alterations. The only alteration which I have been able to note in refer-
ence to prehnite is to chlorite (monoclinic; sp. gr. 2.71-2.725). This
change requires the substitution of magnesium for calcium. Supposing
the chlorite were amesite (hexagonal plates; sp. gr. 2.71), the change might
be expressed:
H.,Ca,Al 2 Si,O 12 +2MgCO 8 +H 2 O=H 4 Mg a Al,SiO 9 +2SiO 2 +2CaCO s +k.
Ignoring the carbonates, the increase of volume of the chlorite and quartz
(rhombohedral; sp. gr. 2.6535) as compared with the prehnite is 3.27 per
cent. The change is one of hydration and desilication, and would be
expected to take place in the zone of katamorphism.
ROCK-MAKING MINERALS. 325
HUMITE GROUP.
CHONDRODITK, HUMITE, AJiD CLIXOH U MITE.
The humite group includes:
Chondrodite:
[Mg(F.OH)] 2 Mg s SiA.
Monoclinic.
Sp. gr. 3.1-3.2.
Humite:
[Mg(F.OH)] 2 Mg 5 SiA 3 .
Orthorhombie.
Sp. gr. 3.1-3.2.
Clinohumite:
[Mg(F.OH)] 2 Mg 7 SiA,.
Monoclinic.
Sp. gr. 3.1-3.2.
In all the above the hydroxide (OH) replaces a part of the fluorine.
occurrence. The humites occur in masses of raagnesian limestones and
rocks bearing carbonates ejected by volcanoes. Chondrodite has a some-
what widespread occurrence in the marbles of eastern United States. In
such cases it is sometimes, at least, a contact mineral. Frequently it is
accompanied by spinel.
Alterations. The mcst frequent alterations of the humites are to serpen-
tine (monoclinic; sp. gr. 2.50-2.65) and brucite (rhombohedral ; sp. gr.
2.38-2.4). In the equations for the alterations the hydroxide will be ignored.
The reactions may be written as follows:
(1) (MgF),Mg s SiA+3H,0=H,Mg,SiA+Mg(OH).,+MgF,+k.
(2) 2[(MgF) 2 Mg 5 Si,0 12 ] +9H,0=3H 4 Mg,SiA+3Mg(OH)-,+2MgF,+k.
(3) (MgF),Mg 7 SiA.+6H,0=2H 4 Mg !) SiA+2Mg(OH ) 2 +MgF 2 +k.
The increase in volume of the serpentine and brucite as compared with
chondrodite, from which it is derived, is 30.15 per cent; as compared with
humite, 35.53 per cent; as compared with clinohumite, 38.39 per cent.
The alterations of humite to serpentine and brucite involve hydration,
expansion of volume, and liberation of heat. They are therefore typical
reactions of the zone of katamorphism.
326 A TREATISE ON METAMOKPHISM.
TOURMALINE.
Tourmaline is a complicated aluminum silicate, which may be of any
one of four different types or intermolecular growths of these types.
According to Clarke, the formulae for these types are a
Tourmaline:
NaHRjAlgBjSi/).,! (R in some cases being lithium and hydrogen).
NaH s Mg 2 Al,B3Si 6 O 3 i (Mg frequently being replaced by Fe).
NaH.M&Al.R.Si.O,,.
NaH 5 Mg 4 Al 5 B,Si 6 O sl .
Rhombohedral.
Sp. gr. 2.98-3.20.
occurrence. Tourmaline rather frequently occurs in the marbles and in
the calcareous schists. It also has a rather widespread occurrence, although
generally not as an abundant mineral, in granites, gneisses, schists, and
granulite. In. these rocks it frequently occurs in such relations to dikes of
igneous rocks, especially of pegmatites, as to suggest that its development
is promoted by contact action. Because of the boron, tourmaline has
generally been regarded as evidence of fumarole action. Certain it is that
boron is not usually a constituent of the ordinary sediments, and to
account for this element, especially where the tourmaline is abundant, as it
occasionally is in the schists, would seem to require its introduction from
an outside source, either by gaseous or by aqueous solutions.
Alterations. Mineral specimens of tourmaline are recorded as altering
into mica, chlorite (monoclinic; sp. gr. 2.71-2.725), and steatite (massive;
sp. gr. 2.794). However, in rocks tourmaline is one of the more permanent
minerals, and the chemical additions and subtractions which occur in the
alterations are so little known, and the exact nature of the tourmaline from
which individual minerals are derived is so uncertain, that it is not thought
advisable to attempt to write all the reactions representing these changes.
If one assumes a definite tourmaline and a definite mica as being produced
from it, it is easy to write a reaction. For instance, supposing that normal
biotite (monoclinic; sp. gr. 2.90) is derived from a tourmaline of the
composition of the last of the four formulae given, that the additional
alkalies are added in the forms of carbonates, that the free boric acid
a Clarke, F. W., The constitution of the silicates: Bull. U. 8. Geol. Survey No. 125, 1895,
pp. 56-57. An alternative form has been proposed by Penfield.
' OCCURRENCE AND ALTERATIONS OF STAUROLITE. 327
passes into borax, and that the excess of alumina separates as gibbsite
(monoclinic; sp. gr. 2.35), the reaction is
4NaH 5 Mg 1 Al 5 B 8 SiAi+4K.CO,-fNa,CO,=
8HKMg 2 Al.,Si 3 O 1 . 1 +3Na. i BjO 7 +4Al(OH) s +5CO.,+k.
The decrease in volume of the biotite as compared with the tourmaline is
6.75 per cent; but if the gibbsite be included the increase of volume
is 3.96 per cent.
STAUEOLITE.
Staurolite:
HFeAl 5 Si. 2 O ls
Orthorhombic.
Sp. gr. 3.65-3.77.
occurrence. Staurolite is similar in its occurrence to garnet, but apparently
requires more intense metamorphic action for it to begin to form. Its most
widespread occurrence is in the schists and gneisses of sedimentary origin.
It also develops in profoundly metamorphosed rocks of eruptive origin, but
it is not known as an original constituent in any eruptive rock. Like
garnet, it may be abundantly developed in the zone of anamorphism in
rocks which are cut by intrusives. The conditions favorable to its formation
are therefore similar to those which produce garnet (see pp. 300-302) and
such minerals as tourmaline, andalusite, sillimanite, and cyanite, with
which it is associated. It is evidently a mineral which derives its materials
from various other minerals, the elements being recombined into the more
compact form of Staurolite under deep-seated conditions.
Alterations. The only alterations recorded for staurolite are to talc
(orthorhombic or monoclinic; sp. gr. 2.75), to chlorite (monoclinic; sp. gr.
2.71-2.725), and to muscovite (damourite) (monoclinic; sp. gr. 2.763.0).
The first two minerals are essentially magnesian ones, although if the
chlorite be aphrosiderite (massive; sp. gr. 2.90) a considerable amount of
iron may be present. It is therefore clear that in the change to talc and
chlorite magnesium must be derived from some other compounds. As the
alterations are those which occur in the zone of katamorphism, it may be
supposed that the magnesium is in the form of carbonate, since magnesium
carbonate is an almost universal constituent of ground waters in the upper
physical-chemical zone. The alteration to muscovite requires an entire loss
of the iron and the addition of potassium. It is therefore clear that some
328 A TREATISE ON METAMORPHISM.
potassium mineral must also be concerned in this alteration. Staurolite
rocks usually contain orthoclase, or at least some potash feldspar. It may
be supposed that these potash feldspars break up into kaolin at the same
time, thus furnishing the potassium necessary for the change.
The change of staurolite to talc may be written as follows:
(1) 2HFeAl s SiAs+3MgCO,+15H 2 O+O=H 2 MgsSi 4 O 1 2+FeA+10Al(OH) 3 +3CO J +k.
The decrease of the volume of the talc as compared with the staurolite is
44.02 per cent; but if the gibbsite (moiioclinic; sp. gr. 2.35) be included
the increase in volume of the two is 90.96 per cent.
In the change to chlorite the most aluminous one is chosen, amesite
(hexagonal plates; sp. gr. 2.71), since staurolite is so heavily aluminous
Moreover, it is supposed that the iron in the chlorite is to the magnesium as
1:3. On these suppositions the reaction may be written:
(2) 2HreAl 5 Si 2 O,3+lOH 2 O+6MgCO,=2H 8 Mg 3 FeAl 4 Si 2 O 18 +2Al(OH) 3 +6CO s +k.
The increase of volume of the chlorite and gibbsite as compared with the
staurolite is 103.58 per cent
The change of staurolite to muscovite may be written:
(3) 3HFeAl 5 Si 2 O u +K 2 CO s +14H 2 O+O=2H 2 KAl s Si 8 O 12 +Fe s O 1 +9Al(OH), > +CO 2 +k.
The decrease in volume of the muscovite as compared with the staurolite
is 24.90 per cent; but if the magnetite (isometric; sp. gr. 5.174) and gibbsite
be included the increase in volume would be 68.08 per cent, and if hema-
tite (rhombohedral; sp gr. 5.225) or limoiiite (amorphous; sp. gr. 3.8)
form, instead of inagnetite, the increase would be still greater.
In the above equations it is entirely possible that the magnesium may
be added in some other form than that given, and the resultant compounds
be different. The same thing may be said of the potassium. It is uncer-
tain what becomes of the excess of aluminum. In the equations the
aluminum is regarded as passing into the gibbsite. However, the presence
of abundant gibbsite is not recorded among the alterations of staurolite,
although frequently corundum (rhombohedral; sp. gr. 4.025) occurs in
connection with it; but it is by no means certain that this corundum is one
of the results of the alteration of the staurolite; indeed, it is more probable
that the corundum formed from gibbsite at the same time the staurolite
developed.
ZEOLITE GROUP. 329
In short, the above reactions are probably as unsatisfactory as any
that have been written, because the text-books do not record what minerals
accompany the talc, chlorite, and muscovite as a result of the transforma-
tion of the staurolite. It is certain that in each case some other minerals
must be produced.
ZEOLITE GROUP.
The zeolites are a great group of hydrous silicates about which there
seems to be no consensus of opinion as to the species in the group, as to
the composition of the species, or as to their classification. Since Groth
and Clarke are among the latest authors to discuss this group, their
formulae are used, Groth's being placed first and Clarke's second when he
differs from Groth. Groth's formulae are put into an empirical form, and
the subordinate constituents which may replace the chief bases are omitted.
The differences between the formula? given and Dana's also are pointed
out. The important rock-making zeolites are as follows, ranged from basic
to acid:
THOMSOMTK, HYDKOSEPHELITE, 1VATBOLITE, MESOLITE, SCOLECITE, ANALCITE, APOPHYLLITE, EPISTILBITE,
HEULAXDITE, ST1LBITE, PHILLIPS1TE, HARMOTOME, GISMOXDITE, CHABAZITE, GMELIMTE, AND
LACMOXTITE.
Thomsonite:
(CaNa-j) Al 2 Si 2 O 8 - 2 i H 2 (Dana agrees with Groth.)
Orthorhombic.
Sp. gr. 2.3-2.4.
Hydronephelite:
HNa 2 Al 8 Si 8 O 12 .3H 2 O
HNa 2 Al 3 Si s O I2 .3H,O
Hexagonal.
Sp. gr. 2.263.
Natrolite:
Na.,Al 2 Si 3 O, .2H 2 O ( Dana agrees with Groth. )
Orthorhombic.
Sp. gr. 2.20-2.25.
Mesolite:
H 2 Na 2 CaAl 4 Si 6 O 21 .4H. ! O. (Dana makes all water that of hydration. )
HgNajCaAl^SijO^. H 2 O.
Monoclinic and triclinic.
Sp. gr. 2.29.
330 A TREATISE ON METAMORPHISM.
Scoleciie:
H 2 CaAl 2 Si,O n .2H 2 O. (Dana agrees with Groth. )
Monoclinic.
Sp. gr. 2.16-2.4.
Analcite:
NajAljSiA^HjO. (Dana agrees with Groth.)
Isometric.
Sp. gr. 2.22-2.29.
Apophyttite:
H,KCa 4 Si 8 O 2 - 4 iH ;1 O. (Dana agrees with Groth. )
Tetragonal.
Sp. gr. 2.3-2.4.
Epistilbite:
inOjB.VHjO. (Dana varies from both, but is nearer Groth. His formula is
Monoclinic.
Sp. gr. 2.25.
Heulandite:
SHjO. (Dana agrees with Groth.)
Monoclinic.
Sp. gr. 2.18-2.22.
StUbtte (desmine):
Ca 4 Al 8 Si 16 O 48 .18H 2 O. (Dana's formula is nearly the same as Clarke's:
H.tNajCaJAljSieO^HjO. )
Ca,Al,Si, 8 O t8 .18H. i O.
Monoclinic.
Sp. gr. 2.094-2.205.
Phillipgite:
(K 2 Na 2 Ca) s Al 6 Si 10 32 .12H 2 O. (Dana's formula is nearly like Clarke's:
(K 2 Ca)Al.,Si 4 O la .4JH 2 O.)
Monoclinic.
Sp. gr. 2.2.
Harmotome:
Ba3Al,Sii O S2 .12H 2 O. (Dana varies from each. His formula is (K 2 Ba)Al.,Si 5 O M .5H,O.)
Ba s Al 6 Si u O 40 .14H 2 O.
Monoclinic.
Sp. gr. 2.44-2.50.
OCCURRENCE OF ZEOLITES. 331
Gismondite:
Ca 3 Al 6 Si,O 24 .12H 2 O. (Dana's formula is one of the molecules given.)
Ca 3 Al 6 Si 4 O 24 .12H 2 O.
Monoclinic.
Sp. gr. 2.265.
Chabuzite:
Ca3Al 6 Si 10 3; ,.16H 2 O. (Dana agrees with Clarke as to the amount of Si. Hia formula
is ( CaXa 2 ) Al 2 Si 4 O 12 .6H 2 0. )
Ca 3 Al 6 Si,. ! O S(i .18H.,O.
Rhombohedral.
Sp. gr. 2.08-2.16.
Gmelinite:
iujOaj.lBHjO. (Dana varies from each. Hia formula ia (Na 2 Ca)Al 2 Si 4 O 12 .6H 2 O. )
Rhombohedral.
Sp. gr. 2.04-2.17.
Laumontite:
'H 4 CaAljSi 4 O, 4 .2H,O. (Dana agrees with Groth.)
Ca,Al.Si 12 i .12H 2 0.
Monoclinif.
Sp. gr. 2.25-2.3(>.
occurrence. The zeolites are not known as original pyrogenic constitu-
ents of igneous rocks. As secondary minerals they are most abundantly
found in basic lavas and allied rocks, including both glassy and crystallized
kinds. The zeolites occur in these rocks, both in the ordinary feldspathic
varieties, such as basalt and trachyte, and in those containing leucite,
nephelite, sodalite, etc., and especially in those which are somewhat vesicu-
lar. The zeolites also develop abundantly in the deep-seated equivalents
of the basalts in such rocks as the diabases, gabbros, etc. The zeolites are
less prevalent, although far from rare, in the diorite and syenite families,
including both the ordinary syenites and nepheline-syenites. Finally, the
zeolites are far from uncommon in the more acid rocks, such as those of
the granite family. In short, the zeolites may occur in almost any variety
of the igneous rocks, but, as already said, are most prevalent in the basic
group. In the sedimentary rocks the zeolites may be allogenic constituents
separate from other minerals, or an alteration product of partly decomposed
minerals. Also, after the deposition of material in the fragmental rocks
the zeolites may develop as alteration products. Hence the zeolites are
constituents of the altered sedimentary rocks, of the semi metamorphosed
332 A TREATISE ON METAMORPHISM.
sedimentary rocks, and of the schists and gneisses of sedimentary origin,
which, after becoming schists and gneisses, have been subjected to agencies
of alteration in the zone of katamorphism.
The zeolites develop from many minerals, but especially from the
plagioclase feldspars and from the leucites, sodalites, nephelites, etc. From
the plagioclases many of the zeolites are produced. The following may be
regarded as derived from anorthite : Thomsonite, gismondite, laumontite,
phillipsite, heulaiidite, epistilbite, stilbite, chabazite, and scolecite. The
following may be regarded as derived from albite: Analcite and natrolite.
Mesolite may be regarded as derived from albite and anorthite together.
Since the intermediate plagioclases contain both the anorthite and the albite
molecules, all of the above minerals may be derived from oligoclase,
andesiiie, labradorite, and bytownite, as may also mesolite. So far as
recorded the derivations of the zeolite minerals from the nephelites, leu-
cites, and sodalites are as follows: Thomsonite from nephelite and sodalite;
hydronephelite from nephelite and sodalite; natrolite from iiephelite,
sodalite, haiiynite, and noselite; analcite from leucite, nephelite, and soda-
lite; stilbite from haiiynite and noselite; chabazite from haiiynite and
noselite. The zeolites are also derived from other minerals as follows:
Analcite from laumontite, natrolite from apatite and chabazite, etc.
It is hardly worth while to consider the occurrence of each of the
zeolites. It may be said, however, that the calcium-bearing zeolites
are most apt to form in the calcareous rocks, and the soda zeolites in
the rocks rich in soda. Thus stilbite, scolecite, and similar minerals are
likely to form in the calcareous rocks and limestones, while hydronephelite,
natrolite, and analcite, and similar minerals, are especially likely to form
from the rocks containing soda feldspars and nephelites, leucites, and
sodalites. The sodium-calcium zeolites, such as thomsonite, mesolite, and
phillipsite, may occur in the calcareous rocks, such as the limestones, in
the igneous soda rocks, such as the nephelite rocks, and in the basalts and
similar rocks.
In the rocks in which they occur the zeolites may be found (1) within
the mass of the rock as alteration products of the minerals; (2) in amyg-
dules, filling the vacuoles of the igneous rocks; and (3) in other openings
of all kinds, such as fractures, the pores of sediments, etc.
ALTERATIONS OF ZEOLITES. 333
The development of the zeolites in nearly all cases requires hydration
and expansion of volume, as shown under the discussions of the particular
minerals from which they form. Their formation, therefore, tends to fill
up the crevices and cracks in rocks, even if no material be furnished from
an extraneous source. It may be that the zeolitization combined with other
alterations furnishes sufficient material to entirely fill the vacuoles of many
arnygdaloids without material being furnished from an extraneous source.
(See pp. 631-634.) However, it is doubtless the case that much of the mate-
rial of the zeolites which is deposited in the belt of cementation is derived
by solution from the belt of weathering. As shown under the individual
minerals from which the zeolites develop, the conditions for the formation
of these minerals are those of the zone of katamorphism, both in the belt
of weathering and in that of cementation.
In the belt of cementation, in which hydration is perhaps the most
characteristic reaction and alterations can take place with expansion of
volume, the zeolites form on a great scale. Conforming with these state-
ments are the observations made by Daubrde" that zeolites can be formed
experimentally in the presence of abundant water at temperatures of about
50 C. Pointing in the same direction is the fact stated by Renard 6 that
phillipsite has extensively formed at the bottom of the sea at temperatures
not far from C.
While the zeolites develop chiefly in the belt of cementation, it is
certain that in very humid regions they form in the belt of weathering.
But it is also certain that to a great extent the zeolites are also destroyed in
the belt of weathering. This is especially the case in hot arid regions.
Alterations. The most comprehensive statement as to the alterations of
the zeolites is that given by Clarke/ His statements are as follows: (1)
Natrolite alters into prehnite (orthorhombic; sp. gr. 2.875); (2) mesolite
alters into prehnite; (3) scolecite alters into prehnite; (4) analcite alters
into albite (triclinic; sp. gr. 2.635), and (5) orthoclase (monoclinic; sp. gr.
2.57) and prehnite; (6) apophyllite alters into pectolite (monoclinic;
sp. gr, 2.73); (7) heulandite alters into albite, and (8) into orthoclase;
Daubre, A., fitudes synthotiquea de geologic expe>imentale, Paris, 1879, pt. 1, pp. 199, 205-207.
* Murray, John, and Renard, A. F., Report of the scientific results of the voyage of H. M. S.
Challenger, 1873-1876; Deep-sea deposits, London, 1891, pp. 400-411.
c Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, pp. 32--15.
334 A TREATISE ON METAMORPH1SM.
(9) stilbite alters into albite, and (10) into orthoclase; (11) ehabazite
alters into natrolite (hexagonal-rhombohedral (Hintze), orthorhombic
(Dana); sp. gr. 2.225); (12) laumontite alters into albite, (13) into ortho-
clase and prehuite, and (14) into analcite (isometric; sp. gr. 2.255).
These alterations may be expressed by the following equations, the
numbers of the equations corresponding to the numbers of the alterations.
In writing these equations, whether Groth's or Clarke's formula is used
depends on which is more nearly analogous to the formula of the mineral
produced.
(1) Na 2 Al 2 Si,0 10 .2H 2 0+2CaC0 3 =H.,Ca 2 AL 1 Si s 12 +Na. 1 C0 3 +C0 2 +H 2 0-k.
(2) H 2 Na 2 CaAl 4 Si 6 2 i.4H 2 O+3CaCO s =2H 2 Ca,Al 2 Si 3 O 12 +Na 2 C0 8 +2CO 2 +3H 2 O-k.
(3) H 2 CaAl 2 Si s O 1I .2H 2 O-rCaCO s =H 2 Ca 2 Al. ! Si3O 12 +2H 2 0+CO 2 -k.
(4) Na 2 Al 2 Si 4 12 .2H 2 O+2SiO 2 =2NaAlSi s O 8 +2H 2 O-k.
(5) 3(Na 2 Al 2 Si 4 12 .2H 2 O)+4CaCO 3 +K 2 C0 8 =
2KAlSi 3 O 8 +2H 2 Ca 2 Al 2 Si 3 O 12 +3Na 2 CO 3 +2CO 2 +4H 2 O-k.
