INTRODUCTION TO THE STUDY OF MINERALS AND ROCKS muiiiiuini: PUBLISHERS OF BOOKS F O *_, Electrical World v Engineering News-Record Power v Engineering and Mining Journal-Press Chemical and Metallurgical Engineering Electric Railway Journal v Coal Age American Machinist v Ingenieria Internacional Electrical Merchandising v BusTransportation Journal of Electricity and Western Industry Industrial Engineer Microphotographs of snow crystals. (After Bentley.} (Frontispiece. ) INTRODUCTION TO THE STUDY OF - MINEEALS AND ROCKS A COMBINED TEXT-BOOK AND POCKET MANUAL BY AUSTIN FLINT ROGERS, ' jy." J>,^ % ,, ^. * . PROFESSOR OF MINERALOGY, STANF(\RK, Vll/V^pSIfi,; ^ r ' '. i ; vl FELLOW, GEOLOGICAL SOCIETY OF AMERICA; FELLOW, MINERALOGICAL SOCIETY OF AMERICA; MEMBER, MINERALOGICAL SOCIETY OF GREAT BRITAIN ; ASSOCIATE EDITOR, AMERICAN MINERALOGIST; FELLOW, AMERICAN ACADEMY OF ARTS AND SCIENCES SECOND EDITION SECOND IMPRESSION McGRAW-HILL BOOK COMPANY, INC. NEW YORK: 370 SEVENTH AVENUE LONDON : 6 & 8 BOUVERIE ST., E. C. 4 1921 ^GOBYRIGHT, 1912, 1921, BY THE Mc4, AsS 3 , AsS4, SbS 3 , SbS4. These are called acid radicals. The most common acids are those containing oxygen and hence are known as oxygen acids or oxy-acids. The oxy-acids are mono- basic (HN0 3 ), dibasic (H 2 SC>4), tribasic (H 3 PO4), or tetrabasic (H 4 SiO4), according as they have one, two, three, or four replace- able H atoms. The polybasic acids, as they are called, are capable of forming condensed acids by subtracting water. This is especially prominent with the silicic acids. Orthosilicic acid is H 4 SiO 4 ; H 4 SiO 4 -H 2 O = H 2 SiO 3 , metasilicic acid; 2H 4 SiO 4 - H 2 O = H 6 Si 2 O 7 , diorthosilicic acid; 2H 4 SiO 4 - 3H 2 O = H 2 Si 2 - Os, dimetasilicic acid. There are also H 4 Si 3 O8 (3H 4 SiO 4 - 4H 2 O), H 8 Si 3 Oio (3H 4 SiO 4 - 2H 2 O), and still other possible silicic acids. CHEMICAL PROPERTIES OF MINERALS 9 The replacement in the oxy-acids of by S gives compounds called sulfo-acids. Thus H 3 AsO 4 is arsenic acid or oxy-arsenic acid, while H 3 AsS4 is sulf arsenic acid. H 3 AsO 3 is arsenious acid, while H 3 AsS 3 is sulf arsenious acid. Various condensed acids, which are entirely analogous to the condensed oxy-acids, are de- rived from the above by the subtraction of H 2 S. Thus we have HAsS 2 (H 3 AsS 3 H 2 S), metasulf arsenious acid, and H 4 As 2 S 7 (2H 3 AsS 3 H 2 S), pyrosulf arsenious acid. Very few of these acids exist either as minerals or prepared compounds, but salts of all of them are known as minerals. Bases are compounds the dilute water solutions of which con- tain hydroxyl (OH) ions. The hydroxyl ions give the basic properties such as soapy feel, and the change of red litmus to blue. The strength of the base is proportional to the number of hydroxyl ions present. The strong bases such as KOH and NaOH are called alkalies. Weak bases are represented by Fe(OH) 3 and A1(OH) 3 . Among the bases represented by min- erals are Mg(OH) 2 , Mn(OH) 2 , Al(OH),, HA1O 2 [A1(OH) 3 -H 2 O], and HFeO 2 [Fe(OH) 3 - H 2 O]. Oxids are compounds of the elements with oxygen. Elements the oxids of which form bases with water are called metals. These oxids are called basic anhydrids for this reason. Elements the oxids of which form acids with water are called non-metals. These oxids are called acid anhydrids. Salts are compounds formed by the union of bases with acids; the metal of the base unites with the non-metal or acid radical of the acid to form the salt, while the hydroxyl of the base unites with the hydrogen of the acid to form water thus: NaOH + HC1 = NaCl + H 2 O. In dilute solutions salts are dissociated into two parts or ions as they are called. The metal forms one ion, called the cation, while the non-metal or acid radical forms the other ion, called the anion. Among salts we may distinguish halogen salts, oxy-salts, and sulfo-salts corresponding to the acids of which they are the derivatives. The following represent salts found as minerals: 10 INTRODUCTION TO THE STUDY OF MINERALS Sulfid, PbS (galena). Selenid, PbSe (clausthalite). Tellurid, PbTe (altaite). Arsenid, FeAs 2 (lollingite). Antimonid, Ag 3 Sb (dyscrasite). Sulfarsenite Ag 3 AsS 3 (proustite). Sulfarsenate Cu 3 AsS 4 (enargite). Sulfantimonite Ag 3 SbS a (pyrargy- rite). Sulfantimonate, CusSbS4 (famati- nite). Sulfoferrite, CuFeS 2 (chalcopyrite). Sulfochromite, FeCr 2 S 4 (daubree- lite). Sulfovanadate, Cu 3 VS 4 (sulvanite). Sulfogermanate, AgsGeSe (argyro- dite). Sulfostannite, PbSnS 2 (teallite). Chlorid, AgCl (cerargyrite). Bromid, AgBr 2 (bromyrite). lodid, Agl (iodyrite). Fluorid, CaF 2 (fluorite). Fluosilicate, K 2 SiF 6 (hieratite). Carbonate, CaCO 3 (calcite). Meta-aluminate, MgAl 2 O 4 (spinel). Metaferrite, FeFe 2 O 4 (magnetite). Metachromite FeCr 2 O 4 (chromite). Metaniobate, Fe(NbO 3 ) 2 (colum- bite). Metatantalate, Fe(TaO 3 ) 2 (tanta- lite). Phosphate, LiFePO 4 (triphylite). Arsenate, FeAsO 4 -H 2 O (scorodite). Vanadate, BiVO 4 (pucherite). Antimonate, Ca 2 Sb 2 O 7 (atopite). Nitrate, NaNO 3 (nitratine). Borate, A1BO 3 (jeremejevite). Sulfate, BaSO 4 (barite). Chromate, PbCrO 4 (crocoite). Selenite, CuSeO 3 -2H 2 O (chalcomen- ite). Tellurite, Fe(TeO 3 ) 3 4H 2 O (dur- denite). Tungstate, CaWO 4 (scheelite). Molybdate, PbMoO 4 (wulfe.nite). Metatitanate, CaTiO 3 (perovskite). Orthosilicate ( Mg, Fe ) 2 SiO 4 (olivine ) . Metasilicate, CaSiOs (wollastonite). Trisilicate, KAlSi 3 O 8 (orthoclase) . Dimetasilicate, LiAl(Si 2 O5) 2 (peta- lite). Diorthosilicate,Pb 3 Si 2 O 7 (barysilite). All the above are normal salts ; that is, all the hydrogen of the acid or hydroxyl of the base has been replaced by metals or by acid radicals respectively. A compound in which only part of the hydrogen of the acid has been replaced by a metal is called an acid salt. Among minerals we have KHSCU, and H 2 CuSiO4, which are called acid potassium sulfate, and acid copper silicate respectively. A compound in which only part of the hydroxyl of the base is replaced by an acid radical is called a basic salt. Among minerals we have Cu 2 (OH) 2 CO 3 , Cu 4 (OH) 6 Cl 2 , Cu 2 (OH)- AsO4, and many others. These three compounds are called basic copper carbonate, chlorid, and arsenate respectively. The formulae of some minerals are written as though they consist of two or more separate molecules. These are called molecular CHEMICAL PROPERTIES OF MINERALS 11 compounds for want of a better name. Among molecular com- pounds are double salts and hydrates or hydrous salts. Double salts are (1) salts composed of two metals with a common acid radical (example, dolomite CaCO 3 -MgCO 3 ), (2) salts of a single metal with two distinct acid radicals (example, arsenopyrite FeS 2 -FeAs 2 ), or (3) salts in- which both the metal and acid radical are different (example, kainite MgSO 4 'KCl*3H 2 O). Acid and basic salts may also be written in the form of double salts. KHSO 4 = K 2 SO 4 -H 2 SO 4 , Cu 2 (OH) 2 CO 3 = CuCO 3 'Cu- (OH) 2 . Another kind of compound is Sb 2 S 2 O, antimony oxy-sulfid, which may be written 2Sb 2 S 3 -Sb 2 O 3 (3Sb 2 S 2 O). Similarly Pb 2 OCl 2 (or PbCl 2 'PbO) is lead oxy-chlorid and Pb 2 Cl 2 CO 3 (or PbCl 2 'PbCO 3 ) is lead chloro-carbonate. Acid and basic salts when heated in the closed tube at a relatively high temperature (usually above 200 C.) give off water and this water is called water of constitution. In other compounds water is more loosely held, and when heated is driven off at a temperature varying from about 100 C. to 200 C. This is the so-called water of crystallization, but this term is a mis- nomer, as water is not necessary for crystallization. Most an- hydrous minerals occur in well formed crystals. Salts which give off a definite amount of water at low temperatures are called hy- drates or hydrous salts. The formula is written as if they contain water as such. Examples: hydrous calcium sulfate, Ca,SO4'2H 2 O (gypsum); hydrous sodium borate, Na 2 B 4 7 '10H 2 O (borax). There may be various hydrates, for example, MgSO 4 *H 2 O (kieser- ite); MgSO 4 -6H 2 O (hexahydrite) , and MgSO 4 7H 2 O (epsomite). The following are examples of complicated salts which occur as minerals: HNa 3 (C0 3 ) 2 '2H 2 0, hydrous acid sodium car- bonate (trona); Fe 4 (OH) 2 (S0 4 ) 5 '17H 2 O, hydrous basic ferric sulfate (copiapite). 3. DERIVATION OF CHEMICAL FORMULA Most minerals have a fairly definite chemical composition and hence may be represented by a formula. The formula is obtained 12 INTRODUCTION TO THE STUDY OF MINERALS by dividing the percentage composition of the various elements or radicals by the corresponding combining weights as found in a table of atomic weights (pages 6-7). The ratio of these expressed in the simplest whole numbers possible gives the empirical formula. Example: An analysis of chalcopyrite from Phoenix ville, Pennsylvania, furnished J. Lawrence Smith the results of column I. I II III IV Cu 32.85-^63.6 =0.516 34.57 Fe 29.93-5-55.8 =0.536 30.54 S 36.10-7-32.1 =1.121 34.89 Pb 0.35 Dividing by the combining weights given in the second column we have the figures of the third column, lead being omitted. These numbers are nearly in the ratio 1:1: 2, hence the formula CuFeS2. The theoretical percentages for chalcopyrite are given in the fourth column. Discrepancies in analyses may be explained in a number of ways. It is very common for similar metals or acid radicals to replace each other in varying proportions. In this case the combining ratios of replacing metals or acid radicals are added together. An analysis of brown sphalerite from Roxbury, Connecticut, gave Caldwell the percentage composition of column I. Dividing by the atomic weights of column II we have the combining ratios of III. The sum of the combining ratios for Zn and Fe (1.033) is to the combining ratio for S (1.039) practically as 1:1. Hence the formula is (Zn,Fe)S, which means that iron replaces zinc in varying amounts. Anal- yses of sphalerite show an iron content varying from nil up to 18 per cent. I II III IV Zn ^3.36 -J-65.4 =0.969 \ Fe 3.60 -7-55.8 =0.063 J S 33.36 -4-32.1 = 1.039 1.039 Analyses of oxids, haloids, sulfids, and sulfo-salts are given as CHEMICAL PROPERTIES OF MINERALS 13 percentages of the elements. This cannot be done with the oxygen salts as there is no way of determining oxygen directly; therefore the percentage composition of the oxygen salts must be expressed either as oxids, or as metals and acid radicals. At present it is customary to use the oxids. This is in accordance with the electro-chemical theory of Berzelius in which dualistic formulae were used. Thus FeSCX was considered as FeOSO 3 , FeO being the base or electropositive radical and SO 3 , the acid or electronegative radical. Although these views are considered antiquated by modern chemists, still the custom is to employ the basic and acid anhydrids in stating the results of analysis. Thus CaSC>4 is given as CaO and SO 3 . The method of giving the metals and acid radical is preferable if haloids or sulfids are present. In the ordinary method there is an excess of oxygen equivalent to the amount of halogen or sulfur present which must be deducted. For example, apatite is Ca 5 F(PO4) 3 . The calculated percentage compositions are: CaO = 55.5, P2O 5 = 42.3, F = 3.8; total = 101.6. The excess over 100 per cent is due to the fact that only part of the calcium is combined with the oxygen, as can be seen by expressing the formula in another way : 9CaO3P 2 (VCaF 2 . The oxygen equivalent of F is >^O with atomic weight of 8. The percentage compositions given above have been figured on the basis of a formula weight of 512.5 (504.5+8). [512.5 : 504.5 : : 101.6 : 100]. A much better way is to express the percentage compositions thus : Ca = 39.7, F = 3.8, P0 4 = 56.5; total = 100.0. In the case of hydrous, acid, or basic salts of any kind, the water percentage is given, as the determination of water is often a practical means of identifying a mineral. 4. VARIATIONS IN THE CHEMICAL COMPOSITION OF MINERALS In the foregoing discussion it was assumed that minerals have a definite chemical composition. Strictly speaking, this is true of only a comparatively few minerals such as some specimens 14 INTRODUCTION TO THE STUDY OF MINERALS of quartz, calcite, and a few others. While other minerals approach definiteness of chemical composition many others are far from being definite. In the first place, it should be emphasized that much of the variation from the theoretical values of chemical formulae is due to mechanical impurities. Many apparently homogeneous substances prove on microscopic examination to be mixtures of two or more substances. For example, wollastonite (CaSiO 3 ) usually seems to effervesce in acids but the effervescence is due to admixed calcite. So-called cupriferous pyrite contains small amounts of chalcopyrite, as may be proved by examining polished surfaces of specimens with the reflecting or metallo- graphic microscope. But complexity in chemical composition is not always proof of mechanical mixtures. In homogeneous minerals the departure from fixed chemical composition can be explained in two or three different ways: (1) by solid solution of which isomorphism is a special case, and (2) by the fact that the mineral is of colloidal origin. 6. ISOMORPHISM Many compounds of similar chemical composition, especially salts with the same acid radicals and related metals, have almost identical crystal forms. Such compounds are said to be isomor- phous. Isomorphous substances have similar form, but except in the isometric system this does not mean that the form is iden- tical. For example, the angle (110 : 110) for barite, BaSO4, is 78 22^', while for celestite, SrSO 4 , the corresponding angle is 75 50', and for anglesite, PbSO 4 , it is 76 16^'. Barite, celestite, and anglesite thus form an isomorphous group. Among promi- nent isomorphous groups of minerals are the following: Pyrite FeS 2 Smaltite (Co,Ni)As 2 Cobaltite CoAsS Gersdorffite NiAsS CHEMICAL PROPERTIES OF MINERALS 15 Ruby Silvers Marcasite FeS 2 Arsenopyrite FeAsS Lollingite FeAs 2 Glaucodot (Co,Fe)AsS Safflorite CoAs 2 Rammelsbergite NiAs 2 Tetrahedrite Cu 3 SbS 3 + z(Fe,Zn) 6 Sb 2 S 9 Tennantite Cu 3 AsS 3 + z(Fe,Zn) 6 As 2 S 9 Proustite Ag 3 AsS 3 Pyrargyrite Ag 3 SbS 3 Corundum A1 2 3 Hematite Fe 2 3 Cassiterite SnO 2 Rutile TiO 2 Diaspore A1(OH) 3 -A1 2 O 3 Goethite Fe(OH) 3 -Fe 2 O 3 Manganite Mn(OH) 3 -Mn 2 O 3 Calcite CaCO 3 Magnesite MgCO 3 Siderite FeCO 3 Rhochrosite MnCO 3 Smithsonite ZnCO 3 Aragonite CaCO 3 Strontianite SrCO 3 Witherite BaCO 3 Cerussite PbCO 3 Fluor-apatite CaioF 2 (P0 4 ) 6 Chlor-apatite Ca 10 Cl 2 (P0 4 )6 Dahllite Ca 10 (C0 3 )(P0 4 )6 Voelckerite CaioO(PO 4 ) 6 Svabite Cai F 2 (AsO 4 ) 6 Pyromorphite PbioCl 2 (P0 4 ) 6 Mimetite PbioCl 2 (AsO 4 ) 6 Vanadinite Pb 10 Cl 2 (V04)6 Barite BaSO 4 Celestite SrSO 4 Anglesite PbS0 4 Alunite KA1 3 (OH) 6 S0 4 Jarosite KFe 3 (OH) 6 SO 4 16 INTRODUCTION TO THE STUDY OF MINERALS Ilmenite FeTiO 3 Geikielite MgTiOg Pyrophanite MnTiO 3 Senaite (Fe,Mn,Pb)TiO 3 Grossularite CaAl 2 (SiO 4 ) Pyrope Mg3Al 2 (SiO 4 )3 Almandite Fe 3 Al 2 (Si0 4 )3 Spessartite Mn 3 Al 2 (SiO 4 )3 Andradite Ca 3 Fe 2 (SiO 4 ) 3 Uvarovite Ca 3 Cr 2 (Si0 4 ) 3 Garnet Many isomorphous compounds are capable of crystallizing together in various proportions and thus form what are known as isomorphous mixtures. There are many such cases among min- erals which fact is very useful in interpreting mineral analyses. Chromite is an isomorphous mixture of FeCr 2 O4, FeFe 2 O4, MgCr 2 04, MgAl 2 04, and FeAl 2 O4, all of which are known as minerals. The formula used to express the chemical composition is as follows: (Fe", Mg) (Cr, Al, Fe /// ) 2 4 , which means that the combined proportion of chromium, aluminum, and ferric iron is twice (molecularly) that of ferrous iron and magnesium together. Among prominent isomorphous mixtures are: sphalerite (Zn,Fe)S, smaltite (Co,Ni)As 2 , columbite (Fe,Mn)(Nb,Ta) 2 O 6 , campylite Pb 5 Cl(As,P) 3 Oi2, endlichite Pb 5 Cl(As,V) 3 Oi 2 , pisanite (Fe,Cu)- SO 4 7H 2 O, wolframite (Fe,Mn)WO 4 , actinolite Ca(Mg,Fe) 3 - (SiO,) 4 , and epidote Ca 2 (Al,Fe) 3 (OH)(SiO 4 ) 3 . The garnets are isomorphous mixtures of the compounds mentioned above. It is rare to find an analysis of garnet that will correspond exactly to any one of these formulae. The physical properties of isomorphous mixtures vary con- tinuously; for this reason the term solid solution is sometimes used. The best test of isomorphism is the ability to form mixed crystals. 6. SOLID SOLUTIONS NOT DUE TO ISOMORPHOUS MIXTURES While most of the variations in the chemical composition of crystalline minerals may be explained by isomorphism there are CHEMICAL PROPERTIES OF MINERALS 17 some cases which cannot be so explained. For example, pyrrho- tite contains a slight excess of sulfur over that required for the formula FeS. Formerly this was expressed by the formula Fe n Sn+i, in which n varies from 5 to 15, in accordance with the law of multiple proportions and the belief that every mineral has a definite composition. This we know to be false and the modern way of expressing the chemical composition of pyrrhotite is by means of the following formula: FeS(S) x , which signifies that pyrrhotite is regarded as a solid solution of sulfur in ferrous sulfid. Another example of solid solution is nepheline, which contains an excess of silica over that required for the formula (Na,K)- AlSi0 4 . The color of many minerals is due to a solid solution of some pigment (either organic or inorganic) in the mineral. 7. MINERALS OF COLLOIDAL ORIGIN Most of the amorphous minerals, such as opal, cliachite, and collophane, are of colloidal origin. They are apparently due to the gradual hardening or setting of a gelatinous mass. Such hardened gels were called porodine by Breithaupt. The mech- anism of the hardening of a gel is not well understood nor is the nature of the gel itself, but some of the hardened gels are common minerals and so deserve our attention. When a substance is precipitated in the form of minute particles intermediate in size between ordinary visible suspensions and solutions, it possesses peculiar properties due to the enormous surface exposed. The phenomenon of surface tension plays an important part in determining these properties. Such a state or condition of a substance is called the colloidal or dispersed state. Colloidal substances, or more accurately speaking sub- stances in the colloidal condition, on account of the great sur- faces exposed, have a tendency to take up or adsorb other sub- stances from the solutions in which they are formed. This adsorption seems to be, to a large extent, selective, and for this 18 INTRODUCTION TO THE STUDY OF MINERALS reason most of the amorphous minerals of colloidal origin ap- proach in chemical composition that of the corresponding crys- talline mineral. This was first emphasized by the Austrian mineralogist, Cornu, in 1909. Cellophane, for example, is very much like crystalline dahllite, SCasCPO^'CaCOs, in chemical composition. Nearly all the well-established amorphous minerals have crystalline equivalents. Opal is the amorphous equivalent of chalcedony and quartz. Cliachite is the amorphous equivalent of gibbsite [Al(OH) J. Turyite is probably the amorphous equiva- lent of hematite. Psilomelane is the amorphous equivalent of hollandite or romanche"ite. Pitchblende is probably the amor- phous equivalent of uraninite. Halloysite is the amorphous equivalent of kaolinite. Cornuite, recently described by the author, is the amorphous equivalent of chrysocolla. Some common minerals such as calcite, barite, fluorite, etc., have no amorphous equivalents, or at least they have not yet been described. Because of adsorption the amorphous minerals vary some- what in chemical composition but there is also variation in the water content. Practically all the amorphous minerals contain water, the reason being that they are hardened hydrogels, that is, gels formed in water solution. While the water is adsorbed when the colloid is first formed, there is a probability of solid solution being formed by the diffusion of the water and also of the other adsorbed substances. The water, however, is variable in amount and so may be represented by: (H 2 O) X . Although practically always present, the water is probably not essential. 8. POLYMORPHISM Something besides chemical composition must be taken in account in the study of minerals, for it is a well-known fact that many chemical substances exist in two or more distinct forms. That is, they occur in crystals with different internal arrange- ments of atoms, usually belong to different crystal systems, and have dissimilar physical properties. CHEMICAL PROPERTIES OF MINERALS 19 Such compounds are called polymorphs. A familiar example is carbon which occurs as graphite in soft, opaque, hexagonal crystals, and as diamond in very hard, transparent, isometric crystals. Polymorphous elements like carbon s are called allo- tropic. Among polymorphous minerals may be mentioned the following: V FeS 2 / Diamond Isometric \ Graphite Hexagonal (Py r ite Isom etric Marcasite Orthorhombic CaCO, Calcite Hexagonal Aragonite Orthorhombic (Orthoclase Monoclinic Adularia Monoclinic Microcline Triclinic Si0 2 a-Quartz Hexagonal (As.3A 2 ) /3-Quartz Hexagonal (A 6 .6A 2 ) Chalcedony (?) Tridymite Hexagonal (Symmetry unknown) Cristobalite Isometric f Rutile Tetragonal (6 = 0.64) TiO 2 1 Octahedrite Tetragonal (6 = 1.77) Brookite Orthorhombic Ca 2 Al 3 (OH)(SiO 4 ) ; Al 2 SiO 5 H 2 M g3 Si 2 O 9 Zoisite Orthorhombic Clinozoisite Monoclinic Kyanite Triclinic Andalusite Orthorhombic (d = 0.986, ^ inches in diameter, is used for fine grinding of minerals. Steel Pliers are used for breaking off fragments of minerals. Platinum Foil. A thin sheet of platinum about % by % inch may be used for sodium carbonate fusions. Dropping Bulbs are useful for reagents that are needed in small amounts, such as cobalt nitrate solution. Small beakers, porcelain crucibles, wash-bottles, etc., may often be used to advantage. 2. REAGENTS USED IN BLOWPIPE ANALYSIS A. Dry Reagents Dry reagents should be kept in wide-mouthed glass bottles. It is convenient to have a set of four to six of these bottles in a wooden stand. Sodium Carbonate, Na 2 CO 3 . Baking " soda " (NaHCO 3 ) may be used instead. Sodium carbonate is used principally for fusions. Borax, Na 2 B4O 7 10B 2 O, is used principally for bead tests. The ordinary commercial salt may be used. Borax glass is simply fused borax, used in silver cupellation. CHEMICAL PROPERTIES OF MINERALS 23 Sodium Metaphosphate, NaPO 3 . This is used for the bead tests, in which salt of phosphorus, HNaNHdPC^ 4H 2 O, is usually employed. It can be made by fusing salt of phosphorus, and is much more convenient, as a salt of phosphorus bead usually drops off the loop of platinum wire when heated. Potassium Acid Sulfate, KBSO4. This is used in bismuth flux, in boric acid flux, and also independently. lodid Flux is made by grinding together 1 part KI, 1 part KHSO4, and 2 parts S. It is used principally on plaster tablets, but also on charcoal. Boric Acid Flux is a mixture of 1 part of finely powdered fluorite (CaF 2 ) with 3 parts of KHSO 4 . Cupric Oxid, CuO. Powdered malachite may be used instead. Tin. Ordinary tin-foil (sheet lead with a thin coating of tin) is used as a reducing agent. Zinc. Zinc in the form of shavings or sheets is used in test- ing cassiterite. Test Lead. Lead in a granulated form such as is used in assaying. Bone-Ash, such as is used in assaying, is moulded into cupels on charcoal. Prepared cupels, 1 inch in diameter, may be used. B. Wet Reagents The following are the more important wet reagents used in the determination of minerals, though occasionally any of the reagents of the chemical laboratory may be found useful. Hydrochloric Acid, HC1. Two parts concentrated acid (sp. gr. 1.20) with 3 parts distilled water is the acid used for general purposes. (5N.) 1 Nitric Acid, HNO 3 . One part concentrated acid (sp. gr. 1.42) with 2 parts water. (5N.) Sulfuric Acid, H 2 SO4. One part concentrated acid (sp. gr. 1.84) with 4 parts of water (5N). It should be diluted with great care by pouring acid into the water rather than the reverse. Citric Acid. As this is a solid it may be used in the field for testing carbonates. A water solution is used. 1 N means a normal solution, i.e., one that contains one gram-equivalent of the sub- stance in one liter (a gram atom of hydrogen is the unit). 24 INTRODUCTION TO THE STUDY OF MINERALS Aqua Regia is a mixture of 3 parts of cone. HC1 and 1 part of cone. HNO 3 . It is made up when needed. Ammonium Hydroxid, NH 4 OH. One part of concentrated NH 4 OH (sp. gr. 0.96) to 4 parts of solution. Ammonium Oxalate, (NH 4 ) 2 CO 4 -2H 2 O. 40 grams of salt to a liter of solution. (HN.) Sodium Acid Phosphate, Na 2 HPO 4 -12H 2 O. 60 grams to a liter of solu- tion. (KN). Ammonium Molybdate, (NH 4 )2MoO 4 . This reagent, which is difficult to prepare, may be made by dissolving 100 grams of MoO 3 in 250 ccm. NH 4 OH (sp. gr. 0.96) with 250 ccm. of water. After cooling, this solution is poured into 750 ccm. HNO 3 (sp. gr. 1.2) with 750 ccm. water while stirring. It may also be prepared by dissolving 150 g. of (NH 4 ) 2 MoO 4 crystals in a liter of distilled water to which a liter of HNO 3 (sp. gr. 1.1) has been added. Silver Nitrate, A 3 NO 3 . 43 grams of the salt to a liter of solution. (KN.) It should be kept in an opaque bottle. Barium Chlorid, BaCl 2 -2H 2 O. 61 grams of salt to a liter of solution. Cobalt Nitrate, Co(NO 3 ) 2 6H 2 O. 73 grams of the salt to a liter of .solution. N.) Alcohol. 95 per cent, ethyl alcohol. C. Additional Reagents used in Qualitative Analysis Acetic Acid, HC 2 H 3 O 2 . 30 per cent. acid. (5N.) Ammonium Carbonate, (NH 4 ) 2 CO 3 . 192 grams to a liter of solution, including 100 ccm. of NH 4 OH. (4N). Ammonium Chlorid, NH 4 C1. 267 grams to a liter of solution. (5N). (The solid reagent is also used for alkali fusions with CaCO 3 .) Ammonium Sulfid, (NH 4 ) 2 S. Saturate cone. NH 4 OH with H 2 S, and add an equal volume of NH 4 OH. Dilute with three volumes of water. (4N.) Ammonium Sulfid, Yellow, (NH 4 ) 2 S Z . This is made by adding flowers of sulfur to (NH 4 ) 2 S. Barium Hydroxid, Ba(OH) 2 -8H 2 O. Used for the detection of carbon dioxide. Calcium Carbonate, CaCO 3 . The solid reagent. Chloroplatinic Acid, H 2 PtCl 6 . This is made by dissolving thoroughly cleaned scrap platinum in aqua regia. Ether -Alcohol. Equal volumes of ether and absolute alcohol. Dimethylglyoxime. A one per cent, solution of the reagent in alcohol. Ferrous Sulfate, FeSO 4 -7H 2 O. Concentrated solution. Lead Acetate, Pb (C 2 H 3 O 2 -3H 2 O.) 95 grams to a liter of solution. CHEMICAL PROPERTIES OF MINERALS 25 Potassium Chromate, K 2 CrO 4 . 49 grams to a liter of solution. Potassium Cyanid, KCN. 33 grams to a liter of solution. (^N.) Potassium Ferricyanid, K 3 Fe(CN) 6 . 55 grams to a liter of solution. Potassium Ferrocyanid, K 4 Fe(CN) 6 -3H 2 O. 53 grams to a liter of solu- tion. (KN.) Potassium Hydroxid, KOH. Solid reagent. Sodium Acetate, NaC 2 H 3 O 2 . The solid dissolved in ten parts of water. Sodium Carbonate, Na 2 CO 3 . Solid reagent. Sodium Cobaltic Nitrite, Na 3 Co(NO 2 ) 6 . This is made by adding 1 part of Co(NO 3 ) 2 solution to 3 parts of acetic acid and 5 parts of a 10 per cent. solution of NaNO 2 . Sodium Hydroxid, NaOH. Solid reagent. Sodium Nitrate, NaNO 3 . Solid reagent. Stannous Chlorid, SnCl 2 -2H 2 O. 56 grams to a liter of solution. (3^N.) Tartaric Acid, C 4 H 4 O 2 . Solid reagent. 3. THE OPERATIONS OF BLOWPIPE ANALYSIS The list of tests given here serves both as an outline to follow with known substances, and also as determinative tables for unknown minerals. As only the more important tests are included decided results must be obtained to be of value. FIG. 1. Oxidizing flame. I. USE OF THE BLOWPIPE. To produce a steady flame, maintain a reservoir of air by keeping the cheeks slightly distended, and by breathing through the nose. Oxidizing Flame (O.F.). The extreme outer tip (Fig. 1) of a small flame produced by a rather strong blast of air is most favorable for oxidation. If a candle, lamp, or Bunsen burner is used, the tip of the blowpipe is held just within the flame. One's ability to produce a good oxidizing flame may be judged by fusing borax on a ^ inch loop of platinum wire and then adding a little MoO 3 . The bead should become colorless. 26 INTRODUCTION TO THE STUDY OF MINERALS Reducing Flame (R.F.). The tip of the inner luminous cone (Fig. 2) of a large flame produced by a gentle blast of air is most favorable for reduction. If a candle, lamp, or Bunsen burner is used, the blowpipe tip is held just outside the flame and the whole flame is directed toward the assay. A borax bead made ame- thyst colored with a little Mn0 2 in O.F. should become colorless when heated in a good reducing flame. The reducing flame should be luminous, but just hot enough to prevent the deposi- tion of soot. II. FLAME TESTS. In the high temperature of the blowpipe flame, many com- pounds are volatilized; and the colors produced are often characteristic. They should be viewed against . a dark background, such as a piece of charcoal. The chlorids, as a rule, are the most volatile compounds of the metals, so FIG. 2. -Reducing flame. HC1 should be used > but in some cases H 2 SO4 is better. Platinum wire is used except for compounds of As, Sb, Pb, and Cu, which should be heated on charcoal. The wire should be cleaned with HC1 after each test, but it should never be placed in a reagent bottle on account of the danger of contaminating the reagent. Red Flames. Purplish red lithium compounds. Crimson strontium compounds. Orange red calcium compounds. Yellow Flames. Intense yellow (masked by blue glass or flame-color screen) sodium compounds. CHEMICAL PROPERTIES OF MINERALS 27 Green Flames. Yellowish green barium compounds. Yellowish green molybdenum compounds. Emerald green copper compounds (without HC1). Bright green (use H 2 SO 4 ) boron compounds. Pale bluish green (use H 2 SO4) phosphates. Pale bluish green tellurium compounds. Pale bluish green antimony compounds. Bluish green zinc compounds. Blue Flames. Azure blue copper compounds (with HC1). Pale blue arsenic compounds. Pale blue lead compounds. Violet Flames. Pale reddish violet (use blue glass or flame-color screen) potassium compounds. (Some potassium compounds such as orthoclase must be fused with Na 2 CO 3 , to obtain the flame test.) The spectroscope must be used to detect such elements as rubidium, calcium, thallium, indium, etc., and also to detect very small amounts of the above mentioned elements. III. OPEN TUBE TESTS. Glass tubes about 4 or 5 inches long and open at both ends are used. The substance, which may be introduced into the tube by means of a folded trough of paper, is placed about 1 inch from one end of the tube. The tube is heated gently in a horizontal position at first and then is gradually inclined while still heating; thus a current of air is produced. If heated too rapidly or too near the end of the tube a closed tube effect is the result. Odor of burning matches (S0 2 ) sulfids and sulfo-salts. Sublimate of minute brilliant crystals (As 2 O 3 ) arsenids and sulfarsenites. Non-volatile amorphous sublimate (Sb 2 O4) on under side of tube antimony sulfid and sulfantimonites. Gray metallic globules (Hg) mercury sulfid. 28 INTRODUCTION TO THE STUDY OF MINERALS IV. CLOSED TUBE TESTS. Glass tubes closed at one end are used. Two closed tubes may be made at the same time by fusing a piece of tubing 5 or 6 inches long at its middle point and pulling it apart when hot. Tubes should be clean and dry before they are used. 1. Change in Appearance. Decrepitates (flies to pieces) characteristic of many minerals. Turns black copper minerals and numerals containing organic matter. Turns dark red iron minerals. Turns yellow lead minerals. Turns yellow (white on cooling) zinc minerals. 2. Formation of Sublimates. Yellow sublimate (S) some sulfids. Black metallic mirror (As) arsenids. Reddish yellow (AsS) arsenic sulfids and sulfarsenites. Reddish brown (Sb 2 S 2 O) antimony sulfids and sulfanti- monites. White volatile sublimate ammonium salts. Water (H 2 0) hydroxids, hydrous, basic, and acid salts. 3. Formation of Gases. Colorless and odorless (C0 2 ) carbonates (detected by Ba(OH) 2 . Colorless and odorless (O) manganese dioxids (detected by glowing charcoal). Brownish-red and pungent odor (NO 2 ) nitrates. V. TREATMENT ON CHARCOAL. The substance, either alone or intimately mixed with some reagent, is heated in a shallow circular cavity at one end of the charcoal, which is made by revolving a coin or end of a knife handle. O.F. or R.F. is used according to the desired effect. CHEMICAL PROPERTIES OF MINERALS 29 1. Evolution of Gas. Odor of burning matches (SO 2 ) sulfids and sulfo-salts (use O.F.). Arsin odor (AsH 3 ) arsenids and sulfarsenites (use R.F.). 2. Formation of Sublimates. (Use O.F.). It is well to run a blank test to observe ash of the charcoal. White sublimate near assay (Sb 2 3 ) antimony compounds. White sublimate far from assay (As 2 O 3 ) arsenic compounds. White sublimate, yellow when hot (ZnO) zinc compounds. White sublimate, yellow near assay (PbSO 4 ) lead sulfid. Yellow sublimate (PbO) lead compounds. Yellow sublimate (Bi 2 O 3 ) bismuth compounds. 3. Reduction with Sodium Carbonate. Mix intimately 1 part of the finely powdered substance with 3 parts of Na 2 CO 3 and fuse in R.F. on charcoal. Magnetic particles (Fe 3 O 4 ,Ni,Co) iron, nickel, and cobalt compounds. Metallic button, gray and malleable (Pb) lead compounds. Metallic button, somewhat malleable but brittle on edges (Bi) bismuth compounds. Metallic button, malleable white (Ag) silver compounds. Metallic button, malleable yellow (An) gold compounds. Metallic button, malleable red (Cu) copper compounds. Metallic button, malleable white (Sn) tin compounds. 4. Fusion Test for Sulfur. An intimate mixture of a finely powdered sulfid or sulfo-salt with about three parts of sodium carbonate is heated in O.F. on a thin sheet of mica placed on charcoal (or on platinum foil if absence of As, Sb, Pb, and Cu is assured). The fused mass placed on a bright silver coin with several drops of water and crushed will give a black stain (Ag 2 S). The reactions are: R"S + Na 2 - CO 3 = Na 2 S + R"CO 3 . Na 2 S + 2Ag + H 2 O + O = Ag 2 S + 2NaOH. Tellurids give the same test as sulfids. 30 INTRODUCTION TO THE STUDY OF MINERALS It is well to run a blank test to see if the gas or sodium car- bonate contains sulfur. Sulfates also give this test if heated in a strong R.F. on char- coal instead of on mica. The addition of powdered charcoal helps the reaction. As the sulfur compounds sink into the charcoal, the same spot cannot be used more than once. 5. Treatment with Cobalt Nitrate. The substance is heated intensely on charcoal before and after adding a dilute solution of cobalt nitrate. In this way cobalt aluminate, cobalt zincate, etc., are formed. Deep blue coloration infusible aluminum compounds and zinc silicates. (Almost any fusible substance will give a blue color, for a cobalt glass is formed.) Bright green coloration zinc compounds, except the sili- cates which give a blue coloration. Bluish-green coloration tin compounds. Pale pink coloration magnesium compounds (not very satisfactory). VI. TREATMENT ON PLASTER WITH IODID FLUX. An intimate mixture of the substance with an equal quantity of iodid flux (2 parts S, 1 part KI, .and 1 part KHS0 4 ) is heated gently at one end of a plaster tablet. In this way iodids of the metals are obtained. Yellow sublimate (PbI 2 ) lead compounds. Orange sublimate stippled with peach-red (SbI 3 ) antimony compounds. Purplish-chocolate sublimate (BiI 3 ) with underlying scarlet bismuth compounds. Scarlet sublimate (dark greenish-yellow if overheated) (HgI 2 ) mercury compounds. VII. TREATMENT ON CHARCOAL WITH IODID FLUX. The same reagent as above, but with charcoal instead of plaster as a support. CHEMICAL PROPERTIES OF MINERALS 31 Greenish-yellow sublimate (Pbl 2 ) lead compounds. Scarlet sublimates (BiI 3 ) bismuth compounds. Faint yellow sublimate (HgI 2 , etc.) mercury, arsenic, and antimony compounds. VIII. BORAX BEAD TESTS. Borax beads are made by fusing borax in a 3 mm. loop of platinum wire formed around the sharpened end of a lead pencil. Great care should be used in O.F. and R.F. Sulfids should be first roasted by gently heating the powdered substance spread out on charcoal. It is well to preserve the beads in a little frame or glass tube for future reference. The colors refer to cold beads, except when otherwise mentioned. Many elements giving colorless or pale yellow beads are not mentioned. Violet Blue Green Red Brown Yellow Colorless Co. O.F.,R.F. Cr O.F.,R.F. Cu O.F. O.F.,R.F. (saturated) R.F. (opaque) Fe R.F. O.F. Mn O.F. R.F. Mo R.F. O.F. Ni O.F. (hot) O.F. (cold) R.F. (turbid gray) Ti R.F. O.F. U R.F. O.F. V R.F. O.F. W R.F. O.F. 32 INTRODUCTION TO THE STUDY OF MINERALS IX. SODIUM METAPHOSPHATE BEAD TESTS. Beads of sodium metaphosphate, NaP0 3 , are made in the same way as with borax. Salt of phosphorus or microcosmic salt, (HNaNH 4 PO4-4H 2 O) may also be used, for on heating it loses NH 3 and H 2 O, and is converted into NaP0 3 . The colors refer to cold beads. Violet Blue Green Red Yellow Colorless Co O.F.,R.F. Cr O.F.,R.F. Cu O.F. (satur- ated) R.F. (opaque) Fe O.F. (pale) O.F.,R.F. Mn O.F. R.F. Mo R.F. O.F. Ni O.F.,R.F. Ti (satur- ated) R. F. O.F. U O.F.,R.F. V R.F. O.F. W R.F. O.F. Silica is insoluble in a NaPO 3 bead, but with silicates the beads dissolve (sometimes coloring the bead), while the silica usually remains as a translucent mass, often the shape of the original fragment, which floats around in the bead. A few other compounds, such as A1 2 O 3 and Ti0 2 , are very slowly soluble in a NaP0 3 bead. CHEMICAL PROPERTIES OF MINERALS 33 X. REDUCTION COLOR TESTS. Saturate several NaPO 3 beads with the finely ground sub- stance, and heat on charcoal with metallic tin in R.F. Dissolve in dilute HC1, add tin, and then boil. Violet solution titanium compounds. Deep blue solution tungsten compounds. Brown solution molybdenum compounds. Green solution chromium, uranium, and vanadium com- pounds. XI. SODIUM CARBONATE BEAD TEST. Beads of sodium carbonate are made the same as borax and sodium metaphosphate. The O.F. is used. The beads are opaque and not clear as with borax. Bluish-green opaque bead (Na 2 MnO4) manganese com- pounds (a very delicate test). Yellow opaque bead (Na 2 CrO 4 ) chromium compounds. Effervescence silica. Na 2 CO 3 + SiO 2 = Na 2 SiO 3 + CO 2 . (The bead will be clear if equal molecular quantities are used.) XII. TREATMENT WITH ACID POTASSIUM SULFATE. The substance is mixed with KHSO4 and heated in a test-tube or closed tube. Red-brown fumes with pungent odor (NO 2 ) nitrates. Colorless gas with HC1 odor (HC1) chlorids. Colorless gas which etches glass (HF) fluorids. Colorless gas with disagreeable odor (H 2 S) sulfids. Colorless, odorless gas (C0 2 ) carbonates. XIII. FUSIBILITY TESTS. Long thin splinters of the mineral about 1 mm. in diameter held with platinum-tipped forceps or wrapped with a coil of platinum wire (in the absence of platinum forceps or wire a splinter may be stuck into a piece of charcoal) are heated in the hottest part of the flame, which is just beyond the tip of the inner 34 INTRODUCTION TO THE STUDY OF MINERALS cone (see Fig. 3) of a small sharp O.F. flame (rather strong blast). Metallic substances should be heated on charcoal as they may contain As, Sb, or Pb which will alloy with the plati- num. Powders or substances which fly to pieces when heated may be ground with a little water into a paste, which after careful drying can be heated in the forceps or on charcoal. Scale of Fusibility 1. Fuses easily in luminous flame or in the closed tube stibnite. 2. Fuses with difficulty in luminous flame or in the closed tube chalcopyrite. 3. Fuses easily in blow- pipe flame almandite FIG. 3. Position of assay in testing fusibility. ^* ... 4. Fuses on edges easily in blowpipe flame actinolite. 5. Fuses on edges with difficulty orthoclase. 6. Fuses only on thinnest edges enstatite. 7. Infusible, even on thinnest edges quartz. Not only the degree, but also the manner of fusion should be noted. The substance may fuse either to a clear, opaque, or colored glass; quietly, with intumescence (bubbling), or with exfoliation (spreading out like leaves of a book). XIV. BLOWPIPE SILVER ASSAY. A qualitative test for silver in ores may easily be carried out by means of the blowpipe. The method is similar to that used in assaying except that it is on a smaller scale. By using an assay centner (100 mg.) of ore and measuring the silver button obtained on an ivory scale made for the purpose, one may obtain quantitative results which, after some practice, are very satisfactory. (1) Mix finely powdered ore intimately with one volume of CHEMICAL PROPERTIES OF MINERALS 35 borax glass (made by fusing borax) and one volume of test lead. If ore contains galena it is not necessary to add test lead. (2) Fuse mixture in a deep cavity in charcoal with a strong R.F. for several minutes. The silver is collected by the lead button. (3) After cooling remove lead from charcoal and hammer off the slag. (4) Add fresh borax glass and heat in O.F. until the quantity of lead is considerably diminished. Again hammer off every particle of the slag. (5) Prepare a cupel by rilling a large cavity in charcoal with very slightly moistened bone-ash and making a smooth concave depression with a mold (the end of a large test-tube will do). Heat cupel gently and remove all loose particles. (6) Carefully place the cube of lead (which should be not more than 2 or 3 mm. in diameter) on the cupel and fuse in O.F. by blowing across the top of it (use a small flame and strong blast and revolve the cupel occasionally) . The oxidation produces a thin film of lead oxid showing interference colors, but when the lead is all absorbed, the film suddenly disappears or "blicks," and a minute sphere of silver, which may also contain gold, remains. The final oxidation of the lead must proceed without interruption, otherwise it may be necessary to repeat the entire operation from the beginning. If the button shows a yellow tinge, gold is present. The silver may be removed by dissolving the button in nitric acid, but if much gold is present it is necessary to add some silver to the button in order to separate the gold from the silver. XV. SOLUBILITY TESTS. In the absence of any special phenomena such as the evolution of a gas, or change in color, the only accurate way of testing solubility is to boil a small amount of the solvent with the sub- stance for some time, and then to filter or decant the clear liquid and evaporate it to dryness. A residue indicates that the sub- stance is soluble (anhydrite furnishes a good example of a soluble mineral which on hasty examination one might call insoluble). If in doubt as to the solubility run a blank test with an equal 36 INTRODUCTION TO THE STUDY OF MINERALS quantity of solvent alone. A water solution of the residue gives a precipitate with a solution of sodium carbonate, except in the case of alkali compounds, but among minerals these are all readily soluble in water or contain other elements that are precipitated. Soluble in water nitrates, some chlorids, some sulfates, some borates, some carbonates. Soluble in HC1 all carbonates, some sulfids, some sulfates, borates, some phosphates, some silicates (see p. 495), iron oxids, and iron hydroxids. Soluble in HN0 3 , but insoluble in HC1 most sulfids and sulfo- salts. Soluble in aqua regia gold and platinum. Soluble in HF silica and nearly all the silicates. Insoluble in acids but soluble in other liquids cerargyrite, soluble in NH 4 OH; anglesite, soluble in NH4(C 2 H 3 O 2 ); sulfur, soluble in CS 2 . Insoluble, but decomposed by fusion with Na 2 CO 3 most silicates, chromite, wolframite, barite, and celestite. For method of treatment see note 4, for silicates, p. 49. Insoluble, not completely decomposed by Na 2 CO 3 fusion, but decomposed by fusion with KOH in a nickel crucible cassiterite, corundum, and rutile. Evolution of Gas. Colorless, odorless gas (CO 2 ) carbonates. Colorless gas with disagreeable odor (H 2 S) some sulfids. Colorless, pungent gas (Cl) with HC1 manganese dioxid. Brown red, pungent gas (NO 2 ) with HN0 3 sulfids and some elements. Color of Solution. Amber solution iron compounds. Green solution copper (especially when iron is present) and nickel compounds. Blue solution copper compounds. Pale red solution cobalt compounds. CHEMICAL PROPERTIES OF MINERALS 37 Insoluble Residue. Gelatinous residue or slimy silica some silicates. White residue (PbSO 4 ), (HSbO 3 ), (AgCl) lead, antimony, and silver minerals. Yellow residue (WO 8 ) calcium tungstate. XVI. WET TESTS AND GROUP -REAGENTS. A. Wet Tests for Metals (Cations) HC1 precipitates AgCl, HgCl, and PbCl 2 . H 2 S in acid solutions precipitates Ag 2 S, PbS, HgS + Hg, Bi 2 S 3 , CuS, HgS, As,S 3 , As 2 S 3 + S, Sb 2 S 3 , SnS, and SnS 2 . NH 4 OH in the presence of HC1 (or NH 4 C1) precipitates Pb(OH) 2 , Hg 2 NH 2 Cl, HgNH 2 Cl,BiO(OH),SbO(OH),Sn(OH) 2r Sn(OH) 2 , Al(OH),, Cr(OH) 3; Fe(OH) 3 , Fe(OH) 2 , and also Ca 3 - (PO 4 ) 2 , CaF 2 , and Ca(BO 2 ) 2 . (NH 4 ) 2 S in neutral solutions precipitates Ag 2 S, PbS, HgS, CuS, Bi 2 S 3 , Sb 2 S 3 , SnS, Al(OH),, Cr(OH 3 ), FeS, FeS + S, ZnS, MnS', CoS, and NiS. (NH 4 ) 2 CO 3 precipitates, from alkaline solutions, carbonates of all the non-alkali metals except Mg. With Ag, Cu, Co, Ni, and Zn the precipitate is soluble in excess. Na 2 HPO 4 precipitates all the metals except the alkalies as phosphates, Hg as basic chlorid, and Sb as oxid. Na 2 CO 3 precipitates all the metals except the alkalies as follows : Ag 2 CO 3 ,Hg 2 CO 3 ,CdCO 3 ,FeCO 3 ,MnC0 3 ,BaCO 3 , SrC0 3 . CaCO 3 , MgCO 3 , Fe(OH) 3 , Al(OH),, Cr(OH) 3 , Sn(OH) 2 , H 2 Sn0 3 , Sb 2 O 3 , Hg 2 OCl 2 , H 3 SbO 4 , and basic carbonates of Pb, Cu, Zn, Co, andNi. H 2 SO 4 (dilute) precipitates PbS0 4 , BaSO 4 , SrS0 4 , CaSO 4 -2H 2 O (incompletely unless alcohol is added), and HgSO 4 (incompletely). NaOH precipitates Ag 2 O, Hg 2 O, HgO, Cu(OH) 2 , Cd(OH) 2 , BiO(OH), SbO(OH) 3 , Sn(OH) 2 , SnO(OH) 2 , Fe(OH) 3 , Fe(OH) 2 , Ni(OH) 2 , Co(OH) 2 , Mn(OH) 2 , Ba(OH) 2 (incompletely), Sr(OH) 2 (incompletely), Ca(OH) 2 (incompletely), Mg(OH) 2 , and the fol- lowing which are soluble in excess: Pb(OH) 2 , Sb 2 O 3 , SbO(OH) 3 , Sn(OH) 2 , SnO(OH) 2 , Al(OH),, Cr(OH) 3 , Zn(OH) 2 . 38 INTRODUCTION TO THE STUDY OF MINERALS (NH 4 ) 2 C 2 O4 precipitates oxalates of all the metals except the alkalies and magnesium from alkaline solutions. B. Wet Tests for Acid Radicals (Anions) With BaCl 2 as a reagent. A white ppt. insoluble in HC1 indicates 864. A white ppt. soluble in HCl, but insoluble in acetic acid indi- cates F. A yellow ppt. soluble in HCl but insoluble in acetic acid indicates CrC>4. A white ppt. soluble in HCl and in acetic acid indicates BO 2 or B 4 O 7 , PO 4 , CO 3 , or AsO 4 . With AgNO 3 as a reagent. A yellow ppt. soluble in HNO 3 indicates PO4. A red or red-brown ppt. soluble in HN0 3 indicates AsO4or CrO4. A white ppt. soluble in HN0 3 indicates BO 2 or 640?. A white ppt. insoluble in HNO 3 indicates Cl. A black ppt. soluble in HN0 3 indicates S. XVII. PREPARATION OF SOLUTION. Water is the first solvent used, and after that either hydro- chloric or nitric acids. For some minerals HCl is the best solvent and for some HNO 3 is the best; therefore it is well to try a small quantity of the mineral with each of these solvents to determine which is the better. For sulfids HN0 3 is the best solvent, but if either lead, antimony, or tin is present, white residues are formed. HCl will precipitate chlorids of silver, lead, and mercury. If the substance is insoluble in both HNO 3 and HCl it may be soluble in aqua regia (1 part HNO 3 +3 parts HCl). Many minerals, especially silicates, are insoluble in aqua regia, and require fusion with Na 2 CO 3 on platinum foil or in a porcelain crucible. A water solution of the fusion will generally contain sodium salts of various acids, while an acid solution of the residue will generally contain the metals. The following minerals are not decomposed by Na 2 CO 3 and CHEMICAL PROPERTIES OF MINERALS 39 require fusion with KOH in a nickel or silver crucible : corundum, (AlaO,), cassiterite, (Sn0 2 ), and rutile, (TiO 2 ). XVIII. QUALITATIVE SCHEME. (For the more common ele- ments) . 1. Add cold dilute HC1 in excess. Ppt. 2. Filtrate 6. 2. Wash ppt. with hot water on filter-paper. Residue 3. Filtrate 5. 3. Add NH 4 OH to residue drop by drop. A blackening indicates Hg. Divide nitrate into two portions 4 and 5. 4. Acidify filtrate with HNO 3 . A white ppt. indicates Ag. 5. Test filtrate with K 2 CrO 4 . A yellow ppt. indicates Pb. 6. Pass H 2 S into warm, slightly acid solution. Ppt. 7. Filtrate 16. 7. Digest ppt. with (NH 4 ) 2 S. Filter. Residue 8. Filtrate 13. 8. Digest residue with hot dilute HNO 3 . Filter. Residue 9. Filtrate 10. 9. Dissolve residue in aqua regia. Boil off Cl. A ppt. with SnCl 2 indicates Hg. 10. Add a little cone. H 2 SO 4 and drive off excess. A white ppt. indicates Pb. Filtrate 11. 11. Add NH 4 OH in excess to filtrate. A white ppt. indicates Bi. Filtrate 12. 12. A blue filtrate indicates Cu. Add KCN until blue color disap- pears; then pass H 2 S. A yellow ppt. indicates Cd. 13. Add dilute HC1 to filtrate. Heat ppt. formed with cone. HC1. A residue indicates As. Filtrate 14. 14. Into the dilute solution, heated to almost boiling, pass H 2 S. An orange red ppt. indicates Sb. Filtrate 15. 15. Into the cool diluted filtrate pass H 2 S. A yellow ppt. indicates Sn. 16. Boil off H 2 S, add a few drops of HNO 3 . Add NH 4 C1 and NH 4 OH. Ppt. 17. Filtrate 22. 17. Dissolve ppt. in least possible amount of HC1. Add 50 % alco- hol and dilute H 2 SO 4 . A crystalline ppt. indicates Ca. Filtrate 18. 18. Boil off the alcohol, make filtrate alkaline with NH 4 OH. Ppt. 19. Reject filtrate. 19. Fuse ppt. with Na 2 CO 3 and NaNO 3 on platinum foil. A bluish- green mass indicates Mn. Digest the fused mass in hot water and filter. Residue indicates Fe. Divide filtrate into two portions, 20 and 21. 20. A yellow filtrate giving red ppt. with AgNO 3 indicates Cr. 21. Acidify with HC1. Add solid NH 4 C1 and boil. A ppt. indi- cates Al. 40 INTRODUCTION TO THE STUDY OF MINERALS 22. Into the warm alkaline filtrate pass H 2 S. Ppt. 23. Filtrate 29. 23. Wash ppt. on filter with cold dilute (1 : 10) HC1. Residue 24. Filtrate 26. 24. Dissolve residue in aqua regia. Evaporate to dryness, add a little water, and make strongly basic with NaOH. Add tartaric acid but not enough to make the solution acid. Heat slightly and pass a 1 3 w & 2 O H 2 S. A ppt. indicates Co. Filtrate 25. 25. Acidify filtrate with HC1. A ppt. indicates Ni. 26. Boil filtrate to remove H 2 S. Add KOH in excess. Ppt. 27. Filtrate 28. 27. Fuse ppt. with Na 2 CO 3 . A bluish-green mass indicates Mn. 28. Add H 2 S to the filtrate and heat. A white ppt. indicates Zn. 29. Evaporate filtrate to rather small volume. Add (NH 4 ) 2 CO 3 and alcohol. After standing an half-hour, filter. Ppt. 30. Filtrate 34. 30. Dissolve ppt. in hot dilute acetic acid and add K 2 CrO 4 . A yellow ppt. indicates Ba. Filtrate 31. 31. Add NH 4 OH and alcohol. A yellow ppt. indicates Sr. Fil- trate 32. 32. Dilute and add (NH 4 ) 2 C 2 O 4 . A white ppt. indicates Ca. Fil- trate 33. 33. Add NH 4 OH and Na 2 HPO 4 . A white ppt. indicates Mg. 34. Evaporate filtrate to dryness. Ignite to drive off ammonium salts. Add NaOH and Na 2 HPO 4 . Heat and add alcohol. A white ppt. indicates Li. Filtrate 5. 35. To the filtrate add Na 3 Co(NO 2 ) 6 . A yellow ppt. indicates K. Note. The original substance must be tested for Na and NH 4 . 4. SELECT BLOWPIPE AND WET TESTS Aluminum, Al. 1. Infusible aluminum minerals (also zinc silicates) when heated intensely before and after adding cobalt nitrate solution give a fine blue color. Fusible minerals may give a blue cobalt glass whether aluminum is present or not. 2. Ammonium hydroxid gives a white gelatinous precipitate, A1(OH) 3 , in solutions containing aluminum. (The ppt. is soluble in KOH or NaOH.) Iron hydroxid, chromium hydroxid, cal- cium phosphate, calcium borate, and calcium fluorid are also precipitated by NH 4 OH along with A1(OH) 3 . The calcium may CHEMICAL PROPERTIES OF MINERALS 41 be removed by means of dilute sulfuric acid and 50 per cent, ethyl alcohol before testing for iron and aluminum. Antimony, Sb. 1. Antimony minerals heated on charcoal in O.F. give a volatile white sublimate (Sb 2 C>4) near the assay and dense white fumes without odor. 2. With iodid flux on a plaster tablet antimony compounds give a peach-red coating or an orange coating stippled with peach-red. 3. In the open tube antimony minerals give a non- volatile, amorphous, white sublimate (Sb 2 04) on the under side of the tube. 4. Compounds of antimony and sulfur give a reddish-brown sublimate (Sb 2 S 2 0) when heated intensely in the closed tube. 5. Concentrated HN0 3 oxidizes antimony sulfids and sulfo- salts to HSb0 3 , a white precipitate soluble in KOH. Arsenic, As. A. Compounds without Oxygen. 1. On charcoal most arsenic minerals give a white volatile coating (As 2 O 3 ) far from the assay and fumes with characteristic odor of arsin (AsH 3 ) (a disagreeable odor something like that of garlic) . 2. In the open tube, minute, brilliant, colorless crystals (As 2 - O 3 ). This sublimate is volatile in contrast with that of Sb 2 O 4 . 3. In the closed tube a black metallic mirror of arsenic. A gray crystalline sublimate may also form. 4. H 2 S precipitates yellow As 2 S 3 , which is soluble in (NH 4 ) 2 - S x , but insoluble in concentrated HC1. B. Ar senates. 5. Arsenates heated intensely in the closed tube with charcoal give a black metallic mirror of arsenic. 6. Nitric acid solutions of arsenates give a yellow precipitate with (NH 4 ) 2 MoO 4 when heated to boiling. If the solution is to be tested for a phosphate, the arsenic must be removed by means of H 2 S (see 4). 42 INTRODUCTION TO THE STUDY OF MINERALS Barium, Ba. 1. Yellowish-green flame (not made blue by HC1). 2. Dilute H 2 S0 4 precipitates white BaSO 4 , a finely divided precipitate insoluble in acids. This, like strontium sulfate, forms in very dilute solutions while calcium sulfate forms only in fairly concentrated solutions. 3. (NH 4 ) 2 CO 3 [or (NH4) 2 C204] gives a white precipitate soluble in acids. (Sr and Ca also.) 4. K 2 Cr04 (or K 2 Cr 2 07) gives a yellow precipitate (dis- tinction from Sr and Ca). Beryllium, Be. 1. Be(OH) 2 is precipitated along with A1(OH) 3 by NH 4 OH. The precipitate is dissolved in dilute HC1 and the solution evaporated nearly to dryness. A little water is added, and also KOH in amount sufficient to dissolve the precipitate which forms at first. The solution is diluted and boiled when Be(OH) 2 separates out. This precipitate heated on charcoal with cobalt nitrate solution assumes a lavender color. Bismuth, Bi. 1. With iodid flux on plaster, a purplish-chocolate sublimate with underlying scarlet. 2. With sodium carbonate on charcoal in R.F., a metallic button brittle on the edges, and also a yellow sublimate (BiO). 3. To a nitric acid solution from which the excess of acid has been evaporated, HC1 is added. On dilution with water, a white precipitate of bismuth oxychlorid (BiOCl) is formed. Boron, B. 1. Borates give a green flame, especially if moistened with H 2 S0 4 . Silicates containing boron give a momentary green flame when heated with boric acid flux (3 parts KHSO 4 to 1 part powdered fluorite, CaF 2 ) . This flame is due to the forma- tion of volatile BF 8 . CHEMICAL PROPERTIES OF MINERALS 43 2. Alcohol added to a solution of a borate will burn with a green flame. 3. Turmeric paper moistened with a HC1 solution of a borate, and dried carefully on the outside of the test-tube containing the boiling solution, becomes reddish-brown. This color is changed to black by NH 4 OH. It is well to run a blank test at the same time. Zirconium solutions give a similar test. Calcium, Ca. 1. In a rather concentrated solution, dilute H 2 S04 precipitates CaS04-2H 2 O, which appears crystalline with the hand lens in contrast with BaSO4 and SrSO 4 . The addition of 50 per cent, ethyl alcohol makes a very complete precipitation. 2. The microchemical gypsum test is the most satisfactory test for calcium. A drop of solution containing calcium is placed on a glass slip and alongside of it FIG. 4. Microchemical a drop of dilute H 2 SO 4 . The two drops are brought into contact. In a few minutes time small crystals of CaSO4-2H 2 O (gypsum) make their appearance. (See Fig. 4.) 3. Yellowish-red flame with HC1. 4. (NH 4 ) 2 C 2 O4 or (NH 4 ) 2 CO 3 gives a white precipitate soluble in acids, as do also Ba and Sr. (Ba gives a yellow precipitate with K 2 CrO4 in the presence of dilute acetic acid. Ca(NO 3 )2 is soluble in ether-alcohol, while Sr(NO 3 ) 2 is insoluble.) 5. Calcium borates, fluorids, and phosphates are all precipi- tated from acid solution on the addition of NEUOH, and hence may be confused with aluminum hydroxid. In this case the calcium may be detected as given in Note 1 above. Carbon, C. A. Carbonates 1. Carbonates effervesce in dilute acids (some in the cold, others only upon heating) with the evolution of a colorless, odor- 44 INTRODUCTION TO THE STUDY OF MINERALS less gas which gives a white precipitate with Ba(OH) 2 or lime- water. 2. Carbonates effervesce in a hot borax bead. When the bead cools, a mass of tiny bubbles may be detected with a lens. 3. Citric acid, a solid, serves as a convenient field reagent. Carbonates effervesce in a water solution of citric acid. B. Hydrocarbons 1. Hydrocarbons, such as asphaltum, albertite, bituminous coal, etc., which are mineraloids rather than true minerals, when heated in the closed tube give oils and tar-like substances with a characteristic disagreeable odor. Chlorin, Cl. 1. To a NaPO 3 bead saturated with CuO (or malachite) a little of the powdered substance is added. On heating, an intense azure-blue flame is obtained. 2. Insoluble chlorids fused first with Na2C0 3 , and then heated with MnO 2 and an excess of KHS0 4 in a closed tube, give free chlorin. 3. In chlorid solutions AgNO 3 gives a white curdy precipitate which is soluble in NH^OH. Chromium, Cr. 1. The borax and sodium metaphosphate beads are emerald green in both O.F. and R.F. 2. The sodium carbonate bead is yellow in O.F. A little KNO 3 or NaN0 3 helps the reaction. 3. Chromate solutions give a dark red precipitate with AgNO 3 and a yellow precipitate with Pb(C 2 H 3 2 )2. Cobalt, Co. 1. The borax and sodium metaphosphate beads are deep blue in both O.F. and R.F. This furnishes a very delicate test for cobalt. 2. Heated on charcoal in R.F., cobalt compounds become magnetic as do also nickel and iron compounds. CHEMICAL PROPERTIES OF MINERALS 45 Copper, Cu. 1. Green flame made azure-blue with HC1. 2. Borax and sodium metaphosphate beads are blue in O.F., and opaque red (due to Cu 2 O) in R.F. if large amounts are used. Metallic copper may also be formed in R.F. In the presence of iron, the O.F. bead is green or bluish-green. 3. On charcoal with Na2CO 3 in R.F., and also with NaP0 3 and metallic tin on charcoal, metallic copper (malleable) is obtained. 4. Solutions of copper minerals are blue (green in the presence of iron). NH 4 OH in excess produces a deep blue coloration. (Nickel solutions give a faint blue coloration with NH^OH.) 5. A slightly acid copper solution touched to a bright surface of iron, such as knife-blade or hammer, gives a coating of metallic copper. Fluorin, F. 1. Fluorids are soluble in concentrated H 2 SC>4 with evolution of HF which etches glass. A lead dish, or watch-glass coated with paraffin, should be used. 2. Fluorin compounds heated in a closed tube with 4 parts of NaPO 3 will etch glass, and deposit a ring of SiO 2 which cannot be washed off with water. 3. Fluorin compounds heated with concentrated H 2 S0 4 and powdered silica give fumes which condense on moistened black paper. 4. Fluorids give a momentary green flame when heated with borax and KHSO4. This flame is due to the formation of vola- tile BF 3 . Gold, Au. 1. With sodium carbonate on charcoal, gold compounds give a malleable yellow button. 2. Gold may be identified in some of its rich ores by panning and washing away light quartz, rock, etc. Mercury is added to 46 INTRODUCTION TO THE STUDY OF MINERALS the concentrates. By grinding in a mortar, an amalgam of gold is obtained. This may be heated on charcoal or in a closed tube, and the mercury driven off. When the residue is heated with a little borax on charcoal a globule of gold is obtained. Hydrogen, H. 1. Minerals with so-called water of crystallization give water when heated in a closed tube at a comparatively low temperature (100-150 C.). With hydrous sulphates of iron, copper, and aluminum the water has an acid reaction which is due to the SO 8;iven off. / 2. /: 1 salts and basic salts give water at comparatively high temperatures (usually above 150 C.). Iron, Fe. 1. On charcoal in R.F., especially with sodium carbonate, iron minerals become magnetic. (This test must be tried after the assay has become cold.) Cobalt and nickel compounds give a similar test. 2. In O.F. the borax bead is amber colored, and in R.F., pale green. 3. NH 4 OH precipitates brownish-red Fe(OH) 3 from solutions containing ferric iron. A few drops of HNO 3 should always be added to the solution to insure oxidation of the iron to the ferric condition. 4. To detect state of the iron, a borax bead made blue with CuO (or malachite) is changed to opaque red by a ferrous com- pound, and to green by a ferric compound. (Use a neutral flame.) 5. To detect the state of iron in insoluble minerals (especially silicates), fuse powdered mineral with a large excess of borax in a test-tube. Break the tube and dissolve finely powdered contents in HC1. Divide the solution in two portions and test one with K4Fe(CN) 6 (ferric compounds give a deep blue precipitate) and the other with K 3 Fe(CN) 6 (ferrous compounds give a dark blue precipitate). CHEMICAL PROPERTIES OF MINERALS 47 Lead, Pb. 1. On charcoal with sodium carbonate in R.F. a malleable button of lead and a yellow coating of PbO. PbS also gives a white coating of PbSO4 2. On plaster with iodid flux, lead compounds give a lemon- yellow coating. 3. From nitric acid solutions containing lead, HC1 precipitates PbCl 2 , which is soluble in the hot solution, but recrystallizes on cooling the solution as white acicular crystals with adamantine luster. Lithium, Li. 1. A purplish-red flame, most intense at first. 2. For separation from the other alkalies, see item 34, page 40. Magnesium, Mg. 1. In the presence of NH 4 OH and NH 4 C1, Na 2 HPO 4 precipi- tates NH 4 MgP0 4 '6H 2 0, which forms slowly. The solution should be cold. Other metals (except alkalies) must be absent as they also give precipitates. 2. White magnesium compounds give a pink color when ignited with cobalt nitrate solution. (This test is not very satisfactory). Manganese, Mn. 1. The sodium carbonate bead is bluish-green and opaque (a very delicate test). 2. The borax or NaPO 3 bead is amethyst colored in O.F. and colorless in R.F. Large amounts of iron interfere with this test. 3. With HC1 manganese dioxids give off chlorin, a gas recog- nized by its penetrating odor. Mercury, Hg. 1. In closed tube with dry sodium carbonate, mercury com- pounds give metallic globules of mercury. 2. On plaster with iodid flux a scarlet sublimate when gently heated. If overheated, the sublimate is dark greenish-yellow. 48 INTRODUCTION TO THE STUDY OF MINERALS 3. Most mercury compounds rubbed on a copper coin with HC1 give a white amalgam. Molybdenum, Mo. 1. The NaP0 3 bead is green in R.F., but colorless in O.F. Several R.F. beads dissolved in HC1 with tin give a brown solution. 2. Na 2 HP0 4 gives a yellow precipitate with hot nitric acid solutions of molybdenum compounds. Nickel, Ni. 1. The borax bead in O.F. is violet when hot, reddish-brown, when cold; while in R.F. the bead is turbid gray. 2. With nickel solutions NaOH gives a pale green precipitate which is insoluble in excess. With NH 4 OH a precipitate is formed which is soluble in excess to a pale blue solution (fainter than copper). 3. A one per cent, solution of dimethyl-glyoxime in alcohol added to an alkaline solution containing nickel, will give a red precipitate. Niobium, Nb. 1. When fused with borax and then dissolved in HC1, the addition of metallic tin gives a deep blue solution similar to that obtained for tungsten; but, unlike the latter, the color disappears on the addition of water. Nitrogen, N. 1. In the closed tube with KHSO 4 , nitrates give brown-red fumes of N02- 2. A concentrated solution of FeS0 4 added to a solution of a nitrate in concentrated H 2 S04 gives a brown ring. Oxygen, O. No direct tests for oxygen are easily made. The dioxids of manganese dissolve in HC1 with the evolution of chlorin, which is recognized by its odor and by its bleaching effect on litmus paper. CHEMICAL PROPERTIES OF MINERALS 49 Phosphorus, P. 1. An excess of (NEU^MoC^ added to a hot nitric acid solution of a phosphate gives a yellow precipitate which is soluble in NH 4 OH. The solution should be only slightly heated, for arsenates give a similar precipitate on boiling. 2. Most phosphates give a bluish-green flame when moistened with H 2 S0 4 . Platinum, Pt. 1. Metallic platinum is insoluble in any single acid, but soluble in aqua regia. In rather concentrated, slightly acid solutions, KC1 gives a yellow precipitate, K 2 PtCl 6 , insoluble in alcohol. Potassium, K. 1. Violet flame, masked by sodium, but visible through a blue glass. (Merwin's flame-color screen is better than a blue glass.) Potassium in silicates may be detected by fusing with Na 2 C0 3 and observing the flame through a blue glass or screen. 2. Sodium cobaltic nitrite, Na 3 Co(N0 2 )6, (see p. 25), gives a yellow precipitate insoluble in alcohol. 3. With H 2 PtCl 6 , potassium solutions give a yellow crystalline precipitate (K 2 Ptde) insoluble in 95 per cent, alcohol. Silicon, Si. 1. In the NaP0 3 bead, silica and the silicates are partially dissolved and usually leave a translucent mass or skeleton of SiO 2 . (A few other minerals such as corundum are soluble with difficulty.) 2. With a small amount of sodium carbonate, silica effervesces and forms a clear mass. The equation is: Na2CO 3 + Si0 2 = Na 2 SiO 3 + C0 2 . 3. Some silicates dissolve in HN0 3 or HC1, and on evaporation leave either a gelatinous mass or a slime of silicic acid. 4. For insoluble silicates a sodium carbonate fusion must be made. The finely powdered mineral is fused on platinum foil, 50 INTRODUCTION TO THE STUDY OF MINERALS or a spiral loop of platinum wire, with three to four parts of sodium carbonate. The fused mass is dissolved in dilute HN0 3 and carefully evaporated just to dry ness. After adding dilute HC1 and boiling, the insoluble silica is filtered off. The filtrate contains the metals, which are commonly Al, Fe, Ca, and Mg. The following is a scheme of separation: Add NH 4 OH and NH 4 C1 Fe( OH) 3 ,Al(OH; 3 : A1(OH) 3 is soluble in KOH. (boiling) Add (NH 4 ) 2 C 2 (X (hot) CaC 2 4 . (cold) Add Na 2 HP0 4 M gNH 4 P0 4 .6H 2 0. 5. For the detection of alkalies in silicates, the very finely powdered substance is intimately mixed with five parts of CaCO 3 and one part of NH^Cl and fused for some time on platinum foil. The sintered mass is digested in hot water and filtered. NH 4 OH and (NH 4 ) 2 CO 3 are added to the filtrate. The precipitate is filtered off and the filtrate evaporated to dryness. The residue is ignited until all the ammonium salts are volatilized, and then the residue is dissolved in a little water. On the addition of H 2 PtCl 6 and 95 per cent, alcohol, a yellow precipitate indicates K. The filtrate is evaporated to dryness and the flame tested for Na. Silver, Ag. 1. With soda on charcoal in R.F., silver minerals yield malle- able metallic globules of silver, which may be tested as under 3. 