(6) H, 4 Ca 4 Si 6 O 23 +Na 2 C0 3 =2HNaCa,Si 3 O 9 +6H 2 O+CO 2 --k.
(7) H 4 CaAl 2 Si 6 O 18 .3H 2 O+Na 2 C0 8 =2NaAlSi 3 O 8 + CaCO 3 ^5H 2 O-k.
(8) H 4 CaAl 2 Si 6 O, 8 .3H 2 O+K 2 CO 3 =2KAlSi 3 8 -rCaCO 3 +5H 2 O-k.
(9) Ca 3 Al 6 Si 18 O 48 .18H 2 O+3Na 2 CO 3 =6NaAlSi s O 8 +3CaCO 3 +18H. i O k.
(10) Ca s Al 6 Si 18 O 48 .18H 2 O+3K 2 CO 3 =6KAlSi s O 8 +3CaCO 3 +18H. ! O-k.
(11) Gas Al.SiuO,,. 18H 2 O+ 2 Al (OH ) 8 +4Na.,CO s =
4H 4 Na 2 Al 2 Si 8 O 12 r3CaCO 8 +COj+13H 2 O-k.
(12) Ca 3 Al 6 Si 12 O 36 .12H 2 O+2Na 2 CO 5 +CO 2 =4NaAlSi 8 O 8 +Al 2 O 3 +3CaCO 9 +12H 2 O-k.
(13) Ca 3 Al 6 Si, 2 O S6 .12H 2 O-rCaOO 3 +K 2 CO 3 =
2KA]Si 3 O 8 +2H 2 Ca 2 Al 2 8i 3 O 12 +2CO 2 +10H 2 O-k.
(14) Ca 3 Al 6 Si, 2 O S6 .12H 2 O+3Na 2 C0 3 =3(Na i Al 2 Si 4 12 .2H 2 O)+3CaCO 3 i-6H 2 O-k.
The decreases in volumes are as follows: Prehnite as compared with
the natrolite, equation (1), 16.12 per cent; prehnite as compared with meso-
lite, equation (2), 15.05 per cent; prehnite as compared with scolecite,
equation (3), 16.66 per cent; albite as compared with the analeite and
quartz, equation (4), 17.25 per cent; orthoclase and prehnite as compared
with analcite, equation (5), 14.09 per cent; pectolite as compared with
apophyllite, equation (6), 19.48 per cent; albite as compared with heulan-
dite, equation (7), 25.03 per cent; orthoclase as compare.d with heulandite,
equation (8), 18.44 per cent; albite as compared witli stilbite, equation (9),
31.67 per cent; orthoclase as compared with stilbite, equation (10), 25.66
per cent; natrolite as compared with ehabazite, equation (11), 4.58 per
ALTERATIONS OF ZEOLITES. 335
cent; albite as compared with laumontite, equation (12), 34.92 per cent;
orthoclase and prelmite as compared with laumontite, equation (13), 17.75
per cent; and analcite as compared with laumontite, equation (14), 4.30 per
cent.
In calculating the volume relations the carbonates, and in equation
(11) the aluminum hydrate, are ignored. If these compounds were taken
into account the decreases in volume would in some cases be somewhat
more, in others somewhat less. To ignore these side compounds seems the
best course, since the added and subtracted salts may be in other forms
than those given.
For the most part the alterations, so far as the bases are concerned,
are remarkably simple, involving only the interchange between the alkalies,
sodium and potassium, or between sodium and the alkaline earth calcium,
or the addition of the bases sodium or calcium. They are all reactions of
dehydration, partial or complete. Many of them are reactions of decar-
bonation and one, equation (4), is a reaction of silication. Presumably
they all take place with the absorption of heat. While nothing can be
ascertained as to the actual conditions under which the changes take place,
one would expect them to occur in the zone of anamorphism, for a number
of the reactions reverse those of the zone of katamorphism. In the latter
zone the alteration of the feldspars into the zeolites is well known. The
reverse changes, those of analcite, heulandite, stilbite, and laumontite into
albite and orthoclase, for which equations are written, would be hardly
likely to take place in the same zone. At any rate the alterations of the
feldspars into the zeolites and the zeolites into the feldspars present a very
interesting case of reversible reactions discussed subsequently. (See
pp. 366-369.)
It is further certain that the zeolites as extensively formed in the belt
of cementation, in the belt of weathering break up, by carbonation and
hydration, into the simpler compounds, such as the carbonates of the
alkalies and the alkaline earths, diaspore or gibbsite, kaolin, and quartz.
Hypothetical reactions could readily be written for these changes similar
to those worked out for zoisite and epidote (pp. 322-323); but since so
little is known as to the definite minerals which are formed from each
zeolite, this is hardly worth while at the present stage of knowledge.
336 A TREATISE ON METAMORPHISM.
MICA GROUP.
MUSCOVITE, PABAC10MTE, BIOTITE, AXD PHLOdOPITE.
The mica group includes the following rock- making species:
Muscovite:
(H,K) AlSiO 4 . (Normal muscovite KH 2 Al 3 Si s O u . )
Monoclinic.
Sp. gr. 2.76-3.0.
Paragcmite:
H 2 NaAl 3 Si s O ls .
Monoclinic.
Sp. gr. 2.78-2.90.
Biotite:
(H,K) 2 (MgFe) 2 Al 2 Si 3 O 12 . (Dana.) (Proportion of Mg:Fe varies widely. Normal
biotite: KHMgiAljSisO^. (Clarke.))
Monoclinic.
Sp. gr. 2.7-3.1.
Phlogopite:
KH 2 Mg,AlSi s O u -
Monoclinic.
Sp. gr. 2.78-2.86.
MUSCOVITE.
Muscovite, as already noted, is hydrogen-potassium-aluminum silicate.
occurrence. Muscovite is an abundant constituent in the plutonic rocks,
but is rather rare as a constituent in the volcanic rocks. It is one of the
most abundant constituents of the metamorphosed rocks, being a chief
mineral in many metamorphosed sedimentary and many metamorphosed
igneous rocks. As a secondary constituent, it is derived from many other
minerals. The more important of these are feldspar, including both ortho-
clase and plagioclase, nephelite, sodalite, leucite, the scapolites, spodumene,
topaz, andalusite, and cyanite. It is also recorded as a pseudomorph after
tourmaline, garnet, beryl, and cordierite. There is little doubt also that
muscovite in the metamorphosed rocks is largely formed from the materials
of the zeolites. Some of the minerals, such as nephelite, sodalite, and
leucite, from which the muscovite is derived, occur only in the igneous rocks.
Others of them, such as the zeolites, occur only in rocks of altered or sec-
ondary nature. Others of the minerals from which muscovite is derived,
such as topaz, cyanite, and andalusite, are chiefly metamorphic constituents.
ALTERATIONS OF MUSCOVITE. ,337
Still others from which muscovite is derived, such as the feldspars, may
be original constituents of the igneous rocks, or they may be original or
secondary constituents of the sedimentary rocks. It is therefore clear that
muscovite has an unusual variety of sources, and consequently it may be
expected in almost any variety of rock except the volcanics. It is, how-
ever, a more characteristic constituent of the acidic and intermediate rocks
than of the basic rocks.
In summary, muscovite is derived from anorthoclase, diaspore, gibbsite,
leucite, microcline, nephelite, orthoclase, plagioclase and orthoclase, scapo-
lites, sodalite, and spodumene. The muscovite damourite is derived from
andalusite, corundum, cyanite, sillimanite, staurolite, and topaz.
Alterations. The minerals to which muscovite alters are not nearly so
abundant as those from which it is derived. One of the most frequent
alterations is that of hydration, a part of the potassium being replaced by
hydrogen; or at the same time it may take up other bases and thus the
mineral may pass into vermiculite, a somewhat indefinite compound to
which no formula can be assigned. Muscovite also alters into serpentine
(monoclinic; sp. gr. 2.50-2.65) and into the steatitic form of talc (massive;
sp. gr. 2.7-2.8). Probably simultaneously with the formation of these
minerals gibbsite (monoclinic; sp. gr. 2.3-2.4) or diaspore (orthorhombic ;
sp. gr. 3.33.5) forms, although the contemporaneous formation of these
minerals is not mentioned. Muscovite also may alter into the soda-mica
paragonite (monoclinic; sp. gr. 2.782.90).
The reactions by which muscovite passes into serpentine and talc are
very uncertain. If the magnesium were supposed to be derived from a
carbonate and all of the silica of the muscovite went into the resultant
compounds, the reactions may be written as follows:
For serpentine:
(1) 2KH 2 Al s Si 3 O 12 +9MgCO 3 +13H 2 O=3H 4 Mg s Si 2 O 9 +6Al(OH) s +K a CO 8 +8CO 2 +k.
For talc :
(2) 4KH 2 Al s Si 3 O 12 +9MgCO,+17H 2 O=3H. ! Mg 3 Si 4 1 2+12Al(OH) 3 +2K 2 CO 9 +7CO 2 +k.
The increase in volume of the serpentine as compared with the muscovite is
16.56 per cent, and the decrease of the talc 25.23 per cent. But if the
magnesium carbonate be contributed by solutions, and the gibbsite remains
as a solid with the serpentine and talc, the increase in volume of the ser-
MON XLVII 04 22
338 A TREATISE ON METAMORPHISM.
pentine and gibbsite as compared with the muscovite is 88.44 per cent, and
of the talc and gibbsite 46.69 per cent.
The change of muscovite to paragonite merely requires the substitution
of sodium for potassium, and may be written as follows :
(3) 2H 2 KAl s SiA a +Na I CO s =2H 2 NaAl,Si 3 O 12 +K 2 CO s +k.
The decrease in volume is 2.67 per cent.
Muscovite under deep-seated conditions is a mineral which is practi-
cally permanent. In fact, under these conditions, as already indicated, it is
produced by the alteration of other minerals. The above alterations of
muscovite, resulting in the formation of vermiculite, serpentine, and talc,
with gibbsite all occur in the zone of katamorphism, and especially in the
belt of weathering. Even under the conditions of the surface belt the proc-
esses of change are exceedingly slow. Corresponding with this position,
the changes take place with increase of volume and liberation of heat.
PARAGONITE.
Paragonite is hydrous sodium-aluminum silicate.
occurrence. Paragonite is not certainly known as an original pyrogenic
constituent in igneous rocks. It is found especially in the metamorphosed
igneous rocks and in the semimetamorphosed and completely metamor-
phosed sedimentary rocks. In many so-called sericite rocks it is probable
that a portion of the micaceous mineral is paragonite rather than muscovite.
Paragonite is especially likely to occur in the metamorphic rocks, instead
of muscovite, where the original rocks, either igneous or sedimentary, bear
a considerable amount of sodium. Very frequently associated with para-
gonite are the heavy metamorphic minerals, such as cyauite, staurolite,
garnet, tourmaline, etc. In certain places muscovite has been noted as pass-
ing to paragonite, and thus the potassium mica is a source for the soda mica.
In summary, paragonite as a metamorphic mineral is derived from
anorthoclase, muscovite, and plagioclases.
Alterations. Alterations of paragonite are not recorded in the standard
text-books. However, there can be little doubt that this mineral undergoes
a set of alterations in the zone of katamorphism, and one would expect
that these alterations would be analogous to those which take place with
muscovite.
MICA GROUP. 339
Biotite is hydrogen-potassium-magnesium-aluminum silicate, a part of
the magnesium frequently being- replaced by iron.
occurrence. Biotite is an original chief constituent of many of the igneous
rocks, both plutonic and volcanic, and ranging from those which are p,cid
to those which are basic. It is a very abundant secondary constituent
in the slates, schists, and gneisses, developing on a great scale in the
metamorphosed rocks, both igneous and sedimentary. As a secondary
constituent it seems usually not to be derived from a single mineral, as is
frequently the case with muscovite, but is produced from material furnished
by two or more minerals. For instance, it is frequently a reaction product
between magnetite and other minerals, the magnetite furnishing the iron
for the biotite, the other constituents being derived from such minerals as
the pyroxenes, arnphiboles, and feldspars. A frequent case is the formation
of biotite from the pyroxenes, feldspars, and magnetite. The feldspars and
feldspathoids frequently furnish the potassia, parts of the alumina, and silica.
The pyroxenes and amphiboles frequently furnish a part of the magnesia,
alumina, and silica. Dolomite is often a source of the magnesia. The
oxides and carbonate of iron are the most frequent sources of this element.
In summary, as a metamorphic mineral, biotite is derived from anortho-
clase, augite, hornblende, microcline, orthoclase, and the scapolites.
Alterations. Perhaps the most frequent alterations of biotite are to
hydrobiotite (probably monoclinic; sp. gr. 2.90, average of biotite) and to
chlorite (monoclinic; sp. gr. 2.80). It also alters into epidote (monoclinic;
sp. gr. 3.25-3.50); rarely it alters into hypersthene (orthorhombic; sp. gr.
3.40-3.50) and sillimanite (orthorhombic; sp. gr. 3.23-3.24); and in some
cases it apparently alters into serpentine (monoclinic; sp. gr. 2.575). Its
alteration into the above minerals may be accompanied by the separation
of quartz (rhombohedral ; sp. gr. 2.6535), and if the biotite be ferriferous,
by the formation of magnetite (isometric; sp. gr. 5.174), or other iron oxide.
The alteration of biotite into serpentine probably requires the simulta-
neous production of kaolin (monoclinic; sp. gr. 2.615) and gibbsite (mono-
clinic; sp. gr. 2.35). Supposing that all the magnesium of normal biotite
340 A TREATISE ON METAMOKPHISM.
goes into the serpentine, and that all the silica not required for the pro-
duction of this mineral passes into the kaolin, the reaction is as follows :
(1)
4H 4 Mg 3 Si 2 O 9 +5H,Al 2 Si 2 O 9 +2Al(OH) 8 +3K 2 UO 3 Tk.
Supposing all the serpentine, kaolin, and gibbsite to remain as solids, and
the potassium carbonate to go into solution, the increase in volume is 14.26
per cent.
The change of biotite into hydrobiotite may be written:
(2) 2HKMg 2 Al 2 Si s O 1 2+7H 2 O+CO 2 =2(H.,Mg 2 Al 2 Si 8 O I2 .3H 2 O)+K. 1 CO s +k.
The increase in volume is 3.8 per cent.
The alteration into chlorite, supposing all the alumina and silica
to remain in the altered mineral, and the additional magnesia to be added
in the form of a carbonate, may be written as follows :
(3) 2KHMg 2 Al 2 Si 3 O 12 +4MgCO s +5H 8 O=2[H. ! Mg 4 Al 2 Si 3 O 1:1 --l(OH)]+K 2 CO s +3CO 2 +k.
The increase in volume, supposing the magnesium carbonate is added in
solution and the potassium carbonate goes into solution, is 22.92 per cent.
The reaction by which biotite passes into epidote is uncertain. If the
ferrous iron of the biotite be changed to sesquioxide during the alteration,
and if the proportion of magnesium to iron be supposed to be 3:1 in the
biotite, and of aluminum to iron 4:1 in the epidote, the reaction may be
written :
(4) 6H 2 K 8 Mg 3 FeAl 4 Si 6 O 2 4+20CaCO s +4CO. 1 +3O=
2(H 5 Ca 10 Al 12 Fe 8 Si 15 O 65 )+6SiO 2 +18MgCO 8 +6K 2 CO 3 +H 2 O+k.
The ratios assumed of the magnesium and iron for the biotite, and of
aluminum and iron for the epidote, are near means. If it be assumed that
the iron of the biotite is not changed to sesquioxide, but that the sesquioxide
of iron for the epidote must be derived from another source, the reaction
takes a very different form. Under such an assumption the epidote may
be produced from normal biotite, and the equation stand as follows:
(5) 30KHMg 2 Al 2 Si s O 12 +6Fe,O,+40CaCO s +35CO, ->- H 2 O=
4(H 5 Ca 10 Al 12 Fe s Si 16 O 65 )+30SiO :i +12AlO(OH)+60MgCO 3 +15K 2 CO a +k.
ALTERATIONS OF BIOTITE. 341
By adding thirteen molecules of water instead of one, twelve molecules of
gibbsite instead of twelve molecules of diaspore (orthorhombic ; sp. gr. 3.40)
will be produced.
In equation (4), supposing the calcium carbonate to be added in
solution and the magnesium carbonate and the potassiuin carbonate to be
removed in solution and the silica to remain as a solid, the decrease in
volume is 14.86 per cent.
In equation (5), supposing the biotite and iron oxide to be solids, the
calcium carbonate to be added in solution, the magnesium' and potassium
carbonates to remain in solution, but the epidote, silica, and diaspore to
remain as solids, the decrease in volume is 18.45 per cent. If gibbsite
instead of diaspore be produced the decrease in volume will not be so much.
If it be supposed that the aluminum passes into spinel (isometric; sp.
gr. 3.8) instead of diaspore or gibbsite, and spinel is known to form in con-
nection with biotite, the number of molecules of magnesium carbonate
would be reduced by six in equation (4); that is, to 54. 6MgALO 4 would
replace the 12A1O(OH). No water would need to be added, and five
molecules of water would be produced. Finally, only twenty-nine mole-
cules of CO 2 would need to be added. Therefore the equation would be:
(6) 30KHMg 2 Al 2 Si 3 I2 +6FejO s +40CaCO s +29CO 2 =
4H 6 Ca 10 Al 12 Fe s Si 15 65 +30SiO 2 +6MgAl 2 O 4 +54MgCO 3 -M5K 2 CO,^5H,O+k.
In this case the volume of the resultant epidote, spinel, and silica, would
be 14.71 per cent less than that of the biotite and 18.15 per cent less than
that of the biotite and hematite.
It is a well-known fact that chlorite secondary to biotite is usually
accompanied by epidote and quartz. Comparing the equation (3) for the
formation of chlorite with equation (4) for epidote, we see why these two
minerals with quartz are frequently formed at the same time. For the
formation of chlorite from biotite additional magnesium is needed. For
the formation of epidote additional calcium is necessary and magnesium is
left over. If instead of magnesium and calcium carbonates being added,
as suggested in equations (3) and (4), only calcium carbonate were avail-
able, the excess of magnesium produced by the passage into epidote may go
into the chlorite, and thus epidote and chlorite be simultaneously produced.
342 A TREATISE ON METAMORPH1SM.
Combining these equations, (3) and (4), and supposing the iron oxide to be
furnished by hematite, the reaction may be written as follows :
(7) 60KHMg 2 Al 2 Si s O 12 +6FeA-40CaOO 3 -r76H 2 O =
30[H 2 Mg 4 Al 2 Si 3 O 12 .4(OH)]+4(H 5 Ca 10 Al 12 Fe 3 Si 15 O 65 )+30SiO. ! +12AlO(OH) +
30K 2 OO 3 +10CO 2 -k.
The increase in volume of the epidote, chlorite, quartz, and diaspore
together, as compared with the biotite and hematite, would be 1.81 per
cent.
If biotite alters to hypersthene and sillimanite it may be presumed, in
order to furnish the necessary iron for the hypersthene, that the biotite is
an iron-bearing one. If the magnesium be to the iron as 3:1, and the
hypersthene be one in which the same ratio prevails, the reaction may be
written as follows:
(8) H 2 K 2 Mg 3 FeAl 4 Si 6 O 2r rCO,=Mg 3 FeSi 4 O]2+2Al 2 SiO 5 +H 2 O+K 2 OO 3 +k.
The decrease in volume of the hypersthene and sillimanite, as compared
with the biotite, is 24.68 per cent.
In the majority of the above reactions a formula for biotite is used
which contains no iron. The majority of biotites in nature do contain
some iron. If this material be present, simultaneously with the formation
of other minerals magnetite (isometric; sp. gr. 5.174), hematite (rhombohe-
dral; sp. gr. 5.225), and the other oxides of iron may be produced. The
abstraction of the iron oxides is accompanied commonly by a change in
color of the altering biotite from brown to green. The presence of these
compounds, however, in subordinate quantities will not alter the main
conclusions as to the volume relations above given.
The alterations of biotite, serpentine, kaolin, and gibbsite, into hydro-
biotite and chlorite, equations (1) to (3). are all reactions which are
known to occur in the zone of katamorphism, corresponding with which
position they are all reactions of hydration, and the first two also of car-
bonation. Where chlorite and epidote form together, the reaction is that
of hydration, and doubtless this change also takes place in the zone of
katamorphism. The formation of hypersthene and sillimanite from biotite
usually occurs in connection with contact reactions of igneous rocks ; it is,
therefore, a reaction requiring high temperature. Also the minerals
OCCURRENCE AND ALTERATIONS OF PHLOGOPITE. 343
sillimaiiite and hypersthene do not form at the surface, but at depth.
Corresponding' with these physical-chemical facts, the reaction is one of
dehydration and reduction of volume.
PHLOGOPITE.
Phlogopite is potassiuin-hydrogen-maguesium-aluminum silicate.
occurrence. Phlogopite has an occurrence which is somewhat different
from that of biotite. It is especially characteristic of metamorphosed
impure carbonates, such as dolomitic marbles. In these rocks it is often
associated with pyroxene, amphibole, etc.
Alterations. The most frequent alterations of phlogopite are to hydro-
phlogopite (monoclinic; sp. gr. 2.303, kerrite) and chlorite (monoclinic;
variety of penninite; sp. gr. 2.649). It is also said to alter to talc (orthor-
hombic or monoclinic; sp. gr. 2.7-2.8). In these last two alterations
gibbsite (monoclinic; sp. gr. 2.32.4) or diaspore (orthorhombic; sp. gr. 3.40)
must simultaneously separate.