2. For the blowpipe silver assay see page 34. 3. On the addition of HC1 nitric acid solutions of silver minerals give a white curdy precipitate which changes to violet on exposure to light and is soluble in NH 4 OH. Sodium, Na. 1. Intense yellow flame masked by a thick blue glass (or Merwin flame-color screen) . This is such a delicate test that only an intense and prolonged coloration indicates sodium as an essential constituent, CHEMICAL PROPERTIES OF MINERALS 51 2. Sodium in insoluble silicates may be detected by the method given under Silicon, note 5. Strontium, Sr. 1. Strontium compounds give a crimson flame, especially with HC1. 2. Dilute H 2 SO4 gives a white precipitate, SrSO-i, with dilute strontium solutions. Barium solutions give the same test, and may be distinguished by the fact that K 2 CrO4 precipitates BaCrO4 from an acetic acid solution while the strontium remains in solution. Sulfur, S. A . Sulfids and Sulfo-salts. 1. The finely powdered substance fused with three parts of sodium carbonate on a sheet of mica (or platinum foil) gives a mass which stains a moistened silver coin. Tellurids and selenids give the same test. 2. In the closed tube some sulfids (e.g. pyrite) give a yellow sublimate of sulfur. 3. In the open tube sulfids give off S0 2 , a colorless gas with the odor of burning matches. 4. A few sulfids (e.g., sphalerite) dissolve in HC1 with the evolution of H 2 S. 5. Sulfids are oxidized to sulfates by nitric acid with the evolu- tion of brown-red fumes of N02- The solution may be tested as under 7 below. B. Sulfates. 6. A sulfate, powdered and thoroughly mixed with 3 parts of soda and a little charcoal powder, is fused on charcoal in R.F. The fused mass will stain a moistened silver coin. Sulfids give the same test, so it is necessary to try the test on mica or platinum first. 7. Sulfate solutions with BaCl 2 , give a white precipitate which is insoluble in HC1. If the mineral is insoluble in acids it must 52 INTRODUCTION TO THE STUDY OF MINERALS be fused with NaoC0 3 , and the water solution of the fusion used. 8. Sulfate solutions will give the microchemical gypsum test with a calcium salt (calcite dissolved in HC1 is convenient). See Calcium, note 2, page 43. Tellurium, Te. 1. A powdered tellurid added to hot concentrated H 2 S04 gives a fine red-violet coloration. 2. Tellurids give a pale bluish-green flame coloration and a white sublimate on charcoal. 3. On plaster with bismuth flux, a purplish sublimate is ob- tained with tellurids. (Like that for bismuth except that there is no underlying scarlet.) Tin, Sn. 1. Cassiterite, wrapped in zinc shavings or placed on a sheet of zinc, and treated with dilute HC1 becomes coated with metallic tin. 2. Tin compounds heated on charcoal in O.F. give a straw- colored coating, Sn0 2 . On addition of Co (NO 3)2 solution and heating in R.F., a bluish-green coloration results. 3. Tin compounds fused on charcoal with sodium carbonate and a little sulfur in strong R.F. give malleable metallic buttons of tin which are oxidized by HN0 3 to a white insoluble powder, H 2 Sn0 3 . Titanium, Ti. 1. The NaP0 3 bead saturated with the finely powdered mineral is violet in R.F. and colorless in- O.F. 2. Fused with Na2CO 3; dissolved in HC1, and the solution heated with metallic tin, titanium compounds give a violet- colored solution due to the formation of TiCl 3 . The solution is usually turbid due to the formation of metatitanic acid, H 2 TiO 3 . 3. A yellow coloration results when a solution of hydrogen peroxide is added to the substance fused with KHS04. This is a very delicate test. CHEMICAL PROPERTIES OF MINERALS 53 Tungsten, W. 1. The NaPO 3 bead is blue in R.F., colorless in O.F. Iron interferes and gives a red bead in R.F. 2. NaP0 3 beads treated on charcoal in R.F. with tin are dis- solved in HC1 and on the addition of metallic tin, a deep blue solution results. Niobates give a similar test but the blue coloration disappears on the addition of water in this case. 3. With soluble tungstates HC1 gives a yellow residue, WO 3 , which is soluble in NH 4 OH. On the addition of tin and boiling, the precipitate becomes blue. Uranium, U. 1. The NaP0 3 bead is a fine green in R.F. and yellowish-green in O.F. Vanadium, V. 1. The NaPO 3 bead is a fine green in R.F.; light yellow in O.F. 2. In the closed tube with KHSOi, vanadates give a yellow mass. Water (see Hydrogen). Zinc, Zn. 1. On charcoal with sodium carbonate, zinc compounds give a white coating which is yellow when hot. With silicates the addition of borax helps. 2. Zinc minerals, when moistened with Co (NO 3)2 solution and intensely ignited, assume a bright green color which is due to the formation of cobalt zincate. Zinc silicates give a blue color like aluminum compounds, but if tried on charcoal the sublimate will turn green. 3. (NH 4 ) 2 S precipitates ZnS in alkaline solutions which is remarkable as being the only insoluble white sulfid. Zirconium, Zr. 1. An HC1 solution of a soda fusion turns turmeric paper orange color. This test is like that for a borate, the absence of which must be proved. THE MORPHOLOGICAL PROPERTIES OF MINERALS Minerals may occur in two essentially different conditions or states: (1) the crystalline and (2) the amorphous. In a crystal- line substance many of the physical properties such as cleavage and hardness, for example, vary with the direction, while on the other hand, in an amorphous substance the physical properties are the same in all directions. Of the various directional or vectorial properties, some vary continuously and can be repre- sented by a curve, while the others have sharp breaks and so are called discontinuous. (See Figs. 299 and 300 on page 148.) A crystalline substance is a homogeneous substance with discon- tinuous vectorial properties (Friedel.) Cleavage is one of the prominent discontinuous vectorial properties. Crystalline sub- stances when formed under favorable conditions in a free space usually take on a geometric form characteristic of the substance. Such crystals are said to be euhedral. Rock crystal (quartz) furnishes a good example. An irregular fragment of quartz is still a crystal, for it has exactly the same physical and chemical properties as the perfect geometric form. A crystal without external faces is said to be anhedral, while crystals with imper- fectly developed faces are said to be subhedral. These terms are necessary in order to avoid the ambiguity in the use of the term crystal. 1. THE AMORPHOUS CONDITION Amorphous substances in free spaces assume a more or less spherical form. In this respect they resemble liquids, for the shape of a liquid free from external influences is spherical. For example, olive oil in a mixture of alcohol and water of exactly the same specific gravity takes on the form of spheres. A variety of opal from Tateyama, Japan, occurs in small spheres. But in most cases amorphous minerals formed in free spaces are in- 54 MORPHOLOGICAL PROPERTIES OF MINERALS 55 fluenced by gravity and for this reason they appear in mam- millary (hemispherical protuberances), botryoidal (more or less separated spheres like a bunch of grapes), and stalactitic (pen- dant like icicles) forms. Colloform is a general term coined by the author to cover all these forms. A colloform structure is also assumed by microcrystalline minerals such as chalcedony. The forms assumed by all amorphous minerals are practically the same, while on the other hand the crystals of each mineral are characteristic of that mineral. The amorphous minerals are hardened hydrogels; all are probably colloidal in origin- (See ~P- 17.) 2. THE GENERAL PROPERTIES OF CRYSTALS In beginning the study of crystals, the student's attention may be directed to crystals of the common minerals such as calcite (Figs. 158-169), quartz (Figs. 180-183), pyrite (Figs. 113- 120), gypsum (Figs. 224-227), and orthoclase (Figs. 212-215). Then, for the time neglecting how they were formed and what they are composed of, their form or geometrical properties may be considered. Euhedral crystals are naturally formed solids bounded by flat, more or less smooth surfaces called faces, which are the result of an internal structure. (The surfaces on cut gems are known as facets.) Intersections of two faces are called edges, and intersections of three or more faces are called vertices. The faces of crystals vary greatly in number, in shape, and in position. On many crystals it will be noticed that there are several kinds of faces. All the faces of one kind on a crystal constitute a form. For example, in Fig. 13 the top and bottom six-sided faces constitute one form, and the six rectangular faces another form. Some crystals have only a single form, but most of them are combinations of two or more forms. It is the great number of combinations possible that gives the variety to crystals, for there is practically no limit to the number of forms possible on a crystal. 56 INTRODUCTION TO THE STUDY OF MINERALS The arrangement of crystal faces in belts of planes with parallel intersection edges called zones is a notable feature of most crystals. For example, the six faces of Fig. 37 constitute a zone. One of the most striking and important properties of crystals is the recurrence of the faces, edges, and vertices according to some fixed law. This property is known as symmetry. It varies for different kinds of crystals, and is the basis for the classi- fication of crystals. On looking over a number of crystals one might fail to see any order, system, or regularity so great is their variety, and might decide that crystals are fortuitous solids. But such is not the case; for between the faces, angles, and zones of crystals there exist exact mathematical relations. Given the angles between a few faces of a crystal, the angles between any two of the many crystal faces possible may be calculated. Crystal faces intersect only at certain definite angles. A facet cut at random on a crys- tal is not a crystal face, for the faces are the result of a definite internal structure. The practical importance of crystallography lies in the fact that a given mineral or artifically prepared compound often occurs in crystals characteristic of that substance, and hence the crystal form may be used in the determination of the substance. 3. THE MEASUREMENT OF CRYSTALS The angles on any crystal are the plane angles of the faces, the interfacial or dihedral angles over the edges, and the solid or polyhedral angles at the vertices. On account of the difficulty of accurate measurement, the plane angles, though characteristic, are little used. The measurement of interfacial angles is the start- ing-point in the description and determination of crystals. An interfacial angle (dihedral angle of geometry) is defined by the plane angle that is formed by cutting a plane normal to the inter- section edge of the two faces. It will be noticed that there are two possible angles to measure: an internal and an external or MORPHOLOGICAL PROPERTIES OF MINERALS 57 supplement angle. For various reasons 1 the supplement angle is the one used in crystallography. In a hexagonal prism, for example, the interfacial angle is read 60 instead of 120. The interfacial angle may be measured approximately by means of a contact goniometer, which, in the simplest type, consists of a semicircular cardboard protractor provided with a celluloid arm (Fig. 5) . The plane of the protractor is placed perpendicular to FIG. 5. Contact goniometer. the intersection edge. One face of the crystal is brought in contact with the arm and the protractor is revolved until the other face is parallel to, but not quite in contact with, it. For more accurate work, especially on minute crystals, the reflection goniometer is used. The principle of measurement is as follows: if a bright face of a crystal is held close to the eye, a 1 U) The sum of the supplement angles in any zone is equal to 360. obtained directly from the reflection goniometer are the supplement angles, to estimate the supplement angle with the eye. (2) The angles (3) It is easier 58 INTRODUCTION TO THE STUDY OF MINERALS reflection of a distant object such as a window bar may be obtained. If the crystal is turned about, reflections are obtained from other faces. The angle through which the crystal is revolved to obtain the images from two adjoining faces is the external or supplement angle. The reflection goniometer, the invention of Wollaston in 1809, originally consisted of a vertical graduated circle with a horizon- FIG. 6. Reflection goniometer. tal axis bearing the crystal carrier. In the modern type of goni- ometer the graduated circle is horizontal and the axis of revolu- tion is vertical. Fig. 6 shows a convenient type of goniometer for student work. A central axis s bears a crystal carrier (axis of the graduated circle) with adjustments which are two sliding motions (q and r) at right angles, and two tipping motions on circular arcs at right angles (o and n). A collimator A with a biconcave slit at the end, and a telescope B which may be set at any angle to the collimator, complete the equipment. A source MORPHOLOGICAL PROPERTIES OF MINERALS 59 of light, such as a Welsbach burner, placed at the end of the collimator furnishes a beam of light which is reflected from the crystal, when in a certain position, into the telescope. Looking into the telescope one sees a biconcave-shaped image (Fig. 7) which may be bisected by the cross-wires in the telescope. Then the reading on the vernier of the graduated circle is taken. The crystal carrier with the mounted crystal is revolved until an image is reflected from another face, and so on for all faces of the zone. A separate set-up must be made for each zone. Another type of goniometer is the two-circle or theodolite goniometer which consists of two graduated circles at right angles. Two angles, one corresponding to the longitude and the other to the co- latitude of a place on the earth's surface, are obtained for each face. The advan- tage of the two-circle goniometer lies in the fact that only one set-up is required for 'all the faces on one-half of a crystal, but the disadvantage is that in monoclinic and triclinic crystals measurements are FIG. 7. Image obtained , j . with reflection goniometer. not always made in zones. In the case of small crystals with dull faces the polarizing microscope with rotating stage may be used to advantage in measuring angles (see Fig. 332, page 174). A simple reflection goniometer may be made from the card- board contact goniometer by fitting a wooden axis through the eyelet, the axis being provided with a wire pointer. Fig. 8 illustrates this device. By holding the goniometer, with the crystal mounted on the end of the axis with wax, so that the intersection edge of the faces is in line with the axis, an image of a distant object, such as a window-bar, on a crystal face is made to coincide with the edge of a table or similar line of reference. The reading of the pointer is taken. Then after obtaining the same image again, the goniometer is held firmly and the axis carrying the crystal is rotated until a similar image 60 INTRODUCTION TO THE STUDY OF MINERALS is obtained from an adjacent face. The supplement angle is the difference between the two readings, and so on for other faces of the zone. As the protractor includes but 180, only part of the zone can be measured at one time. 4. THE SYMMETRY OF CRYSTALS The repetition or recurrence of the faces, interfacial angles, and vertices of crystals in accordance with some fixed law is called symmetry. Symmetry is perhaps the most im- portant property of crystals, for among natural objects it is a pro- perty peculiar to crystals (that is, if the term is used in an exact mathe- matical sense) and besides furnishes the basis for the classification of crystals. At the same time it should be emphasized that a few crystals lack symmetry of any kind (e.g., hydrous cal- FIG. 8. Simple reflection goniometer. cium thiosul- fate, CaS 2 3 -6H 2 O). (See Fig. 9.) The symmetry of a crystal may be defined by the operations necessary to bring it into coincidence with its original position. The symmetry operations are rotation about an axis, reflection in a plane, a combination ,. .,, . / FIG. 9. A crystal (Ca- of rotation with reflection (rotatory-reflec- s 2 o 3 -6H 2 O) devoid of tion), and inversion about the center. symmetry. If a solid can be revolved about some line through its center so that similar faces recur a certain number of times in a complete revolution, that line is called an axis of symmetry (denoted by MORPHOLOGICAL PROPERTIES OF MINERALS 61 A n ). In crystals the period of the axis (the value of n in the symbol A n ) or the number of times of recurrence is either two FIG. 10. FIG. 11. FIG. 12. Figures illustrating axes of symmetry. FIG. 13. (2), three (3), four (4), or six (6). An axis about which similar faces recur every 180 is said to be a two- fold axis (A 2 ); every 120, a three-fold axis (A 3 ) ; every 90, a four -fold axis (A4) ; and every 60, a six-fold axis (A 6 ). Figs. 10, 11, 12, and 13 illustrate these various axes of symmetry. The vertical lines through the centers of the lower figures are the axes of symmetry. The plane figures above are plans showing the amount of rotation neces- sary to bring the figures into self-coincidence. A plane that divides a solid into two parts so that similar faces occur on opposite sides of the plane is called a plane of symmetry (denoted by P). Unless the crystal is mis-shapen, one half is the mirror image of the other half. Fig. 14 represents an orthoclase FIG. 14. Plane of symmetry. 62 INTRODUCTION TO THE STUDY OF MINERALS crystal in which the shaded area is a plane of symmetry. The faces are either perpendicular to this plane, or occur in pairs, one on each side of it. A crystal may have a number of planes of symmetry. A cube, for example, has nine, three parallel to opposite faces and six through opposite edges. A solid is said to have a center of symmetry (denoted by C) if a line drawn from any point through the center encounters an exactly similar point on the opposite side. The operation is called inversion. Figure 15 represents a crystal of axinite with a center of symmetry. Every face has a similar parallel face on the opposite side of the crystal. This is the easiest test for a center of symmetry. The recurrence of similar faces by rota- tion about an axis, combined with reflec- tion in a plane normal to the axis, is called composite symmetry. The two operations, rotation and reflection, take place simul- taneously; therefore the symbol & n is used. The period of the axis is always even and in crystals the two possible cases are ^4 and ./Pe. In Fig. 16 the vertical line is an axis of 4-fold composite symmetry, for the upper part of the crystal revolved 90 becomes a reflection of the lower part. Similarly the vertical line in Fig. 17 is an axis of 6-fold composite symmetry, for the crystal revolved 60 becomes a reflection of the lower part. 1 It should be observed that ^ includes A 2 , and ^ 6 includes A 3 , so these may be written > 4 (A 2 ) and > 6 (A 8 ). In Fig. 15 the front part of the crystal when revolved 180 be- comes the reflection of the rear part (dotted lines). This is true, however, of any direction in the crystal, so that & 2 becomes < ^2. It is more logical to use C than JP 2 , for a single opera- tion (inversion) is involved. 1 A twin-model of calcite with {OOOl} as twin-plane may be used to show composite 6- fold symmetry. FIG. 15. Center of symmetry. MORPHOLOGICAL PROPERTIES OF MINERALS 63 Center, axis, plane, and composite axis with plane are collec- tively known as elements of symmetry. In crystals the ele- ments of symmetry are combined in various ways. With the limitation imposed by the rationality of indices, or with the assumption of a crystal structure made up of particles at small, finite distances apart, only ax"es of 2-, 3-, 4-, and 6-fold symmetry are possible, and in fact no other axes of symmetry have ever been found in crystals. The elements of symmetry, then, are as FIG. 16. FIG. 17. Figures illustrating composite symmetry. follows: A 2 , A 3 , A4, AG, P, C, -5*4, and JP&. Various methods of combining the elements of symmetry with each other lead to the result that only thirty-one combinations are possible among crystals. These thirty-one combinations of symmetry elements plus the crystal division without any symmetry constitute the thirty-two crystal classes. L* 4^ \* , so In the above discussion the term similar faces has been used so often that an explanation is necessary. By similar faces are meant faces which are more or less alike in shape, size, and ap- pearance. On crystals which have been formed quietly without 64 INTRODUCTION TO THE STUDY OF MINERALS disturbing influences, similar faces have the same size and shape. But on account of various external influences similar faces are rarely exactly of the same size and shape. The effect of external influences may be illustrated by alum which crystallizes in octa- hedrons. Alum crystallizing on the bottom of a beaker will form in more or less flattened octahedrons like Fig. 18, while if it crystallizes about a weighted string suspended in the solu- tion, the crystals will be more or less perfect octahedrons like Fig. 19. The irregularity in the size and shape of similar faces is one difficulty in the study of crystals. While the faces may vary, the FIG. 18. FIG. 19. Alum crystals. angles are constant (within certain limits), as is expressed in the law of constancy of interf acial angles : In all crystals of the same substance, the angles between corresponding faces are constant. (Steno, 1669.) In order to determine the symmetry of a crystal it is necessary in many cases to measure the interfacial angles. Thus the crystals represented in cross-section by Figs. 20, 21, 22 are bounded by hexagonal prisms and have an axis of six-fold symmetry if the interfacial angles are all 60. On the other hand, the crystal represented in cross-section by Fig. 23, though apparently a hexagonal prism, is a combination of two forms (a rhombic prism and a pinacoid), and has an axis of two-fold MORPHOLOGICAL PROPERTIES OF MINERALS 65 symmetry and not one of six-fold symmetry because the inter- facial angles are 62 and 56 instead of 60. Another property of crystals used in determining symmetry is the physical character of the faces; and for this reason geomet- rical crystallography is by no means merely a branch of ge- ometry. Similar faces are those with the same luster, the same 62 60' 60 60 FIG. 20. FIG. 21. FIG. 22. kind of striations, pits, or other markings. Geometrically a cube has nine planes of symmetry, three parallel to the cube faces, and six through opposite cube edges, but a cube of pyrite with striations like those of Fig. 24 has only three planes (those parallel to the cube faces). A crystal of sphalerite represented FIG. 24. Pyrite. FIG. 25. Sphalerite. FIG. 26. Apophyllite. by Fig. 25 is geometrically an octahedron, but from the stand- point of crystallography it is a combination of two tetrahedrons, one with smooth faces, the other with striated faces. Figure 26 illustrates another good example of this kind. Apophyllite occurs in crystals which are apparently cubes modified by the octahedron. Close examination, however, shows that the side 66 INTRODUCTION TO THE STUDY OF MINERALS faces are striated and have a vitreous luster, while the top and bottom faces are smooth and have a pearly luster. The forms, then, are a pinacoid and square prism (tetragonal prism) instead of a cube. The apparent octahedron is in reality a double- ended square pyramid (tetragonal bipyramid in the language of crystallography.) A more general method of determining the symmetry of a crystal is by means of etch-figures. When a crystal is acted upon by a solvent, the action is not uniform, but begins at certain points and proceeds more rapidly in some directions than in others. If the action is stopped at the right time, the faces of the crystal are usually found to be covered with little angular _ figures of definite shape and orientation called etch-figures. The etch-figures are usually shallow depressions bounded by minute faces, but in some cases they are elevations. The fact that these faces are often general forms (see p. 72) enables one in many cases to determine the crystal class. Without etch- ing it would have been impossible to assign man y crystalline substances to their proper crystal class. For example, the representa- tives of classes 9, 12, 23, and 24 (see p. 80,) were assigned to these classes, solely on account of the etch-figures and the assign- ment of many crystals, both minerals and prepared compounds, to their crystal class has been checked by the etch-figures. For example, the etch-figures on an etched prismatic crystal of nepheline is assigned to the hexagonal pyramidal class (A 6 ) solely on account of the etch-figures (Fig. 27). The shape of the etch-figures varies with the solvent, time, and temperature, but whatever their shape they are practically always the same in symmetry. On similar faces the etch-figures are alike and on dissimilar faces they are unlike, hence we have an exact method of determining the forms present on the crystal (see Fig. 28). The faces of etch-figures lie in well developed F on MORPHOLOGICAL PROPERTIES OF MINERALS 67 zones, but they often have high indices. There is no rule to follow in obtaining etch-figures as it is simply a question of ease of solution. Crystals soluble in water may furnish them by passing a moistened cloth over the surface, while some refractory minerals such as topaz require fused potassium hydroxid. Crystals are sometimes found with natural etch-figures. Diamond crystals, for example, very often have triangular etch- figures on the octahedral faces (see Fig. 378, p. 214). Closely related to etch-figures there are often found growth- figures produced, not by solution, but by growth, and these may FIG. 28. Etch-figures on diopside. (Modified from Ries.) FIG. 29. Growth-figures on quartz crystal. indicate the symmetry of the crystal. On quartz, for example, these are sometimes found on the rhombohedral f aces r{ 1011 J and z{OlTl) as illustrated in Fig. 29. Optical characters are also useful in determining the true symmetry of a crystal. For example, stilbite crystals (see Fig. 574, p. 409) are apparently rhombic bipyramidal, but optical examination of a thin-section parallel to the cleavage face b proves them to be monoclinic prismatic crystals twinned on the c face, i.e., they are composite crystals made up of two individuals. 68 INTRODUCTION TO THE STUDY OF MINERALS 6. THE FORMS OF CRYSTALS The similar faces of a crystal constitute a form, the word "form" being used here in a special technical sense. Similarity FIG. 30. Pedion. FIG. 31. Pinacoid. FIG. 32. Dome. FIG. 33. Sphenoid. of faces on some crystals may be observed at a glance, but for others not only careful examination and measurement, but also ^~~~ s v^ ~~^H^>i X - ^ P^l H^ i i k b^-J i -j Is-.---... ^ "-i - f ~ - FIG. 34. FIG. 35. FIG. 36. FIG. 37. Rhombic prism. Trigonal prism. Tetragonal prism. Hexagonal prism. etching with some solvent is necessary. Similar faces will have the same kind of etch-figures as has been mentioned in the pre- ceding section. MORPHOLOGICAL PROPERTIES OF MINERALS There are many kinds of forms. The most logical method is to name the forms according to geometrical principles, regardless of their position with respect to axes of reference (see p. 73). A single face is called a pedion (Fig. 30). Two parallel faces constitute a pinacoid (from the Greek word for a board) (Fig. 31). A form composed of two non-parallel faces is known as a dome (from the Latin word for house) if astride a plane of symmetry (Fig. 32), but a sphenoid 1 (from the Greek word for wedge), if not astride a plane of symmetry (Fig. 33). Y. ...I ..----,-1 FIG. 38. FIG. 39. FIG. 40. Ditrigonal prism Ditetragonal prism. Dihexagonal prism. Next we have three, four, six, eight, or twelve similar faces in one zone. These are called prisms, and are distinguished accord- ing to their cross-section as trigonal (Fig. 35), rhombic (Fig. 34), tetragonal (Fig. 36), hexagonal (Fig. 37), ditrigonal (Fig. 38), ditetragonal (Fig. 39), and dihexagonal (Fig. 40). Pyramids are forms consisting of three or more similar faces intersecting in a point. They are defined by the shape of the cross-section just as the prisms are. See Figs. 41 to 44, and 49 to 51. Bipyramids 1 These two (dome and sphenoid) are known by different names because one results from reflection in a plane, and the other from revolution of 180 about an axis. FIG. 45. Rhombic. FIG. 46. Trigonal. FIG. 47. Tetragonal. FIG. 48. Hexagonal. MORPHOLOGICAL PROPERTIES OF MINERALS 71 are double-ended forms which may be imagined to be formed by placing two pyramids end to end. They are defined by cross- section just as prisms and pyramids are. See Figs. 45 to 48 and 52 to 54. FIG. 55. FIG. 56. FIG. 57. Trigonal trapezohedron. Tetragonal trapezohedron. Hexagonal trapezohedron. Trapezohedrons are double-ended forms with the symmetry A n -nA 2 . They are distinguished as trigonal (Fig. 55), tetragonal (Fig. 56), or hexagonal (Fig. 57), according to the period n of the axis A n . Bisphenoids are forms consisting apparently of two sphenoids placed together symmetrically. They are called rhombic (Fig. 62), or tetragonal (Fig. 16), according to cross-section. A rhomb ohedron is a form consist- ing of six rhombic faces, three at each end of a six-fold axis of com- posite symmetry (Fig. 17). It is like a cube symmetrically distorted along one of its diagonals. Scale- nohedrons are double-ended forms consisting of scalene triangular faces meeting in zigzag lateral edges. They are distinguished by their cross-section as tetragonal (Fig. 58) or hexagonal (Fig. 59). There are fifteen more kinds of forms which are restricted to the isometric system. Some of these, such as cube, octahedron, and tetrahedron are simple, but as most of them are rather FIG. 58. Tetragonal scalenohedron. FIG. 59. Hexagonal scalenohedron. 72 INTRODUCTION TO THE STUDY OF MINERALS complicated, their description is deferred until the isometric system is studied. Of the forty-eight kinds of forms possible, all but four have been found on crystals. The four are the tetragonal trapezohe- dron, the ditetragonal pyramid, the hexagonal trapezohedron, and the dihexagonal pyramid. They are possible forms because they each have the symmetry of one of the 32 crystal classes. See table on p. 80. The thirty-two forms which result from the symmetry opera- tions performed on a given face are general forms. The other sixteen forms result when the face occupies a special position FIG. 60. FIG. 61. Congruent tetrahedra. FIG. 62. FIG. 63. Enantiomorphous rhombic bisphenoids with respect to the elements of symmetry. Thus a hexagonal prism results in class 23 when a face is parallel to the A 6 . Of the above mentioned forms, the pyramids, prisms, pina- coid, dome, sphenoid, and pedion cannot occur by themselves; and for that reason are called open forms. All the others are called closed forms because by themselves they enclose space. Two forms are said to be congruent if one of them may be made coincident with the other by rotation. For example, the tetrahedra of Figs. 60 and 61 are congruent. Two forms are said to be enantiomorphous if they are non-superposable and the mirror-image of each other. (The right hand and the left hand, for example, are enantiomorphs) . Thus the rhombic bisphenoids of Figs. 62 and 63 are enantiomorphous. Two forms are said to be complementary when their combination is geometrically indis- MORPHOLOGICAL PROPERTIES OF MINERALS 73 tinguishable from another kind of form. For example, the two tetrahedra of Fig. 25 are complementary, for geometrically they form an octahedron. Another method of naming forms used by some crystallographers is based upon the position of faces with respect to the axes of reference (see below). Thus a pinacoid is denned as a form that cuts one axis and is parallel to the other two. A prism is defined as a form that is parallel to the vertical axis and cuts the other two. A form that is parallel to one of the lateral axes and cuts the other two is a dome. A form that cuts all three axes is in general called a pyramid. A pyramid developed at only one end of the verticle axis is known as a hemimorphic pyramid. In the monoclinic and triclinic systems the names of forms are based upon the analogy of these systems with that of the orthorhombic. For example, in the monoclinic system {hkl} is called a hemi- pyramid because there are one-half as many faces as in the corresponding form of the orthorhombic system; while { hkl} in the triclinic system is a tetarto-pyramid, as there are two faces instead of eight. In the monoclinic system { hOl } is a hemi-dome because { hOl \ in the orthorhombic system is a dome. In the triclinic system JMOJ is a hemi-prism consisting of two opposite parallel faces instead of the four faces of the prism {hkO} of the orthorhombic system. But, as was said before, the logical names of forms are based upon their symmetry and shape, and not upon their position with respect to the axes of reference. 6. THE NOTATION OF CRYSTAL FACES Crystal measurement proves that exact mathematical relations exist between crystal faces. To make use of this fact the posi- tion of crystal faces is defined by the method of analytic geome- try, which consists in referring them to three (in one case, four) suitably chosen coordinate axes passing through the center of the crystal. These axes are sometimes called crystallographic axes, but they should be called axes of reference to distinguish them from axes of symmetry. The selection of these axes is more or less arbitrary, but they are chosen so as to yield the simplest relations possible. Therefore they are usually either axes of symmetry, normals to planes of symmetry, or lines parallel to prominent edges. Any face may be defined by its intercepts on the axes of refer- ence which in the most general case intersect at oblique angles. In Fig. 64 the axes are the dot-and-dash lines OX, OY, and OZ 74 INTRODUCTION TO THE STUDY OF MINERALS intersecting at the origin, 0. The intercepts of the plane ABC (extension of the face m) are OA, OB, and OC. The intercepts of the plane HKL (ex- tension of the face n) are OH, OK, and OL. Now it has been found that the ratios OA: OH, OB: OK, andOC:OLonany one crystal are practi- cally always simple rational numbers such as 1:3, 1:2, 1:1, 3:2,2:1, B 4: 1, etc. This, the Y second fundamental law of geometrical crystallo- graphy, is known as the law of rational indices (Hauy, 1784). The ratios OA: OB: OC and FIG 64 OH: OK: OL are, on the other hand, in general irrational. 1 In Fig. 64 the ratios O A : OH = 1: 2, OB: OK = 1:1, OC:OL = 3:2. In the case of the mineral barite, the relative intercepts of some of the faces are as follows (the letters refer to Fig. 65). Face Relative intercepts Weiss symbols Miller symbols m 0.815: 1: oo d: b: oo<} 110 a 0.815: 00:00 &:*>b:t 100 u 0.815: oo :l. 313 d: >b: 6 101 d 0.815: oo :0. 656 d: oob:^6 102 I 0.815: oo : 0. 328 d: b:%6 104 c oo : co:i.313 oo# : 0=6: 6 001 o oo : 1:1.313 &\ b: 6 Oil y 0.815:0.5:0.656 &:%S:M 122 z 0.815: 1:1.313 d: b: 6 111 ' On isometric crystals even these ratios are rational and on tetragonal and hexagonal crystals two of the three ratios are rational. This fact is expressed as the law of rational symmetric intercepts (Friedel, 1905). MORPHOLOGICAL PROPERTIES OF MINERALS 75 As the expressions for the intercepts are cumbersome, a very simple method of notation is suggested by the fact that these values for different faces are in the ratio of sample rational numbers (or infinity) . We may select the expression for one of these faces as a standard, and represent the other faces by the numbers or infinity. In barite the face z with the intercepts 0.815: 1:1.313 has been taken as the unit face. This establishes the axial ratio as d:b:6 = 0.815: 1:1.313. The symbols for the other faces may be written as in the third column. These are called Weiss symbols from the name of the German crystallographer who proposed this method (1818). The general expression for a face in this method of notation is na:pb:mc, where n, p, and ra are simple numbers or fract- ions, or infinity and are called coefficients. As the order a, b, c is always understood, these letters may be omitted and as infinity is incon- . . , ,. , , , FIG. 65. Barite crystal. venient in mathematical calcula- tions, the reciprocal values of the ratios may be used. We then have the symbols of the fourth column. If OA : OB : OC are the intercepts of a unit face, the symbol of another face with the inter- cepts OH : OK : OL is hkl in the expression OH : OK : OL = ^ -~ : /I K ~T = h : k'~r ^ e tnree smi pl es t whole numbers h, k, I, that ex- press this ratio are called the Miller indices, as Miller, formerly professor of mineralogy at the University of Cambridge, was the first to make extensive use of this method. The Miller symbol hkl is a kind of algebraic expression standing for certain numbers and so is called a type symbol. Besides a face hkl that cuts all three axes of reference, we have the faces hkO, hOl, and Okl, each of which intersects two axes and is parallel to the third and /iOO (100), OfcO(OlO), and 002(001) each of which intersects one axis 76 INTRODUCTION TO THE STUDY OF MINERALS and is parallel to the other two. These constitute the seven so- called type symbols. They are represented in Fig. 66. Figure 67 represents an olivine crystal with seven actual type forms a { 100} , &{010),c{001},w{110},d {101},fc{021),p{lll}. As in analytic geometry, the front, right, and top ends of the axes are considered as positive, while the back, left, and bottom ends are considered as negative. A negative index is indicated by a line over the letter. There are eight planes which cut the axes at the same relative distances, but in different octants. They are hkl, hkl, hkl, hkl, hkl, hkl, hkl, and hkl. These symbols as just written 7 FIG. 66. The seven type faces. FIG. 67. Olivine crystal. are face-symbols, but the symbol of one face hkl may be taken to represent the form. The form -symbol is usually written with brackets {hkl} to distinguish it from a face-symbol hkl or (hkl) . In order to determine the type symbol it is only necessary to write the indices h } k, I, in the order of the axes a, b, c, and to substitute if the face is parallel to an axis. In writing type symbols the reciprocal idea may be disregarded except for the zero. The determination of the symbol involves calculation by means of trigonometry, or the corresponding graphic solution. A MORPHOLOGICAL PROPERTIES OF MINERALS 77 simple case is illustrated by Fig. 68, which represents the vertical prism zone of cerussite. Here we have a rectangular zone of (hkQ) faces, a, m, r, b, where a is (100) and b is (010), (the axes of reference are parallel to these two faces) . Assuming m to be (110), the problem is to determine the symbol of r. Move r parallel to itself until r' and m' intersect the a-axis at a common point. Then the intercept of r' on the 6-axis, it may be seen, is one-third of that of m. The intercepts of the r-face are la: lib: o6 or ^{d: ^b: %6. The Miller indices are (130) (read one, three, zero). The law of the rationality of the indices, which has been estab- lished by the measurement of thousands of crystals, is the foundation of geome- tric a 1 crystallography. After the axial ratio for a given substance has once been established by a unit face, all the other possible faces may be pre- dicted, for their interfacial angles can be calculated by the formulae of plane or spherical trigonometry. Why the symbol (111), in the case of barite for example, does not represent a face that cuts the three axes at equal lengths is one of the most difficult points for the student of crystallography to comprehend. Several illustrations may clear up this point. Imagine two cities laid out according to different plans. In one, the blocks are 475 feet long and 325 feet wide, and in the other 650 feet long and 300 feet wide. A pedestrian on inquiring about a certain building in either place might be directed to go two blocks north and three blocks east. Yet the actual distance for him to walk in the two cities would be different, for the lengths of the blocks are different. Altho the lengths of the blocks are on c\ 6 _ - FIG. 68. Graphic determination of indices. 78 INTRODUCTION TO THE STUDY OF MINERALS record in the city engineer's office, the pedestrian is not directly concerned with them, but only with the directions given him. The axial ratios for crystals are established and on record in reference books, but in the description of the various crystal faces and forms, use is always made of Miller indices or other symbols rather than of the intercepts of the faces. Another analogous case that will appeal to the student of chemistry is the law of definite proportions and the law of multiple proportions. In the chemical formulae, CuO and Cu 2 O, CuO means that there are 63.6 parts (by weight) of copper and 16 parts of oxygen, while Cu 2 means that there are 127.2 (2 X 63.6) parts of copper and 16 parts of oxygen. The atomic weights have been determined and are given in tables, but they are not expressed in chemical formulae. In order that the symbols may be as simple as possible, it has been found convenient to have six kinds of sets of axes of refer- ence; crystals of every known substance may be referred to some one of these sets. The axes of reference differ in their inclina- tions to each other and the unit lengths on the axes also differ in their relative length. For crystals with a single axis of 3-fold symmetry or 6-fold symmetry it is more convenient to use four axes of reference (see p. 102). 7. THE CLASSIFICATION OF CRYSTALS The Crystal Classes. The modern classification of crystals is based upon symmetry. Only axes of 2-fold, 3-fold, 4-fold, and 6-fold symmetry have ever been found on crystals. With this limitation it may be proved mathematically that only thirty-one combinations of symmetry elements are possible. These thirty- one divisions together with the one division devoid of symmetry constitute the thirty-two crystal classes. Examples of all of these but one (A 3 .P) have been found either among minerals, or compounds made in the laboratory. It is interesting to note that just as Mendele*ef, the Russian chemist, predicted the exist- MORPHOLOGICAL PROPERTIES OF MINERALS 79 ence and even the properties of several chemical elements by the discovery of the periodic law, so Hessel, a German mineralo- gist, in 1830 predicted the thirty-two possible crystal classes when representatives of only about half of them were known. The table on page 80 gives the name of the class, the number of faces in the general form,' the symmetry, and a typical ex- ample. The name of the form with the symbol {hkl} (h-k-h + k-l in the hexagonal system) , or the general form as it is called, gives the name to the class. In contradistinction, the other forms are called limit forms. This term may be explained by considering a pyramidal face (hkl) in the rhombic bipyramidal class. By increasing its intercept on the vertical axis it becomes steeper and steeper, its limit in this case being the prism (hkO). By decreasing its intercept on the vertical axis it becomes less steep, its limit in this case being the pinacoid (001). If its intercept on the 6-axis is increased it gradually passes into (hQl), another limit form, while if its intercept on this axis is decreased it becomes (010). Similarly by increasing its intercept on the a-axis it passes into its limit (Qkl) and then by decreasing its intercept it becomes (100). The forms corresponding to the type symbols in any class may be found from the symmetry by a graphical method. Indicate a- and &-axes by two dot-and-dash lines at right angles (oblique angles in the triclinic system). Their intersection is the projection of the c-axis. Then indicate the symme- try elements in their proper positions by the following conventions: A full line represents a plane of symmetry. A plane of symmetry parallel to the plane of the paper may be indicated by a heavy circle of convenient diameter. Denote axes of 2-, 3-, 4-, and 6- fold symmetry by small ellipses, triangles, squares, and hexagons, respectively. As an example, let it be required to find the forms represented by the type symbols of the rhombic pyramidal class with the symmetry, A-2P. In Fig. 69 the two planes of symmetry are represented by two full lines which coincide with the projection of the axes of reference a and b. Their intersection is the axis of 2-fold symmetry. Pro- jections of the faces in the upper octants are small crosses. (For faces in the lower octants circlets may be used.) Faces parallel to the vertical axis may be indicated by arrows. The general form of this class is a rhombic pyramid, for the symmetry requires (hkl), (hkl), and (hkl) to accompany 80 INTRODUCTION TO THE STUDY OF MINERALS Table of the Thirty-two Crystal Classes 1 Faces f No. Name of Class in general Symmetry Example 1 form V gl 1 2 Asymmetric 1 Pinacoidal 2 No symmetry. c (CaS 2 Q 3 -6H 2 O) Albite jl 3 4 Sphenoidal Domatic 2 2 At P (Sucrose [sugar]) Clinohedrite 5 Prismatic 4 ArP-C Gypsum 6 Rhombic bisphe- 4 3A S Epsomite 1 ^ noidal 6)0 7 Rhombic pyram- 4 A 2 -2P Calamine +3 P idal O^ 8 Rhombic bipy- 8 3A 2 -3P-C Barite ramidal 9 Tetragonal bi- 4 A 2 (>4> (Ca 2 Al 2 SiO7) sphenoidal 10 Tetragonal py- 4 A4 [B a (SbO) 2 (C 4 H4O6)- ramidal 2H 2 O] 15 11 Tetragonal scale- 8 A 2 (^P4)-2A 2 -2P Chalcopyrite (5 nohedral 1 12 Tetragonal trap- ezohedral 8 A 4 -4A 2 (NiSO4-6H 2 0) g 13 Tetragonal bi- 8 A4-P-C Scheelite H pyramidal 14 Ditetragonal py- 8 A4-4P (AgF-H 2 O) ramidal . * 15 Ditetragonal bi- pyramidal 16 A4-4A 2 -5P-C Zircon 16 Trigonal pyram- 3 As (NaIO4-3H 2 O) idal 17 Rhombohedral 6 AaC^PsJC Phenacite / 18 Trigonal trapezo- 6 A 3 -3A 2 a-Quartz hedral 19 Ditrigonal py- 6 A 3 -3P Tourmaline ramidal - 20 Hexagonal scale- 12 A 3 (JP 6 )-3A 2 -3P-C Calcite 01 nohedral A *-P . XI 22 Trigonal bi- pyramidal Ditrigonal bi- 12 A3 r Benitoite 1 23 pyramidal Hexagonal py- 6 A 8 Nepheline ramidal 24 Hexagonal tra- 12 A 6 -6A 2 /8-Quartz 25 pezohedral Hexagonal bipy- 12 A-P-C Apatite ramidal 26 Dihexagonal py- 12 Ae-6P lodyrite ramidal 27 Dihexagonal bi- 24 A6-6A 2 -7PC1 Beryl pyramidal 28 Tetartoidal 12 4A 3 -3A 2 Ullmannite n 29 Gyroidal 24 3A4-4As-6A 2 Sylvite "S 30 Diploidal 24 4Aj(4iP6)'3A2'3P-C Pyrite 31 Hextetrahedral 24 4A 3-3A2(3^P <)'6P Tetrahedrite 32 Hexoctahedral 48 3A4-4A 3 (4^ > )-6A 2 -9P-C Galena Names in parentheses are prepared compounds of the laboratory. MORPHOLOGICAL PROPERTIES OF MINERALS 81 \ Xhkl \hkl (hkl). For a face (Okl), the symmetry requires (Qkl); the form {0/cZ}, then, is a dome. Similarly { hOl } is a dome; [hkO] is a rhombic prism; {lOO} h^ and JOIO} are each pinacoids, while J001 j is a pedion consisting of a single face. In the lower half of the crystal {hkl} is a rhombic pyramid; JO/J), and {hOl}, domes; while {OOT} is a pedion. The forms on the lower half of the crystal in this case are inde- pendent of those on the upper half. Calamine, an example of a crystal belonging to the rhombic pyramidal class, is shown in Fig. 70. Here the forms are c{00l), *{30l[, i{03lj, Xhkl J^ nf Xhkl 6{010), rajllO}, and v{l2l). 010 001 hOl Okl V 100 a The Crystal Systems, ^ Although the thirty-two classes are fundamental in the classifica- F I Q - 69. Graphic method of deter- ,, , -. . , . . mining the possible forms in a crystal tion of crystals, it is convenient c i ass to assemble them in larger groups called crystal systems. Six crystal systems are generally recog- nized. It is not always possible to determine the crystal class by inspection, but the crystal system is usually apparent in well-formed crystals. If directions fixed by symmetry (either axes of sym- metry or lines normal to planes of symmetry) are chosen for axes of reference it is found that all equivalent faces are represented by indices which differ from each other only in their order of succes- sion and sign. If this be done, one symbol (enclosed in brackets) may stand for all the faces of a form. The classification of crystals into systems is largely one of convenience to bring out the relation of crystal form to physical properties, but for all prac- tical purposes in elementary work it may be said to rest upon the character of the axes of reference fixed by sym- Fio. 70 . - Calamine. 82 INTRODUCTION TO THE STUDY OF MINERALS metry. Accordingly the following six systems are recognized: Isometric, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. Crystals with three like or interchangeable directions of sym- metry at right angles to each other are referred to the isometric system. These three directions, which are either 2-fold or 4- fold axes of symmetry, constitute the axes of reference for this system. Crystals with a single 4-fold axis (or a single composite 4-fold axis, ^4) are referred to the tetragonal system. The other two axes of reference, which may, or may not, be directions fixed by symmetry, are interchangeable and are at right angles to each other and also at right angles to the principal axis. Crystals with a single 3-fold or 6-fold axis of symmetry (includ- ing JP&) are referred to the hexagonal system. Four axes of reference are usually employed, one (A 3 or Ae) at right angles to three interchangeable ones which are in one plane and intersect each other at angles of 120. The three lateral axes of reference may or may not be directions of symmetry. Crystals with three unlike or non-interchangeable directions of symmetry at right angles and no other directions of symmetry are referred to the orthorhombic system. These three directions are the axes of reference. Crystals with a single direction fixed by symmetry, not pre- viously included, are referred to the monoclinic system. This direction is an axis of reference; the other two are in a plane normal to it but are in general oblique to each other. Crystals without any directions fixed by symmetry are referred to the triclinic system. There are three non-interchangeable axes of reference, in general at oblique angles to each other. In the case of each of the six systems, at least some of the directions fixed by symmetry are used for axes of reference. If there are not enough axes of reference, then lines parallel to prominent edges or perpendicular to prominent faces are chosen. MORPHOLOGICAL PROPERTIES OF MINERALS 83 The forms of the crystal class with the highest grade of symmetry in each system are sometimes called holohedral or whole forms, while many of the forms of the classes of lower grade of symmetry are called hemihedral or half forms, because they have half the number of faces of the holohedral forms. There is a geometrical resemblance between these two kinds of forms. A tetrahedron, for example, is said to be the hemihedral form of an octahedron, for it may be derived by extending alternate faces and suppres- sing the others as shown in Fig. 71. A cube has no hemihedral form, or rather the hemihedral and holohedral cubes are geometrically identical, for the supression of alternate octants still leaves the cube. The same is true of the rhombic dodecahedron. The symmetrical suppression of the faces of the general forms of the six holohedral classes gives rise to twenty-six divisions. These, together with the six holohedral divisions, lead to the thirty- two classes before mentioned. The general forms of the five isometric classes may be derived from the hexoctahedron thus: The suppres- sion of faces of alternate octants gives the hextetrahe- dron, the suppression of alternate faces gives the gyroid, the suppression of faces in pairs astride the planes of symmetry gives the diploid, while the com- bination of any two of these methods gives a twelve- , \ G ' 7. 1 ' 7 ^ e .,, ii 1, derivation of the sided figure called the tetartoid. As this form has tetrahedron from the one-fourth the number of faces of the hexoctahedron, octahedron, it is called a tetartohedral or quarter form. The idea of hemihedrism implies that the six crystal systems are funda- mental, whereas we know that the crystal classes are more fundamental. Hence the terms involving hemihedrism are now of historical interest only. The class with the highest symmetry in each system may be called holo- symmetric instead of holohedral. 8. CRYSTAL DRAWING Before the crystal systems and classes are described in detail the method of drawing crystals will be explained. Crystal drawings are parallel projections made by drawing parallel lines from the vertices of the crystal to the plane of projection. If the projectors are perpendicular to the plane of projection, we have an orthographic projection; if the pro- jectors are inclined to the plane of projection, we have a clino- graphic projection. 84 INTRODUCTION TO THE STUDY OF MINERALS The orthographic projection is especially useful in the graphic determination of the indices of crystal faces and axial elements. An orthographic projection is easily made from the interfacial angles, without any knowledge of the axial ratios, simply by drop- ping perpendiculars from the vertices of the crystal to the plane of projection which is usually an actual or possible crystal face. All faces normal to the plane of the drawing appear as lines inclined to each other at their true angles. Horizontal edges of the crystal appear in their true length, but oblique edges are foreshortened. Orthographic projections lack the appearance of solidity given by clinographic projections, but by combining two orthographic projec- tions made on planes at right angles to each other, a plan and elevation are obtained, which together give a good idea of the crystal habit. Fig. 72 is a plan and front elevation of a topaz crystal with the forms c{ 001 } , 2/{041), ,{110), /{120}, w{lll}, and i{ 223 } . The plan 1 (top figure) was constructed by laying off the interfacial angles mm, ml, and II and by placing i and u in the same zone with c and m. The elevation (lower figure) was constructed from the interfacial angle cy and by observing zonal relations. The fact that corresponding points in the plan and elevation lie on the same vertical line greatly facilitates the construction. For example, the directions of the intersection edge ul in the elevation and iy in the plan are deter- mined automatically, provided a supplementary elevation is made. 1 The third angle projection is used. FIG. 72. Plan and elevation of topaz crystal. MORPHOLOGICAL PROPERTIES OF MINERALS 85 From one or two orthographic projections of a crystal it is possible to derive graphically the Miller indices of the faces and the geometrical constants (axial ratio and axial angles). Ex- amples will be shown under each crystal system. (See pages 97, 101, 111, 115, 120 and 123.) The orthographic projection may be used for graphic determinations but for general descrip- tive purposes (text-books and articles) clinographic projections are preferable, for they give the appearance of solidity. The clinographic parallel projection or so-called parallel per- spective is used instead of a true perspective because the paral- lelism of edges or the occurrence of crystal faces in zones is one of the prominent features of crystals and should be retained in the drawing. The clinographic projection is made on a vertical plane by inclined projectors taken so that one sees both the top and the right side of the crystal. The first step in producing a clinographic projection of a crystal is to make an isometric axial cross. The method of making an isometric axial cross is shown in Fig. 73. The upper right-hand part (a) of the figure shows the rotation of the plan of the axial cross 18 26' to the left. (This angle is chosen because its tan- gent is J-^). The left-hand portion of the figure (6) shows the projection of an elevation of the axial cross by projectors inclined 9 28' (taken because its tangent is J^j) from the horizontal on a vertical plane. The lower right-hand part of the figure (c) shows the method of obtaining the axial cross (dot-and-dash lines). The OC-axis is given in its full length, but both the OA-axis and the OB-axis are foreshortened. The isometric axial cross is modified for the other systems. In the tetragonal system the unit length on the vertical axis is either greater or less than that on the lateral axes. The unit lengths of the axial cross of a vesuvianite, for example, are: OA: OB: 0.537 X OC. In the hexagonal system there are three lateral axes. The method of determining these lateral axes is shown in Fig. 74. The line OS is made equal to 1.732 X OA (Fig. 73), and S is joined with B and B'. BS and B'S are bisected 86 INTRODUCTION TO THE STUDY OF MINERALS at the points Q and R. ROR', QOQ', and BOB' are the unit lengths of the three lateral axes. The unit length on the vertical axis is modified according to the value on record. In the ortho- rhombic system both the unit lengths on the A -axis and OC-axis FIG. 73. Construction of isometric axial cross in clinographic projection (modified from French). are modified. For example, the three values for barite are 0.815 X OA : OB : 1.313 X OC. In the monoclinic and triclinic systems the angles between the axes are also modified. For the angle between a and c the position of the a-axis is changed as follows: On the axis OC (Fig. 75) the distance OM = cos X OC is laid off and on the axis OA MORPHOLOGICAL PROPERTIES OF MINERALS 87 the distance ON = sin /? X OA is laid off. Then the 6-axis is the line QOQ' ', QO being the diagonal of a parallelogram MQNO. The lengths of the axes are modified as in the other systems. R \ FIG. 74. Hexagonal axial cross. FIG. 75. Monoclinic axial cross. FIG. 76. Linear projection of a scapolite crystal. After the axes are projected in their proper positions, crystals consisting of a single form are drawn by finding the intersection 88 INTRODUCTION TO THE STUDY OF MINERALS of the faces on the axial cross and connecting them with lines. For crystals with two or more forms, use is made of the linear projection. In the linear projection each face is represented by a line. The lines are the intersections of faces, shifted parallel to themselves so that they cut the vertical axis (c) at unity, with FIG. 77. no 100 110 010 FIG. 78. r*/ FIGS. 77-78. Clinographic projection of a scapolite crystal. the plane of projection which is a plane through the center of the crystal, perpendicular to the c-axis. Figure 76 shows a linear projection of the scapolite crystal of Fig. 78. It is necessary to plot a linear projection of the crystal on these axes by taking the reciprocal of the Miller indices and then making the third term MORPHOLOGICAL PROPERTIES OF MINERALS 89 equal to unity. The desired direction of the intersection edge of the two faces is a line joining the intersection of the linear projection of the two faces with the extremity of the vertical axis. Figure 77 shows the method of construction of the clino- graphic drawing of the scapolite crystal of Fig. 78. The dot-and- dash lines are the axes of reference; the heavy lines, the linear projection constructed on the axial cross; while the dotted lines are the directions of the intersection edges. The direction inter- section of faces like (111) and (110), which do not intersect, is simply the direction of the lines. 9. ISOMETRIC SYSTEM The isometric systems includes all crystals with three inter- changeable axes of reference at right angles. All crystals of this system have four axes of 3-fold symmetry; many of them also have three axes of 4-fold sym- metry. The axes of reference are de- signated as in Fig. 79 with 01, running front and back, a 2 , running right and left, and a 3 in a vertical position. As all isometric crystals have identi- cal angles for corresponding FlG " 79.-Isometric axes of reference. forms, there are no axial elements to be determined. There are five classes in the isometric system (see p. 80) but of these only the three that are of much practical importance will be discussed. Isometric crystals, unless much distorted, are of about equal dimensions in all directions and this fact aids in their identifica- tion. Highly modified crystals may approach a sphere in general appearance. 90 INTRODUCTION TO THE STUDY OF MINERALS Hexoctahedral Class. 3A 4 4A 3 (4> 6 ) 6A 2 9P C (Holohedral) The crystals of this class have the maximum degree of symme- try possible in crystals. The 4-fold axes are mutually perpen- dicular and lie at the intersections of three of the planes of symmetry (axial planes). The other six planes of symmetry (diagonal planes) intersect in the four axes of 3-fold symmetry. The 4-fold axes of symmetry are the axes of reference. List of Forms in the Hexoctahedral Class Cube 6 faces { 100 } Octahedron 8 faces { 111 Dodecahedron 12 faces { 110 Tetrahexahedron 24 faces { hkO Trisoctahedron 24 faces { hhl } Trapezohedron 24 faces { hkk } Hexoctahedron 48 faces { hkl } [In the above symbols h>k>l] Cube {100}. The cube (or hexahedron) is a six-faced form with interfacial angles of 90. The ideal form is shown in Fig. 80. The cube is a common form on galena, fluorite, cuprite, and halite. Octahedron {lllj. As its name implies, this is an eight-faced form. Each face is an equilateral triangle in the ideal form. FIG. 80. {100}. FIG. 81. {111}. FIG. 82. {110}. FIG. 83. {hkQ}. The interfacial angles are 70 32'. (Fig. 81.) It is a common form on magnetite, spinel, and diamond. Dodecahedron {110}. This form (Fig. 82) consists of twelve faces, each rhombic in shape. It is often called the rhombic dodecahedron to distinguish it from the regular dodecahedron of MORPHOLOGICAL PROPERTIES OF MINERALS 91 geometry, which is a crystallographically impossible form. The interfacial angles are 60 and 90. It is especially common on garnet. Tetrahexahedron {hkO(. This form is so called because it apparently consists of a four-faced pyramid on each cube face. Figure 83 represents the form {210 j . It is occasionally found on fluorite. Trapezohedron {hkk}. Each face is a trapezoid. This form is sometimes called the tetragonal trisoctahedron to distinguish it from the next mentioned form, the trigonal trisoctahedron. Figure 84 represents the form {211} which is common on garnet, leucite, and analcite. Trisoctahedron jhhl). Each face is an isosceles triangle. With this form the intercept on the third axis is greater than the FIG. 84. \hkk\. FIG. 85. [hhl\. FIG. 86. [hkl\. intercepts upon the other two, which are equal, while with { hkk } the intercept on the third axis is less than the intercepts upon the other two. Figure 85 represents the trisoctahedron { 22 1 ( , a form which occurs on some crystals of galena. Hexoctahedron {hkl}. The general form of the hexoctahedral class consists of forty-eight faces, the symbols of which may be derived from the form symbol by taking six permutations of letters and eight permutations of signs. Fig. 86 represents the hex- octahedron {321(. It sometimes occurs on fluorite crystals as illustrated in Fig. 98. Combinations. The cube, octahedron, and dodecahedron are much more common than the other forms. They occur alone and in combination with each other. See Figs. 423-426, page 267. 92 INTRODUCTION TO THE STUDY OF MINERALS The hexoctahedron, trisoctahedron, and tetrahexahedron usually occur as small faces modifying simple forms. Galena, garnet, fluorite, and magnetite are given as typical examples for study and practice. Examples Galena. Usual forms : a { 100 j , o { 1 1 1 j . Interfacial angles : aa(100 : 010) = 900'; oo(lll: 111) = 70 32'; oo(100:lll) = 54 44'. Figures 87 to 91 represent usual combinations varying from the cube alone to the octahedron alone. FIG. 87. FIG. 88. FIG. 89. FIG. 90. FIGS. 87-91. Galena. FIG. 91. Garnet. Usual forms: d{ 110}, n{ 211} . Interfacial angles: dd{ 110: 101) = 60 O';nw(21 1:121) = 3333^';/m(211:2ll) = 48 ll^';oX110:211) = 30 0'. Figures 92 to 95 represent usual combinations varying from the dodecahedron alone to the trapezohedron alone. FIG. 92. FIG. 93. FIG. 94. FIGS. 92-95. Garnet. FIG. 95. Fluorite. Usual forms : a ( 100 ) , / { 3 10 } , t { 42 1 ) . Cleavage parallel to {111}. Interfacial angles: oa(100: 010) = 90; a/(100:310) = 18 26'; 4)-6P The 2-fold axes are mutually perpendicular. The planes of symmetry are diagonal to the 2-fold axes. (Tetrahedral hemihedral) The axes of 2-fold symmetry (these are also axes of composite 4-fold symmetry, ^4) are the axes of reference. Cube Dodecahedron Tetrahexahedron Tetrahedrons Deltohedrons Tristetrahedrons Hextetrahedrons List of Forms in the Hextetrahedral Class 6 faces {100J 12 faces ) 110 } 24 faces hkO 4 faces 111 . ("I) 12 faces 12 faces hhl hkk , {hhl J }, )hkk| [In the above symbols 24 faces jhkl (, jhkl 94 INTRODUCTION TO THE STUDY OF MINERALS The first four forms are geometrically different from the cor- responding forms in the hexoctahedral class and hence are described below. Tetrahedrons {111}, {HI}. This is the regular tetrahedron of geometry the interfacial angles being 109 28' (Fig. 103). The positive and negative forms are_ exactly alike except in position. The two forms {111} and {111} in equal combination form an octahedron geometrically and therefore they are said to be complementary. The tetrahedron occurs on tetrahedrite and sphalerite. Deltohedrons jhhl} , {hhl} . These two forms are also positive and negative according to the octant in which they occur. Fig. FIG. 103. {111}. FIG. 104. {hhl}. FIG. 105. \hkk}. FIG. 106. {hkl}. 104 represents a positive form. The name refers to the deltoid shape of the faces. Tristetrahedrons {hkk}, {hkk}. These forms resemble three- faced pyramids built upon each tetrahedral face, hence the name, tristetrahedron. The two forms, which occur in alternate octants, are distinguished as positive and negative. Fig. 105 represents a positive form. Hextetrahedrons {hkl} , {hkl} . The general form is a 24-faced form called the hextetrahedron as it apparently consists of a 6-faced pyramid built upon each face of a tetrahedron. (Fig. 106.) The two forms given are distinguished as positive and negative. They occur in alternate octants. Combinations. Crystals of this class usually have a tetra- hedral aspect. The best example is furnished by tetrahedrite. MORPHOLOGICAL PROPERTIES OF MINERALS 95 Sphalerite also belongs to this class, but the crystals are usually distorted. Example Tetrahedrite. Usual^ forms: o{ 111), o^lTl}, n{ 211), d{ 110} . Inter- facial angles: oo(lll:TTl)_= 109 28'; nn(211:121) = 33 33^'; no(211: 111) = 19 28'; do(110:lll) = 35. 16'. Figs. 107 to 110 represent usual types of tetrahedrite crystals. FIG. 107. FIG. 108. FIG. 109. FIGS. 107-110. Tetrahedrite. FIG. 110. Diploidal Class. 4A 3 (4> 6 ) 3A 2 3P C (Pentagonal hemihedral) The planes of symmetry are mutually perpendicular. Their intersections are the 2-fold axes. The 3-fold axes are also composite 6-fold axes. The axes of 2-fold symmetry are the axes of reference. List of Forms in the Diploidal Class Cube 6 faces { 100 [ Octahedron 8 faces {ill} Dodecahedron 12 faces { 110 } Pyritohedrons 12 faces { hkO } , { khO } Trisoctahedron 24 faces { hhl Trapezohedron 24 faces { hkk Diploids 24 faces jhkl , {khl} [In above symbols h>k>l] Of these forms, all but the pyritohedron and diploid are geo- metrically similar to those of the hexoctahedral class. Pyritohedrons {hkO}, jkhO}. The pyritohedron is so named because it is common on the mineral pyrite. The two forms given are arbitrarily distinguished as positive and negative. On 96 INTRODUCTION TO THE STUDY OF MINERALS pyrite the most common form is the positive pyritohedron {210}, represented by Fig. 111. The faces of the pyritohedron are not regular pentagons. A form with twelve faces each a regular pentagon is impossible as a crystal form, for it has axes of 5-fold symmetry (see p. 136). Diploids {hkl}, (khl). The general form is a 24-faced form, the faces of which lie in pairs astride the planes of symmetry, hence the name, diploid, which means double. The two con- FIG. 111. \hkO}. FIG. 112. {hkl} gruent forms {hkl} and {khl} are distinguished as positive and negative. Figure 112 represents the positive diploid (321). Pyrite is the only common example of this class. Examples Pyrite. Usual forms: a(lOO}, e{210}, o{lll), s{32l}, n{21l}. Inter- facial angles: k] MORPHOLOGICAL PROPERTIES OF MINERALS 99 Pinacoid {001} (Basal pinacoid). This form consists of two parallel faces, an upper one and a lower one (Fig. 124). Tetragonal Prism {100} (Prism of the second order). This is an open form similar to {110} except in position (Fig. 125). Tetragonal Prism {110} (Prism of the first order). This is a-n open form with four faces euch parallel to the vertical axis (Fig. 126). Ditetragonal Prism {hkO[ (Ditetragonal prism). An open form consisting of eight faces, each parallel to the vertical axis (Fig. 127) . The faces meet in angles which are alternately equal. Tetragonal Bipyramid {hOlj (Pyramid of the second order). A form consisting of eight faces each parallel to one lateral axis (Fig. 128). This form and \hhl\ are identical except in position. 124(001}. 125(100}. 126(110}. 127{hkO}. 128 {MM}. 129{hhl\. 130{hkl}. FIGS. 124-130. The seven type forms of the ditetragonal bipyramidal class. Tetragonal Bipyramid {hhl} (Pyramid of the first order). This form cuts the lateral axes at equal distances (Fig. 129). The faces are isosceles triangles in the ideal form. Ditetragonal Bipyramid jhkl} (Ditetragonal pyramid). The general form consists of sixteen faces; the faces in the ideal form are scalene triangles (Fig. 130). The angles over alternate polar edges are equal. Combinations, The bipyramids are closed forms, but the prisms and pinacoids are open forms, and hence must occur in combination. In habit, tetragonal crystals are usually prismatic, pyramidal, or tabular, but equidimensional pseudo-octahedral and pseudo-cubic crystals are not uncommon. 100 INTRODUCTION TO THE STUDY OF MINERALS Examples Zircon, apophyllite, and vesuvianite are given as typical examples for study and practice. Zircon. 6 = 0.640. Usual forms: m{ 110}, a{ 100), p{ lll} L u{33l}, x{3ll}. Interfacial angles: mp(110:lll) = 47 50'; mm(110:ll0) = 90 m m FIG. 131. FIG. 132. FIG. 133. FIGS. 131-134. Zircon. FIG. 134. 0'; mo(110:100) = 45 0'; a/>(100jlll) = 61 40'; ww(110:331) = 20 12'; xx(3l 1:311) = 32 57'; pp(lll:lll) = 56 40'. Figures 131 to 134 rep- resent the usual combinations and habits. Apophyllite. 6 = 1.251. Usual forms: a) 100} , cjOOl), p{lll}, j/J310j. Cleavage parallel to c{00l). Interfacial angles: co(001:lll) = 60 32'; ap(100:lll) =52 0'; pp(lll:lTl) =76 0'; ay(100:310) = 18 26'. Figures 135 to 138 represent the usual combinations and habits. < c FIG. 135. FIG. 136. FIG. 137. FIGS. 135-138. Apophyllite. FIG. 138. Vesuvianite 6 = 0.537. Usual forms: p{ 111}, m{ 110}, a{ 100}, cjOOl}, <{33l(, s{31l}. Interfacial angles: pp(lll:lll) = 5039'; cp(001:lll) = MORPHOLOGICAL PROPERTIES OF MINERALS 101 37 13^'; ap(100:lll) = 64 40^'; am(100:110) = 45 0>; m<(110:331) 23 41M'j as(100:311) =35 10'. Figures 139 to \43 illustr^ ' crystals. ' * m FIG. 139. FIG. 140. FIGS. 139-142.- FIG. 141. -Vesuvianite. FIG. 142. Graphic Determination of Indices and Axia Ratio in the Tetra- gonal System. A plan and elevation of a zircon crystal are shown in Fig. 143. The unit bipyramid { 111 } is the p face; the problem is to determine the symbol of u and the axial ratio arc. In the elevation, lines parallel to the projections of p and u are drawn FIG. 143. Plan and elevation of a zircon crystal. through the point x to intersect the c-axis. Then the distance os is equal to 3 times the distance or. Therefore the symbol of u is Io!:la 2 :3c or (331). The distance or is equal to about 0.64 of the distance ca\ (in the plan) ; therefore the axial ratio a : c is 1 : 0.64. (It will be noted that ox is the foreshortened ca\.) 102 INTRODUCTION TO THE STUDY OF MINERALS FIG. 144. Hexagonal axes of reference. it. THE HEXAGONAL SYSTEM 1 The hexagonal system includes all crystals with a single axis of 3-fold or 6-fold symmetry. (In two classes the 3-fold axis is. also a composite axis of 6-fold symmetry.) Four axes of refer- ence, three interchangeable ones in a plane at right angles to the fourth, are used. The positive ends of the three lateral axes make angles of 120 with each other, as shown in Fig. 144. The index on the third axis is always equal to the sum of the first two with the sign changed, so that the Miller symbol for the general form is {/i-A;-/i+A;-Z},inwhich h is always greater than k. The axes may be designated 01 : o 2 : a a : c, in which the unit lengths on a\, a z , and a 3 are unity and the unit length on 6 either greater or less than unity. The axis of 3-fold or 6-fold symmetry is always taken as the c-axis. Dihexagonal Bipyramidal Class. A 6 -6A 2 -7P-C (Holohedral) The 2-fold axes are normal to the 6-fold axes. There are six vertical planes of symmetry at angles of 30 apart. The other plane of symmetry is perpendicular to these six. List of Forms in the Dihexagonal Bipyramidal Class (Basal pinacoid) (Prism of 1st order) (Prism of 2d order) (Dihexagonal prism) (Pyramid of 1st order) (Pyramid of 2d order) (Dihexagonal pyramid) [In the above symbols h>k.] 1 Classes 16, 17, 18, 19, and 20 (see p. 80) of the hexagonal system constitute a rhombo- hedral subsystem. They may be referred either to the four axes of reference mentioned or to three interchangeable axes at equal oblique angles (like the legs of a 3-legged stool). These five classes are sometimes treated as a separate system, but they are so closely related to the other seven classes that they are here retained in the hexagonal system. Pinacoid Hexagonal prism Hexagonal prism Dihexagonal prism Hexagonal bipyramid Hexagonal bipyramid Dihexagonal bipyramid 2 faces {0001} 6 faces {1010} 6 faces J1120} 12 faces (h-k-h+k-0[ 12 faces jhOhl) 12 faces )h-h-2h-l} 24 faces {h-k-h+k-l} MORPHOLOGICAL PROPERTIES OF MINERALS 103 This form consists of two usually regular hexagons. Pinacoid {0001J (Basal pinacoid). opposite parallel faces which are (Fig. 145.) Hexagonal Prism (1010) (Prism of the first order), faces are in a vertical zone and intersect at angles of 60. 146.) The (Fig. FIG. 145 {0001}. FIG. 146 {1010}. FIG. 147 {1120}. FIG. 148 [h-k-h+k-Q]. Hexagonal Prism {1120} (Prism of the second order). This form is similar to {1010} except in position. (Fig. 147.) Dihexagonal Prism jh-k-h+k-0}. All the faces are in a vertical zone, each being parallel to the vertical axis. Alternate angles are equal. (Fig. 148.) FIG. FIG. 150{h'h-2h'l}. FIG. 151{h-k-h+k-l\. Hexagonal Bipyramid jhOhl) (Pyramid of the first order). The faces cut two of the lateral axes, but are parallel to the third. (Fig. 149.) Hexagonal Bipyramid {h h 2h 1} (Pyramid of the second order). The faces cut two of the lateral axes at equal but greater dis- tances than the third lateral axis. (Fig. 150.) This form differs from jhOhl} only in position. 104 INTRODUCTION TO THE STUDY OF MINERALS Dihexagonal Bipyramid {h-k-h-fk-1} (Dihexagonal pyra- mid). This form consists of 24 faces (scalene triangles in the ideal form), each of which cuts the four axes at unequal distances. The angles over alternate polar edges are equal. (Fig. 151.) Combinations. The habit is prismatic, pyramidal, or tabular. Simple combinations are the rule in this class. As beryl is the only common mineral belonging to this class, it is the only ex- ample given for practice. Example Beryl. 6 = 0.498. Usual forms : c { OOOlj , m { 10TO } , p { 1011 } , s { 1121 } , t>{213l}. Interfacial angles: ?nm(1010:0110) = 60 0'; cs(0001:1121) = FIG. 152. FIG. 153. FIG. 154. FIGS. 152-155. Beryl. FIG. 155. 44 56'; cp(0001:1011) = 29 57'; my(1010:2131) = 37 49'; ms(1010:1121) = 52 17'. Figures 152 and 153 are the ordinary combinations. Figure 154 has in addition the general form VJ2131). Figure 155 represents beryl of tabular habit, which is rare as compared with the prismatic habit. Hexagonal Scalenohedral Class. A 8 Cfl>6)-3A2-3P-C (Rhombohedral hemihedral) The planes of symmetry intersect each other in the 3-fold axis and the 2-fold axes are diagonal to the planes of symmetry. The lateral axes of reference are the axes of 2-fold symmetry and the c-axis, the axis of 3-fold symmetry. The 3-fold axis is also a composite 6-fold axis. 1 1 It is possible to refer crystals of this class and the next two classes to three interchange- able axes of reference at oblique angles to each other. In this case, the Miller symbol has three indices hkl and the axial element is a, the oblique angle between the axes, which varies for each particular mineral. (See footnote on p. 102.) MORPHOLOGICAL PROPERTIES OF MINERALS 105 List of Forms in the Hexagonal Scalenohedral Class Pinacoid 2 faces Hexagonal prism 6 faces Hexagonal prism 6 faces Dihexagonal prism 12 faces Rhombohedrons 6 faces {0001} {1120} {1010} {hOhl}, {Ohhl} (Basal pinacoid) (Prism of 2d order) (Prism of 1st order) (Dihexagonal prism) (Rhombohedrons) (Pyramid of 2d order) (Scalenohedrons) Hexagonal bipyramid!2 faces Scalenohedrons 12 faces jh-k-h+k-l}, [In the above symbols h>k] Pinacoid {0001} (Basal pinacoid). There are two faces at opposite ends of the vertical axis. (Fig. 145.) Hexagonal Prism {1010} (Prism of the first order). There are six faces in one zone meeting at angles of 60. (Fig. 146.) Hexagonal Prism {1120} (Prism of the second order). This form is exactly like {1010} except in position. (Fig. 147.) Dihexagonal Prism (h-k-- h+kO}. There are twelve faces in a vertical zone. (Fig. 148.) Alternate angles are equal. Rhombohedrons {hOhl}, {Ohhl}. A rhombohedron consists of six rhombic faces, and is like a cube distorted in the direction of one of its diagonals. A rhombohedron is distinguished as acute or obtuse according to whether the supplement angle over the polar edges is greater or less than 90. The rhombohedron with faces in the middle front, right rear, and left rear dodecants is called positive and has the symbol {hQhl}, while the rhombohe- dron with faces in the right front, left front, and middle rear dodecants is called negative and has the symbol {Qhhl}. Figure 156, an obtuse positive rhombohedron, represents the cleavage rhombohedron of calcite. Hexagonal Bipyramid {h-h-2h-l( (Pyramid of the second order). This form consists of twelve faces, each an isosceles FIG. 156 {hOhl}. FIG. 157 [h-k-h+kt] 106 INTRODUCTION TO THE STUDY OF MINERALS triangle. (Figure 150.) Hexagonal bipyramids are very rare forms for calcite. 158 162 159 163 160 fPS m P m 164 167 168 FIGS. 158-169. Calcite. 165 169 Scalenohedrons {h-k-h+k-l(,jk-h-k+h-l}. The general form of this class is a 12-sided figure, each face of which is a scalene triangle. There are three kinds of edges : short polar, long MORPHOLOGICAL PROPERTIES OF MINERALS 107 polar, and middle edges, each with their characteristic interfacial angles. The {h-k-h+k-1} form is called positive and the {k-h k+h-1) form, negative. Figure 157 is a positive scalenohe- dron. Example Calcite. 6 = 0.854._ Usual forms:' c{0001}, ra{1010}, a {1120}, e{0ll2}, r{10ll},/{0221}, M0332}, M{4041}, VJ2131}, 2/13251}, *{2134} L Cleavage parallel to r. Interfacial angles: ee(0112: 1012) = 45 3'; em(0112:_1010) = 63 45'; rr(10Tl:Il01) _= 74 55'; m(1011:10lO)_ = 45 23^'; #(0221:2021) = 101 9';/w(0221:0110)_= 26 53'; MM (404 1:4 401) = 1 14^10' ; Mm (4041: lOlO) = 14 13'; M(0332:3302) = 9T 42'; iw(2131:2311)_= 75 22'; tw(2131:3l21) ^35 36'; w(2131:1231) = 47 1'; ^(3251:3521) _= 70 59'; 3^(3251:5231) =45 32'; vy (213 1:3251) ^8 53'; n>(1011:2131)_ = 29 1^'; mv(1010:2131) = 28 4'; #(2134:3124) =20 36>'; k] 108 INTRODUCTION TO THE STUDY OF MINERALS Pedions {0001}, {0001}. Each of these forms consists of a single face, a positive pedion at the upper end of the crystal and a negative pedion at the lower end. Trigonal Prisms {1010}, {0110}. These two forms differ only in position. Figure 170 shows {OlTOJ. FiG._170 {0110}. FIG. 171 {h-k-h+k-0}. Fio._172 lohhlj. FIG. 173 {h-k-h+k-1} Hexagonal Prism {1120}, (Second order prism). Ditrigonal Prisms {h-k-hTk.O}, {k.h.k+h.O}. The angles over alternate angles are equal. Fig. 171. Trigonal Pyramids {hOEl}, {hOH}, {OhEl}, {Ohhl} (Hemimor- phic trigonal pyramids of the first order). Each of these forms consists of three faces. They are distinguished as positive and negative, and upper and lower. Figure 172 represents an upper negative trigonal pyramid. Hexagonal Pyramids {h-h-2h-l}, {h-h-2hl} (Hemimorphic hex- agonal pyramids). There are six faces, each of which cuts two lateral axes at equal but greater distances than the third lateral axis. Ditrigonal Pyramids {h-k-h+k-lj, {h-k-h+k-i}, {k-h-k+hl}, {k-h-k+h.i} (Hemimorphic ditrigonal pyramids). The general form is a six-faced pyramid with alternate angles equal. The four forms indicated are the positive upper, positive lower, negative upper, and negative lower pyramids. (Fig. 173.) Example Tourmaline, a complex boro-silicate, is the best representative of this class. Tourmaline. jd = 0.447. _Usual forms: mjlOlpj, wi{OlTO}, a{ll20J, rflOll), njOlll), o{022l), e{0112), ci{000l}, z{l232). Interfacial MORPHOLOGICAL PROPERTIES OF MINERALS 109 angles: rr(1011:1101)_ = 46^ 52'; rar(1010a011^ = 62 40'; mo(1010: 1120) = 30 0' ; aa(1120 : 1210) = 600' ; ee(OlT2 : T012) =252' ; em 1 (OlT2 : OlTO) : =75 30M'; oo(0221 : 2021) =77 0'. (Figs. 174-177.) m FIG. 174. FIG. 175. FIG. 176. FIGS. 174-177. Tourmaline. FIG. 177. Trigonal Trapezohedral Class. A 3 .3A 2 ( Trapezohedral tetartohedral) The 2-fold axes are perpendicular to the 3-fold axis. The axes of symmetry are the axes of reference. List of Forms in the Trigonal Trapezohedral Class Pinacoid Hexagonal prism Ditrigonal prisms Trigonal prisms Rhomb oh edrons Trigonal bipyramids Trigonal trapezohedrons Trigonal trapezohedrons [In the above symbols h> k.] The two geometrically new forms for this class are the trigonal bipyramid and trigonal trapezohedron. Trigonal Bipyramids. Two kinds of trigonal bipyramids are possible for each value of h. They differ only in position. Fig. 178 shows the form {h. h. 2h.l}. Trigonal Trapezohedrons. The trigonal trapezohedron is a double-ended 6-faced form with the symmetry A 3 .3A 2 . Four 2 faces 6 faces 6 faces 3 faces 6 faces 6 faces 6 faces 6 faces {h-k-h+k-0} {1120} {hOhl}, {h-h-2h-l} fh-k-h+k-l} {h+k-k-h-l} {0001} {1010} {k-h-k+h-0} {1210} {Ohhl} {2h-h-h-l} {k-h-k+h-ll {k-h+k-h-l} 110 INTRODUCTION TO THE STUDY OF MINERALS different trapezohedrons are possible for any given value of h and k. Figure 179 represents the form {h-k-h+k-l}. Example Quartz. 6 = 1.099. Usual forms: r{1011}, z{OlTl}, m{10TO}, s{1121|, x {5161} , xi { 6151 } . Interfacial angles, mm(10TO:OlTO) = 60 0'^ wr(10lO: 1011) = 38 13'; mr(0110:1011) = 66 52'; rz(1011:0111) = 46 16'; rr(10Tl:Tl01) = 85 46';_ ms(10lO:- 1121) = 37 58'; _m.T(1010:5161) = 12!'; mxi (10TO:6l51) = 12 1'. Figures 180-183 represent some of the common varieties of quartz crystals. FIG. 178. FIG. 179. Graphic Determination of In- dices and Axial Ratio in the Hexagonal System. Graphic determinations in this system are illustrated by the plan and elevation of a quartz crystal shown in Fig. 184. The unit face r is (lOTl), z is (OlTl), and m is (1010). What is the symbol of Ml In the side elevation, draw lines FIG. 180. FIG. 181. FIG. 182. FIGS. 180-183. Quartz. FIG. 183. through n parallel to the projections of the r and M faces. These intersect the vertical axis in the points p and q. The distance oq is three times the distance op; therefore the symbol of the Af-face is: \a\ : oa 2 : ~~ ls:3c or 3031. MORPHOLOGICAL PROPERTIES OF MINERALS 111 The axial ratio 6 (a = l) is the distance op in terms of oai (in the plan). \ FIG. 184. Plan and elevation of a quartz crystal. 12. THE ORTHORHOMBIC SYSTEM The orthorhombic system includes all crystals with three non- interchangeable directions of symmetry at right angles to each other. The axial ratios are a: b:6. Conventionally the unit length of b is unity, and the unit length of a always less than unity. FIG. 185. Axes of ref- erence for topaz. FIG. 186. Axes of ref- FIG. 187. Axes of ref- erence for barite. erence for cerussite. These values for the axial ratios differ for every orthorhombic substance. Figures 185, 186, and 187 represent the unit lengths of the axes for topaz, barite, and cerussite respectively. 112 INTRODUCTION TO THE" STUDY OF MINERALS Of the three classes of the orthorhombic system, only one is discussed here. Rhombic Bipyramidal Class. 3A 2 3P C (Holohedral) The three planes of symmetry are mutually perpendicular, and their intersections are the axes of 2-fold symmetry. The three axes of 2-fold symmetry are the axes of reference. The selection of the c-axis is arbitrary, but of the other two, the unit on a is always shorter than the unit on b. List of Forms in the Rhombic Bipyramidal Class Pinacoid Pinacoid Pinacoid Rhombic prism Rhombic prism Rhombic prism 2 faces {001} 2 faces {010} 2 faces {100} 4 faces jhkO} 4 faces jhOl} 4 faces {Okl} (Basal pinacoid) (Brachypinacoid) (Macropinacoid) (Rhombic prism) (Macrodome) (Brachydome) Rhombic bipyramid 8 faces {hkl} (Rhombic pyramid) 1 Pinacoid {001} (Basal pinacoid). This form may be called the top pinacoid (Fig. 188). The symbol is written J001J instead of 1 001} , for only one form of the kind is possible in this class. Pinacoid {010} (Brachypinacoid). This form, consisting of two parallel faces, one on the right and one on the left, may be called the side pinacoid (Fig. 189). The symbol is written (OlOj instead of {OfcO}. Pinacoid {100} (Macropinacoid). This form may be called the front pinacoid as it consists of two opposite parallel faces, one in front and one behind (Fig. 190). As there is only one pinacoid of this kind possible, the symbol {100} is used instead of {hOO} . Rhombic Prism (hkO) (Rhombic prism). An open form consisting of four vertical faces (Fig. 191). For each substance crystallizing in the orthorhombic system a whole series of prisms is possible ranging from {010} to { 100} . The unit prism is { 110} . 1 These names are used by some authors. MORPHOLOGICAL PROPERTIES OF MINERALS 113 Rhombic Prism {hOl} (Macrodome). A horizontal open form composed of four faces each parallel to the 6-axis (Fig. 192). There is also a series varying from {001} to {010} for all possible values of h and I. Rhombic Prism {Oklf (Brachydome). A horizontal open form composed of four faces each parallel to the a-axis (Fig. 193). There is a series of all possible rational values of k and I. U! FlQ. 188. (001 1 189. (010 1 190. {100} 191. {hkO}. I 192.{/iOZ{ 193. {Okl}. 194. {hkl\. FIGS. 188-194. The seven type forms of the rhombic bipyramidal class. Rhombic Bipyramid {hkl} (Rhombic pyramid). The general form of this class consists of eight faces, which in the ideal form are scalene triangles (Fig. 194) . For any one substance there is a great variety of forms possible depending upon various simple rational values of h, k, and I. If h and k are equal we have {hhl} , of which there is a series with varying values of I. As these forms are in a vertical zone with the unit prism {110}, they are called bipyramids of the unit-series. {111} is the unit bipyramid. Combinations. Only the bipyramids can occur alone. All other crystals are combinations of two or more forms. There 8 114 INTRODUCTION TO THE STUDY OF MINERALS are manifold combinations and consequently a great variety in the habit. The most common are tabular, prismatic, and pyramidal, but some crystals cannot be placed under either of these. Pseudo- hexagonal orthorhombic crystals are common, but careful meas- urement distinguishes them from hexagonal crystals. Examples Examples of orthorhombic crystals are numerous among both minerals and prepared compounds. Barite (BaSOO and topaz (Al2F2SiO4) are given as typical examples for study and practice in working out the forms. Barite. d:b:6 = 0.815:1:1.313. Usual forms: c{001 }, w{110}, &{010}, o { 01 1 } , u { 101 } , d {JL02 } , I { 104 } . Cleavage parallel to c and m. Inter! acial angles: ram(110:lTO) =78 22>^'; cw(001:110) = 90; co(001:011) = FIG. 195. FIG. 196. FIG. 197. FIG. 198. FIGS. 195-198. Barite. 111 FIG. 199. FIG. 200. FIG. 201. FIGS. 199-202. Topaz. FIG. 202. 52 43'; cw(001:101) =58 10^'; cd(001:102) =38 51'; cZ(001:104) = 21 56'. Figures 195 to 198 are usual combinations. Figure 65, page 75, is more complex with a { 100 } , z [ 111 } , and y { 122 } in addition to the above. Topaz. d:b:6 = 0.528:1:0.477. Usual forms: m{110}, Z{120|, c{001}, /{021 },?/ {041 },u{ 111}, o{221},i {223}. Cleavage parallel to c. Interfacial MORPHOLOGICAL PROPERTIES OF MINERALS 115 angles; mmfllO:110} =55 43'; K(120:120) =86 49'; mZ(110:120) = 19 44'; c/(001:021) = 43 39'; cjKOO 1:041) = 62 21'; a(001:223) = 34 14'; cw(001:lll) = 45 35'; co(001:221) = 63 54'; wu(lll:lll) = 39 0'; oo(221:221) = 49 38'. Figures 199 to 202 represent usual types of topaz crystals. The lower part of these figures represents cleavage; doubly terminated crystals are very rare. Graphic Determination of Indices and Axial Ratio in the Ortho- rhombic System. Figure 203 represents a barite in plan and side elevation. The unit faces are ra(llO) and w(101). What are FIG. 203. Plan and elevation of a barite crystal. the axial ratios a:b:6t The intercept of the m-face in the plan gives us oa, which is the unit length of the a-axis in terms of ob (the unit length on the 6-axis). In the side elevation the line through, a parallel to the u face determines the distance oc which is the unit-length of the c-axis in terms of ob of the plan. 7. THE MONOCLINIC SYSTEM The monoclinic system includes all crystals in which there is a single direction fixed by symmetry not previously included (not A 3 , A4, or Ae). Three non-interchangeable axes of reference are used, one at right angles to the other two, which are in general in- clined to each other. The axial elements are a : b : 6 and p, the angle between the a- and c-axes (see Fig. 204). In a few 116 INTRODUCTION TO THE STUDY OF MINERALS cases /? is equal to 90. The unit length on the a-axis may be either shorter or longer than that on the 6-axis which is taken as unity. The crystal is held so that / the a-axis points down and toward j / the observer. a/ Prismatic Class. A 2 P-C (Holohedral) The axis of symmetry is normal to the plane of symmetry. The axis of 2-fold symmetry is the 6-axis. The axes d and 6 are in the plane of symmetry, but their position is more or less arbitrary. They are usually taken parallel to prominent edges or faces. FIG. 204. Monoclinic axes of reference. Pinacoid Pinacoid Pinacoid Pinacoids Rhombic prism Rhombic prism 2 faces 2 faces 2 faces 2 faces 4 faces 4 faces {001 } {010 } {100} {hOl }, {hkO} {Okl } (Basal pinacoid) (Clinopinacoid) (Orthopinacoid) (Hemi-orthodomes) (Prism) (Clinodome) Rhombic prisms 4 faces {hkl }, {hkl} (Hemi-pyramids) FIG. 205. FIG. 206. FIG. 207. FIG. 208. FIG. 209. FIG. 210. FIG. 211. FIGS. 205-211. The seven type forms in the prismatic class. Pinacoid {001) (Basal pinacoid). This form is usually known as the basal pinacoid, but its faces are inclined and not perpen- dicular to the c-axis, Fig. 205. MORPHOLOGICAL PROPERTIES OF MINERALS 117 Pinacoid {010} (Clinopinacoid) . This may be called the side pinacoid, but it is also known as the clinopinacoid, Fig. 206. Pinacoid {100} (Orthopinacoid). This form may be called the front pinacoid, but it is also known as the orthopinacoid, Fig. 207. Rhombic Prism {hkO} (Prism). An open form consisting of four faces each parallel to the vertical axis, Fig. 208. Rhombic Prism {Okl} (Clinodome). An open form consist- ing of four faces, each parallel to the a-axis. The a-axis is some- times called the clino-axis hence the name clino-dome, Fig. 210. Pinacoids {hOlj, {hOl}. (Hemi-orthodomes) . These forms, each composed of two opposite parallel faces parallel to the 6- axis (often called the ortho-axis), are independent of each other. Figure 209 represents IhOl}. Rhombic Prisms {hkl}, {hkl} (Hemi-pyramids) . These two forms occur independently, but together they constitute a figure that resembles a pyramid; hence the name hemi-pyramid is some- times used. Figure 211 represents an {hkl} form. Combinations. All monoclinic crystals are necessarily com- binations of two or more forms, as all the forms are open ones. As in the orthorhombic system, the habits are diversified. If the angle /? is close to 90 there is often marked resemblance to ortho- rhombic crystals, but this result may also be due to equal de- velopment of front and back faces. Prismatic crystals are usually elongated in the direction of the c-axis, but occasionally in the direction of the 6-axis, as in the case of epidote, and in the direction of the a-axis, as in orthoclase. Examples Many minerals and also artificially prepared substances crystallize in this class. Orthoclase (KAlSi 3 O 8 ), diopside (CaMgSi 2 O 6 ), augite (R n SiO 3 ), and gypsum (CaSO 4 .2H 2 O) are given as good examples for study and practice. Microcline is triclinic, but is so close to the monoclinic in angles that it may readily pass for orthoclase. Orthoclase. a:b:t = 0. 658 :_1: 0.555; ft = 63 57'. Usual forms: c{001}, 6{010}, m{110|, z{130}, z{T01}, y{20l], n{021}, o{Tllj. Cleavage 118 INTRODUCTION TO THE STUDY OF MINERALS parallel to c and b, also imperfect cleavage parallel to m. Interf acial angles : wm(110:ll0) =61 13'; &z(01(hl30)^ =_29 24'; ac(001:I01) =50 16'; cz/(001 :201) = 80 18'; az(edge HO, ITO:T01) = 65 47' [a(TOO) is a possible face truncating the edge lTO:TTO]; cn(001:021) =44 56'; 6o(010:Ill) = 63 8'; 6c(010:001) = 90 0'; cm(001:110) = 67 47'. Figures 212 to 215 represent usual types of crystals. FIG. 212. FIG. 213. FIG. 214. FIGS. 212-215. Orthoclase. Diopside. ti:b:6 = 1.092:1:0.589; = 74 10'. JJsual forms: c{001}, 6{010}, a{100}, i{1101, p{lll}, o{221}, d{101}, A{311|, a{lll}. Inter- facial angles: mm(110:110) = 92 50'; o6(10p:010) = 90 0'; ac(100: 001) = 74 10^66(010:001) = 90 O';_pp(lll:lll) = 48 29';_cp(001:lll) = 33 50'; ss(Tll:TTl) = 59 11'; oo(221:22D = 84 11'; AA(311:311) = 37 50'; cd(001:101) = 31 20'. Figures 216-219 represent typical crystals of FIG. 216. K FIG. 217. FIG. 218. FIGS. 216-219. Diopside. MORPHOLOGICAL PROPERTIES OF MINERALS 119 diopside. The striations on Fig. 216 are due to polysynthetic twinning with {001 } as twin-plane. Augite. Axial elements, usual forms, and interfacial angles practically the same as for diopside. Figures 220 to 223 represent the common types of augite crystals. Twins with { 100 } as twin-plane are common (see Fig. 223). m FIG. 220. FIG. 221. FIG. 222. FIGS. 220-223. Augite. FIG. 223. Gypsum. &:b:& = 0.690:1^0.412; ft = 80 42'. Usual forms: w{110}, , b {010}, njlll}, e{103}. Cleavage parallel to 6. Interfacial angles: mm(110:110) = 68 30'; 6m(010:110) = 55 45'; K(lll:lTl) = 36 m FIG. 224. N/ FIG. 225. FIG. 226. FIGS. 224-227. Gypsum. FIG. 227. 12'; 6n(010:ni)_= 69 20'; W(010:lll) = 71 54'; 6e(010:103) = 90 0'; ae(edge 110,110:103) = 87 49'. The usual combination is bml, but with varying habit as represented in Figs. 224 and 225. Figure 227 represents a twin crystal with {100} as twin-plane. 120 INTRODUCTION TO THE STUDY OF MINERALS Graphic Determination of Indices and Axial Elements in the Monoclinic System. An example of graphic determination is shown in Fig. 228, which is a plan and side elevation of an orthoclase (or microcline) crystal. The unit faces are m(110) and z(101); the problem is to determine the symbols of z and y and also the axial elements a : b : 6 and /?. The a-axis is drawn parallel to c(001) face (it appears foreshortened in the plan). A line is drawn from a in the plan parallel to the z-face. Its intersection on the 6-axis determines the distance os, which is FIG. 228. Plan and elevation of an orthoclase (or microcline) crystal. hence the symbol of z is la : >& : c which reduces to (130). Similarly in the elevation, a line through a parallel to y intersects the c-axis at the point, t. As the distance ot = 2 or (or is the intercept of the unit face x) the symbol of y is 1 a : b : 2c or (201). The symbol of o proves to be 111 for it is common to the two zones [010 : 101] and [001 : llO]. The distance oa (in the elevation) is the unit length of the a-axis in terms of ob in the plan (the 6-axis), and the distance or in the elevation is the unit length of the c-axis in terms of ob also. MORPHOLOGICAL PROPERTIES OF MINERALS 121 14. THE TRICLINIC SYSTEM The triclinic system includes all crystals in which there are no directions fixed by symmetry. The axes of reference are three non-interchangeable axes, in general at oblique angles, which are taken parallel to three promi- nent edges. The axial elements are a : b : 6 (the unit on b being 6 j unity, and the unit on a usually ^^ \f less than unity) and the angles ^~ "~^/ a, j8, and 7 between the axes b a /> *^ and c, a and 6, a and b respec- / , tively. Figure 229 represents a possible triclinic axial cross. The triclinic system includes _ J FIG. 229. Triclinic axes of reference. two classes, one with a center of symmetry and the other without any symmetry whatever. As no known mineral is devoid of symmetry, only the pinacoidal class is considered here. &*ct/L*, fa. ~ Pinacoidal Class. C (Holohedral) The choice of axes is arbitrary, but they are usually taken parallel to the intersection edges of the three most prominent faces. In some cases, as in the triclinic feldspars, directions corresponding to those in the monoclinic feldspar, orthoclase, are chosen. List of Forms in the Pinacoidal Class Pinacoid 2 faces {001} (Basal pinacoid) Pinacoid 2 faces {010} (Brachypinacoid) Pinacoid 2 faces {100} (Macropinacoid) Pinacoids 2 faces jhkO}, {hkO} (Hemi-prisms) Pinacoids 2 faces {Okl}, {Okl} (Hemi-brachydomes) Pinacoids 2 faces {hOlj, {hOl} (Hemi-macrodomes) Pinacoids 2 faces { hkl } , { hkl } , { hkl } , { hkl } (Tetarto-pyramids) 122 INTRODUCTION TO THE STUDY OF MINERALS All forms are pinacoids each of which consists of two opposite parallel faces. Combinations. The appearance of triclinic crystals depends largely upon the obliquity of the axes. Many of them closely approach monoclinic crystals in angles. This is especially the case with the plagioclase feldspars. xamples Comparatively few minerals crystallize in this class. The only common ones are the plagioclase feldspars, rhodonite, kyanite, and microcline. Albite (NaAlSi 3 O 8 ) is selected as the best mineral for study. Albite crystals are usually so small that measurements must be made by the reflection gonometer. Microcline is triclinic but is so close to orthoclase in angles that it may pass for mono- clinic. (The optical properties, especially the ob- lique extinction on the 001 cleavage face, prove that it is triclinic.) Albite. a:b:6 = 0.633:1:0.556; a =93 58'; = 63 39'; 7 = 87 31'. Usual forms: mjllO), Mj_110j, c{001}, 6J010}, x[101}, 2/{201}, /{130}, z{130}, n{021}, p{lll}, ojlll}. Cleavage parallel FIG. 230. Albite. to c and b. Interfacial angles: mM(110:110) = 59 16^'; m/(110:_130) = 30 24', Mz(110:130) = 29 36'; m6(110:010) = 60 58'; cz(001:101) = 52 6>^'; c?/(001:201) = 81 53'; Figure 230_represents_an albite crystal with the forms: c{001}, and Graphic Determination of the Indices in the Triclinic System. Figure 231 'shows a plan and side elevation of an albite crystal. The unit faces are m(110), M (110), and #(101) ; the problem is to determine the symbols of/, z, y, and p. (c = 001; b = 010). The determinations are made just as they were in the case of Fig. 228; the only difference is that the 6-axis is not normal to the 6(010) face. _The following symbols are obtained/ = (130); z = (130); 2/= (201); p = (Til). MORPHOLOGICAL PROPERTIES OF MINERALS 123 15. COMPOSITE CRYSTALS AND CRYSTALLINE AGGREGATES Loose, isolated crystals are comparatively rare in nature. They usually occur in groups. The grouping may be in parallel FIG. 231. Plan and elevation of an albite crystal. position (see Fig. 232), in the most irregular manner, or in the third condition of partial parallelism. Twinning The peculiar sort of grouping in partial paral- lelism is known as twinning ; crystals so grouped are called twin-crystals. Many crystals are found to be composed of two parts, one half of which apparently has been revolved 180 about a line called the twin-axis. These may be called rotation twins. Other crystals have two portions symmetrically placed with reference to a plane called the twin-plane. These may be called reflection twins. In a third type the two individuals are symmetrical to a point, though neither of the crystals has a center of symmetry. These are called in- FIG. 232 Octa- hedra in parallel 124 INTRODUCTION TO THE STUDY OF MINERALS version twins. The face of union of the two individuals is called the composition-face. It may or may not be the twin-plane. The twin-plane is always a crystal face or a possible crystal face, but never a plane of symmetry. The twin-axis is always FIG. 233. Contact twin. FIG. 234. FIG. 235. Penetration twin. Cyclic twin. FIGS. 233-236. FIG. 236. Polysynthetic twin. a possible crystal edge or a line normal to a possible crystal face, but it is never a 2-fold, 4-fold, or 6-fold axis of symmetry. Two general types of twin-crystals are distinguished: (1) con- tact twins with a definite composition face and (2) penetration twins with an indefinite or irregular com- position face. Figure 233 is a diagram- matic representation of a contact twin and Fig. 234, that of a penetration twin. In the case of contact twins the twin -law is defined with respect to a twin-plane, while in penetration twins it is defined with re- spect to a twin-axis. In addition to twins composed of two in- dividuals, there are also multiple twins made up of three or more parts. If the same face serves as twin-plane for a series of individuals we have a polysynthetic twin (Fig. 236). But if different faces (of the same form) are twin-planes we have a cyclic twin (Fig. 235). A polysynthetic twin may consist of a large number of in- dividuals and some of these may be so narrow that they appear FIG. 237. Plagioclase. MORPHOLOGICAL PROPERTIES OF MINERALS 125 as striations. Cleavages of calcite and of plagioclase often show twinning striations. In calcite, the rhombohedron {OlT2} is the twin-plane, and so on the cleavage face { 1011} the striations are parallel to the long diagonal as represented in Fig. 251. In plagioclase the twin-plane is usually 6{010), and so the twin striations appear on the c{001f cleavage face as narrow bands parallel to the (001 :010) edge as shown in Fig. 237. I I in m FIG. 238. Gypsum. FIG. 239. Augite. m \ \ FIG. 240. Hornblende. FIG. 241. Orthoclase. Examples Figure 238 represents a twin of gypsum with { 100 { as twinning plane. In Figs. 239 (augite) and 240 (hornblende), { 100} is also twin-plane. Figure 241 represents a Carlsbad twin of orthoclase. This is a penetration twin with the c-axis as twin-axis. Figu re 242 represents a cruciform penetration twin of staurolite. In Fig. 243 a contact twin of aragonite with ?w{110} as twin-plane is shown. A pene- tration trilling of cerussite is illustrated by Fig. 244. Figure 245, a twin of marcasite, apparently has an axis of 5-fold symmetry. The angles in this case would be exactly 72 (^ of 360), but accurate measurement proves four of them to be 74 55' instead. Figures 246 to 249, inclusive, represent various kinds of rutile twins, but in each case (101) is the twin-plane. Figure 246 is a simple contact twin; Fig. 247 shows twin striations. A single band inserted in twinning position like Fig. 248 is called a twin-seam. Figure 249 is a cyclic twin. These four figures are orthographic parallel projections made on the (100) plane. 126 INTRODUCTION TO THE STUDY OF MINERALS The next row of figures illustrates four of the five twin- laws known for calcite. Figure 250 is the scalenohedron {2131} twinned on { 0001 } . Figure 251 represents a calcite cleavage with twin lamellae inserted parallel to {0112}, which is the most FIG. 242. Staurolite. FIG. 243. Aragonite. FIG. 244. Cerussite. FIG. 245. Marcasite. common twin-law for calcite. Figure 252 is a calcite twin with {1011} as twin-law, while Fig. 253 is a scalenohedron twinned on {0221}, one of the rare twin-laws for calcite. m FIG. 246. FIG. 247. FIG. 248. FIGS. 246-249. Rutile. FIG. 249. The next four figures represent twins of the isometric system. Figure 254 (with 1 1 1 as twin-plane) is called the spinel twin be- cause it is so common for the mineral spinel. Figure 255 repre- sents twin striations observed on cubic cleavages of galena. Here MORPHOLOGICAL PROPERTIES OF MINERALS 127 the twin-plane is the trisoctahedron {441}. A penetration twin of fluorite with the cube diagonal as twin-axis is represented in Fig. 256, while Fig. 257 is a twin of pyrite with the a-axis as twin- FIG. 250. FIG. 251. FIG. 252. FIGS. 250-253. Calcite. FIG. 253. Many apparently simple crystals are in reality twins. In such cases optical tests are usually necessary to reveal their composite character. Twins are usually recognized by the presence of reentrant angles, but there are exceptions to this general rule, as for example, hornblende, Fig. 240, p. 125. FIG. 254. Spinel. FIG. 255. Galena. FIG. 256. Fluorite. FIG. 257. Pyrite. The tendency of twinning is to raise the grade of symmetry apparently. This is especially the case with pseudo-hexagonal orthorhombic minerals such as aragonite, witherite, and cerussite. See Figs. 472, 473, 474, and 478. 128 INTRODUCTION TO THE STUDY OF MINERALS Crystalline Aggregates Most minerals consist of crystal aggregates which do not possess definite crystal faces. That such minerals are crystal- line, however, may be determined by certain physical properties, particularly the optical properties. The kinds of crystalline aggregates are distinguished by certain terms which are con- stantly used in the description of minerals. A mineral made up of plates is called lamellar (example, barite). If the layers are readily separated the term micaceous is used (example, hematite) . An aggregate of more or less parallel imperfect crystals is called columnar (example, aragonite) ; and the same on a smaller scale is fibrous (example, gypsum). Fibrous radiating aggregates of crystals are known as spherulites (ex- ample, chalcedony). A flat columnar aggregate is said to be bladed (example, kyanite). The term granular needs no explana- tion (example, magnetite). The forms assumed by many aggregates derive their names from some natural object. Nodular is the term used for irregular rounded lumps (example, pyrite). Mammillary refers to low rounded prominences (example, smithsonite) . Botryoidal is from a Greek word meaning a bunch of grapes (example, chal- cedony). Reniform means kidney-shaped (example, hematite). The last three terms are so closely related that it is often difficult to decide which term to use. The term colloform was pro- posed several years ago by the author for the more or less spherical forms assumed by amorphous and metacolloid minerals in free spaces. Pisolitic is the term used for an aggregate of shot-like masses (example, cliachite), while oolitic is similar except that the spheres are smaller, about like fish-roe (example, calcite). Stalac- titic indicates that the mineral is found in icicle-like forms (ex- ample, calcite). Dendritic means branching like a tree (example, copper). Concretions are more or less spherical masses formed by the tendency of matter to gather around a center (example, siderite). A geode is a hollow concretion usually lined with MORPHOLOGICAL PROPERTIES OF MINERALS 129 crystals (example, quartz.) A vug is a cavity in a rock or vein lined with crystals, 16. CLEAVAGE AND PARTING Many crystals have the property of breaking with smooth surfaces in certain directions which are parallel either to actual FIG. 258. Cubic cleavage. FIG. 259. Octahedral cleavage. FIG. 261. Feldspar cleavage FIG. 262. Barite cleavage. FIG. 263. Calcite cleavage. or possible crystal faces. This important property is known as cleavage. (It really belongs among the physical properties but 130 INTRODUCTION TO THE STUDY OF MINERALS is so intimately related to the crystal form that it is discussed here.) Galena, which usually crystallizes in cubes, has a cubic cleavage (Fig. 258), while fluorite, which also crystallizes in cubes, has an octahedral cleavage (Fig. 259). (The octahedron is a form sometimes found on fluorite). Cleavage is defined accord- ing to the direction as cubic, rhombohedral, prismatic, etc., and according to the character of the surface, such terms as imperfect, good, perfect, and very perfect being used. Thus the micas have a very perfect cleavage parallel to (001) (Fig. 260), while the feldspars have a perfect cleavage parallel to (001) and good cleavage parallel to (010) (Fig. 261). In barite, an orthorhombic mineral, the cleavage is perfect in one direction parallel to (001), and a little less perfect in two direc- tions parallel to (110) (Fig. 262). In gypsum there is a very perfect cleavage parallel to (010), an imperfect cleavage with conchoidal surface parallel to (100), and an imperfect cleavage with fibrous surface parallel to (1 1 1) . The relation of a cleavage fragment of gypsum to a crystal is shown in Fig. 264. Here the inner rhombic figure is the result of cleavage. FIG. 264. Relation A line normal to the paper is an axis of two- of cleavage to euhedrai fo^ S y mme try for the cleavage fragment, as crystal of gypsum. - J & . & well as for the crystal. But, in general, cleavage shows a greater degree of symmetry than crystal form for the simple reason that the presence or absence of a center of symmetry cannot be established by cleavage alone. For example, tetrahedral cleavage cannot be distinguished from octahedral cleavage. In calcite, whatever the shape of the crystal, the cleavage is perfect rhombohedral in three directions at angles of 74 55' to each other. Figure 263 represents a cleavage of calcite with three surfaces and intersecting cleavage traces on each. Cleavage is a fairly constant property of minerals, and is MORPHOLOGICAL PROPERTIES OF MINERALS 131 invaluable in the rapid recognition of minerals. Such minerals as calcite, fluorite, feldspars, amphiboles, and gypsum are dis- tinguished principally by their cleavage. On the other hand, such minerals as quartz and garnet possess practically no cleavage. They break with an irregular fracture. In chalcedony the fracture is co'nchoidal (curved like the interior of a shell). Other terms applied to fracture such as splintery, hackly, even, and uneven are self-explanatory. The term parting is applied to a separation due to some molecular disturbance, such as twinning. Cleavage may be obtained in any part of a crystal in the given direction; the size and the number of the cleavage particles are limited FIG. 265. Halite parting. FIG. 266. Calcite parting. only by the mechanical appliance available. Parting, on the other hand, takes place only along certain definite planes, those of the molecular disturbance. In a cubic cleavage of rock-salt, if pressure is applied in a direction normal to a vertical diagonal plane, a surface normal to the direction of pressure is developed, (the shaded plane in Fig. 265) . In this case we have an example of dodecahedral parting. If pressure is applied by a dull knife edge normal to the obtuse edge of a cleavage rhombohe- dron of calcite, a small portion of the calcite will be reversed in position, forming a twin with (0112) as twinning plane. This phenomenon is known as gliding. In this case the small portion of calcite in Fig. 266 may be easily removed; the parting is par- 132 INTRODUCTION TO THE STUDY OF MINERALS allel to (01 12) , a plane normal to the direction of the pressure and is produced by secondary twinning. In ice, gliding may take place in a direction normal to the c-axis (ice crystallizes in the hexagonal system). This may explain in part the movement of glaciers. One case is known in which the direction of parting is not a possible crystallographic plane. Some cleavage pieces of plagio- clase show well-defined parting almost, but not quite, parallel to {001} See Fig. 267. This parting is due to pericline twinning, a method of twinning in which the 6-axis is the twin-axis. In the triclinic system, if the 6-axis is a twin-axis, the com- position face of a twin is not a possi- ble crystal face. Prominent examples of parting are the following: basal parting (001) in FIG. 267. Pericline parting in diopside (Fig. 216, page 118), basal parting (001) in stibnite, octahedral parting (111) in magnetite, and rhombohedral parting (0112) in calcite. 17. THE INTERNAL STRUCTURE OF CRYSTALS The law of simple rational indices is the foundation stone of geometrical crystallography. In addition to the formulation of the law given on page 74 it may be stated in another way. If from a point, lines parallel to the intersection edges of three prominent non-parallel faces of a crystal be drawn, and a plane parallel to a fourth chosen face also be drawn there may be con- structed from the four points 0,A,B,C (Fig. 268) thus established, a series of parallelepipeds of indefinite extent with OAGBECDF as a unit cell. Such a network of points constitutes a space - lattice. The particular one shown in Fig. 268 is triclinic, the distances OA, OB, and OC are unequal, and the angles between them oblique. Besides the four planes mentioned, many others may be drawn by connecting any three points of the space-lattice. MORPHOLOGICAL PROPERTIES OF MINERALS 133 Now the possible faces of a crystal are parallel to the possible planes of the space-lattice. This is a geometrical expression of the law of rational indices without any theory whatever as to the internal structure of crystals. FIG. 268. Triclinic space-lattice. A point to be considered is the limitation of the term "simple" in the expression "law of simple rational indices. " All the planes possible within the limits of Fig. 268 are those with simple indices, but it is clear that possible planes of the lattice may be represented by as large numbers as we choose if the lattice is sufficiently ex- tended. Now what are the actual facts? We find that the 134 INTRODUCTION TO THE STUDY OF MINERALS crystal faces of common occurrence are those with simple indices : 0, 1, 2, 3, 4, 5, and 6, rarely above 10. For example, if we take the { hhl } faces of all orthorhombic minerals (97 in number) , we find that those with 4 as the highest index occur on 16 different min- erals, those with 6 as the highest index occur on 5 different minerals, those with 7 as the highest on 3 minerals, and those with 9 as the highest on 2 minerals. Forms such as {9.9.10}, {12.12.11}, and {5.5.19} occur on only one mineral. Geomet- rically expressed, the faces of most frequent occurrence are those of the greatest reticular density. This is shown by Fig. 269 which 100 FIG. 269. Diagram showing the reticular density of {hkO} faces in an ortho- rhombic crystal. may be taken to represent some of the possible hkO forms in the orthorhombic system. This fact, which is independent of any theory, is known as the law of Bravais. Whether a crystal actually possesses a space-lattice or not is another question. In 1905, Friedel, a French crystallographer, formulated the law of rationality of symmetric intercepts (see page 74), which added to the law of rationality of indices practi- cally proved the existence of the space-lattice. Direct proof, however, was not furnished until the work of Laue in 1912. The problem of crystal structure resolves itself into two more or less independent questions: (1) the nature of the constituent MORPHOLOGICAL PROPERTIES OF MINERALS 135 particles of the crystal and (2) their arrangement in space. The second problem is essentially a mathematical one. It is simply necessary to find all the possible arrangements of points in space, for the constituent particles, whatever their nature, may be represented by points. All the possible space-lattices -belong to one of seven types of symmetry viz.: C, A 2 P C, 3A 2 3P C, A 4 -4A 2 5P-C, A 3 (^ > 6 )-3A 2 .- 3PC, A 6 6A 2 -7P-C, and 3A 4 -4A 3 (4> 6 )-6A 2 -9P-C. It will be noted that the only symmetry axes present are those with periods FIG. 274. FIG. 275. FIGS. 270-275. Diagrams showing possible axes of symmetry in a space- lattice. of 2, 3, 4, and 6, and the fact that only these axes of symmetry have been found on crystals makes it probable, apart from any other considerations, that crystals have a regular internal struc- ture. The diagrams of Figs. 270-275 will make it clear that only these symmetry axes are possible in a space-lattice. Let a\ t 0,2, 03, etc. represent the points of a space-lattice and the projection of the lines of the space-lattice. Let ai 2 be the smallest possible distance between them (the distances are not infinitesimal). Rotation around symmetry axes of 180, 120, 90, and 60 re- spectively will then give us the points a 3 in Fig. 270, as and a* 136 INTRODUCTION TO THE STUDY OF MINERALS in Fig. 271, as, a*, and a 5 in Fig. 272, and as, GU, as, a 6 and ai in Fig. 273. These figures show that axes of 2-, 3-, 4-, and 6-fold symmetry are possible. Next take the case of a 5-fold axis of symmetry. In Fig. 274 a rotation of 72 (J of 360) about a\ brings a 2 to as, and a similar rotation about a 2 brings a i to a. But asa4 is smaller than the original distance aia 2 , which is contrary to hypothesis. There- fore an axis of 5-fold symmetry is impossible. If, in a similar way, we take an angle of rotation less than 60 (Fig. 275), the new points as and a* result, but the distance asa4 is also smaller than the original distance, a^. Therefore axes of n-fold symmetry with n greater than 6 are impossible. The first substantial contribution to the subject of crystal structure was made by Bravais, a French physicist in 1850. He found that fourteen space-lattices are possible. They include, in addition to the seven primary lattices enumerated below, Primary Space-lattices Other Space-lattices Cubic (Fig. 276) Centered cube (Fig. 277) Hexagonal prism (Fig. 281) Face-centered cube (Fig. 278) Rhombohedron (Fig. 282) Centered square prism (Fig. 280) Square prism (Fig. 279) Centered rectangular prism (Fig. 286) Rectangular prism (Fig. 285) Rhombic prism (Fig. 283) Monoclinic parallelepiped (Fig. 288) Centered rhombic prism (Fig. 284) Triclinic parallelepiped (Fig. 289) Clinorhombic prism (Fig. 287) seven others which may be derived by combining two lattices in a symmetrical manner. For example, the centered square prism (Fig. 280) is made up of two interpenetrant square prism lattices, one derived from the other by shifting it in the direction of its diagonal for a distance equal to one-half of its diagonal. This operation is known as translation. Now the fourteen space- lattices of Bravais are the only possible ones that may be ob- tained by adding translations to the symmetry-operations, provided no translations less than one-half the distance between the points of a primary lattice are used. Every Bravais space- lattice necessarily has a center of symmetry. MORPHOLOGICAL PROPERTIES OF MINERALS 137 FIG. 276. Fro. 277. FIG. 278. FIG. 279. FIG. 280. FIG. 281. FIG. 282. FIG. 283. FIG. 284. FIG. 285. FIG. 286. FIG. 287. FIG. 288. FIG. 289. FIGS. 276-289. The fourteen space-lattices of Bravais. 138 INTRODUCTION TO THE STUDY OF MINERALS Sohncke in 1879 discovered a new kind of symmetry opera- tion applicable to points in space, viz., a screw -axis of symmetry, which is an axis around which a spiral movement takes place. It may be produced by combining an ordinary axis of symmetry with a translation along the direction of the axis. An example of a 4-fold screw-axis of symmetry is shown in Fig. 290. The point b may be derived from a, c from b, d from c, a^ from d, and so on, by a clockwise rotation of 90 combined with a translation t, equal to J^ aai. A screw-axis may be either right-handed or FIG. 290. Screw-axis of symmetry. FIG. 291. Glide-plane of symmetry. left-handed; it is thus possible to account for exactly similar, but enantimorphous, crystals such as those of quartz (see Figs. 420-21, p. 260). By adding screw-axes to the 14 Bravais space-lattices, Sohncke proved that 57 kinds of arrangements of points are possible. These are called point-systems. It was soon realized that Sohncke's work was incomplete, and the Russian crystallographer Fedorov a little later found that a new type of symmetry was necessary, viz. a glide-plane of symmetry, which is the result of combining reflection in a plane of symmetry with a translation parallel to the plane. An MORPHOLOGICAL PROPERTIES OF MINERALS 139 example is shown in Fig. 291. The dotted lines are the traces of a glide-plane of symmetry, for with a translation equal to t, this plane becomes a plane of symmetry. (A tessellated pavement furnishes a good example in two-dimensional space) . The result of combining translations, screw-axes, and glide-planes with the ordinary symmetry-operations of finite figures furnished Fedorov (1890) with 230 possible kinds of space-groups. Each of these space-groups belongs to one of the 32 point-groups. Schoenflies (1891) and Barlow (1894) arrived at the same conclusion in- dependently. The mathematical side of the problem of crystal structure was thus firmly established, and Friedel's recognition of the law of the rationality of symmetric intercepts in 1905 practi- cally proved that a space-lattice exists in crystals. Visible proof of the space-lattice, however, was not forthcoming until 1912. The results obtained in the next few years have opened up one of the most interesting fields in the whole realm of science. The first experimental work on crystal structure originated in an attempt to determine the nature of .X-rays. It was doubtful whether X-rays consisted of a wave-motion or were material in nature. Laue, a Swiss physicist, conceived the idea that a crystal might act as a 3-dimensional diffraction grating for X-rays if the latter consisted of a wave-motion. In order to test this, a beam of X-rays was directed upon a crystal plate of sphalerite (isometric ZnS), and a photograph was obtained which showed a central circular spot surrounded by elliptical spots of varying intensity arranged in a symmetrical manner. Two radiograms (X-ray photographs) of sphalerite are shown in Figs. 292a and 2926. Fig. 292a was taken from a crystal plate cut parallel to a cube face and Fig. 2926 from a plate cut parallel to a tetrahedral face. The experiment had succeeded and proof was furnished that the mysterious X-rays discovered by Roentgen are the result of a wave-motion similar to, but with much shorter wave-length than, that of light. The central spot of the photograph is pro- 140 INTRODUCTION TO THE STUDY OF MINERALS FIG. 292a-6. Radiograms of sphalerite (after Frederick and Knipping), MORPHOLOGICAL PROPERTIES OF MINERALS 141 duced by the direct beam of X-rays and the other spots are due to secondary beams produced by reflections from internal planes of particles. (The actual particles are of course too small to show separately, for millions of them are present even in a minute crystal.) The fact that the intensity of the spots is pro- portional to the reticular density of the plane just as the relative frequency of occurrence of crystal forms makes it possible to determine the actual position of the constituent particles. This work has been undertaken by a number of investi- gators, notably the English physicist, W. H, Bragg, and his son, W. L. Bragg. The Braggs introduced a method different from that of Laue. They mounted the crystal upon an X-ray FIG. 293. Explanation of X-ray diffraction effects (after the Braggs). spectrometer, an instrument in which the telescope of the ordi- nary spectrometer is replaced by an ionisation chamber, and used a beam of X-rays of given wave-length analogous to a beam of monochromatic light. (In the Laue experiments the general radiation corresponding to white light was used.) As the mounted crystal is revolved about the axis of the spectrometer, effects are obtained over a considerable range of angles. At certain angles the leaf of the electroscope attachment moves and the intensity of the movement may be read off on a suitable scale. The reason that the leaf of the electroscope moves only at certain angles may be shown by a consideration of Fig. 293. The horizontal lines pp, etc., represent traces of internal planes in the crystal with the spacing d. A, A\, Az, A*. . . .are a train of 142 INTRODUCTION TO THE STUDY OF MINERALS 100 X-ray waves of wave length X. (It is to be noted that d and X are of the same order of magnitude which makes it possible to use a crystal as an X-ray diffraction grating.) The reflected wave BC is the result of reflections from successive planes B, B', B" ', B'", etc., provided the distance ND is equal to X or a multiple of X, for then all the trains of reflected waves are in the same phase and the intensity is equal to the sum of their ampli- tudes. If the distance ND is not equal to X or n\ there is no re- flection. It is clear then that reflection takes place only at certain angles X = 2d sin 0i; 2X = 2d sin 2 ; 3X = 2d sin 63; etc. (ND = 2d sin B) . For a different crystal face the value d will be differ- ent and consequently the angle 6. The readings obtained can then be used to interpret the crystal structure. The re- sults obtained by the Braggs for sphalerite are shown in Fig. 294 for the three im- portant faces of the isometric system viz., the cube {100}, ** * ^ M* 6b the dodecahedron { 1 10} , and FIG. 294.-X.ray spectrometer readings the octa hedron {ill}. For for sphalerite (after the Braggs). the octahedral or tetrahedral plane there are four readings, corresponding to spectra of the first, second, third, and fourth order of the diffraction grating. Now there are three different kinds of space-lattice possible in the isometric system: (1) the cube (Fig. 276), the centered cube (Fig. 277), and the face-centered cube (Fig. 278). The distance of adjacent reticular planes in the three cases for the prominent faces are as follows: 1 1 1 no in Cube lattice Centered cube lattice : V3 1 Face-centered cube lattice MORPHOLOGICAL PROPERTIES OF MINERALS 142 The ratios of the sines of the angles for sphalerite given in Fig. 294 are practically as l:\/2:\/f> which proves that the con- stituent particles of a sphalerite crystal lie at the points of a face-centered cubic lattice. The Braggs conclude that equally spaced planes parallel to the cube contain Zn and S atoms alter- nately (Fig. 295), that equally spaced planes parallel to the dodecahedron contain both Zn and S atoms (Fig. 296), and that unequally spaced planes parallel to the octahedron (or tetra- hedron) contain Zn and S atoms alternately, also that the Zn-Zn distance is four times the Zn-S distance (Fig. 297). The internal structure of sphalerite must then be that shown in Fig. 298. (The lines of the figure are of course imaginary. They are i 1 I 1 1 i 1 1 1 1 n S 2 n S Z n 2 n Z 3 S n 2 > r n Z 5 < n Z iS Z nS Zn i F to. 295. FIG. 2< )6. FIG . 297. FIGS. 295-297. Spacing of planes of atoms in sphalerite (after the Braggs). drawn simply to show the relation of the atoms to each other) . The zinc atoms lie on a face-centered cubic lattice and the sulfur atoms on another face-centered cubic lattice distant one-fourth of the diagonal of the cube apart. The symmetry elements of the structure are as follows: There are dodecahedral planes of symmetry through the Zn and S atoms at intervals of J^A/2a, where a is the dimension of the edge of the unit cube, and halfway between these, glide-planes of symmetry with a translation of HVf a in the direction between the central Zn and the lower front right Zn. Lines normal to the cube planes at the projection of all the zinc and sulfur atoms are composite planes and four- fold axes of symmetry with the planes at intervals of Ha. There are also two-fold screw-axes of symmetry with translation of %a 144 INTRODUCTION TO THE STUDY OF MINERALS Pio. 298. The internal structure of sphalerite. MORPHOLOGICAL PROPERTIES OF MINERALS 145 along vertical and horizontal lines half-way between vertical and horizontal lines of Zn-atoms. No centers of symmetry are pres- ent for the three-fold axes of symmetry lying along -the cube diagonals are polar. The space-lattice shown in Fig. 298 thus accounts for the symmetry of sphalerite: 4A 3 -3A 2 (3^P4)-6P. The Laue radiogram of Fig. -292a apparently shows four planes of symmetry (axial as well as diagonal) intersecting in a central axis of 4-fold symmetry but Friedel has proved that a radiogram does not tell us whether a center of symmetry is present or not. The unit cube of the sphalerite structure contains 4 sulfur atoms and 4 zinc atoms, for the zinc atoms at the 8 corners each belong equally well to the seven adjacent cubes and each of those at the centers of the cube faces belong equally well to an adjacent cube. [(8 X M) + (6 X }4) = 4]. This does not mean that the molecular formula of sphalerite is 4 ZnS for it is probable that the molecule does not exist in crystalline solids, but only in gases, liquids; and amorphous solids. The structure of diamond is similar to that of sphalerite except that all the points of Fig. 298 are carbon atoms. The dodeca- hedral planes are planes of symmetry with intervals of J4\/2a and the cube planes at intervals of Ya are glide-planes of sym- metry with translation of J^a along a line normal to a cube edge. Composite four-fold axes of symmetry are at the projections of all the carbon atoms and corresponding to the two-fold screw-axes of Fig. 298 we have instead, four-fold screw-axes with translations of ya in the direction of the cube edges. The three-fold axes of symmetry are no longer polar as in the case of sphalerite. The work of the Braggs proves that diamond belongs to the hexocta- hedral or holosymmetric class of the isometric system. In practically all of the discussions of crystal structure the idea of a unit is prominent. The term " particle" often is used instead of molecule because of the possibility that the unit of structure is a collection of molecules instead of a single molecule. Some interpret the ^T-ray work of the Braggs to mean that molecules as such do not exist in crystals, but in the opinion of 10 146 INTRODUCTION TO THE STUDY OF MINERALS others this conclusion is premature. The molecule, if such exists, may escape detection by X-ray methods. In any event the X-ray experiments prove that not only the units of structure but also the individual atoms are arranged in space-lattices which was predicted by Groth in 1905. The sphalerite crystal of Fig. 298, for example, is built up of two interpenetrant space-lattices, one of Zn atoms and the other of S atoms. The nature of the unit of crystal structure still remains in doubt. The X-ray analysis of crystals combined with a mathematical study of the possible arrangements of points in space has fur- nished us with a means of determining the stereochemistry of the solid or crystalline state and has thus thrown new light on the structure of matter. This work also promises to be of value in settling many doubtful questions concerning crystals. It has truly opened up one of the most interesting fields in the whole realm of science. THE PHYSICAL PROPERTIES OF MINERALS Two classes of physical properties are recognized. Physical properties such as specific gravity are independent of the direction and are called scalar properties, while others, such as cohesion and the effect of light, heat, and electricity, can be represented by a line of given length and direction, hence the term vectorial properties. Vectorial properties may be divided into two general groups: continuous and discontinuous. Properties that can be repre- sented by a smooth curve such as the curve of hardness (Fig. 299), are continuous vectorial properties, and those that vary discon- tinuously such as etch-figures (Fig. 300) are discontinuous vec- torial properties. The following is a tabulation of some of the prominent physical properties. Scalar Properties Specific Gravity Specific Heat Vectorial Properties Continuous Hardness Elasticity Optical, p. 156 Ellipsoidal I Thermal Properties Magnetic Electr c Discontinuous Cleavage, p. 129 Etching, p. 66 The discontinuous vectorial properties, cleavage and etching, are so closely related to the morphological properties that they have been described under that heading rather than under the physical properties. Of all the continuous vectorial properties the optical properties 147 148 INTRODUCTION TO THE STUDY OF MINERALS are the most important from both the theoretical and practical standpoints and for that reason they are treated in a separate division. Fm. 299. Curve of hardness on cube face of fluorite. FIG. 300. Etch-figure on cube face of fluorite. 