The reactions for these changes are as follows:
For hy drophlogopite :
(l)2H 2 KMg 3 AlSi 3 O 12 +7H 2 O+CO,=2(H 3 Mg 3 AlSi 3 O 12 .3H 2 O)+K 2 C0 3 +k.
For chlorite:
(2)2H 2 KMg 3 AlSi 3 O 12 +6MgCO !> +7H,O=2[H3Mg 6 AlSi 3 O 12 .6(OH)]+K 2 CO,+5CO 2 +k.
For talc and gibbsite:
(3)4H 2 KM g3 AlSi 3 12 +6H 2 0+-tC0 2 =3H 2 Mg 3 SiA2+4Al(OH) 3 +3MgC0 3 +K :i C0 3 +H 2 0+k.
(4)2H 2 KMg 3 AlSi 3 I2 +C0 2 +4H 2 0=H 2 Mg 3 Si 4 I2 +H 4 M g 3Si 2 9 +2Al(OH) 3 +K. 1 C03+k.
In equations (3) and (4) if diaspore instead of gibbsite were produced
less water would be needed.
Disregarding the carbonates, the increase in volume for hydrophlogo-
pite, equation (1), is 26.89 per cent; for chlorite, equation (2), is 41.02
per cent. The decrease for talc and gibbsite, equation (3), is 7.79 per
cent, and for talc and diaspore 18.27 per cent. The increase in volume of
the serpentine, talc, and gibbsite, equation (4), is 5.23 per cent.
All of these reactions are those of hydration and solution. They are
characteristic of the zone of katamorphism.
344 A TREATISE ON METAMOKPHISM.
According to' Clarke, penninite, one of the chlorites, is composed of
one molecule of biotite-chlorite and one molecule of phlogopite-chlorite;
and clinochlore is composed of two molecules of biotite-chlorite and one
molecule of phlogopite- chlorite. It is therefore easy to combine the equa-
tion given under biotite for the production of chlorite with the one under
phlogopite producing chlorite, and thus produce penninite (pseudorhombo-
hedral and monoclinic; sp. gr. 2.62.85) and clinochlore (monoclinic; sp.
gr. 2-2.5). However, as the alterations for the production of chlorite from
biotite and of chlorite from phlogopite, reactions of hydration, carbona-
tion, and liberation of heat occur in the zone of katamorphism, it may
be said that where penninite and clinochlore are produced from biotite and
phlogopite the physical-chemical reactions are of the same class as those
which have been given for hydrobiotite and hydrophlogopite.
CLINTONITE GROUP.
MAROARITK, i II I.OIM1OI n. AND OTTRKUTK.
The clintonite group includes the following rock-making minerals:
Margarite:
H 2 CaAl 4 Si s O ls .
Monoclinic.
Sp. gr. 2.99-3.08.
Chlariloid:
H 2 (MgFe)Al 2 Si0 7 .
Monoclinic (G) or triclinic (D).
Sp. gr. 3.52-3.57.
Ottrelite:
H 2 (FeMn)Al 2 Si 2 O 9 .
Monoclinic or triclinic.
Sp. gr. 3.3.
occurrence. The most common development of margarite is in connection
with corundum. In a number of cases it is recorded that the alumina of the
margarite is directly furnished by the corundum. Margarite is also found
as a metamorphic mineral in schists and gneisses, associated with the heavy
minerals staurolite, tourmaline, etc. As a metamorphic mineral, margarite
is also recorded as being derived from diaspore and gibbsite.
CHLORITE GROUP. 345
Chloritoid and ottrelite both occur in the slates, schists, and gneisses
which are derived from the argillaceous sediments as a product of or
connected with deep-seated, and especially deep-seated regional metamor-
phism, and often contact action. They are thus heavy minerals which
develop from the simpler constituents in the argillaceous sediments in the
zone of anamorphism, their formation resulting in condensation.
Alterations. The only alteration of the clintonite group recorded is that
of margarite to dudleyite. However, as no definite formula for this mineral
is given, it is not practicable to write an equation representing the
transformation.
While no other alterations of the clintonite group are mentioned, there
is no doubt that in the upper zone of metamorphism, especially in the belt
of weathering, the chloritoids, ottrelite, and margarite are decomposed into
simpler compounds, as are the other silicates.
CHLORITE GROUP.
AMESITE, rORUXDOPHILITE, ri:u< n MUM I I . i l.i \,n II I inn . AND PENMNITE.
The minerals of the chlorite group, according to Tschermak, may be
regarded as isomorphous mixtures of amesite (H 4 Mg 2 Al 2 Si0 9 ) and serpen-
tine (H 4 Mg 3 Si 2 9 ) molecules, although Clarke dissents from this conclusion.
Tschermak gives the range of the various orthochlorites as follows :
Amesite: At to At 4 Sp.
Corundophilite: At 4 Sp to At 7 Sp :) .
Prochlorite (ripidolite) : At,Sp 3 to At 3 Sp 3 .
Clinochlore: At 3 Sp 3 to At Sp.
Penninite: At Sp to At 2 Sp 3 .
These would correspond to the following compositions:
Amesite:
H 4 Mg 2 Al 2 Si0 9 to H 20 Mg 11 Al 8 Si 6 45 .
Monoclinic.
Sp. gr. 2.71.
Corundophilite:
HjoMgnAlgSieOu to H 40 Mg 23 Al H Si 13 O llo .
Monoclinic.
Sp. gr. 2.90.
346 A TREATISE ON METAMORPHISM.
Prochlorite:
H 40 Mg 2S Al u Si 13 90 to H 20 Mg 12 Al 6 Si,0 45 .
Monoclinic.
Sp. gr. 2.78-2.96.
dinochlore:
H 20 Mg 12 Al 6 Si 7 O, 5 to H 8 Mg 5 Al 2 Si 3 O 18 .
Monoclinic.
Sp. gr. 2.65-2.78.
Penninile:
H 8 Mg 5 Al 2 Si,0 18 to H M Mg ls Al 4 Si 8 45 .
Pseudorhombohedral and monoclinic.
Sp. gr. 2.60-2.85.
A considerable part of the magnesium, as shown by the analyses,
may be replaced by iron, the analyses of corumlophilite showing as high
as 15 per cent of monoxide of iron; of prochlorite, from 15 to 25 per cent,
running even higher than the magnesia. The percentage of monoxide of
iron in clinochlore and penninite is usually much less. The alumina may
be replaced in part by sesquioxide of iron, although the proportion of this
replacement is not nearly so great as that of the magnesia by the iron mon-
oxide, the iron sesquioxide generally not running beyond 2 or 3 per cent.
It is apparent that the specific gravities of the chlorites do not regularly
grade from lower to higher, as in the feldspars. Doubtless such a regular
gradation would occur provided the chlorites were pure magnesium min-
erals, corresponding to the formulae above given. The high specific gravities
which the intermediate minerals in the group, corundophilite and prochlo-
rite, may have are doubtless explained by their frequent high content of
iron monoxide.
occurrence. Chlorite is the most abundant and widespread of all the
secondary silicates. As a secondary mineral it is probably subordinate
only to quartz. Chlorite is nowhere known as a pyrogenic constituent of
igneous rock. It is very abundant in many of the altered igneous rocks,
both plutonic and volcanic, including lavas and tuffs, being especially
abundant in the so-called green-schists. Also it is very abundant in many
amphibolites. In the altered igneous rocks which are changed under mass-
static conditions it is one of the most abundant secondary constituents,
being especially prevalent in the basic rocks, such as the greenstones. It
is also found in the acid rocks. Chlorite occurs as a plentiful allogenic
constituent in all kinds of mechanical sedimentary rocks. It develops as a
ALTERATIONS OF CHLORITE. 347
very abundant secondary constituent in the metamorphosed sedimentary
rocks, such as slates, schists, and gneisses. Frequently chlorite may occur
more abundantly adjacent to intrusive rocks than elsewhere. The most
characteristic associated secondary minerals are epidote, serpentine, talc,
zeolites, kaolin, magnesite, iron oxides, aluminum oxides, etc.
In the discussion of the individual minerals it has been shown that
chlorite is one of the abundant derivation products of the following minerals :
Almandite, augite, garnet, hornblende, iolite, prehnite, pyrope, staurolite,
tourmaline, and vesuvianite. Being essentially a magnesium-aluminum
silicate, it is especially likely to be derived from the heavily magnesian
minerals, of which the olivine, pyroxene, amphibole, mica, and garnet
groups are the more important. As already seen, corundophilite and pro-
chlorite may contain a large percentage of iron monoxide, and therefore
one would naturally expect these chlorites to form from the minerals which
also contain a large percentage of iron monoxide, as, for instance, olivine,
actinolite, etc. In many cases the mineral from which the chlorite is
derived does not contain a sufficient-amount of magnesium. In such cases
this substance is derived from adjacent minerals, or is brought in in solution.
It has been supposed in such cases that the magnesium is transported as a
carbonate. However, the principles of its development would be in no
way changed if any other salt of magnesium, such as magnesium chloride,
were substituted for the carbonate.
In the discussion of the individual minerals it is shown that chlorite
develops especially in' the upper physical-chemical zone, and particularly in
the belt of cementation. Under conditions of quiescence it develops at
very considerable depth ; but in proportion as interior movement occurs it is
likely to develop in smaller quantity or not at all, its place as a metamor-
phic mineral being taken by the magnesian mica biotite.
Alterations. The alterations of chlorite, like those of other minerals, are
largely dependent upon the zones or belts in which the mineral is located.
The only definite alteration products of chlorite which are recorded are
those which Tschermak has called enophite and berlanite. The first is said
by him to be a serpentinous variety of chlorite. No formula for either has
been determined, and therefore it is not possible to write equations repre-
senting the transformation. Rosenbusch says that the last stage of the
alteration of chlorite is into an aggregate of limonite, carbonate, and quartz.
This degeneration is especially characteristic of the belt of weathering. As
348 A TREATISE ON METAMORPHISM.
usual, no attempt is made to write equations for these degenerative changes;
but if one knew definitely the composition of the original mineral and that
of the minerals which were produced in a given case, it would be easy to
write equations for the change and to calculate the volume relations.
While the alterations of chlorite in the zone of anamorphism are not
recorded, it is certain that the chlorite of chloritic rocks under the condi-
tions of the lower physical-chemical zone pass into other constituents, since
chlorite is almost always rare or absent in both the sedimentary and the
igneous rocks which have recrystallized in the lower zone and have not
been later affected by changes in the upper zone.
Therefore in the lower zone chlorite and some of the material of the
associated minerals recombine and reproduce minerals from which chlorite
was originally derived, or other minerals. There is little doubt that chlorite
furnishes a considerable part of the elements for such minerals as the micas,
feldspars, amphiboles, pyroxenes, and even the olivines, which develop in
the zone of anamorphism, and also it is probable that the chlorite furnishes
a part of the constituents for certain of the heavy metamorphic minerals,
such as garnet, cliutonite, staurolite, tourmaline, etc.
SERPENTINE-TALC GROUP.
I
SEKPEXTIXE AND TALC.
The serpentine-talc group includes:
Serpentine:
H^MgjSijjCV (A part of the Mg may be replaced by Fe, and where the amount of Fe is
considerable this mineral is called bastite.)
Monoclinic.
Sp. gr. 2.50-2.65.
Talc:
H s M gs Si 4 12 .
Orthorhombic or monoclinic.
Sp. gr. 2.7-2.8.
Serpentine and talc, like chlorite, are both hydrous magnesium silicates.
Indeed, as has been pointed out, Tschermak regards the serpentine mole-
cule with the amesite molecule (H 4 Mg 2 Al 2 SiO 9 ) in variable proportions to
constitute the chlorites. Serpentine is more hydrous and more basic than
talc. Since the serpentine molecule is similar to some of the chlorites,
one would expect that the occurrence of the two would be very similar,
and such is the fact.
OCCURRENCE AND ALTERATIONS OF SERPENTINE. 349
SERPENTINE.
occurrence. Serpentine occurs in substantially all the varieties of rocks in
which chlorite is found, but it is most abundant as a secondary constituent
in the igneous rocks which are very heavily magnesian, especially the
pyroxenites, peridotites, and similar rocks. Locally it may be so abundant
as a secondary constituent in rocks of this class, especially those bearing
olivine, as to form- serpentine rocks. Serpentine develops very abundantly
in the sedimentary rocks which are rich in magnesian constituents, both in
detrital material from basic igneous rocks, and in limestones, and in various
transition varieties. Serpentine is a product of the zone of katamorphism,
including both the belt of cementation and the belt of weathering.
As shown under the discussion of the various minerals, it is secondary
to actinolite, biotite, bronzite, chondrodite, clinohumite, diopside, enstatite,
hornblende, humite, hypersthene, muscovite, oliviue, pyrope, sahlite, and
spinel. Of these the most important is olivine, and of second importance
are the pyroxenes and amphiboles. In many cases the constituents out of
which serpentine is formed are derived not from a single mineral, but from
various minerals, in which case the serpentine may replace nonmagnesian
minerals, as feldspar, or may form in veins.
Alterations. Serpentine, where long exposed to the conditions of the belt
of weathering, is likely to break up into various minerals, of which brucite
(rhombohedral ; sp. gr. 2.39), magnesite (rhombohedral ; sp. gr. 3.06), opal
(amorphous; sp. gr. 2.15), and quartz (rhombohedral; sp. gr. 2.6535) are
the more important. By hydration and loss of magnesia it passes into
webskyite (amorphous; sp. gr. 1.771).
The reaction by which serpentine passes into magnesite, brucite, and
quartz may be written thus:
(1) H 4 Mg 3 Si 2 O 9 +CO 2 =MgCO a +2Mg(OH). ! +2SiOj+k.
The increase in volume is 13.02 per cent. In case opal or hydromagnesite
were formed the increase in volume would be somewhat greater, and the
reaction would involve hydration as well as carbonation. It is of course
possible that both brucite and magnesite may not always be formed simul-
taneously. If brucite and not magnesite be formed the equation is
(2) H 4 Mg s Si 2 9 +H 2 O=3Mg(OH). 1 +2SiO a +k.
350 A TREATISE ON METAMORPHISM.
The volume of the brucite and quartz is 9.82 per cent greater than the
serpentine- If magnesite and not brucite be formed the equation is
(3) H 4 Mg 3 Si 2 O 9 +3CO.,=3MgCO 3 +2SiO 2 +2H 2 O- k.
The volume of the magnesite and quartz is 18.84 per cent greater than that
of the serpentine.
Brauns" gives the formula for the development of webskyite as follows:
(4) 3(H 4 R,Si 2 9 ) RO+12aq.=2(H 6 R 4 Si 3 13 +6aq.)
Where serpentine contains iron as a base, partly replacing the magne-
sium, the iron is oxidized and may be hydrated, thus producing hematite
or limonite.
The breaking up of serpentine occurs especially in the belt of weath-
ering, the transformation representing one of the final changes in the
degeneration of the silicates. Alterations of serpentine in the zone of
anamorphism are not recorded. But the general absence of serpentine in
the schists and gneisses of sedimentary origin profoundly metamorphosed
in the zone of anamorphism is conclusive evidence that the serpentine which
once was in these rocks, and the associated secondary minerals, have
recombined to produce heavy minerals of the classes from which serpentine
and those other secondary minerals were originally produced. One could
readily form equations for such alterations by reading the equations by
which serpentine is formed from right to left. (See Table C, pp. 37f>-394.)
occurrence. Talc is practically coextensive in its occurrence with chlorite
and serpentine, but in its distribution is more nearly allied to serpentine
than to chlorite. Therefore it is found in almost every variety of rock long
subjected to alterations in the belt of weathering; but it is especially
abundant in the heavily magnesian rocks. Steatite, which is nearly pure
talc, is usually derived from the pyroxenites or peridotites. However, talc is
so abundant in many schists as to give them the name talcose, or even talc-
schists. Also, like serpentine, it occurs abundantly in the dolomite-bearing
rocks and in dolomite.
Brauns, R., Studien iiber den Palaeopikrit von Amelose bei Biedenkopf und dessen Umwand-
lungsprodukte: Neues Jahrbucli, supp.-vol. 5, Stuttgart, 1887, p. 322.
OCCURRENCE AND ALTERATION OF GLAUCONITE. 351
Talc forms in the upper zone of metamorphism. In this respect it is
like chlorite and serpentine. It is especially likely to form under condi-
tions of weathering. The minerals from which talc is derived are as
follows: Actinolite, andalusite, anthophyllite, bronzite, cyanite, diopside,
enstatite, gehlenite, hypersthene, muscovite, olivine, phlogopite, pyrope,
sahlite, sca|)olites, sillimanite, spinel, staurolite, topaz, and tremolite. The
manner of formation is given under the various minerals. Of these minerals
the more important are the uonaluminous amphiboles and pyroxenes, both
orthorhombic and monocliuic. It also forms rather abundantly from olivine,
mica, and garnet.
Alterations. Alterations of talc are not recorded. It appears to be one
of the end products of rock alteration in the belt of weathering. However,
I have no doubt that when the talc-bearing rocks are buried so deeply as to
pass into the zone of anamorphism and there alteration takes place, talc,
like chlorite, serpentine, and other minerals, is destroyed, and that from it
alone, or from it and other minerals, the classes of heavy minerals from
which the talc was originally produced are again formed.
GLAUCONITE.
Glauconite:
Essentially a hydrous silicate of iron and potassium. Definite formula unknown. The
potassium ranges from 1.85 to 6.56 per cent.
Amorphous.
Sp. gr. 2.2-2.4.
occurrence. Glauconite occurs in sediments of many kinds and ages.
Where it is so abundant as to give the rock a green color it is known as
greensand. Greensands are especially prevalent in the Cretaceous.
Alterations. Since no definite formula for glauconite can be given it is
impracticable to write equations representing the transformations. But
since glauconite is almost, if not quite, unknown in the schists and gneisses
formed in the zone of anamorphism it seems certain that under the condi-
tions of that zone the glaucouite is broken up, its constituents passing into
other minerals.
352 A TREATISE ON METAMORPHISM.
KAOLIN GROUP.
Kaolinite is the only important rock-making mineral of this group.
Kaolinite:
H 4 A1 2 Si 2 O 9 .
Monoclinio.
Sp. gr. 2.6-2.63.
occurrence. Kaolinite is a secondary product in all classes of igneous
rocks and occurs as an important constituent in all sedimentary rocks except
the pure sandstones and the pure limestones. Kaolinite and quartz are the
chief constituents of the clays, and kaolinite is a very abundant constituent
of muds and grits.
Kaolinite is a product which forms extensively in the zone of katamor-
phism in the belt of cementation and in the belt of weathering. It is likely
to be produced as a result of the decomposition of any of the aluminous
minerals. It has been noted as having been produced from the following
minerals: Andalusite, anorthoclase, biotite, cyanite, epidote, leucite, micro-
cline, nephelite, orthoclase, plagioclases, scapolites, sillimanite, sodalite,
topaz, and zoisite. Of these, undoubtedly the most important are the feld-
spars, and especially the acid feldspars.
Alterations. No alterations of kaolinite are recorded. It is certain, how-
ever, that where the kaolin-bearing sediments are deeply buried the mineral
becomes dehydrated, that such bases as the alkalies and alkaline earths and
iron replace the hydrogen, and that various anhydrous silicates or silicates
low in hydrogen are produced. It is certain that in the zone of anamor-
phism the minerals which in the upper physical-chemical zone have broken
up into kaolinite as one of the products may recombine to a large extent
and reproduce the original minerals.
SUMMARY OF ALTERATION OF SILICATES.
While the important groups of the rock-forming silicates have been
treated separately, it may be well in closing the section to class together
the groups of the original minerals which have a somewhat similar chemical
composition and therefore alter into somewhat similar products.
These classes are called by the petrographers (1) the feldspathoid
class, (2) the transition class, and (3) the ferromagnesian class. The felds-
pathoid class includes the feldspar, nephelite, sodalite, leucite, and
wernerite groups. The only rock-forming minerals belonging to the
SUMMARY OF ALTERATION OF SILICATES. 353
transition class are those of the inuscovite group. The ferromagnesian
class includes the pyroxene, araphibole, chrysolite, biotite-phlogopite, and
clintonite groups.
(1) In the upper physical-chemical zone, that of katamorphism, the
more common alteration products of the feldspathoid class are the kaolins,
the zeolites, the epidotes (including both zoisite and epidote proper), diaspore,
gibbsite, and quartz. Probably all of these minerals form in both belts of
the zone, but the development of the zeolites and the epidotes is more
characteristic of the belt of cementation than of the belt of weathering.
Indeed, as pointed out under these minerals, under long-continued condi-
tions of the belt of weathering these minerals break up into carbonates
of the alkalies and alkaline earths, hydrous and anhydrous, oxides of
aluminum and iron, quartz, and, probably, also kaolin. In the zone ot
anamorphism the more common alteration products of the feldspathoid
class are muscovite (damourite) and scapolite. The nephelite, sodalite, and
leucite groups alter into the feldspars.
(2) In the zone of katamorphism muscovite alters into serpentine,
talc, and vermiculite (hydromuscovite). In the belt of weathering the
serpentine and vermiculite may break up into simpler compounds of the
same character as those which form from the zeolites and epidotes.
In the lower physical-chemical zone, that of anamorphism, muscovite
is one of the minerals commonly produced, and therefore does not usually
alter. But by profound and deep-seated metamorphism, the material of
muscovite may pass into the heavy ferromagnesian minerals, such as garnet,
staurolite, etc. %
(3) The ferromagnesian silicates may be divided into two great
divisions those which are nonaluminous, and those which are aluminous.