1. SPECIFIC GRAVITY The density of a substance compared with the density of water under standard conditions (4C.) is called the specific gravity. A specific gravity of 3 means that the substance weighs three times as much as an equal volume of water. The specific gravity is the weight of a substance divided by the weight of an equal volume of water. Five methods of finding the specific gravity are described. (a) A rough, but rapid, method is to weigh out a gram of the mineral and then find its volume with a burette. Care must be taken to eliminate air-bubbles. If one gram is used, the specific gravity is the reciprocal of the volume. (6) A more accurate method is based on the fact that a body immersed in water loses in weight an amount equal to the weight of the water displaced. The substance has the weight A in air, say. Suspended by a fine thread in a vessel of water, it has the weight W. Then G = A_W> wnere G is the specific gravity. Numerous precautions must be taken to insure accuracy. (c) A convenient specific gravity balance for the practical identification of minerals is that represented in Fig. 301, which was designed by the author. It consists essentially of a brass THE PHYSICAL PROPERTIES OF MINERALS 149 beam supported near one end by a knife edge. The short arm carries two pans; the lower one is immersed in water. The end of the long arm rests within a guard, which limits the motion of the balance. The long arm of the beam is graduated so that the specific gravity may be read off directly. This may be done by always placing the counterpoise in the notch near the end of the long arm when weighing in air. Whatever the weight of the counterpoise, its distance (x) from the fulcrum and the distance (y) of the counterpoise from the fulcrum when weighing T* in water are connected by the equation, G = , G being the x y specific gravity. The distance y for various values of G is de- FIG. 301. Specific gravity balance. termined and the corresponding value for G is marked on the beam. Thus in the balance figured, x is 15 inches. Then if G is 2, y is 7.5; so 2 is marked at a point 7.5 inches from the fulcrum. If G is 3, y is 10; so 3 is marked at a point 10 inches from the fulcrum. The balance is adjusted by a device just above the fulcrum. When in adjustment the balance will look like the figure, the lower pan being immersed in water and the long arm of the balance free. The mineral is placed on the upper pan and the counterpoise in the notch near the end of the long arm. Wire loops are added to the hook of the counterpoise until the mineral is balanced. Then the mineral is transferred to the lower pan. (It is well to moisten the mineral before immersion so as to free it of air bubbles.) The mineral will lose weight, so 150 INTRODUCTION TO THE STUDY OF MINERALS the counterpoise is moved toward the fulcrum until balance is restored. The specific gravity is indicated directly on the beam. This method is rapid, and the results are accurate enough for the practical purposes of determination. (d) For accurate work a pycnometer or specific gravity flask may be used. The pycnometer itself is first weighed (A). The coarsely powdered mineral is introduced into the pycnometer and another weighing (B) made. The flask is then filled with distilled water, and air bubbles are eliminated by boiling. After cooling, the weight (C) is taken. Then the flask is emptied and filled with distilled water, weight (D). Then G = ^ n -L. # _ A _ rf With proper precautions this method is very accurate. Fibrous or porous minerals should be finely powdered, otherwise the value is too low. (e) A number of heavy liquids are useful in determining the specific gravity. Methylene iodid (CH 2 I 2 ) has a specific gravity of 3.3 and may be diluted with benzol (sp. gr. = 0.98) ; this forms a liquid with any desired intermediate specific gravity. A water solution of potassium mercuric iodid (KI.HgI 2 ), also called Thou- le"t solution, has a specific gravity of 3.19 and may be mixed with water in any proportion. The specific gravity of a mineral may be determined by diluting these liquids until fragments of the mineral neither sink nor float, but remain suspended. A West- phal balance is then used to determine the specific gravity of the liquid. The heavy liquids are especially useful in separating mixtures of minerals for the purpose of analysis. The specific gravities of the common and important minerals are given below. MINERALS ARRANGED ACCORDING TO SPECIFIC GRAVITY 1.0 1.7 Sylvite Ice 1.8 2.1 1.6 1.9 Chabazite Carnallite 2.0 CHRYSOCOLLA Ulexite SULFUR GRAPHITE THE PHYSICAL PROPERTIES OF MINERALS 151 HALITE Scapolite APATITE Hy d rom agnesite Vivianite Diopside Kainite 2.7 FLUORITE OPAL Andesine Forsterite Stilbite Anorthite Jarosite 2.2 Beryl HORNBLENDE Analcite Bytownite PYROXENE Chalcanthite CALCITE Sillimanite Chrysotile Labradorite 3.3 Cristobalite PLAGIOCLASE Augite Halloysite Scapolite Axinite Heulandite TALC Clinozoisite Natrolite Turquois Enstatite 2.3 2.8 OLIVINE Apophyllite CHLORITE 3.4 Glauconite COLLOPHANE CALAMINE Tridymite DOLOMITE EPIDOTE GYPSUM Lepidolite Hypersthene Nitratine MUSCOVITE Vesuvianite Sodalite Pyrophyllite 3.5 2.4 Phlogopite Diamond Brucite Sericite Rhodochrosite Golem anite Wollastonite Titanite Gibbsite 2.9 Topaz Lazurite ANHYDRITE 3.6 2.5 Aragonite GARNET ANTIGORITE BIOTITE Kyanite CLIACHITE Datolite Rhodonite Garnierite Prehnite Spinel Leucite 3.0 3.7 2.6 Cryolite Staurolite Adularia Dahllite Strontianite Albite Tremolite 3.8 Alunite 3.1 Azurite CHALCEDONY Andalusite GARNET Kaolinite Anthophyllite LIMONITE Microcline Chondrodite PSILOMELANE ORTHOCLASE Glaucophane SIDERITE Nepheline Magnesite 3.9 Oligoclase Spodumene Brochantite QUARTZ TOURMALINE Celestite PLAGIOCLASE 3.2 MALACHITE 152 INTRODUCTION TO THE STUDY OF MINERALS 4.0 Pentlandite Anglesite CORUNDUM Pyrolusite 6.4 GARNET 4.9 Bismuthinite PSILOMELANE Marcasite 6.5 SPHALERITE 5.0 CERUSSITE 4.1 PYRITE 6.6 Carnotite 5.1 6.7 Turyite BORNITE Wulfenite Willemite Franklinite 6.8 4.2 MAGNETITE Pyromorphite CHALCOPYRITE 5.2 Vanadinite PSILOMELANE HEMATITE 6.9 Rutile Stibiconite 7.0 4.3 5.3 CASSITERITE Goethite 5.4 Pitchblende Manganite 5.5 7.1 Turyite Cerargyrite 7.2 Witherite 5.6 Mimetite 4.4 Columbite 7.3 CHROMITE 5.7 Argentite Enargite CHALCOC1TE 7.4 SMITHSONITE Jamesonite Wolframite 4.5 5.8 7.5 BARITE Pyrargyrite GALENA STIBNITE 5.9 Iron Turyite Columbite Pitchblende 4.6 6.0 8.0 Covellite ARSENOPYRITE CINNABAR PYRRHOTITE Cuprite Pitchblende Zircon Scheelite 8.8 4.7 6.1 Calaverite Ilmenite Polybasite COPPER Molybdenite 6.2 10.5 TETRAHEDRITE Columbite SILVER Turyite Smaltite 15 to 19 4.8 Stephanite GOLD Hausmannite 6.3 Platinum 2. HARDNESS The resistance that a substance offers to abrasion is called hardness. It is not a property that is capable of exact definition THE PHYSICAL PROPERTIES OF MINERALS 153 or measurement, but comparative tests are expressed in terms of a so-called scale of hardness. The scale of hardness consists of ten minerals ranging from talc, a mineral which has a soapy feel and is very easily scratched by the finger nail, up to diamond, the hardest known substance. The scale of hardness is as follows: Scale of Hardness 1 Talc 2 Gypsum Finger Nail 3 Calcite 4 Fluorite 5 Apatite Knife Blade 6 Orthoclase 7 Quartz 8 Topaz 9 Corundum 10 Diamond The finger nail is about 2J, for it scratches gypsum, but is scratched by calcite. A knife blade is about 5}^, for it scratches apatite, but is scratched by orthoclase. The hardness of a mineral is judged both by its effect on the minerals of the scale and their effect upon it. If a mineral scratches fluorite but is scratched by apatite, it has a hardness of 4J^. Two minerals of the same hardness will scratch each other. Great care should be used in determining the hardness. A foreign substance embedded in the mineral will often give too high a value. A soft mineral leaves a " chalk-mark" on a harder one, so the mark left by a mineral should be a distinct groove. Minerals made up of grains or fibers often appear too low simply because the particles are forced apart. Thus a sandstone made up of sand grains with hardness of 7 may appear to have a hard- ness of about 3 simply because the grains are loosely cemented. The value recorded in the description of minerals is the maximum value for well-crystallized varieties. 154 INTRODUCTION TO THE STUDY OF MINERALS In crystals the hardness varies with the direction, as do prac- tically all the physical properties except specific gravity. This is shown in Fig. 299, p. 148. The amount of abrasion of the crystal is determined for a number of directions by mounting the crystal on an instrument known as a sclerometer. The values are plotted and connected by a smooth curve known as the curve of hardness. The most remarkable case of variation of the hardness with direction is probably that of kyanite (tri- clinic A^SiOs). Kyanite in a direction parallel to the c-axis has a hardness of 4J^, while at right angles to this the hardness is about 7. For most faces calcite has a hardness of 3, but on the basal pinacoid (0001) the hardness is about 2, as this face is easily scratched by the finger nail. 3. LUSTER Luster is the term applied to the quality of light reflected from a substance. Metallic luster is the brilliant luster of metals possessed by most sulfid minerals such as galena, pyrite, etc., as well as some oxids such as hematite and magnetite. Minerals with metallic luster are opaque even on the thinnest edges. Adamantine is the brilliant luster of transparent or translucent minerals with high index of refraction. Examples are diamond (n = 2.41) and cerussite (n = 1.80-2.07). Vitreous is the luster of broken glass possessed by most transparent or trans- lucent minerals such as quartz, calcite, etc. Pearly luster is due to continued reflection from a series of parallel plates, and is possessed by minerals with eminent cleavage such as gypsum and talc. Silky luster is due to fibrous structure and is illustrated by chrysotile and fibrous gypsum. Waxy, greasy, pitchy, and dull are self-explanatory terms used to describe luster. 4. COLOR The term color is a general one, and for convenience usually includes black (which really means the absence of color) and white (the union of all colors). The solar spectrum is the standard for THE PHYSICAL PROPERTIES OF MINERALS 155 non-metallic colors. The term hue refers to a particular part of the spectrum. Thus we speak of an orange hue or a green hue. A given spectrum hue illuminated (or mixed with white) becomes a tint, while a given hue with insufficient illumination (or mixed with black) becomes a shade. Thus pink is a tint of red, and olive green a shade of yellow. Besides tints and shades produced from the spectrum we have other colors formed by mixing the spectrum hues with various grays (mixtures of black and white). In some minerals such as cinnabar, orpiment, malachite, and azurite the color is a property of the substance and hence is constant. The color of a metallic mineral is usually quite con- stant, but as these minerals are susceptible to tarnish a fresh fracture should always be observed. But in the majority of non-metallic minerals the color is due to some impurity which usually exists in very small amounts, and is often present as a pigment in solid solution. Thus quartz, calcite, and fluorite are colorless when pure, but they are found in various tints and shades of practically all hues which may vary even in the same specimen. 6. STREAK The streak of a mineral refers to the color of its powder. It may be determined by rubbing a corner of the mineral on a piece of unglazed porcelain or tile called a streak-plate. In the absence of a streak-plate a smooth piece of light colored flint or chert may be used. A thin slab of novaculite also makes an excellent streak-plate. The streak, though colorless for most non-metallic minerals and dark-gray or black for many metallic ones, is especially valuable in the determination of a few common minerals such as hematite (streak, red-brown) and limonite (streak, yellow-brown) . THE OPTICAL PROPERTIES OF MINERALS Among the physical properties, the optical properties take first rank in the accurate description and determination of all minerals that transmit light in thin layers, for the mineral may be deter- mined even in the absence of distinct crystals and when occurring in small quantities. Most of the optical determinations can be made by means of a special form of microscope known as the polarizing microscope, but for the more accurate determination of the optical constants the refractometer, the goniometer, and the axial angle apparatus must be used. Minerals for optical determinations may be prepared in three different forms: (1) oriented sections or sections cut in definite crystallographic directions, (2) thin sections in which the con- stituent minerals are cut at random (by this means minerals in fine grained rocks may be determined), and (3) fragments, cleavage flakes, and minute crystals. As fragments are easily prepared simply by crushing the mineral, the method is a general one for the examination of all but opaque minerals. The polariz- ing microscope should have a place in the mineralogical laboratory and should supplement the blowpipe in the determination of minerals. It is safe to say that, generally speaking, it is impossible to deter- mine the less common non-metallic minerals without the use of the polarizing microscope. 1. THE NATURE OF LIGHT It is now generally believed that light consists of a vibratory motion or some kind of disturbance in the ether, a hypothetical medium which is supposed to pervade all space and even material bodies. 1 The wave-motion, as it is called, is regarded as the 1 Light is a form of energy and as we pan not conceive of energy being transmitted without Borne kind of medium, physicists postulate a something different from ordinary matter which they call the ether. 156 THE OPTICAL PROPERTIES OF MINERALS 157 resultant of simple harmonic motion and a uniform linear motion at right angles to this. Simple harmonic motion is uniform motion in a circular path as it would appear on a diameter of the circle. The circle of reference, as it is called, is shown in Fig. 302 (adgj). A point moving from a to d appears to move from a' to d. This constitutes a periodic vibration with varying velocity which may be represented by the swinging of a pendulum. If this is compounded with linear motion from A to A', we have the harmonic curve ADGJA', which is a sine curve represented by the equation y = a sin -,x, in which a is OD, the amplitude j/_X 6' /\f x^ c J E \ u3 g3 z B O F \ \^7 V-w A G \ / \z A; " ! V H X I T K / L FIG. 302. Wave-motion. and JJT, the angular velocity in the circle. Fig. 302 illustrates the wave-motion at a given instant. Thus light consists, to the best of our knowledge, of a periodic vibration transverse to the direction of transmission, though we know nothing of its physical nature. It may be illustrated in a rough way by the waves observed along the sea-shore. A float- ing object, in general, simply moves up and down, while the wave as a whole advances toward the shore. The maximum displacement, OD (Fig. 302), is called the amplitude of the vibration. The period is the interval of time necessary for a complete vibration. The point with the maximum upward displacement, D, is called the crest of the wave and the point with maximum downward displacement, J, the trough. 158 INTRODUCTION TO THE STUDY OF MINERALS The distance between two successive crests or troughs, or a corresponding distance such as AA', is called the wave-length (denoted' by the Greek letter \ [lambda]) . The number of vibra- tions executed in a unit of time defines the frequency. By phase is meant the relative position of any two points. Two points such as A and A' are in the same phase when they are in the same relative position and moving in the same direction. Two points such as A and G are said to be in opposite phase when they are in the same relative position but moving in opposite directions. A ray of light is a line used to indicate the direction of trans- mission of the wave-motion, but in practice beams of light must be employed. A wave-front is the surface determined by all the parts of a system of waves which are in the same phase. A ray is perpendicular to its wave-front in an optically isotropic medium. The intensity of light depends upon the amplitude of the vibra- tions, and the color of the light depends upon the wave-length of the vibrations or, more accurately, upon the frequency, for the wave-length is altered when the light enters any material medium. The wave-length for the violet end of the spectrum is 380/x/* (millimicrons or millionths of a millimeter) and for the red end of the spectrum, 760/-IM (millionths of a millimeter). White light is the sum of light of the various waves which together produce the solar spectrum. For this reason monochromatic light (light of approximately one wave-length) must be employed in all accurate optical work. The simplest method of obtaining mono- chromatic light is to ignite a sodium salt on platinum wire in a dark room. Yellow light with a wave-length of 589^ is pro- duced. For red light a lithium salt is used and for green light a thallium salt. In the laboratory it is more convenient to use a colored screen which gives light with a considerable range of wave-lengths, but for quantitative work strictly monochromatic light must be used. THE OPTICAL PROPERTIES OF MINERALS 159 2. REFRACTION OF LIGHT When a beam of light passes from one medium into another, in general there is a change of direction, which is due to the fact that there is a change of velocity in the light waves. This is shown in Fig. 303. Parallel rays pp' on passing from air, say, into a section, arrive at the points rr' together. But while the impulse along the ray p' is going the distance rV in the air, that along the ray p has gone the distance rs, the velocity in the sec- tion being less than in air. The wave-front in the section is then ss' constructed by drawing a line from s f tangent to an arc with radius rs (rs being the velocity in the action as compared with velo- city r's' in air). The beam of light is bent toward the perpendicular. This phenomenon is known as refraction. A familiar illustration is the apparent bending of a stick in water. In Fig. 304 the angle i is called the angle of incidence, and the angle r, the angle of refraction. The radii of the two con- centric circles are proportional to the indices of refraction of the two sub- stances (air and a liquid in this case). The value of r for any given value of i may be found by extending the incident ray until it cuts the smaller concentric circle. From this intersection a perpendicular is dropped to the bounding surface of the two media. A line drawn from the intersection of this perpendicular with the larger circle to the center gives the direction of the refracted ray. There is found to be a constant relation between the sines of these angles, for whatever the direction of transmis- sion, - - = n (a constant). The constants, which is called the sin r index of refraction, depends upon the substance and upon the kind of monochromatic light used. The index of refraction for the violet end of the spectrum is greater than for the red end of FIG. 303. The relation of velocity to index of refraction. 160 INTRODUCTION TO THE STUDY OF MINERALS the spectrum as shown in Fig. 305. For diamond the dispersion or difference between the values of the refractive indices for opposite ends of the spectrum is very large (n v n r = 0.063), hence the " fire " of the diamond. For fluorite, on the other hand, the dispersion is very small (n v n r = 0.006), hence fluorite FIG. 304. FIG. 305. is often used in making achromatic microscope lenses. The follow- ing list gives the indices of refraction (yellow light) for some of the common minerals: Fluorite Gypsum Quartz Barite Apatite Corundum Cerussite Sphalerite 1.434 1.520-1.529 1.544-1.553 1.636-1.647 1 . 635-1 . 638 1.759-1.767 1.804-2 ..078 2.369 The minerals in the above list with the exception of fluorite and sphalerite are doubly refracting and so the index of refrac- tion ranges from a minimum to a maximum depending upon the direction of the light rays. Substances with a high index of refraction (1.9 or over) have the brilliant appearance called adamantine luster, minerals with an index of refraction lower than 1.7 have ordinary vitreous luster, while those between 1.7 and 1.9 (e.g. corundum) have i subadamantine luster. THE OPTICAL PROPERTIES OF MINERALS 161 Several particular directions of transmission should be men- tioned. In the formula - - = n, if i = 0, r =0; so for normal sin T incidence there is no refraction. If i 90, the equation becomes sin r = -; r for this particular value is called the critical angle, The critical angle like the index of refraction is a constant for the substance. A graphic determination of this angle is shown in Fig. 306. The indices of refraction of the two substances are the radii of two concentric circles, AB being the boundary between the two substances. A tangent is dropped from the intersection of the inner circle with the boundary line. A radius is then drawn through the point where this tangent intersects the outer circle. The angle PON is the critical angle. Rays of light passing from the denser (lower) medium to the rarer (upper) medium along the line PO will graze the surface OA. Rays of light passing from the denser into the rarer medium at angles greater than the critical angle cannot enter the rarer medium, but are reflected back into the denser medium as illus- trated by the dotted line of Fig. 306. This phenomenon is called total reflection. An empty test-tube immersed in a beaker of water has a peculiar silvery appearance caused by total reflection. This silvery reflection disappears when the test-tube is filled with water. The reason for the silvery appearance may be shown by constructing the critical angle for water and air on a drawing of the test-tube in a beaker of water. The great brilliancy of the cut diamond, as compared with natural crystals of the uncut diamond, is due principally to the fact that the facets are arranged so that most of the light is totally reflected, for the critical angle for diamond is very small (24 as compared with 48 for ordinary glass). Direct Determination of the Index of Refraction The index of refraction is the fundamental optical constant, and its determination is one of the best means of identifying a given n 162 INTRODUCTION TO THE STUDY OF MINERALS mineral or other substance, provided of course that it transmits light. The most direct procedure for determining the index of re- fraction is called the prism method. The crystal cut in the form of a prism of about 60 (internal angle) is mounted on a reflec- tion goniometer or spectrometer. A narrow beam of light from the collimator is refracted both on entering and emerging from the prism, as shown diagrammatically in Fig. 307. If the prism C N M FIG. 307. Determination of index of refraction by the prism method. were not present the beam of light, CO, would reach the point M but instead it is deviated out of its course and reaches a point such as N. The direct reading with the telescope at M is taken and then the telescope is moved to the left. The crystal is then revolved on the axis of the goniometer until the refracted image is in the field of view. The crystal and telescope are then manipu- lated, one with each hand, until the image first moves in one direc- tion and directly afterwards in the opposite direction. This momentary and stationary position of the image determines the THE OPTICAL PROPERTIES OF MINERALS 163 angle of minimum deviation, d, from which the index of refrac- tion may be calculated by the equation n = - ~ sin where p is the internal angle of the prism. In doubly refracting crystals two values of the index of refraction will in general be obtained. The index of refraction of a liquid may be determined by placing it in a hollow prism and proceeding as above. Other methods of determining the index of refraction depend upon finding the value of the critical angle. For this purpose an instrument known as a refractometer is used. Now the criti- Fio. 308. Determination of the index of refraction by total reflection. FIG. 309. Determination of the index of refraction by grazing incidence. cal angle may be determined in two different ways: (1) by total reflection proper and (2) by grazing incidence. The first method is shown in Fig. 308. The crystal or vessel with enclosed liquid is placed on the upper plane surface of highly refracting glass (a necessary condition is that the glass must have a higher index of refraction than the substance to be tested) and diffused light is directed upward on one side of the glass hemisphere. On striking a substance with lower index of refraction some of the rays will enter the substance but those that meet it at an angle greater than the critical angle will be re- flected back into the hemisphere. This will cause half of the field of the telescope to be partially dark and the other half light as 164 INTRODUCTION TO THE STUDY OF MINERALS shown in Fig. 308. If the sharp line of demarcation is placed on the cross hairs of the telescope, the critical angle may be read off. directly. In the other method, that of grazing incidence, the light enters the crystal, or liquid in the vessel, from the side, as shown in Fig. 309. In this case the upper side of the telescope field is dark, for no totally reflected light is allowed to fall upon the lower part of the hemisphere. In the Abbe* refractometer the principle of total reflection proper is used, while in the Pulfrich refractometer the method of grazing incidence is employed. Either of these methods may be used for solids or liquids. FIG. 310. The Smith refractometer. A convenient refractometer for the approximate determination of the refractive indices, especially for cut gem stones, is one devised by G. F. H. Smith of the British Museum. This refracto- meter is shown diagrammatically in cross-section in Fig. 310. It consists of a metal frame holding a hemisphere, h, of highly refracting glass (n = 1.79), a totally-reflecting prism, P, and lenses at V, I 2 , and I s . The substance, c, the index of refraction of which is sought, is placed over the glass-hemisphere, h and in close contact with it by means of a drop of a-monobromnaphthalenc or methylene iodid. As the glass hemisphere has a greater index of refraction than the substance (this is a necessary condition), rays of light entering at V are in part totally reflected back into THE OPTICAL PROPERTIES OF MINERALS 165 the hemisphere, the rays represented by the dotted lines entering the substance. The totally reflected rays fall on a scale, S, en- graved on glass and are reflected by the prism P into the eye-piece at P. The field of vision appears as in the circle above the eye- piece; one-half of it is light and the other half dark. After the instrument has been calibrated the index of refraction may be read off directly on the scale. The reading in the figure indicates a doubly refracting substance with indices of refraction of 1.559 and 1.588. Accurate observations should be made in mono- chromatic light, but examination in white light will indicate the amount of dispersion. Indirect Determination of the Index of Refraction A simple, but indirect, method of determining the index of refraction of a mineral is by means of the Becke test. Frag- ments of the mineral are embedded in a liquid of known index of refraction and examined on the stage of a microscope with the diaphragm of the substage partially closed. (In the absence of a diaphragm the substage should be lowered.) On focusing sharply on the edge of the mineral and then throwing it slightly out of focus by raising the microscope tube, a blurred white line will appear on the side of the substance having the greater index of refraction. If the fragment is small enough, the fragment as a whole, not simply the border, will become brighter if its index is greater than that of the liquid and darker if the index is less than that the liquid. On lowering the microscope tube the white line appears on the side of the substance having the smaller index of refraction. The explanation of the Becke test is given in Figs. 311-312. Two fragments are shown em- bedded in a liquid. The fragment on the left has an index of refraction greater than that of the liquid, while with the one on the right the opposite is the case. The microscope tube is supposed to be raised. In the first case the rays of light on striking the oblique boundary are reflected back into the mineral, for the critical angle is exceeded. In the other case the rays 166 INTRODUCTION TO THE STUDY OF MINERALS go on through the mineral because the critical angle is not ex- ceeded. If then the focus is changed to the dotted line shown, it is apparent that there will be a concentration of light toward the fragment in the first case, but not in the second. By using FIG. 311. FIG. 312. FIGS. 311-312. Explanation of the Becke test. a number of liquids the index may be obtained within certain limits. The most useful liquids are as follows : Petroleum . 450 Turpentine . 472 Clove Oil . 530 a-monobromnaphthalene . 658 + Methylene Iodide . 742 These liquids may be mixed with each other to form liquids of intermediate indices of refraction. The index of refraction of the liquid must be determined on a refractometer. The Abbe* refractometer is especially convenient because only one or two drops of the liquid are necessary and the index may be read off directly. It is convenient to have a series of 30 liquids ranging from 1.45 to 1.74 and differing from each other by 0.01. These liquids should be kept in bottles with double ground glass stoppers so that the index of refraction will remain constant. Some idea of the index of refraction may also be judged by the appearance of the fragment in the liquid. If the fragment and the liquid have about the same index of refraction, the fragment THE OPTICAL PROPERTIES OF MINERALS 167 will appear smooth and will scarcely be visible; it is said to have low relief (Fig. 314). If, on the other hand, the indices of re- fraction of the two substances are quite different, the surface of the fragment will appear rough and the borders dark. In this case the mineral is said to have high relief. It should be noted that the fragment has high relief whether its index is greater (Fig. 315) or less (Fig. 313) than that of the liquid. In the above explanations it is assumed that the velocity is the same in all directions of the substance. This is true only of amorphous and isometric substances, and even then only under normal conditions. Doubly refracting substances are treated in a later section. FIG. 313. FIG. 314. FIG. 315. FIGS. 313-315. High and low relief of mineral fragments embedded in a liquid. 3. POLARIZED LIGHT That the light transmitted through a slice of a colored tourma- line crystal cut parallel to the c-axis has acquired peculiar proper- ties may be seen by looking at one tourmaline through another similar tourmaline. (The little device known as " tourmaline tongs" shows this very well.) When similar directions of the two tourmalines of the right depth of color are parallel, a maxi- mum amount of light is transmitted, while if similar directions of the tourmalines are perpendicular no light at all is transmitted. The simplest explanation is that the light is vibrating in one plane only. A simple experiment proves that the vibration plane is the one that includes the c-axis (the axis of 3-fold symmetry). A beam of light from an arc lamp after traversing the tourmaline 168 INTRODUCTION TO THE STUDY OF MINERALS crystal and a colloidal suspension made by adding an alcoholic solution of resin to water has a maximum intensity when the plane of the c-axis is normal to the line of sight, but is almost invisible when the plane of the c-axis coincides with the line of sight. In ordinary light the vibra- tions are in all planes, while the light transmitted by the tourma- line slice is in but one plane. This light is known as polarized light. Figure 316 is a diagram- matic representation of ordinary FIG. 316. Polarization by absorption, light and polarized light as trans- mitted by tourmaline. The light reflected from non-metallic surfaces such as glass or polished wood is more or less polarized. If the light reflected from a sheet of glass is examined with the tourmaline, it will be found that for a certain angle of incidence (56 for ordinary glass) light is almost extinguished when the c-axis of the tourmaline FIG. 317. Polarization by reflection. is parallel to the plane of incidence, while a maximum amount of light is transmitted when that direction is perpendicular to the plane of incidence. This means that the reflected light is partially polarized, and that the plane of vibration is perpendi- cular to the plane of incidence as shown in Fig. 317. Another method of producing polarized light is by continued THE OPTICAL PROPERTIES OF MINERALS 169 refraction through a series of parallel glass plates. The emerging light is partially polarized and vibrates in the plane of incidence. Figure 318 shows this, as well as the fact that the reflected light is polarized and that the vibrations are in a plane normal to the plane of incidence. The most practical method of obtaining polarized light is by means of a Nicol prism, but this involves a discussion of double refraction which is the next topic. FIG. 318. Polarization by refraction. 4. DOUBLE REFRACTION If a dot is viewed through a clear cleavage rhombohedron of calcite (Iceland spar), two images of the dot are seen; on revolution of the calcite one dot remains stationary, while the other dot appears to revolve around the fixed one. A ray of light thus gives rise to two rays, the ordinary ray (the fixed one) and the extraordinary ray (the one that revolves). In Fig. 319 the image of the ordinary ray is marked o, and that of the extra- ordinary e. This phenomenon is known as double refraction and though possessed by most minerals, calcite is practically the only transparent mineral in which is it marked enough to be seen with the naked eye. Light that emerges from a doubly refracting substance such as calcite has acquired peculiar properties, as may be demonstrated 170 INTRODUCTION TO THE STUDY OF MINERALS by examining this double image with a tourmaline section cut parallel to the c-axis. When the c-axis of the tourmaline is parallel to the long diagonal of the calcite rhomb (Fig. 320) only the image due to the ordinary ray, o, is seen, but when the FIG. 319. FIG. 320. FIG. 321. FIG. 322a. FIG. 3226. FIGS. 319-3226. Double refraction in calcite. c-axis of the tourmaline is parallel to the short diagonal of the cal- cite rhomb (Fig. 321) only the image due to the extraordinary ray, e, is seen. Hence for the ordinary ray the vibrations are in FIG. 323. FIG. 324. FIG. 325. FIG. 326. FIG. 327. FIGS. 323-327. Experiment with two Iceland spar cleavages. the plane of the long diagonal and for the extraordinary ray the vibrations are in the plane of the short diagonal. See Figs. 322a and 3226. If the double image formed by a piece of Iceland spar be viewed THE OPTICAL PROPERTIES OF MINERALS 171 through another piece of Iceland spar, in general four images are visible, pairs of which wax and wane in turn as one of the Iceland spars is revolved. In certain positions 90 apart, only two images appear. Figures 323-327 show diagrammatically the changes that take place. The symbols o and e refer to images produced by 'the first rhomb, while with the second rhomb o gives rise to o and o e , and e to e and e e . This behavior is good evidence, if not proof, that in doubly refracting crystals, light is polar- ized and is vibrating in two planes which are at right angles to each other. In Figs. 323-327 the short lines through the circlets indicate the vibra- tion planes. 6. THE NICOL PRISM The principal device for producing polarized light is a Nicol prism, so-called from the name of its inventor, Nicol. It is a piece of apparatus to which we are indebted for much of our knowledge of crystal optics. A clear piece of calcite or Ice- land spar (this variety is obtained almost exclu- sively from cavities in the basalt at a certain locality in Iceland) of suitable dimensions is cut through in a plane at right angles to the principal section and 93 to the terminal faces. After polishing, the two halves are cemented by Canada balsam and mounted. Figure 328 represents a vertical cross-section of a Nicol prism together with a horizontal plan. Now as the refractive index of the ordinary ray of calcite is 1.658 and that of the balsam about 1.54, it will be seen from the figure that the ordinary ray o, in passing from the calcite to the balsam cement does not enter the balsam, but is totally reflected and meets the surface at an angle greater than the critical angle FIG. 328. Nicol prism. 172 INTRODUCTION TO THE STUDY OF MINERALS which is about 68. The extraordinary ray, e, passes on through the balsam cement but slightly affected by the balsam, as for this particular direction of trans- mission its index of refraction is 1.516. Hence there emerges from the upper surface of the nicol, plane polarized light with vibrations parallel to the short diagonal of the calcite rhomb as indicated in Fig. 328. If a Nicol prism is examined with a tourmaline, darkness re- sults when the c-axis of the tourmaline is parallel to the long diagonal of the calcite rhomb. If two nicols have their short diagonals parallel, light goes through unaffected, except that the intensity is diminished. If one of the nicols is revolved, the light gradually fades until their short diagonals are at right angles, when darkness results. 6. THE POLARIZING MICROSCOPE The polarizing microscope, also often called the petrographic microscope (Fig. 329), differs from an ordinary microscope in the addition of a rotating stage for measuring angles and of two Nicol prisms, one above, the other below, the stage. Two nicols are necessary, for the effects due to polarized light cannot usually be distinguished except by another nicol. The lower nicol is called Fio. 329. Polarizing microscope size). THE OPTICAL PROPERTIES OF MINERALS 173 the polarizer and the upper one, the analyzer. The lower one fits into a socket so it may be rotated, but ordinarily it is set so that its vibration plane is at right angles to that of the upper nicol. Then the field should be dark and the nicols are said to " crossed." In some cases we use the lower nicol alone and then its vibra- tion plane should be known. A convenient method of making this determination is to examine crushed fragments of fibrous tourma- line under the microscope. The tourmaline prisms become dark when their long axis is perpendicular to the vibration plane of the lower nicol, as shown in Fig. 330. A thin section of biotite may also be used to de- termine the vibration plane of the lower nicol. (See p. 202.) Adjustments of the Polarizing Microscope The following adjustments of the polarizing microscope must be made in the order indicated. 1. " Crossing" of the nicols. 2. Determination of the plane of vibration of the lower nicol. 3. Placing of the cross-hairs of the eye-piece parallel to the vibration planes of the nicols. 4. Centering the stage (see below). In order to use the rotating stage, its center must coincide with the optical center of the microscope tube. The method of centering the stage may be explained by referring to Fig. 331. A and B are centering screws 90 apart, located either on the stage or on the microscope tube, preferably on the latter. The two perpendicular lines across the field represent the cross-hairs. An object o on a glass slide is placed at the intersection of the cross-hairs, Suppose on revolution of the stage it appears to FIG. 330. Tourmaline fragments polarized light. 174 INTRODUCTION TO THE STUDY OF MINERALS revolve in the dotted circle until it resumes its original position. Then revolve the stage 180, correct for one-half the error by the centering screws-, the other half by moving the slide on the stage. In the figure the error is oo'> the components of which in the direc- tion of the cross-hairs are o'e and o'd. They by the centering screw A, o' is first moved the distance %o'e and by the centering screw B, o' is moved the distance %o'd. The object, o, then takes the position c and the slide itself must be moved the distance co, when the stage should be approximately centered. It may be necessary to repeat the operation several times for accurate centering. FIG. 331. Method of centering the stage. FIG, 332. Method of measuring an angle To measure a plane angle in a thin section or the interfacial angle of a small, flat crystal the stage is centered with the inter- section of the two edges at the center of the cross-hairs. A reading is made when one edge of the crystal is parallel to the east-west cross-hair, say, then the stage is revolved until the other edge (dotted line in Fig. 332) is parallel to the same cross- hair, but on the opposite side of the center, when another reading is taken. The difference between the two readings is the supple- ment or external angle (a in the figure) . The microscope is often used for measuring very small distances THE OPTICAL PROPERTIES OF MINERALS 175 such as the dimensions of minute crystals. For this purpose a special eye-piece (micrometer eye-piece) containing a scale etched on glass is used. On the stage of the microscope a scale reading hundred ths of a millimeter is placed. It is then necessary to see how many hundredths of a millimeter each divi- sion of the eye-piece is equivalent to. 7. INTERFERENCE COLORS If thin plates of singly refracting (isometric) crystals are examined between crossed nicols there is no result, for the original field remains dark. But if thin plates of doubly refracting crystals are examined between crossed nicols there result the beautiful color effects known as interference colors. In order to explain these colors it is necessary to consider the results obtained in examining the doubly refracting plates in monochro- matic light. If two light waves or train of waves of the same wave-length travel along the same path, after they have traveled different distances in another medium, they combine in general to produce a new wave, the ordinate at any point of which is equal to the sum of the ordinates of the two original waves. This phenomenon is called interference. 1 There are two special cases of importance. (1) The two waves have the same amplitude and a path difference of J^X. As can be seen from Fig. 333 they neutralize each other and darkness is the result. That is, under certain conditions two light waves can combine so as to produce darkness. (2) The two waves have the same amplitude and a path difference of X. In this case the new wave will have the same wave length, but the amplitude will be doubled (Fig. 334). For intermediate cases such as path dif- ference of %X, the resultant wave will have an intermediate amplitude (Fig. 335). Interference results when light waves from the same source go over the same path, one in advance of the other. There are two methods of obtaining interference, one by 1 The interference in this case is destructive; constructive interference is produced by diffraction. 176 INTRODUCTION TO THE STUDY OF MINERALS the use of thin films, the other by the use of doubly-refracting crystals. Let Fig. 336 represent a thin film of air in a selenite cleavage. AB and A'B' are oblique incident rays. On reaching the surface of the film they are partly reflected and partly refracted. So that for a point B' there will be two rays traveling along B'D'; one of them is the reflected ray of A'B', the other the reflected ray Path difference = ^ Path difference = FIGS. 333-335. of the refracted ray EC. These two rays are in a position to interfere for one of them has traveled a greater distance than the other. For while one has traveled A'B' the other has traveled ABE. Hence the ray from AB is the distance ECB' behind the ray from A'B'. If we use monochromatic light and adjust the thickness of the film so that the retardation, or lagging of one ray behind the THE OPTICAL PROPERTIES OF MINERALS 177 other, is equal to J^X, we have, for that particular thickness, dark- ness, while for a retardation of X we have light of maximum inten- sity. Therefore with a wedge-shaped film in monochromatic light we have a series of dark bands, for retardations of n/2 X and n\ produce the same effect as ^\ and X respectively. Now let us consider the case 'of doubly-refracting crystals. A series of parallel light waves from the polarizer or lower nicol enter the crystal and are broken up into two sets of waves, one vibrating in the plane of the paper, say, and the other in the plane normal to the paper. See Fig. 337. At certain points on the upper surface there will emerge two sets of waves traveling FIG. 336. FIG. 337. in the same path but vibrating in planes at right angles to each other, and oblique to the planes of vibration of the nicols. In order that they may interfere it is necessary to reduce the vibra- tions to one plane and for this purpose an analyzer or upper nicol is necessary. The effects produced depend upon the relative positions of the nicols, upon the position of the crystal with reference to the nicols, and upon the path difference of the two sets of polarized light waves. Figure 338 explains diagrammatically what happens when a doubly refracting crystal is examined between the crossed nicols of a polarizing microscope. A ray of light entering the lower 12 178 INTRODUCTION TO THE STUDY OF MINERALS nicol is broken into two rays, e and o. One of these (o) is totally reflected and disappears. The remaining ray (e) is broken into two rays (e' and o') by the mineral plate. These two rays, which FIG. 338. Diagrammatic representation of doubly refracting crystal examined between crossed nicols. are vibrating at right angles to each other, are each broken into two rays (e", o", and e'" , o'"), by the upper nicol. One from each of these (o" and o'"} is totally reflected and finally there emerge from the top of the upper nicol two sets of waves (e" and THE OPTICAL PROPERTIES OF MINERALS 179 e'") vibrating in the same plane and these interfere with each other. With crossed nicols darkness results when the path difference is X. In Fig. 339, PP' is the vibration plane of the lower nicol and AA' of the upper nicol, RR' and SS', the vibration planes of the crystal plates. The distances or and os represent vibrations in the same phase. The components of these in the plane of the upper nicol are op and ocr, which are opposite and equal. Hence FIG. 339. FIG. 340. FIG. 341. they annul each other. With crossed nicols there is maximum light when the path difference is J^X. In Fig. 341, r' and s represent vibrations of J^X path difference. Their components in the plane of the upper nicol are p' and a, equal but on the same side of the origin. Hence the intensity is doubled. For a path difference of J^X, we have the intensity shown in Fig. 340. Thus the intensity varies between for a retardation of X, and a maximum for retardation of J^X. As can be seen from Fig. 342, the retardation produced by a, 180 INTRODUCTION TO THE STUDY OF MINERALS wedge-shaped section of a crystal will vary from point to point. A retardation of n\ will give darkness and a retardation of n/2 X will give a maximum intensity. Hence a wedge examined in mono- chromatic light beween crossed nicols will appear as a series of parallel dark bands interspersed with colored spaces. For white light we have the combined effect of all the colors of the spectrum. Some idea of the interference colors seen in white light may be gained by a study of Fig. 343. (This diagram may be colored by the student.) The top of the figure represents a wedge-shaped section as viewed in various kinds of monochro- matic light, there being a dark band at positions which give retardations of n\. For each of these colors a medium value of FIG. 342. Wedge of a doubly-refracting crystal between crossed nicols. the wave length is chosen as follows: red, 700^; orange, 620/x/x; yellow, 560juju; green, 515MMJ blue, 460/xju; violet, 410/^/4. The top row of figures gives the value of retardations in n,u. The lower part of the figure indicates the interference colors as seen in white light. Let us consider the colors in succession. The intensity of different parts of the spectrum varies and has an influence in determining the color. Yellow-green is the most intense, and violet, the least intense part of the spectrum. The interference color chart, as it is called, begins with darkness, succeeded by dark gray which gradually becomes lighter. At about 250jUM all the colors combine to form white light. At for yellow) yellow is at a maximum, but mixed with THE OPTICAL PROPERTIES OF MINERALS 181 A 100 200 300 400 500 800 700 800 900 1000 1100 1200 1300 1400 1500 1000 1700 1300 1900 2000 2100 Orange Je'low Green Blue Violet * ? A o is! s I| 1 2 2 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1800 1700 1300 1900 2000 2100 First Order Second Order Third Order Fourth Order FIG. 343. The derivation of the interference color chart. white it gives straw yellow. At 310/i/i and at 350/i/i, orange and red respectively, are at a maximum, but the great intensity of the yellow modifies these colors and places them further to the right, for at 360/iM the color is bright yellow. At about 550juM> violet is the color. Though of weak intensity, violet is produced here because the other colors are practically extinguished. As can be seen from the diagram the colors follow in order: blue, green, yellow, and red. At about 11 00/-iM> violet appears again. At this point only red, blue, and violet are near a maximum. But red and blue together produce violet. Then in order we have blue, green, yellow, and red again. These same colors are repeated a second time but become paler and then pass into neutral tints (largely pink and pale green) and finally into high-order white which resembles ordinary white light. 182 INTRODUCTION TO THE STUDY OF MINERALS There is a repetition in the colors, but they gradually become fainter. The colors from black up to the first violet (A = 550/i/i) are called first -order colors, from this violet up to the second violet (A= 1100/i/z), second -order colors, and so on. In white light seven err eight orders may be distinguished, but in mono- chromatic light there is no limit to the number of orders as determined by the dark bands. By trial it may be found that the interference color depends upon three factors: (1) the double refraction which is a constant for the crystal, (2) the orientation or direction in which the crystal is cut (e.g., in a thin section of sandstone the quartz grains, cut in various directions, have a great variety of interference colors), (3) the thickness, as may be seen in a quartz or gypsum wedge. The formula A = t(ni n^) gives the relation between A, the retardation in W, t, the thickness of the plate, and (n\ n 2 ), the double refraction ; HI and n 2 are the two values of the indices of refraction for a particular section. For a given substance with known indices of refraction the thickness may be measured and the interference colors predicted. Or the thickness may be measured, the retardation determined from the color chart, and the double refraction calculated. Or the thickness may be cal- culated that will give a certain interference color for a crystal with known double refraction. In the interference color chart of Fig. 344, the horizontal lines represent the thickness from 0.00 to 0.06 mm.; ordinary rock slides and fragments are from 0.03 to 0.05 mm. in thickness. The vertical lines give retardations in /*/*, while the diagonal lines represent the amount of the double refraction. A crystal of 0.03 mm. thickness and double refraction of 0.02 has a retardation of 600^ (0.000600 = 0.03 X 0.02), and gives a second-order indigo blue interference color. Fig. 344 may be colored to correspond to the colors given at the bottom of Fig. 343; the names of the common minerals may be written in the blank space to the left in the appropriate position according to the double refraction. 183 184 INTRODUCTION TO THE STUDY OF MINERALS 8. VIBRATION OR EXTINCTION DIRECTIONS If a section of a doubly refracting crystal is revolved between crossed nicols there is darkness four times in a complete revolu- tion. This is called extinction and is simply due to the fact that for these particular positions the crystal has no effect upon the dark field produced by the crossed nicols. The two directions in the crystal parallel to the vibration planes of the two nicols are called extinction directions or vibration directions. These two directions are directions of the two plane polarized waves pro- duced in doubly refracting crystals by the plane-polarized light of the lower nicol. FIG. 345. Parallel extinction. FIG. 346. Oblique extinction. FIG. 347. Symmetrical extinction. Now according to the position of these directions with refer- ence to the crystal outlines we have various kinds of extinction characteristic of crystals of the various systems. In case the directions are parallel to the outline we have parallel extinction. This is represented by the convention of Fig. 345, the cross-hairs of the microscope being parallel to the vibration-planes of the nicols, and the crystal placed in the position of darkness. If the directions are not parallel to the outline we have oblique extinction (Fig. 346). The particular case in which these direc- tions make equal angles with the edges of the crystal is called symmetrical extinction (Fig. 347). THE OPTICAL PROPERTIES OF MINERALS 185 The angle between an extinction direction and a prominent crystallographic direction of a crystal (usually the c-axis) is called the extinction angle, and is characteristic of certain crystals in certain directions. The extinction angle is determined by taking a reading when the outline is parallel to one of the cross- hairs (the stage being centered) and then revolving the stage until maximum darkness results, when another reading is taken. The difference between the two readings is the extinction angle. In Fig. 346, the extinction angle indicated by the arrow is 15. It may be noticed that there are two possible extinction angles which are complementary. The smaller angle ( < 45) is usually taken. Accurate determinations of the extinction angle are made in monochromatic light. A convenient determination in white light may be made by using a gypsum plate which gives a field showing red of the first order. This is called the sensitive tint, for the least change gives either orange-red or violet-blue. When inserted in the slot provided for test-plates, a doubly refracting crystal appears the same tint of red as the red field only when it is in the extinction position. 9. THE DETERMINATION OF THE INDICES OF REFRACTION IN DOUBLY-REFRACTING CRYSTALS For a section of a doubly-refracting crystal cut in any direction there are in general two values of the index of refraction, one for each of the two vibration directions at right angles to each other. These two directions are the extinction positions for the particular section. After bringing the crystal plate or fragment into extinction, one index of refraction is determined with reference to the set of liquids described on p. 166. Then after revolving the stage of the microscope 90, the other index is determined in exactly the same way. See Fig. 348. The two values of the index of refraction may be designated fti and r& 2 , and their difference (n\ n 2 ) is the double refraction or birefringence for the particular section. The vibration direc- 186 INTRODUCTION TO THE STUDY OF MINERALS tion of the two rays of the doubly-refracting substance are at right angles to each other. For a given doubly-refracting crystal cut in various direction a good many different values of HI and n 2 may be obtained. The maximum of all possible values of n\ is denoted by n y , and the minimum of all possible values of n 2 is denoted by n a . Some particular section, which may be recognized by the fact that it has the highest interference color for a given thickness, will furnish both n y and n a . In addition to n y and n a , a great many intermediate values may be obtained. In orthorhombic, mono clinic, and triclinic crystals (these are collectively called biaxial) a section cut normal to the plane of a and y furnishes an important inter- mediate (not a mean) value known as %. The fact that the value np always lies between FIG. 348. To illustrate the deter- (except in the rare cases in mination of indices of refraction in wn ich it is exactly equal to one doubly-refracting crystals. , or both of them) the two values of HI and n z found on any section enables one to determine it by trying fragments in various liquids until a liquid is found, the index of refraction (n 3 ) of which is greater than that for various fragments (n z >ni>nz] and also another liquid, the index of refraction (n*) of which is less than that of various fragments (n4& of a biaxial crystal con- taming a, 7, and the optic axes. 192 INTRODUCTION TO THE STUDY OF MINERALS axis appears uniformly bright between crossed nicols for all positions of rotation. The optic axes always lie in the plane of 7 and a, which is therefore called the plane of the optic axes or the axial plane; the acute angle between the optic axes is called the axial angle (denoted by the term 2F). In Fig. 353, OA and OA' are the optic axes and AOA' the axial angle. The optic axes are always symmetrically placed with respect to 7 and a. SYNOPSIS OF THE OPTICAL PROPERTIES AND CONSTANTS FOR THE CRYSTAL SYSTEMS g 1 S All sections No inter- i ft ft "p. ISOMETRIC remain dark n ference 1 03 figure c 1 I TETRAGONAL UNIAXIAL soid a spheroid of Basal sec- tion dark. Parallel ex- tinction in most other sections n a , n y Interference figure with dark cross and colored circles p. 3 HEXAGONAL o 1 Parallel ex- Symmetrical % ORTHORHOM- tinction in dispersion. BIC most sec- tions. ! Parallel ex- Interference Horizontal, j '^ tinction in figure with inclined, or 3 'C 100, 001, dark hyper- crossed 35 MONOCLINIC and hOl n a , n0, n y bola and dispersion. 2V ^ sections. colored I Oblique in ellipses and 13 others. lemniscates. Q I TRICLINIC Oblique ex- Asymmetric tinction in dispersion. all sections THE OPTICAL PROPERTIES OF MINERALS 193 The line bisecting the axial angle is called the acute bisectrix (denoted by Bx a ). The line bisecting the obtuse angle between the optic axes is called the obtuse bisectrix (denoted by Bx ). There are two divisions of biaxial crystals according to whether the acute bisectrix is 7 or a. The former are called positive (Bx a = 7), and the latter, negative (Bx a = a); this is a purely arbitrary designation. Figure 353 represents a positive crystal. The determination of the optical character may be made by testing for the faster and slower ray in a section known to be perpendicular to the acute bisectrix. It may also be determined in this kind of section by obtaining an interference figure and testing it with a mica plate. The optical properties for the crystal systems may be tabu- lated as in the preceding table. 12. INTERFERENCE FIGURES The tests mentioned up to this point have been made by using ordinary parallel light or polarized parallel light. A unique series of effects, important in the identification and description of minerals, may be obtained by examining suitable sections in convergent polarized light. For this purpose either a polari- scope or a polarizing microscope may be used. A polariscope is an instrument consisting essentially of an analyzer and a polarizer with slight magnifying power and strongly convergent lenses both above and below the stage. If the polarizing microscope is used, a high power objective (Focal length = 3 to 5 mm.) and also a condensing lens placed just below the stage must be substituted for the ordinary set-up. Either the eye-piece must be removed, or a special lens, called the Bertrand lens, must be inserted in the microscope tube be- tween the analyzer and the eye-piece. The color effects seen when basal sections of uniaxial crystals and sections of biaxial crystals cut normal to the acute bisectrix, are examined in convergent light between crossed nicols are known as interference figures. 13 194 INTRODUCTION TO THE STUDY OF MINERALS FIG. 354. FIG. 355. FIG. 356. FIG. 357. FIGS. 354-357. INTERFERENCE FIGURES OBTAINED BETWEEN CROSSED NICOLS IN MONC CHROMATIC LIGHT. (After Hauswaldt.} FIG. 354. Uniaxial interference figure (calcite). Plate cut normal to the optic axis, FIG. 355. Uniaxial interference figure (calcite). Plate cut oblique to the optic axis. FIG. 356. Biaxial interference figure (aragonite). Plate cut normal to the acut bisectrix. Normal position. FIG. 357. Biaxial interference figure (aragonite). Plate cut normal to the acut bisectrix. Diagonal position. THE OPTICAL PROPERTIES OF MINERALS 195 With isometric crystals no interference figures are obtained, for there is no double refraction. Double refraction is necessary for the production of interference figures. In fact, interference figures are simply the result of interference colors, due to varying double refraction in different directions, combined and modified by the darkness due to crossed'nicols. FIG. 358. Explanation of a uniaxial interference figure. Basal sections of uniaxial crystals examined in monochromatic convergent light between crossed nicols give a dark cross with dark concentric rings (Fig. 354). The explanation is as follows: (see Fig. 358). Strongly convergent rays of light traverse the crystal in various oblique directions and the effect is the same as if rays of parallel light were transmitted through a wedge of the crystal. Therefore, along any radius we get a dark band 196 INTRODUCTION TO THE STUDY OF MINERALS where the retardation is n\ for the particular light used. Mid- way between the bands we get the maximum color. As the structure is the same all around the c-axis in uniaxial crystals, the dark bands are circular. It remains to explain the dark cross, which, it should be noticed, is stationary on rotation of the section on the stage. The crystal, it may be imagined, is made up of innumerable parts, each with extinction directions at right angles. These parts are arranged radially around a center, and on rota- tion of the stage, as vibration directions of successive parts be- come parallel to the vibration directions of the nicols, darkness results for that particular part. As new radii are always coming into the extinction position there is always a dark cross, the arms of which are parallel to the vibration directions of the two nicols and so remain fixed. That the optic axis is simply a direction in the crystal may be proved by the fact that the interference figure remains the same when the crystal plate is moved about on the microscope stage. For sections not quite parallel to the basal plane, the dark cross on rotation of the stage is eccentric, but the arms always remain parallel to the vibration planes of the nicols and revolve in the same direction in which the stage is rotated (see Fig. 355). With ordinary white light we still have the black cross, but the dark rings become colored rings, 1 the colors of which vary from black at the center, through gray, white, yellow, red, blue, green and so on, until after six or seven orders there is practically white light. The number of rings depends upon the thickness and also upon the double refraction. In very thin sectons there may be no rings visible. Very thick sections show the full number of rings. Quartz with weak double refraction shows for ordinary thickness no rings at all, while calcite with very strong double refraction shows a large number of rings. The optical character of a uniaxial crystal may be determined from the interference figure by inserting in the slot of the micro- : The rings may appear to be dark on account of the weak intensity of the violet portions but the borders at least are colored. THE OPTICAL PROPERTIES OF MINERALS 197 scope tube just above the objective, a mica plate with the slower ray, 7, in the 45 position. The interference figure is changed, the dark cross disappears and two dots appear in two oppo- site quadrants, as represented diagrammatically in Fig. 359. If the imaginary line joining the two dots is perpendicular to 7 of the mica plate, the crystal is positive (as in Fig. 359a), while if parallel to 7 of the mica plate, it is negative (as in Fig. 3596). This is due to the fact that the interference colors " go up "in two opposite quadrants and "go down" in the other two quadrants. The rings, then, are not continuous, but broken, and the two rings nearest the center form the two dots. FIG. 359a. FIG. 3596. FIGS. 359a-3596. Uniaxial interference figures with superimposed mica plate. Sections of biaxial crystals cut normal to the acute bisectrix show an interference figure like that of Fig. 356, with a black cross and two sets of concentric ellipses passing into 8-shaped curves (lemniscates) . In monochromatic light the rings are dark and in white light, colored. On revolving the section on the stage, the dark cross opens up and passes into hyperbolae as shown in Fig. 357, which represents the 45 position. The line joining the centers of the ellipses is the trace of the axial plane; the centers of the ellipses represent the emergence of the optic axes. The biaxial interference figure may be explained by means of the diagrammatic Fig. 360. For monochromatic light the optical structure of biaxial crystals is symmetrical to three planes at 198 INTRODUCTION TO THE STUDY OF MINERALS right angles to each other. One of these is the plane of the paper, while the other two are represented by their traces, the vertical and horizontal lines of the figure, which are also vibration planes of the two nicols. The two circlets are traces of the optic axes. The extinction directions for various parts of a crystal may be obtained by bisecting, internally and externally, the angles formed by joining any point with the traces of the optic axes. The dotted lines represent this procedure for one point. In similar manner the small crosses were obtained for different FIG. 360. Explanation of a biaxial interference figure. parts of the crystal. In the normal position, a black cross will be formed along the vertical and horizontal lines. On revolving the section the black cross disappears. In the 45 position brush-like hyperbolae are formed by the darkness of different parts along the hyperbolae of the figure. In the 90 position the cross is restored. The number of rings depends upon the strength of the double refraction and upon the thickness, but the distance between the vertices of the hyperbolae remains constant whatever the thickness. The optical character is determined by means of a quartz THE OPTICAL PROPERTIES OF MINERALS 199 wedge on which is marked the slower ray 7. The quartz wedge is inserted in the slot, thin end first, when the interference figure shows hyperbolae. Then when 7 is parallel to the trace of the axial plane, the ellipses appear to expand for positive crystals and to contract for negative crystals. When 7 is perpendicular to the trace of the axial plane, the ellipses contract for positive crystals and expand for negative crystals. A section of a biaxial crystal normal to an optic axis shows a series of concentric rings crossed by a dark bar which revolves in an opposite direction from the rotation of the stage. The axial angle of a biaxial crystal may be measured by means of. an axial angle apparatus, which is practically a reflection gonio- meter plus Nicol prisms. If a suitable crystal is mounted, so that it can be rotated around its direc- a > tion as an axis, between horizontal crossed nicols, arranged so that the interference figure shows hyperbolae, the apparent axial angle can be determined by reading the circle when the vertices of the hyper- bolae are tangent to the cross-wires. Figure 361 represents a section of a biaxial crystal parallel at the axial plane. It will relation between true and be seen that the angle measured is not 27, apparent axial angles " but another angle which is called 2E and related to it by the following equation: sin E = n^ sin 7. -(rip being the index of re- fraction in the direction of the optic axis). The value 2E is often recorded, as it is obtained directly. Another, but less accurate, method of measuring the axial angle is based upon the fact that the distance apart of the vertices (d in Fig. 361) of the hyperbolae of an interference figure is proportional to the value of 2E. This determination may be made by using a micrometer eye-piece in the microscope. The distance apart of the branches of the hyperbolae of a substance with previously determined value of 2E is measured. This determines the constant C in the equation sin E = d/C. So 200 INTRODUCTION TO THE STUDY OF MINERALS for other crystals, if the same microscope and combination of lenses are used, the value E may be calculated from the measure- ment of d and substitution of C in the above formula. 13. OPTICAL PROPERTIES OF TWIN-CRYSTALS One-half of a twin-crystal has the same position with respect to the other half that it would have if it were rotated 180 about an axis from its original position. From this it can be seen that the extinction angles in two halves of a twin-crystal are equal, but opposite in sign, if the section is cut normal to the twin-plane. Thus in a cleavage of a gypsum twin the extinction directions are inclined to each other. Consequently if examined between crossed nicols, one-half of such a crystal is dark, while the other half is light (see Fig. 362). On rotation, the light and dark parts interchange. FIG. 362. Fm. 363. FIG. 364. FIGS. 362-364. Sections of twinned crystals examined between crossed nicols. A section of a polysynthetic twin such as plagioclase (see Fig. 363) shows a series of dark and light bands; the extinction directions in alternate bands are parallel. Many orthorhombic crystals such as aragonite are pseudo- hexagonal by twinning, and basal sections between crossed nicols are divided into six sectors like Fig. 364, opposite pairs of which extinguish together. Basal sections of aragonite, like Fig. 364, examined in convergent polarized light, show a biaxial interference figure in each sector but are arranged so that the axial THE OPTICAL PROPERTIES OF MINERALS 201 planes are parallel to the outline. An optical examination reveals the composite nature of many apparently simple crystals. 14. ABSORPTION AND PLEOCHROISM The color of a transparent substance is due to the residual color of the spectrum left after the substance has absorbed a certain part of it. Many colored anisotropic crystals have the property of absorbing different amounts or kinds of light in different directions. Absorption has reference to the amount or intensity of light absorbed and hence may be tested in mono- chromatic light, while pleochroism refers to the kind of light absorbed and so necessarily must be tested in white light. FIG. 365. Dichroscope. A prismatic crystal of epidote from the Sulzbachthal in Tyrol of suitable thickness will appear green in a certain position, while on revolving it 90 about its long axis it will appear brown. Few substances show such a marked change as this; at any rate it is not always possible to turn a crystal about and look through it in various directions. The determination of pleochroism and absorption is made either by means of one nicol of a polarizing microscope, in which case minute crystals may be examined, or by means of the dichro- scope when large crystals are available. A dichroscope is simply a piece of Iceland spar set in a cylin- drical frame that is provided with a small aperture at one end and either open or provided with a lens at the other end. Figure 365 is a diagrammatic representation of the dichroscope. When held up to the light the dichroscope shows two images of the aperture side by side, for the diameter of the aperture is made so that 202 INTRODUCTION TO THE STUDY OF MINERALS the images do not overlap. If a pleochroic crystal is viewed through the dichroscope, two colored images are seen simulta- neously. The reason for this is that in a doubly refracting calcite crystal we have two sets of light waves vibrating in planes at right angles to each other. The color of some crystals, as we have seen, varies with the direction, but by using a Nicol prism we may observe the two colors successively, one when the vibration plane of the nicol is parallel to the length of the crystal, and one when perpendicular to the length of the crystal. These colors are called axial colors. FIG. 366. Biotite (in rock section). Fio. 367. Calcite (cleavage fragments). In uniaxial crystals there are two axial colors, hence the term dichroic is used. In biaxial crystals there are three axial colors, hence the term trichroic. In order to determine the axial colors for particular directions it is necessary to ascertain the vibration plane of the lower nicol. For this purpose a rock-section containing biotite may be used. Biotite sections showing cleavage have very strong absorption. On revolving the section, the biotite becomes very dark every 180. The cross-wire which is parallel to the cleavage traces of* the biotite when it is darkest represents the vibration plane of the lower nicol as illustrated in Fig. 366. Here the arrow indicates the vibration plane. If a rock-section containing biotite is not available, the test may be made by examining minute cleavage fragments of calcite THE OPTICAL PROPERTIES OF MINERALS 203 obtained by pounding to a coarse powder almost any kind of cal- cite. If mounted in oil of cloves or Canada balsam, the calcite rhombs have a marked relief when their long diagonals are parallel to the vibration plane of the lower nicol, and but slight relief when their short diagonals are parallel to this direction as represented in Fig. 367. The- vibration direction of the lower nicol is indicated by the arrow. Fragments of fibrous tourmaline may also be used for the same purpose. The prismatic fragments appear dark when their length is perpendicular to the vibration plane of the lower nicol. (See Fig. 330, p. 173.) FIG. 368. FIG. 369. FIG. 370. FIGS. 368-370. Pleochroism of glaucophane. The trichroism of a biaxial crystal is beautifully illustrated by the soda amphibole, glaucophane, as seen in thin rock-sections under the microscope. Three kinds of cross-sections may be distinguished as follows: pseudo-hexagonal (Fig. 368), stout rectangular (Fig. 369), and thin with oblique ends (Fig. 370). These sections are respectively almost perpendicular to the c-, a-, and 6-axes. These sections when rotated on the stage of the microscope show respectively the following pairs of colors: neu- tral and violet, blue and violet, blue and neutral. It will be seen that the color for the a-axis is the neutral tint, for the mineral has this color when the a-axis is parallel to the vibration plane of the nicol (represented by the arrow). Similarly the violet is the 204 INTRODUCTION TO THE STUDY OF MINERALS color for the 6-axis, and blue for the c-axis. The three axial colors may be combined in an axial cross. In glaucophane it has been found that b = 0, and a (almost) = a, and c (almost) = 7. The absorption may be indicated by the following: 7 >/?>. This is called the absorption scheme and means that more light is absorbed in the 7 or c-direction than in the ]8 or 6-direction, and more in the /3 or 6-direction than in the a or a-direction. Sometimes Gothic letters are used instead of a, /?, and 7. 15. SUGGESTED OUTLINE OF TESTS TO ILLUSTRATE THE OPTI- CAL PROPERTIES OF MINERALS The following outline will serve as an introduction to the study of crystal optics. Most of the slides are made from cleavage flakes and fragments, but thin sections are necessary for some of the examples. The fragments are produced by pounding (not grinding) small chips of the mineral to coarse powder on an anvil or in an agate mortar. The largest fragments that will pass through a 100-mesh sieve are selected. For temporary slides, clove oil is a convenient mounting medium. Per- manent slides may be made by using a solution of Canada balsam in xylol. The xylol gradually evaporates. On account of cleavage and structure a great many mineral fragments have a more or less characteristic shape. Those without cleavage are irregular (see Fig. 377). Form. Euhedral. Calamine crystal. (Measure angles and determine faces). Subhedral dolomite in sedimentary dolomitic limestone. Anhedral quartz in sandstone. Cleavage fragments (See Figs. 371-376.) Triangular cleavage fragments fluorite. (Fig. 371.) Rectangular cleavage fragments anhydrite. (Fig. 372.) Rhombic cleavage fragments calcite. (Fig. 373.) Prismatic cleavage fragments tremolite. (Fig. 374.) Acicular cleavage fragments wollastonite. (Fig. 375.) Platy cleavage fragments (not previously included) orthoclase. (Fig. 376.) Irregular fragments quartz. (Fig. 377.) Inclusions. Regularly arranged labradorite, phlogopite. Intergrowth perthite (microcline with albite). Alteration olivine to antigorite. THE OPTICAL PROPERTIES OF MINERALS Index of Refraction and Relief. High relief, n< clove oil fluorite. Low relief, n< (about =) clove oil orthoclase. High relief, n> clove oil anhydrite. 205 FIG. 377. FIGS. 371-377. Cleavage fragments as observed under compound microscope. Pleochroism and Absorption. Pink to red pale blue to indigo tourmaline. 206 INTRODUCTION TO THE STUDY OF MINERALS Blue to purple glaucophane. Pink to pale green hypersthene. Relief varies with the direction calcite. (High relief when long diagonal of rhomb is parallel to the vibration plane of the lower nicol.) Isotropic. Amorphous opal, volcanic glass. Isometric fluorite. Anisotropic anhydrite, quartz, apatite, etc. Interference Colors. Due to thin films cleavage cracks in gypsum, calcite, or fluorite. Effect of thickness Gypsum wedge (made by shaving down a cleavage flake of gypsum.) {Low-order colors apatite. Bright colors anhydrite. High-order colors calcite. Effect of orientation Quartz in sandstone. Extinction. Parallel wollastonite. Symmetrical calcite, dolomite. Oblique, small extinction angle hornblende, tremolite. Oblique, large extinction angle augite, diopside. Elongation. Parallel to faster ray stilbite. Parallel to slower ray wollastonite. Aggregate Polarization chrysocolla. Spherulitic Structure chalcedony. Optical Anomalies. Amorphous, but doubly refracting cellophane. Isometric mineral with double refraction leucite. Anomalous interference colors vesuvianite, chlorite. Twinning. Simple gypsum . Polysynthetic plagioclase. Crossed microcline. Interference Figures. Uniaxial positive. Brucite, cleavage; quartz (basal section). Uniaxial negative. Calcite, basal parting; wulfenite, tabular crystal. Biaxial positive, small axial angle. Chlorite, cleavage. THE OPTICAL PROPERTIES OF MINERALS 207 Biaxial positive, large axial angle. Topaz, cleavage. Biaxial negative, small axial angle. Phlogopite, cleavage. Biaxial negative, large axial angle. Muscovite, cleavage. Biaxial, normal to optic axis, shows axial bar. Epidote, (001) cleavage; diopside (001) parting. Optical Orientation. Muscovite crystal (determine position of a, /3, and 7 with respect to crystallographic axes a, b, and c). 16. LIST OF MINERALS ARRANGED ACCORDING TO INDICES OF REFRACTION An arrow after a mineral name indicates that the mineral is doubly refracting. If the arrow points downward, the lower value (n a ) is the one given, and the highest value (n T ) may be found in a place further on in the list. Other possible values lie between these two extremes. The fact that no arrow is placed after a mineral name means that the mineral is either optically isotropic (amorphous or iso- metric) or has such a weak double refraction (less than 0.01) that the extreme values are included within a single division. Some amorphous and isometric minerals are variable in com- position and so may be found in two or more divisions. < 1 . 45 CALCITE J, Ulexite J, Cryolite Carnallite ] 1.51 FLUORITE Chabazite Adularia j Ice Cristobalite Chalcanthite J, Nitratine J Natrolite | ORTHOCLASE J 1 . 45 Sodalite Ulexite j OPAL 1.49 1.52 1 . 46 Heulandite Adularia f Carnallite j Kainite j GYPSUM CHRYSOCOLLA J, Stilbite Hydromagnesite J 1 . 47 Sylvite Kainite | Natrolite j 1 . 50 Magnesite J, Tridymite DOLOMITE-J, Microcline 1-48 Lazurite ORTHOCLASE | Analcite Leucite Strontianite 208 INTRODUCTION TO THE STUDY OF MINERALS Witherite J, 1.53 Albite Apophyllite Aragonite j CHALCEDONY J, Gibbsite j Nepheline J, 1.54 Chalcanthite j CHALCEDONY | Chrysotile | HALITE Hydromagnesite f Oligoclase Nepheline f QUARTZ J, TALC | 1.55 Andesine Chrysotile f Gibbsite f Halloysite QUARTZ | Scapolite J, 1.56 ANTIGORITE j Beryl Brucite J, CHRYSOCOLLAt Kaolinite Labradorite Lepidolite J, MUSCOVITE | Phlogopite J, Sericite J, 1.57 ANHYDRITE J, Anorthite J, ANTIGORITE | BIOTITE | Bytownite CLIACHITE Pyrophyllite Vivianite j 1.58 Anorthite f Brucite f CHLORITE Colemanite \, COLLOPHANE Garnierite Nitratine j Pyrophyllite TALCf 1.59 CHLORITE COLLOPHANE Lepidolite f MUSCOVITE | Scapolite f Sericite | 1.60 Chondrodite J, COLLOPHANE Rhodochrosite | Phlogopite | 1.61 ANHYDRITE | CALAMINE Colemanite j COLLOPHANE Dahllite Prehnite SMITHSONITE | Topaz | Tremolite j Turquois | 1.62 Celestite COLLOPHANE Dahllite | Datolite | Glaucophane | Topaz | Vivianite | Wollastonite I 1.63 Andalusite Anthophyllite \, BARITE I BIOTITE | CALAMINE | Chondrodite | Celestite | Forsterite J, HORNBLENDE SIDER1TE | TOURMALINE Tremolite f Wollastonite T 1.64 APATITE BARITE t Glaucophane f Prehnite t 1.65 Anthophyllite j CALCITE | HORNBLENDE MALACHITE [ Spodumene J, TOURMALINE t Turquois | 1.66 Datolite f Enstatite J, OLIVINE I Sillimanite t 1.67 Axinite Diopside J, Enstatite | Forsterite f Spodumene f Strontianite f Witherite THE OPTICAL PROPERTIES OF MINERALS 209 .68 1.73 Hausmannite Aragonite j Augite f HEMATITE DOLOMITE t Brochantite J, Jarosite Sillimanite f EPIDOTE | LIMONITE .69 Grossularite MALACHITE | Hypersthene J, Rhodonite | Mimetite OLIVINE t Staurolite j Pitchblende Willemite j 1.74 Polybasite .70 (Methylene lodid) Pyrargyrite Diopside f Anglesite Pyromorphite Hypersthene f Azurite Rhodochrosite | Willemite | Brochantite f Rutile .71 Carnotite Scheelite Augite J, CASSITERITE SIDERITE I Clinozoisite J, Cerargyrite SMITHSONITE Kyanite J, CERUSSITE SPHALERITE Magnesite f CHROMITE Staurolite f .72 CINNABAR SULFUR Clinozoisite f CORUNDUM Titanite Kyanite f Cuprite Turyite Rhodonite J, Diamond Vanadinite Spinel EPIDOTE t Wulfenite Vesuvianite Goethite Zircon GARNET 14 MINERALS 1. ELEMENTS. 2. SULFIDS. 3. SULFO-SALTS. 4. HALOIDS. 5. OXIDS. 6. ALUMINATES, FERRITES, ETC. 7. HYDROXIDS. 8. CARBONATES. 9. PHOSPHATES, NITRATES, BORATES, ETC 10. SULFATES. 11. TUNGSTATES AND MOLYBDATES. 12. SILICATES. MINERALOIDS (GLASS AND HYDROCARBONS) 210 PART II THE DESCRIPTION OF IMPORTANT MINERALS AND MINERALOIDS Introductory About a thousand or so distinct kinds of minerals or mineral species are recognized by the mineralogist. Most of these are very rare, many of them being found only at the single, original locality in which they were discovered. In this book 175 minerals are considered. These include all the common minerals, most of those of economic importance, and a few others which are added so as to give the student a comprehensive view of the mineral kingdom as a whole. The student may occasionally encounter a mineral not included in the list of 175 and the larger reference books such as Dana's System and Hintze's Handbuch must be consulted. There is also a possibility of finding a new mineral, but the chance is very remote for bona fide new minerals are being found and described at the rate of only about ten or so a year by mineralogists the world over. By mineral species we mean all specimens with essentially the same chemical composition (some variation must be allowed as stated on page 13) and the same crystal form. Each mineral species has a distinctive name in addition to the chemical name of the substance of which it is composed. The name connotes certain physical properties in addition to chemical composition, for polymorphous modifications and amorphous equivalents are recognized as distinct minerals. Most mineral names end in -ite. This custom has its origin in the practice of the Greeks and Romans of adding the suffix -ites or -itis (originally from the Greek lithos, a stone) to a word which was descriptive of the mineral, or indicated its use or the locality in which it was found. Other endings used especially an 212 INTRODUCTION TO THE STUDY OF MINERALS by Haiiy are ~ane, -ase, -ene, -ine, -ose, and -ote. Names in use before the custom of using the termination ite was adopted are quartz, opal, topaz, garnet, mica, diamond, galena, beryl, gyp- sum, zircon, and hornblende. It seems fortunate that we have some variety in our mineralogical nomenclature especially since the termination -ite has been used for rock names (granite, syenite, etc.) and for artificial compounds (aloxite, quercite, etc.). Dana in 1837 used a binomial nomenclature for minerals like that at present used for plants and animals. Thus the genus Baralus included Baralus ponderosus (barite), E. prismaticus (celestite), B.fusilis (witherite), andB. rubefaciens (strontianite). The classification used then was based upon external characters or physical properties. This gradually gave way to the chemical classification of Berzelius and the Swedish chemists. The modern chemical classification is one in which the minerals are arranged according to the acid radical. The principal classes of minerals are: elements, sulfids, sulfo-salts, haloids, oxids, hydroxids, carbonates, phosphates, nitrates, borates, sulfates, tungstates, and silicates. Within each of these divi- sions the minerals are arranged as far as possible in isomorphous groups. Thus calcite (CaCO 3 ), dolomite (CaMg[CO 3 ]2), magne- site (MgCO 3 ), siderite (FeCO 3 ), rhodochrosite (MnCO 3 ), and smithsonite (ZnCO 3 ) are included in the calcite group of rhom- bohedral carbonates, for they crystallize in the hexagonal system and have rhombohedral cleavage and similar optical properties. Aragonite (CaC0 3 ) and cerussite (PbCO 3 ) together with the carbonates of strontium and barium constitute another group of orthorhombic carbonates, while malachite [Cu 2 (OH) 2 CO 3 ] and azurite [Cu(OH) 2 (CO 3 ) 2 ] and hydromagnesite [Mg 4 (OH) 2 (CO 3 ) 3 - 3H 2 O] must be considered separately because they are basic carbon- ates unlike in crystal form and physical properties. Certain naturally occurring homogeneous substances not definite enough in chemical composition to be called minerals are considered under the term mineraloid. They include volcanic glass and the hydrocarbons. 1. ELEMENTS A. Non-metals Diamond C GRAPHITE C SULFUR S B. Metals GOLD Au SILVER Ag COPPER Cu Platinum Pt Iron Fe Of the eighty or more known elements only about twenty occur uncombined as minerals, if we leave out of consideration the free gases of the atmosphere. The elements occurring free and uncombined are: carbon, sulphur, selenium, tellurium, phos- phorus, arsenic, antimony, bismuth mercury, copper, silver, gold, lead, iron, palladium, iridium, osmium, tantalum, and tin. From a chemical standpoint the elements may be divided into two classes : the metals and the non-metals. The metals include such elements as copper, silver, gold, lead, iron, and platinum. Some of these occur as alloys such as electrum (Au,Ag), amalgam (Ag,Hg), nickel-iron (Fe,Ni), and iridosmine, (Ir,Os). The non- metals include such elements as oxygen, hydrogen, nitrogen, phosphorus, carbon, and sulfur. Arsenic, antimony, and bismuth are intermediate in their properties between metals and non-metals and are usually called semi-metals or metalloids. Diamond, C Form. Diamond is practically always found in small loose crystals with rounded faces and curved edges. It crystallizes in the isometric system, probably in the hexoctahedral class. Though many of the isometric forms have been observed, the 213 214 INTRODUCTION TO THE STUDY OF MINERALS only common well-defined one is the octahedron. Figure 378 represents a typical crystal with grooved edges and triangular markings. Spinel twins of diamond are rather common. The internal structure of diamond determined by X-ray analy- sis is like that of sphalerite (Fig. 298, p. 144) except that all the atoms are carbon atoms. Cleavage. Perfect octahedral. This fact enables the dia- mond-cutter to save considerable work. H. = 10 (the hardest known substance). Color. Diamond is usually colorless or faintly colored, though brilliantly colored blue, green, and red stones are known. One variety known as carbonado is black and opaque. Luster. The luster is the brilliant luster known as adamantine. The rough uncut crystals have a peculiar greasy appearance. Optical Properties. The index of re- fraction is very high (n = 2.4 175 for sodium light) which accounts for its brilliancy. The "fire" of the diamond is accounted for by its strong dispersion; the index of refraction for the red end of the spectrum FlQ ' 3 7 cr ltd * a m n d is 2 ' 402 ' while f r the violet end {i is 2 ' 465 * Diamonds are transparent to X-rays, while imitations are opaque. Chemical Composition. Pure carbon. Upon heating the diamond in an atmosphere of oxygen it is converted into C02- Blowpipe Tests. Infusible. Insoluble in acids. Distinguishing Features. Diamond is distinguished from similar minerals by its superior hardness, its adamantine luster, and its comparatively high specific gravity. The peculiar rounded crystals with an apparently oiled sur- face are unlike those of any other mineral. Uses. On account of its great hardness, brilliancy, and rarity, diamond stands as the gem mineral, par excellence. Among the famous historic diamonds are the Kohinoor, 186 carats; the ELEMENTS 215 Regent, 137 carats; the Star of the South, 254 carats; the Im- perial, 457 carats; and the Excelsior, 969 carats. Of colored diamonds the most famous are the Tiffany (orange-yellow), the Hope (greenish-blue), the Dresden (bluish-green), and the Paul I. (ruby-red). The largest diamond on record is the Cul- linan (since named the Star of Africa) found in 1905 at the Pre- mier mine in the Transvaal. This diamond weighed 3106 metric carats (621.2 grams or about 1^ pounds avoirdupois). Diamonds are also used as an abrasive in cutting and polishing precious stones, glass, and other materials. The center of the diamond cutting industry is Amsterdam. Several mines within an area of ten square miles at Kimberley, South Africa, have furnished the world's principal supply of diamonds since their discovery in 1867. A black, opaque, non-cleavable variety of diamond is used for diamond drills. It is found only in Bahia, Brazil, and is known as carbonado or " black diamond." Occurrence. 1. In volcanic necks and dikes of a rock known as kimberlite (locally called "blue-ground"). Kimberlite is an altered peridotite composed of fragments of pyrope, pyroxene, biotite, olivine, etc., in a matrix of serpentine. The origin of the diamond is in doubt, but many believe it to be of igneous origin. Diamonds have recently been found in a peridotite dike in Pike County, Arkansas. The stones are small in size but of very good quality. 2. In alluvial deposits associated with heavy minerals. Among these localities may be mentioned southern India, (where diamonds were first found), the states of Bahia and Minas Geraes, Brazil (one locality is known as Diamantina), and scat- tered localities throughout the United States. In the Great Lakes region, diamonds are found in glacial drift. In California small diamonds have frequently been found in sluice-boxes along with gold. 3. In the chromite of serpentinized dunite in the Tulameen 216 INTRODUCTION TO THE STUDY OF MINERALS district, British Columbia, minute diamonds have recently been found. 4. In meteorites from Canon Diablo (Arizona), minute dia- monds have been found. Moissanite, SiC, the same as the arti- ficial carborundum, is also found there. A peridotite meteorite which fell at Novo-Urei, Russia, in 1886 also contained diamonds. Moissan obtained small diamonds by cooling in water a block of soft iron saturated with carbon. GRAPHITE, C Form. Graphite occurs occasionally in six-sided tabular crystals, but more often in foliated masses, minute disseminated scales, or earthy lumps. Cleavage. Cleavage in one direction. H = 1 to 2. Sp. gr. 2.1 . Color. Dark gray to black. Luster, metallic. Streak, gray. Opaque even in thinnest fragments. Sectile. Chemical Composition. Graphite is a modification of carbon. It grades from pure carbon to earthy varieties which yield a large amount of ash on combustion. Blowpipe Tests. The tests for graphite are largely negative on account of its refractory nature. Infusible. Insoluble in acids. Distinguishing Features. Graphite resembles molybdenite but is distinguished by its lower specific gravity, negative NaPO 3 bead test, negative sulfid test, and difference in streak on glazed porcelain or glazed paper. (Molybdenite has a greenish gray streak.) It is distinguished from hematite by its streak and lower specific gravity, from magnetite by its inferior hardness and failure to be attracted by a magnet, and from hydrocarbons by its non-volatility in the closed tube. Uses. Graphite is used in the manufacture of lead pencils (varying hardness is due to admixed clay), lubricants for ma- chinery, refractory crucibles, and electrical supplies. Austria and Ceylon are the chief producers. ELEMENTS 217 Artificial graphite is now made from anthracite in electric furnaces at Niagara Falls. Occurrence. 1. In crystalline limestones, doubtless formed by the recrystallization of the organic matter of sedimentary lime- stones. Franklin, New Jersey. 2. In schists and gneisses, . of ten as an essential constituent. Hague, New York. 3. In veins, in granites and granulites, which proves that its origin may be independent of previous life. Ceylon. 4. In coal-beds, the coal is often converted into graphite near the contact with igneous intrusions. 5. In meteorites. Paramorphs of graphite after diamond, called cliftonite, are also found in meteorites. (On heating diamond out of contact with air it is converted into graphite) . SULFUR, S Form. Sulfur occurs in crystals, incrustations, dissemina- tions, and compact masses. The crystals are good examples of the orthorhombic system. Usual forms: p{lll}, c{001), n{011|, and s { 1 13 } . The habit is usually pyramidal, with {111} as the dominant form. Interfacial angles: pp(lll : ill) = 94 52', cp(001 : 111) = 71 40',cs(001 : 113) = 45 10', cn(001 : Oil) = 62 17'. Figure 379 is the common type of crystal. FIG. 379. Sulfur H. = IK to 2%. Sp. gr. 2.07 . crystah Color. Yellow, sometimes with orange, brown, or green tinge. Luster. Resinous to adamantine. Optical Properties. w 7 (?.24) - w (1.95) = 0.29. Fragments are irregular with high-order interference colors. Chemical Composition. Sulfur, often with such impurities as clay and asphaltum. Some varieties contain selenium. Blowpipe Tests. Fuses easily (114 C.) and burns with a blue flame, giving off sulfur dioxid. If impure, a residue is left. 218 INTRODUCTION TO THE STUDY OF MINERALS Insoluble in acids. Soluble in carbon bisulfid and on evapora- tion minute crystals are formed. Distinguishing Features. Sulfur is not apt to be confused with any other mineral. It may, however, be overlooked when occurring as an impregnation of tuff or clay, as the color is apt to be gray instead of yellow. This material will burn and leave a residue. Uses. Sulfur is used in the manufacture of gunpowder, matches, for vulcanizing rubber, and for the production of sulfur dioxid, which is used in paper manufacturing and bleaching. It was formerly used in the manufacture of sulfuric acid, but that is now made from pyrite. The island of Sicily and southwestern Louisiana are the principal sources of sulfur. At the latter local- ity the sulfur is obtained by dissolving it in superheated steam and pumping the solution to the surface. Occurrence. 1. As a sublimate in the crevices around vol- canoes (called solfataras) or as an impregnation of tuffs. Formed by the incomplete oxidation of hydrogen sulfid. 2H 2 S + 02 = 2 S + 2H 2 0. 2. In sedimentary rocks (limestones, travertine, and marl) with gypsum, and occasionally with celestite. Girgcnti, Sicily. 3. As a decomposition product of sulfids such as galena, stibnite, sphalerite, and pyrite. 4. Formed by some bacteria and algae in sulfate-bearing waters. Sulfur furnishes one of the best examples of polymorphism. The orthorhombic modification, known as a-sulfur, is the one that occurs so extensively in nature. The monoclinic modifica- tion (0-sulfur) formed when sulfur solidifies from fusion has been noted in nature several times, but it rapidly changes to the orthorhombic form on standing. GOLD, AM Form. Though usually finely disseminated through contain- ing rock and only apparent on assaying, gold also occurs in rolled ELEMENTS 219 grains and scales, occasionally in large nuggets, and rarely in crystals and imperfect crystal aggregates. Like many of the metals, gold crystallizes in the isometric system, the octahedron being the only common form. Sheets of gold (leaf-gold) with raised triangular markings are not uncommon. H. = 2J> to 3. Sp. gr. = 15 to 19 (according to purity). Pure gold has a specific gravity of 19.3. Color. Deep to pale yellow. The pale yellow variety containing over 20 per cent, silver is known as electrum. Luster, metallic. Very malleable. Chemical Composition. Gold, always alloyed with more or less silver, sometimes also with Bi, Cu, Fe, Pd, and Rh. The purity of gold is expressed by the amount of gold in 1000 parts. For example, gold with 13 per cent, silver has a fineness of 870. The following analyses give an idea of the variation in the chemical composition. Au Ag Misc. Sp. Gr. South Australia. ...... Urals 93.5 87.4 6.5 12.1 Cu = 0.1 18.8 17.4 Peru Verespatak 79.9 66 4 20.1 33 2 SiO 2 =04 16.6 15 Blowpipe Tests. Fusible at 3. With mercury forms an amal- gam. Soluble in aqua regia; the silver-bearing varieties leave a residue of AgCl. Distinguishing Features. Pyrite and chalcopyrite are some- times mistaken for gold but they are easily distinguished by their brittleness, lower specific gravity, and by the fact that they are soluble in nitric acid. Yellow, decomposed scales of mica are even more like gold in appearance, but a careful examination reveals their true nature. Uses. Native gold is the source of most of the gold of com- merce, but some is derived from the gold tellurid (Calaverite). 220 INTRODUCTION TO THE STUDY OF MINERALS In order of their production the countries are: Transvaal, United States, Australia, Russia, and Canada. The States in order are: California, Colorado, Alaska, Nevada, and South Dakota. Occurrence. 1. In quartz veins along with pyrite, chalco- pyrite, galena, sphalerite, arsenopyrite, etc. 2. In placers along streams (ancient river-channels in some cases) associated with heavy minerals such as magnetite, ilmen- ite, garnet, zircon, platinum, etc. Prominent localities are Alaska, California, Brazil, Colombia, Urals, and Australia. 3. In the quartzite conglomerate (''banket") on the Rand, Transvaal, South Africa. The origin of the gold is in doubt. 4. In the gossan or oxidized zone as a secondary mineral or mechanically released. At Cripple Creek, gold pseudomor- phous after calaverite is found in the oxidized zone. SILVER, Ag Form. The characteristic occurrences of silver are in wire form , thin sheets, skeleton crystals, dendritic groups, and masses. The cube is the only common crystal form, but the crystals are very small. H. = %. Sp. gr. 10.5+. Color. Tin-white to pale yellow, but is usually dull and tarnished. Luster, metallic. Malleable. Chemical Composition. Silver, often with some gold and copper. Blowpipe Tests. Easily fusible (2) to a malleable button. Soluble in HNO 3 . HC1 gives a white precipitate (AgCl), which turns violet on standing and is soluble in .NH 4 OH. Distinguishing Features. Silver resembles some of the other native metals but is easily distinguished by the color of a freshly cut surface. Uses. Native silver is the source of some silver, although most of the supply is derived from the sulfid and sulfo-salts. The Cobalt (Ontario) district now furnishes about one-tenth of the ELEMENTS 221 world's supply of silver, the silver being largely in the form of the native metal. Occurrence. 1. In veins associated with cobalt and nickel minerals. Cobalt, Ontario, is a prominent locality. 2. In veins with argentite, pyrargyrite, polybasite, stephanite, etc., and usually the last mineral to be formed and probably by ascending solutions. 3. In the gossan or oxidized zone, often associated with cerussite and limonite, and doubtless formed by descending solutions. Leadville, Colorado, and the Coeur d'Alene district in Idaho are prominent localities. In contrast with gold, silver is practically never found in placers. COPPER, Cu Form. Copper is found in small disseminated grains, in sheets, and occasionally in large masses. Copper crystallizes in the iso- metric system, but the crystals are usually distorted and asso- ciated in dendritic groups. The forms {100}, {111}, {110}, and {210} can sometimes be made out. H. = 2%. Sp. gr. 8.8 . Color. Copper-red, often tarnished and also encrusted with alteration products such as cuprite and malachite. Metallic luster. Malleable. Chemical Composition. Copper, often with a little silver. Blowpipe Tests. Fusible (3) to a malleable globule. Soluble in dilute HNO 3 to a green solution with the evolution of NO 2 . With an excess of NH 4 OH, the nitric acid gives a deep blue coloration solution. Distinguishing Features. The color of freshly cut copper is distinctive. Uses. Native copper is an important source of copper in but one locality (Upper Peninsula of Michigan), where immense quantities of copper ore are produced from very low grade ores. 222 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. In the oxidized zone of many copper mines formed by the reduction of copper compounds in solution, which in turn were formed by the oxidation of chalcopyrite. 2. In amygdaloidal diabases and basalts associated with calcite, datolite, prehnite, epidote, zeolites, and sometimes with silver. The Upper Peninsula of Michigan is the type locality. Platinum, Pt Form. Platinum is found in rounded grains, scales, and irregu- lar lumps. Cubic crystals have been found but are exceedingly rare. H. = 4K- Sp. gr. 15-19 (21, if pure). Color. Light steel gray. Luster metallic. Malleable. Some varieties are magnetic because of the high iron content. Chemical Composition. Platinum, often alloyed with iron and metals of the platinum group (Ir, Os, Pd, Rh). Pt Fe Pd Rh Ir Os Cu Urals 80.9 2.3 1.6 11.1 tr 1.0 S = 0.8 Colombia California 84.8 79.8 8.3 9.4 1.0 0.3 2.1 3.4 1.0 4.3 1.0 1.1 0.6 0.3 Blowpipe Tests. Infusible. Soluble in aqua regia. From this solution KC1 precipitates K 2 PtCl 6 , a yellow crystalline powder composed of minute octahedra. Distinguishing Features. Platinum is recognized by its high specific gravity, and its refractory nature. Uses. Platinum is used largely for chemical apparatus and in jewelry, but also for some industrial purposes. Native platinum is the only source of platinum. The Ural Mountains furnish practically the entire supply. Occurrence. 1. In placer deposits along with gold, magnetite, ilmenite, zircon, diamond, etc. Prominent localities are the ELEMENTS 223 Ural Mountains, Colombia, British Columbia, northern Cali- fornia, and southern Oregon. 2. In peridotites or dunites with chromite, olivine, and ser- pentine. These rocks are the original source of the platinum of placers. Urals and British Columbia. Iron, Fe to (Fe,Ni) Form. Iron is found in compact or spongiform masses and in disseminated grains. Iron crystallizes in the isometric system, but distinct crystals are exceedingly rare. Many meteoric irons have an octahedral structure which is revealed by etching a polished surface with nitric acid, and is due to varying solubility of several different alloys of iron and nickel. H. = 4K- Sp.gr. 7.5 . Color. Steel-gray to iron-black, often covered with iron-rust. Luster, metallic. Malleable. Attracted by the magnet. Chemical Composition. Iron, usually alloyed with nickel. It usually contains small amounts of cobalt, copper carbon, sul- fur, phosphorus, etc. Fe Ni Co Cu s Mn C Silicates, Insol. Terrestrial iron, Greenland 91.7 1.7 0.5 0.1 0.1 1.4 3.9 Meteoric iron, Siberia 88.4 10.7 0.5 0.1 tr. 0.1 0.1 0.5 Blowpipe Tests. Infusible. Amber borax bead in O.F., bottle green in R.F. Soluble in HC1 with the evolution of hydrogen. Iron becomes copper-coated in a solution of copper sulfate. Distinguishing Features. Iron is recognized by its oxidized surface and bright metallic interior. Occurrence. 1. In meteorites, either as the main constituent (iron meteorites) or associated with silicates such as olivine (iron-stone meteorites) . 224 INTRODUCTION TO THE STUDY OF MINERALS 2. Terrestrial iron is usually formed by the reduction of iron compounds. The only prominent locality is Disco Island off the west coast of Greenland, where large masses are found in a basalt. This iron constitutes a natural steel as it contains the requisite amount of carbon and shows the microscopic structure characteristic of steel. The basalt is associated with coal beds and the coal has doubtless served as a reducing agent of the iron in the basaltic magma. 2. SULFIDS STIBNITE Sb 2 S 3 Bismuthinite, Bi 2 S 3 Molybdenite, MoS 2 Argentite, Ag 2 S GALENA, PbS CHALCOC1TE, Cu 2 S SPHALERITE, ZnS Pentlandite (Fe,Ni)S CINNABAR, HgS Covellite, CuS PYRRHOTITE, FeS(S) x j PYRITE, FeS 2 I Smaltite, (Co,Ni)As 2 j Marcasite FeS 2 1 ARSENOPYRITE, FeAsS Calaverite, AuTe 2 The sulfids and their analogues, the selenids, tellurids, arse- nids, etc., are derivatives of H 2 S, H 2 Se, H 2 Te, H 2 As, etc. They may be considered as salts of these acids or as sulfanhydrids of sulfo-salts, just as the oxids are anhydrids of the oxy-salts. They are mostly sulfids of the heavy metals. The sulfids may be divided into two classes: (1) the sulfids of the semi-metals, and (2) the sulfids of the metals. Minerals under each of these are arranged according to increasing number of sulfur atoms in the formula. Prominent isomorphous groups are indicated by brackets. STIBNITE, Sb 2 S 3 Form. Stibnite is found in prismatic or acicular crystals, in columnar or bladed aggregates, and in granular masses. Crys- tals are orthorhombic (bipyramidal class). The habit is long is 225 226 INTRODUCTION TO THE STUDY OF MINERALS prismatic with {110} as the dominant form (110:lTO = 89 34'). Crystals are often highly modified and always vertically striated. Cleavage. Perfect in one direction parallel to the side pina- coid (010). It may also have parting parallel to (001) which is manifested in cross lines on the cleavage surfaces. H. = 2. Sp. gr. 4.5 . Color. Lead gray. Luster, brilliant metallic on fresh sur- face. Streak, lead gray. Chemical Composition. Antimony sulfid, Sb 2 S 3 ; (Sb = 71.4 per cent.). Blowpipe Tests. On charcoal easily fusible (1), gives a pale greenish-blue flame, dense white fumes, and a white sublimate close to the assay. In the open tube gives S0 2 and a white non- volatile sublimate (Sb 2 4 ) on the under side of the tube. In the closed tube gives a dark red sublimate of antimony oxysulfid (Sb 2 S 2 0). Decomposed by HNOs with the separation of a white precipi- tate of metantimonic acid (HSbO 3 ). Distinguishing Features. Stibnite differs from similar min- erals in its perfect cleavage in one direction, and its easy fusi- bility. (It can be fused in the flame of a match). It is often coated with pale yellow stibiconite, a product of oxidation. Uses. Stibnite is the principal source of antimony which is used extensively in the manufacture of various alloys. Some of these alloys, such as type-metal, are made directly from anti- monial lead ores. China is the principal producer of stibnite. Occurrence. 1. As a vein mineral often associated with pyrite, sphalerite, galena, cinnabar, and realgar in a gangue of quartz, barite, or calcite. Prominent localities for specimens of stibnite are Felsobanya, Hungary, and Shikoku, Japan. At the latter locality magnificent crystals over a foot in length are found. Bismuthinite, Bi 2 S 3 Form. Bismuthinite is isomorphous with stibnite, and greatly resembles it. SULFIDS 227 Cleavage. In one direction parallel to the length. H. = 2. Sp. gr. 6.4 . Color. Lead gray, often with a peculiar yellowish tarnish. Chemical Composition. Bismuth sulfid, Bi 2 S 3 ; (Bi = 81.2 per cent.). Some varieties contain Se. Blowpipe Tests. Easily fusible (1), gives on charcoal a metallic button (malleable, but brittle on the edges) and a yellow coating. Heated with iodid flux on plaster it gives a purplish chocolate sublimate with underlying scarlet. Soluble in HNO 3 . On diluting the solution with water a white precipitate is formed. Distinguishing Features. Resembles stibnite and is only distinguished from it by blowpipe or chemical tests. Uses. Bismuthinite is probably the most important bis- muth mineral. Bolivia is the chief producer. Occurrence. 1. As a vein mineral associated with bismuth and chalcopyrite especially. Molybdenite, MoS2 Form. Molybdenite is usually found in foliated masses or in disseminated scales, and occasionally in hexagonal crystals of tabular habit. Cleavage. In one direction parallel to {0001}. H. = IY 2 . Sp. gr. 4.7 . Color. Bluish lead gray. The streak on glazed porcelain or glazed paper (the thumb nail is a fair substitute) has a greenish tinge. Cleavage plates are flexible and sectile. Chemical Composition. Molybdenum sulfid, MoS 2 (Mo = 60.0 per cent.). Blowpipe Tests. Infusible. On charcoal gives a white subli- mate which is copper red near the assay. Green NaPOs bead in R.F., colorless in O.F. Decomposed by HNOa with the formation of a white sublimate (MoO 3 ) which is soluble in NH 4 OH. 228 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Distinguished from graphite by higher specific gravity and streak on glazed paper. Uses. Molybdenite is the chief source of molybdenum which is used as an alloy with steel. Australia is the leading producer of molybdenite. Occurrence. 1. In pegmatites and surrounding rocks, espe- cially granite. 2. In tin-stone veins with cassiterite, wolframite, topaz, etc. 3. In contact-met amor phic zones between limestones and granites associated with epidote, chalcopyrite, etc. Argentite, Ag 2 S Form. Occurs massive, incrusting, more rarely in rough crys- tals. The crystals are isometric, the only common form being the cube. Cubes are often arranged in parallel position. H. = 2%. Sp. gr. 7.3 . Color. Dark lead gray, dull black on exposed surface. Luster, metallic. Very sectile. Chemical Composition. Silver sulfid, Ag 2 S; (Ag = 87.1 per cent.) Blowpipe Tests. Easily fusible (lj^). On charcoal yields a malleable button of silver. Soluble in HN0 3 with the separation of S. HC1 gives a white precipitate (AgCl) soluble in NH 4 OH. Distinguishing Features. Distinguished by its perfect sec- tility and metallic luster. Uses. Argentite, the silver glance of the miner, is an impor- tant ore of silver on account of the high silver content. Occurrence. As a vein mineral associated with other silver minerals, and pyrite, galena, sphalerite, etc. Freiberg, Saxony. GALENA, PbS Form. Galena occurs in well-formed crystals as well as in cleavable and granular masses. Crystals are isometric (hexocta- hedral class) . The usual forms are the cube { 100 } , the octahedron SULFIDS 229 (111), more rarely the dodecahedron {110}, and the trisocta- hedron {221}. The habit is usually cubic, cubo-octahedral, or octahedral, as shown by Figs. 380-384. Small octahedral crystals are sometimes found in parallel position on large cubic crystals. Cleavage. Perfect cubic cleavage. H. = 2^. Sp. gr. 7.5 . Color. Lead gray, often tarnished. Metallic luster. Chemical Composition. Lead sulfid, PbS (Pb = 86.6 per cent.) May contain zinc, silver, free sulfur, and other impurities. If the silver is chemically combined, it is present in amounts less than 0.1 per cent, according to Guild. Blowpipe Tests. Easily fusible at 2, giving on charcoal a malle- able button, a yellow sublimate (PbO) near the assay, and a white sublimate (PbSCM farther from the assay. FIG. 380. FIG. 381. FIG. 382. FIG. 383. FIG. 384. FIGS. 380-384. Galena crystals. The silver may be obtained by cupellation (see p. 34). Decomposed by HC1 with the separation of PbCl 2 , a white crystalline precipitate soluble in hot water. Decomposed by HNOa with the separation of S and PbSO4. Distinguishing Features. It is an easy mineral to recognize on account of its cleavage and high specific gravity. Uses. Galena is the most important ore of lead; argentifer- ous galena is one of the most important silver ores. The silver is for the- most part present as included silver minerals. South- eastern Missouri and the Coeur d'Alene district of Idaho are the most important sources of galena. 230 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. In veins associated with sphalerite, chalcopy- rite, pyrite, etc., often with barite or fluorite, a gangue mineral. In the north of England galena occurs with fluorite, barite, cal- cite, and sphalerite in veins in Sub-carboniferous limestone. 2. In sedimentary rocks such as limestones, shales, and sand- stones often associated with sphalerite. In southeastern Mis- souri galena is disseminated through Ordovician limestone. 3. In contact-metamorphic zones with sphalerite. CHALCOCITE, Cu 2 S Form. Usually fine-grained compact masses, rarely in pseudo- hexagonal orthorhombic crystals, which are sometimes twinned. H. = 2>i Sp.gr. = 5.78. Color. Dark lead-gray with black tarnish. Metallic luster. Chalcocite is sub-sectile (i.e., it can be cut with a knife, but not so readily as argentite). Chemical Composition. Cuprous sulfid, Cu 2 S; (Cu = 79.8 per cent.). It may contain a little cupric sulfid in solid solution. A little iron is usually present, due to admixed bornite, chalcopyrite, or pyrite. Blowpipe Tests. Fuses at 2. Unaltered in the closed tube. In the open tube gives the odor of SO 2 . In R.F. on charcoal gives metallic copper. Soluble in HNOs giving brown-red fumes, residue of S, and a green solution. Distinguishing Features. A compact massive mineral, dis- tinguished by its imperfect sectility, especially from tetrahedrite, which is very brittle. Uses. Chalcocite is a valuable ore of copper on account of the high percentage of copper. At the Bonanza Mine, Kennecott, Alaska, enormous quantities of chalcocite ore running 70 per cent, copper are being mined. It is a prominent mineral in the ores at Butte, Montana, and also occurs in the disseminated "porphyry copper" ores in Arizona, Utah, and Nevada. SULFIDS 231 Occurrence. 1. As a product of downward secondary enrich- ment, formed at the expense of pyrite, chalcopyrite, or bornite. Bingham, Utah. 2. As a vein mineral associated with pyrite, chalcopyrite, bornite, and covellite and is often formed as a replacement of these minerals by ascending solutions. Butte, Montana. On heating orthorhombic chalcocite (/3-Cu 2 S) to a temperature of 91C or above, it changes to an isometric form (-Cu 2 S). Some specimens of chalcocite show structures on polished sur- faces which prove them to be paramorphs of /3-Cu 2 S after -Cu 2 S. This makes it practically certain that some chalcocite has been formed by hydrothermal ascending solutions. SPHALERITE, ZnS Form. Sphalerite crystallizes in the hextetrahedral class of the isometric system. Crystals are usually distorted and diffi- cult to decipher. The common habits are tetrahedral and dodec- ahedral; the usual forms are a{100), djllO}, o{lll), o FIG. 385. FIG. 386. FIGS. 385, 386. Sphalerite crystals. and raj 311}. Figs. 385 and 386 represent typical crystals. Crystals are often twinned on the {111} face and twinning striations due to polysynthetic twinning on this face are sometimes observed. The internal structure of sphalerite as determined by JY-ray analysis (Bragg and Bragg) is shown in Fig. 298, p. 144. Cleavage. Very prominent (dodecahedral at angles of 60). H. = 3K to 4. Sp. gr. 4.0 . 232 INTRODUCTION TO THE STUDY OF MINERALS Color. The color of sphalerite varies from white to black, depending upon the amount of iron present. The usual color is yellowish-brown or reddish-brown. The luster varies from adamantine to submetallic, and the streak from pale yellow in yellow varieties to dark brown in the black varieties. Optical Properties, n = 2.37. Fragments are triangular, pale yellow to brown, and isotropic (dark between crossed nicols). The high index of refraction accounts for the adamantine luster. Chemical Composition. Zinc sulfid, ZnS; (Zn = 67.0 per cent.). Usually contains iron (up to as high as 20 per cent.) which replaces zinc isomorphously, as shown in the formula (Zn,Fe)S. Cadmium is another common impurity present in part as a isomorphous replacement of zinc and in part as an associated cadmium sulfid (xanthochroite) occurring as an amor- phous yellow incrustation on sphalerite. The rare elements in- dium and gallium were discovered in sphalerite. The following are typical analyses : Zn Fe S Misc. White: Franklin, New Jersey 67.5 32.2 Cd = tr Yellow: Schemnitz, Hungary 65.2 0.5 32.8 Cd = 1.5 Brown: Roxbury, Connecticut 63.4 3.6 33.4 Dark brown: Westphalia 58.2 8.2 33.4 Cu = 0.1; Pb = tr Black: Felsobanya, Hungary 50.0 15.4 33.3 Cd = 0.3; Pb = 1.0 Blowpipe Tests. Fusible with difficulty (5) . On charcoal gives a white sublimate, which is yellow when hot. This sublimate heated intensely with cobalt nitrate solution gives a green color (cobalt zincate). The presence of cadmium is indicated by an iridescent coating on charcoal. Soluble in HC1 with the evolution of H 2 S. Distinguishing Features. The perfect cleavage together with the adamantine luster will distinguish sphalerite from all other common minerals. It somewhat resembles siderite and occasion- ally garnet. SULFIDS 233 Uses. Sphalerite is the most important ore of zinc. The Joplin district of southwest Missouri is the principal locality in this country. Occurrence. 1. As a vein mineral associated with galena, chalcopyrite, pyrite, and other sulfids in a gangue of quartz, calcite, barite, fluorite, dolomite, etc. - 2. As a replacing or accessory mineral in sedimentary rocks, especially limestones. 3. As a contact-metamorphic mineral. Magdalena mines, New Mexico. Pentlandite (Fe,Ni)S Form. A massive mineral very much like pyrrhotite in appearance. Cleavage. Octahedral. H. = 3H to 4. Sp. gr. 4.8 . Color. Bronze-yellow like pyrrhotite. Opaque, non-magnetic. Chemical Composition. Iron and nickel sulfid (Fe,Ni)S. (Ni = 20 to 40 per cent.) Blowpipe Tests. Easily fusible (at 2) to a magnetic globule which gives the bead tests for iron and nickel. Soluble in nitric acid to a green solution. When made alka- line the solution gives a red ppt. with dimethylglyoxime. Distinguishing Features. It very much resembles pyrrhotite but is distinguished by its octahedral cleavage (or parting) and by its non-magnetic character. On a polished surface containing pyrrhotite it is not affected by HC1, while the pyrrhotite is. Uses. Pentlandite is the chief ore of nickel. It is extensively mined in the Sudbury district, Ontario, Canada where it occurs intimately associated with pyrrhotite and chalcopyrite. This district is the most important producer of nickel ore; its only rival is New Caledonia. Occurrence. 1. As a magmatic sulfid associated with, and formed later than, pyrrhotite. Sudbury District, Ontario, Canada. 234 INTRODUCTION TO THE STUDY OF MINERALS CINNABAR, HgS Form. Cinnabar usually occurs disseminated through the rock and in massive and earthy forms. It may also occur in minute crystals in cavities. The crystals are hexagonal and of variable habit. Cleavage. Perfect, often parallel to length of the crystal. Even in massive varieties reflections from minute cleavage planes usually may be seen with a lens. H. = 2^. Sp. gr. 8.0 . Color. Scarlet to dark red, sometimes black when impure. Luster, adamantine in typical specimens. Streak, vermilion. Optical Properties. n T (3.20) - n a (2.85) = 0.35. Fragments are red, and irregular with high order interference colors. The very high index of refraction accounts for the adamantine luster. Chemical Composition. Mercuric sulfid, HgS; (Hg = 86.2 per cent.). Clay and organic matter are often present as impurities. Blowpipe Tests. Volatile if pure. In the closed tube with dry sodium carbonate, cinnabar gives a sublimate of metallic mercury (little globules when rubbed with a wire). Soluble in HNO 3 . Distinguishing Features. Cinnabar is distinguished from other red colored minerals by its high specific gravity, adamantine luster, and cleavage. Uses. Practically the only ore of mercury. The principal producing localities are Almaden, in Spain, Idria, in Austria, New Almaden and New Idria in California. Occurrence. 1. In deposits formed near the surface. The more common associated minerals are pyrite, marcasite, stibnite, and mercury. The gangue minerals are chalcedony, opal, barite, and calcite. Covellite, CuS Form. Tabular hexagonal crystals are rare; the mineral usually occurs as a dissemination or incrustation. H. = IY 2 to 2. Sp. gr. = 4.68. SULFIDS 235 Color. Deep indigo blue, sometimes almost black. Luster, metallic pearly to earthy. Chemical Composition. Cupric sulfid, CuS; (Cu = 66.4 per cent.). Blowpipe Tests. Fusible at 2^- In the closed tube gives a sublimate of sulfur (distinction from chalcocite). On charcoal burns with a blue flame, gives off S(>2, and leaves a residue of metallic copper. Soluble in HNO 3 to a green solution. Distinguishing Features. The dark blue color is distinctive for a metallic mineral. Uses. One of the minor ores of copper. It is prominent in some of the Butte mines. Occurrence. 1. A characteristic mineral of the zone of downward secondary sulfid enrichment. In occurs associated with chalcopyrite, bornite, and chalcocite. 2. In veins with other copper minerals and probably formed by ascending solutions. Butte, Montana. PYRRHOTITE, FeS(S) x Form. Pyrrhotite is usually massive and without distinct cleavage, though it may have a platy structure. Pseudohexa- gonal orthorhombic crystals of tabular habit are known, but are very rare. H. = 4. Sp. gr. 4.6 . Color. Bronze-yellow. Luster, metallic. Attracted by the magnet but usually only slightly so. Chemical Composition. Ferrous sulfid, usually with a little excess of sulfur present in solid solution and expressed by the formula FeS(S)*. (Fe = 60-63.6 per cent.). It frequently contains nickel which is present as admixed pentlandite. Blowpipe Tests. Fuses at 3 to a magnetic globule, gives fumes of SO 2 . In the closed tube gives little or no sulfur (dis- tinction from pyrite). Soluble in HNO 3 . 236 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. The bronze color and slightly magnetic character of pyrrhotite are distinctive. It can be dis- tinguished from pyrite by the fact that it is scratched by the knife. Uses. The pyrrhotite itself has no special uses, but the inti- mately associated pentlandite which occurs with it at Sudbury, Canada, is the world's chief source of nickel. Occurrence. 1. In plutonic basic igneous rocks, such as gabbros and norites, as a late magmatic mineral formed by the replacement of the silicate minerals. 2. In high-temperature veins and replacement deposits. 3. In contact-metamorphic deposits. 4. In meteorites. This variety, FeS, without an excess of sulfur, is known as troilite. PYRITE GROUP ISOMETRIC Pyrite, FeS 2 ; smaltite, CoAs 2 ; chloanthite, NiAs2; cobaltite, CoAsS; and gersdorffite, NiAsS, constitute an isomorphous group as they crystallize in the diploid class of the isometric system in cubes and pyritohedrons, and have a hardness of 5^ to 6^ and a specific gravity of 5 to 6.5. There are also intermediate com- pounds such as (Co,Fe)As 2 , (Ni,Fe)As 2 , and (Co,Fe)AsS. The general formula, then, can be written (Fe,Co,Ni) (As,Sb,S) 2 . PYRITE, FeS 2 Form. Pyrite is often well crystallized and furnishes the typical example of the diploid class of the isometric system. The most common forms are the cube a {100}, pyritohedron e{210}, octahedron 0(111} , diploid s{321), diploid {421}, and trapezohedron {211}. The habit is nearly always either cubic, pyritohedral, or octahedral. Figures 387 to 394 represent typical crystals. The cube faces are commonly striated as in Fig. 407. The {hkO} forms are so characteristic of pyrite that they are called pyritohedrons. Of these, the pyritohedron {210J is the most common. Penetration twins of the pyritohedron SULFIDS 237 with the a-axis as the twinning axis are occasionally found. (See Fig. 257, page 127.) H. = 6 to 6^. Sp. gr. 5.0 (5.027 if pure) Color. Brass yellow. Luster, metallic. Streak, greenish to brownish-black. Chemical Composition. Iron disulfid, FeS 2 ; (Fe = 46.6 per cent.). May contain copper, cobalt, nickel, arsenic, and gold; but of these only the cobalt and nickel are chemically combined. lllllll FIG. 387. FIG. 388. FIG. 389. FIG. 390. FIG. 391. FIG. 392. FIG. 393. FIGS. 387-394. Pyrite crystals. FIG. 394. In the so-called cupriferous pyrite, the copper is in the form of chalcopyrite. The following are typical analyses: Fe S Cu Misc. French Creek, Penn 44.2 54.1 0.05 Arnsberg, Germany 46.4 51.4 1.0 Mn = 0.5; Co = 0.1; As = 0.6 Cornwall, Penn 44.5 53.4 2.4 Blowpipe Tests. Fusible at 3 to a magnetic globule. On char- coal it burns with a blue flame and gives off SO2. In the 238 INTRODUCTION TO THE STUDY OF MINERALS closed tube, pyrite gives a sublimate of sulfur and leaves a mag- netic residue. Soluble in cold HNO 3 , but no sulfur separates unless the acid is heated. Distinguishing Features. Pyrite is distinguished from pyr- rhotite and chalcopyrite by superior hardness, and from marca- site, its dimorph, by crystal form and solubility in cold HN0 3 . Uses. Pyrite is used extensively in the manufacture of sul- furic acid, which is the basis of many chemical industries. Spain, Norway, Portugal, United States, and Italy are the principal producers. Pyrite is also an important low-grade copper ore at many localities. The copper is present as disseminated chalco- pyrite. Pyrite is also often gold-bearing. Occurrence. 1. As a vein mineral associated with other sulfids. 2. As a secondary mineral in igneous rocks, especially in the country rock around ore deposits. 3. As a dissemination in sedimentary rocks, such as shales and limestones, often replacing organisms. 4. As bedded deposits in metamorphic rocks, perhaps formed from original pyrrhotite. 5. As a contact-metamorphic mineral often associated with hematite and magnetite. Smaltite, (Co,Ni)As 2 Form. Smaltite is usually massive without any cleavage, but occasionally is found in cubic crystals. H. = 5^. Sp.gr. 6.2 . Color. Tin white to steel gray. Luster metallic. Chemical Composition. Cobalt and nickel arsenid, (Co,Ni)- As 2 , varying from CoAs 2 (Co = 28.1 per cent.) to NiAs 2 (Ni = 28.1 per cent.). If the latter predominates, the mineral is called chloanthite. Iron and sulfur are usually present in small amounts. The following analyses illustrate the range in composition. SULFIDS 239 Co Ni Fe As S Misc. Atacama, Chili 24.1 1.2 4.1 70.8 0.1 Cu = 0.4 10 1 8 5 5 1 69 7 4 7 Cu = 0.9; Bi = 1.0 Schneeberg, Saxony 4.2 24.9 0.7 68.4 1.1 Bi = 0.2 Blowpipe Tests. On charcoal gives off arsin and fuses at to a magnetic globule, which colors the borax bead blue. In the closed tube it gives an arsenic mirror if strongly heated. In the open tube it gives minute octahedral crystals of As2O 3 . Soluble in HNO 3 to a rose-red solution. Distinguishing Features. Smaltite resembles arsenopyrite and can only safely be distinguished by blowpipe or chemical tests. Uses. Smaltite is the chief ore of cobalt which is used principally as a blue pigment (a cobalt silicate called smalt.) Cobalt, Ontario, is the principal producer. Occurrence. 1. As a vein mineral usually with silver or bismuth and in a gangue of calcite. Smaltite is often coated with erythrite, a hydrous cobalt arsenate known as "cobalt bloom." MARCASITE GROUP ORTHORHOMBIC The following minerals: marcasite, FeS 2 ; arsenopyrite, FeAsS; lollingite, FeAs2,* glaucodot (Co,Fe)AsS; safflorite, CoAs2; and rammelsbergite, NiAs 2 , constitute an isomorphous group parallel to the pyrite group. They are orthorhombic in crystallization, have a hardness of 5 to 6^, and are tin-white to brass-yellow in color. Only the first two of these minerals are considered, as the others are rare. Marcasite, Form. Marcasite occurs in orthorhombic crystals, in crystal- line aggregates, and in rounded concretionary masses. Crystals are usually tabular in habit and often elongated in the direction 240 INTRODUCTION TO THE STUDY OF MINERALS of the a-axis. Figures 395 to 397 represent typical crystals with the forms: c{001), m{110), t>{013}. ram(110:lTO) = 74 55'. Twins with mjllO} as twin-plane are common. H. = 6to6^. Sp. gr. 4.9 . Color. Pale brass yellow with a greenish tinge. Almost tin- white when cleaned with dilute HC1 (distinction from pyrite which is yellow). Luster metallic. Chemical Composition. Iron disulfid, FeS 2 ; (Fe = 46.6 per cent.). Analyses often show small amounts of arsenic. Blowpipe Tests. The same as for pyrite, except that it is de- composed by cold nitric acid with the separation of sulfur. m FIG. 395. FIG. 396. Fio. 397 FIGS. 395-397. Marcasite crystals. Distinguishing Features. Marcasite is distinguished from pyrite by crystal form and by difference in color and behavior with nitric acid. Uses. If found in sufficient quantity, marcasite could be used in the manufacture of sulfuric acid. Occurrence. 1. In sedimentary rocks or associated with coal beds, often in concretions. Dover, England. 2. As a vein mineral formed near the surface at a low tem- perature and probably from acid solutions. Joplin district, Missouri. ARSENOPYRITE, FeAsS Form. Arsenopyrite is found in well-formed crystals, as well as in disseminated grains and compact masses. The crystals SULFIDS 241 are orthorhombic, similar in habit and angles to marcasite. Figures 398 and 399 represent typical crystals with the forms m(110l, c{001}, and u{QU}. H. = 5K to 6. Sp.gr. 6.0 . Color. Tin-white to light steel-gray. Luster, metallic. Chemical Composition. Iron arsenid-sulfid, FeAsS; (Fe = 34.3, As = 46.0, S = 19.7). It often contains cobalt and grades into glaucodot. Blowpipe Tests. On charcoal fuses (at 2) to a magnetic glob- ule and gives off arsin. In the closed tube on gentle heating gives a red sublimate (AsS), but on further heating an arsenic mirror is formed. In the open tube minute crystals of As 2 03 are deposited and SO 2 also formed. m FIG. 398. FIG. 399. FIGS. 398, 399. Arsenopyrite crystals. Soluble in HNOs with the separation of S. Distinguishing Features. Crystals of arsenopyrite resemble marcasite but are distinguished by color of fresh surface. Mas- sive arsenopyrite resembles smaltite and often can only be dis- tinguished by blowpipe tests. Uses. Arsenopyrite is the chief source of the white arsenic (As 2 Os) of commerce. At Deloro, Canada, arsenopyrite is a gold ore. Occurrence. 1. As an intermediate-temperature vein mineral associated with pyrite, chalcopyrite, galena, and other sulfids. Mother Lode of California. 2. In high-temperature veins. 3. In granite-pegmatites. 16 242 INTRODUCTION TO THE STUDY OF MINERALS Calaverite, AuTe2 Form. Calaverite usually occurs in small, striated, elongated crystals along seams. The crystals are complex monoclinic; some of the faces have such very high indices that the law of rational indices has been questioned. Fracture, subconchoidal. No cleavage. H. - 2H- Sp. gr. 9.0 . Color. Pale brass yellow, somewhat resembling pyrite. Metallic luster. Chemical Composition. Gold tellurid, AuTe2j (Au = 44 per cent.). It always contains some silver, usually from 2 to 4 per cent. Blowpipe Tests. Easily fusible (at 1) on charcoal to a yellow button of gold, giving dense white fumes and coloring the flame bluish-green. The powdered mineral dropped into hot con- centrated H 2 S04 gives a purplish-red coloration. Soluble in aqua regia with the separation of a little AgCl. Distinguishing Features. Calaverite is distinguished from pyrite by its inferior hardness and elongated crystals. Uses. An important ore of gold. At Cripple Creek, Colorado it is the chief source of gold. It also occurs in West Australia associated with sylvanite ( Au AgTe 4 ) . The name is derived from Calaveras county, California, where it was first found in the Stanislaus Mine. Occurrence. 1. As a vein mineral. At Cripple Creek, fluorite is a common associate. 3. SULFO-SALTS CHALCOPYRITE, CuFeS 2 BORNITE, Cu 6 FeS 4 Jamesonite Pb4FeSbeSu Pyrargyrite, Ag 3 SbS 3 TETRAHEDRITE, Cu 3 SbS 3 + s(Fe, Zn) 6 Sb 2 S 9 Stephanite, Ag 5 SbS 4 Polybasite, (Ag,Cu)i 6 Sb 2 Sii Enargite, Cu 3 AsS 4 Under the sulfo-salts are included certain compounds of sulfur, salts of hypothetical acids which may be derived from ordinary oxygen acids by replacing S for O, as these two elements are similar chemically. Three classes of these compounds may be distinguished: (1) Sulfoferrites, derivatives of H 3 FeS 3 analogous to ferrous acid, H 3 FeO 3 ; (2) Sulfarsenites and sulfantimonites, derivatives of H 3 AsS 3 and H 3 SbS 3 analogous to arsenious acid, H 3 AsO 3 and antimonous acid, H 3 SbO 3 ; (3) Sulfarsenates and sulfantimon- ates, derivatives of H 3 AsS 4 and H 3 SbS 4 , analogous to arsenic acid, H 3 AsO 4 , and antimonic acid, H 3 SbO 4 . There are also condensed acids derived from the above mentioned acids by the subtraction or addition of H 2 S, just as condensed acids may be derived from oxygen acids by the subtraction or addition of H 2 0. Thus chalcopyrite, CuFeS2, is a salt of HFeS 2 derived from H 3 FeS 3 (H 3 FeS 3 - H 2 S = HFeS 2 ). Jamesonite is a salt of H 5 Sb 3 S 7 (3H 3 SbS 3 - 2H 2 S). Stephanite is a salt of H 5 SbS 4 (H 3 SbS 3 +H 2 S). Polybasite is a salt of H 16 Sb 2 S n (2H 3 SbS 3 +5H 2 S). The sulfo-salts are sometimes considered as double sulfids. Thus pyrargyrite Ag 3 SbS 3 is written 3Ag 2 S-Sb 2 S 3 . About sixty sulfo-salt minerals are known, but most of them are rare. 243 244 INTRODUCTION TO THE STUDY OF MINERALS CHALCOPYRITE, CuFeS 2 Form. Chalcopyrite occurs in crystals, in masses, and dis- seminated through the rock. The crystals belong to the tetra- gonal system, scalenohedral class, but are pseudotetrahedral and pseudo-octahedral in form. Fig. 400 represents a common type of crystal with the forms pjlll) and z{201j. This is a tetragonal bisphenoid and is distinguished from a tetrahedron by the striations. H. to 4. Sp. gr. 4.2 . FIG. 400 . Chal- copyrite crystal. Color. Brass yellow, often with an iridescent tarnish, hence the name " peacock copper." Metallic luster. Chemical Composition. Cuprous sulfoferrite, CuFeS 2 ; (Cu = 34.5 per cent.). Variations from this formula are usually due to admixed pyrite. Blowpipe Tests On charcoal fusible (at 2) to a magnetic globule which heated with sodium carbonate gives a copper button. In the closed tube it decrepitates and gives a sublimate of sulfur. Soluble in HNOs to a green solution with the separation of sulfur from which NH.4OH gives a red-brown precipitate and blue solution. Distinguishing Features. Chalcopyrite is distinguished from pyrite by difference in color and inferior hardness, and from gold by its brittleness. Uses. Chalcopyrite is one of the principal ores of copper and the most widely distributed copper mineral. Occurrence. 1. As a vein mineral associated with pyrite, galena, sphalerite, tetrahedrite, bornite, etc. 2. In basic igneous rocks with pyrrhotite, as a late mag- matic mineral. Sudbury, Canada. 4. In fahlbands of schists and gneisses. 5. As a contact mineral with magnetite and hematite. SULFO-SALTS 245 BORNITE, Cu 5 FeS 4 Form. Bornite occurs in masses and disseminated specks, very rarely in rough cubic crystals. H. = 3. Sp. gr. 5.1. Color. A red-brownish bronze with purple tarnish. Metallic luster. Slightly sectile. Chemical Composition. Copper sulfoferrite, Cu 5 FeS 4 ; (Cu = 63.3 per cent.). Analyses vary widely due to intermixture with chalcopyrite and chalcocite. Blowpipe Tests. Fusible (at 2J^) on charcoal R.F. to a mag- netic globule. In the closed tube gives a faint sublimate of sulfur. Soluble in HNO 3 to a green solution with the separation of S from which NH 4 OH gives a red-brown precipitate and a blue solution. Distinguishing Features. The peculiar color of bornite and its purple tarnish distinguishes it from all other minerals. Uses. Bornite is an important ore of copper. It is usually intimately associated with chalcopyrite or chalcocite. Butte, Montana. Occurrence. 1. As a vein mineral associated with chalcocite, chalcopyrite, and pyrite. Butte, Montana. 2. As a late magmatic mineral associated with chalcopyrite. Engels mine, Plumas county, California. 3. As a contact-metamorphic mineral between limestones and igneous rocks. Jamesonite, Pb 4 FeSb 6 Si 4 Form. Jamesonite occurs in delicate capillary crystals and in columnar #nd compact masses. Cleavage. If cleavage is distinct, it is transverse to the length of the crystals. H. = 2M. Sp.gr. 5.7+ . Color, Lead-gray. Metallic luster. 246 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Lead and iron sulfantimonite, Pb 4 - FeSb 6 S J4 (Pb = 40.3 per cent.). Blowpipe Tests. On charcoal easily fusible (at 1) giving white and yellow coatings. In the closed tube gives a yellow subli- mate of sulfur and a dark-red sublimate of Sb 2 S 2 O. With sodium carbonate on charcoal it gives a lead button. Soluble in HC1 with the evolution of H 2 S. On cooling the solution needle crystals of PbCl 2 separate. Decomposed by HNOs with the separation of a white residue (HSbO 3 ). Distinguishing Features. Jamesonite resembles stibnite, but has no distinct cleavage parallel to the length of the crystals. Occurrence. 1. As a vein mineral. Sevier county, Arkansas. Pyrargyrite, Ag 3 SbS 3 Form. Pyrargyrite often occurs in small well-defined crystals belonging to the ditrigonal pyramidal class. The habit is usually prismatic, and the hexagonal prism, { 1120}, the dominant form. Opposite ends of the crystal are differently terminated. Figure 401 repre- sents a typical crystal; the striations indicate the hemimorphic character. H. = 2J. Sp. gr. 5.8 . Color. Dark red to black. Translucent red on thin edges. Streak purple-red. Luster metallic-adamantine. Optical Properties. n 7 (3.08) - n a (2.88) = 0.20. Fragments are irregular and red in color, with high order interference colors. Chemical Composition. Silver sulfantimonite, Ag 3 SbSa ; (Ag = 59.9 per cent.). Arsenic replaces antimony to some extent. The corresponding sulfarsenite is called proustite. Together they constitute the ruby silver group. Blowpipe Tests. On charcoal fuses easily (at 1) to a globule of silver sulfid giving a white sublimate. This globule with sodium SULFO-SALTS 247 carbonate in R.F. gives a silver button. Heated intensely in the closed tube, it gives a slight red sublimate. Decomposed by HNO 3 with the separation of sulfur and a white residue. Distinguishing Features. Pyrargyrite is distinguished from other silver minerals (except .proustite, Ag 3 AsS 3 ) by the red fragments and streak, and from cuprite and cinnabar by blow- pipe tests. Uses. A valuable ore of silver, often associated with argentite, stephanite, and polybasite. Occurrence. 