In the zone of katamorphism the most common alteration products of the
nonaluminous ferromagnesian silicates are talc and serpentine. The
nonaluminous pyroxenes and amphiboles ordinarily pass into talc; the
chrysolites ordinarily pass -into serpentine. The transformations in these
directions are explained by the fact that the pyroxenes, amphiboles, and talc
are metasilicates, while the oliviues and serpentines are orthosilicates. The
metasilicates naturally pass into metasilicates, and the orthosilicates into
orthosilicates. In the zone of katamorphism the heavily aluminous ferro-
magnesian silicates alter into chlorites and epidotes. The pyroxenes and
amphiboles which are not heavily aluminous frequently split up into a com-
354 A TREATISE ON METAMORPHISM.
bination of chlorite and talc, the aluminous part of the original molecule
going to the chlorite and the nonaluminous part into the talc. The
development of epidote is largely if not wholly confined to the belt of
cementation. But chlorite apparently forms both in the belt of cementa-
tion and in the belt of weathering, especially where there is abundant
vegetation. Under extreme and long-continued conditions of the belt of
weathering the serpentines, chlorites, and epidotes are likely to further
. degenerate, breaking up into carbonates of the alkalies and alkaline earths,
anhydrous or hydrous oxides of aluminum and iron, quartz, and kaolin.
Or in the belt of weathering these end products may directly develop from
the metasilicates without the serpentines, chlorites, and epidotes as inter-
mediate products. In the zone of anamorphism the pyroxenes change to
amphiboles; the pyroxenes and amphiboles both alter to biotite; the
olivines change to the amphiboles, anthophyllite, tremolite, and actinolite.
The biotite group does not ordinarily alter. But by profound metamor-
phism the material of the biotites, amphiboles, pyroxenes, etc., may pass into
the still heavier class of minerals represented by the garnets, staurolites, etc.
THE TITANATES.
TITANITK AND PKROV8KITE.
As rock-making constituents only two titanates of importance, titanite
and perovskite, are here treated, ilmenite being given under the oxides.
TManite:
CaTiSiO 5 .
Monoclinic.
Sp. gr. 3.4-3.56.
Peronkiti:-
CaTiO s .
Isometric or pseudoisometric.
Sp. gr. 4.017-4.039.
occurrence. Titanite occurs as a rather subordinate but widespread min-
eral as an original pyrogenic constituent of igneous rocks, and also in the
schists and gneisses. So far as observed, titanite as a secondary constituent
is derived from ilmenite and rutile. These alterations are discussed under
those minerals.
ALTERATIONS OF TITANITE. 355
Alterations. Titaiiite alters into rutile (tetragonal; sp. gr. 4.18-4.25),
octahedrire (tetragonal; sp. gr. 3.82-3.95), and perovskite (isometric or
pseudoisometric ; sp. gr. 4.017-4.039).
Rutile and octahedrite may be supposed to be produced by the follow-
ing reaction:
(1 ) CaTi8iO 5 +CO.,==TiO 2 +CaCO 3 +SiO,+k.
The expansion of volume is 39.22 per cent for rutile, provided all of the
compounds separate as solids, and 42.07 for octahedrite.
Perovskite may be supposed to be produced by the simple breaking
up of titanite, according to the reaction:
(2) CaTiSiO 5 =CaTi< ) s
The expansion of volume is 0.14 per cent provided the silica also separates
as a solid.
Information as to the natural conditions under which these changes
take place is not obtainable from the papers giving the above minerals as
secondary to titanite. From the character of the first change one would
expect, however, that it would occur in the zone of katamorphism, and
especially in the belt of weathering'. Under such conditions there would be
a reason for the change, for there carbonation of the silicates, with libera-
tion of heat and with expansion of volume, is the rule. As so frequently
indicated before, the freed silica may be taken into solution, and if this
occurs the volume is decreased. Under what conditions the second reaction
is likely to have taken place I can only conjecture from its nature. I
should expect it to occur in the zone of katamorphism.
PEROVSKITE.
occurrence. Perovskite occurs as an original constituent in eruptive
rocks, and also in the metamorphic rocks, such as the schists and gneisses.
As a secondary mineral it has been observed as a product secondary to titan-
ite. It may be suspected that in the schists and gneisses it forms in the
zone of anamorphism by the union of rutile and octahedrite or brookite,
with calcium carbonate; but this is a pure conjecture, as the details of its
formation are not found in literature.
Alterations. The mineral does not readily alter into other compounds,
although it has been observed to alter into an undetermined substance, and
it is said to alter into ilmenite (hexagonal-rhombohedral ; sp. gr. 4.75).
356 A TREATISE ON METAMORPHISM.
THE PHOSPHATES.
APATITE.
The only important rock-making mineral among the phosphates is
apatite.
Apatite:
CaF.Ca 4 P 3 O 12 , or CaCl.Ca 4 P 3 O 12 , or a mixture of the two.
Hexagonal.
Sp. gr. 3.17-3.23.
occurrence. Apatite is one of the most widespread, if not the most wide-
spread, of all the subordinate constituents of rocks. It is a common, if not
an almost universal, constituent of the plutonic rocks, occurs almost as
broadly in the volcanic rocks, and is found in many varieties of unaltered
or little altered, sedimentary rocks, such as limestones, shales, sandstones,
etc.; and, finally, it is almost everywhere found in the metamorphosed
igneous and sedimentary rocks.
Alterations. The only alteration which is recorded for apatite is to osteo-
lite, which is reported as having the same composition as apatite, except
that there has been a loss of part or all of the fluorine or chlorine.
I It is certain, however, that in the belt of weathering of the zone of
anamorphism apatite is slowly dissolved. This is shown by comparative
analyses of the weathered with the unweathered varieties of the same
rock. This fact has been frequently noted in reference to the iron ores,
because here the presence or absence of phosphorus is of such great impor-
tance. It may be stated that in the iron ores it is the general rule that those
parts of deposits which have been long subjected to weathering bear a
smaller proportion of apatite than the continuations of these same deposits
in the belt of cementation.
The depletion of the surface; rocks in apatite would seem to furnish an
adequate source for the apatite in veins, this mineral being taken into
solution near the surface and redeposited deeper down, thus being trans-
ported from the belt of weathering to the belt of cementation.
ANHYDRITE AND GYPSUM, 357
THE SULPHATES.
ANHYDRITE AND GYPSUM.
The only important rock-making sulphates are anhydrite and gypsum.
Anhydrite;
CaSO t
Orthorhombic.
Sp. gr. 2.899-2.985.
Gypsum:
CaSO 4 .2H 2
Monoclinic.
Sp. gr. 2.314-2.328.
ANHYDRITE.
occurrence. As explained below, the main source of anhydrite is by the
alteration of gypsum in the zone of anamorphism. Although I do not
know the facts, I conjecture that the anhydrite deposits of Switzerland
have had such a history.
Alterations. 'Phe chief alteration of anhydrite is to gypsum (monoclinic;
sp. gr. 2.314-2.328). The reaction is:
(1) CaSO 4 +2H 2 O=CaS0 1 .2H 2 O+k
The increase in volume is 60.30 per cent. This alteration is one which
takes place ill the zone of katamorphism. An interesting case is that of
Bex, Switzerland, where the transformation from anhydrite to gypsum has
taken place completely to a depth of from 18 to 30 meters, and where
below this depth the material is anhydrite. The change of anhydrite to
gypsum is with liberation of heat, expansion of volume, hydration, lowering
of specific gravity, and lessening of symmetry,- and thus stands as a rare
example of all the tendencies of the upper physical-chemical zone.
occurrence. The most important source of gypsum is as a chemical
precipitate, especially in salt lakes having no outlets. It therefore natu-
rally occurs in association with halite, calcite, and mechanical detritus.
Gypsum also is produced in a subordinate way through fumarole action.
The calcium sulphate for the gypsum in either case is produced by the
reaction of sulphuric acid or sulphates upon calcium-bearing salts. Com-
358 A TREATISE ON METAMO11PHISM.
monly the sulphate is formed by the oxidation of a sulphide. A common
method is the production of iron sulphate by oxidation of pyrite, marcasite,
or pyrrhotite, which reacts upon calcium carbonate, thus producing gypsum.
The reaction is
CaCO s +FeSO 4 +2H 2 O=CaSO 4 .2H 2 O+FeCO3+k.
The development of gypsum by this method is illustrated at many mines.
Finally gypsum may be formed by the hydration of anhydrite. All
these methods of formation of gypsum are characteristic of the zone of
katamorphism, and especially of the belt of weathering.
Alterations. An important alteration of gypsum is to anhydrite (ortho-
rhombic, sp. gr. 2.899-2.985). - The reaction is
(1) CaSO 4 .2H 2 O=CaS0 4 +2H.,O-K.
The decrease in volume is 37.62 per cent. The other important alteration
of gypsum is into calcite (rhombohedral, sp. gr. 2.713-2.714). The
reaction is
(2) CaSO 4 .2H 2 O-CO 2 =CaCO,+H 2 SO 4 +H 2 O+k.
The H,S() 4 produced may simultaneously react upon some other compound.
The decrease in volume is 50.29 per cent.
Unless beds of gypsum have been deeply buried the alteration to
anhydrite has not extensively occurred. It is a reaction of diminution of
volume, absorption of heat, dehydration, increase in specific gravity, and
increase in symmetry, and therefore is one of the very rare cases which
illustrate all the tendencies of the lower physical-chemical zone. The
change of gypsum to calcite is a reaction with liberation of heat and con-
densation of volume. The change takes place near the surface, especially
in the belt of weathering, where carbon dioxide is abundant, and may also
occur in the lower zone. It therefore stands, in its physical-chemical
relations, in the same position as dolomitization. (See p. 240.)
MINERALS. 359
SECTION 4. GENERAL STATEMENTS.
PHYSICAL-CHEMICAL FACTORS ON WHICH NATURE OF ALTERATIONS
DEPENDS.
As inferences from the foregoing' treatment it may be said that the more
important physical-chemical factors on which the alteration of an individual
mineral depends are (1) the chemical composition of the mineral, (2) the
chemical composition of the adjacent minerals, (3) the chemical composi-
tion of the circulating solutions, (4) the specific gravity, (5) the symmetry,
(6) the heat effect of the reaction, and (7) the pressure and volume.
CHEMICAL COMPOSITION.
Certain chemical compounds are stable under a great variety of
conditions; others are stable only under certain definite conditions; and thus
the chemical composition influences the stability of minerals. As an illus-
tration of minerals which have stability under widely varying conditions
may be mentioned quartz, which forms alike from a magma and from water
solutions, and also at the surface and at great depth. Nephelite and soda-
lite are examples of minerals which can exist only under a comparatively
narrow range of conditions.
CHEMICAL COMPOSITION OF ADJACENT MINERALS.
It has been seen, that mineral particles may react upon one another,
either through the medium of contained solutions or by direct rearrange-
ment under the influence of pressure. Therefore, it is clear that the nature
of a mineral which is mainly secondary to another mineral is influenced
by the chemical compositions of adjacent minerals.
CHEMICAL COMPOSITION OF CIRCULATING SOLUTIONS.
It has already been shown that the secondary minerals are dependent
not only upon the adjacent minerals, but upon the material carried by the
underground solutions. The amount of such material is dependent upon
the vigor of the circulation. As explained on pages 507-518, 655-656,
764-766, the material added or abstracted may be great in the zone of
katamorphism, but is usually rather limited in amount in the zone of
anamorphism.
360 A TREATISE ON METAMOKPHISM.
SPECIFIC GRAVITY.
Apparently high specific gravity is favorable to stability. This is what
one would expect, for high specific gravity involves a comparatively close
arrangement of the atoms. Where the atoms are near together the
molecular attraction is great, and in order to break up the combination
this force must be overcome. This principle is illustrated by dimorphous
and trimorphous compounds. Diamond (av. sp. gr. 3.52) is more stable than
graphite (av. sp. gr. 2.16), and graphite is more stable than carbon (sp. gr.
1.9, charcoal). Pyrite (av. sp. gr. 5.025) is more stable than marcasite (av.
sp. gr. 4.870). Cyanite (av. sp. gr. 3.615) is more stable than sillimanite
(av. sp. gr. 3.235), and sillimanite is more stable than andalusite (av. sp. gr.
3.18). Quartz (av. sp. gr. 2.6535) is more stable than tridymite (av. sp. gr.
2.305). This last instance well illustrates the principle; for the symmetry
of quartz and tridymite is the same, and this variable factor included in
the previous illustrations is excluded. The same is true of andalusite
and sillimanite of the aluminum-silicate series. As pointed out on page
112, the more condensed a compound, or, in other words, the higher the
specific gravity, the less energy is potentialized. In the change from a
lower to a higher specific gravity energy is liberated. In this we have the
physical explanation of the greater stability of minerals of high specific
gravity. To form minerals of higher specific gravity from those of lower
specific gravity releases energy. To reproduce minerals of lower specific
gravity from those of higher specific gravity requires the expenditure of
energy. An exception to the above rule as to increase of stability with
increase of specific gravity is furnished by calcite and aragonite. Calcite
(av. sp. gr. 2.7135) is more stable than aragonite (av. sp. gr. 2.94), but in
this case the factor of symmetry enters, which is discussed under the next
heading.
SYMMETRY.
Apparently the greater the symmetry the more stable is the mineral
likely to be. This principle is illustrated by substances which are dimor-
phous or trimorphous.
Pyrite (isometric) is more stable than marcasite (orthorhombic).
Diamond (isometric) is more stable than graphite (hexagonal), and this
is more stable than amorphous carbon. Kelvin suggests that soft
a Lord Kelvin, Popular lectures and addresses, Macmillan & Co., London, 1894, vol. 2, p. 428.
SPECIFIC GRAVITY AND SYMMETRY. 361
phosphorus as compared with red phosphorus, and prismatic sulphur as
compared with octahedral sulphur, contain potential energy. When the
change from the first to the second takes place, energy is evolved, and
consequently the second form is more stable. These changes are in the
direction of higher symmetry, and Kelvin's argument applies equally well
to all the changes in which minerals pass from a lower to a higher degree of
symmetry. To reproduce minerals of lower symmetry would require the
expenditure of energy. Therefore we have an energy cause why minerals
with high symmetry are more stable. They contain less potential energy.
Their formation is under the apparent law of the universe of dissipation of
energy.
SPECIFIC GRAVITY AND SYMMETRY.
Where specific gravity and symmetry work together, as in a number
of the illustrations mentioned, there seem to be no exceptions to the rule
of increase of stability with increase of specific gravity and increase in
symmetry.
But in those instances in which the specific gravity and symmetry are
opposed to each other it can not be predicted which will be the dominant
factor. For instance, calcium carbonate crystallizes as calcite (hexagonal-
rhombohedral; sp. gr. 2.7135) and aragonite (orthorhombic ; sp. gr. 2.94).
The former is the more stable. In this case it seems that symmetry is the
dominant factor. In the aluminum silicate which crystallizes as andalusite
(orthorhombic; sp. gr. 3.18), sillimanite (orthorhombic; sp. gr. 3.235), and
cyanite (triclinic; sp. gr. 3.615), the latter is the most stable. In this case
it appears that the specific gravity is the determining factor.
It is believed that when the energy relations of these changes become
known it will be found that in each of these cases "the more stable molecules
contain less potential energy. If this be true, calcite, considering both its
specific gravity and its symmetry, contains less energy than aragonite, and
cyanite less than andalusite or sillimanite. If this conjecture be true, all
compounds are subject to a 'common law. That mineral forming from a
compound is most stable in which the minimum energy is contained.
The relations of symmetry and specific gravity raise some very
interesting questions as to the arrangement of the molecules in minerals.
Pressure undoubtedly tends to produce the most compact arrangement.
362 A TREATISE ON METAMORPHISM.
(See pp. 182-186.) According to Slichter, the most compact possible
arrangement of spherical molecules is that which gives a rhpmbohedral unit
having face angles equal to 60 and 120. " One might therefore conclude,
other things being equal, that minerals having hexagonal crystallization would
be those which have the closest arrangement of molecules and therefore the
highest specific gravity; but plainly there are other factors entering into
the problem, for, as already pointed out, aragonite (orthorhombic) has a
higher specific gravity than calcite (-hexagonal), and cyanite (tri clinic)
has a higher specific gravity than sillimanite and andalusite (orthorhombic).
On the other hand, diamond (isometric) has a higher specific gravity than
graphite (hexagonal). This is an especially interesting case, since the
cubical arrangement of molecules, the one ordinarily appealed to to
explain isometric symmetry, is the most open of all possible arrangements.
From the foregoing it appears perfectly clear that besides the manner of the
arrangement of the molecular particles the distance of the molecules from
one another enters as a very important factor. Also the shape of the
molecules, the closeness of the arrangement of their atoms, and the com-
plexity of the molecules themselves doubtless enter as important factors
into the density of minerals.
HEAT REACTIONS.
Other things being equal, within the lithosphere reactions take place
which give the greatest liberation of heat. This law is best illustrated
at or near the surface, where the reactions usually occur in accordance
with it. The reactions of oxidation, hydration, and carbonation are there-
fore dominant However, the law of reactions with liberation of heat
becomes less and less able to control as the pressure becomes con-
siderable. Where the pressure is great, as noted under the next heading,
it determines the reaction without respect to whether heat is absorbed or
liberated, and in many cases the reactions take place with the absorption
of heat, so far as the chemical factors are concerned. If all the physical
factors also were included, all reactions would take place with the dissipa-
tion of energy. (See p. 57.)
"felichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Kept.
U. S. Geol. Survey, pt. 2, 1899, pp. 306-310.
PHYSICAL-CHEMICAL FACTORS 363
PRESSURE AND VOLUME.
Pressure lessens the volume and therefore tends to preserve and to pro-
duce minerals which have a high specific gravity. Where the pressure
is small this factor is relatively inefficient and consequently other factors
usually control, and many minerals of low specific gravity form. But even
where the pressure is small it is not unimportant, at least retarding reactions
which would otherwise occur. This is illustrated by a partially altered
rock described by Merrill, which seemed solid when confined by the
surrounding rock, but which when brought to the surface from a depth of
a few feet, and thus freed from pressure, rapidly decomposed and disinte-
grated. Where the pressure is great it is likely to be a determinative
force, Controlling the reactions. At the depths of the zone of anamorphism
the uniform production of anhydrous or slightly hydrous minerals of higher
average specific gravity than those formed in the zone of katamorphism is
clear evidence of the dominance of pressure.
In this connection it will be well to mention the mineral groups, with
their specific gravities, which are more characteristic of the zones of
katamorphism and anamorphism.
The characteristic products of the zone of katamorphism are: Of the
oxides, (1) those of silicon, opal, chalcedony, and quartz (sp. gr 2.1 to
2.654); (2) those of iron, including the hydrous and anhydrous ferric-
oxides (sp. gr. 3.80 to 5.225); (3) the hydrous aluminum oxides, gibbsite
and diaspore (sp. gr. 2.35 and 3.40); of the carbonates, calcite and dolo-
mite (sp. gr. 2.7135 and 2.85); of the silicates, (1) the epidote-zoisite group
(sp. gr. 3.25 to 4.20); (2) zeolite group (sp. gr. 2.04 to 2.40); (3) chlorite
group (sp. gr. 2.60 to 2.96); (4) serpentine-talc group (sp. gr. 2.50 to 2.80);
and kaolin group (sp. gr. 2.6 to 2.63). (See pp. 519.-520, 621-627.)
The characteristic important mineral groups formed in the zone of
anamorphism are as follows: Of the sulphides, pyrite, and pyrrhotite (sp.
gr. 5.025 and 4.61); of the oxides, (1) those of silicon, chert, chalcedony,
and quartz (sp. gr. 2.6 to 2.654); (2) those of iron, hematite, magnetite, and
ilmenite (sp. gr. 5.225, 5.174, and 4.75); (3) those of aluminum, corundum
(sp. gr. 4.025); (4) those of titanium, rutile, octahedrite, and brookite (sp.
gr. 4.215, 3.885, and 3.975); of the silicates, (1) the feldspar group (sp. gr.
"Merrill, George P., Rocks, rock weathering, and soils, Macmillan Co., New York, 1897, pp.
252-253.
364 A TREATISE ON METAMORPHISM.
2.54 to 2.76); (2) pyroxene group (sp. gr. 2.68 to 3.58): (3) amphibole
group (sp. gr. 2.9 to 3.713); (4) garnet group (sp. gr. 3.41 to 4.30); (5) chrys-
olite group (sp. gr. 3.2 to 4.1); (6) scapolite group (sp. gr. 2.566 to 2.74);
(7) aluminum silicate group (sp. gr. 3.16 to 3.67); (8) humite group (sp. gr.
3.1 to 3.2); (9) tourmaline (sp. gr. 3.09); (10) staurolite (sp. gr. 3.71); (11)
mica group (sp. gr. 2.7 to 3.1); (12) clintonite group (sp. gr. 2.9 to 3.57).
The average specific gravity of the mineral groups above mentioned
as products of the zone of katamorphism is 2.948. The average specific
gravity of the mineral groups of the zone of anamorphism is 3.488. It
thus appears that the average specific gravity of the minerals which develop
in the zone of anamorphism is 18 per cent greater than that of the
minerals in the zone of katamorphism. This comparison is of course very
roughly approximate, since the various minerals are not present in equal
quantities. Probably the percentage is too great, since the heavy sulphides
and the very heavy silicates are given equal weight with the abundant but
lighter quartz, feldspars, pyroxenes, amphiboles, and micas. The com-
parison, however, shows beyond question that a given mass of material
occupies much less space in the lower physical-chemical zone than in the
upper physical-chemical zone.
It is shown under the next heading that many of the reactions written
for the minerals of the zone of katamorphism may be read in reverse order
when the resultant minerals are buried so deep as to be in the zone of
anamorphism. The lighter minerals characteristic of the zone of kata-
morphism reunite to produce heavier minerals of the zone of anamorphism,
such as the feldspars, the micas, the pyroxenes, the amphiboles, the chryso-
lites, andalusite, etc. . Furthermore, where the pressure is great enough
these minerals rearrange themselves again in whole or in part so as to
produce still heavier minerals, such as garnet, staurolite, tourmaline,
sillimanite, cyanite, etc. This great change takes place within the narrow
range of less than 10,000 meters.