1. As a vein mineral often formed at a late stage by ascending solutions. Tonopah, Nevada. TETRAHEDRITE, Cu 3 SbS 3 +z(Fe,Zn) 6 Sb 2 S 9 Form. Tetrahedrite occurs in masses, and also in crystals belonging to the hextetrahedral class of the isometric system. FIG. 402. FIG. 403. FIG. 404. FIG. 405. FIGS. 402^405. Tetrahedrite crystals. The common forms are the tetrahedron oflll}, the tristetrahe- dron w{211}, and the dodecahedron djllOj. Figures 402 to 405 represent typical crystals. H. = 3^. Sp. gr. 4.7. Color. Dark iron-gray. Metallic luster. Brittle. Chemical Composition. Copper sulfantimonite, Cu 3 SbS 3 -|-z (Fe,Zn) 6 Sb 2 S 9 , where x = Ko to ^. The copper is often replaced by silver and the antimony by arsenic. It grades into tennantite, the corresponding sulfarsenite. The following analyses illustrate the wide variation in chemical composition. 248 INTRODUCTION TO THE STUDY OF MINERALS Cu Sb S As Fe Zn Ag Misc. Fresney d'Oisans 45.4 28 8 24 5 tr 1 3 Pb = 1 38 23 9 25 8 2 9 8 7 3 6 Machetillo, Chili Cabarrus Co., N. C Poracs, Hungary 36.7 30.7 32.8 20.7 17.8 30.2 25.3 25.5 24.9 6.5 11.6 1.2 1.4 5.9 6.9 2.5 2.9 10.5 0.1 Hg = 5.6 Blowpipe Tests. Easily fusible (at lj^) giving dense white fumes and a white sublimate near the assay. The residue heated with sodium carbonate in R.F. gives metallic copper. In the closed tube it gives a dark red sublimate (Sb 2 S 2 O). Soluble in HN0 3 to a green solution with the separation of sulfur and a white residue, HSbO 3 . Distinguishing Features. Tetrahedrite is apt to be confused with chalcocite. It is very brittle, while chalcocite is somewhat sectile. Uses. Tetrahedrite is an ore of copper and silver known to miners as "gray copper." Highly argentiferous tetrahedrite is called freibergite. Occurrence. 1. As a vein mineral often associated with chal- copyrite, galena, sphalerite, and siderite. Cornwall, England, and Oruro, Bolivia, are prominent localities. 2. In fahlbands of schists. Fahlore is a synonym of tetrahe- drite. Stephanite, Ag 5 SbS 4 Form. Stephanite occurs disseminated, compact massive, more rarely in crystals. The crystals are orthorhombic, but pseudohexagonal and short prismatic in habit. H. = 2%. Sp.gr. 6.2. Color. Dark gray to black. Very brittle. Metallic luster. Chemical Composition. Silver sulfantimonite, Ag 5 SbS4,' (Ag = 68.5 per cent.). SULFO-SALTS 249 Blowpipe Tests. On charcoal easily fusible (at 1) to a globule giving dense fumes and a white sublimate of Sb 2 Os. The globule heated with sodium carbonate R.F. gives a silver button. Decomposed by HN0 3 with the separation of sulfur and a white residue. Distinguishing Features. Stephanite resembles argentite and tetrahedrite. Argentite is sectile, and tetrahedrite harder than stephanite. Uses. A valuable ore of silver. It was a prominent mineral in the Comstock Lode of Nevada. Stephanite is known to miners as " brittle silver." Occurrence. 1. As a vein mineral formed at a late stage by ascending solutions. Polybasite, (Ag,Cu) 16 Sb 2 Sn Form. Polybasite usually occurs in monoclinic pseudo-hexag- onal crystals of tabular habit with triangular striations on the basal pinacoid. H. = 2H. Sp.gr. 6.1 + . Color. Iron black. Metallic luster. Optical Properties. n>1.93. Very thin fragments are deep red translucent. Chemical Composition. Silver and copper sulfantimonite (Ag,Cu)i 6 Sb 2 Sn; (Ag = about 70 per cent.). Polybasite usually contains arsenic and thus grades into pearceite, (Ag,Cu)j 6 As 2 Sn. Blowpipe Tests. On charcoal fuses easily (at 1) to a globule and gives a white sublimate. The globule heated with sodium carbonate gives a metallic button. In order to get pure silver it is necessary to cupel the button. Decomposed by HNO 3 with the separation of sulfur and a white residue (HSbO 3 ). Distinguishing Features. The tabular crystals with triangular markings are distinctive. Uses. Polybasite is an ore of silver. It occurs at Tonopah, Nevada, and at several mines in Colorado. 250 INTRODUCTION TO THE STUDY OF MINERALS Occurrence. 1. As a vein mineral, formed at a late stage by ascending solutions. Aspen, Colorado. Enargite, Cu 3 AsS4 Form. Enargite occurs in columnar masses and occasionally in prismatic orthorhombic crystals. Cleavage. Prominent (in two directions at angles of 82 to each other). H. = 3. Sp. gr. 4.4. Color. Dark gray to black. Chemical Composition. Copper sulfarsenate, CiisAsSij (Cu = 48.3 per cent.). It usually contains a little antimony and a little iron. The corresponding sulfantimonate is a rare mineral known as famatinite. Blowpipe Tests. Easily fusible at 1 ; sulfur dioxid masks the odor of arsin. In the closed tube gives a sublimate of sulfur. In the open tube it deposits minute octahedral crystals of As 2 O 3 with adamantine luster and 862 gas is formed. Soluble in HNO 3 . Distinguishing Features. The columnar structure and good cleavage are distinctive. It is distinguished from stibnite by its darker color. Uses. Enargite is an ore of copper, occurring at Butte, Montana, and at many localities in South America. Near Butte white arsenic (As 2 3 ) is recovered from smelter smoke. Occurrence. 1. In veins and replacement deposits formed at intermediate depths and temperatures. At Tintic, Utah, enargite is the original source of several copper arsenate minerals. 4. HALOIDS The haloids comprise chlorids, bromids, iodids, and fluorids which are salts of HC1, HBr, HI, and HF respectively. Com- paratively few haloids occur in nature, but several of them are very common minerals. All the minerals are normal anhydrous salts with the exception of carnallite. A. Normal anhydrous haloids HALITE, NaCl Sylvite, KC1 Cerargyrite, AgCl FLUORITE, CaF 2 Cryolite, Na 3 AlF 6 B. Basic and hydrous haloids Carnallite, KMgCl 3 6H 2 O HALITE, NaCl Form. Halite occurs in crystals, and in cleavable, granular, and fibrous masses. Crystals are isometric, usually cubes (Fig. 406), sometimes hopper-shaped (Fig. 407), rarely in octahedrons or cubo-octahedrons (Fig. 408). Cleavage. The perfect cubic cleavage is a marked feature of halite. A dodecahedral {110} parting is developed by pressure applied on the cube-edges by a hammer or in a vise (Fig. 265, page 131). H. = 2M- Sp. gr. 2.1. Color. Colorless and white, often reddish or gray, and some- times deep blue in patches. Optical Properties. Isotropic. n = 1.54. Recrystallizes from a water solution in squares, often hopper-shaped (Fig. 490), which are dark between crossed nicols and have low relief in clove oil. 251 252 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Sodium chlorid, NaCl; (Na = 39.4 per cent.)- Halite may contain MgCl2,MgSO 4 ,CaCl 2 , and Ca- SO 4 . It is to these impurities that the deliquescence of table salt is due. Blowpipe Tests. Fuses easily (at 1), giving an intense yellow flame. With CuO in NaPO 3 bead it gives an azure-blue flame. Soluble in cold water. Distinguishing Features. Distinguished from most minerals by its cubic cleavage. It has a pleasant saline taste and not the sharp taste of sylvite. Uses. Halite is the chief source of sodium compounds used extensively in the manufacture of soap and glass, and also as FIG. 406. FIG. 407. FIG. 408. FIGS. 406-409. Halite crystals. FIG. 409. table salt and as a preservative. Salt brines furnish bromin. Salt is obtained (1) directly by the mining of rock-salt as at Petite Anse, Louisiana, and Lyons, Kansas; (2) by pumping brine to surface and evaporating as at Syracuse, New York, and Hutchinson, Kansas; and (3) by solar evaporation as at Great Salt Lake, Utah. Occurrence. 1. Occurs in beds associated with anhydrite, gypsum, and occasionally with other chlorids and sulfates. These deposits are formed by the evaporation of sea- water. Important localities are Stassfurt, Germany; Wieliczka, Poland; Cheshire, England; western New York; Saginaw, Michigan; and central Kansas. HALOIDS 253 Sylvite, KC1 Form. Sylvite occurs in cleavable and granular masses and in well-formed cubic or cubo-octahedral (like Fig. 408) crystals. Etch-figures indicate that.sylvite belongs to the gyroidal class of the isometric system. Cleavage. Perfect cubic cleavage. Color. Colorless or white, sometimes with bluish opalescence. Taste, sharp saline. H. = 2. Sp. gr. 2.0. Optical Properties. Isotropic. n = 1.49 (clove oil). Re- crystallizes from water solution in square crystals with a tendency toward skeleton crystals (Fig. 410). These are dark between crossed nicols and have moderate relief in clove oil. Chemical Composition. Potassium chlorid, KC1; (K = 52.4 per cent.). It may contain NaCl. Blowpipe Tests. Fuses easily (at 1J^), coloring the flame violet. With CuO in a NaP0 3 bead it gives an azure-blue flame. Soluble in cold water. Distinguishing Features. It is re- cognized by its cubic cleavage and its sharp saline taste. Uses. Used as a fertilizer and a source of potassium salts. Occurrence. 1. In salt beds with halite, anhydrite, kainite, and carnallite. It is sometimes a secondary mineral formed from carnallite (KMgCl 3 -6H 2 O). Stassfurt, Germany, and Mulhouse, Alsace. 2. As a volcanic sublimate on lava. Vesuvius. FIG. 410. Sylvite recrystallized. Cerargyrite, AgCl Form. Cerargyrite usually occurs as a thin crust or seam, but small cubic crystals are also found. H. = 2. Sp. gr. 5.5. 254 INTRODUCTION TO THE STUDY OF MINERALS Coloiv Gray, greenish, or violet. Luster, waxy to adaman- tine. Very sectile. Optical Properties. Isotropic. n = 2.06. May be ham- mered to a thin sheet which is translucent, but dark between crossed nicols. A few drops of NH 4 OH will give minute octahe- dral crystals (AgCl). Chemical Composition. Silver chlorid, AgCl (Ag = 75.3 per cent.). Blowpipe Tests. On charcoal fuses easily (at 1), giving a silver button. A fragment touched with a NaPO 3 bead saturated with CuO gives an intense azure-blue flame. Insoluble in acids, but soluble in NH 4 OH. Distinguishing Features. Cerargyrite is easily overlooked but on close inspection its appearance and sectile character are distinctive. Uses. An ore of silver in the western United States, Mexico, and Chili. The miner's name for it is "horn silver." Occurrence. 1. A mineral characteristic of the upper part of ore deposits. It is formed by the action of chlorid-bearing meteoric waters on other silver minerals, and therefore is promi- nent in arid regions. Poorman mine, Idaho. FLUORITE, CaF 2 Form. Fluorite usually occurs in cleavable masses, but also often in distinct crystals. The crystallization is isometric (hexoctahedral class). Usual forms: a{ 100}, /{310J, {421}, djllO), o{lll}. The habit is practically always cubic; the other forms are subordinate. Octahedral crystals are rare. At some localities, apparent octahedra are built up of minute cubes in parallel position. Figures 411 to 414 represent typical crystals of fluorite. Figure 414 is a penetration twin with the cube diagonal as twin axis. Vicinal faces with high indices such as {32-1-0} are often found on these crystals. Cleavage. One of the most important characters of fluorite is the perfect octahedral cleavage. On a cube this will show as HALOIDS 255 triangular faces at the vertices, or at least as cracks in this direction, as shown in Fig. 411. H. = 4. Sp. gr. 3.2 (3.18, if pure) Color. Usually colorless or some tint or shade of violet or green, rarely yellow, brown, blue or pink. The color is prob- ably due to hydrocarbons. Some crystals from Cumberland are green by transmitted light, but blue by reflected light. This property, also possessed by some aniline colors such as red-ink, is known as fluorescence, a name derived from the mineral fluorite. Some varieties of fluorite are also phosphorescent, that is, after being heated, continue to emit light in the dark. FIG. 411. FIG. 412. FIG. 413. FIG. 414. Optical Properties. Iso tropic, n = 1.434, hence high relief in clove oil. Fragments are triangular, colorless, and dark be- tween crossed nicols. (See Fig. 371, page 205.) Chemical Composition. Calcium fluorid CaF 2 ; (F = 48.9 per cent.). Impurities are usually calcite, dolomite, barite or quartz. Free fluorin has been detected in some fluorite. Fluorite is the only common fluorid occurring in nature. Blowpipe Tests. In the closed tube decrepitates. Fuses (at 3) to an enamel coloring the flame red. Distinguishing Features. Fluorite is distinguished by the cubic crystals, octahedral cleavage, and specific gravity, which is a little higher than the average non-metallic mineral. Soluble in H 2 SO4 with evolution of HF, which etches glass. Dilute H 2 SO 4 added to a hydrochloric acid solution of the mineral gives a crystalline precipitate of hydrous calcium sulfate. Uses. The main use of fluorite is a flux in iron smelting and 256 INTRODUCTION TO THE STUDY OF MINERALS foundry work. Western Kentucky and southern Illinois are the principal sources of fluorite in this country. Minor uses are the manufacture of enamels, opalescent glass, and hydrofluoric acid. Moissan in his work on fluorin used vessels made of fluorite. Occurrence. 1. As a vein mineral associated with galena, sphalerite, calcite, and barite. Typical localities are fissure veins in the limestone of western Kentucky and lead mines in the north of England, where magnificent museum specimens are found. 2. In tin-stone veins associated with cassiterite, apatite, topaz, and lepidolite. Zinnwald, Bohemia. 3. In limestones. St. Louis, Missouri. Cryolite, 3NaF-AlF 3 Form. Massive and in pseudo-cubic (monoclinic) crystals often in parallel position. Cleavage. Imperfect in three directions at nearly right angles. H. = 2K- Sp. gr. 3.0 . Color. White, sometimes brown. Translucent. Optical Properties. n$ = 1.36. Low relief in water (for water, n = 1.333). Double refraction very weak. Fragments are roughly rectangular or irregular. Interference colors are first order gray. Chemical Composition. Sodium aluminum fluorid, 3NaF- A1F 3 - (Al = 12.8 per cent., Na = 32.8). Blowpipe Tests. Easily fusible ( at 1) giving an intense yellow flame. Soluble in H 2 S0 4 with the evolution of HF. Distinguishing Features. The translucent white masses resemble fluorite but lack the good cleavage of the latter. Frag- ments placed in water look as though they had partially dissolved. Uses. Formerly used as a source of aluminum, but now used as a bath in the electrolytic production of aluminum from bauxite. It is also used in the manufacture of sodium and aluminum salts at Natrona, Pa. The mineral is shipped from Greenland. Occurrence* 1. In granite pegmatites. The most important HALOIDS 257 locality is Ivigtut, in southern Greenland, where an immense vein-like mass of cryolite containing siderite, sphalerite, galena, etc., occurs in a porphyritic granite. It also occurs at St. Peter's Dome in El Paso county, Colorado. Carnallite, KMgCl 3 .6H 2 O Form. Massive or granular. Crystals (orthorhombic) are very rare. Cleavage. No cleavage, but has conchoidal fracture. H. = 1. Sp.gr. 1.6 . Color. Colorless or reddish. Luster, greasy. Very deli- quescent. Optical Properties. 717(1. 49) - na(1.46) = 0.03. Recrystallized from water solu- tion, it forms in order (1) isotropic squares of KC1, (2) rectangular twinned crystals of KMgCl 3 -6H 2 O, and (3) streaked aggregates of MgCl 2 (Fig. 415). Chemical Composition. Hydrous potas- sium magnesium chlorid KMgCl 3 -6H 2 O or KCl-MgCl 2 -6H 2 O; (KC1 = 26.8 per cent.) (H 2 O = 39.0 per cent.). Blowpipe Tests. Fusible at 1^, coloring the flame violet. With CuO in a NaPO 3 bead it gives an azure-blue flame. Gives abundant water in the closed tube. Soluble in water. Distinguishing Features. Carnallite is distinguished by its bitter taste and lack of cleavage. Uses. Carnallite is used as a fertilizer and in the manufacture of potassium salts. KC1 crystallizes out of a water solution of carnallite. Occurrence. 1. In salt beds associated with anhydrite, halite, sylvite, and kainite. Stassfurt, Prussia, is the most prominent locality. FIG. 415. 17 5. OXIDS QUARTZ, Si0 2 CHALCEDONY, SiO 2 OPAL, Si( Tridymite SiO 2 Cristobalite SiO 2 Ice, H 2 O Cuprite, Cu 2 O f CORUNDUM, A1 2 8 \ HEMATITE, Fe 2 O 3 Turyite, Fe 2 O 3 (H 2 O) f CASSITERITE, SnO 2 \ RutUe, TiO 2 Pyrolusite, MnO 2 Stibiconite, Sb 2 O 4 (H 2 O) x Among the oxids are some of the most common and widely distributed minerals. The silica minerals are placed first, and after them the monoxids, R 2 and RO, the sesquioxids, R 2 O 3 , and the dioxids, RO 2 in the order named. The minerals of the spinel group, sometimes considered as double oxids of the type ROR 2 3 , are placed in a separate division, the aluminates, etc. QUARTZ, SiO 2 Form. Crystals of quartz are very common, both large and small, loose and attached. There are crystalline aggregates of various kinds as well as massive, granular, and compact varieties. Quartz crystallizes in the trigonal trapezohedral class of the hexagonal system. 6 = J..099. Usual forms: rjlOll}, z{OlTl[, ra{10lO}, s{1121} z[5161}. Interfacial angles: rar(10TO:10ll) = 38_ 13';jr(10ll:Tl01) = 85 46'; rz(10ll: Oil!) = 46 16'; ms(WlO: 1121) = 37 58_'; raz(1010 : 0111) = 66 52'; mx(WW 5161) = 12 1'; mm(10lO : OlIO) = 60 0'. Figures 416-421 258 OXIDS 259 represent typical crystals. The habit varies from prismatic to pyramidal. The two rhombohedrons r and z are often in equal combination (Figs. 416 , 419) and apparently form a hexagonal bipyramid. The s face at alternate vertices proves the trigonal character. Figures 420 and 421 are more complex with x } { 6151 J and.si{2lll}. Cleavage, practically absent (an imperfect cleavage parallel to r is occasionally noticed). H. = 7. Sp.gr. 2.66 . Color, more often white or colorless, but may be any color. Luster, vitreous. Transparent to translucent, rarely opaque. FIG. 416. FIG. 418. FIG. 418. FIG. 419. Optical Properties. n 7 (1.553) - n a ( 1.544) = 0.009. Double refraction rather weak. Fragments are irregular, with low relief in clove oil (n > clove oil) and upper first-order interference colors. In thick basal ( J_ to c-axis) sections quartz shows rotary polari- zation, i^. } in monochromatic light a section only becomes dark by rotating one nicol. For red light the angle of rotation is 13 for each millimeter of thickness. . Sections from crystals like Fig. 421 rotate the plane to the right, and those from crystals like Fig. 420 rotate the plane to the left. Chemical Composition. Silica or silicon dioxid, SiO 2 . Varia- 260 INTRODUCTION TO THE STUDY OF MINERALS tions in analyses are due to inclusions such as chlorite, tourma- line, rutile, etc. Blowpipe Tests. Infusible even on the thinnest edges. When fused with an equal volume of sodium carbonate, effervesces and gives a colorless glass (Na 2 C0 3 + SiO 2 = CO 2 + Na 2 - SiO 3 .) Insoluble in a NaPO 3 bead. Insoluble in ordinary acids. Soluble in HF. FIG. 420. Left-handed quartz crystal. l FIG. 421. Right-handed quartz crystal. Distinguishing Features. Quartz often resembles calcite but is distinguished by its superior hardness and lack of cleavage. From the other forms of silica it is distinguished by its indices of refraction. Uses. Quartz in the form of rock crystal, amethyst, and smoky quartz is used for ornamental purposes, in the form of rock crystal for optical apparatus, in the form of sand for glass-making, and in the form of pulverized quartz for pottery and porcelain and as an abrasive. 1 This figure is drawn as if the axes had been rotated to the right instead of to the left, as ordinarily. This brings out the enantimorphous relation to the right-handed crystal. OX IDS 261 Occurrence. 1. As normal constituent of the acid igneous rocks (rhyolites and granites). (/3-quartz.) 2. As an abnormal constituent of the basic igneous rocks, espe- cially basalts (quartz basalts). 3. As a vein mineral, often the gangue of ores. Quartz is the most common vein mineral., (a-quartz). 4. As the chief constituent of sandstones and quartzites. 5. As a replacement mineral in various rocks, often occurring as pseudomorphs after various minerals, and as petrifactions. 6. As the chief constituent of river and beach sands. CHALCEDONY, SiO 2 Form. Chalcedony occurs in compact masses and in cavities in colloform crusts. Although chalcedony is never found in distinct crystals, it is crystalline as the examination of thin sec- tions or fragments in polarized light will show. Fracture, more or less conchoidal. No cleavage. H. = 7. Sp. gr. 2.6 . Color, colorless, white, or any color, often banded and varie- gated. Red and brown varieties are called jasper, and the banded and variegated varieties, agate. Translucent to opaque. Luster, waxy to dull. Optical Properties. n 7 (1.543) - n a (1.532) = 0.011. Double refraction rather low. Fragments are irregular with n slightly lower than clove oil. The aggregate structure with low order interference colors in spots and streaks is highly characteristic of chalcedony and usually distinguishes it from quartz. Chemical Composition. Silica, SiO 2 . Blowpipe Tests. Same as for quartz except that it gives a small amount of water in the closed tube. Distinguishing Features. Chalcedony is distinguished from most minerals of similar appearance by its greater hardness. From the other silica minerals it is distinguished by its dull luster. (Quartz has vitreous luster and opal, greasy luster.) 262 INTRODUCTION TO THE STUDY OF MINERALS Uses. Agate, chrysoprase (apple-green, translucent chalce- dony), and jasper are used as ornamental stones. Occurrence. 1. As a secondary mineral in seams and cavities of various rocks, especially the volcanic igneous rocks. 2. As chert or flint and jasper occurring in concretions, lenses or layers in sedimentary rocks. The origin is doubtful. In the Joplin district the zinc ores occur in a brecciated chert, which covers large areas. 3. As a low-temperature vein mineral, often the gangue of gold, silver, and mercury ores. OPAL, SiO 2 (H 2 O) x Form. Opal usually occurs in seams and cavities, but is also disseminated and massive. It is one of the typical amorphous minerals and so in cavities it often has a colloform structure. Fracture, conchoidal. No cleavage. H. = 5K to 6M- Sp. gr. 2.1+ (very light). Color, white, colorless, or almost any color. Usually translu- cent. Luster, more or less greasy. Optical Properties. Isotropic. n = 1.45. Fragments are irregular, usually dark between crossed nicols, and have high relief in clove oil (n less than clove oil, Becke test). Some varie- ties, notably hyalite, show weak double refraction, which is due to strain. Opal is often intimately mixed with chalcedony. Chemical Composition. Hydrous silica SiO 2 - (H^O^ with water varying from 3 to 12 per cent. Like most amorphous minerals it is very apt to contain impurities. The following are typical analyses: SiOi H 2 A1 2 3 Fe 2 Oa CaO MgO Waltsch Bohemia 95 5 3 0.8 0.2 Washington Co , Ga 91 9 5.8 1.4 0.9 Faroe Islands 88.7 8.0 1.0 0.5 1.5 Meronitz, Bohemia 83.7 11.5 3.6 1.6 0.7 OXIDS 263 Blowpipe Tests. Infusible, but becomes opaque. In the closed tube yields water. Insoluble in the ordinary acids. Soluble in HF and also soluble in KOH. Distinguishing Features. Opal may resemble chalcedony but has a lower specific gravity,- is a little softer, and usually has a greasy luster. Uses. The opal with play of colors known as precious opal and also the red or fire-opal are well known gems. The best precious opals are found in New South Wales and in Hungary, while the fire-opal is found principally in Mexico. Occurrence. 1. As a characteristic mineral in cavities and along the seams of igneous rocks. 2. In volcanic tuffs as a replacement of wood (opalized wood or wood opal). 3. As siliceous sinter (geyserite) formed around hot springs and geysers. Yellowstone National Park is a prominent locality. 4. As the principal constituent of diatomaceous earth or diatomite. Diatoms and radiolaria secrete casts of opal silica. Tridymite, SiO 2 Form. Tridymite usually occurs in the form of minute crystals. The habit is pseudohexagonal tabular; twinned crys- tals are common. The high-temperature /3-tridymite is hexa- gonal but on cooling to the low-temperature a-tridymite (probably orthrombic) it retains the hexagonal form. H. = 7. Sp. gr. = 2.27. Color, colorless. Optical Properties. n T (1.473) - n(1.469) = 0.004. Frag- ments are six-sided plates or irregular with fair relief in clove oil. The double refraction is very weak. Chemical Composition. Silica, SiO 2 , the same as that of quartz. Blowpipe Tests. Infusible before the blowpipe. Insoluble in ordinary acids. 264 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Optical tests are necessary to dis- tinguish tridymite from quartz and cristobalite. The indices of refraction of tridymite are less than 1.480, while those of cristo- balite are greater than 1.480. Occurrence. 1. In volcanic igneous rocks usually in cavities and probably produced by hot gases after the main period of rock formation. Obsidian Cliff, Yellowstone National Park. Cristobalite, SiO 2 Form. Cristobalite occurs in spherical aggregates or in minute pseudo-octahedral crystals. The high-temperature /?- cristobalite is isometric, but on changing to a-cristobalite it retains the isometric form. It is sometimes found in pseudo- morphs after tridymite. H. = 7. Sp. gr. = 2.33. Color. White subtranslucent. Optical Properties. n y (1. 487) -n(l. 484) = 0.003. Fragments are irregular with fair relief in clove oil. The double refraction is very weak. Chemical Composition. Silica, Si02, the same as that of tridymite and quartz. Blowpipe Tests. Infusible before the blowpipe. On heating it becomes somewhat transparent and on cooling it suddenly becomes subtranslucent again. This change is due to the change of the high-temperature /3-cristobalite to a-cristobalite. Insoluble in ordinary acids. Distinguishing Features. The characteristic behavior before the blowpipe distinguishes it from similar minerals. In occurrence and general characters it is much like tridymite, but its indices of refraction are greater than 1 .480 while those of tridymite are less than 1.480. Occurrence. 1. In volcanic igneous rocks usually in cav- ities and probably formed by hot gases after the main period of rock formation. Tehama County, California. OXIDS 265 Stability Relations of the Silica Minerals Silica exists in at least seven well-defined polymorphous forms, each of which is stable between certain temperature limits as shown in Fig. 422. Ordinary vein quartz is stable up to 575C. I!// 8} i J/ o ' / / 58 Variable g J | Temperature 3 S FIG. 422. Stability diagram of the silica minerals. (After Fenner.) At that temperature there is a sudden change in the indices of refraction and some of the other properties, and it passes into the form known as j8-quartz. This form is the one char- 266 INTRODUCTION TO THE STUDY OF MINERALS acteristic of igneous rocks, especially rhyolite porphyry and granite porphyry. No specimens of 0-quartz exist at ordinary temperature for it has changed to a-quartz. At 870C. 0-quartz changes suddenly to /3 2 -tridymite, and at 1470C. this changes to 0-cristobalite. At 1625C. cristobalite melts to silica glass. On cooling, /3-cristobalite changes at tempera- tures ranging from 220 to 275C. to another form called a-cristobalite. Similarly /3 2 -tridymite changes to /Si-tridymite at 163C. and this again to a-tridymite at 117C. The cristobalite and tridymite in mineral collections are each the a-forms but usually retain the crystal habit of the /3-forms. The rela- tion of chalcedony to the other forms of silica is uncertain and opal can not be treated at all from the standpoint of the phase rule. It is a two-phase system, for it consists of solution of water in amorphous silica and is of colloidal origin. The diagram of Fig. 422 has been worked out in the Geophysical Laboratory of the Carnegie Institution of Washington. Ice, H 2 O Form. Ice, the solid form of H 2 0, occurs in frost and snow crystals and in massive and granular forms. Ice (and snow) crystallizes in the dihexagonal pyramidal class of the hexagonal system. Snow crystals are skeleton crystals of great variety. The frontispiece shows a number of microphotographs of snow crystals. H. = IK- Sp. gr. = 0.9167. Color. Colorless to white, bluish in thick layers. Luster, vitreous. Optical Properties. n 7 (1.313) = n a ( 1.309) = 0.004. Optically positive. Occurrence. 1. In the form of snow, frost, and hail. 2. On the surface of rivers, ponds, and lakes. 3. In the polar regions. 4. In glaciers. OXIDS 267 Cuprite, Cu 2 O Form. Cuprite is found in crystals, in crystalline aggregates, and in fine-grained masses. Crystals are isometric; the common forms are the cube (a), octahedron (o), and dodecahedron (d). The habit is usually determined by one of these forms. (Figs. 423-426.) Capillary cuprite found in Arizona proves to be elongate cubes. H. = 3M to 4. Sp.gr. 6.0 . Color. Dark red to brownish-red. Translucent to opaque. Streak, brownish-red. Luster, metallic-adamantine. Imperfect cleavage. Optical Properties. Isotropic. n = 2.85. Fragments are ir- regular, translucent red, and dark between crossed nicols. FIG. 423. FIG. 424. FIG. 425. FIG. 426. Chemical Composition. Cuprous oxid, Cu 2 O; (Cu = 88.8 per cent.). Iron oxid is the most frequent impurity. Blowpipe Tests. On charcoal fuses (at 2*^) to a copper button. Soluble in HNOs to a green solution. Distinguishing Features. Cuprite is recognized by the isometric crystals, adamantine luster, and absence of perfect cleavage. Uses. Cuprite is a valuable copper ore on account of the high percentage of copper. Bisbee, Arizona, is an important locality. Occurrence. 1. In the oxidized zone of ore deposits asso- ciated with other copper minerals, especially native copper. Bisbee, Arizona. 268 INTRODUCTION TO THE STUDY OF MINERALS HEMATITE GROUP HEXAGONAL Corundum and hematite form a perfect isomorphous group, although intermediate compounds are lacking. With them is sometimes placed ilmenite, but it is more properly considered a ferrous metatitanite, FeTiO 3 . It belongs to a different crystal class, the trigonal rhombohedral class. CORUNDUM, A1 2 3 Form. Corundum is found in rough, loose crystals, in cleav- able masses and disseminated through rock in small crystals or grains. The crystals belong to the scalenohedral class of the x/ FIG. 427. FIG. 428. FIG. 429. hexagonal system. Usual forms: cfOOOl}, rflOll), a|1120}, rc{2243}. Interfacial angles^ cr(0001 :1011) = 57 _34', rKlOll: IlOl) = 93 56', cri(0001 : 2243) = 61 11'; nn(2243 : 4223) = 51 58'. Habit prismatic (Fig. 428), tabular (Fig. 427), and steep pyramidal (Fig. 429). The trigonal character is shown by the r faces at alternate vertices and by the triangular striations on the basal pinacoid c. Cleavage. There is often parting parallel to c and r. The rhombohedral parting greatly resembles cubic cleavage (rr = 93 56'). H. = 9. Sp. gr. 4.0 . 'OXIDS 269 Color. Bluish-gray is the most common color, but brown, red, pink, green, bright blue, and white colors are not at all rare. Usually translucent. Luster, sub-adamantine. Optical Properties. n T (1.767) - w a (1.759) = 0.008. Double refraction rather low. Fragments are irregular with first-order interference colors and index of refraction greater than methylene iodid. Large deep-colored fragments or small crystals are pleochroic. Chemical Composition. Alumina or aluminum oxid, A1 2 O 3 ; (Al = 52.9 per cent.). Emery is a dark-colored mixture of corundum with magnetite, hematite or spinel. Blowpipe Tests. Infusible. When intensely heated with Co(NO 3 )2 solution it becomes deep blue. Insoluble in acids. Decomposed by fusion with KHSO 4 . Distinguishing Features. Corundum is recognized by its extreme hardness (it is often altered on the exterior to soft mica- ceous product) , by its cleavage, and by its high specific gravity. Uses. Certain varieties of corundum are valuable gems. Ruby, the transparent red corundum, is even more valuable than diamond. Sapphire is the blue transparent corundum. Colorless stones are known as white sapphires. The best rubies come from Burma, and the best sapphires from Ceylon. Artificial rubies and sapphires are now produced synthetically in Paris. They are with difficulty distinguished from natural stones. Corundum is also used as an abrasive, either as the pure cleav- able mineral or as the mixture known as emery. Corundum is mined in Ontario, Canada, and emery in Asiatic Turkey. Artificial corundum is now made by heating bauxite in the electric furnace. It is sold under the trade names alundum and aloxite. Occurrence. 1. In certain igneous rocks such as syenites and nepheline syenites in which an excess of A1 2 O 3 has crystallized out as corundum, just as an excess of SiO 2 crystallizes as quartz in granites. Craigmont, Ontario. 270 INTRODUCTION TO THE STUDY OF MINERALS 2. In peridotites along the borders of adjacent rocks. In North Carolina the country rocks are gneisses, but the mode of origin is doubtful. 3. In crystalline limestones (Burma, New York, New Jersey). 4. In lamprophyre dikes, probably the result of absorption of shale and subsequent recrystallization of the excess of alumina. Yogo Gulch, Montana. Emery is associated with limestone at Naxos, Greece, and perhaps is the metamorphic equivalent of bauxite. 5. In sands and gravels. The gem-bearing gravels of Ceylon furnish sapphire and other varieties of corundum. HEMATITE, Fe 2 O 3 Form. Hematite is found in a variety of forms : small crystals in cavities, micaceous, fibrous, oolitic, and massive compact. FIG. 430. FIG. 431. FIGS. 430-432. Hematite crystals. FIG. 432. Some of the red massive minerals called hematite are probably turyite, its amorphous equivalent. Crystals are hexagonal, usually tabular or low rhombohedral in habit (Figs. 430, 431). The island_of Elba furnishes good crystals with the forms w{10l4|, rjlOTl), and n{2243| repre- sented in plan by Fig. 432. cr(0001 : lOll) = 57 37. H. = 6. Sp. gr. 5.2. Color. Iron-black to dark red. Streak, brownish-red. Luster, metallic to dull. Opaque, but translucent red in very OX IDS 271 thin scales. These scales are dark between crossed nicols (basal sections). Chemical Composition. Ferric oxid, Fe 2 O 3 ; (Fe = 70.0 per cent.). The iron is sometimes partly replaced by titanium and magnesium. Blowpipe Tests. Fusible with difficulty (5^). On charcoal in R.F. becomes magnetic. Gives bead tests for iron. Slowly soluble in concentrated HC1. Distinguishing Features. Hematite is distinguished from magnetite, ilmenite, and limonite by its brownish-red streak and from turyite by its crystalline nature, greater hardness, and absence of water. Uses. Hematite is the principal ore of iron ; the Lake Superior district furnishes the principal domestic. supply. Occurrence. 1. In basic igneous rocks as a late magmatic mineral. Engels Mine, Plumas county, California. 2. In cavities of lavas as a volcanic sublimate. Vesuvius. 3. In contact-metamorphic deposits often associated with magnetite and pyrite. 4. As a metasomatic replacement of cherty iron carbonate. This origin is assigned to the Lake Superior hematite. 5. In metamorphic rocks often forming hematite schists and quartz-hematite schists. 6. As an alteration product of other iron minerals. The fibrous pencil-ore of England is supposed to be formed by the dehydration of limonite. Martite is a pseudomorph of hema- tite after octahedral crystals of magnetite. Turyite, Form. The typical occurrence of turyite (formerly called turgite) is in colloform crusts usually with a fibrous structure. It also occurs in massive forms. It is probably the amorphous equivalent of hematite. H. = 5 to 6. Sp. gr. = 3.5-5.0. Color, black to dark red. Streak, dark cherry-red. 272 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties, n, variable 2.4-2.6. Fragments are thin splinters which under crossed nicols in direct sunlight show a deep red color. Chemical Composition. Ferric oxid with a variable amount of water, Fe 2 O3(H 2 0)x-(H 2 O usually = 4 to 6 per cent.). Blowpipe Tests. Fusible with difficulty. In the closed tube decrepitates and gives water. Soluble in dilute HC1. Distinguishing Features. Turyite is distinguished from hema- tite by the presence of water, and from goethite and limonite by its red streak. Occurrence. 1. In the oxidized zone of various ore-deposits, often associated with goethite and limonite. RUTILE GROUP TETRAGONAL Cassiterite (SnO2) and rutile (Ti02) together with plattnerite (PbO 2 ), polianite (MnO 2 ), zircon (ZrSiO 4 or ZrO 2 -SiO 2 ), and thorite (ThSi04 or ThO 2 -Si0 2 ) are isomorphous; all are tetrag- onal dioxids of tetravalent metals. CASSITERITE, SnO 2 Form. Cassiterite is found in crystals, crystalline and reni- form masses, pebbles, and grains (stream-tin). Crystals are tetragonal and prismatic or pyramidal in habit. Twins are common. H. = 6>^. Sp. gr. 7.0 . Color, black or brown. Luster, adamantine. Optical Properties. n y (2.09) -- n a (1.99) = 0.10. Double refraction strong. Fragments are irregular with high-order interference colors and high relief even in methylene iodid. Some varieties are pleochroic. Chemical Composition. Tin oxid, SnO 2 ; (Sn = 78.6 per cent.). Blowpipe Tests. Infusible: Fused with sodium carbonate, sulfur, and a little powdered charcoal gives a metallic button OXIDS 273 and a straw-colored coating near the assay. The coating heated with Co (NO 3 ) 2 solution assumes a bluish-green color. Placed on zinc and treated with dilute HC1, the mineral gives a coating of tin, which takes a good polish when rubbed. Insoluble in acids. Distinguishing Features. The high specific gravity and adamantine luster serve to distinguish cassiterite from other minerals. Uses. Cassiterite is practically the only source of tin. The Malay States lead in the production of tin, with Bolivia second. Occurrence. 1. In tin-stone veins associated with topaz, wol- framite, arsenopyrite, lepidolite, and fluorite. Granite is the country rock. Zinnwald, Bohemia, is a prominent locality. 2. In greisen (quartz-muscovite rock) and other rocks affected by the intrusion of pegmatites, but rare in the pegmatites themselves. 3. In rhyolites and quartz porphyries often accompanied by topaz. Durango, Mexico. 4. In sands and gravels. (Stream-tin). Rutile, TiO 2 Form. Rutile is found in embedded grains or crystals, as acicular inclusions or in a massive form. Crystals are tetragonal and usually prismatic in habit. Usual forms: p{lll[, ejlOl), a{100), m{110}. Interfacial angles: pm(l 11 : 100) = 47 40'; ea(101 : 100) = 57 13'; ee(lQl : Oil) = 45 2'. Figures 433 to 436 represent various types of twinned crystals with e{101} as twin- plane. Cleavage. Imperfect prismatic. H. = 6-6^. Sp. gr. 4.2 . Color. Red, brownish-red to black. Streak, pale brown. Luster, metallic-adamantine. Optical Properties. n y (2 . 90) - n a (2 . 62) - . 28. Fragments are yellow and irregular with high-order interference colors and high relief even in methylene iodid. 18 274 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Titanium oxid, TiO 2 ; (Ti = 60.0 per cent.). Iron is usually present. Blowpipe Tests. Infusible. Gives a violet NaPO 3 bead in R.F. (Use very fine powder and first heat in O.F.) Insoluble in acids. Distinguishing Features. Distinguished from cassiterite by lower specific gravity. The red color and metallic-adamantine luster are distinctive. Uses. Rutile is used as coloring matter for porcelain and as a source of ferro-titanium. Nelson County, Virginia, is an im- portant locality. FIG. 433. FIG. 434. FIG. 435. FIG. 436. Occurrence. 1. As a constituent of high-temperature veins (or pegmatites) associated with apatite and scapolite. The coun- try rock is gabbro. Kragero, Norway. 2. As a secondary mineral in various rocks such as gneisses, schists, and clays. The rutile is set free by the decomposition of titanium-bearing silicates, especially the pyroxenes. Rutile is also an alteration product of titanite and occurs as a paramorph after brookite (an orthorhombic form of TiO 2 ). Pyrolusite, MnO 2 (H 2 O), Form. Pyrolusite occurs in fibrous and columnar forms, in acicular crystals, in crusts, in masses, and along seams in den- OXIDS 275 dritic forms. Crystals are prismatic but indistinct, and probably always pseudomorphous after manganite. H. = 1 to 2. Sp. gr. 4.8 . Color, black. Streak, black. Luster, metallic to dull. Opaque. Chemical Composition. Manganese dioxid with a little ad- sorbed water, MnO 2 (H 2 O)x; (Mn =63.2 per cent.). Blowpipe Tests. Infusible. In closed tube it gives a small amount of water (usually about 2 per cent.). Gives manganese bead tests. Soluble in HC1 with the evolution of chlorin. Distinguishing Features. Distinguished from other manga- nese oxids by its inferior hardness and small water content. Uses. Pyrolusite is one of the prominent ores of manganese, but it is usually mixed with psilomelane or manganite. It is also used in the manufacture of chlorin and has other minor uses. Occurrence. 1. Pyrolusite is probably in most cases formed by the dehydration of manganite. Its occurrence is similar to that of the other manganese oxids. Hants county, Nova Scotia. Stibiconite, Sb 2 O 4 (H 2 O) a! Form. Stibiconite occurs massive or as a coating. It is never found crystallized, but is sometimes pseudomorphous after stibnite. H.=4 to 5. Sp. gr. 5.2 + . Color, pale yellow. Luster, dull. Optical Properties. n = 1.61-1. 75. Fragments are irregular, color pale yellow, and isotropic. Chemical Composition. Amorphous antimony tetroxid with adsorbed or dissolved water; Sb 2 O4(H 2 O)x (Sb about 75 per cent.). Blowpipe Tests. Infusible. In the closed tube gives water. Insoluble in HC1. 276 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Stibiconite is recognized by its pale yellow color and high specific gravity. It is usually associated with stibnite. Uses. A minor ore of antimony but abundant in certain localities. Occurrence. 1. A secondary mineral often found with stibnite and resulting from its oxidation. 6. ALUMINATES, FERRITES, ETC. Spinel, MAGNETITE, Franklinite, CHROMITE, Hausmannite MgAl 2 O 4 FeFe 2 O 4 (Zn,Mn)Fe 2 O 4 (Fe,Mg)(Cr,Al) 2 4 Mn 3 O 4 These minerals are sometimes considered as oxids, but they are probably salts of certain unfamiliar acids. Spinel is magnesium metaluminate derived from HA10 2 (H 3 A1O 3 H 2 O). Magnetite is ferrous metaferrite; iron acts both as an acid and as a base. Chromite is essentially ferrous metachromite, derived from HCr0 2 (HaCrOa - H 2 O). SPINEL GROUP ISOMETRIC Four of the enumerated minerals belong to the spinel group, which is one of the best known examples of isomorphism, for many intermediate compounds exist. The minerals of this group are isometric and usually crystallize in octahedrons. Besides the minerals mentioned there are also hercynite (FeA^O^, gahnite (ZnAl 2 O 4 ), and jacobsite (MnFe 2 O 4 ). The general formula, then, is: (Mg,Fe,Mn,Zn)(Al,Fe,Cr,Mn) 2 O 4 . The following analyses illustrate the range and variation in composition. Analyses of Minerals of the Spinel Group MgO FeO MnO ZnO A1 2 O 3 Fe 2 Os Mn 2 O 3 CrzOs Misc. Spinel 24.6 4.6 69.7 1.6 Spinel pleonaste) 19.9 11.6 68 5 Spinel (picotite). . 23.6 3.9 53.9 11.4 7.2 Hercynite 2.9 35.7 61.2 Magnetite 3.0 26.1 tr 70.6 TiOz = 0.3 Magnetite 2.1 27.7 0.4 1.1 68.5 0.6 Gahnite 0.1 1.1 39.6 49.8 Si0 2 = 0.6 Franklinite 10.5 23.1 63.4 4.4 Jacobsite 6.4 20.7 68.3 4.0 Chromite 4.4 25.0 0.9 7.2 59.2 SiO 2 = 3.2 Chromite 14.1 18.0 0.5 12.1 56.5 277 278 INTRODUCTION TO THE STUDY OF MINERALS Spinel, MgAl 2 O 4 Form. Spinel is practically always found in crystals or grains, usually disseminated, but sometimes loose in sands and gravels. Crystals are isometric, the octahedron being the only common form (Fig. 437). Contact-twins with {111} as twinning plane are so common that this twin-law is known as the spinel law (Fig. 438). H. = 8. Sp. gr. 3.6 to 4.0, depending upon composition. Color, black and dark shades of gray, brown, and green; also red and blue. Usually translucent. Luster, sub-adamantine. Optical Properties. Isotropic. n = 1.72. Fragments are ir- regular and dark between crossed nicols. The usual color of the fragments is green (pleonaste) and coffee-brown (picotite). FIG. 437. FIG. 438. Chemical Composition. Magnesium metaluminate, MgAl 2 4 or MgOAl 2 3 ; (MgO = 28.2 per cent.). The magnesium is of ten replaced by ferrous iron, and the aluminum by chromium and ferric iron. The iron-bearing spinel is called pleonaste and the chrome-bearing spinel, picotite. Blowpipe Tests. Infusible, but the color may change on heat- ing. Turns blue when heated with cobalt nitrate solution. Insoluble in hydrochloric and nitric acids. Decomposed by fusion with potassium acid sulfate. Distinguishing Features. Distinguished from most minerals by its octahedral form and superior hardness. The black variety is distinguished from magnetite by its non-metallic luster ALUMINATES, FERRITES, ETC. 279 and non-magnetic character. From ruby, the red variety is distinguished by optical tests. Uses. A red variety called spinel-ruby is used as a gem. Occurrence. 1. As a contact mineral in crystalline limestone associated with phlogopite, chondrodite, corundum, and graphite. Amity, New York, is a prominent locality. 2. As an accessory mineral in various igneous and meta- morphic rocks. Pleonaste occurs with emery, and picotite, with serpentine. 3. In gem-bearing gravels. (Ruby spinel.) Ceylon. MAGNETITE, FeFe 2 O 4 (or Fe 3 O 4 ) Form. Magnetite occurs in loose and attached crystals, in compact and granular masses, and in the form of sand. Crystals FIG. 439. FIG. 440. FIG. 441. belong to the hexoctahedral class of the isometric system. The only common forms are the octahedron o, the dodecahedron d, and the trapezohedron raj 311}. The habit is octahedral, more rarely dodecahedral, but almost never cubic. Figures 439, 440, and 441 represent typical crystals. Cleavage. Some specimens have octahedral parting. H. = 6. Sp. gr. 5.1 . Color, black. Streak, black. Opaque. Luster metallic. Strongly attracted by the magnet and sometimes is a magnet itself (lodestone). 280 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Ferrous metaferrite, FeFe 2 4 or Fe 3 O 4 . (FeO = 31.0; Fe = 72.4 per cent.). May contain magnesium, manganese, or titanium. Blowpipe Tests. Fusible with difficulty (5^). Gives bead tests for iron. Soluble in concentrated HC1. The hydrochloric acid solution of the borax fusion gives tests for both ferrous and ferric iron. Distinguishing Features. Magnetite is distinguished from all other black minerals by its strong magnetism. Hematite and chromite are often similar but are recognized by differences in streak. Uses. Magnetite is an important ore of iron, mined in New York, New Jersey, and Pennsylvania, and in Scandinavia it is the principal iron ore. Occurrence. 1. A very common and widely distributed accessory constituent of igneous rocks. 2. In ore-deposits due to magmatic segregation at the end of the magmatic period. The Scandinavian magnetite has this origin. 3. As a contact mineral between igneous rocks and limestones often occurring with pyrite and hematite. 4. In lenses and layers in schists and gneisses. 5. As an alteration product of iron-bearing silicates in more or less altered igneous rocks (serpentines, for example). 6. In detrital deposits as the main constituent of the so-called black sands which are prominent on the Pacific Coast. Franklinite, (Zn,Mn)Fe 2 O 4 Form. Franklinite occurs in disseminated crystals or in granu- lar aggregates. The crystals are usually octahedrons, modified by the dodecahedron (like Fig. 426, p. 267). H. = 6. Sp. gr. 5.1. Color, black. Opaque. Luster, metallic. Streak, dark brown. Slightly magnetic. Chemical Composition. Zinc and manganese metaferrite, ALUMINATES, FERRITES, ETC. 281 (Zn,Mn)Fe 2 O4 or (Zn,Mn)O-Fe 2 O 3 . Some analyses show fer- rous iron and manganic manganese. A typical analysis is given on p. 277. Blowpipe Tests. Infusible. In O.F. the borax bead is ame- thyst (Mn), while in R.F. it is green (Fe). On charcoal with sodium carbonate it gives a white coating of ZnO and a magnetic residue. Soluble in HC1 with the evolution of a little chlorin. Distinguishing Features. Franklinite resembles magnetite and chromite,butmay usually be distinguished by its association with willemite and zincite (a dark red mineral with the composi- tion: ZnO). Uses. Franklinite, extensively mined in Sussex county, New Jersey, is used for the production of zinc white. The residue is used for the production of spiegeleisen, an iron-manga- nese alloy. Occurrence. 1. In crystalline limestone with willemite, zincite, and rhodonite. Sussex County, New Jersey, is practi- cally the only locality for this mineral. This deposit was prob- ably formed by the metamorphism of a sedimentary limestone containing calamine and some manganese mineral. CHROMITE, (Fe,Mg)(Cr,Al) 2 4 Form. Chromite occurs disseminated and in compact masses, rarely in small octahedral crystals. H. = 5^. Sp. gr. 4.4 . Color, black. Streak, dark brown. Luster, submetallic or metallic. Opaque. Some varieties are slightly magnetic on account of the presence of the magnetite molecule in solid solution. Optical Properties. Isotropic n>1.93. Thin fragments are irregular, usually translucent brown, and dark between crossed nicols. Chemical Composition. Ferrous and magnesium metachro- 282 INTRODUCTION TO THE STUDY OF MINERALS mite and metaluminate. Ferric iron may also be present. Two typical analyses are given on page 277. Blowpipe Tests. Infusible. Gives chromium bead tests. Fused with sodium carbonate it gives a magnetic mass. Insoluble in acids. Decomposed by sodium carbonate with the formation of sodium chromate, which is soluble in water. Distinguishing Features. The submetallic luster is distinc- tive. It is usually associated with antigorite. Uses. Chromite is the only source of the salts of chromium such as potassium chromate, potassium dichromate, and lead chromate. Chromite bricks are used as a furnace-lining for certain kinds of smelting. Ferro-chrome is an alloy used in making chrome-steel. New Caledonia and Rhodesia are the principal sources of chromite. Important deposits exist in California. Occurrence. In peridotites and derived serpentines as an original or residual constituent. Ore deposits may be due to magmatic segregation. Woods mine, Lancaster county, Penn- sylvania. 2. In serpentines, probably derived from chromium-bearing olivine in the process of the serpentinization of peridotite. Lake county, California. Hausmannite, Mn 3 O 4 Form. Hausmanite is usually a massive mineral but some- times it occurs in euhedral tetragonal crystals with the tetra- gonal bipyramid {111} as the dominant form. Cleavage, fairly distinct. H = 5 to 5}^. Sp.gr. 4.8 . Color, steel gray to brownish, reddish black. Streak, chestnut brown. Luster submetallic. Optical Properties n 7 (2.45) - w(2.15) = 0.30 (Larsen). Fragments are dark red and doubly refracting when examined in direct sunlight between crossed nicols. ALUMINATES, FERR1TES, ETC. 283 Chemical Composition, probably a manganese manganite, MnMn 2 O 4 (= Mn 3 O 4 ). Mn = 72 per cent. Blowpipe Tests. Infusible. It gives the Mn bead tests. Soluble in HC1 with the evolution of chlorin. Distinguishing Features. It is distinguished from the man- ganese dioxide minerals by its streak and absence of water. Uses. Hausmannite is one of the minor ores of manganese. It has been mined at Batesville, Arkansas. Occurrence. 1. A hydrothermal replacement of limestone. Often altered to psilomelane. 7. HYDROXIDS Goethite, Fe 2 O 3 H 2 O Manganite, Mn 2 O 3 H 2 LIMONITE, H 2 Fe 2 4 (H 2 0) x Gibbsite, A1(OH) 3 CL1ACHITE, Al 2 O 3 (H 2 O)a; Brucite, Mg(OH) 2 PSILOMELANE, 4MnO 2 (Ba,K 2 )O -(H 2 O)*(?) The hydroxids or hydrous oxids are in part normal hydroxids such as Mg(OH) 2 . Others may be derived by subtracting water from the normal compounds. For example, 2Fe(OH) 3 2H 2 = Fe 2 O 3 -H 2 O, goethite. Goethite, Fe 2 O 3 H 2 O Form. Goethite is found in small acicular crystals, in bladed crystal aggregates, and in scaly or fibrous masses. Crystals are orthorhombic, but are usually too minute to decipher. Cleavage, in one direction parallel to the length. H. = 5J. Sp.gr. 4.3 . Color, yellowish-brown to nearly black. Streak, yellowish- brown like that of limonite. Luster, metallic-adamantine. Optical Properties. n 7 (2.40) - rc a (2.26) = 0.14. Thin fragments are prismatic, and translucent brown with parallel extinction. Chemical Composition. Ferric oxid monohydrate, Fe 2 O 3 -H 2 O (H 2 O = 10.1 percent.). Itusually contains a little manganese. Fibrous varieties contain an excess of water over that required by the formula. Blowpipe Tests. Fusible with difficulty (5^). In the closed tube turns red and gives off water. Bead tests for iron. Soluble in HC1. 284 HYDROXIDS 285 Distinguishing Features. The yellow-brown streak and fibrous or bladed structure are distinctive. It is distinguished from limonite by the fact that it is crystalline. Uses. As an ore of iron it is classed as brown hematite along with limonite. Occurrence. 1. In iron-ore 'deposits along with limonite and hematite. 2. As inclusions in various minerals such as feldspars, quartz, etc. Manganite, Mn 2 O 3 H 2 O Form. Manganite is found in prismatic crystals and in colum- nar and fibrous masses. Crystals are orthorhombic, prismatic in habit, and striated parallel to their length. Cleavage, in one direction (010) parallel to the length of the crystals. H. = 4. Sp.gr. 4.3 . Color. Iron-black or dark gray . Streak, dark brown. Luster, submetallic. Opaque. Chemical Composition. Manganic oxid monohydrate Mn 2 O 3 -- H 2 O; (H 2 = 10.3 per cent.). Blowpipe Tests. Infusible. In the closed tube gives water. Bead tests for manganese. Soluble in HC1 with the evolution of chlorin. Distinguishing Features. Manganite is distinguished from psilomelane by its crystalline structure and inferior hardness, and from pyrolusite by its greater hardness, brown streak, and higher water content. Uses. Manganite is an ore of manganese occurring along with pyrolusite and psilomelane. Occurrence. 1. As a vein mineral. Ilefeld in the Harz Mts. is a prominent locality. 2. As a secondary mineral in residual clays associated with psilomelane. 286 INTRODUCTION TO THE STUDY OF MINERALS LIMONITE, H 2 Fe 2 O 4 (H 2 0) x Form. Limonite occurs in colloform crusts and in compact, pisolitic, nodular, porous, and earthy masses. It is often pseudo- morphous after other iron minerals, especially pyrite. H. = 5%. Sp. gr. 3.8 . Color, yellow, brown, or black. Streak, yellowish-brown. Luster, submetallic to dull. Optical Properties, n = 2.0 2.1. Fragments are ir- regular, and either iso tropic or doubly refracting. Chemical Composition. Ferric hydroxid, H 2 Fe 2 O 4 (H 2 0) a; (Fe= 50 to 60 per cent.). (H 2 O = 13 to 18 per cent.). Often impure from the presence of manganese oxid, phosphates, clay, sand, and organic matter. Blowpipe Tests. Fusible with difficulty (5^). When heated in R.F. it becomes magnetic. In the closed tube it turns red and yields water. Iron bead tests. Soluble in HC1. Distinguishing Features. Limonite is distinguished from hematite by its red streak and from goethite by the absence of fibrous structure and by its optically isotropic character. Uses. Limonite is a prominent ore of iron and in the United States ranks next to hematite in importance. Occurrence. 1. As a secondary mineral in veins and ore deposits formed by the oxidation of pyrite. It constitutes an important part of the gossan or "iron-hat." 2. As a metasomatic replacement of limestone. 3. As sedimentary bedded deposits, often with an oolitic structure and perhaps formed from original oolitic siderite. The "minette" ores of Lorraine and Luxembourg belong here. 4. As bog iron ore formed by the oxidation of FeH 2 (CO 3 )2 in solution in marshes (probably by iron bacteria). 5. As a pigment and stain in various rocks. Gibbsite (Hydrargillite) A1(OH) 3 Form. Gibbsite occurs in minute pseudohexagonal crystals, often lining cavities and in stalactitic and incrusting forms. In HYDROXIDS 287 thin sections it is sometimes seen as a crystalline aggregate pseudomorphous after feldspars. Cleavage in one direction. H. = 2^ to 3K- SP- gr- 2.4 . Color. Colorless, white, gray, and pale colors. Optical Properties. 71/1.558) - w tt (1.535) = 0.023. Frag- ments are platy or prismatic to acicular with oblique extinction. The inference colors vary from first-order gray up to lower second- order. Chemical Composition. Aluminum hydroxid, A1(OH) 3 ; (H 2 O = 34.6 per cent.). Blowpipe J^ests. Infusible. When heated with cobalt nitrate solution it becomes blue. In the closed tube yields water. Insoluble in dilute HC1. Uses. Gibbsite is one of the constituents of bauxite, which is used as a source of aluminum and aluminum salts. Distinguishing Features. Some varieties resemble chalce- dony from which it is distinguished by inferior hardness. Gibb- site is crystalline, while cliachite is amorphous. Occurrence. 1. Occurs along with cliachite in bauxite, a rock produced from clay by desilication. Bauxite, Arkansas. 2. Occurs in limonitic iron ores. Clove mine. Dut chess county, New York. CLIACHITE, A1 2 O 3 (H,O) * Forms. Cliachite is an amorphous mineral which is the prin- cipal constituent of the rock called bauxite. It occurs in pisolitic forms, more rarely in clay-like masses. The crystalline equiva- lent of cliachite is gibbsite (A1(OH) 3 ). H. - 1 to 3. Sp. gr. 2.5 . Color, white, yellowish- white, pale red, or brownish-red. Lus- ter, dull and earthy. Optical Properties, n about 1.57. Fragments are irregu- lar and isotropic. Associated doubly-refracting particles are usually gibbsite. 288 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Hydrous aluminum oxid, A1 2 3 - (H 2 0)x. Cliachite often contains iron (up to 15 per cent.). Blowpipes Tests. Infusible. Heated intensely with cobalt nitrate solution it becomes blue. In the closed tube gives abundant water. If pure, it completely dissolves in the NaP0 3 bead, while clay, on the other hand, leaves a residue of silica. Soluble in HC1 with difficulty. Distinguishing Features. Cliachite is easily recognized on account of the pisolitic structure. The clay-like variety can only be distinguished by proving the absence of silica. Uses. Bauxite is now practically the only ore of aluminum. It is mined in Georgia, Alabama, Arkansas, and Tennessee. Bauxite is also used in the production of aluminum salts, aluminum oxid (used as an abrasive), and for bauxite bricks. Occurrence. 1. Cliachite is the principal constituent of bauxite, a rock produced by the desilication of clays, which in Arkansas were formed from nepheline syenites. Brucite, Mg(OH) 2 Form. Brucite is occasionally found in crystals, but more often in foliated masses and sometimes in fibrous seams. Crys- tals are hexagonal and tabular in habit with the basal pinacoid and several rhombohedrons. Cleavage, in one direction parallel to (0001). H. = 2M- Sp. gr. 2.4. Color, white or greenish- white. Luster, pearly or silky. Optical Properties. n T (1.58) -n a (1.56) = 0.02. Cleavage flakes give a positive uniaxial interference figure without rings unless very thick. The fibrous variety gives acicular fragments with parallel extinction and negative elongation. Chemical Composition. Magnesium hydroxid, Mg(OH) 2 or MgOH 2 O; (H 2 O = 31.0 per cent.). It often contains iron and manganese. The manganese is due to the isomorphous replace- ment of Mn(OH) 2 . The latter occurs as a mineral called pyrochroite. HYDROXIDS 289 Blowpipe Tests. Infusible, but glows. Heated with cobalt nitrate solution it turns pink. In the closed tube yields water and becomes opaque. Soluble in HC1. Distinguishing Features. Distinguished from gypsum by its fusibility and absence of calcium. ' Its occurrence is characteristic. Occurrence. 1. As a secondary mineral in serpentine often associated with magnesite and dolomite. Texas, Pennsylvania, is a prominent locality. 2. In crystalline limestones associated with chondrodite and spinel. It has been produced by the hydration of periclase (MgO). Crestmore, Riverside County, California. PSILOMELANE, 4MnO 2 (Ba,K 2 )O - Form. Amorphous. Psilomelane is usually a compact mas- sive or earthy mineral without any hint of crystalline structure. Occasionally it shows a colloform surface in open spaces. H. = 3 to 6. Sp. gr. 3.0-4.5. Color, black to brown. Streak, brownish-black to brown. Luster, submetallic to dull. Opaque. Chemical Composition. Impure hydrous manganese dioxid, perhaps 4MnO 2 -(Ba,K 2 )O-H 2 O-; (MnO 2 = 70 to 90 per cent. H 2 O = 3 to 9 per cent.). It usually contains barium and potas- sium and sometimes lithium, cobalt, copper, or iron. Blowpipe Tests. Infusible. In the closed tube gives water and also oxygen. Manganese bead tests. Soluble in HC1 with the evolution of chlorin. Distinguishing Features. Distinguished from pyrolusite and manganite by the absence of crystalline structure and from limonite by the streak. Uses. Psilomelane is an important ore of manganese and is also used as a source of chlorin. An earthy cobalt-bearing va- riety (asbolane) is used as a source of cobalt compounds. Occurrence. 1. In residual clays formed during the process of weathering. Batesville, Arkansas. 2. In bog deposits often associated with limonite. 19 8. CARBONATES A. Normal Anhydrous Carbonates CALCITE, * CaC0 3 DOLOMITE, CaMg(CO 3 ) 2 Calcite J Magnesite, MgCO 3 Group SIDERITE, FeCO 3 Rhodochrosite, MnCO 3 SMITHSONITE, ZnCO 3 Aragonite, CaCO 3 Aragonite J Strontianite, SrCO 3 Group ] Witherite, BaCO 3 CERUSSITE, PbCO 3 B. Basic Carbonates MALACHITE, Cu 2 (OH) 2 CO 3 Azurite, Cu 3 (OH) 2 (CO 3 )2 Hydromagnesite, Mg 4 (OH) 2 (CO 3 )3-3H 2 O The carbonates are not many in number, but they include some of the most common minerals with which the mineralogist has to deal. All the important normal carbonates fall into two well- defined isomorphous groups: the calcite group (rhombohedral) and the aragonite group (orthorhombic). These two groups are said to be isodimorphous, as calcite and aragonite are dimorphous. CALCITE GROUP HEXAGONAL The calcite group of rhombohedral carbonates is a well charac- terized group of familiar minerals. These minerals crystallize in rhombohedral and scalenohedral crystals with cleavage parallel to the faces of a rhombohedron of about 75. All except dolo- mite belong to the ditrigonal scalenohedral class of the hexago- nal system. Dolomite belongs to the rhombohedral class, but is similar to the other minerals in angles and other properties. All the minerals of this group are uniaxial and optically negative, and 290 CARBONATES 291 have very strong double refraction. Many isomorphous mix- tures are known; and some of them have received special names (see breunnerite, ankerite, and mesitite below). The following analyses are representative of the minerals mentioned and illustrate isomorphism. CaO MgO FeO MnO ZnO CO 2 Misc. Calcite 56.0 0.4 43.5 H 2 O = 0.1 Calcite 48.7 0.9 0.4 6.8 0.4 40.8 H 2 O = 0.3 Dolomite 31.4 21.2 47.7 Dolomite 29.6 17.6 '6.7 45.6 Dolomite (Ankerite) . . . 28.4 10.2 17.2 44.2 Magnesite .... 47.3 0.8 51.5 H 2 O - 0.5 Magnesite (Breunnerite) .... 41.8 6.5 0.6 50.3 Magnesite (Mesitite) . . 1.3 28.1 24.2 45.8 Siderite 0.2 59.6 1.9 38.0 Siderite 2.4 50.4 7.5 38.6 gangue = 0.3 Rhodochrosite 0.6 0.4 0.4 59.9 38.3 Smithsonite 0.4 0.1 64.1 34.7 CdO = 0.6; CdS = 0.3 CALCITE, CaCO 3 Calcite has played a very prominent part in the history of mineralogy. The discovery of cleavage in calcite led to the establishment of crystallography as an exact science by Hatiy, and the discovery of double refraction in calcite led to the de- velopment of crystal optics. The invention of the Nicol prism, which is made of calcite, has made possible the identification of fine-grained mineral aggregates and rocks. Form. Calcite is found in well defined crystals (often large in size), in crystalline crusts and druses, in cleavable masses, in various imitative forms, such as stalactitic, pisolitic, and oolitic, in granular masses, and sometimes in fibrous forms. Calcite is the type example of the ditrigonal scalenohedral class of the hexagonal system. In number of forms and variety of their combinations, calcite is unsurpassed among minerals. Over 300 well established forms, most of them rhombohedrons and scalenohedrons, are known. 6 = 0.854. 292 INTRODUCTION TO THE STUDY OF MINERALS Usual forms (in order of their abundance) : m{ 1010 j , c{0001 } , e{01JL2),/{0221}, r{10Tl}, M {4041}, w{2131}, a{1120), 2/J3251), <{2134). Interfacial angles: ee(OlT2:I012) = 45_3'; em{Oll2:10lO} = 63 45';_rr(1011:1101) = 74 55'; m(1011:10lO) = 45 23%'; .#(0221: 2021) = 101 9'; fm (022 1:0 110) = 26 53'; MM(4041: 4401) = 114 10 / ;_Mm(4041:10TO = 14_13'; jw(2131:23Tl) = 75_22 / ;_w(2131:3121) = 35 _36' ; jwf 213 1:1231) = 47 I'^rv (1011:2131) = 29_6K^mz;(1010:2131) = 28 4' ;^2/(325 1:5231) = 45 32^; 3/^(3251:3521) = 70 59'; (?134:2314) = 41 55'; #(2134:3124) = 20 36>^'; mm(10lO:OlTO) = 60 0'; ma(10TO: 1120) = 30 0'. FIG. 442. FIG. 443. FIG. 444. FIG. 445. Figures 442-461 represent typical calcite crystals. These figures illustrate the great variation in habit. The habit is variable. Figures 442-445 are simple forms. Figure 442, e{OlT2), is an obtuse rhombohedron, while Fig. 443, /{0221}, is an acute rhombohedron. Figure 445 is a pseudo-cubic rhom- bohedron /i{0332) with the angle hh (0332:3302) = 91 42 r . The dotted lines in each case represent the cleavage which is a great help in orienting a crystal. The unit rhombohedron rjlOllj alone is rare, but it is very frequently the dominant form as in Figs. 446 and 447. The combination em (Figs. 448 and 449) is said to be the most frequent combination. Figures 450-453 are common types. The bottom of Fig. 451 CARBONATES 293 FIG. 446. FIG. 450. FIG. 454. m FIG. 447. FIG. 448. FIG. 449. FIG. 451. FIG. 452. FIG. 453. FIG. 455. FIG. 456. FIG. 458. FIG. 459. FIG. 460. FIGS. 446-461. Calcite crystals. FIG. 457. FIG. 461. 294 INTRODUCTION TO THE STUDY OF MINERALS represents cleavage. The faces of e { 01 12 } are very often striated as in Fig. 453. Figures 454 to 457 represent plans of common types of calcite crystals. The trigonal symmetry is apparent. Five twinning laws are known for calcite. (1) {0001} as twin-plane. Figure 458 represents a scalenohedron twinned according to this law. (2) {01 12) as twin-plane. This is of ten polysyn thetic twinning with striations parallel to the long diagonal (Fig. 459). 3. {lOll} as twin-plane. Figure 460 represents the combination {lOTOj, {Oll2} twinned according to this law. The vertical axes of the two parts of ths crystal are almost at right angles. (4) {0221} as twin-plane. This is represented by Fig. 461. (5) {2021} as twin-plane. This twinning law is very rare. Cleavage, perfect rhombohedral in three directions at angles of 74 55'. There is often parting parallel to {Oll2} and this is sometimes better developed than the cleavage itself. H. = 3. Sp. gr. 2.72 . Color. Calcite is usually colorless, FIG. 462. cleavage frag- white or am b er , but may be any color. ments of calcite. . J Luster, vitreous. Optical Properties. w 7 ( 1.658) - n a (1.486) = 0.172. The strong double refraction is one of the most prominent characters of calcite. It may be observed in Iceland spar, the clear trans- parent cleavable variety. Fragments are rhombic (Fig. 462) with symmetrical extinction and very high-order interference colors. The rhombs often have striations parallel to the long diagonal. These are due to polysyn thetic twinning produced by pounding the fragments. The relief varies with the direction. As shown in Fig. 462, the rhombs have a high relief when the long diagonal is parallel to the vibration plane of the lower nicol. It gives the microchemical gypsum test with dilute H 2 S04 (Fig. 4, p. 43). Chemical Composition. Calcium carbonate, CaCO 3 ; (CaO = CARBONATES 295 56.0 per cent.) . The common replacing elements are iron, magne- sium, and manganese. The amber color is due to a small amount of organic matter. Clay, sand, bitumen, and other mechanical impurities may be present. Blowpipe Tests. Infusible, glows, and gives a yellowish-red flame coloration. In the closed tube whitens, gives off CO 2 , and leaves a residue of CaO. Easily soluble in large fragments in cold dilute HC1 with vigor- ous effervescence. In concentrated solutions, dilute H 2 SO 4 gives a white crystalline precipitate (CaS04-2H 2 0). Distinguishing Features. Calcite is distinguished from dolo- mite by its lower specific gravity and by its ready effervescence in cold HC1. The colored varieties are distinguished from siderite and rhodocrosite by lower specific gravity. From aragonite it is distinguished by perfect rhombohedral cleavage and by its failure to give a lilac color when heated in a test-tube with cobalt nitrate solution. Uses. Limestones are extensively used for building and orna- mental stones, in the manufacture of cement, as ballast and road material and as a flux in smelting. Iceland spar is used in optical apparatus, especially the polarizing microscope. Occurrence. 1. As a vein mineral, often forming the gangue of ores. The north of England furnishes fine crystallized speci- mens of calcite. 2. As travertine, calcareous tufa, and cave-deposits (stalactites and stalagmites). Calcium carbonate is soluble in carbonated water, and on the escape of CO 2 , due to release of pressure, calcite crystallizes out. 3. As a biogenic mineral forming limestones, organisms such as molluscs, brachiopods, corals, and crinoids contributing their shells or other hard parts. 4. As a characteristic mineral in cavities of the basic igneous rocks, especially basalt, and often associated with the zeolites. Iceland spar occurs in large cavities in basalt in Iceland. 5. As a prominent mineral in seams and cavities of sedimentary 296 INTRODUCTION TO THE STUDY OF MINERALS rocks, especially limestone. In the Joplin district fine amber- colored calcite crystals occur in chert breccias in the zinc mines along underground water courses. Crystal Cave at Joplin is completely lined with calcite crystals from 1 to 2 feet in length. It is probable that the calcite was formed when the cave was completely filled with a water solution of calcium carbonate. 6. As the principal constituent of crystalline limestones, which were formed from sedimentary limestones by metamorphism. The crystalline limestones often contain diopside, tremolite, wollastonite, garnet, spinel, graphite, etc. 7. As a paramorph after aragonite. Calcite is the stable form of calcium carbonate under ordinary conditions. DOLOMITE, Ca(Mg,Fe)(CO 3 ) 2 Form. Dolomite is found in crystals, in crystal druses, in cleavable masses, and in granular and massive forms. FIG. 463. FIG. 464. FIG. 465. FIG. 466. The crystals belong to the rhombohedral class of the hexagonal system and have a lower grade of symmetry than calcite crystals, though this is not often apparent on inspection. The only com- mon kind of dolomite crystal is the simple unit rhombohedron, often curved and more or less saddle-shaped (Fig. 463) . Figure 464 is a rhombohedron modified by c{0001[ and Af{4041}. Crystals like Fig. 465 wth cjOQOl j and M"{4041} are found em- bedded in anhydrite and gypsum. Several twinning laws are known for dolomite, but the only common one is polysynthetic twinning with {0221} as twin-plane which gives rise to striations on the cleavage faces parallel to CARBONATES 297 both the short diagonal and the long diagonal of the rhomb (Fig. 466) . This test can often be used to distinguish dolomite from calcite in crystalline limestones. Cleavage, rhombohedral like that of calcite, but often curved. H. = 3J^ to 4. Sp. gr. = 2.83 - 3.00 (varies with iron content) . Color, white, pink or gray, but rarely colorless. Luster, pearly or vitreous. It is often called pearl spar. Optical Properties, n/1.682) - n a (1.503) = 0.179. Double refraction very strong. Fragments are rhombic with symmetri- cal extinction and very high-order interference colors. The relief varies with the direction as in calcite. It gives the micro- chemical gypsum test with dilute H 2 SO4. In fragments it is distinguished from calcite by the absence of striations parallel to the long diagonal. Chemical Composition. Calcium magnesium carbonate, Ca- Mg(CO 3 )2; (For normal dolomite CaO = 30.4 per cent.; Mg = 21.7 per cent. MgCO 3 = 45.6). It often contains iron. The highly ferriferrous variety is known as ankerite. Ferriferous varieties are represented by the formula Ca(Mg,Fe)(CO 3 ) 2 . Analyses are given on page 291. Dolomite is a double salt with equal molecular quantities of CaCO 3 and MgC0 3 , and not an isomorphous mixture of these two compounds. Blowpipe Tests. Infusible and colors the flame yellowish red. The iron-bearing varieties darken in the closed tube and also become magnetic when heated in R.F. on charcoal. Large fragments are only slightly attacked by cold dilute HC1. (Dolomite can thus be distinguished from calcite.) In con- centrated solutions dilute H 2 S04 gives a white crystalline pre- cipitate. Distinguishing Features. The curved unit rhombohedral crystals and pearly luster usually distinguish dolomite from calcite. Rhodochrosite is heavier than pink dolomite. Uses. Dolomitic limestones, like ordinary limestones, are 298 INTRODUCTION TO THE STUDY OF MINERALS used for building and ornamental purposes, and also for furnace linings. Occurrence. 1. As a characteristic mineral in cavities of lime- stones. The pink dolomite of the Joplin district is a prominent example of this occurrence. 2. As the essential constituent of dolomitic or magnesian lime- stones. These limestones are formed from ordinary limestones by the process known as dolomitization. This consists of the partial replacement of calcium carbonate by magnesium carbon- ate in a way not fully understood. As there is a shrinkage of about 10 per cent., these dolomitic limestones are often porous. 3. As a vein mineral often associated with calcite, as in the north of England. 4. As the principal constituent of the crystalline dolomitic limestones. These consist either of dolomite or of a mixture of dolomite and calcite. Other characteristic minerals are tremo- lite, phlogopite, chondrodite, forsterite, spinel, antigorite, and talc. Antigorite and talc are secondary minerals formed from the other silicates. Magnesite, MgCO 3 Form. Magnesite occurs in cleavable or compact porcelain- like masses. The so-called amorphous magnesite is microcrys- talline (metacolloid) and sometimes has a colloform surface. Euhedral crystals of magnesite are exceedingly rare. Cleavage. Rhombohedral cleavage is sometimes prominent. Color, usually white or gray. Luster, vitreous to dull. H. = 4 to5j. Sp. gr. 3.1 . Optical Properties. n 7 (1.717) - n tt (1.515) = 0.202. Frag- ments are rhombic with symmetrical extinction, or irregular with aggregate structure. The interference colors are very high- order. Chemical Composition. Magnesium carbonate, MgCOs ; (MgO = 47.6 per cent.). Iron and calcium are often present in small amounts. The massive varieties often contain magnesium silicate. CARBONATES 299 Blowpipe Tests. Infusible., Turns pink when heated with cobalt nitrate solution. Soluble in hot HC1 with effervescence. Distinguishing Features. Magnesite is usually distinguished by the compact white masses, but some varieties show good cleavage and are distinguished by the comparatively high specific gravity. It may be necessary to prove that calcium is not an essential constituent. Uses. Dead-burned magnesite is used as a refractory lining. The light-burned or caustic magnesite is used as a plaster and in the manufacture of wood pulp. Austria-Hungary and Greece are the principal producers of magnesite. Occurrence. 1. In veins in serpentine (compact massive variety). Numerous localities in California. 2. In beds as a replacement of limestone or dolomite (cleavable variety). Stevens county, Washington. SJDERITE, FeCO 3 Form. Siderite is found in small crystals in cavities, in cleav- able masses, in colloform crusts, and in compact masses. The crystals are varied in habit; the common forms are the unit rhombohedron (1011 : 1101 = 73 2H')i the rhombohedron {OlT2}, and the rhombohedron {0221}, the latter often modified by the pinacoid {0001}. Figure 467 is the unit rhombohedron rflOll}. Crystals may be curved like those of dolomite. Cleavage, rhombohedral, like that of calcite. H. = 3^ to 4. Sp. gr. 3.8 . Color, various tints and shades of brown and gray. Optical Properties. rc T (1.87) - n tt (1.63) = 0.24. Strong double refraction. Fragments are rhombic with symmetrical extinction and very high-order interference colors. In methylene iodid the relief changes with the direction, but in both positions the index of refraction is greater than that of methylene iodid. 300 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. Ferrous carbonate, FeCO 3 ; (FeO = 62.1 per cent.). Calcium, magnesium, and manganese are usually present in small amounts as replacing elements. Clay- ironstone is an impure massive siderite containing argillaceous material. Blowpipe Tests. Fuses with difficulty. In the closed tube it darkens. Heated on charcoal in R.F. it becomes magnetic. It is soluble in hot HC1 with effervescence and the solution gives tests for ferrous iron. Distinguishing Features. Siderite is distinguished from brown calcite by its comparatively high specific gravity, and from sphalerite by its rhombohedral cleavage and vitreous luster. Uses. Siderite is one of the minor ores of iron. The clay- ironstone variety has been mined extensively in England and to some extent in Ohio, Pennsylvania, and Maryland. Occurrence. 1. As a vein mineral, often the gangue of other ores. 2. As a metasomatic replacement of limestone. 3. As " clay-ironstone " concretions in shales and as " black- band" ore, a carbonaceous variety. 4. As a secondary mineral in cavities of basalt. Rhodochrosite, MnCO 3 Form. Rhodochrosite occurs in rhombohedral crystals and in cleavable masses. The unit rhombohedron with (1011 : 1101) = 73 0' is the only common kind of crystal (Fig. 468). Cleavage, rhombohedral, like that of calcite. H. = 4. Sp. gr. 3.5 . Color, pink, red, brownish-red or reddish- gray. Optical Properties. n T (1.82) - n a (1.60) = 0.22. Double refraction strong. Fragments are rhombic with symmetrical extinction and have very high-order interference colors. CARBONATES 301 Chemical Composition. Manganous carbonate, MnCO 3 ; (MnO = 61.7 per cent.). Calcium and iron are usually present as replacing elements. Blowpipe Tests. Infusible. In the closed tube it darkens and decrepitates. The borax bead in O.F. is amethyst color. Soluble in hot HC1 with effervescence. Distinguishing Features. Rhodochrosite is distinguished from pink calcite and dolomite by its higher specific gravity. It even more resembles rhodonite (MnSiOs) but is distinguished by its rhombohedral cleavage and inferior hardness. Uses. Rhodochrosite is sometimes an ore of manganese. Occurrence. 1. A characteristic vein mineral often serving as the gangue of silver ores. Lake county, Colorado. SMITHSONITE, ZnCO 3 Form. Smithsonite usually occurs in colloform incrustations and in porous masses. Crystals of smithsonite are comparatively rare and as a rule are much rounded. Cleavage, imperfect rhombohedral and often curved. H. = 5. Sp. gr. 4.4 . Color, white, gray, yellow, brown; sometimes blue or green. Optical Properties. n y (l.S2) -n a (1.62) =0.20. Strongdouble refraction. Fragments are rhombic with symmetrical extinction and very high-order interference colors. The index of refraction is greater than that of methylene iodid. Chemical Composition. Zinc carbonate, ZnCO 3 ; (ZnO = 64.8 per cent.; Zn = 52.1 per cent.). Iron is the most frequent replacing element. Blowpipe Tests. Infusible. Heated with cobalt nitrate solu- tion on charcoal, it gives a green coating. In the closed tube it turns yellow. Soluble in HC1 with effervescence. Distinguishing Features. It resembles calamine but is distinguished by the absence of crystals with sharp edges. It is harder than the other carbonates. 302 INTRODUCTION TO THE STUDY OF MINERALS Uses. Smithsonite is one of the minor ores of zinc and is often associated with calamine, a basic zinc silicate. It is called " dry- bone" in the Wisconsin-Illinois-Iowa zinc district. Occurrence. 1. Usually found in the oxidized zone and formed from sphalerite. It also occurs as a metasomatic replace- ment of limestone, and is often pseudomorphous after calcite and dolomite. Marion county, Arkansas, is a prominent locality. ARAGONITE GROUP ORTHORHOMBIC The aragonite group is an isomorphous group consisting of aragonite CaCO 3 ; strontianite, SrC0 3 ; witherite, BaCO 3 ; and cerussite, PbCO 3 . The crystals are orthorhombic, but pseudo- hexagonal (110 : 110 = 62 64), and are usually prismatic in habit. Twinning on the unit prism { 110} is very common and also accounts for the pseudohexagonal character of some crystals. Optically the minerals are biaxial with a small axial angle. The double refraction is very strong as in the calcite group. Aragonite, CaCO 3 Form. Aragonite occurs in acicular crystals, in columnar and fibrous masses, and in incrusting and stalactitic forms. Fibrous masses are especially common for aragonite. The crystals are usually prismatic or acicular in habit, and belong to the bipyramidal class of the orthorhombic system. Usual forms: mjllO), 6 {010}, ft {Oil}. Interfacial angles: mm(110 : HO) = 63 48'; w&(110 : 010) = 58 6'; &fc(010 : Oil) = 54 13K'. Figures 469 and 470 are drawings of typical crystals. The pseudohexagonal character can be seen from Figure 471. Twins with raj 110} as twin-plane are common. Figure 472 is a contact twin and Fig. 473, a penetration-trilling with slight reentrant angles. Cleavage, imperfect parallel to the length of the crystals (010 face). Calcite has perfect cleavage oblique to the length of the crystals. H. = 3J. Sp. gr. 2.9 . CARBONATES 303 Color, colorless, white, or amber; also other pale tints. Luster, vitreous; faint resinous on fracture. Optical Properties. n y = (1.68) - n a (1.53) = 0.15. Double refraction very strong. Fragments are prismatic with parallel extinction, negative elongation, and very high-order interference colors. With dilute H 2 SO4, the hydrochloric acid solution gives the microchemical gypsum test. Chemical Composition. Calcium carbonate, CaCO 3 ; (CaO = 56.0 per cent.). It often contains a small amount of SrCO 3 . Blowpipe Tests. Infusible, but becomes opaque and falls to pieces when heated. m m FIG. 469. FIG. 470. FIG. 471. FIG. 472. FIGS. 469-473. Aragonite crystals. FIG. 473. Soluble in cold dilute acids with effervescence. In a concen- trated solution, dilute H 2 SO 4 gives a crystalline precipitate of CaSO 4 -2H 2 O. Finely powdered aragonite heated in a test-tube with cobalt nitrate solution becomes lilac colored, while calcite is practically unchanged. Distinguishing Features. Aragonite and calcite are dis- tinguished by differences in crystal form, cleavage, and specific gravity and if these fail, by the cobalt nitrate test. Occurrence. 1. As a secondary mineral in basic igneous rocks such as basalt. 2. As a secondary mineral in seams and cavities of limestones along with calcite. 304 INTRODUCTION TO THE STUDY OF MINERALS 3. In clays and marls along with gypsum. Prominent locali- ties are Girgenti in Sicily and the Pyrenees. 4. As a hot-spring deposit. Carlsbad, Bohemia. This recalls the fact that aragonite often forms as a crust in a tea-kettle. 5. As a mother-of-pearl layer of mollusc shells. 6. As a secondary mineral in serpentine, produced by the decomposition of the pyroxene of the original peridotite. Strontianite, SrCO 3 Form. Strontianite occurs in acicular crystals and also in columnar and fibrous masses. The crystals are pseudohexag- onal orthorhombic and resemble those of aragonite. H. = 3J to 4. Sp. gr. 3.7 . Color, colorless, white, or pale tints. Optical Properties. 7^(1.67) - n a (1.52) = 0.15. Like those of aragonite, but gives faint test, if any, for microchemical gypsum. Chemical Composition. Strontium carbonate, SrCO 3 ; (SrO = 70.1 per cent.). It usually contains some CaCO 3 , which may be detected by the microchemical gypsum test. Blowpipe Tests. Infusible, but swells up and gives a crimson flame when heated after it is moistened with HCl. Soluble in HCl with effervescence. In dilute solutions, H 2 SO 4 gives a finely divided white precipitate which distinguishes Strontianite from aragonite. Distinguishing Features. The fibrous or columnar structure and high specific gravity are characteristic. It may be mistaken for celestite, which has about the same specific gravity, but the solubility in HCl will differentiate them. Uses. The strontium hydroxid used in sugar refining is made from Strontianite. Occurrence. 1. As a secondary mineral produced from celestite. 2. In veins in calcareous marl. Hamm in Westphalia, Ger- many, is a prominent locality. CARBONATES 305 Witherite, BaCO 3 Form. Witherite occurs in granular or columnar masses in crystalline druses, and in distinct crystals. The crystals are pseudohexagonal twins of pyramidal habit, often resembling quartz crystals. (See Fig. 474.-) H. = 3K. Sp. gr. 4.3 . Color, white or gray. Luster, faint resinous or vitreous. Optical Properties. n y (1.67) - n(1.52) = 0.15. The optical properties are like those of aragonite. Chemical Composition. Barium carbonate, BaCO 3 ; (BaO = 77.7 per cent.). Blowpipe Tests. Easily fusible, giving a yellowish-green flame. Soluble in HC1 with effervescence. In dilute solution, H 2 S04 gives a finely divided white precipitate. Distinguishing Features. The high specific gravity and effer- vescence in acids distinguish witherite from other minerals. Occurrence. 1. In veins with galena. Barite is a secondary mineral. The North of England is the only prominent locality for witherite. CERUSSITE, PbCO 3 Form. Cerussite occurs in fibrous and reticulated forms, in compact masses, and also in crystals which are orthorhombic and pseudo-hexagonal. The habit is usually tabular parallel to the side pinacoid 6(010} (Fig. 476), or pyramidal with pflll) and i{ 021 } in about equal development (Fig. 475). Other promi- nent forms are:m{110), ajlOO}, r{ 130}, fc{011J. Twinning (ra{100[ as twin-plane) is common both as simple contact twins like Fig. 477 and as interpenetrant twins like Fig. 478. H. = 3^. Sp.gr. 6.5. 20 306 INTRODUCTION TO THE STUDY OF MINERALS Color, colorless, white, gray, and other tints. Luster, adaman- tine. Very brittle. Optical Properties. n 7 (2.07)-n a (1.80) = 0.27. Double re- fraction very strong. Fragments are mostly irregular with very high-order interference colors, and index of refraction greater than methylene iodid. A nitric acid solution gives octahedral crystals of lead nitrate. Chemical Composition. Lead carbonate, PbCOs, PbO = 83.5 (Pb = 77.5 per cent.). Blowpipe Tests. Fuses easily on charcoal. In the closed tube it decrepitates and becomes yellow. Heated on charcoal in R.F., cerussite gives a metallic button and a yellow coating (PbO). FIG. 475. FIG. 476. FIG. 477. FIG. 478. Soluble in HN0 3 with effervescence. Soluble in hot HC1, but on cooling needle crystals of adamantine luster (PbCl 2 ) separate. Distinguishing Features. Distinguished from most other min- erals by adamantine luster and high specific gravity. It may usually be distinguished from anglesite by the presence of twin- ned crystals, but chemical tests may be necessary. Uses. Cerussite is an ore of lead and sometimes is argentiferous. Occurrence. 1. Usually derived from galena and occurring especially in the gossan of ore-deposits. Prominent localities are Broken Hill, New South Wales, and Coeur d'Alene district, Idaho. In the Joplin district cerussite pseudomorphs after galena are found. CARBONATES 307 MALACHITE, Cu 2 (OH) 2 CO 3 Form. For malachite the characteristic recurrences are mam- millary crusts, fibrous masses, and acicular crystals. The crystals are monoclinic, but are usually very small and indistinct. H. = 3K to 4 ' Sp. gr. 3.9 . Color, emerald green. Optical Properties. n T (1.91 )- w(1.66) = 0.25. Fragments are prismatic with oblique extinction (23). Interference colors are masked by the green color of the mineral. Arrow-head twins like those of gypsum (Fig. 238) are common among the fragments. Chemical Composition. Basic copper carbonate, Cu 2 (OH)CO 3 or CuCO 3 -Cu(OH) 2 ; (Cu = 57.4 per cent.; H 2 = 8.1 per cent.). Blowpipe Tests. Easily fusible at (3) giving a green flame which is made blue by HC1. In the closed tube it turns black and gives off water. Soluble in acids with effervescence and gives a green solution. Distinguishing Features. It is distinguished from other copper minerals of green color by its effervescence in acids. Chryso- colla is blue-green and lacks the fibrous structure of malachite. Uses. Malachite is sometimes an ore of copper together with azurite, cuprite, and chrysocolla, which are collectively called oxidized ores. Polished malachite with a concentric fibrous structure is used as an ornamental stone. Occurrence. 1. A characteristic mineral of the upper oxi- dized zone of copper deposits. It constitutes the so-called " cop- per-stain, " found in outcrops. Malachite is often associated with azurite and is sometimes pseudomorphous after it. Bisbee, Arizona, is a prominent locality. Azurite, Cu 3 (OH) 2 (CO 3 ) 2 Form. Azurite occurs in crystals in crystalline coatings and nodular groups of crystals. Crystals are monoclinic, prismatic class, and are usually short prismatic or tabular in habit, often 308 INTRODUCTION TO THE STUDY OF MINERALS highly modified. The best specimens come from Chessy in France, hence chessylite, the French name for azurite. H. = 4. Sp. gr. 3.8. Color, deep azure blue. Optical Properties. ri>1.83. Fragments are irregular, blue in color, but not pleochroic. Interference colors are masked. Chemical Composition. Basic copper carbonate, Cu 3 (OH) 2 - C0 3 ) 2 or 2CuC0 3 -Cu(OH) 2 ; (Cu = 55.2 per cent., H 2 O = 5.2 percent.). Blowpipe Tests. The same as for malachite. Distinguishing Features. The dark azure blue color is dis- tinctive. It differs from malachite not only in color, but also in water content. Uses. Azurite is one of the so-called oxidized copper ores. Occurrence. A characteristic mineral of the oxidized zone. It is usually associated with malachite and has been formed after the malachite. Bisbee, Arizona, has furnished fine speci- mens of azurite. Hydromagnesite, Mg 4 (OH) 2 (CO 3 ) , 3H 2 O Form. Hydromagnesite occurs in crystalline crusts, thin seams, and earthy, chalk-like masses. Crystals, which are very small, are usually like Fig. 479 and are monoclinic and usually twinned on {100}. H. = 1 to 3. Sp. gr. 2.1. Color, white,. Luster, often pearly. Optical Properties. n 7 (1.545) - n(1.523) = 0.022. Frag- ments and crystals are prismatic with parallel extinction, nega- tive elongation, and low-order interference colors. Chemical Composition. Hydrous basic magnesium carbonate, Mg 4 (OH) 2 (C0 3 ) 3 -3H 2 O or 3MgCO 3 -Mg(OH) 2 3H 2 O; (H 2 = 19.8 per cent.). Blowpipe Tests. Infusible. In the closed tube it gives water. Soluble in HC1 with effervescence. CARBONATES 309 Distinguishing Features. .Hydromagnesite is distinguished from magnesite by its inferior hardness and abundant water. Occurrence. 1. As a secondary mineral in serpentine. Ala- meda county, California (Fig. 479). FIG. 479. 2. In crystalline limestones, formed directly from brucite, which in turn was formed from periclase (MgO). Crestmore, Riverside county, California. 3. In spring deposits. Atlin, British Columbia. 9. PHOSPHATES, NITRATES, BORATES, ETC. Apatite Group Ilmenite, Columbite, APATITE, Dahllite, Pyromorphite, Mimetite, Vanadinite, COLLOPHANE, Turquois, Carnotite, Nitratine, Colemanite, Ulexite, Pitchblende, FeTi0 3 (Fe,Mn)(Nb,Ta) 2 O 6 Cai F 2 (PO 4 )6 Ca 10 (C0 3 )(P0 4 )6 Pb 10 ClHP0 4 ) 6 PbioCl 2 (As04) 6 Pb 10 Cl 2 (V0 4 )6 8Cas(P04),-Ca(CO*F,)(H,0), H 5 [Al(OH) 2 ] 6 -Cu(OHXP0 4 ) 4 K 2 (U0 2 ) 2 (V0 4 ) 2 -8H 2 NaN0 3 Ca 2 B 6 On-6H 2 O NaCaB 5 O 9 8H 2 O (UO,),U04-(H,0), About 150 phosphate minerals are known but most of them are rare. Basic phosphates of iron and of copper are especially numerous. The compounds are practically all orthophosphates, that is, salts of H 3 PO 4 . Ilmenite and columbite properly belong to other divisions, but are placed here for convenience. Of the few mineral nitrates known, nitratine (sodium nitrate) is the only one of importance. Only a few borates are prominent as minerals. H 3 BO 3 is boric acid. Borax is a salt of H 2 B 4 O 7 , which is derived thus : 4H 3 BO 3 5H 2 = H 2 B 4 O 7 . Colemanite is a salt of H 4 B 6 On (6H 3 B0 3 - 7H 2 O). These are called tetra- and hexa-boric acids respectively. Ilmenite, FeTiO 3 Form. Ilmenite occurs in tabular hexagonal crystals, in flat plates without definite outline, in disseminated grains, in com- 310 PHOSPHATES, NITRATES, BORATES, ETC. 311 pact masses, and in the form of sand. The crystals resemble those of hematite in habit and angles, but the crystal class is different, for ilmenite belongs to the rhombohedral class. H. = 5 to 6. Sp.gr. 4.7 + . Color, black. Luster, submetallic to metallic. Streak, black to brownish-red. Slightly magnetic. Chemical Composition. Ferrous metatitanate, FeTiO 3 ; (FeO = 47.3 per cent.), analogous to a metasilicate. Ilmenite usually contains ferric iron and also magnesium. It grades on the one hand into hematite and on the other into MgTi0 3 (geikielite) . Analyses of the Minerals of the Ilmenite Group FeO MgO MnO TiO 2 Fe 2 3 Ilmenite ... . 22.4 0.5 0.3 23.7 53.7 Ilmenite . . . 36.5 0.6 2.7 45.9 14.3 Ilmenite (Picroilmenite) 24.4 14.2 56.1 5.4 Geikielite 6.3 28.5 63.8 1.9 Blowpipe Tests. Infusible. The sodium carbonate fusion dissolved in HC1 and boiled with metallic tin gives a violet solution. Slowly soluble in HC1. Decomposed by fusion with KHS0 4 . Distinguishing Features. Ilmenite may be mistaken for hematite or magnetite. It fails to give the red streak of the former and the strong magnetism of the latter. When it is intimately mixed with hematite and magnetite, polished surfaces are necessary to identify it. Occurrence. 1. As an accessory constituent of igneous rocks, especially diabases. 2. As a magmatic segregation in igneous rocks intergrown with magnetite and forming the so-called titaniferous magnetites. 3. As a prominent constituent of sands, especially the " black sands." 312 INTRODUCTION TO THE STUDY OF MINERALS Columbite, (Fe,Mn) (Nb,Ta) 2 O 6 Form. Columbite usually occurs in orthorhombic crystals of short prismatic or tabular habit. Cleavage, fair in two directions at right angles. H. = 6. Sp. gr. 5.5-6.5. Color, black, often iridescent. Luster, submetallic. Chemical Composition. An iron and manganese meta-niobate and meta-tantalate grading from (Fe,Mn)Nb 2 O 6 to (Fe,Mn) Ta 2 6 . The latter mineral is called tantalite. Blowpipe Tests. Fusible on the edges with difficulty (5^). On charcoal with sodium carbonate in R.F. it gives a magnetic residue. The sodium carbonate bead in O.F. is opaque bluish- green (Mn.). The borax fusion dissolved in HC1 with tin gives a deep blue color. Distinguishing Features. Columbite resembles wolframite and gives almost identical blowpipe tests, but the latter has higher specific gravity and more perfect cleavage. Insoluble in acids. Uses. Tantalum, which is used as a filament for incandescent lights, is obtained from columbite and tantalite. Occurrence. 1. In granite pegmatites, associated with beryl, lepidolite, spodumene, etc. The Etta mine in the Black Hills, South Dakota, is the most prominent locality in the United States. APATITE GROUP HEXAGONAL The apatite group is a well-established isomorphous group of minerals, for several isomorphous mixtures are known as given in the analyses below. Besides fluor-apatite 3Ca 3 (PO4) 2 -CaF 2 and chlor-apatite, 3Ca 3 (PO 4 ) 2 -CaCl 2 , there are dahllite, 3Ca 3 (PO 4 ) 2 - CaCO 3 , voelckerite, 3Ca 3 (PO 4 ) 2 -CaO, and a rare mineral called svabite, 3Ca(AsO 4 ) 2 -CaF 2 . The following are typical analyses of the minerals of this group (given in the form of metals and acid radicals instead of the usual form). PHOSPHATES, NITRATES, BORATES, ETC. Analyses of Minerals of the Apatite Group 313 Ca Pb P04 AsO4 V04 Cl F Misc. Fluor-apatite, Portland 39 5 55 4 ?, 3 8 1 .1 Chlor-apatite, Norway 37 2 54 2 R 3.1 Voelckerite, Zillerthal 40 4 57 5 6 O = 1.4, Dahllite (Podolite) Russia 36 5 52 2 H 2 O = 0.1 COs = 53, Pyromorphite, Freiberg 4 6 67 26 8 1 9 Fe2Os = 3.0 Pyromorphite, Schemnitz 2 75 1 21 3 2 5 Pyromorphite, Roughten Gill. . . Mimetite, Bohemia Endlichite, New Mexico Vanadinite, Arizona Svabite, Jakobsberg 0.2 30.1 71.7 70.6 68.2 71.7 2.8 15.1 0.8 tr 1.9 0.5 10.9 26.7 16.1 61.7 13.7 24.7 2.3 2.5 2.5 2.4 0.1 2.0 1.7 2.6 APATITE, CaioF 2 (PO 4 )6 Form. Apatite is found in crystals and in massive forms. The crystals are hexagonal and belong to the bipyramidal class FIG. 480. FIG. 481. FIG. 482. FIG. 483. as there is but one plane of symmetry which is horizontal. The habit is usually prismatic with the following forms: mjlOlO}, c|0001[, andzjlOll}. Interfacial angles: raz(10TO:10Tl) = 49 42'; aa(10Tl:OlTl) = 37 44^'- Figures 480 to 483 represent usual types of crystals. In Fig. 482, the general form ^{2131} is present in addition to s{ 1121} and other forms. Cleavage, imperfect basal parallel to {0001}. H. = 5. Sp. gr. 3.2 . 314 INTRODUCTION TO THE STUDY OF MINERALS Color, usually reddish-brown or green, more rarely white, colorless, gray, or violet. Luster, subresinous. Optical Properties. w 7 ( 1.646) - n a (1.641) = 0.005. Frag- ments are irregular, and colorless with low first-order interference colors. It gives the microchemical gypsum test with dilute H 2 SO 4 . Chemical Composition. Apatite is an isomorphous mixture of the tricalcium fluophosphate, CaioF 2 (PO 4 ) 6 with smaller amounts of CaioCl 2 (P0 4 ) 6 , CaioO(TO 4 ) 6 , and Caio(C0 3 )(PO 4 ) 6 . For CaioF 2 (TO 4 ) 6 (Ca = 39.7 per cent., F = 3.8 per cent., P0 4 = 56.5 per cent.). Some varieties contain manganese. Blowpipe Tests. Fusible on edges (5M) and gives a yellowish- red flame which is made green with H 2 SO 4 . Soluble in HNO 3 , sometimes with slight effervescence on heat- ing. The solution gives a yellow precipitate with an excess of (NH 4 ) 2 Mo04 solution when warmed. As NH 4 OH gives a white precipitate of calcium phosphate, the best test for calcium is dilute H 2 S0 4 which in the presence of 50 per cent, alcohol gives a crystalline precipitate of CaS0 4 -2H 2 O. Distinguishing Features. Apatite is distinguished from most similar minerals by its crystal form, hardness (just less than a knife blade), and subresinous luster. Collophane is amorphous, has lower specific gravity, and effervesces in acids. Uses. Massive apatite is used to some extent in the prepara- tion of superphosphate for fertilizer, but most of the superphos- phate is made from so-called phosphate rock which is largely collophane. Occurrence. 1. As an accessory constituent of igneous rocks, very common and widely distributed but in small quantities. It is the original source of the phosphates of sedimentary rocks. 2. In high-temperature veins with phlogopite, calcite, and diopside. 3. In metamorphic rocks. 4. In granite pegmatites. PHOSPHATES, NITRATES, BORATES, ETC. 315 Dahllite, Caio(CO 3 )(PO 4 )6 Form. Dahllite occurs in colloform fibrous incrustations on cellophane, rarely in minute hexagonal crystals. H. = 5. Sp. Gr. = 3.0 . Color, white or pale tints. Optical Properties. n., (1.623) - w a (1.619) = .004. Frag- ments are prismatic to acicular with parallel extinction, nega- tive elongation, and low to middle first order interference colors. Chemical Composition. Calcium carbonate-phosphate, Cai - (C0 3 )(PO 4 )6, analogous to fluor-apatite. (Ca = 38.9 per cent. PO 4 = 55.3, CO 3 = 5.8.) Water is usually present, but it is probably mechanically held as in chalcedony. Blowpipe Tests. Fusible with difficulty, but usually decrepi- tates. In the closed tube it gives a small amount of water. Soluble in cold dilute HNO 3 with slight effervescence; effer- vesces vigorously in hot HN0 3 . The solution gives tests for calcium with dilute H 2 S04 and alcohol, and for P0 4 with ammo- nium molybdate. Distinguishing Features. Dahllite is distinguished from cellophane by its crystalline character and small water content, and from apatite by its effervescence in HN0 3 . Uses. Some dahllite occurs in phosphorites (the hard rock phosphate of Florida, for example) along with cellophane and is used for superphosphate. Occurrence. 1. In phosphorite or so-called phosphate rock as an incrustation on cellophane. Marion county, Florida. Pyromorphite, Pbi Cl 2 (PO 4 ) 6 Form. Pyromorphite usually occurs as small crystals and earthy crusts. The crystals are hexagonal and prismatic in habit; c{0001} and m{1010) are the only common forms (Fig. 484). H. = 3H to 4. Sp. gr. 6.8 . Color, green or brown. Luster, adamantine. 316 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. n y (2.061) -n a (2.049) = 0.012. Frag- ments are irregular, colorless or pale green, and have low first- order interference colors. With HNO 3 isometric crystals of lead nitrate are deposited. Chemical Composition. Lead chlorid-phosphate, PbioCl 2 - (P0 2 ) 4 ; (Pb = 76.3 per cent.). In some varieties Ca replaces Pb (p. 313), and in others (V0 4 ) replaces (PO 4 ). Blowpipe Tests. Easily fusible (at 2) on charcoal to a globule with apparent crystal faces. With sodium carbonate on char- coal it yields a lead button. With a NaPO 3 bead saturated with CuO it gives an azure-blue flame (Cl). Soluble in HNO 3 . An excess of (NH 4 ) 2 MoO 4 gives a yellow ppt. with the nitric acid solution. Distinguishing Features Pyromorphite is distinguished from other minerals with adaman- tine luster by its hexagonal form. Occurrence. 1. As a secondary mineral formed from galena. Phoenixville, Pennsylvania, is a prominent locality. Both pyromorphite pseudomorphs after galena and galena pseudomorphs after pyromorphite have been noted, the former from the Joplin district and the latter from Huelgoat, France. Mimetite, Pbi Cl 2 (AsO 4 ) 6 Form. Mimetite usually occurs in rounded hexagonal crystals. H. = 3M- Sp. gr. 7.2 . Color, yellow, orange, or red. Luster adamantine. Optical Properties. n T (2.13) - n a (2.l2) = 0.01. Fragments are irregular yellow, and have low first-order interference colors. With HNO 3 isometric crystals of Pb(NO 3 ) 2 are deposited. Chemical Composition. Lead chlorid-arsenate, Pbi Cl 2 (AsO 4 ) 6 ; (Pb = 69.5 per cent.). It grades on the one hand into pyromor- phite and on the other into vanadinite. Blowpipe Tests. Easily fusible (at 1^) on charcoal and gives FIG. 484. PHOSPHATES, NITRATES, BORATES, ETC. 317 a white sublimate and a metallic button. In the closed tube heated with charcoal it gives an arsenic mirror test for an arsen- ate. With CuO in NaPO 3 bead it gives an azure-blue flame (Cl). Soluble in HNO 3 . With (NH 4 ) 2 MoO 4 the nitric acid solution gives a yellow precipitate on boiling (phosphates give the pre- cipitate on slight warming) . In order to determine the presence of the phosphate radical it is necessary to remove the arsenic by means of H 2 S. Distinguishing Features. Mimetite is difficult to distinguish at sight. The adamantine luster and high specific gravity suggest that it is a lead mineral. Occurrence. 1. As a secondary mineral in lead mines. Cum- berland, England. Vanadinite Pb ]0 Cl 2 (VO 4 ) 6 Form. Vanadinite practically always occurs in small hexag- onal crystals of prismatic habit. The common forms are : c { 000 1 } and_ra{10lOj (Fig. 485). The general form {2131}, a hexagonal bipyramid, is sometimes present. H. = 3. Sp.gr. 6.8 + . Color, usually red, but also yellow and brown. Luster, adamantine. Optical Properties. n 7 (2.35)-n a (2.30) - 0.05. Fragments are irregular yellow or orange color, and give bright interference colors. Chemical Composition. Lead chlorid-van- adate, PbioCl 2 (VO 4 )6; (Pb = 72.7 per cent.). It often contains (PO 4 ) and (As0 4 ) as replacing radicals. Endlichite is intermedi- ate between vanadinite and mimetite (see analyses, page 313). Blowpipe Tests. Easily fusible (at 1^) on charcoal, giving a white sublimate and a metallic globule. In the closed tube with KHSO 4 it gives a yellow mass. The NaPO 3 bead is green in R.F. and yellow in O.F. It gives the Cl test with a NaPO 3 bead saturated with CuO. m 318 INTRODUCTION TO THE STUDY OF MINERALS Soluble in HN0 3 . Uses. Vanadinite is one source of the vanadium used as alloy with steel and of various compounds used in dyeing and the manufacture of ink. Distinguishing Features. Vanadinite is distinguished from most minerals by the adamantine luster and hexagonal crystals. It is distinguished from pyromorphite by its color. Occurrence. 1. A secondary mineral formed from galena and often associated with wulfenite. Yuma county, Arizona, is a prominent locality. Vivianite, Fe 3 (PO 4 ) 2 .8H 2 O Form. Vivianite occurs in small crystals, in nodules, and in earthy masses. Crystals are monoclinic with varying habit. Cleavage, perfect in one direction (010). H. = 2. Sp.gr. 2.6 . Color, deep blue or bluish-green, but colorless if unaltered. Optical Properties. n 7 (1.62) - w(1.57) = 0.05. Fragments are prismatic with either parallel or oblique (28J^) extinction. Pleochroic from blue to colorless or blue to green. Chemical Composition. Hydrous ferrous phosphate, Fe 3 - (PO 4 ) 2 -8H 2 O; (H 2 O = 28.7 per cent.). It usually contains some ferric iron as the result of oxidation. Blowpipe Tests. Easily fusible (2) to a black magnetic glob- ule. In the closed tube gives water and whitens. It gives the borax bead test for ferrous iron. (A bead made blue with CuO becomes opaque red in a neutral flame.) Soluble in HNO 3 or HCL (NH 4 ) 2 MoO 4 gives a yellow ppt. Distinguishing Features, The dark blue color and inferior hardness are distinctive. Occurrence. 1. As a secondary mineral in veins associated with pyrite and pyrrhotite. Ibex mine, Leadville, Colorado. 2. In clay and marl beds sometimes replacing fossils and in soils often around the roots of trees. At Mullica Hill, New Jersey, it replaces fossil belemnites. PHOSPHATES, NITRATES, BORATES, ETC. 319 COLLOPHANE,3Ca 3 (PO 4 ) 2 .nCa(C03,F 2 )(H 2 O) x Form. Amorphous. Usually massive, often concretionary or oolitic and sometimes banded. It is often a replacement of bone. Colloform crusts may be present in open spaces. H. = 3 to 5. Sp. gr. 2.6 2.9 (variations are dependent upon purity and porosity). Color, white, yellow, brown, gray, or black (coloration is due to an organic pigment). Optical Properties, n = 1.58-1.62. Fragments are irregular, colorless to brown, translucent, with high relief in clove oil and low relief in cinnamon oil. Between crossed nicols the mineral is either dark or has low first-order interference colors (the double refraction is due to strain). Chemical Composition. Hydrous calcium carbonophosphate with n in the above formula varying from 1 to 2. Some speci- mens approach the formula of dahllite, 3Ca 3 (PO 4 ) 2 -CaCO3. Organic matter, aluminum, and iron are usually present, and sometimes the sulfate radical. Calcite is the principal mechani- cal impurity. The following analyses give some idea of the purer forms of cellophane: Analyses of Cellophane Ca Al Fe Mg P0 4 CO 3 SO 4 F H 2 Misc. Sombrero. . 36 2 6 52 3 5 4 5 Pouzillac. 35 5 1 3 50 5 1 9 7 1 03 Nauru 34 7 5 3 1 2 48 8 2 5 7 o 01 Alberta (Fossil Bone). 34.7 0.4 1.4 tr. 50.5 6.4 0.1 0.9 3.5 0.2 Blowpipe Tests. Fuses (at 5 to 6) with difficulty on the edges, glows, and turns white. In the closed tube it turns dark and gives water (1 to 8 per cent.). It is soluble in cold dilute nitric acid with slight to moderate effervescence; when the acid is heated there is vigorous effer- vescence. A hot nitric solution gives a yellow precipitate with 320 INTRODUCTION TO THE STUDY OF MINERALS an excess of ammonium molybdate solution. The solution gives the calcium test with dilute H 2 SO 4 and 50 per cent, alcohol (see p. 39 for separation of calcium, etc.). Distinguishing Features. Cellophane is a difficult mineral to recognize without chemical and optical tests on account of its variable appearance. A mineral showing bone structure is almost certain to be cellophane. It is distinguished from opal and chalcedony by inferior hardness, from calcite by its greater hardness and optical tests, from apatite by its amorphous nature, decided effervescence in acids, and optical tests. It is apt to be overlooked because of its indefinite character. Uses. Phosphorite or so-called phosphate rock, the chief constituent of which is cellophane, is used in the manufacture of superphosphate, a valuable fertilizer made by treating the crude material with sulfuric acid. The United States and Tunis are the largest producers. The states in order of production are Florida, Tennessee, and South Carolina. Large deposits occur in southeastern Idaho and adjoining portions of Utah and Wyoming and extend into Montana. Occurrence. 1. In bedded deposits, often formed by the re- placement of limestones. Marion county, Florida. 2. In recent deposits. Nauru Island, Pacific Ocean. 3. As a replacement of fossil bones. The bone structure is retained and cavities are often filled with calcite or chalcedony. The organic matter has largely disappeared, but some of it re- mains as a pigment. Turquois, H 5 [Al(OH) 2 ] 6 Cu(OH)(P0 4 )4 Form. Turquois occurs in seams and incrustations, and is apparently amorphous, but the polarizing microscope proves it to .be crystalline and at one locality it has been found in minute triclinic crystals. H. = 6. Sp. gr. 2.7. Color, usually bluish-green but varies from blue to green. Optical Properties. n 7 (1.65) - w a (1.61) = .04. Fragments PHOSPHATES, NITRATES, BORATES, ETC. 321 are irregular, bluish or greenish with aggregate polarization in low first-order interference colors. Chemical Composition. Acid and basic aluminum copper phosphate, probably H 5 [Al(OH 2 )]6-Cu(OH)(P0 4 )4, (H 2 = 19.5 per cent). Blowpipe Tests. Infusible,' but turns dark when heated and gives a green flame which is made blue by HC1. In the closed tube at a high temperature it gives water and turns dark. Soluble in HC1. Distinguishing Features. The only common mineral resem- bling turquois is chrysocolla from which it is distinguished by superior hardness. The association of chrysocolla with other copper minerals is also an aid in its identification. Uses. Turquois is used extensively as a gem. Blue stones are more valuable than green ones. Occurrence. 1. In seams of volcanic rocks such as trachytes and rhyolites. Los Cerrillos Mts., New Mexico (in andesite). Carnotite, K(UO 2 ) 2 (VO 4 ) 2 8H 2 O Form. An apparently amorphous mineral occurring in earthy masses, impregnations, or as incrustations. Carnotite, however, is crystalline and minute tabular orthorhombic crystals have been described. H. = rather soft. Sp. gr. = 4.1 . Color, canary yellow. Optical Properties. n 7 (1.95) - w a (1.75) = 0.20. Fragments are yellow and doubly-refracting. Chemical Composition. Hydrous potassium uranyl vanadate, K 2 (UO 2 ) 2 (VO 4 ) 2 ;8H 2 O (H 2 O = 14.5 per cent.). Calcium re- places part of the potassium. The corresponding calcium mineral is called tyuyamunite. Blowpipe Tests. Fusible (at 2J) to a black non-magnetic slag. It gives a yellow NaPO 3 bead in O.F. which becomes a fine green in the R.F. In the closed tube it darkens and gives water. 21 322 INTRODUCTION TO THE STUDY OF MINERALS Soluble in cold dilute acids. Distinguishing Features. Carnotite is easily recognized by its bright canary yellow color. Uses. Carnotite is used for the production of vanadium, uranium, and radium salts. Occurrence. 1. As an incrustation or impregnation in sand- stones, sometimes associated with fossil wood. It is evidently a secondary mineral derived from some pre-existing minerals. Western Colorado and eastern Utah. Nitratine, NaNO 8 Form. Nitratine or "soda niter" is found in crystalline and granular masses. It crystallizes in rhombohedrons with almost the same angles as the unit or cleavage rhombohedron of calcite (lOTl :0lll = 73 30'). Cleavage, rhombohedral like caleite. H. = l^ to 2. Sp. gr. 2.3. Color, white or colorless. Very deliquescent. Optical Properties. w 7 (i.587) - n a (1.336) = 0.251. It recrystallizes from water solution in rhombic shaped crystals (Fig. 486) with symmetrical extinction and FIG. 486. Nitratine re- i_- i_ j j. i i~-u crystallized. very high-order interference colors which greatly resemble calcite cleavage fragments. The indices of refraction, however, are much lower than those of calcite. Chemical Composition. Sodium nitrate, NaN0 3 . lodin may be present in the form of Ca(IO 3 ) 2 , which imparts a yellow tint. Blowpipe Tests. Easily fusible (at 1), giving an intense yellow flame. With KHS04 in the closed tube it gives red-brown fumes of NO 2 . Soluble in water. Distinguishing Features. Nitratine is distinguished by its cooling, saline taste, and by the optical tests which are much like those of calcite. PHOSPHATES, NITRATES, BOKATES, ETC. 323 Uses. Nitratine is used as a fertilizer and also in the manu- facture of potassium nitrate. Chile furnishes the world's supply. Occurrence. 1. In superficial beds ("caliche") in arid regions. Occurs in northern Chile and to a slight extent in California and Nevada. 2. In caves. Holmdale, Idaho. Colemanite, Ca 2 B 6 Oir5H 2 O Form. Colemanite occurs in crystals, which often line geodes, and in crystalline and compact masses. The crystals are mono- clinic and are often highly modified. Cleavage, perfect in one direction parallel to (010). H. = 4M- Sp. gr. 2.4 . Color, colorless or white. Optical Properties. n 7 (1.61) - n a (1.58) = 0.03. Fragments are irregular plates with bright interference colors. Pseudo- hexagonal crystals of boric acid separate from the hydrochloric acid solution (Fig. 487). Chemical Composition. Hydrous cal- cium hexa-borate, Ca 2 B 6 On-5H 2 O; (B 2 O 3 FIG. 487. Boric acid. = 50.9 per cent., H 2 O = 21.9 per cent.). Blowpipe Tests. Fuses easily (at 1^) with exfoliation, coloring the flame green. In the closed tube it gives water. Soluble in hot HC1. Boric acid separates out on cooling. Distinguishing Features. Colemanite is distinguished from most minerals by its perfect cleavage in one direction. From gypsum it is distinguished by its greater hardness. Uses. Colemanite is the principal source of borax and boric acid. It is obtained in San Bernardino, Inyo, and Los Angeles counties, California. O ccurrence. 1 . In shales and probably formed by the replace- ment of ulexite. Calico, San Bernardino county, California. 324 INTRODUCTION TO THE STUDY OF MINERALS Ulexite, NaCaB 5 O 9 8H 2 O Form. Ulexite is found in rounded fibrous masses, locally called " cotton-balls," and also in compact translucent masses. H. = 1 - 3. Sp.gr. 1.6 . Color, white. Luster, silky to vitreous. Optical Properties. n 7 (1.520) - - rc(1.500) = .020. Frag- ments are acicular with parallel extinction, negative elongation, and low first-order interference colors. Chemical Composition. Hydrous sodium calcium penta- borate, NaCaB 5 O 9 -8H 2 O; (H 2 O = 35.5 per cent.). Blowpipe Tests. Easily fusible (1) with intumescence, color- ing the flame an intense yellow. In closed tube gives water. Soluble in hot HC1; H 3 BO 3 separates out on cooling. Uses. It is a source of borax in northern Chile. Distinguishing Features. The soft rounded masses with fibrous structure and silky luster are characteristic. Occurrence. 1 . In playas or the dried-up lakes of arid regions associated with borax, gypsum, and halite. Esmeralda County, Nevada. Pitchblende, (UO 2 )2UO 4 (H 2 O), Form. Pitchblende is usually massive and sometimes has a colloform surface. It is an amorphous mineral. Uraninite is the crystalline equivalent. H. = 5K- Sp. gr. 6.5 to 8.0. Color, dark brown to black. Luster, submetallic or pitch- like. Streak usually olive-green. Optical Properties. n> 1.74. Fragments are irregular, trans- lucent brown, and isotropic. Chemical Composition. Probably uranyl uranate (UO 2 )sr U0 4 (H 2 O) ;E . Lead and radium are present, the latter in very small quantities. Blowpipe Tests. Infusible. The NaPO 3 bead is yellowish- green in O.F. and green in R.F. It gives water in the closed tube. PHOSPHATES, NITRATES, BORATES, ETC. 325 Soluble in HN0 3 . With NH 4 OH the solution gives a yellow precipitate. Distinguishing Features. This mineral has no very distinctive characters outside of its high specific gravity and pitchy luster. Uses. Uraninite is the source of uranium compounds and also of radium compounds. Occurrence. 1. In veins with metallic sulfids. Joachimsthal, Bohemia. 10. SULFATES A. Normal Anhydrous Sulfates [BARITE, Baso 4 Barite Group | Celestite, SrSO 4 [ Anglesite, PbSO 4 ANHYDRITE, CaSO 4 B. Basic and Hydrous Sulfates Alunite Group Kainite, Brochantite, GYPSUM, Chalcanthite, Alunite, Jarosite, MgS0 4 KC13H 2 Cu 4 (OH) 6 SO 4 CaSO 4 2H 2 O CuSO 4 6H 2 O KA1 3 (OH) 6 (S0 4 ) 2 KFe 3 (OH) 6 (S0 4 ) 2 A large number of sulfate minerals, most of them basic and hydrous salts, are known, but comparatively few are of much importance. They are all salts of H 2 S0 4 . No sulfites, pyrosul- fates, thiosulfates, or persulfates are known among minerals. BARITE GROUP ORTHORHOMBIC In crystal habit, angles, and cleavage barite, celestite, and anglesite are similar, and thus constitute an isomorphous group. One would expect to find anhydrite in this group, but it differs in angles and especially in cleavage. There are isomorphous mix- tures of BaSO 4 and SrSO 4 , also BaS0 4 and PbS0 4 . The following are analyses of minerals of this group. Analyses of Minerals of the Barite Group BaO SrO CaO PbO SO, Misc. Strontium-bearing barite Lead-bearing barite (Hokutolite) Celestite 43.8 48.9 13.9 54 7 0.1 1 4 17.8 36.9 32.2 43 8 3.7 Ign. - 0.6 Anglesite ... 74 25 7 HjO - 0.3 326 SVLFATES BARITE BaSO 4 327 Form. Barite occurs in crystals, in crested groups, in lamellar, nodular, fibrous, and granular masses. Rhombic bipyramidal class: d: 5: 6 = 0.815:1:1.313. Usual forms: c{001), m(HQ}, o{011}, w{101j, d{102], 1(104} t z(lll} t y{ 122). Interfacial angles: mm (110:110) = 78 22'; co (001:011) = 52 43'; cu (001:101) = 58 10>^'; cd (001:102) = 38 51 J'; cl (001:104) = 21 56^'; cz (001:111) = 64 19'; oy (011:122) = 26 1'. The habit is usually tabular parallel to {001} , as rep- resented in Figs. 488 to 491, but prismatic crystals are also common. (See Fig. 203, p. 115). FIG. 488. FIG. 489. FIG. 490. FIGS. 488-491. Barite. FIG. 491. Cleavage, parallel to c{001} and to mfllO). The cleavage form is like Fig. 488, with two right angles and one oblique angle (78 22 r ) . H. = 3. Sp. gr.4.5. Color, colorless, white, gray and tints of brown, .blue, green, etc. Optical Properties, w r (1.647) - rz a (1.636) = 0.011. Frag- ments are rhombic with symmetrical extinction or rectangular with parallel extinction. The interference colors are bright. Chemical Composition. Barium sulfate, BaS0 4 ; (BaO = 65.7 per cent.) Strontium and lead often replace part of the barium. Blowpipe Tests. Fusible (at 4), coloring the flame yellowish- green. Unaltered in the closed tube, but usually decrepitates. The water solution of the sodium carbonate fusion gives a white precipitate with BaCl 2 , which is insoluble in HC1. The acetic acid solution of the residue gives a yellow precipitate with K 2 CrO 4 . Insoluble in acids. 328 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Barite can usually be recognized by its lamellar structure and high specific gravity. Uses. Barite is used in the manufacture of paint as a substi- tute for white lead and also in the manufacture of barium salts. Occurrence. 1. As a gangue mineral in veins, associated with galena, sphalerite, dolomite, calcite, etc. 2. As lenticular masses in clays overlying limestones. These are residual deposits formed by the weathering of the limestone. 3. As a metasomatic replacement of limestone and in cavities in limestones. Celestite, SrSO 4 Form. Celestite occurs in crystals, in cleavable masses, and in fibrous seams. The crystals are like barite in habit, forms, and angles. Crystals one and a half feet in length have been found on the Island of Put-In-Bay, Lake Erie. Cleavage, perfect parallel to {001}, and imperfect parallel to {110}. H. = 3. Sp.gr.3.9. Color, colorless, white, pale blue; sometimes red. Optical Properties. ^(1.631) = n(1.622) = 0.009. Frag- ments are like those of barite. Chemical Composition. Strontium sulfate, SrSCU; (SrO = 56.4 per cent.). Calcium and barium sometimes are present. Blowpipe Tests. Fusible (at 4) giving a crimson red flame with HC1. The water solution of the sodium carbonate fusion gives a white ppt. with BaCl 2 , which is insoluble in HC1. The acetic acid solution of the residue fails to give a precipitate with K 2 CrO4, but the addition of NH 4 OH and alcohol causes a yellow ppt. to form. Insoluble in acids. Distinguishing Features. Celestite greatly resembles barite but can often be distinguished by its imperfect prismatic cleavage. It is not as heavy as barite. Uses. Celestite is used to some extent in the manufacture of fire-works. SULFATES 329 Occurrence. 1. In cavities in limestone. Near Austin, Texas. 2. In marl with sulfur and gypsum. Girgenti, Sicily. Anglesite, PbSO 4 Form. There are two characteristic occurrences of anglesite : in crystals in cavities, and in masses with a banded structure. The crystals are orthorhombic and of varied habit. See Figs. 492-494, in which ra{110(, a{100), c{001}, d{102|, ojOll}, zjlll), y{!22}. The angles are almost the same as for barite. Unlike cerussite it never occurs in twin-crystals. m FIG. 492. FIG. 493. FIGS. 492-494. Anglesite. FIG. 494. Cleavage, imperfect and not important. H. = 3. Sp. gr. 6.3 . Color, colorless, white, or gray. Luster, adamantine to dull. Optical Properties. r? 7 (1.893) - n a (1.877) = 0.016. Frag- ments are irregular with bright interference colors (cerussite has very high-order colors). Chemical Composition. Lead sulfate, PbSO 4 ; (PbO = 73.6 per cent., Pb = 68.3 per cent.). Blowpipe Tests. Easily fusible (at lj) on charcoal to a white globule. In R.F. on charcoal gives a metallic button. Soluble in HNO 3 with difficulty. Soluble in NH^CaHsOa) (made by neutralizing acetic acid with ammonium hydroxid). 330 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. In its general appearance and associates, anglesite resembles cerussite from which it is usually distinguished by the similarity of its crystals to those of barite and by the absence of twinned crystals. It also has weaker double refraction than cerussite. Uses. Anglesite is one of the minor ores of lead. Occurrence. 1. As a secondary mineral in the oxidized zone of lead mines. It often accompanies cerussite and is some- times pseudomorphous after galena. It frequently occurs as band surrounding galena and between it and cerussite. Phoenix- ville, Pennsylvania. ANHYDRITE, CaSO 4 Form. Anhydrite occurs in cleavable and granular masses and but rarely in orthorhombic crystals. Cleavage, in three directions at right angles (pseudo-cubic). H. = 3 to 3^. Sp. gr. 2.9 . Color, colorless, white, gray, bluish, or reddish. Luster, pearly on cleavage faces. Optical Properties. n 7 (1.61) - n(1.57) = 0,04. Fragments are square and rectangular with parallel extinction and bright interference colors. There are often twinning striations parallel to the diagonals of the squares. With dilute HC1, microchemical gypsum is formed (Fig. 4, p. 43). Chemical Composition. Anhydrous calcium sulfate, CaS0 4 ; (CaO = 41.2 per cent.). Blowpipe Tests. Fuses (at 3) and colors the flame yellowish- red. In the closed tube it may yield a little water due to partial hydration to gypsum. Soluble with difficulty in HC1. Distinguishing Features. Anhydrite is recognized by its pseudo-cubic cleavage and by its moderate specific gravity (heavier than calcite and lighter than barite). It is heavier and harder than gypsum and only soluble with difficulty in HC1. Occurrence. 1. In bedded deposits due to the direct deposi- SULFATES 331 tion of sea-water and often associated with halite. Ellsworth county, Kansas. 2. In veins or vein-like deposits. Beaver county, Utah. 3. In cavities in limestone. Lockport, New York. Kainite, MgSO 4 KC13H 2 O Form. Kainite usually occurs in granular masses. H. = 2y 2 . Sp. gr. 2.1 . FIG. Color, white, colorless, gray, or reddish. Optical Properties. n T (1.52) - n(1.49) = 0.03. Recrystal- lizes from water solution in the following order: (1) K 2 Mg (804)2' 6H 2 O, prismatic crystals with oblique ex- tinction. (2) KC1, isotropic squares. (3) MgSO 4 -7H 2 and MgCl 2 -6H 2 0, confused streaky aggregates. Figure 495 represents the three stages of crystallization. The equation is 3(MSO 4 -KC1 3H 2 O) + 10H 2 O = K 2 Mg(SO 4 ) 2 6H 2 H- KC1 + MgS0 4 7H 2 O + MgCl 2 6H 2 O. Chemical Composition. Hydrous mag- nesium sulfate and potassium chlorid. MgSO 4 -KCl-3H 2 O; (H 2 = 21.7 per cent.). Blowpipe Tests. Easily fusible (at 2), violet. In the closed tube gives water. Soluble in water. The solution gives wet tests for Mg, SO 4 , and Cl. Distinguishing Features. Kainite is distinguished from halite and nitratine by the absence of cleavage and by the bitter taste. Uses. Kainite is extensively used as a fertilizer and as a source of potassium salts. Stassfurt, Prussia, is practically the only producer. Occurrence. 1. A secondary mineral of the Stassfurt salt deposits resulting from the action of magnesium sulfate on carnallite, (KMgC] 8 -6H 2 O.). 495. Kainite crystallized. coloring the flame 332 INTRODUCTION TO THE STUDY OF MINERALS Brochantite, Cu 4 (OH) 6 SO 4 Form. Brochantite is found in small prismatic crystals, in drusy crusts, and in fibrous masses. H. = 3K. Sp. gr. 3.9 . Color, emerald green. Optical Properties. w 7 (1.803) - - w a (1.730) = 0.073. Frag- ments are prismatic with parallel extinction. Chemical Composition. Basic copper sulfate, Cu 4 (OH) 6 SO 4 orCuSO 4 -3Cu(OH) 2 ; (Cu = 56.2 per cent.; H 2 O = 12 per cent.). Blowpipe Test. Fusible at 3^. In the closed tube it turns black and gives off water. Uses. At Chuquicamata, Chile, it is the principal ore of copper. Insoluble in water. Soluble in HNOs without effervescence. Distinguishing Features. Brochantite greatly resembles malachite. It is, however, soluble in acids without effervescence. From chalcanthite it is distinguished by the fact that it is not soluble in water. Occurrence. 1. A secondary mineral associated with other copper minerals in the oxidized zone. Chuquicamata, Chile. GYPSUM, CaSO 4 2H 2 O Form. In form gypsum is variable. It occurs in embedded and attached crystals, in cleavage and crystalline masses, and in fibrous and granular masses. Monoclinic system. Prismatic class: d:b:6 = 0.689:1:0.412; ft = 80 42'. Usual forms: rajllO}, 1(111} , 6{010), e{103[. Interfacial angles: mm(HQ: 110) = J>8_30';j/(lll: 111) = 3612'; mZ(110 : 111) = 49 9'; ae(edge 110-ITO:I03) = 87 49'. The habit is usually tabular parallel to the side pinacoid { 010) . Figs. 496 to 498 represent typical crystals. Twins with { 100} as twin- plane are common (Fig. 498). Cleavage, perfect in one direction parallel to {010}, also imperfect conchoidal parallel to (100), and fibrous parallel to SULFATES 333 {111}. A cleavage fragment is oriented with respect to the crystal outline as shown in Fig. 499. H. = 2to2>^. Sp.gr. 2.3+ . Color, colorless, white, amber, gray, pink, etc. Luster, vitreous, silky, or pearly. Optical Properties. n 7 (1.529) - n tt ( 1.520) = x 0.009. Frag- ments are prismatic, acicular, or platy with bright interference colors and extinction angles of 0, 13^, or 37^. Recrystallizes from dilute HC1 solution as microchemical gypsum (Fig. 4, p. 43). m I m FIG. 496. FIG. 497. FIG. 498. FIGS. 496-499. Gypsum. FIG. 499. Chemical Composition. Hydrous calcium sulf ate, CaSO 4 ' 2H 2 O ; (H 2 O = 20.9 per cent.). Massive gypsum may contain calcite, anhydrite, clay, sand,, or organic matter. Blowpipe Tests. Easily fusible (at 3) to a white enamel, giving a yellowish-red flame. In tne closed tube it becomes opaque and gives off water at a low temperature. Easily soluble in dilute HC1 (distinction from anhydrite) and slightly soluble in water. Distinguishing Features. Gypsum is distinguished from most minerals of similar appearance by its inferior hardness and low specific gravity. In addition, it is distinguished from anhydrite by its easy solubility, high water content, and by optical tests. 334 INTRODUCTION TO THE STUDY OF MINERALS The optical tests are valuable, for gypsum and anhydrite can be recognized in the presence of each other. Uses. Gypsum is extensively used as plaster and as a fertilizer to neutralize black alkali (sodium carbonate) in arid regions. France and the United States are the principal sources of gypsum. In this country, New York, Iowa, Michigan, and Ohio lead in the production. Occurrence. 1. As bedded deposits associated with salt and limestone and formed directly by the evaporation of inland seas. 2. As a secondary mineral in various rocks formed principally by the action of sulfuric acid or ferrous sulfate (produced by the oxidation of pyrite) on calcium carbonate. 3. As a hydration product of anhydrite. At the Ludwig mine in Lyon county, Nevada, the hydration has reached a depth of 400 feet. 4. As gypsum earth (or gypsite) deposits formed by solution in fresh water and reprecipitation. Chalcanthite, CuSO 4 -5H 2 O Form. Chalcanthite occurs as an incrustation or as fibrous seams. Artificial crystals of this substance furnish us excellent examples of triclinic crystals. H. = 2^. Sp. gr. 2.2. Color, blue. Optical Properties. n Y (1.54) - w a (1.51) = 0.03. Itrecrystal- lizes from water solution in pale blue prismatic crystals with oblique extinction (10 to 15). Chemical Composition. Hydrous copper sulfate, CuSO4-5H 2 O ; (Cu= 25.4 H 2 O = 36.1 per cent.). It often contains iron. Blowpipe Tests. Fusible at 3. In the closed tube turns white, then black, and yields abundant water. Soluble in water. The water solution placed on metallic iron (knife-blade) gives a film of copper. Uses. In a few places it has been used in silver extraction- SULFATES 335 Distinguishing Features. The color and disagreeable metallic taste are distinctive. Occurrence. 1. A secondary mineral often found in aban- doned mine drifts. The Bluestone mine in Lyon county, Nevada, is a prominent locality. ALUNITE GROUP HEXAGONAL The following minerals form a well-defined isomorphous group : Alunite, K 2 A1 6 (OH) 12 (SO4)4 Natroalunite, Na 2 Al6(OH) 12 (SO4)4 Jarosite, K 2 Fe 6 (OH)i 2 (SO 4 )4 Natrojarosite, Na 2 Fe 6 (OH)i 2 (SO 4 )4 Plumbojarosite, PbFe 6 (OH)i 2 (SO 4 ) 4 Carphosiderite, H 2 Fe 6 (OH)i 2 (SO 4 )4 These minerals crystallize in the hexagonal scalenohedral class of the hexagonal system. Some rare phosphate and sulfato-phosphate minerals are also probably isomorphous with the above listed minerals as has been shown by Schaller. Alunite, KA1 3 (OH) 6 (SO 4 ) 2 Form. Alunite occurs in small crystals in cavities or dis- seminated through the rock mass occasionally in veins and often in fine grained masses. The crystals belong to the hexagonal system. The habit is usually tabular with the pinacoid c{0001} and the rhombohedron rjlOll) as represented in Fig. 500. H. = 4. Sp. gr. 2.8 . Color, colorless, white, gray, or reddish. Optical Properties. n 7 (1.59) - n a (l.57) = 0.02. Fragments are irregular with bright interference colors. Small crystals are triangular, dark between crossed nicols (basal sections), and give a positive uniaxial interference figure in convergent light. Chemical Composition. Basic potassium aluminum sulfate, KA1 3 (OH) 6 (S04) 2 (K 2 O = 11.4 per cent, H 2 O = 13.0 per cent.). Sodium may replace part of the potassium. 336 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Infusible and turns blue with cobalt nitrate solution. In the closed tube it gives water which has an acid reaction. Soluble in H 2 SO 4 with difficulty. It is insoluble in water, but after roasting it is converted into water-soluble alum. Distinguishing Features. Alunite is difficult to recognize at sight. Uses. Alum is obtained from the alunite rock by roasting and leaching with water. It has recently been mined near Marys- vale, Utah. Occurrence. 1. A product of hydro- thermal alteration, probably brought about by ascending acid solutions. Goldfield, Nevada. Jarosite, KFe 3 (OH) 6 (SO 4 )2 Form. Jarosite occurs in small crystals in cavities or in massive to earthy forms. FIG. 500. Alunite. The crystals vary from hexagonal tabular to pseudo-cubic rhombohedral (lOTl -IlOl) = 9045'. Figure_500 is a plan of a typical crystal with the forms cjOOOl [ and r { lOll } . H = 3. Sp.gr. = 3.2 + . Color, yellow to brown. Optical Properties. w 7 (1.77)-w(1.74) = 0.03. Fragments are irregular with third and fourth order interference colors, or are hexagonal tabular crystals and dark between crossed nicols. Chemical Composition. Basic potassium ferric sulfate KFe 3 - (OH) 6 (SO 4 )2-(H 2 O = 10.8 per cent). Sodium or hydrogen may replace part of the potassium and thus it grades into natrojaro- site and carphosiderite. Blowpipe Tests. Fuses with difficulty (at 4.5) to a dark magnetic mass. In the closed tube turns dark and gives water which reacts acid to litmus paper. Soluble in HC1 to an amber-colored solution which reacts for ferric iron and the sulfate radical. SULFATES 337 Distinguishing Features. The blowpipe and wet tests are distinctive. Occurrence. 1. In the oxidized zone of ore-deposits. Tintic District, Utah. 2. In sedimentary rocks. Near Coalinga, California. 22 11. TUNGSTATES AND MOLYBDATES Wolframite (Fe,Mn)WO 4 Scheelite / Scheelite, CaWO 4 Group I Wulfenite, PbMoO 4 WOLFRAMITE GROUP MONOCLINIC FeO MnO W0 3 Ferberite (Colorado) 23 9 7 73 9 Wolframite (Burma) 13 8 9 4 74 8 Wolframite (Zinnwald) 9 6 14 8 75 6 Htibnerite (Colorado) 1 6 21 8 76 6 Wolframite, (Fe,Mn)WO 4 Form. Wolframite occurs in crystals and in crystalline aggre- gates. The crystals are monoclinic and are usually tabular parallel to {100}. Cleavage, perfect in one direction parallel to {OlOj. H. = 5to5>. Sp. gr. 7.4 . Color, black or dark brown. Luster, sub-metallic. Opaque. Chemical Composition. An isomorphous mixture of iron and manganese tungstates (Fe,Mn)WO 4 , varying from FeWO4 (ferberite) to MnW0 4 (hubnerite) (WO 3 = 76.4 per cent.). Blowpipe Tests. Fusible (at 3) to a magnetic globule. The sodium carbonate fusion is bluish-green (Mn). Soluble in aqua regia with the separation of WO 3 , a yellow resi- due. Uses. Wolframite is the principal source of tungsten. Bur- ma, the United States, and Portugal are the chief producers of wolframite. Distinguishing Features. Wolframite is distinguished by its cleavage in one direction combined with its high specific gravity. 338 TUNGSTATES AND MOLYBDATES 339 Occurrence. 1. A vein mineral especially in tin-stone veins associated with cassiterite, scheelite, etc. Zinnwald, Bohemia. 2. In granite pegmatites. Black Hills, South Dakota. SCHEELITE GROUP TETRAGONAL Besides CaW0 4 (scheelite) .and PbMo0 4 (wulfenite), there are also CaMo04 (powellite) and PbW0 4 (stolzite) which are similar crystallographically. Isomorphous replacement is illustrated by the following analyses: CaO PbO W0 3 MoOs Misc. Scheelite (Carrock Fells) Scheelite (Zinnwald) 19.3 20 3 80.0 71 1 0.3 8 2 Scheelite (Chili) 18 1 75 8 CuO = 33- SK>2 = 07 Wulfenite (Eureka Co., Nevada) . . 1.0 61.1 39.3 FezOs = 0.4 Scheelite, CaWO 4 Form. Scheelite occurs in both crystals and massive form. Crystals are tetragonal (Fig. 501) and belong to the tetragonal bipyramidal class (one plane of symmetry perpendic- ular to an axis of fourfold symmetry). The habit is pyramidal with pflll) or e{ 101) as the dominant form. Angles: (111:111) = 7955H'; (101:011) = 72 FIG. 501. Scheelite. Cleavage, distinct parallel to {011} (in four directions). H. = 4Kto5. Sp.gr. 6.0 + . Color, white, gray, or pale colors. Luster, sub-adamantine. Optical Properties. r? T (1.93) - n a (1.92) = 0.01. Fragments are irregular with bright interference colors. Chemical Composition. Calcium tungstate, CaW0 4 ; (WOa = 80.6 per cent.). The tungsten is often partially replaced by molybdenum. The copper in the third analysis above is due to partial alteration to cuprotungstite (CuWO 4 -2H 2 O). 340 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Fusible with difficulty (at 5). The NaPO 3 bead is blue in R.F. Decomposed by HC1 with the separation of a yellow residue (WO 8 ) soluble in NH 4 OH. Distinguishing Features. Scheelite is recognized by its high specific gravity and sub-adamantine luster. Uses. Scheelite is one of the prominent ores of tungsten which is used to harden steel. Atolia, San Bernardino county, California, is the chief producer of scheelite. Occurrence. 