Since in this mere outer film of the earth a great diminution in the
volume of the minerals has taken place, it is thought to be highly probable
that, even if the average chemical composition of the interior of the earth
be supposed to be the same as the crust, the pressure is such that the min-
erals may further rearrange themselves into still more compact products,
thus probably producing minerals of a different kind and higher specific
PRESSURE AND VOLUME. 365
gravity than any with which we are acquainted. Indeed, the interior
pressures increase so rapidly with depth that rearrangement might, occur
again and again. Therefore, even if the average chemical composition be
the same deep within the earth as at the surface, in the centrosphere, in
consequence of high pressure, there may be a set of silicate minerals which
have as high a specific gravity as the average density of the earth, viz,
5.67. If the accepted theory as to the distance between molecules be cor-
rect, viz, that molecules of ordinary liquids at the surface of the earth do not
occupy more than one-third of the total volume," there is ample room
between them for the condensed rearrangement suggested. From the fore-
going it appears that we do not necessarily appeal to a great preponderance
of heavy metals deep within the earth to explain its average high specific
gravity. It may be very largely explained by the condensation of the
material due to pressure. If, as suggested by Chamberlin, the average
specific gravity of the material of the earth be that of meteoric falls, the
average change in specific gravity would be from 3.69 to 5.67 as a result of
pressure. The great increase in the average specific gravity of minerals
with increase of pressure in the crust of the earth would seem to make the
estimate of the change in average specific gravity of the minerals from 3.69
to 5.6.7, as a result of the very great pressures deep within the earth, a very
modest one.
\Yhile I have no doubt that the condensation of the earth material into
heavier compounds as a result of pressure is a partial explanation of the
high specific gravity of the earth, I by no means urge this as the sole cause.
Indeed, it is probable that the segregation of heavy material toward the
center and lighter material toward the surface has steadily continued
throughout geological time, and therefore the difference in composition is a
very important factor in the difference in density at the surface and the
center. But I do not venture even a guess as to the relative importance of
the two factors of condensed compounds and segregation of material in
explaining the increase in density of the material of the earth with increase
of depth.
oNernst, W., Theoretical chemistry, trans, by C. S. Palmer, Macmillan & Co., London, 1895,
p. 196.
366 A TREATISE ON METAMORPHISM.
REVERSIBLE REACTIONS.
On the foregoing pages numerous reactions have been written by
which the minerals characteristic of the zone of katamorphism are pro-
duced; very few reactions have been written by which the minerals of the
zone of anamorphism are reproduced. It is certain that when the minerals
formed in the belts of weathering 1 and cementation are altered under the
conditions of the zone of anamorphism the minerals characteristic of that
zone develop; therefore it is believed that many of the reactions for the
development of the minerals of the zone of katamorphism are reversible.
To illustrate, in the zone of katamorphism olivine may alter into the min-
erals serpentine, magnetite, magnesite, and quartz, according to the fol-
lowing equation :
3Mg s FeSi 2 O 8 +3CO 2 +4H 2 0+O=2H 4 Mg 3 Si 2 O 9 +FeA+3MgCO s 4-2SiO 2 +k.
It is believed that when these four minerals are brought together in proper
proportions under favorable conditions in the deep-seated zone the reverse
reaction occurs, and that the equation may be read from right to left
instead of left to right, thus reproducing the olivine.
The above illustration is chosen because the change from left to rio-lit
involves carbonation, desilication, hydration, and oxidation; and the change
from right to left involves silication, decarbonation, dehydration, and deoxi-
dation. Of course, where deoxidation takes place in the zone of anamor-
phism some reducing agent must be present to utilize the abstracted oxygen.
The principle of the reversibility of the reactions in the two opposing zones
is actually illustrated in a few cases where the products of the zone of kata-
morphism have been observed to alter in the zone of anamorphism. For
instance, it is recorded (p. 261) that aualcite is derived from albite according
to the following equation:
2NaAlSi 8 9 +2H 2 0=Na 2 Al 2 Si t O I 2-2H 2 0+2Si0 2 +k;
whereas we find (p. 334) that aualcite alters to albite by the reaction:
Na 2 Al 2 Si 4 O I2 -2H 2 O+2SiO 2 =2NaAlSi 3 8 +2H 2 O-k.
Ill other words, the reaction is exactly reversible; for while the k is plus
in the first equation and minus in the second,. it is on opposite sides in the
two equations. The feldspars alter into many zeolites, and a number of
REVERSIBLE REACTIONS. 367
the zeolites alter into the various feldspars. The above reaction chances
to be the only one given for these groups which is exactly reversed. This
is a consequence of the fact that reactions are written only for recorded
alterations. There can be no doubt that practically all the equations
representing the recorded alterations of the feldspars (pp. 261-263) into
the zeolites, and all the reactions representing the recorded alterations of
the zeolites (pp. 333-334) into the feldspars, are reversible. For instance,
we have anorthite altering into gismondite as follows (p. 262):
3CaAl 2 Si 2 O 8 +12H 2 O=Ca.,A] 6 Si 6 O 24 .12H 2 O+k.
Can one doubt that if gismondite passes into the zone of anamorphism
dehydration may take place and anorthite be reproduced!
Another line of evidence pointing to the reversibility of the reactions
in the two zones is the frequent recorded association of corundum with
diaspore and gibbsile, the latter minerals being secondary to the corundum.
Can it be doubted that these hydrates may be dehydrated in the zone of
anamorphism and reproduce corundum! Of ' course this particular change
may not occur alone. At the same time the dehydration takes place the
alumina may unite with silica and form andalusite, sillimanite, or cyanite,
or the alumina may enter into some other silicate.
Bearing in the same direction are the experiments made by Daubre"e
upon serpentine." It is well known that both enstatite and olivine alter
into serpentine. Daubree found that by the fusion of serpentine it split up
into enstatite and olivine, according to the following equation:
H 4 Mg 2 Si 2 O 9 +Heat=MgSiO 3 +MgSiO 4 +2H 2 0+k.
Finally, my chief reason, in addition to those already given, for belief
in the reversibility of the reactions in the two zones lies in the actual
compositions of the unmetamorphosed sediments and their metamorphosed
equivalents. The unmetamorphosed pelites are composed largely of the
lighter hydrous minerals of the belt of weathering and the belt of cementa-
tion. It is true that with these, as already explained, there are also
considerable, or even dominant, quantities of residual undecomposed
anhydrous minerals; but it is certain that the metamorphosed equivalents
of these pelites contain none of the minerals which are characteristic of
a Daubree, A., Experiences synthe'tiques relatives aux me^orites: Comptes rendus des stances
de 1'academie des sciences, vol. 62, Paris, 1866, p. 661.
368 A TREATISE OX M KTAMORPHISM.
the belts of weathering and cementation, and the only possible conclusion
is that these minerals have recombined and reproduced the heavier minerals
of the lower physical-chemical zone. That this is so is shown by the fact
that, barring the water and the carbon dioxide which are liberated in the
process of alteration, the average chemical compositions of the unaltered
pelites and their metamorphosed equivalents are nearly the same.
While it is held that the reactions are reversible, it is not supposed
that this is often exactly the case for a given rock. In order that this
should even approximately take place, it would be necessary that there be
no change of average composition in the zone of katamorphism, and this is
never the case. The minerals formed in the zone of anamorphism depend
not only upon the minerals of the zone of katamorphism present, but upon
their proportion and many other factors. What is meant by the reversi-
bility of the reactions is that, when compounds produced in the zone of
katamorphism from a given mineral are together in proper proportions
and conditions in the zone of anamorphism, the original mineral may be
reproduced.
If this law of the reversibility of reactions in the two zones be true,
the question naturally arises why so few of the reversing reactions in
the zone of anamorphism have been recorded. The answer lies in the
difference in the readiness with which observations may be made in the
two zones. The reactions of the belts of weathering and cementation of
the zone of katamorphism have been more fully described, because they
are constantly taking place at or near the surface under conditions of
ready observation. Many of the reverse reactions have not been fully
described, because they occur at depth, and because in areas of strong
metamorphic action they have been complete. Usually gradation from
practically complete reactions to very incomplete reactions in the zone of
anamorphism is comparatively rapid. But notwithstanding the very imper-
fect observations of the zone of anamorphism, the general reversibility of
the reactions in the two zones seems as certain as if it were established by
observation, and it is believed that it will be established by observation.
If the conclusions of the foregoing paragraphs be correct it is evident
that there is an almost entirely neglected field of observation in metamor-
phism that by which the minerals of the zone of anamorphism are produced
from the minerals of the zone of katamorphism.
TABLES. 369
For the reasons given above, I conclude: It is believed that most of
the equations which represent the reactions in the zone of katamorphism are
reversible in the zone of anamorphism; and so far as there is expansion of
volume and liberation of heat in the upper zone, just so far is there condensation
of volume and absorption of heat in the lower zone.
SECTION 5. TABLES.
In order to present compactly the essential facts as to the alterations
of each mineral, a set of tables is here given.
Table A gives the mineral sources of each of the minerals.
Table B gives the minerals to which each mineral alters.
Table C gives the equations representing the alterations of each of the
minerals into other minerals and shows the volume changes.
Table D classifies the alterations of the minerals under processes and
gives their various combinations, with volume changes.
TABLE A. Sources of minerals.
Acmite is derived from arfvedsonite.
Actinolite is derived from ankerite, bronzite, hypersthene, olivine, parankerite,
sahlite.
Albite is derived from : analcite, heulandite, laumontite, plagioclases (with or-
thoclase), sodalite, spodumene, stilbite.
Allophane is derived from anorthoclase, microcline, orthoclase.
Amesite is derived from pyrope.
Analcite is derived from laumontite, leucite, nephelite, plagioclases, sodalite.
Anhydrite is derived from gypsum.
Anthophyllite is derived from bronzite, hypersthene, olivine.
Aphrosiderite is derived from garnet. .
Augite is derived from hornblende.
Bastite is derived from actinolite, anthophyllite, bronzite, cummingtonite,
hyperethene, sahlite.
Berlanite is derived from chlorite.
Beta-spodnmene is derived from spodumene.
Biotite is derived from anorthoclase, augite, hornblende, microcline, ortho-
clase, seapolites.
Biotite-chlorite is derived biotite.
Brucite is derived from chondrodite, clinohumite, humite, serpentine.
Breunerite is derived from olivine.
Calcite is derived from actinolite, ankerite, anthophyllite, aragonite, augite,
diopside, dolomite, epidote, fluorite, garnet, grossu-
larite, gypsum, hauynite, hornblende, noselite, paran-
kerite, sahlite, seapolites, tremolite, zoisite.
Chabazite is derived from hauynite, noselite, plagioclases.
Chalcedony is derived from augite, sahlite.
Chlorite is derived from almandite, augite, biotite, garnet, hornblende, iolite,
phlogopite, prehnite, pyrope, staurolite, tourmaline,
vesuvianite.
MON XLVII 04 24
370 A TREATISE ON METAMORPHISM.
TABLE A. Sources qf, minerals Continued.
Cimolite is derived from anorthoelase, microcline, orthoclase.
Chlorophyllite is derived from iolite.
Chromite is derived from olivine.
Clinochlore is derived from biotite (with phlogopite).
Corundum is derived from diaspore, gibbsite.
Cyanite is derived from andalusite, corundum, diaspore, gibbsite.
Cymatolite is derived from spodumene.
Damourite is derived from andalusite, corundum, cyanite, microcline, orthoclase,
sillimanite, staurolite, topaz.
Diaspore is derived from biotite, corundum, garnet (conjectural ) , gibbsite, haiiyn-
ite, muscovite, nephelite, noselite, phlogopite, scapo-
lites, sodalite.
Diopside in derived from dolomite.
Dolomite is derived from ankerite, calcite, parankerite.
Dudley ite is derived from margarite.
Enophite is derived from chlorite.
Enstatite is derived from .pyrope.
Epidote is derived from anorthoclase, augite, biotite, garnet, hornblende, micro-
cline, orthoclase, plagioclases, scapolites.
Epistilbite is derived from plagioclases.
Eucryptite is derived from spodumene.
Fassaite is derived from gehli-nite.
Garnet is derived from vesuvianite.
Gibbsite is derived from anorthoclase, andalusite, biotite, cancrinite, corundum,
cyanite, epidote, garnet (conjectural), haiiynite, mi-
crocline, muscovite, nephelite, noselite, orthoclase,
phlogopite, plagioclases, pyrope, scapolites, silliman-
ite, sodalite, topaz, tourmaline, zoisite.
Gismondite is derived from plagioclases.
Grossularite is derived from gehlenite.
Griinerite is derived from siderite.
Gypsum is derived from anhydrite.
Halloysite is derived from anorthoclase, microcline, orthoclase.
Hematite is derived from actinolite, ankerite, anthophyllite, biotite, bronzite,
garnet, greenalite, griinerite, hornblende, hyper-
sthene, ilmenite, limonite, magnetite, marcasite, oli-
vine, parankerite, pyrite, serpentine, siderite.
Hercynite is derived from olivine.
_Heulandite is derived from plagioclases.
Hornblende is derived from augite, garnet.
I lydrobiotite is derived from biotite.
I 1 yd romagnesite is derived from brucite.
HydromUflCOVite is derived from nephelite, scapolites, sodalite.
Hydronephelite is derived from nephelite, sodalite.
Hydrophlogopite is derived from phlogopite.
Hydrotalcite is derived from olivine.
Hypersthene is derived from almandite, biotite, garnet.
Ilmenite is derived from perovskite, rutile.
Kaolin is derived from andalusite, anorthoclase, biotite, cyanite, epidote,
garnet (conjectural), leucite, microcline, nephelite,
orthoclase, the plagioclases, the scapolites, silliman-
ite, sodalite, topaz, and zoisite.
Laumontite is derived from anorthite.
TABLES. 371
TABLE A. Sources of minerals Continued.
Lepidomelane is derived from arfvedsonite.
Limonite is derived from actinolite, ankerite, anthophyllite, arfvedsonite, biotite,
bronzite, chlorites, epidote, garnet, greenalite, griin-
erite, hematite, hornblende, hypersthene, ilmenite,
magnetite, marcasite, olivine, parankerite, pyrite,
pyrrhotite, serpentine, siderite.
Magneeite is derived from garnet, olivine, pyrope, serpentine.
Magnetite is derived from actinolite, ankerite, arfvedsonite, augite, biotite, bronz-
ite, diopside, garnet, greenalite, griinerite, hematite,
hornblende, hypersthene, iluienite, marcasite, olivine,
parankerite, pyrite, pyrrhotite, sahlite, siderite.
Malacon (hydrous zircon) is derived from zircon.
Marcasite is derived from hematite.
Margarite is derived f rom corundum, diaspore, gibbsite.
Meionite is derived fr< >ni grossularite.
Mesolite is derived from plagioclases.
Mica is derived from spinel, tourmaline, vesuvianite.
Microcline is derived from spodunu ne.
Muscovite is derived from anorthoclase, diaspore, gibbsite, leucite, microcline,
iiepbeliti', orthoclase, plagioclase and orthoclase,
scapolites, sodalite, spodumene.
See also Damourite.
Natrolite is derived from apatite, chabazite, hauynite, nephelite, noselite, plagio-
clases, sodalite.
Nephelite is derived from leucite, sodalite ( conjectural).
Newtonite is derived from anorthosite, microcline, orthoclase.
Octahedrite is derived from ilmenite, titanite.
Opal is derived from olivine, serpentine.
Orthoclase is derived from analcite, heulandite, leucite, laumontite, atilbite.
Osteolite is derived from apatite.
Paragonite is derived from : anorthoclase, muscovite, plagioclases.
Peetolite is derived from apophyllite.
Penninite is derived from biotite (with phlogopite).
Perovskite is derived from titanite.
Phillipsite is derived from plagioclases.
Phlogopite-chlorite is derived from phlogopite.
Finite is derived from iolite.
Prehnite is derived from analcite, laumontite, mesolite, natrolite, plagioclases,
scolecite.
Pyrite is derived from marcasite, pyrrhotite.
Pyrophyllite is derived from anorthoclase, microcline, orthoclase.
Quartz is derived from actinolite, anorthite, anorthoclase, anthophyllite, augite,
biotite, bronzite, chalcedony, chlorites, cummington-
ite, diopside, enstatite, epidote, garnet, grossularitc,
hornblende, hypersthene, microcline, olivine, opal,
orthoclase, 'plagioclases, prehnite, pyrope, sahlite,
scapolites, serpentine, tridymite, zoisite.
Rutile is derived from brookite, ilmenite, octahedrite, titanite.
Sahlite is derived from ankerite, parankerite.
Scapolites are derived from plagioclases.
Scolecite is derived from plagioclases.
Serpentine is derived from actinolite, biotite, bronzite, chondrodite, clinohumite,
diopside, enstatite, hornblende, humite, hypersthene,
muscovite, olivine, pyrope, sahlite, spinel.
372 A TREATISE ON METAMORPHISM.
TABLK A. Sources of minerals Continued.
Siderite is derived from art" vedsonite, garnet, hematite, hornblende, limonite,
magnetite, olivine.
Sillimanite is derived from andalusite, biotite, corundum, cyanite, diaspore, gibbsite.
Smaragdite is derived from diallage.
Sodalite is derived from nephelite.
Spinel is derived from almandite, biotite, corundum, diaspore, garnet, gibbsite,
olivine, pyrope.
Steatite is derived from andalusite, cyanite, muscovite, sillimanite, topaz, tour-
maline.
Stilbite is derived from haiiynite, noselite, plagioclases.
Talc is derived from actinolite, andalusite, anthophyllite, bronzite, cyanite,
diopside, enstatite, gehlenite, hypersthene, musco-
vite, olivine, phlogopite, pyrope, sahlite, scapolites,
sillimanite, spinel, staurolite, topaz, tremolite.
Titanite is derived from ilmenite, rutile.
Thomsonite is derived from nephelite, plagioclases, sodalite.
Tremolite is derived from diopside, dolomite, olivine.
Vermiculite is derived from muscovite.
Webskyite is derived from serpentine.
Wollastonite is derived from calcite, dolomite.
Zoisite is derived from corundum, diaspore, gibbsite, grossularite, plagioclases.
TABLE B. Alteration products of minerals.
Actinolite alters to bastite, calcite, hematite, limonite, magnetite, serpen-
tine, talc, quartz.
Albite alters to analcite, gibbsite, kaolin, niarialite, natrolite, quartz.
Almandite alters to chlorite, hypersthene, spinel.
Analcite alters to albite, orthoclase, prehnite.
Andalusite alters to cyanite, kaolin, gibbsite, muscovite (damourite) , silli-
manite, talc (steatite).
Anhydrite alters to gypsum.
Ankerite alters to actinolite, calcite, dolomite, hematite, limonite, mag-
netite, sahlite.
Anorthite alters to gibbsite, gismondite, kaolin, laumontite, meionite,
prehnite, quartz, scolecite, thomsonite, zoisite.
Anorthoclase alters to allophane, biotite, cimolite, damourite, epidote, gibbs-
ite, halloysite, kaolin, muscovite, newtonite, para-
gonite, pyrophylite, quartz.
Anthophyllite alters to bastite, calcite, hematite, limonite, quartz, talc.
Apatite alters to osteolite.
Apophy llite alters to pectolite.
Aragonite alters to calcite.
Arf vedsonite alters to . acmite, lepidomelane, limonite, magnetite.
Augite alters to biotite, calcite, chalcedony, chlorite, epidote, horn-
blende, magnetite, quartz.
Beta-spodumene alters to albite, eucryptite, muscovite.
Biotite alters to ... chlorite, diaspore, epidote, gibbsite, hematite, hydro-
biotite, hypersthene, kaolin, limonite, magnetite,
quartz, serpentine, sillimanite, spinel.
Biotite (with phlogopite) alters to penninite, clinochlore.
Bronzite alters to actinolite, anthophyllite, bastite, hematite, limonite,
magnetite, quartz, serpentine, talc.
Brookite alters to rutile.
Brucite alters to hydromagnesite.
TABLES. 373
TABLE B. Alteration products of minerals- Continued.
Calcite alters to dolomite, wollastonite.
Cancrinite alters to calcite, gibbsite, natrolite.
Chabazite alters to natrolite.
Chlorite alters to berlanite, enophite, limonite, quartz.
Chondrodite alters to brucite, serpentine.
Clinohumite alters to brucite, serpentine.
Corundum alters to diaspore, cyanite, gibbsite, margarite, muscovite (da-
mourite), sillimanite, spinel, zoisite.
Cummingtonite alters to bastite, quartz.
Cyanite alters to kaolin, gibbsite, muscovite (damourite), talc (steatite).
Diallage alters to calcite, chlorite, epidote, feldspar, magnetite, quartz,
smaragdite.
Diaspore alters to corundum, cyanite, margarite, muscovite, sillimanite,
spinel, zoisite.
Diopside alters to calcite, magnetite, quartz, serpentine, talc, tremolite.
Dolomite alters to calcite, diopside, tremolite, wollastonite.
Enstatite alters to quartz, serpentine, talc.
Epidote alters to calcite, gibbsite, kaolin, limonite, quartz.
Fluorite alters to calcite.
Garnet alters to aphrosiderite, calcite, chlorite, diaspore (conjectural),
epidote, gibbsite (conjectural), hematite, hornblende,
hypersthene, iron oxide, kaolin (conjectural), limon-
ite, magnesite, magnetite, quartz, giderite.
Gehlenite alters to fassaite, grossularite, talc.
Gibbsite alters to corundum, cyanite, diaspore, margarite, muscovite,
sillimanite, spinel, zoisite.
Grossularits alters to calcite, meionite, quartz, zoisite.
Griinerite alters to hematite, limonite, magnetite.
Gypsum alters to anhydrite, calcite.
Haiiynite alters to calcite, chabazite, diaspore, gibbsite, natrolite, stilbite.
Hematite alters to limonite, magnetite, marcasite, pyrite, siderite.
Heulandite alters to albite, orthoclase.
Hornblende alters to augite, biotite, calcite, chlorite, epidote, hematite,
magnetite, quartz, serpentine, siderite.
Humite alters to brucite, serpentine.
Hypersthene alters to actinolite, anthophyllite, bastite, hematite, limonite,
magnetite, quartz, serpentine, talc.
Ilmenite alters to hematite, limonite, magnetite, octahedrite, rutile,
titanite.
lolite alters to chlorite, chlorophyllite, pinite.
Laumontite alters to albite, analcite, orthoclase, prehnite.
Leucite alters to analcite, kaolinite, muscovite, nephelite, orthoclase.
Limonite alters to hematite, siderite.
Magnetite alters to hematite, limonite, siderite.
Marcasite alters to , hematite, limonite, magnetite, pyrite.
Margarite alters to dudleyite.
Marialite alters to biotite, kaolin, muscovite, quartz, talc.
Meionite alters to biotite, calcite, epidote, gibbsite, kaolin, muscovite.
Mesolite alters to prehnite.
Microcline alters to allophane, biotite, cimolite, damourite, epidote, gibbsite,
halloysite, kaolin, muscovite, newtonite, pyrophyl-
lite, quartz.
Muscovite alters to diaspore, gibbsite, paragonite, serpentine, talc (steatite),
vermiculite.
374 A TREATISE ON METAMORPHISM.
TABLE B. Alteration products of minerals Continued.
Natrolite alters to prehnite.
Nephelite alters to albite (conjectural), analcite, diaspore, gibbsite, hydro-
muscovite (pinite), hydronephelite, kaolin, inusco-
vite, natrolite, sodalite, thomsonite.
Noselite alters to calcite, chabazite, diaspore, gibbsite, natrolite, stilbite.
Octahedrite alters to rutile.
Olivine alters to actiuolite, anthophyllite, breunnerite, chromite, hema-
tite, hercynite, hydrotalcite, limonite, magnes-ite, mag-
netite, opal, quartz, serpentine, siderite, spinel, tremo-
lite.
Opal alters to chalcedony, chert, quartz.
Orthoclase alters to allophane, biotite, cimolite, damourite, epidote, gibbsite,
halloysite, kaolin, muscovite, newtonite, pyrophyl-
lite, quartz.
Parankerite alters to actinolite, calcite, dolomite, hematite, limonite, magne-
tite, sahlite.
Perovskite alters to ilmenite.
Phlogopite alters to chlorite, diaspore, gibbsite, hydrophlogopite, talc.
Phlogopite (with biotite) alters to clinochlore, penninite.
Plagioclases alter to analcite, chabazite, epidote, epistilbite, gibbsite, gis-
mondite, heulandite, kaolin, laumontite, mesolite,
natrolite, paragonite, phillipsite, prehnite, quartz,
scapolites, scolecite, stilbite, thomsonite, zoisite.
Prehnite alters to chlorite, quartz.
Pyrite alters to hematite, limonite, magnetite.
Pyrope alters to amesite, chlorite, enstatite, gibbsite, magnesite, quartz,
serpentine, spinel, talc.
Pyrrhotite alters to limonite magnetite, pyrite.
Rutile alters to hematite, ilmenite, titanite. ,
Sahlite alters to actinolite, bastite, calcite, chalcedony, magnetite, quartz,
serpentine, talc.
Sea polite alters to biotite, calcite, diaspore, epidote, gibbsite, hydro-
muscovite (pinite), kaolin, muscovite, quartz, talc.
Scolecite alters to prehnite.
Serpentine alters to brucite, hematite, limonite, magnesite, opal, quartz,
webskyite.
Siderite alters to griinerite, hematite, limonite, magnetite.
Sillimanite alters to cyanite, kaolin, gibbsite, muscovite (damourite), talc.
Sodalite alters to albite (conjectural), analcite, diaspore, gibbsite, hydro-
muscovite (pinite), hydronephelite, kaolin, musco-
vite, natrolite, nephelite (conjectural), thomsonite.
Spinel alters to mica, serpentine, talc.
Spodumene alters to albite, beta-spodumene, cymatolite, eucryplitite, micro-
cline, muscovite.
Staurolite alters to chlorite, gi I ilisitc. magnetite, muscovite (damourite), talc.
Stilbite alters to albite, orthoclase.
Titanite alters to calcite, octahedrite, perovskite, quartz, rutile.
Topaz alters to gibbsite, kaolin, muscovite, talc (steatite).
Tourmaline alters to biotite, chlorite, gibbsite, mica, steatite.
Tremolite alters to calcite, talc.
Vesuvianite alters to chlorites, garnets, micas.
Zircon alters to hydrous zircon (malacon).
Zoisite alters to calcite, gibbsite, kaolin, quartz.
TABLES.
375
IH eo
ifi eo o
/
3?
ss
S 3 3 S
f
S
S
cS S S
z s
-
O q
t
to o co
Ss fi .-i
\ + \
8 "
+ |
4-
CO
4- + + 4
4
4
a
4
: 2 :
H CO
f 7
. f
II
1
: 'S :
S
o
a
2
<d
E
J i o
2
- i
. a 1
B
I
Q
0)
.c
i S
cr
S |
1 I
aT p
i
o
2
tt
1
* S s I
te . H J
1 1 | '*
2
1
2
ite. nuartz.
i
a
2 of
11
* s
2 2 S
- 1
E
2
'1
]
C
C
2
"o
g
2 2 m
II
1
-I
3
2
P-.
Sill
I
r
j
I
<
c
i
y
S S 1
6 |
c
Z
1 j
. g B
o
. S
i
1
p
3
1
^
^
C
i
E
i
1
I
2
tinolite
do
I
c
bite, anorthite
s
Q
C
GJ O C
IS
c
J
c
O
3
2 2
2 2
2
3 1
c 5
8 i
5 '.E
^c
E
3
5
2
j
Z
3
^
j
<
<
3
*(
" -^
i "
\
<
-t
S
:
*
a
S^ t? -
COB
1
1
B
E
j
' ^ C
4-4-3
5
g
1
H
jhemical reactions.
|
jj
J
4
C
6CaMg a FeSi 4 O 1 o+4H2O+6CO2+3O=4H 2 Mg ;j Si 4 O 1 2+6CaCO 3 +3Fe 2 O 3 +8SiO 2
Ca(MgFe)3Si4Oi2+2H 2 O+CO. 2 =H4(MgFe)3Si 2 O9+CaCO 3 +2SiO 2 ,or1
6"
1
1
5 r
t ^
O ;
'1 -
^, c
i
f
Q' 3
^L *
B
B
II
V -
^ -
7t
U **
i
I
.1
-
;B
" i
i
r j
t
- -
I,
- -
> =
7
-
; s
' ?
6CaAl 2 Si 2 O 8 +4NaA18i 3 O 8 +6KAlSi 2 O 6 4-48H 2 O4-2CO2=l
3(K2Ca2Al 6 Si|2O 1K .14H 2 O)4-2Na 2 CO34-4Al(OH) 3 )
4NaAlSi 3 O 8 +3CaAl2Si 2 O 8 4-21H 2 O4-2CO 2 =3(H 4 CaAl2Si <) Oi 8 .3H 2 O)4-2Na5CO J
4NaAlSi 3 O 8 4-3CaAl2Si 5 O 8 4-21H 2 O4-2CO 2 =3(H 4 CaAl 2 8i O 18 .3H 2 O)4-2Na 2 CO.
JMu Altii-O.^Qrii Al .Qi,n,4-'>dTT^n4-9Pn r.i..Al^Si,,rt.,U 1SRO4-9Kn^^O..4
6NaAlSi 3 O 8 4-CCaAl28i 2 O 8 4-3CO 2 4-45H 2 O= 1
w
o
i
o c
q 5
K 5
CO C
+ B
|i
1 =
S 2
2 i
^ C
il
CM *
S
c
1
c
\
;<
- 3
;
i
i
-
f
'
B
c-
" lO l5 O 8 !S 8 IV t UN-IOWN4- 8 O t !SIVBNE
f s O!S94-(o 5 H' B 9 I8 v IV')0 5 BN 8 H)2= i! OO+O ! H8I4- 8 O ! !S s IVBOE4- 8 O' : ISIVMt
j
^
-
C
J
<
B
c
1
B
"<
c.
4
<
*
i
2
x (Na A !Si a O e ) + 4 (CaAl 2 Si 2 O d ) + K AlSi 3 O 8 +2H 2 O - I
x(NaAlSi s O 8 )+2(HCa. 2 Al3Si 3 O, 3 ) 4-H2KAl 3 Si,,O 12 4-2SiO 2 1
IVnt fnrTtiiiliitp/H
1
^
e
1
^
2
376
A TREATISE ON HETAMORPHISM.
00 <C
. at 2
S
S S
S
t* 1C
S S
R
!r* c^
W l>
?
s s
iO 00
<O OT
3 3
11
'" ""*"
^1
4
T
3 S3
l i
4- 1
s ^
4- 1
a s
1
1
1 *
4- 4
CO i-t
S S
|
c
1
n
-
i
c
2
Products.
S _
- a
3 =
g. |
1 j
s
11 1
a
T
I
ase, prehnite
teatite)
1
1
gibbsite
ite (damourit
ite (damourit
g
_a
j
"S
I
a
c
a
j
0)
4J *-
2
S "^
^
<o
S 2
ite, gibbsite .
Site, gibbsite
II
< s
5
X
: |
8 i
5 S
g
'-v.
IKaolin
[Musciiv
II
J
c
S C
1 ^
cS <
s
|
t
g 1
i 1
3 tr
' "v
Sic
1 I
oj c- c
_3 B a
1 ^
5 S
(Phillips
Heulan
a
1
a
1
.
'f
,1
i
^
I
Almandite, pyrope
iln
j
I
; |
c
-S
1
<
' i a
' .-s 1
"S !
r
c c
< *
C
c
c
4
1
<
Ankerite, quartz..
...do...
c
"
-c
I
g
I
<
Anorthite
...do..
4
2
o
o t
Anorthite, albite..
5
4A1(OH):,.
Chemical reactions.
4Fe 3 Al 2 SI 3 li .2Mg 3 AI 2 Si 3 12 4-15H 2 0-3H, Fe 4 MgoAl 4 Si 4 26 4-6810.
Fl>.Al.S(..O..9Ml7..Al.Si.n 3MirFpSi.n,-l-SMA].n.-l-SSin.
3(2Mg 3 Al 2 Si 3 O 1! .Fe 3 Al 2 Si 3 Oi 2 .Ca 3 Fe 2 Si 3 Oi 2 ) +4CO 2 = 1
5CaMg 2 FeSi 4 0,o.2[(Mg 4 Fe i! )(A] 9 Fe3)Si 6 36 ]+4CaC0 3 4-4Si0 2 J
No.A1.3i.r>.. 9H.O-l.9Sm,, 9NftA]Si.O.4.9H.O
3(Na 2 Al 2 Si 4 O 12 .2H 2 O)4-4CaCO 8 +K 2 CO 3 =2KAlSi 3 O 8 4-2H..Ca 2 Al 2 Si 3 Oi 2 4-3Na 2 (
4AUSi() = -l-3MirCO,-H3HO-H.MK.Si,O, n +8AlmiIU4-3f;O.
'
B
i i
=
' 1
B
1
' d
m
-
C
3
<
E
C
*
u
! s
i
: 5
: S
:
k
4
3
<
It
1
t
,
'
j
'. C
9
1
: |
d
c
> C
1
1
7
^aFeC 2 O 6 .CaMgC 2 O 6 +4SiO 2 -MgFeCa 2 Si4O 1 o+4CO2
naFeC,O, n .CaMtr,C J O, n +SSiO.-MroFp rfl.fti,O,^RrO n
i
(
\
; H
- i
"I
i
4
;i
c
'
_
|
|
.
|
1
I
CaAl 2 Si.,O 8 +7H*O-Ca 3 Al 6 Si 6 O 2 4.7H 2 O
CaAloSi.,O H2H n O-OaoALSLO 15HO
CaA1.8i 2 O 8 4-7H,O +CO 2 -H,CaAl 2 Sl,On.2H 2 O4-CaCO 3 4-2Al(OH )
CaAl 2 Si 2 O 8 4-4NaAlSl 3 O 8 4-6KAlSi 2 O 6 4-48H 2 O4-2CO 2 =l
3(K 2 Ca 2 Al a Sii 2 O 3 i ] .14H 2 O)4-2Na2C0 3 4-4Al(OH) 3 J
iJaAlSi 8 O 8 4-3CaAl 2 8i. i O 8 +21H 2 O4-2CO 2 =3(H 4 CaAl 2 Si c O 18 .3H 2 O)4-2Na 2 CO 3 4
1
a
1
TABLES.
377
)
1
s
?
3
>
g
I
t
i
5
k
P
|
S
A
S
*
i
3
^
1
H
Volume
change.
. 3 S?
S ?38288SB 828
S^S ** Q
o 31 55 ^
!*'
* + + + +II+ + +I ill
S 8 S S
4
Products.
: : a : : : : : : S : : :
Epistilbite, gibbsite
Stilbite, gibbsite
Chabazite. eibbsite. auart2
1 1 i
" 1
o* aj - <u
1 1 i i c
3 3
* S ]
2 S ^ -2 "^
, gibbsite, quartz . . .
e, kaolin, gibbsite ..
e, gibbsite, quartz . .
te
ao
Albite, zoisite.muscovite.q
(Kaolin, quartz
(Kaolin
(Gibbsite, quartz
3 :
'= a
! | j i
3! 1 !
1 1 I 1
1 i o. "5
Talc, limonite...
s i i 1
-H 35 35 * K
8 , '3
O) < S CM S
liil
o 'S. 'S. 2
S w W S
Source.
Anorthite, albite
...do...
c
c
albite
8
Anorthoclase, gibbsite
.do
Anorthoclase, hematite, calcite..
i
o
t
.
3
a
a
1
ii
s ~
S 3" 3
111
S S S
a a c
<<*
C
c
t
Anorthite,
...do...
( Anorthite.
Anorthite,
*3
A c
"=
Chemical reactions.
4NaAlSi 3 O 8 +3CaAl 8 Si s O 8 J-21H.,O+2CO.,-3(H 4 CaAl 2 Si 6 O 18 .3H 2 O) +2NaCO 3 -t 4A1(OH) 3
4NaAlSi 3 O 8 +3CaAl 2 Si 2 O,+24H 2 O+2CO. ! -Ca 3 Al e (Si 3 O 8 )6.18H 2 O+2Na.CO 3 +4Al(OH)3
6NaAlSi 3 O 8 +6CaAl 2 Si 2 O 8 +3CO+45H 2 O= 1
h2Al(OH) 8 +CaCO......
, '
x(NttAlSi3O 8 )+2(HCa2Al3Si3Oi3) + H 2 KAl 3 Si 3 O 12 +2SiO 2 )
2(2NaA18i 3 O B .KAlSi 3 O 8 )+6HO+3CO=-3H 4 Al 2 Si.O+12SiO 2 +K s CO3+2Na.,CO 8
2(2NaAlSi 3 O 8 .KAlSi 3 O 8 )+9H 2 O+3CO 2 -6Al(OH)3+18SiO 2 +K.CO 3 +2Na 2 CO3
/
2NaAlSi,O 8 . K AlSi. i O 8 +6Al(OH) 3 -KH 2 Al 3 Si 3 O 12 +2NaHAl3Si 3 O, 2 +6H 2 O
2NaAlSi3O,,.KAlSi 3 O 8 +MgCO 3 +FeCO3+5Al(OH)3= 1
HKMgFeAl 2 Si 3 O 12 +2NaH 2 Al.^i 3 O 12 +5H 2 O+2C0 2 |
2(3NaAlSi3O 8 .KAlSi3O 8 )+2Fe 2 O3+8CaCO3+2H 2 O= 1
I
ll
+ "
8" -
|c
$i
1 1
ii
a s
"*" !
c*
i
!
j
- 1
5 -.
v
2[Ca s Al 6 (Si0 4 )s(Si 3 8 )3.18H 2 0]+3Na 2 C0 3 +6Al(OH) 8 +6Si0 2 |
3CaAl 8 SioO 8 +9H 2 O + COo-2CaAl.,Si 3 Oio.3H 2 O+2Al(OH) 3 +CaCO8
4NaA18i3O 8 +3CaAl 2 Si 2 O 8 +13H 2 O+CO 2 =2(H 8 Na 2 CaAl 4 Si e O s4 .H 2 O)+6SiO 2 -
4CaAl 2 Si.O 8 +8H 2 O-2HoCa 2 Al^i3O l2 +4Al(OH ) 3 +2SiO 2
dr.a Al-St~n, J-SHn~TTPn. Al-Si.O 0-TT. AUSin
d
5
e-
, -
I
' '
\\
.- z
<
,1
1
. ^
p
^
- -
: =
i
'
, t
a
4CaAl 2 Sl 2 O 8 +Fe 2 O 3 +6H 2 O=H 2 Ca 4 Al 4 Fe 2 Si,O 20 +H 4 Al 2 Si 2 O+2Al(OH) 3 ...
4CaAUSi 2 Og+7H 2 O+Fe 2 O 3 -H 2 Ca 4 Al 4 Fe^i 6 O 2l i+4Al(OH)3+2Si0 2
3Ca A USi.O 8 + CaCO, - Ciu Al 6 Si 9 O6+ CO 2 . . .
378
A TREATISE ON METAMORPHISM.
,,
. S 8
3 S 3 S 3 S
8 g S
S S 55
O CO 00 if
-j. ?O t-
'^ + 4~
1 -f + -
-1- -J- -^-
4- + -
1
>
*
;
: : : as
4 Ja
II' O
: : a f .i
:
]
Products.
' ' R.I
: : 2 S s
[ tite, magnesite.
i S
"^> O
1 | a
g S 2
I S g |
E S p. a *
St * -3
a S S s S
1
_c
a
!
- ~
I
Chlorite
Enidote. quartz . .
N
3
o-
2 S |
UJ
j
|
M i i j ] 1 l
~ j
|
i
5
iftillJfil
ill 1111 1*1
> PH O ""J J S >J U
S
1
I
s
2"
1
Is g
2
A
C
s =
III! t *
. a g t f
3 - c
a -a -a *
S3 S
:
_
a
1
c
j
e
^
c
"*
- . i^j ^
i
i
~7
BE
i
d
1
CO
+
a
3
M
o
M +
,'
o 6
II
- ?: "
(N '
+ ^
S
o a -
reactions
-4'
|1
o
a 8" t
fsl
a g
o +
+ os Q
+ o 2
1
1
M
O
j
i
1
i
5 1
il
> &c
s 1
< a
4* it tfi M
fa o" + o
+ -* 1 "i
OQ ( Cfa ogj 4,
oT < fa 'JI
5C0 2 =4H 4 Mg 3 Si 2 9
O 2 =2(H 2 Mg,Al 2 Si 3
f5H 2 O=2[H 2 Mg,A
CO 3 +4CO 2 +3O=
^
m
v fa cS* m
+ + d 3
f I
t ifl i
S fill 1
lilS -
1 f . a* ^
epfCajMg.FeSioOuJ.Mg,
IStHCosAlsFfSijO,:,!
2[CasMgFeSi 4 0, : .(MgFe
Ca,Mg,Fei,,O sl .(Mg
2 [Ca 6 Mg 4 Fe 2 Si, 2 30 . Mg 4 F
2[H,K 6 Mg,,Ff 4 Al a Fc 3
(Not formulated) . . .
(Not formulated) . . .
6KHMgAI 2 Si 3 O 12 +18H 2 C
2HKMg.Al 2 Si 3 12 +7H 2
2HKMg.Al 2 Sl 3 12 +4MgC
6HjK 2 Mg 3 FeAl 4 Si O 24 +2C
I
g
s
I
Q
s
-
TABLES.
379
-I
1/5 JH IT
x S '& "
3
?i
3 f; 3
S
? f: S
S 3 g g
i
n
S 00 1" CC
7 77
rt ?! 3 S
+ 1 + +
S
i
S? ?! 2
+ + +
T 4
i 7
co ad ri i
- ^H rH
+ + I 1
?i
1
. ,
' * ^_
. T .
. ^ .
e
J
3
3-
2 *
A
|
quartz
!
2
'S
i
5
=
'S s
E S
c
1
&
i
s
g
, s
"
Epidote, qnartx.,
Epidote, spinel,
.do...
i 'S i
1 . 'i
I SI =
1 i 1 ;
2 *c >- "5
33 K E-
(Tale, hematite]
(Tale, limonite |
(Serpentine, hen
[Serpentine, hen
(Bastite
iBastite, quartz.
Anthoohvllito .
1 2
V E
**! ff
Hydromagnesit
Natrolite, gibbs
Dolomite
...do...
ene and bronzitc
; ;
2"
B
Q
S
: i :
i
8,
A
M
I
Biotite, hematite
{Biotite
Rintito homatito
!
c
o 5 ;
~ .-
c
4
c
r:
IBronzite or hypersthe
quartz.
Brookite.. .
I 1 a i
IIS'*
C a) n!
U
5
1
i
gr. hypersthene
_c
1
30KHMg 2 Al 2 Si: ! O,2+GFe2O 3 +40CaCO 3 +35CO 2 +H 2 O= 1
4H 5 Ca 1 oAl 1 2Fe 3 Si 15 O 5+30SiO 2 +12AlO(OH)+60MgCO 3 +15K.,CO3J
30KHMgoAl2Si3Oi.+6Fe2O3+40CaCO 3 +29COs= 1
E 03 5 X+O ! H+ ! O!SnVP+ !l O*!S3J E SH= s Oa+ K O'!S>tVajI E K s M"H
HO)OIVr,l+ i; O!S08+ 59 O OI IS E 3d !1 IV llI B0 5 HJ-+[(HO)l'' 51 (V : !S r -|V f aK- r ll](H:
=O : H9i+ 8 OOBOW+ E O ;; .J9+ !;l O' : !S ;: lV ! 8NH}I09
i
*
-
i
p
-
;,
* f
'~, \
> 1
4 -
c ~:
3
C
5
c
1
-
c
c
1
S
j
C
-
c
s
; :
c
' \
I
I :
\ i
i :
\ =
i 1
> B
3 C
3 B
1 (X
-
r
1
1 1
4
'
)
;
1 j
f i
? \
s
f
i
i
2
5
; i \
+ J
( i
- o *
3 J ^
3 'S -
J fa G
1 1 1
4Mg(OH) 2 +3CO.,-Mg2(CO 3 ) 3 .2Mg(OH).3H.O
H Na (i Ca(NaCO 3 )2Al s Si 9 O s e+6H 2 O=3(Na2Al 2 Si : iOio.2HO)+2Al(OH) 3 +Ca(
2CaC0 3 +Mg=CaMg(C0 3 ) 2 +Ca
a.
A
1
h
be
i
> 8
>
f
p
r
>
!
i
380
A TREATISE ON METAMORPHISM.
TABLE C. Chemical reactions and volume changes Continued.
Volume
change.
fc 1 <
1 +
:::::::::::::::+ + + +
Products.
S :
I ! ! i I
a ....
:::::: s : s
' ' - 1 1 S 1 i
N| 11 ! tllli
illiilsll
|_|_J_|_J. 5 I 5 I 5 !
Dolomite .
rtn
J j t
p ^ c
J f lijlllf:
Source.
s
f
Calcite
...do..
(Calcite, quartz
Icalcite
Chabazite
Chalcedonv and ch
s : 2 : 2 :
I I S ? s 1 1
* 1 I HUM
o o 66600
Chemical reactions.
]
c
j
j
s_
2CaCO 3 +MgC0 3 =CaMgCsO,+CaC0 3
CaCO, + MitCO,-CaMK(CO), ..
H
d
i
d
*oc
H
S
&
Ca s Al (1 Si,jOa8.18H 2 O+2Al(OH)a+4Na.CO 3 =4H < NaoAl.Pi,O,.+3CaCO 3 +CO.-)-18HO
(Recrystallization) ...
(Reactions variable) . .
"5
3
f
1
>_
(MgF) 2 Mg 3 SL0 8 +3H 2 0=H,Mg 3 Si 2 0,+Mg(OH)j+MgF.,
(Not formulated)...
(MgF) 2 Mg 7 Si 1 Oie+6H.,0=2H,Mg 8 Si.O!,+2Mg(OH)e+Mg:F 2 . . . .
AUO,+H.O-2rAlO.(OH'jl ...
Alo() 3 +3H 2 O-2Al(OH) 3 .
TABLES.
381
la
"a a
85
3
1
lO C>1 1 <N Tf CO O
w to o 61 eo 55 J5
t
t-
<M
S -S
s s
S : : :
: : : 2
1
c
II
^
1 s ~
4- 4- + 1 4- I 4-
S
4
c5
- 1
4 +
2
1
: : : ?
>
1
Products.
1
E
1
|
1
- 7
1
f M H M
1
S :
2 =
1 o I o S = 2
, I * " I * 1
S S N f
^
^
c
|
1
,
- 1
1
[Talc (steatite), gibbsiu-
fKaolin
(Kaolin, gibbsite
fMnscovite (damourite).
(Muscovite (damourite),
Chlorite
Epidote
Magnetite . . .
Feldspar
Smaragdite..
Quartz
j
&
Sillimanite . . .
Source.
i
'i
\
t
E
' S
i
f
"22
^ S .5
i f f
& a
q
i
i
I
*
1
E
E
t
1
4
E s a a s s s
3 3 3 3 .3 3 C
1 I 1 f 1 1 1
S E E E E E S
8 5 3 5 8 5 S
.
a
c
c
i
j
C
t
i
0)
I
M
Q
Chemical reactions.
1
|
I
1
H
s
I c
2
B ^
,d
a
H
! =s
' c
, 3
<
' d
H
f
3Al 2 O s 4-6SiOo4-K,CO 3 4-2H 2 O-2H 2 KAl s Si 3 Ou4-CO.
2Al.O 3 4-2SiO.,4-CaCO s 4-H 2 O-H 2 CaAl,Si.iOi 2 +CO 2
3Al.,O 3 +6SiO 2 +4CaCO 3 4-H.O-HoCa 4 Al 6 Si 6 O 2e 4-4CO
3(MgFe)8iO : ,4-2H.,O = n,(MgFe) 3 Si<,Oi,4-SiO 2 , or]
1
c7
-
i
i
?
=
i
~
H
7
; :
?
H
! C
: 1
= S
' q
w
' 4
. i
s
' K
4
C
3
4
w
i
B
c
, i
1
B
c
1C
4
C
2
J
c
i
c
jj
1
3
: i
=5
a
2
s
4
a
a
1
1
1
1
i
H
]
c
s
q
a
S
1
s
i
1
-;
S
J-
=
C
" O 5 H4-'O!SIV-O!84-(HO)OIV?;
382
A TREATISE ON METAMORPHISM.
3
S g
z
CO
s ?
5o S g
S 3 S
S 8
8
S
8
c
Volumi
change
f '
35 S
I i
1
O
o5
1
00 O
+ 4
3 "= S
1 .
a =
i +
B
-1
1 , *
Products.
i
> T
I
, rlllcite, i|ll!irtx
entitle, qtmrtz
entitle, quart/, caii-ite .
lolite
lolite, caleite
loto I
VOlU' or hnlll . .
"
1
^ Ji
| I 5 1
lo
llolite, Udllnstonite
g
CJ
a
g
S
5 1
cT
.S *
lOphylHtfi)
ilr, L-ibbsiir. kiM'liii. li
d
I
c
i 1
1
3
it &
CO CJ
6- OD
s ;=.
'
S, S H
i
1
I
.
t ,
^ ?
,^. _,,- - ^
1
!
e. nuiirtz . .
p ~
iV a
A t
S *
a
i a
s
t
ft) fl?
1
aT u
^
*
8 i
s
P*
1
c
E
O $ -y
'CD.
ill
o c o
S S
c o
2 a
4
i
:
j
S c
a S a
o o
C G
^
1 ^
1
u
i
Chemical reactions.
S
H
d
a
-
S
c
c
*
C 1
6A1O(OH I + i>SiOj+ K CO3-2H s KAl3Si 3 O,j+CO s +H.,O
j Airi((i^ii_i-')ui,u.pi.r l r m R-PH 4i.si^n.j-rn.,j-TT^ri
a
c^
3
, c.
-
1
= c
, 'I
j
ll
> C
1 i
- -
-
) C
:
j
C
1 4
C
C.
5 ^
: 1
\
',
1
;
j
c
^
' !
c
! !
i
> 6
S
tl
. 1
" i
: 1
} &
t
2CaMgSU) G + MrO :i .-^^MK: { Si 4 (),. J + CaCO 3
, \"ni r, .i-itiiii.-n.'ii i
j
} i
' i
-t
H
;
'5
?
?
i
' I
?
-
J C
5
' '
p
-
i
c t
?
-1
'
i
, ,-
: 'i
S c
' 1
a
i
' I
-
i !
a
V
t>
'
1
j
r
!
'. 1
^.
-.
i
-
5
!
H
[
5
-
p
:
a
c
4
V
-
C
!
5
a
H
j
i
V
s
-
-
1
i
!
i
t
i
i
f
I
"53
O
K
TABLES.
383
Volume
change.
S
f
^
| S S S
S S S
J S S
. '
; s
i 1
S ii
1 ;
i-
.
S
!
w
8
S
s
1
T
31
a
j -
1 Ci
3 S S 5
3 S SS ?
I i
Se-i f
o C
> ^ 3C c<
* to 55 *!
1 1 1
g
CJ
S
' ,
. jj
i 1
: | e
1
t
<
:
!
"S
!
j 1
"
i
.
|
1
~I \
S a
" 2 '-3
- -3
-
c
*
1
>
>
a
1
3 p
s
(.Pyroxene, e
fNnt rp^'nrd
a
3
i
c c
a, o a
S ra x
o 1 Z
I
frH <! &
' 5
3
> s
g. o.
_H_ w
q
E
1 I
c* c
P
! I
i
' I
Grossularite
C
" 1
iFassaitef
J i
N
Spinel
Sillimanite
Cviiiiit,'
. ^ ^ t
3V M {
if
i !
o &
V
s
s"
^ 1
1
1
: *
3
c
03
cf -S
Source.
te, calcite, q
almandite a
i i
J9 s
60
DO
a
"*
^. s
o> >.
o.
S
H
C
&
a
magnesite
quarts
11
!v N'
oi a!
D 3
o- a-
i
I
1
1 t
1
1
'E +
0> QJ C
1 1 *
fe O
+j "aT o
0) *
G 'S 2
30<
2^ 2_
( melan
[Garnet
s -
1 1
IGehleni
IGehleni
l^phlpnil
j
I
*
i i "=
3 3
S S
* a ^
2 6
3 "5
d'
CO
! d 1
: i
: + /^
+
d 1
1
t
. *
1
1
: si c
: 3 *
q
a ir
1
1
"i \
i 1 9
j S 1 <
; % d
t s
* #
!
-
;
t- iC w
+ + O
a
1
lulated^...
1
E
i
+2CaCO 3 +5SiO 2 =2Mg 3 CaSi 4 12 +2CC
3 O 12 .2Mg 3 Al 2 Si 3 O 12 +15H2O=3H 10 Fe 4 )
O,.,.2Me,Alai,O,o-3Ms-FeSi,O^-3Ms
3 O 12 .Ca 3 Fe2Si 3 O] 2 +5CO 2 +H 2 O=2HCa
O,2.Mg 3 Al 2 Si 3 Oi 2 .Ca 3 Fe 2 Si 3 O 12 +H 2 O-f
ioAl 2 FeSi 3 O, 3 +2CaCO 3 +3MgCO 3 +3Si
2 Si 3 Oi2.Fe 3 Al 2 Si,,O 12 .Ca 3 Fe2Si 3 O 12 )+4(
g2FeSiA2.2[(Mg4Fe 2 )(Al Fe 3 )Si6O 36
nulatedt . ..
<
1
II
d
5
1
3
i
9
4-
I
C
II
9
!
il
j+MgCO 3 =MgA! 2 O 4 +C0 2 +3H 2 O ...
j+SlO.- Al.,SiOj+3H,O
,+SiO.,=AI.SiO-,+3HoO ...
^40S O 5tg9 [V t B;) : H =fof)BOI-+ r -(|iS'.i M
oo+ 51 o l; !S t iv> i .rH rl: ooi t o+ i; o!sr,+ f
00+ :I O E !S E [V>l :; H<-= E 0.rM+ r -0!S') + F
_;
Jj
H
235
'% ? 5
: 1 1
S 1
a l?
Oi C
5
E
,c
C
EC
O
000
o o o
Z
g
C
S 5?
8 o
cs
N
g
J
o
i
5
c<
< f f
3 5
13
1
!&>
I
s
o
s
I
^
I
00
S
S
fvi
E
S
i
a
j
pa
E^
384
A TREATISE ON METAMOKPHISM.
s s
06 JS
: g
1
A t
eO ?a OC
^> t~ y
~
S
3
1
Volum<
change
1
S i
+ 4
s g
4- +
1 V
S
1
g
1
4- 4
s c -
1 4- l
es
4
,<
1
H
s
2
O 1
i
S :
: 2
1 1
S s
?
3
s t
= 1
1
2 t
g
4
1
3
1
i
r
\
(Not recorded)
(Not recorded)
(Not recorded)
Meionite, caleit
Zoisite. calcito.
| JJ
] 111
2 -2 a i
& a 1
[Magnetite]
Anhvdrite...
4
2
Natrolite, gibbs
Stilbite. eibbsit
'& 1
2" o o i
S E '5 ^
1 MI
W -' ^J ^
e
e
1
1
I
-,
i
1
1
1
2 |
t
< C
(Chlorite, epidott
^
5
-
=.
2 !
B
S,
o
q fe
s
\
e
Glauconite
S 2 S c
a o '
3 o o
E 'E i
5
i 1 =
o a e
1
1
C
C
\
^
Hauynite..
...do..
ll
a I f j
8 1 j 4
K a
c
c
4
Heulandite
fin
a
c
c
s
c
1
s
O
ffl
1
+
1
i
i
+
.
TT
8
T
5
o
3
f \
: t'
d
. f
1
x. 3
4- * : :
1
9
o*
-
1
- CO "
7 1
%
o ~ ' '
1 ?
c
4 B
1 I
SJ
S~
<5 E
33 r
t
4
o ; ;
? Si:
Q
" s
S
2 =Ca,Al 8 Si l! O., f ,45CaCO 3 4-3
n,j-H.n-9nrn.Ai,si.n.4-s
Fe2Si 3 O,.+5CO 2 4-H 2 O=2HCa
U s Si 3 O, 2 .Ca ; ,Fe 2 Si 3 O l2 4 H 2 04-
l 3 0, 3 4-2CaC0 3 +3MgC0 3 4-3Si
B
Cs
4
C
q
H
5
CC
i
) Al 2 Si 3 O, 2 4-2CO 2 48H 2 O=2(I
)Al 2 Si 3 Oi 2 436HO + 6CO 2 =
,.18H 2 O4-12Al(OH) 3 4-3CaCO
) A lSi 3 O, 2 4- 24 H 2 O + 6CO.=
Si,0 8 ) 3 .18H 2 04-4Al(OH) 3 4-C
'eO 3 .3H 2 O
n.-uro.
B
1
;i
!
, =
E
e*
4
1
' a
c
1
,O 4 NajCO,, = 2Na AlSi 3 O 8 + Ci
,n 4-K.rn VK A isi,o.,4.nr:
Mg,Fe 2 Al 8 FeSiO s6 4-21H 2 O4
i;0) +2HC%Al 2 FeSi 3 13 44
1
1
. !
Id
55 3
1'
u 2
* m cf
j? * ai ir
.". ee o> i
U v d ~
r - Is t =
' <** O 3*
l |*I 1
1 n =
! a 3 ?
'J
1
C
a
C
c
c.
4
. =
S
c-
' C
!
2Sa.,Ca(NaSO 4 .A
6Na 2 Ca(NaSO,.A
'i < -S IQ S .S
$ 3' | 1 1 1
* 8 a 1 % i
35315?
< < e + j
M fc S *
1 f ^ I 1
Z / - -
* rj f
1 c
4
'
a
'" 1
C
C
-
. '
3
c*
4
' \
u
H,CaAl 2 SiOi g .3H
H.PBAUi,O.,SH
J
, c?
' 1
f
JJ
ic
fT
1
c
'
o
I
--.
I
s
ca
TABLES.
385
Volume
change.
S
1 *
8
1
^-v-
>rt ^
+ +
1
+
V
4
r-
9
oj
S
1
auartz..
or both
Products.
Biotite.calcite...
1
j
t
1
1 [Serpentine)
K With other
[Magnetite J
Serpentine, brucite
Talc, magnetite, quartz
i
"^
E
4
=
S
en
(SeeBronzite.)
Titanite
Titanite. magnetite
ITitanite, hematite)
\ tone
ITitanite, limonite. |
Rutile. maenetite ...
|
1
1
E
Of
Rutile limonite...
Octahedrite. maenetitp
I Octahedrite, hematite.
;
:
i
j
*Z
2
\
3
&
a
8
1
"3
]
[
c
c
4
j
O
T:
1 1
%
o
r^
'3
c c
oi a
4
c
T
c
c
c
c
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H 6 K 6 Mg 8 Fe 4 Al 8 Fe 4 Sii 8 O,2+2
8Ca ( MgFe) 3 Si 4 12 .18(MgFe)2( Al
30HK(MgFe) 2 (AlFe) 2 Si 3 Oi2-
8CaMg 2 FeSi 4 O,2.6Mg 4 Fe 2 Al 8 Fe 4 S
10H 3 K 3 Mg,Fe 2 Al 4 Fe 2 Si 9 O 36 +
Ca2Mg 3 Fe 3 Si 8 O, 4 . ( MgFe) 2 ( AlFe)
2 [CaMgFeSi 4 O, 2 . (MgFe) ( A
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386
A TREATISE ON METAMORPHISM.
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SiioO 37 +3H 2 O=H 8 (MgFe),Al 8 Si, 0, or
10 O 37 +3H 2 O=H 8 Mg 3 FcAl 8 Sii O 4 o.
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2H 2 0+2Na5C0 3 +C0 2 =4NaAlSl 3 O 8 +Al 2
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6 Si, 2 3 6.14H 2 0)+2Na 2 C0 3 +4Al(OH) 3 ..
a 4 C0 3 +2H 2 0=Na 2 Al 2 Si < 12 .2H 2 0+K 2 CC
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C+3CO.-4FeC0 3 +3H 2
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A TREATISE ON METAMORPHISM.
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TABLES.
389
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a . g ?
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Chemical reac
+C0 2 =H 4 Al s Si a 9 +4Si0 2 +K 2 CO J .
D 0-rr.oiit in nmrtiint TT^H wHHpd HT
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+ 7H s O+COj=2( H 3 Mg 3 AlSi 3 O 12 .3H.
+ 6MgCO a +7H,0 = 2[H a MKr,AlSi a O,,
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390
A TREATISE ON METAMORPHISM.
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.. Talc, gibbsite ".
. . Talc, diaspore
. . Talc, serpentine, gibbaite
fPenninite-.l
Mine orhot.h
IclinochloreJ
.. (Not recorded)
.. Chlorite, quartz
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Chemical reaction
4H 2 KMg 8 AlSiA 2 +6H,0+4C0 2 =3H 2 Mg 3 Si 4 12 +4Al(OH
4H 2 KMg s A18i s Oi a 4-2H 2 O+4CO=3H 2 Mg 3 Si 4 O 12 +4AlO(OE
2H 2 KMg 8 AlSi,0, 2 4C0 2 4-4H a O=H 2 Mg 3 Si 4 12 4-H 4 Mg 3 Si !
(Not formulated)
HjCa^ Al 2 Si 3 Oi, +2MgCO 3 4- H 2 O = H 4 Mg 2 Al.iSiO 9 + 2SiO 2 +2
4FeSj+22O4-3H s O=2Fe s 3 .3H a O4.8SO i!
FeS 2 +6O=.FeS0 4 4-SO>or FeS 2 +SO+H 2 O=FeSO 4 4-H 2 S 1
4FeSO 4 4-2O4-7H 2 O=2Fe.iO 3 .3H 2 O+4H 2 SO 4
3FeSo+16O-Fe 3 O 4 4-6SO 2
3FeS a 4-4HO4-4O=Fe 3 O 4 +4H 2 S4-2SO..
4Mg 3 Al 2 Si 3 0, 2 4-15H 2 04-3C0 2 =3H 2 Mg 3 Si 4 12 +3MgC034-8,
4Mg 3 Al s Si 3 O, a 4-6H 2 0=3H 2 Mg 3 Si 4 12 +3MgAl ! 4 4-2Al(OH
Mg 3 Al 2 Si 3 O 12 4 6H a O=H 4 Mg 3 Si,,0 9 4-2Al(OH) 3 4-Si0 2
Mg 3 Al 2 Si 3 12 4-2H 2 04-CO a =H 4 Mg.,Al 2 SiO94-MgC0 3 +2SiO,
3Mg 3 Al 2 Si 3 O,j4-8H 2 0=H 16 Mg 9 Al 6 Si 5 0364-4Si0 2
Mg 3 Al 2 8i 3 0,.=2MgSi0 3 4-MgAl 2 4 +SiO,
4Fe 3 Al 2 8i 3 O 12 .2Mg 3 Al 2 Si 8 Oi 2 +15HoO=3H 10 Fe 4 Mg 2 Al 4 Si 4 O 2
Fe 3 Al 2 Si 3 O 12 .2Mg 3 Al 2 Si 3 O 12 =3MgFeSi 2 O 6 +3MgAl 2 O 4 4-3Si(
3[2Mg 3 Al 2 Si 3 Oi 2 .Fe 3 Al 2 Si 3 O 12 .Ca 3 Fe 2 Si 3 Oi 2 ]4-4CO 2 =
5CaMg 2 FeSi 4 O 1 2.2[(Mg4Fe 2 )(Al 9 Fe 3 )Si 6 036]+4CaCO 3 -
Ca3Al 2 Si3Oi2.Mg3Al 2 Si3Oi2.Ca3Fe 2 Si3O 12 +H 2 O+5CO 2 =2HC
Fft,.S,-l-inHS HFeSo4-10H
3Fe n Si 2 +1160=llFe 3 O 4 436SO 2 or 3Fe 11 S 12 +36H 2 O4-8O=
4Fe,iS,.+33HoO+162O=ll(2FenO,.3H.O)4-48SO ...
1
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TABLES.
591
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A TREATISE ON METAMORPHISM.
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TABLES.
393
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A TREATISE ON METAMORPHISM.
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TABLES. , 395
TABLE D. Classification of alterations, with volume changes.
INDEX TO CLASSIFICATION.
Pa3.3.
Carbonation 396
Carbonation and defluoridation 396
Carbonation and dehydration 396
Carbonation, dehydration, and desulphation 396
Carbonation, dehydration, and desilication 396
Carbonation and deoxidation 396
Carbonation, deoxidation, and dehydration 396
Carbonation and desilication 396
Carbonation and hydration 397
Carbonation, hydration, and dechloridation 398
Carbonation, hydration, and desilication 398
Carbonation, hydration, and desulphation 399
Carbonation, hydration, oxidation, and desilication 399
Carbonation, hydration, and silication 399
Carbonation, oxidation, dehydration, and desilication 399
Change of symmetry and molecular change 399
Chloridation 400
Deboration and decarbonation 400
Decarbonation 400
Decarbonation and titanation 400
Dehydration 400
Dehydration and decarbonation 401
Deoxidation 401
Desilication 401
Hydration 402
Hydration and decarbonation 402
H ydration and dechloridation 403
Hydration, dechloridation, earbonation, and desilication 403
Hydration, dechloridation, and decarbonation 403
Hydration and defluoridation 403
Hydration and desilication 404
Hydration, desilication, and decarbonation 401
Hydration and oxidation 404
Hydration, oxidation, and desilication 405
Hydration and silication 405
Molecular division '. 405
Oxidation 405
Oxidation and decarbonation 405
Oxidation, decarbonation, and desulphidation 406
Oxidation and desulphidation 406
Oxidation, hydration, and decarbonation 406
Oxidation, hydration, and desulphidation 406
Oxidation and titanation 405
Silication 406
Silication and decarbonation ' 407
Silication and dehydration 407
Silication, dehydration, and decarbonation 407
Silication, hydration, and decarbonation 408
Silication, oxidation, and decarbonation 408
Substitution of bases 408
Sulphidation 408
Sulphidation, deoxidation, and earbonation 408
396
A TREATISE ON METAMOKPHISM.
TABLE D. Classification of alteration*. >/ it h volume changes Continued.
CARBONATION.
Source.
Products.
Volume
change.
Brucite .
Hydromagnesite
CARBONATION AND DEFLUORIDATION.
Per cent.
+ 73.08
Fluorite .
Calcite
CARBONATION AND DEHYDRATION.
+ 47.66
Biotite i Hypersthene, Billimanite - 24. 68
Laumontite \ Albite - 34.92
|
CARBONATION, DEHYDRATION, AND DESULPHATION.
Gypsum Calcite - 50.29
CARBONATION, DEHYDRATION, AND DESILICATION.
Biotite Epidote, spinel, quartz - 14. 71
Biotite, hematite do - 18.15
Serpentine Magnesite, quartz + 18. 84
CARBONATION AND DEOXIDATION.
Magnetite j Siderite +101.30
CARBONATION, DEOXIDATION, AND DEHYDRATION.
Limonite Siderite + 22. 27
CARBONATION AND DESILICATION.
Almandite, melanite, and pyrope Hornblende, calcite, quartz + 24. 55
Grossularite Meionite, calcite, quartz + 54. 62
Melanite (see Almandite).
Pyrope (see Almandite).
Serpentine Magnesite, brucite, quartz + 13.02
Titanite Octahedrite, oaleite, quartz + 42. 07
Do Rutile, calcite, quartz + 39.22
TABLES.
397
TABLE D. Classification of alterations, with volume changes Continued.
CARBOXATION AND HYDRATION.
Source.
Products.
Volume
change.
Epistilbite, gibbsite . . ... . -
Per cent.
+37.14
Do -
Heulandite, gibbsite
+37. 14
Do
Stilbite, gibbsite
+43.50
Phillipsite
+31. 98
Phillipsite, gibbsite
+40.61
Laumontite, gibbsite ..........
+33. 65
Do .
Scolecite, gibbsite
+35.23
Biotite . .
+17.26
Muscovite, albite . .. .. .....
.76
Biotite - ...
Biotite-chlorite . ....
+22. 92
Do
Hydrobiotite
+ 3.80
Do
Serpentine, gibbsite, kaolin
+14. 26
Enstatite
Talc
+ 9.93
Orthoclase -
38. 57
Do
Orthoclase, kaolin . ....
10.58
Do
Orthoclase, muscovite ......... _.__......
-12.43
Leucite, albite, anorthite (see Albite).
Meionite
Kaolin, calcite
+35.40
Do
Muscovite, calcite ... - .
+29.42
Nephelite
Analcite, diaspore
+ 5.49
Do
Analcite, gibbsite
+19. 68
Do
Hydronephelite.
+23. 49
Do
Muscovite
38. 46
Do
Muscovite, kaolin . .
16. 50
Do
Natrolite, diaspore
+15.00
Do
Natrolite, gibbsite
+24. 46
Do
Finite, kaolin .. ..-.
-13.00
Phlogopite
Hydrophlogopite
+26. 89
Do
Talc, diaspore ..
18.27
Do
Talc, gibbsite
- 7.79
Do
Talc gibbsite, serpentine
+ 5.23
Talc, magnesite, gibbsite
+75. 91
Muscovite, inicrocline _.
+31. 74
Talc
.83
Do
Talc calcite . .......... ..
+25. 61
398
A TREATISE ON METAMORPHISM.
TABLE D. Classification of alterations, with volume changes Continued.
CARBONATION, HYDRATION, AND DECHLORIDATION.
Source.
Products.
Volume
change.
Sodalite
Analcite, diaspore
Per cent.
20.77
Do
Analcite, gibbsite
10. 11
Do
Hydronephelite
7.25
Do
Muscovite, kaolin
37.07
Do
Natrolite, diaspora *
13. 62
Do
Natrolite, gibbsite
6 52
CARBONATION, HYDRATION, AND DESILICATION.
Actinolite ......
Bastite
18 06
Do
Bastite, calcite, quartz
+38 67
Albite
+ 1 58
Do
4 89
Albite, anorthite
Chabazite gibbsite quartz
+46 76
Do ...
Mesolite gibbsite quartz
+24 96
Do
Mesolite, gibbaite quartz calcite
+30 19
Anorthoclase .
Gibbsite
68 02
Do
Gibbsite quartz
3 30
Do
Kaolin
52 19
Do
Kaolin, quartz
9 56
Biotite, hematite
Epidote, quartz diaspore
18 45
Diopside _
Serpentine quartz
-4- 44
Do
Serpentine, quartz, calcite
+56 32
Do
Talc .
30 13
Do
Talc, calcite, quartz
+48 74
Epidote
-t-fiq ns
Grossularite
Zoisite, calcite, quartz
+40 49
Grossularite, melanite
Epidote, calcite, quartz
+40 88
Grossularite, melanite, pyrope
Epidote, calcite, quartz, magnesite
+39 53
Hornblende
Chlorite epidote calcite siderite quart/
+25 39
Melanite (see Grossularite).
Orthoclase or microcline
hematite.
Gibbsite, quartz
6 61
Do
Kaolin
54 44
Do
Kaolin quartz
1O C7
Do
Muscovite, quartz
-15. 58
TABLES.
399
TABLE D. Classification of alterations, with volume changes Continued.
CAEBONATION, HYDRATION, AND DESILICATION Continued.
Source.
Products.
Volume
change.
Pyrope
Pyrope (see Grossularite).
Sahlite
Do
Zoisite
Amesite, magnesite, quartz
Bastite, quartz
Bastite, quartz, calcite
Calcite, gibbsite, kaolin, quartz
Percent.
+62.26
+ 1.93
+56. 41
+66.22
CARBONATION, HYDRATION, AND DESULPHATION.
Haiiynite Chabazite, gibbsite - 7. 46
Do Natrolite, gibbsite, calcite + 4. 99
Do Stilbite, gibbsite, calcite + .46
Noselite Natrolite, gibbsite 16. 44
CARBONATION, HYDRATION, OXIDATION, AND DESILICATION.
Actinolite Talc 36.51
Do Talc, calcite, hematite, quartz +20.33
Augite Chlorite, epidote, quartz, hematite + 8. 58
Do Chlorite, epidote, quartz, hematite, magnesite. +15. 43
Olivine Serpentine, magnetite, magnesite, quartz +37. 13
Sahlite Serpentine, magnetite, calcite, quartz +37. 50
Do Talc, magnetite, calcite, quartz +27. 88
CARBONATION, HYDRATION, AND SILICATION.
Hornblende, quartz Biotite, calcite +41. 13
CARBONATION, OXIDATION, DEHYDRATION, AND DESILICATION.
Biotite Epidote, quartz 14. 86
CHANGE OF SYMMETRY AND MOLECULAR CHANGE.
Andalusite Cyanite -12.03
Aragonite Calcite + 8.35
Bronzite or hypersthene Anthophyllite + 8. 70
Marcasite.. Pyrite -2.98
p. gr. hypersthene.
400
A TREATISE ON METAMORPHISM.
TABLE D. Classification of alterations, with volume changes Continued.
CHLORIDATION.
Source.
Products.
Volume-
change. .
Albite
Marialite
Per cent.
+10. 29
Albite halite
do
+ 1.84
Sodalite
+33. 14
Nechelite. halite . .
..do..
+15.64
DEBORATION AND DECARBONATION.
Biotite
- ti. 75
Do
Biotite, gibbsite
+ 3.96
DECARBONATION.
Meionite
3.78
Spinel
29. 17
DECARBONATION AND TITANATION.
Rutile, siderite.
Ilmenite.
-34.77
DEHYDRATION.
Albite gibbsite . .
Paragonite
18 85
Anorthoclase, gibbsite
Muscovite, paragonite
20 04
Diaspore -
Corundum
28. 18
Gibbsite
. ...do
61.81
Do
Diaspore
46. 82
Gypsum ....
Anhydrite . .
37. 62
Heulandite
Albite ...
25. 03
Do
Orthoclase
18.44
Ijaumontite
Analcite
4 30
Linionite . .
Hematite
37 78
Opal
Chert, chalcedony
Do
Quartz
22 81
Orthoclase or microcline, gibbsite
Muscovite (damourite)
20 81
Stilbite
Albite
31 67
Do
Orthoclase
25.66
TABLES.
401
TABLE D. Classification of alterations, with 'volume changes Continued.
DEHYDRATION AND DECARBONATION.
Sourci'.
Products.
Volume
change.
Analcite
Orthoclase prehnite
Percent.
14 09
Anorthoclase, gibbsite
Biotite, paragonite
10. 91
Apophvllite
Pectolite
19 48
Chabazite
Natrolite
4 58
Diaspore, magne.site.... ......... ......
Spinel .
40. 39
( lililisite, iiiagiu'siti*
do
60.12
Ijaumontite
Ortboclase, prehnite
17 75
Mesolite
Prehnite
15. 05
Natrolite
do
16. 12
Orthoclase or microclint', magnesite, sider-
Biotite
22 33
ite, gibbsite.
Scolecite
Prehnite .
16.66
HYDRATION, DESILICATION, AND DECARBONATION.
Anorthocla^e, calcite, hematite
Epidote, quart/
28 30
Biotite, hematite
Biotite-chlorite, epidote, quartz, diaspore
+ 1 81
Orthoclase or microclino, calcite, hema-
Epidote, quartz
33. 73
tite.
Orthoclase or microcline, magnesite, sid-
Biotite, quartz
22.64
erite.
1
JEOXIDATION.
Hematite
Magnetite
2 38
1
)ESILICATION.
Almandite, pvrope .
Hvpersthene, npinel quartz
+12 66
Pvrope
Knstatite, spinel, quartz
+13. 51
Titanite
Perovskite quartz
+ 14
MON XLVII (.
402 A TREATISE ON METAMORPHISM.
TABLE D. Classification of alterations, with volume changes Continued.
HYDKATION.
Source.
Products.
Volume
changes.
Kaolin
Per cent.
3. 15
+ 61.87
+_ 60.30
+ 52.76
+ 34.65
7.77
Do
Kaolin, gibbsite
Gypsum
Gismondite
Do
Thomson! te
Do
Zoisite, kaolin
Anorthite hematite ....
Epidote, kaolin, gibbsite
+ 3.60
+ 8. 64
+ 39.25
+161. 83
+ 10. 11
+ 84.01'
+ 60.72
.86
Cancrinite - - -
Natrolite, gibbsite, calcite
Diaspore
Do
Gibbsite
Kaolin
Do .....
Kaolin, gibbsite
Hematite . ...
Limonite
lolite ( cordierite) - -
Chlorophyllite
Leucite ' ... . ..
Analcite
+ 10. 74
1.62
Meionite, hematite . ......
Epidote, gibbsite
Nephelite . . .
Thomsonite
+ 24.60
+ 36.84
Pvrone
Talc, spinel, gibbsite
Serpentine. .. . . . .
Webskyite
Sillimanite
Kaolin 1 47
Do
Kaolin, gibbsite . .
+ 64.67
+ 24.05
Zircon
Malacon (hydrous zircon)
HYDRATION AND DECARBONATION.
Andalusite
Talc (steatite)
32 37
Do
Talc, gibbsite
+ 97.67
9 55
Do
Muscovite (damourite)
Do
Muscovite, gibbsite
+ 55.47
+ 22.92
23 12
Biotite
Chlorite
Cyanite
Talc (steatite)
Do
Talc, gibbsite
+124. 71
+ 2.83
+ 76.74
+ 16. 50
+ 88.44
- 25.23
+ 46.69
Do..
Muscovite (H^mniirite)
Do *
Muscovite, gihhsite
Muscovite
Serpentine
Do
Do
Talc
Do..
Talc, gibbsite . .
TABLES.
403
TABLE D. Classification nf alterations, with volume change Continued.
HYDRATION AND DECARBONATION Continued.
Source.
Products.
Volume
change.
Per cent.
Phlogopite Chlorite 4 41. 02
Sillimanite Muscovite (damourite) 7. 98
Do Muscovite, gibbsite + 58. 16
Do Talc (steatite) - 31. 20
Do Talc, gibbsite +101. 09
Staurolite.. Chlorite (amesite), gibbsite +103.58
HYDRATION AND DECHLORIDATION.
Sodalite.. ..:...? Thomsonite
- v 6. 41
HYDRATION, DECHLORIDATION, CARBONATION, AND DESILICATION.
Marialite Muscovite, quartz
HYDRATION, DECHLORIDATION, AND DECARBONATION.
- 16.74
Marialite . . Kaolin, talc
7.69
HYDRATION AND DEFLUORIDATION.
Chondrodite Serpentine, brucite.
Clinohumite do
Humite . . do
+ 30.15
+ 38.39
-(- 35.53
404
A TREATISE ON METAMORPHISM.
TABLE D. Classification of alterations, with volume changes Continued.
HYDRATION AND DESILICATION.
Source.
Products.
Volume
change.
Albite
Analcite, quartz
Per cent.
+20. 82
Do
Natrolite, quartz
+19. 95
Albite anorthite orthoclase
Albite, zoisite, muscovite, quartz
Aphrosiderite, quartz
+50.98
Zoisite, gibbsite, quartz
4.58
Do
Prehnite, gibbsite, quartz
+14. 85
Anorthite hematite - . . .
Epidote, gibbsite, quartz .
+ 6.57
Anthophvllite ... .
Bastite
+12.09
Do ...
Bastite, quartz
+34 09
Bronzite or hypersthene ....
Bastite ,
+22. 77
Do
Bastite, quartz
&+15. 65
o _(-46 87
Cummingtonite
Bastite
+f4 20
Do
Bastite, quartz
+36 76
Enstatite . ...
Serpentine
+ 14 25
Do
Serpentine, quartz
+38 36
Orthoclase (we Albite).
Prehnite
Chlorite, quartz
+ 3 27
Pyrope
do
+56 02
Do
Serpentine, gibbsite quartz
+81 61
Pyrope (see Almandite).
Serpentine .. ...
Brucite, quart/
+ 9 82
HYDRATION AND OXIDATION.
Anthophyllite
Talc, hematite
+11.41
Do
Talc, limonite
Bronzite or hvpersthene
Talc, hematite
Do
Talc, limonite
Do
Talc, magnetite
--|-14 68
*
Magnetite
Limonite .
" +21. 73
+18. 20
+64 63
Olivine
Serpentine, magnetite
+29 96
a Sp. gr. hypersthene.
6Sp. gr. bronzite.
c Average sp. gr. bronzite and hypersthene.
TABLES.
405
TABLE D. Classification of alterations, with volume changes Continued.
HYDRATION, OXIDATION, AND DESILICATION.
Source.
Products.
Bronzite or hypersthene Serpentine, hematite
Do Serpentine, hematite, quartz. .
Hypersthene Talc, magnetite, quartz
Do j Serpentine, magnetite, quartz .
Olivine do
Do do
HYDRATION AND SILICATION.
Hornblende, quartz Biotite, epidote
MOLECULAR DIVISION.
Spodumene (beta-spodumene) Eucryptite, albite
OXIDATION.
Ilinenite Octahedrite, hematite
Do Octahedrite, magnetite
Do Rutile, hematite
Do Rutile, magnetite
Magnetite Hematite
OXIDATION AND TITANATION.
Rutile, magnetite Ilmenite, hematite
OXIDATION AND DECARBONATION.
Ankerite Hematite
Do Magnetite
Parankerite Hematite
Do Magnetite
Siderite Hematite
Do Magnetite
Sp. gr. hyp^rsthene.
406
A TREATISE ON METAMORPHISM.
TABLE D. Classijfioation of alterations, with volume changes Continued.
OXIDATION, DECARBONATION, AND DESULPHIDATION.
Source.
Products.
Volume
change.
Per cent.
Siderite, raarcasite Magnetite 47. 14
Siderite, pyrite do 46. 67
|
OXIDATION, HYDRATION, AND DECARBONATION.
Ankerite Limonite
Meionite Epidote, gibbsite + 7. 55
Parankerite Limonite
Siderite do -18. 22
Staurolite Muscovite (damourite) 24. 96
Do Muscovite, magnetite, gibbsite +68. 08
Do v Talc 44.02
Do Talc, gibbsite +90. 96
OXIDATION, HYDRATION, AND DESULPHIDATION.
Marcasite Limonite - 0. 14
Pyrite do + 2.93
Pyrrhotite.v do +24.68
j .
OXIDATION AND DESULPHIDATION.
Marcasite Magnetite 39.34
Pyrite do 37. 48
Pyrrhotite do 24. 27
SILICATION.
Corundum, quartz Cyanite - 6. 59
Do Sillimanite ; + 4.38
Gehlenite Grossularite - 4. 42
Gehlenite, quartz do 18.56
Olivine, quartz Anthophyllite - 1. 48
Nephelite, quartz Albite .41
TABLES.
407
TABLE D. Classification of alterations, with volume changes Continued.
SILICATION AND DECARBONATION.
Source.
Products.
Volume
change.
Per cent.
Ankerite or parankerite, quartz Actinolite 32. 72
Do Actinolite, calcite 22. 62
Do Sahlite 37.27
f o_io.77
Bronzite or h ypersthene, calcite, quartz . . Actinolite ! ft
Calcite ! Wollastonite +10.81
Calcite, quartz I do 31.48
Dolomite Diopside + 2.03
Dolomite, quartz do 40.11
Dolomite Tremolite, calcite - + 9. 89
Dolomite, quartz do 25. 20
Dolomite Tremolite, wollastonite +14. 00
Dolomite, quartz I do 33.09
Forsterite, calcite, quartz Tremolite 12. 29
Olivine, calcite, quartz Actinolite 13. 34
Rutile, calcite, quartz Titanite 28.17
Siderite, quartz Grunerite .' 32. 53
SILICATION AND DEHYDRATION.
Analcite, quartz Albite 17. 25
Diaspora, quartz ' Cyanite 22. 61
Do : Sillimanite v -13.52
Gibbsite, quartz Cyanite 49. 61
Do Sillimanite , -43.68
SILICATION, DEHYDRATION, AND DECARBONATION.
Diaspore, quartz, calcite Margarite 14.08
Do Zoiaite 29. 44
Diaspore, quartz, K..CO, Muscovite 54. 21
Gibbsite, quartz, calcite Margarite 38.92
Do Zoisite -43.06
Gibbsite, quartz, K 2 CO 3 Muscovite -64.99
<" Sp. gr. bronzite.
l>Sp. gr. hyperathene.
408
A TREATISE ON METAMOKPHISM.
TABLE D. Classification of alterations, with volume changes Continued.
SILICATION, HYDRATION, AND DECARBONATION.
Source.
Products.
Volume
change.
Per cent.
Corundum Margarite
Corundum, quartz, calcite do 1.22
Corundum Zoisite . +261. 34
Corundum, quartz, calcite do - 23.58
Corundum , Muscovite (damourite) +264. 25
Corundum, quartz, K 2 CO S ^ do + 1.62
SILICATION, OXIDATION, AND DECARBONATION.
Ilmenite Titanite + 76. 35
Ilmenite, calcite, quartz Titanite, magnetite 22. 35
SUBSTITUTION OF BASES.
Augite Hornblende + 4. 30
Augite, siderite, magnesite ' Hornblende, calcite + 6. 14
Calcite Dolomite 12. 30
Diopside Tremolite + 5.68
Diopside, magnesite Tremolite, calcite + 10. 55
Hornblende Augite 4.13
Leucite Orthoclase, nephelite 7. 59
Muscovite Paragonite 2.67
Olivine, anorthite Actinolite, spinel 7.18
Sahlite Actinolite + 7.28
Sahlite, siderite, magnesite Actinolite, calcite + 10. 81
Spodumene Beta-spodumene + 24. 72
SULPHIDATION.
Pyrrhotite Pyrite 4. 21.13
. : . j
SULPHIDATION, DEOXIDATION, AND CARBONATION.
Hematite Marcasite, siderite + 78. 73
Do Pyrite, siderite _|_ 76. 12
Re c'a UCB
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