1. In veins, especially in tin-stone veins with cassiterite, fluorite, topaz, etc. Wulfenite, PbMoO 4 Form. Wulfenite usually occurs in crystals which are tetra- gonal and usually tabular in habit. Figure 502 is a plan of the common type of crystal with cjOOl } and u{ 102} . H. = 3 Sp. gr. 6.7+ . Color, yellow, orange, or red. Luster, adamantine. Optical Properties. n 7 (2.40) -- n(2.30) = 0.10. Fragments are irregular and yellow with rather high interference colors. Thin tabular F T Wulfenite 2 ~~ crystals give a negative uniaxial interference figure in convergent light. Chemical Composition. Lead molybdate, PbMo0 4 ; (MoO 3 = 39.3 per cent.). Blowpipe Tests. Easily fusible (at 2) on charcoal, giving a metallic button. The NaPO 3 bead is green in R.F. Decomposed by HC1. Distinguishing Features. Wulfenite is distinguished .by its tabular crystals, yellow to red color, and adamantine luster. Uses. Wulfenite is one of the sources of molybdenum. Occurrence. 1. In the oxidized zone of veins often associated with vanadinite. Yuma county, Arizona. 12. SILICATES Feldspars Feldspathoid Group Pyroxene Group Amphibole Group GARNET Olivine Group PLAGIOCLASE /Ab = NaAlSi 3 O 8 \ \An = CaAl 2 Si 2 O 8 / PYROXENE ORTHOCLASE, (K,Na)AlSi 3 O 8 Adularia, KAlSi 3 O 8 . Microcline, KAlSi 3 O 8 Albite, Abioo to Ab 9 o Oligoclase, Ab 90 Abio to Ab 7 o An 3 o Andesine, Ab 70 An 30 to Ab 50 An 50 Labradorite, Abso An 5 o to Ab 20 An 7 o Bytownite, Ab 30 An 70 to Abio An 90 Anorthite, AbioAbgo to Anioo Leucite, KAl(SiO 3 ) 2 Nepheline, (Na,K)AlSiO 4 -(NaAlSi 3 O 8 )* Sodalite, Na 4 AI 3 Cl(SiO 4 ) 3 Lazurite, Na 5 Al 3 S(SiO 4 ) 3 Enstatite, MgSiO 3 Hypersthene, (Mg,Fe)SiO 3 Diopside, Ca(Mg, Fe)(SiO 3 ) 2 Augite m CaMg(SiO 3 ) 2 + n(Mg,Fe) (Al,Fe) 2 Si 2 6 Rhodonite, MnSiO 3 Anthophyllite, (Mg,Fe)SiO 3 Tremolite, Ca(Mg,Fe) 3 (SiO 3 ) 4 HORNBLENDE, mCa(Mg,Fe) 3 (SiO 3 ) 4 + n(Al,Fe)(F,OH)SiO 3 Glaucophane, NaAl (SiO 3 ) 2 (Fe, Mg) SiO 3 Beryl, Be 3 Al 2 (SiO 3 ) 5 Wollastonite, CaSiO 3 ... Spodumene, LiAl(SiO 3 ) 2 Grossularite, Ca 3 Al 2 (SiO 4 ) 3 Almandite, Fe 3 Al 2 (SiO 4 ) 3 Pyrope, Mg 3 Al 2 (SiO 4 ) 3 ' Andradite, Ca 3 Fe 2 (SiO 4 ) 3 OLIVINE, (Mg,Fe) 2 Si0 4 Forsterite, Mg 2 SiO 4 WiUemite, Zn 2 SiO 4 CALAMINE, Zn 2 (OH) 2 SiO 3 Scapolite, m(3CaAl 2 Si 2 O 8 CaCO 3 ) + n(3NaA!Si 3 O 8 -NaCl) 341 342 INTRODUCTION TO THE STUDY OF MINERALS Epidote Group Vesuvianite, Ca 6 Al 3 (OH,F)(SiO 4 )6 Zircon, ZrSiO 4 Topaz, Al 2 (F,OH) 2 SiO 4 Andalusite, Al 2 SiO 5 Kyanite, Al 2 SiO 5 Sillimanite, f EPIDOTE, Ca 2 (Al,Fe'") 3 (OH)(SiO 4 ) 3 \ Clinozoisite, Ca 2 Al 3 (OH)(SiO 4 ) 3 Prehnite, H 2 Ca 2 Al 2 (SiO 4 ) 3 Staurolite, FeAl 5 (OH)(SiO 6 ) s TOURMALINE, R 9 Al 3 B 2 (OH) 2 Si 4 O 19 Datolite, CaB(OH)SiO 4 Axinite, HCa 2 (Fe,Mn)Al 2 B(SiO 4 ) 4 MUSCOVITE, H 2 KAl 3 (SiO 4 ) 3 Sericite, H 2 KAl 3 (SiO 4 ) 3 Mica Group i Lepidolite, LiKAl 2 (OH,F)(SiO 3 ) 3 BIOTITE, (HK) 2 (Mg,Fe)Al 2 (Si0 4 ) 3 [ Phlogopite, H 2 KMg 3 Al(SiO 4 ) 3 CHLORITE, H 8 (MgFe) 6 Al 2 (SiO 6 )3 ANTIGORITE, H 4 Mg 3 Si 2 O 9 Chrysotile, H 4 Mg 3 Si 2 O 9 TALC, H 2 Mg 3 (Si0 3 ) 4 Chondrodite, Mg 6 (F,OH) 2 (SiO 4 ) 2 Kaolinite, H 4 Al 2 Si 2 O 9 Halloysite, H 4 Al 2 Si 2 O 9 (H 2 O) X Garnierite, H 2 (Ni,Mg)SiO 4 H 2 O CHRYSOCOLLA, CuSiO 3 2H 2 O? Glauconite, K Fe /// (SiO 3 ) 2 -(H 2 O) x ? ApophyUite, (H,K) 2 Ca(SiO 3 ), H 2 O Heulandite, H 4 CaAl(SiO 3 ) 3H 2 O Stilbite, H 4 (Ca,NaKAl 2 (SiO 3 ) 6 -4H 2 O ZEOLITES I Chabazite, (Ca,Na 2 )Al 2 (SiO 3 ) 4 6H 2 O I Analcite, NaAl(SiO 3 ) 2 H 2 O ( Natrolite, Na 2 Al 2 Si 3 Oi -2H 2 O Titanite, CaTiSiO 5 About a fourth of the known minerals are silicates, though many of them are very rare. They are the most important rock- forming minerals and thus make up the bulk of the earth's outer shell. Among the important rock-making minerals are the feldspars, the pyroxenes, the amphiboles, and the micas. These, SILICATES 343 together with quartz, constitute about 87 per cent, of the earth's outer shell, according to F. W. Clarke. Many of the silicates are complex in composition and the establishment of chemical formulae of some of them has baffled the skill of many eminent chemists. In a chemical discussion of the silicates the starting point is H 4 SiO 4 , which is called ortho- silicic acid. The compound H 2 SiO 3 , derived thus (H 4 SiO 4 H 2 = H 2 SiO 3 ), is called metasilicic acid. A large number of orthosilicates and metasilicates are known among minerals, but many silicates cannot be placed in either of these divisions; so the assumption has been made that other silicic acids are possible. Among them are H 6 Si 2 O 7 (2H 4 Si0 4 H 2 O), diorthosilicic acid; H 2 Si 2 O 5 (H 2 SiO 3 + SiO 2 ), dimetasilicic acid. In a similar way H 4 Si 3 O 8 , H 8 Si 3 O ]0 , H 6 SiO 5 , and H ]0 Si 2 9 may be derived. Min- erals corresponding to all these acids are known. Another method of nomenclature formerly used by chemists and still employed by metallurgists is based upon the ratio of the oxygen of silica to that of the bases. R 2 SiO 4 may be written 2ROSiO 2 . Here the oxygen ratio is 1 : 1, so orthosilicates are called unisilicates. Metasilicates, RSiO 3 or ROSi0 2 , are called bisilicates. Polysilicates have the formula ROnSi02 and sub- silicates, nRO-SiO 2 , where n is greater than 2. The difficulty of assigning formulae to many silicate minerals lies in the fact that it is often impossible to decide upon the valence and grouping of the basic elements. Many silicates give water when heated in a closed tube, but it is often difficult and sometimes impossible to determine whether hydroxyl (OH), hydrion (H), or so-called water of crystallization (H 2 O) is present. H 2 Zn 2 SiO 5 is the empirical formula for the mineral calamine. It may be an acid oxy-orthosilicate, H 2 (Zn 2 0)SiO 4 , a basic metasilicate, Zn 2 (OH) 2 Si0 3 , or an acid salt of H 6 Si0 5 , one of the possible silicic acids. The silicates are treated as far as possible in more or less well- defined groups. Because of their importance the feldspars are given first. 344 INTRODUCTION TO THE STUDY OF MINERALS FELDSPARS A series of monoclinic and triclinic silicates of aluminum with either potassium, sodium, or calcium, collectively known as the feldspars, forms the most important group of rock-form- ing minerals. In fact feldspar is the most abundant substance of which we have direct knowledge, for it constitutes about 60 per cent, of the solid crust of the earth. The feldspars are non-metallic minerals with a hardness of about 6, and cleavage in two directions at, or nearly at, right angles, and a specific gravity of 2.6-2.7. The important feldspars are: orthoclase, and microcline, which are polymorphous forms of and plagioclase, which is an isomorpKous mixture of NaAlSi 3 O 8 and CaAl 2 Si 2 O 8 . The following are typical analyses of the various feldspars: Analyses of the Feldspars K 2 Na 2 CaO A1 2 S Si0 2 Misc. Ort Ad Mi hoclase 11.7 14.0 13.5 4.3 1.0 1.6 0.5 1.3 18.8 17.9 19.6 64.6 65.7 64.8 BaO = 0.4; ign. = 0.1 Fe 2 Os = tr. ign. = 0.2 ilaria jrocline | Plagioclase Albite 0.5 1.3 1.0 tr 0.6 11.1 8.5 6.2 4.4 1.8 0.2 0.4 4.8 8.1 12.0 16.1 19.3 19.3 23.8 26.6 29.6 31.1 36.8 68.8 61.3 58.0 54.2 46.9 44.0 FezOs = 0.1 Fe 2 Os = 0.4 MgO = 0.1; ign. = 0.1 Fe 2 3 = 1.3; H 2 O = 1.0 MgO = 0.2; ign. = 0.1 Andesine Labradorite Bytownite ORTHOCLASE, (K,Na)AlSi 3 O 8 ) Form. Orthoclase occurs in attached and embedded crystals, in cleavable masses, and disseminated through rock masses. The crystals furnish one of the best examples of the monoclinic prismatic class. Axial ratio: d :b :6 = 0.658 : 1 : 0.555; (3 = 63 57'. Usual forms: cfOOl), 6J010}, zjlOlj, ?/{201), m{110), z{130}, ojTll), rcj021). Interfacial angles: bc(010 : 001) = 90 0'; mm(110 : 110) = 61 13'; mc(110 : 001) = 67 47'; ex- SILICATES 345 (001 : 101) = 50 16M'; q/(001 : 201) = 80 18'; mz(110 : 130) = 29 59^'; 6o(010 : 111) = 63 8'; cn(001 : 021) = 44 56^'. The habit is usually elongate in the direction of the a-axis (Figs. 503-505), or elongate in the direction of the c-axis and tabular parallel to {010} (Fig. 506). FIG. 503. FIG. 504. FIG. 505. FIGS. 503-506. Orthoclase. There are three common twinning laws for orthoclase: (1) the Carlsbad law in which the c-axis is the twin-axis (usually penetration twins with b{ 010} as the composition face) (Fig. 507), (2) the Baveno law in which n{021} is the twin-plane (Fig. 508), m FIG. 507. FIG. 508. FIGS. 507-509. Orthoclase twins. FIG. 509. and (3) the Manebach law in which cfOOlj is the twin-plane (Fig. 509). Cleavage in two directions at right angles, parallel to { 001 } and {010}. There is also imperfect cleavage (or parting) parallel to {110} which assists in orienting cleavages and imperfect crystals. H. = 6. Sp. gr. 2.57 . 346 INTRODUCTION TO THE STUDY OF MINERALS Color, white, colorless, gray, pink, red. It sometimes shows a play of colors. Optical Properties. rc T (1.526) - njl.519) = 0.007. Frag- ments are plates usually with one set of parallel straight edges (Fig. 510). Extinction on 001 = 0; on 010 = 5. The inter- ference colors are middle first-order (gray and straw-yellow). Chemical Composition. Potassium aluminum trisilicate, (K, Na)AlSi 3 8 ; (K 2 = 16.9, A1 2 O 3 = 18.4, Si0 2 = 64.7inKAlSi 3 8 .) Sodium replaces part of the potassium. Blowpipe Tests. Fusible with difficulty (5) . When fused with powdered gypsum on platinum wire it gives a violet flame. ' Insoluble in ordinary acids. Distinguishing Features. Distinguished from other minerals by rectangular clea- vage and from minerals of the plagioclase group by the absence of twinning striations. Its distinction from microcline is difficult without optical tests. Occurrence. 1. In acid plu tonic igneous rocks, especially granites and syenites. 2. In acid volcanic igneous rocks, rhyo- lites and trachytes. This is usually sani- dine, a transparent variety of orthoclase. 3. In certain rare basic igneous rocks.- 4. In gneisses, partly as a remnant of igneous rocks, partly recrystallized. 5. In arkoses or feldspathic sandstones (Portland, Connecti- cut) and in some beach sands (Pacific Grove, California). Adularia, KAlSi 3 O 8 Form. Adularia occurs in distinct monoclinic crystals with practically the same forms and interfacial angles as orthoclase. The habit of the crystals is usually pseudo-orthorhombic because of the equal development of c{001[ and zjTOl) (see Fig. 511) and as a consequence the cross section is rhombic. FIG. 510. Orthoclase cleavage fragments. SILICATES 347 Cleavage in two directions at right angles, parallel to { 001 } and {010}. H. = 6 Sp. gr. 2.57 . Color, colorless or white. Optical Properties. ^ 7 (1.524) - n tt (1.518) = 0.006. Frag- ments are plates with one set of parallel edges with extinction angles of on 001, and 5 on 010. Chemical Composition. Potassium aluminum silicate KA1- Si 3 8 (K 2 = 16.9 per cent. ; A1 2 O 3 = 18.4; Si0 2 = 64.7). Sodium is practically absent which fact distinguishes adularia from orthoclase. Blowpipe Tests. Fusible with difficulty (at 5) to a colorless glass. Insoluble in ordinary acids. Distinguishing Features. Adularia can usually be distinguished from orthoclase by the crystal habit. The crystals have a rhombic cross section Fl< ? ,. * Adularia. as the 6{010j face is absent or very narrow. Adularia is usually colorless and transparent, and orthoclase translucent. Uses. A variety of adularia known as moonstone on account of its beautiful internal reflections is used as a gem. It is obtained in Ceylon. Occurrence. 1. As a vein mineral formed at a comparatively low temperature in deposits near the surface which are gold- or silver-bearing. Guanajuato, Mexico. 2. In cavities and seams of schists and gneisses. Zillerthal, Tyrol. Microcline, KAlSi 3 O 8 Form. In crystal form microcline is almost like orthoclase, but it is triclinic with the angle (001 : 010) 7= 89 30' instead of 90. Cleavage. In two directions at practically right angles (89 30'). H. = 6. Sp. gr. 2.5 . 348 INTRODUCTION TO THE STUDY OF MINERALS Alblte Microcline Color, white, gray, reddish, green. The green variety is called amazon-stone. Optical Properties. n 7 ( 1.529) - n a (1.522) = 0.007. Extinc- tion on 001 = 15; on 010 = 5. Fragments are plates with middle first-order interference colors, and are usually dis- tinguished from orthoclase by the "gridiron" structure caused by polysynthetic twinning in two directions at right angles. Chemical Composition. Potassium aluminum trisilicate KA1- Si 3 O 8 , usually intergrown with albite, NaAlSi 3 O 8 (Fig. 512). This intergrowth which is known as perthite or microperthite is probably formed by the unmixing of the solid solution, (K,Na)- AlSi 3 O 8 , brought about by a lowering of the temperature. Blowpipe Tests. The same as for orthoclase. Uses. The feldspar used in the manufacture of porcelain and pottery is largely the micro- cline-albite intergrowth known FIG. 512. Microcline-aibite as perthite. Maine, North intergrowth (perthite). Carolina, and Pennsylvania are the principal producers. Distinguishing Features. Microcline can often be distin- guished from orthoclase by the fact that it is intergrown with albite. Otherwise it must be distinguished by optical tests. Occurrence. 1. In granite pegmatites. Near Florissant, Colo- rado. 2. In granites (but not in rhyolites). 3. In gneisses. PLAGIOCLASE The plagioclase groups of feldspars constitutes perhaps the best defined isomorphous group to be found among minerals. There is a perfect gradation in properties from the albite end of the group with the formula NaAlSi 3 O 8 to the anorthite end of the SILICATES 349 group with the formula CaAl 2 Si 2 Os. Intermediate members of the group are designated by Ab TO An n ; Ab denotes the albite FIG. 513. Albite twinning. FIG. 514. Pericline twinning. 001 001 molecule and An, the anorthite molecule. Crystals are triclinic, but with angles near those of orthoclase. The angle (001: 010), for example, . varies from 86 24' for albite to 85 50' for anorthite, while for orthoclase the corresponding angle is 90 0'. The plagioclases have good cleavage parallel to {001}, and fair cleav- age parallel to {010}. There is also imperfect cleavage parallel to {110} and fllO}. Twinning is rarely absent in the plagioclases. The most common twinning is known as albite twin- ning, in which {010} is the twin- plane. This is usually poly- synthetic and the twin striations, which are always parallel to the (001:010) edge, (Fig. 513) show best on the {001} cleavage face. Figure 515 shows in plan and elevation a cleavage fragment of plagioclase twinned on the albite law. The narrow planes marked 001 are placed at angles of about 010 FIG. 515. Plan and elevation of f 350 INTRODUCTION TO THE STUDY OF MINERALS 8 with the 001 planes. In pericline twinning, which is also polysynthetic, the 6-axis is the twin-axis. This kind of twinning shows best on the {010} cleavage (Fig. 514), and the angle the striations make with the (001 : 010) edge differs for the various plagioclases. This angle is given in the column of the following tabulation labeled " angle of rhombic section." In optical properties the plagioclases are similar. They are all biaxial with 2V varying from 77 to 90. The indices of refraction vary from 1.536 for albite to 1.588 for anorthite. The double refraction is rather weak (0.007 to 0.012). Typical analyses are given on page 344. The following tabulation illustrates the continuous variation in properties. The positive angles are clockwise and negative angles counter-clockwise. Extinc- Extinc- Angle of n y n a tion on tion on rhombic Sp. 001 010 section gr. Albite { Ab . 01igoclase..{ Ab9Am Ande 8 ine...{ Ab7An ' Labradorite{ AblAni Bytownite A ' Am Anorthite.. Ab ' An ' .536 .541 .552 .562 .572 .582 .588 .525 .532 .545 .555 .563 .571 .576 + 2H -14 -31 -40 + 23 + 15M -16 -27 -36 -37H + 22 + 10 + 3 - 1 - 9 -10 -16 2.62 2.64 2.66 2.69 2.73 2.74 2.76 Albite, Ab (NaAlSi 3 O 8 ) to Ab 90 Ani Form. Albite occurs in small crystals, in lamellar masses, and intergrown with microcline. The crystals are triclinic, pinacoidal class, and are usually tabular parallel to {010} . The usual forms are the same as for orthoclase. Figure 513 represents the common type of crystal with cjOOl}, 6{010[, raj 110), Af{lTO},andz|T01}. Albite, pericline, and Carlsbad twins are all common and some- times two or more of these are combined on one crystal. Cleavage, perfect parallel to (001) and less perfect parallel to {010}. H. = 6. Sp. gr. 2.62 . SILICATES 351 Color, white, colorless, or gray. Optical Properties. n 7 (1.536 to 1.541) - n a (1.525 to 1.532) = 0.009. Fragments are plates with middle first-order interference colors and extinction angles of about 3 on (001) and 15 to 23 on (010). The index of refraction is less than oil of cloves. Chemical Composition. Abioo to Ab 9 o Ani ; (for NaAlSi 3 O 8 Na 2 O = 11.8, A1 2 O 3 = 19.5, SiO 2 = 68.7). A little calcium is usually present as albite grades into oligoclase. Blowpipe Tests. Fusible (at 4) to a colorless glass and colors the flame yellow. Insoluble in ordinary acids. Distinguishing Features. Albite may resemble barite but is distinguished by its greater hardness. From the other plagio- clases it is only safely distinguished by optical tests. Occurrence. 1. In granite pegmatites associated with tour- maline, lepidolite, spodumene, etc. 2. In veins and seams especially in the hydrothermal meta- morphic rocks. 3. In certain soda-rich igneous rocks, usually intergrown with microcline. This intergrowth is known as perthite. Oligoclase, Ab 90 Ani to Ab 7 oAn 30 Form. Oligoclase occurs in cleavable masses and disseminated through rock masses, but unlike albite is rarely found in distinct crystals. Cleavage, perfect parallel to { 001 J , less perfect parallel to { 010 } . H. = 6. Sp. gr. 2.65 . Color, white, colorless, greenish, or reddish. Optical Properties. w Y (1.641 to 1.552) - n a (1.532 to 1.545) = 0.008. Fragments are plates with middle first-order interference colors and extinction angles of about 3 to (001) and about 153/ to (010). The fragments usually show polysynthetic twinning. Chemical Composition. Sodium and calcium aluminum silicate, AbgoAnjo to AbyoAnso. 352 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Fusible at 4. Insoluble in ordinary acids. Distinguishing Features. Distinguished from other plagio- classes by optical tests (or by a quantitative chemical. analysis). Occurrence. 1. In acid and intermediate igneous rocks, such as the granite-rhyolite series and the diorite-andesite series. Strange to say, oligoclase is more common in granites than albite. Andesine, Ab7 An 3 o to Form. Andesine occurs in cleavable masses or disseminated crystals through rock masses. Distinct loose crystals are rare. Cleavage, in two directions at angles of 86 14'. H = 6. Sp. gr. 2.69 . Color, colorless, white, and various tints. Optical Properties. n 7 (1.552 to 1.562) -n(1.545 to 1.555) = .007. Fragments are plates with middle first-order interfer- ence colors and extinction angles of to 4J^ on (001) and to 16 on (010). They usually show polysynthetic twinning. Chemical Composition. Sodium and calcium aluminum sili- cate, AbroAnao to Ab 5 oAn 5 o. Blowpipe Tests. Fusible (at 4). Insoluble in ordinary acids. Distinguishing Features. Distinguished from the other plagio- clases by optical tests (or by a quantitative chemical analysis) . Occurrence. 1. In intermediate igneous rocks such as diorites and andesites. Labradorite, Ab 6 oAn 5 o to Ab 30 An7o Form. Labradorite occurs in embedded crystals and in cleavable masses, but very rarely in distinct loose crystals. Albite twinning is the common kind of twinning. Cleavage, perfect parallel to {001}, less perfect parallel to {010}. H. = 6. Sp. gr. 2.71 . Color, gray or white, often showing a play of colors which is an optical effect due to minute inclusions. SILICATES 353 Optical Properties. ^(1.562 to 1.572) - n a (l.555 to 1.563) = 0.009. Fragments are plates with middle first-order interference colors and extinction angles of 4J^ to 14 (001) and 16 to 27 (010) . The fragments usually show poly synthetic twinning and often minute inclusions arranged in rows. Chemical Composition. Calcium-sodium aluminum silicate, Ab 5 oAn 5 o to AbaoAn7o. Blowpipe Tests. Fusible at 4. Soluble with difficulty in HC1. Distinguishing Features. Labradorite is distinguished from orthoclase by the twinning striations on the cleavage surfaces. Uses. Labradorite rock is used as an ornamental stone. Occurrence. 1. In basic igneous rocks such as gabbros, dia- bases, and basalts, associated with olivine, augite, hypersthene, ilmenite, and magnetite. 2. As the principal constituent of anorthosite, a basic plutonic igneous rock composed practically of labradorite. Adirondack Mts., New York. Bytownite, AbsoAnyo to Abio Ango Form. In cleavable masses and disseminated through rock like the other plagioclases. Cleavage, in two directions at angles of about 86. H. = 6. Sp. gr. 2.74 . Color. Colorless, white, or gray. Optical Properties. n y (1.572 to 1.582) -n a (1.563 to 1.571) = .010. Fragments are plates with upper first-order inter- ference colors and extinction angles of 14 to 31 on (001) and 27 to 36 on (010). Chemical Composition. Calcium and sodium aluminum sili- cate, AbaoAnro to Abio An 9 o. Blowpipe Tests. Fusible at 4^. Slightly soluble in HC1. Distinguishing Features. Bytownite can be distinguished 23 354 INTRODUCTION TO THE STUDY OF MINERALS from labradorite only by optical tests Cor by a quantitative chem- ical analysis). Occurrence. 1. In basic igneous rocks especially gabbros. Duluth, Minnesota. Anorthite, AbioAn 9 o to An (CaA^SiOs) Form. Anorthite occurs in cleavable masses and occasionally in euhedral crystals. Cleavage. Like the other plagioclase feldspars, in two direc- tions at angles of about 86. H. = 6. Sp. gr. 2.76 . Color, white, gray, or reddish. Optical Properties. n 7 (1.582 to 1.588) -n(1.571 to 1.576) = .012. Fragments are plates with upper first-order interference colors and extinction angles of 31 to 40 on (001) and 36 to 37^ on (010). Chemical Composition. Calcium aluminum silicate, usually with a little sodium, Abio An 9 o to An I0 o. (For An, CaO = 20.1 ; A1 2 O 3 = 36.7; SiO 2 = 43.2.) Blowpipe Tests. Fusible at 4%. Slowly soluble in HC1 and gives a jelly of silicic acid upon evaporation. Distinguishing Features. Optical tests (or a quantitative chemical analysis) are necessary to distinguish anorthite from other members of the plagioclase group. Occurrence. 1. A comparatively rare mineral in basic igneous rocks. Miyake, Japan. FELDSPATHOID GROUP Leu cite, nepheline, sodalite, and lazurite are collectively known as feldspathoids, for they are alkaline aluminum silicates which play the same role in some rare igneous rocks that the feldspars do. SILICATES 355 Leucite, KAl(SiO 3 ) 2 Form. For leucite the characteristic form is well-defined embedded crystals. The crystals are isometric; the only com- mon form is the trapezohedron {211} (Fig. 516). Cross-sec- tions are eight-sided (Fig. 517). H. = 5J^ to 6. Sp. gr. 2.5. Color, white or gray. Optical Properties, n = 1.50. Isotropic. Fragments are ir- regular and are either dark between crossed nicols or have very low first-order interference colors. Chemical Composition. Potassium aluminum metasilicate KAl(Si0 3 ) 2 . (K 2 = 21.5 per cent.). A little sodium is sometimes present. Blowpipe Tests. Infusible. Decomposed by HC1 with the separation of powdery silica. Distinguishing Features. Leucite is distinguished by its equidimensional crystals, which are FlG - 516 ' FlG ' 517 ' .. , . J ... FIGS. 516-517. Leucite. never formed in cavities. Uses. Rocks containing large amounts of leucite are a possible source of potassium salts. Occurrence. 1. In certain volcanic rocks in which leucite takes the place of the feldspars or occurs with feldspars. (Exceed- ingly rare in plutonic rocks.) Leucite is rare in the United States, but occurs in large quantities in the Leucite Hills, Wyoming. Nepheline, (Na,K)AlSiO 4 -(NaAlSi 3 O 8 ) a: Form. Nepheline occurs in embedded crystals or grains and in massive forms. Crystals are hexagonal, and short prismatic in habit. The cross-sections are six-sided and rectangular. H. = 5^ to 6. Sp.gr. 2.6 + . Color, white, gray, or reddish. Luster, greasy. 356 INTRODUCTION TO THE STUDY OF MINERALS Optical Properties. n 7 (1.543). - w( 1.538) = 0.005. Frag- ments are irregular with low first order interference colors. Chemical Composition. Essentially sodium aluminum ortho- silicate, NaAlSiCX, but with^an excess of silica which is probably present as NaAlSiaOs. Potassium replaces part of the sodium. Blowpipe Tests. Fuses at 4 to a colorless glass. Gelatinizes with HC1. Distinguishing Features. Nepheline resembles quartz but is distinguished by its inferior hardness. It lacks the cleavage of the feldspars. Occurrence. 1. In nepheline syenites, phonolites, and other rare soda-rich igneous rocks. It is often associated with ortho- clase, but never with quartz. Magnet Cove, Arkansas. Sodalite, Form. Sodalite usually occurs in disseminated or massive forms, but isometric dodecahedral crystals are sometimes found. Cleavage, indistinct dodecahedral. H. = 5^ to 6. Sp. gr. 2.3 . Color, blue, gray, or colorless. Optical Properties, n = 1.483. Isotropic. Fragments are irregular and colorless, and dark between crossed nicols. Chemical Composition. Sodium aluminum chlorid and ortho- silicate, Na 4 Al 3 Cl(Si0 4 )3 or 3NaAlSi0 4 'NaCl. Blowpipe Tests. Fusible with intumescence to a colorless glass. The NaPO 3 bead with CuO gives an azure blue flame. Gelatinizes with HC1. Distinguishing Features. Blue sodalite is distinguished from lazurite by the absence of associated pyrite. Unless it shows the blue color, it is a difficult mineral to recognize. The chlorin test in a NaPO 3 bead will distinguish it from most other minerals. Uses. Some varieties of sodalite rocks are used for ornamental stones. Occurrence. 1. In soda-rich igneous rocks such as nepheline syenites and phonolites. SILICATES 357 Lazurite, Form. Lazurite usually occurs in compact massive form, more or less mixed with calcite, pyrite, and other silicates. This mix- ture is known as lapis lazuli. H. = 5to5M- .Sp. gf. 2.4 . Color, deep blue. Streak, pale blue. Optical Properties, n about 1.50. Isotropic. Fragments are irregular, deep blue, non-pleochroic, and dark between crossed nicols. Chemical Composition. Sodium aluminum sulfid and ortho- silicate, Na 5 Al 38(8104)3 or 3NaAlSiO4-Na 2 S. It usually contains calcium and the sulfate radical, both due to isomorphous re- placement. Blowpipe Tests. Fuses (at 3) with intumescence to a white glass. Soluble in HC1 with gelatinization and with the evolution of H 2 S. Distinguishing Features. Lazurite, or more properly lapis- lazuli, is distinguished by its blue color and by the presence of pyrite. The latter also distinguishes it from imitation stones. Uses. Lapis lazuli is a valuable ornamental stone. It was the "sapphire" of the ancients. The paint called ultramarine was formerly lapis lazuli, but it is now made artificially. Occurrence. 1. As a contact mineral in crystalline limestones associated with diopside and other silicates. The quarries at Badakshan in Afghanistan, the principal source of lapis lazuli, are the oldest known mines in existence. PYROXENE GROUP The following hiinerals constitute a mineral group, though they are not strictly isomorphous; for enstatite and hypersthene are orthorhombic and rhodonite is triclinic, while the others are monoclinic. 358 INTRODUCTION TO THE STUDY OF MINERALS Typical analyses of the more important members of the pyrox- ene group are given in the following tabulation. Analyses of Minerals of the Pyroxene Group 1 MgO FeO CaO AhOa Fe 2 O 3 SiO 2 Misc. Enstatite 36.9 3.2 1.3 58.0 ign. = 0.8 Bronzite 29.7 10.1 1.3 58.0 MnO = 1.0 Hypersthene 21.3 21.3 3.1 0.4 51.4 Diopside 17.3 1.9 25.0 0.5 1.0 54.8 Diopside 16.1 5.0 24.9 1.5 0.6 52.8 Diopside 10 12 3 22 1 2 1 3 51 1 MnO = 0.1; ign. = 0.3 Diallage 16 4 8 4 20 3 3.8 50 2 Augite 16.0 4.1 19.0 9.8 4.5 46.9 Augite 7.6 9.4 12.3 21.5 3.8 42.2 Na 2 O = 3.0 Augite 13.2 4.3 21.3 8.2 3.7 46.5 TiO 2 = 2.8 Enstatite, MgSiO 3 Form. Enstatite usually occurs in lamellar or fibrous-lamellar masses. The mineral is orthorhombic, but distinct crystals are very rare. Cleavage, distinct in several directions. H. = 5>^ to 6. Sp. gr. 3.3 . Color, bronze, gray, or brown. Luster, metalloidal. Optical Properties, n 7 (1.67)- n a (1.66) = 0.01. Fragments are prismatic with parallel extinction, low first order interference colors, and positive elongation. Chemical Composition. Magnesium metasilicate; (MgO = 40.0 per cent.). Ferrous iron usually replaces part of the mag- nesium. Ferriferous enstatite is called bronzite. Blowpipe Tests. Fusible on thin edges (at 6). Insoluble in acids. Distinguishing Features. Enstatite can usually be dis- tinguished by its peculiar bronze-yellow, but non-metallic, appearance. Occurrence. 1. In basic igneous rocks such as peridotites and gabbros. 2. In meteorites, SILICATES 359 Hypersthene, (Fe,Mg)SiO 3 Form. Hypersthene usually occurs in cleavable masses or is disseminated through rock masses. Cleavage, good cleavage in one direction. H. = 5J. . Sp. gr. 3.4 . Color, dark brown or greenish brown. Optical Properties. w 7 (1.70) - w a (1.69) = 0.01. Fragments are prismatic with parallel extinction, bright interference colors, and positive elongation. Hypersthene is usually pleochroic; it changes from pink to green. Chemical Composition. Iron and magnesium metasilicate, (Fe,Mg)Si0 3 . Blowpipe Tests. Fusible (at 5) to a black glass. On charcoal in R.F. it becomes magnetic. Soluble with difficulty in HC1. Distinguishing Features. Hypersthene is difficult to dis- tinguish without optical tests. Occurrence. 1. In basic igneous rocks, especially gabbros and norites. PYROXENE On account of the difficulty of distinguishing some of the members of the pyroxene group they are often grouped under FIG. 518. Cross-sections of pyroxene (diopside and augite). the name, common pyroxene or pyroxene proper. Pyroxene (in this sense) is a silicate of aluminum, iron, calcium, and magne- sium, which is often found in monoclinic prismatic crystals (see Figs. 519-526) with the typical cross-sections shown in Fig. 518. It comprises two fairly well-defined minerals, diopside and augite. 360 INTRODUCTION TO THE STUDY OF MINERALS Diopside, Ca(Mg,Fe)(SiO 3 ) 2 Form. In crystals, granular masses, disseminated through rocks, but rarely fibrous or columnar. Monoclinic system. Prismatic class, d : b : 6 = 1.092 : 1 : 0.589; ft = 74 10' L Usual forms: a{ 100}, 6{010), mjllO), c{001), p{lll}, o{221), AJ311}, d{101}. Interfacial angles: wm(110 : 110) = 92 50'; pp(lll : ill) = 48 29'; oo(221 : 221) = 84 11'. The habit is usually prismatic in the direction of the c-axis. Figs. 519 to 522 represent typical crystals. The cross-section of FIG. 519. FIG. 520. FIG. 521. FIGS. 519-522. Diopside. crystals is characteristic; it is usually four- or eight-sided as represented in Fig. 518. Cleavage, imperfect in two directions at angles of 87 10' and 92 50' (parallel to the unit prism { 110) ). There is often parting parallel to {001} which is more prominent than the cleavage. The variety diallage has well-defined parting parallel to jlOOj. H. = 4 to 6. Sp.gr. 3.2 + . Color, white, gray, or green. Optical Properties. n 7 (1.70) - n a (1.67) = 0.03. Fragments are prismatic and colorless or pale green with bright interference colors and an extinction angle of 20-30. A thin parting flake SILICATES 361 parallel to {001} will give an interference figure consisting of an axial bar with concentric rings. Diallage has parallel extinction and positive elongation. Chemical Composition. Calcium magnesium-ferrous meta- silicate varying from CaMg(SiO 3 ) 2 to CaFe(SiO 3 ) 2 . Small amounts of aluminum, ferric .iron, and manganese may also be present. Blowpipe Tests. Fusible at 4 to a colorless or pale green glass. Insoluble in acids. Distinguishing Features. Diopside is distinguished by its crystal form and imperfect prismatic- cleavage. It is usually distinguished from augite by basal parting and lighter color. Occurrence. 1. In crystalline limestones as a contact mineral associated with garnet. 2. In schists and other metamorphic rocks, both in the rock mass and in seams. 3. In gabbros and peridotites (the variety diallage). Augite, mCaMg(SiO 3 ) 2 + n(Mg,Fe)(Al,Fe) 2 SiO 6 Form. Augite usually occurs in embedded crystals. The crystals are monoclinic, prismatic class, with the forms: ajlOOj, 6{010[, mUlO}_,_s{Illj. Interfacial angles: mm(110:ll0) = 92 50', 3s(lll:lll) = 59 11'. The habit is usually prismatic (Figs. 523-526) and either square or octagonal in outline (see Fig. 518). Twins with a {100} as twin-plane are common (Fig. 526). Cleavage, imperfect in two directions parallel to {110}, and at angles of 92 60' and 87 10'. H. = 5M- Sp. gr. 3.3 . Color, dark green to black. Optical Properties. n 7 (1.73) - n(1.71) = 0.02. Fragments are prismatic with bright interference colors and large extinction angles (25 to 40). The thin fragments are only slightly pleo- chroic, if at all. This usually distinguishes augite from horn- blende. 362 INTRODUCTION TO THE STUDY OF MINERALS Chemical Composition. An isomorphous mixture of CaMg (SiO 3 ) 2 and (Mg,Fe)(Al,Fe) 2 SiO 6 in varying proportions. The presence of aluminum and ferric iron distinguishes augite from diopside. Sodium and titanium are sometimes present. Blowpipe Tests. Fusible at 4 to a black glass. Insoluble in acids. Distinguishing Features. Augite is distinguished from horn- blende by its square or octagonal cross-section and imperfect prismatic cleavage. It is darker colored than diopside. Occurrence. 1. In basic igneous rocks, especially basalts and diabases, often as phenocrysts like Figs. 523-526. Bohemia. 2. In basaltic tuffs. m 6 o m FIG. 523. FIG. 526. FIG. 524. FIG. 525. FIGS. 523-526. Augite. Rhodonite, MnSiO 3 Form. Rhodonite is found in cleavable and compact masses, and occasionally in euhedral crystals. The crystals are triclinic, but similar to diopside and augite in angles. Cleavage, in two directions at angles of 92J^ (parallel to 110) and also an additional parting parallel to (100); the angle (100: 110) is 48 33'. H. = 5>^. Sp. gr. 3.6 . Color, pink or red, often stained black by manganese oxids. Optical Properties. n Y (1.74) - w(1.72) = 0.02. Fragments are prismatic with bright interference colors and large extinction angles (20 to 25). SILICATES 363 Chemical Composition. Manganese metasilicate, MnSiO 3 . Calcium is usually present and sometimes iron. Blowpipe Tests. Fusible at 3 to a dark glass. Partially soluble in HC1. Distinguishing Features. Rhodonite is distinguished from orthoclase and microcline by its high specific gravity and from rhodochrosite by its greater hardness. Uses. Conpact rhodonite is used as an ornamental stone, especially in Russia. Occurrence. 1 . In high-temperature veins with garnet. Broken Hill, New South Wales. 2. In crystalline limestones with willemite, franklinite, and zincite. Franklin Furnace, New Jersey. AMPHIBOLE GROUP The amphibole group is parallel to the pyroxene group, but the triclinic members are so rare that no account of them will be given here. The amphiboles differ from the pyroxenes mainly in the prism and cleavage angle, which is 56 (and 124) instead of 87 (and 93) . For many of the pyroxenes there are corresponding amphiboles, but they cannot be regarded as dimorphous minerals. For example, diopside is CaMg(SiO 3 ) 2 , while the corresponding tremolite is CaMg 3 (SiO 3 )4. Tremolite and hornblende contain a small amount of water of constitution, while diopside and augite, if unaltered, contain none. Analyses of Amphiboles CaO MgO FeO AhOa Fe 2 O 3 Si0 2 Ti0 2 H 2 O NazO K 2 F Anthophyllite. . 0.2 28.7 10.4 0.6 58.0 1.7 Tremolite 13.2 24.1 0.6 1.8 57.7 0.1 1.6 0.5 0.2 0.4 Tremolite (acti- nolite) 12.1 21.2 5.5 1.2 0.8 56.3 1.8 0.2 0.3 0.1 Hornblende .... 9.8 12.6 10.5 8.3 6.9 43.8 0.8 0.6 3.4 1.3 1.8 Hornblende. . . . 11.5 11.2 14.3 11.6 2.7 42.0 1.5 0.6 2.5 0.1 0.8 Hornblende .... 12.0 14.2 2.2 17.6 7.2 39.9 1.7 0.4 3.2 0.2 0.1 364 INTRODUCTION TO THE STUDY OF MINERALS Anthophyllite, (Mg,Fe)SiO 3 Form. Anthophyllite usually occurs in lamellar, fibrous, or asbestiform masses, which are often radiating. Crystals are orthorhombic but terminal faces have never been found. Cleavage, perfect in two directions at angles of 54 and 126. H. = 5-6. Sp. gr. = 3.1. Color, gray to brown. Optical Properties. W T ( 1.657) -- n a (1.633) = 0.025. Frag- ments are prismatic with parallel extinction and positive elonga- tion. The interference colors range up to low second-order. Chemical Composition. Magnesium, iron metasilicate (Mg,- Fe) SiO 3 . Aluminum is of ten present and in one variety (gedrite) is prominent. Most specimens of anthophyllite contain a little water. A typical analysis is given on page 363. Blowpipe Tests. Anthophyllite is fusible on thin edges to a black glass. In the closed tube it may give a little water. Insoluble in acids. Distinguishing Features. Anthophyllite is distinguished from the other amphiboles by the absence of calcium and by the parallel extinction in fragments. Uses. Anthophyllite in its finely fibrous form is one of the varieties of asbestos. It is quarried at Sails Mountain, Georgia. Occurrence. 1. A typical metamorphic mineral occurring in schists and gneisses. Tremolite, Ca(Mg,Fe) 3 (SiO 3 ) Form. Tremolite occurs in long prismatic crystals and in columnar and fibrous aggregates. Crystals are monoclinic with the prism j 110) and the pinacoid {OlOj , but rarely have terminal faces. The axial ratios and interfacial angles are like those of hornblende. Characteristic cross-sections are shown in Fig. 527. Cleavage, in two directions at angles of 56 and 124 parallel to {110}. The cleavage is more perfect than that of diopside. H. = 5%. (Fibers may appear to be lower). Sp. gr. 3.0 . SILICATES 365 Color, white, gray, or green. The green varieties are some- times called actinolite. Optical Properties. n 7 (1.636) - w a (1.611) = 0.025. Frag- ments are prismatic or acicular with bright interference colors, positive elongation, and extinction angles of 10 to 15. Chemical Composition. Calcium magnesium-iron metasilicate Ca(Mg,Fe) 3 (SiO 3 )4. Ferriferous varieties with more than 2 or 3 per cent, of FeO are called actinolite. Aluminum and ferric iron are very low and this is the principal chemical distinction between these minerals and hornblende. Blowpipe Tests. Fusible at 4 to a glass. Insoluble in acids. Distinguishing Features. Tremolite is easily recognized in typical specimens by its characteristic cleavage. It is distin- l in FIG. 527. Cross-sections of tremolite (inc. actinolite) and hornblende. guished from hornblende only by its color, and optical characters. Uses. The fibrous tremolite is one kind of asbestos. Occurrence. 1. In crystalline dolomitic limestones. Lee, Massachusetts. 2. In schists, often associated with talc. St. Lawrence County, New York. 3. As a hydrothermal alteration product of pyroxene. This is a fibrous variety known as uralite which usually contains some iron and aluminum and thiis grades into hornblende. HORNBLENDE, wCa(Mg,Fe) 3 (SiO 3 ) 4 +n(Al,Fe)(F,OH)SiO 3 Form. Hornblende occurs in well-defined crystals, in cleav- ages, in disseminated crystals and grains, and in bladed aggre- gates. Monoclinic system. Prismatic class. Axial ratio : a : b : 6 = 366 INTRODUCTION TO THE STUDY OF MINERALS 0.551 :1 : 0.293; ft = 73 58'. Usual forms: wjllO), 6{_010), a{100}, r{011|, p{101}. Interfacial angles: mm(110 : 110) = 55 49', rr{011 :011} = 31 32', rp{011 : 101} = 34 25'. Habit short to long prismatic, usually pseudohexagonal or rhom- bic in cross-section. (See Fig. 527.) The common type of hornblende crystal is that of Fig. 528. Cleavage, perfect in two directions at angles of 56 and 124, parallel to {110}. H. = 5>^. Sp. gr. 3.2 . Color, dark green, or dark brown to black. Optical Properties, n Y (1.653) - n a (1.629) = 0.024. Frag- ments are prismatic, and green or brown in color. The extinction angle varies from 5 to 20 and the elongation is positive. Pleochroism is a marked feature of hornblende. The colors vary from pale to deep green, from yellowish-green to bluish-green, from brown to greenish-brown, or from pale to deep ^ brown. By the pleochroism and the extinction F JP- ^, 28 ;~ angle, hornblende may easily be distinguished from .Hornblende. . . augite, which it often greatly resembles. Chemical Composition. A complex metasilicate of calcium, magnesium, ferrous iron, aluminum, and ferric iron with fluorin and hydroxyl. The formula given above was established by Pen- field and Stanley (see analyses, page 363). Blowpipe Tests. Fusible at 4 to a black glass. In the closed tube it gives a little water at a high temperature. Insoluble in acids. Distinguishing Features. Hornblende is distinguished from pyroxene by its six-sided cross-section and by its perfect pris- matic cleavage of 56. Optical tests may be necessary to dis- tinguish hornblende from pyroxene. Occurrence. 1. In volcanic igneous rocks such as andesites and certain basalts. 2. In plutonic igneous rocks, especially granites, syenites, and diorites, rarely in gabbros and peridotites. SILICATES 367 3. In diabases and gabbros as a magmatic alteration product of augite and other pyroxenes. 4. In schists and gneisses often forming rock masses, horn- blende schists and amphibolites. Glaucophane, NaAl(SiO 3 ) 2 - (Fe,Mg)SiO 3 Form. Glaucophane occurs in small disseminated crystals and in fibrous masses. Crystals are prismatic in habit with { 1 00 } , {010}, and {110}, but are rarely terminated. The cross-section FIG. 529. FIG. 530. FIG. 531. FIGS. 529-531. Pleochroism of glaucophane. as seen in thin rock sections is pseudohexagonal or rhombic (Fig. 527). Cleavage, parallel to { 110), i.e., in two directions at angles of 56 and 124. H. 6to6M- Sp. gr. 3.1 . Color, blue to blue-black. Optical Properties. n y (l.Q4) - w(1.62) = 0.02. Fragments are prismatic and blue in color with pleochroism from blue to violet. The extinction is practically parallel and the elonga- tion positive. In thin sections glaucophane shows the pleochroism illustrated by Figs. 529-531. Chemical Composition. Sodium, aluminum, iron-magnesium metasilicate, NaAl(SiO 3 ) 2 -(Fe,Mg)SiO 3 . Glaucophane is one of the group known as soda amphiboles. 368 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Easily fusible (at ,3) to a dark glass; gives an intense yellow flame. Insoluble in acids. Distinguishing Features. The blue to blue-black color and occurrence in schists are distinctive. Occurrence. 1. In schists and gneisses often constituting the main part of the rock. Glaucophane schists and glauco- phane gneisses are especially abundant in the Coast Ranges of California. Beryl, Be 3 Al 2 (SiO 3 )6 Form. For beryl the characteristic forms are crystals and columnar masses. Beryl crystallizes in the hexagonal system, FIG. 532. FIG. 533. FIG. 534. FIGS. 532-535. Beryl. FIG. 535. and is one of the few examples of the di hexagonal bipyramidal class^ Axial ratio: 6 = 0.498. Usual forms: c{0001}, m{l()To), a{1120}, p{10ll}, s{1121}._Interfacial angles: cp(_0001 :_1011) = 29 56^; cs{0001 : 1121J = 44 56 r ; ma(1010 : 1120) = 30 0'. The habit is usually prismatic (Figs. 532-534), but some- times tabular (Fig. 535). Crystals are often very large. H. = 7^to8. Sp.gr. 2.7 . Color, usually various tints of green, but sometimes white, yellow, pink, or blue. SILICATES 369 Optical Properties. W T (1.570) - n a (1.564) = 0.006. Frag- ments are irregular with low first-order interference colors. Chemical Composition. Beryllium aluminum metasilicate, Be 3 Al 2 (SiO 3 )6. The alkalies, sodium, lithium, and caesium, often partly replace beryllium. Blowpipe Tests. Fusible on thin edges (at 6). Insoluble in acids. Distinguishing Features. Beryl is usually distinguished by crystal habit. It is harder than apatite and not as heavy as corundum. Uses. The deep green variety, emerald, is a valuable gem. The best emeralds are found at Muzo in Colombia. Sea- green (aquamarine) and pink varieties are also used as gems. Occurrence. 1. In granite pegmatites associated with topaz, albite, lepidolite, spodumene, etc. San Diego county, California, 2. In mica schists and gneisses. North Carolina. 3. In calcite veins in limestone. Muzo, Colombia. Wollastonite, CaSiO 3 Form. Wollastonite is found in cleavable, columnar, fibrous, and compact masses. Euhedral crystals, which are rare, are monoclinic and are elongate in the direction of the 6-axis. Cleavage, in two directions (001 and 100) at angles of 84^. H. = 4^ to 5. Sp.gr. 2.8 . Color, white or gray. Optical Properties. w 7 ( 1.633) - w a (1.621) = 0.012. Frag- ments are acicular with bright interference colors, parallel extinction, and positive elongation. Chemical Composition. Calcium metasilicate, CaSiO 3 . Blowpipe Tests. Fuses (at 4) to a white glass, giving a yellow- ish-red flame. It is decomposed by HC1 with the separation of powdery silica and usually effervesces because of admixed calcite. Distinguishing Features. Wollastonite resembles tremolite 24 370 INTRODUCTION TO THE STUDY OF MINERALS but differs in cleavage. It may be necessary to use optical tests to distinguish them. Occurrence. 1. In crystalline limestones at the contact with igneous rocks, often associated with garnet, diopside, etc. Lewis county, New York. 2. In calcareous inclusions in volcanic rocks. Spodumene, LiAl(SiO 3 ) 2 Form. Spodumene occurs in rough monoclinic crystals and in cleavable masses. The habit of the crystals is prismatic and usually tabular parallel to {100}. Cleavage, in two directions at angles of 93 and 87. There is also parting parallel to { 100} at times, which causes the mineral to break into plates. H. = 6^. Sp. gr. 3.1 . Color, white, gray, colorless, lilac, greenish. Optical Properties. n 7 (1.67) - n a (1.65) = 0.02. Fragments are prismatic with first-order interference colors and oblique extinction of 20 to 25. Chemical Composition. Lithium aluminum metasilicate, LiAl (Si0 3 ) 2 . Sodium often replaces part of the lithium. Blowpipe Tests. Fuses (at 3J^) to a clear glass, giving a purple- red flame. Insoluble in acids. Distinguishing Features. Distinguished from feldspars by its higher specific gravity, and from tremolite by differences in cleavage. Uses. Spodumene has been used to some extent as a source of lithium salts. A transparent lilac variety called kunzite is used as a gem and also a transparent emerald-green variety known as hiddenite. Occurrence. 1. In granite pegmatites associated with albite, lepidolite, tourmaline, etc. Pennington county, South Dakota. One crystal from the Etta Mine in this county measured 14 meters in length. SILICATES 371 GARNET GROUP (Ca,Mg,Mn,Fe) 3 (Al,Fe,Cr) 2 (SiO 4 ) Grossularite, Almandite, Pyrope, Mg 3 Al 2 (SiO 4 )3 Andradite, Ca 3 Fe 2 (SiO 4 )3 Form. Garnet is found in distinct crystals, which are usually embedded, in granular or compact masses, and in the form of sand. Garnet crystallizes in the hexoctahedral class of the isometric system. The usual forms are the dodecahedron d{ 110} and the trapezohedron WJ211J. The hexoctahedron {321[ is sometimes found, but the cube and octahedron are exceedingly rare forms for garnet. Figs. 536-539 illustrate commonly occurring garnet FIG. 536. FIG. 537. FIG. 538. FIGS. 536-539. Garnet. FIG. 539. crystals ranging from dodecahedral habit to trapezohedral habit. Interfacial angles: dd(110 : 101) =_60; dddlO : HO) = 90, nn (211 : 121) = 3333J';nn(211 : 2ll) = 48 11^'; dn(110 : 211) = 30. Cleavage. Usually absent, but some varieties show parting. H. - 7. Sp. gr. varies from 3.5 to 4.2. Color, various tints and shades of red, brown, yellow, green, and occasionally black. Optical Properties, n varies from 1.74 to 1.88. Iso tropic. Fragments are irregular, colorless or pale red, and dark between crossed nicols, but some varieties have weak double refraction. Chemical Composition. The general formula of garnet is (Ca,Mg,Mn,Fe) 3 (Al,Fe,Cr) 2 (Si0 4 )3. The four most common minerals of the group are: 372 INTRODUCTION TO THE STUDY OF MINERALS Grossularite, Ca 3 Al 2 (SiO 4 )3 1 . 736 to 1 . 763 Almandite FesAMSiOOs 1 . 778 to 1 . 815 Pyrope, Mg 3 Al 2 (SiO 4 )3 1 .741 to 1 . 760 Andradite, Ca 3 Fe2(SiO 4 )3 1 . 857 to 1 . 887 Sp. gr. 3.53 4.25 3.51 3.75 Two other rare minerals of the group are also known, viz.; spessartite, Mn 3 Al 2 (SiO4)3, and uvarovite CasC^SiO^a. It is rare to find a garnet that corresponds exactly to any one of these as can be seen from the following analyses: Analyses of Garnets CaO MgO FeO MnO AhOa Fe 2 Os Cr 2 O 3 Si0 2 Grossularite Pyrope 33.9 5 1.7 17 9 8 1 5 20.2 22 4 4.9 5 5 39.5 40 9 Almandite 1 .4 3 6 33.8 1 .1 22 7 37 6 Almandite 2.4 3.7 29.5 4.8 19.2 4.9 35.9 5 5 7 37 2 20 9 35 7 Andradite 31 5 tr 3 2 2 30 4 35 3 Uvarovite 31.6 1.5 5.7 2.0 21.8 36.9 Blowpipe Tests. Fusible from 3 to 4. Insoluble before fusion, but after fusion (alone, not with Na 2 CO 3 ) it gelatinizes with HC1. Distinguishing Features. Garnet is recognized by crystal habit, absence of cleavage, rather high specific gravity, and by its hardness. Uses. Garnet is used as an abrasive, especially for finishing wood and leather. New York has furnished the total domestic supply. Spain has also produced abrasive garnet from alluvial deposits. Some varieties are used for gems. Occurrence. 1. In crystalline limestones especially at con- tacts, associated with wollastonite, diopside, vesuvianite, etc. (grossularite and andradite). 2. In schists and gneisses (almandite). 3. In eclogites with pyroxenes or amphiboles. SILICATES 373 4. In granites and granite pegmatites (almandite) . 5. In peridotites and derived serpentines (pyrope). 6. In nepheline- and leucite-bearing lavas, such as phonolites, etc. (melanite variety of andradite). 7. In sands. OLIVINE GROUP ORTHORHOMBIC The olivine group is a group of orthorhombic orthosilicate minerals with the general formula R 2 "Si0 4 in which R" may be Mg, Fe, or Mn. The minerals of this group are: forsterite, Mg 2 SiO 4 ; fayalite, Fe 2 SiO 4 ; olivine, (Mg,Fe) 2 SiO 4 ; tephroite, Mn 2 SiO 4 ; and moriticellite, CaMgSi0 4 . Olivine is an isomor- phous mixture of magnesium and ferrous silicates, and monti- cellite, a double salt. Olivine is the only common mineral of the group. The following analyses show the range in composition of these minerals : MgO FeO CaO MnO Si0 2 Misc. Forsterite Olivine 54.4 50.3 1.5 8.5 0.8 42.8 41.2 ign. = 0.8 Olivine. . 44.1 17.5 39.2 Olivine Fayalite 30.6 2 1 28.1 65.5 1.4 1.2 38.9 32.4 Tephroite Monticellite 1.4 22.0 1.1 5.6 1.0 34.9 65 6 30.2 37.9 ign. =0.4; ZnO=0.3 OLIVINE, (Mg,Fe) 2 SiO 4 Form. For olivine the characteristic occurrences are granular masses or disseminated crystals and grains. Crystals are ortho- rhombic and are usually tabular in habit. Figure 540 represents a crystal with all the seven type forms of the rhombic bipyramidal class. a{100|, 6{010), c{001), m{110), d{101}, /c{021), pflll}. H. = 6>^ to 7. Sp. gr. 3.3 . 374 INTRODUCTION TO THE STUDY OF MINERALS Color, yellowish green to bottle green. Optical Properties. n y ( 1.699) - n a (1.662) = 0.037. Frag- ments are irregular and colorless with bright interference colors. Chemical Composition. Magnesium and iron orthosilicate (Mg,Fe) 2 SiO 4 , (Feb = 5.0 to 30.0 per cent.). Blowpipe Tests. Infusible. Gelatinizes with HC1. Distinguishing Features. Olivine is usually recognized by c its bottle-green color and absence of cleavage. /j '~f\\ Uses. Clear transparent olivine is used as Zj lP\ ^ a gem under the name peridot. Occurrence. 1. In pericfotites with enstatite or diallage. The olivine is usually partially altered to serpentine. An igneous rock com- posed practically of olivine alone is called dunite. 2. In basalts, olivine diabases, and olivine gabbros as an essential constituent. 3. In tuffs and volcanic bombs. 4. In meteorites. Pallasite is a meteorite rock with olivine filling the cavities in a spongy mass of iron. Forsterite, Mg 2 SiO 4 Form. Forsterite usually occurs in disseminated anhedral or subhedral crystals. Euhedral, orthorhombic crystals like olivine in form and angles are known, but are rare. H. = 6H- Sp. gr. 3.25 . Color, gray to pale green or yellow. Optical Properties. n y ( 1.670) - n a (1.635) = 0.035. (For pure artificial Mg 2 SiO 4 ). The indices of refraction of the natural mineral are a little higher on account of the presence of Fe 2 SiO4). Fragments are irregular with bright interference colors. Chemical Composition. Magnesium orthosilicate Mg 2 SiO 4 (MgO = 57.1, Si0 2 = 42.9), with a little ferrous iron replacing the magnesium. With increasing iron it grades into olivine. SILICATES 375 Blowpipe Tests. Infusible before the blowpipe. Soluble in HC1 with gelatinization. Distinguishing Features. It is distinguished from olivine by lower iron content and by lower indices of refraction. From other magnesium silicates it is distinguished by the absence of water. Occurrence. 1. In crystalline limestones as the product of dedolomitization. A common associate is spinel. Bolton, Mass- achusetts. 2. In contact-metamorphic zones with magnetite. Phillips- burg, Montana. Willemite, Zn 2 SiO 4 Form. This mineral is usually crystalline massive or granular massive. Crystals are hexagonal and prismatic in habit with the hexagonal prism {1120}, and the rhombohedron {10ll{. H. = 5>i Sp.gr. 4.1 . Color, pale red, yellow to green. Optical Properties. n 7 (1.717) - n a (1.693) = 0.024. Fragments are irregular with bright interference colors. Chemical Composition. Zinc orthosilicate, Zn 2 Si0 4 ; (Zn = 58.0 per cent.). Manganese often replaces part of the zinc. Blowpipe Tests. Fusible (at %) with difficulty. With cobalt nitrate solution on charcoal the assay turns blue and the sublimate on the coal, green. Willemite is distinguished from calamine by the absence of water in the closed tube. Gives a fine jelly when the HC1 solution is heated. Uses. Willemite is a source of zinc white and also of spelter. Distinguishing Features. Willemite is often distinguished by its association with franklinite (black) and zincite (red). Occurrence. 1. In crystalline limestone intimately mixed with franklinite and zincite. It perhaps has been formed by the metamorphism of calamine present in the original sedimentary limestone. Franklin Furnace, New Jersey. 376 INTRODUCTION TO THE STUDY OF MINERALS 2. In the oxidized zone of zinc deposits. (Calamine, however, is much more common in the oxidized zone). CALAMINE, Zn 2 (OH) 2 SiO 3 Form. Calamine occurs as drusy crystalline coatings, more rarely in botryoidal and stalactitic forms with a spherulitic structure, and also massive. Crystals are orthorhombic, pyra- midal class. The habit is usually tabular parallel to {010} and the two ends of the crystal are differently terminated. Figure 541 represents a typical crystal. The usual forms are: c{001(, &{010), m{110[, i{031|, s{101}, {301}, 0J121J. Interfacial angles: c(001: 301) = 61 20>^'; cs(001: 101) = 31 23'. Cleavage, perfect parallel to the length of the crystal. . H. = 5. Sp. gr. 3.4 . Color, colorless, white, and pale colors. Optical Properties. n T (1.64) - w(1.61) = 0.03. Fragments are irregular or prismatic with parallel extinction. Crystals have parallel extinction and positive elongation. The interference colors are bright. Chemical Composition. Basic zinc metasilicate, . Zn 2 (OH) 2 Si0 3 or ZnSiO 3 -Zn(OH) 2 ; (Zn = 54.2 per cent., H 2 O = 7.5 per cent.). The common impurities are iron and aluminum. Blowpipe Tests. Fusible on the edges (at 5). In the closed tube decrepitates and gives off water. Heated with cobalt nitrate solution on charcoal, the assay becomes blue and the sublimate on the coal, green. Soluble in HC1, giving a fine jelly on partial evaporation. Distinguishing Features. Calamine resembles smithsonite and is often distinguished from it by the sharp, well-defined crystals, and by the cleavage parallel to the length of the crystals. Uses. Calamine is one of the ores of zinc. FIG. 541. Calamine. SILICATES 377 Occurrence. 1. A mineral characteristic of the oxidized zone, usually derived from sphalerite and often associated with smith- sonite. Granby, Newton county, Missouri, is a prominent locality. Scapolite, m(3CaAl 2 Si 2 O 8 CaCO 3 ) + n(3NaAlSi 3 O 8 -NaCl). Form. Scapolite occurs in rough crystals, in cleavable, columnar, and massive forms. Crystals are tetragonal (tetra- gonal bi pyramidal class), prismatic in habit, and often resemble diopside crystals. The usual forms are: ajlOOj, ra{110}, r{lll}, and {101}. In- terfacial angles : rr(lll :Tll) = 43 45', rar(110:lll) =58 12'; am(100: 110) = 45. Figure 542 represents a typical scapolite crystal. Cleavage, imperfect parallel to {100} and {110}, so in four directions in one zone at angles of 45. H. = 5^. Sp. gr. 2.56 to 2.77. Color, white, gray, greenish, or reddish. Optical Properties. n 7 (1.595 to 1.550) - n a (1.557 to 1.542) = 0.038 to 0.018. Fragments are prismatic with parallel extinction, bright interference colors, and negative elongation. Chemical Composition. An isomorphous mixture of calcium aluminum carbonate-silicate with sodium aluminum chlorid- silicate in varying proportions. Blowpipe Tests. Easily fusible (at 3) to a white glass with intumescence coloring the flame yellow. Partially decomposed by HC1. Some varieties give slight effervescence when acid is hot. Distinguishing Features. Scapolite resembles diopside and the feldspars but is distinguished by its tetragonal crystals and by optical tests. Occurrence. 1. In crystalline limestones at the contacts with igneous rocks and associated with diopside, garnet, and other silicates. FIG. 542. Scapolite. 378 INTRODUCTION TO THE STUDY OF MINERALS 2. In gabbros along the border of apatite veins. Formed from plagioclase by the pneumatolytic process known as scapo- litization. Vesuvianite, Ca 6 Al3(OH,F)(SiO 4 )5 Form. For vesuvianite the characteristic form is striated columnar masses, but crystals belonging to the tetragonal system are also common. 6 = 0.537. The usual forms are w{110}, a{100), c{001}, p{lll}, Z{331[, s{311}. The inter- facial angles are: cp(001 : 111) = 37 13^'; op(100 : 111) = 64 40^'; pp(lll:lTl) = 50 39'; mt(110 : 331) = 23 41J; asClOO : 311) = 35 10'; aw(100 : 110) = 45 0'. The habit is FIG. 543. FIG. 544. FIG. 545. FIGS. 543-546. Vesuvianite. FIG. 546. prismatic or low pyramidal and the cross-section is usually square or octagonal. Figs. 543-546 are drawings of typical vesuvianite crystals. H. = 6J. Sp. gr. 3.4 + . Color, yellow, brown, or green. Optical Properties. n y ( 1.723) - n(1.722) = 0.001. Frag- ments are irregular with anomalous interference colors. The weak double refraction may be proved by means of a gypsum plate. Chemical Composition. Basic calcium aluminum orthosili- cate, probably Ca6Al 3 (OH,F)(SiO 4 )5. Iron replaces part of the aluminum and magnesium, part of the calcium. Boron is found in some varieties. Blowpipe Tests. Fuses (at 3) with intumescence to a colored SILICATES 379 glass. In the closed tube at a high temperature it gives a little water (about 2 per cent.). Slightly decomposed by HC1. After fusion (alone, not with Na 2 CO 3 ) it gelatinizes with HC1. Distinguishing Features. Vesuvianite is distinguished from most other minerals by its crystal form. In massive specimens the weak double refraction with peculiar interference colors is the best test to apply. Uses. Calif ornite, a massive jade-like variety, is used as a semi-precious stone. It occurs in Siskiyou and Fresno counties, California. Occurrence. 1. In crystalline limestones at the contacts with igneous rocks, and associated with garnet, diopside, wollastonite, etc. Crestmore, California. 2. In calcareous inclusions in volcanic rocks. Vesuvius. Zircon, ZrSiO 4 Form. Zircon is practically always found in crystals which are either embedded or occur loose in sands. Zircon is one of the best examples of the ditetragonal bipyram- idal class of the tetragonal system. 6 = 0.640. Usual forms: mfllOj, _a{100), pflll), ^{331}, x{131}. Interfacial angles: pp(lll : 111) = 56 40'; mpfllO : 111) = 47 50'; ap(100 : 111) = 61 40'; mu(UQ : 331) = 20 12'. The habit is low pyramidal or prismatic (Figs. 547 to 550). H. = 1Y 2 . Sp.gr. 4.7+ . Color, usually brown but also red, yellow, and colorless. Lus- ter, adamantine. Optical Properties. w 7 (1.993) - n(1.931) = 0.062. Frag- ments are irregular with fourth-order interference colors. Chemical Composition. Zirconium orthosilicate, ZrSiO4. Blowpipe Tests. Infusible, but loses color. Practically insoluble in acids. Distinguishing Features. Zircon is easily distinguished by its crystal form and high specific gravity. 380 INTRODUCTION TO THE STUDY OF MINERALS Uses. Certain kinds of zircon called hyacinth are used as gems. It is one of the few minerals that rivals the diamond in brilliance and "fire. " Zircon is the source of zirconia (ZrO 2 ) used as a glower in the Nernst lamp. P I P m m FIG. 547. FIG. 548. FIGS. 547-550.- FIG. 549. -Zircon. FIG. 550. Occurrence. 1. As an accessory mineral in igneous rocks, especially the "acid" rocks rich in soda such as syenites and soda granites. El Paso county, Colorado. 2. As a constituent of sands and gravels. Ceylon. Topaz, Al 2 (F,OH) 2 SiO 4 Form. Topaz occurs in well-defined crystals and in cleavable masses. It crystallizes in the bipyramidal class of the ortho- rhombic system, d :b : 6 = 0.528 :: 0.477. Usualforms: m{ 110) , Z(120}, c{001},/{021}, 5 or A1 2 - 3 -SiO 2 . Sillimanite, andalusite, and kyanite all have the same composition, but differ in physical properties. Blowpipe Tests. Infusible. It turns blue when heated with cobalt nitrate solution. Insoluble in acids. Distinguishing Features. The bladed structure and variation of hardness with direction are characteristic. Occurrence. 1. In schists and gneisses as the product of regional metamorphism. It is often associated with staurolite and garnet. Lincoln county, North Carolina. Sillimanite, Al 2 SiO 5 Form. Sillimanite occurs in prismatic and acicular crystals and in fibrous masses. It crystallizes in the orthorhombic system, but distinct crystals are rare. The prism faces with (110 : 110 = 88) are usually the only forms present. Cleavage, perfect in one direction parallel to {010}. H. = 6K- (Fibers may appear to be lower). Sp. gr. 3.23 . Color, brown, gray, or white. Optical Properties. w T (1.681) - n tt U.660) = 0.021. Frag- ments are prismatic or acicular with parallel extinction and posi- tive elongation. The interference colors are bright. Chemical Composition. Aluminum silicate, Al 2 SiO5 or A1 2 - (VSiO 2 , with the same composition as andalusite and kyanite. 384 INTRODUCTION TO THE STUDY OF MINERALS Blowpipe Tests. Infusible. It turns blue when heated with cobalt nitrate solution. Insoluble in acids. Distinguishing Features. Sillimanite may be distinguished from similar minerals by optical tests. Uses. Jade-like varieties were used for implements by prehistoric man in Europe. The artificial mineral is now a commercial product. Occurrence. 1. In gneisses and schists, as the result of meta- morphism. Norwich, Connecticut. EPIDOTE GROUP MONOCLINIC The epidote group is a group of basic orthosilicates of calcium and aluminum, with essentially the general formula (Ca,Fe")2- (Al,Fe' / ',Mn /// ,Ce,Cr // 3 (OH)^SiO 4 )3. The only pure compound is the mineral clinozoisite (fouqueite) with the formula Ca 2 - Al 3 (OH)(SiO4)3. Epidote is an isomorphous mixture of this compound with Ca 2 Fe 3 ( OH) (8104)3, a possible mineral yet to be discovered. Piedmontite is a manganiferous epidote and allanite, a cerium-bearing epidote. EPIDOTE, Cas(Al,Fe) 8 (OH)(SiO 4 )3 Form. In crystals, in columnar aggregates, and in granular masses. The crystals are monoclinic, prismatic in habit, and are elongated in the direction of the 6-axis instead of the c-axis, as in most monoclinic minerals. Usual forms: a {100), b {010}, cjOOlj, w{l20), rjTOl}, rcjlll}. Interfacial angles: ac(100:001) = 64 37'; i Sp.gr. 2.9 + . Color, colorless, white, pale green. Optical Properties. w 7 ( 1.670) - n a (1.626) = 0.044. Fragments are irregular with bright interference colors. Chemical Composition. Basic calcium boron orthosilicate, CaB(OH)SiO 4 ; (H 2 O = 5.6 per cent.). Blowpipe Tests. Easily fusible (at 2) with intumescence to a glass, coloring the flame green. In the closed tube it gives water. Gelatinizes with HC1. Distinguishing Features. Datolite is usually recognized by the complex crystals without cleavage and by its occurrence in cavities of igneous rocks with the zeolites. Occurrence. 1. Found in cavities of diabases and basalts associated with the zeolites, apophyllite, prehnite, pectolite [HNaCa 8 (SiO*)a], etc. Axinite, HCa 2 (Fe,Mn)Al 2 B(SiO 4 )4 Form. Axinite occurs in crystals and crystalline aggregates. The crystals furnish one of the best examples of the triclinic FIG. 563. Datolite. 392 INTRODUCTION TO THE STUDY OF MINERALS system. The habit is usually tabular and the cross-sections wedge-shaped. Figure 564 represents a typical crystal. H. = 6>^ to 7. Sp. gr. 3.3 . Color, violet, brown, smoky gray. Optical Properties. n 7 (1.68) - w a (1.67) = 0.01. Fragments are irregular with upper first-order interference colors. Thick fragments are pleochroic. Chemical Composition. An acid calcium iron-manganese alu- minum boron silicate, HCa 2 (Fe,Mn)Al 2 B(Si0 4 )4. Blowpipe Tests. Easily fusible (at 2^) with intumescence to a dark glass. In the closed tube at a high temperature it gives a little water. When fused with KHS0 4 and CaF2 it gives a green flame. Insoluble in HC1. Gelatinizes after fusion. Distinguishing Features. The acute- angled triclinic crystals are characteristic. Occurrence. 1. At the contact of granites with basic lime-rich rocks such as schists and impure limestones. FIG. 564. Axinite. MICA GROUP The micas are acid orthosilicates of aluminum with magnesium, ferrous iron, and the alkalies. When heated to a high tempera- ture they give from 2 to 5 per cent, of water. The micas are monoclinic, but pseudohexagonal in habit. Dis- tinct terminated crystals are very rare. The very perfect cleav- age parallel to {001} is the most striking feature of the micas. They are optically biaxial with varying axial angle. In the absence of crystal faces, cleavage plates of the micas may be ori- ented by means of the percussion figure in connection with the interference figure. A sharp, quick blow with a dull conical point develops a six-rayed star. If the interference figure lies along one of the rays then that ray is the direction of the 6-axis. SILICATES 393 These are micas of the first class and are represented by Fig. 565. If the interference figure lies between two rays of the percussion figure, then the third ray is the direction of the 6-axis. These are micas of the second class and are represented by Fig. 566. Muscovite and lepidolite are micas of the first class, while biotite and phlogopite are micas of the second class. FIG. 565. Mica of the first class. FIG. 566. Mica of the second class. MUSCOVITE, H 2 KAl 3 (SiO 4 )3 Form. Muscovite usually occurs in cleavages and scaly masses and but rarely in well-defined crystals. The crystals are tabular in habit, and pseudohexagonal or pseudorhombic, but are really monoclinic. Figure 567 represents a muscovite crystal in plan and side elevation. The side elevation proves it to be monoclinic. Cleavage, very perfect in one direction parallel to {001}. H. = 2^. Sp.gr. 2.83 . Color, in thick sheets various tints of green, yellow, brown, and red. Thin sheets are colorless and transparent. Optical Properties. n 7 (1.597) - w a (1.560) = 0.037. Cleav- age flakes give low first-order interference colors (n$ n a = .004) and in convergent light, a fine negative biaxial interference figure with large axial angle (2E = 60 to 75). a = 0; 6 = 7; c:a = 1. Chemical Composition. An acid potassium aluminum ortho- silicate, H 2 KAl 3 (Si04) 3 ; (H 2 O = 4.5 per cent.). The potassium is partially replaced by sodium. Some varieties contain an excess of silica over that required by the formula. 394 INTRODUCTION TO THE STUDY OF MINERALS FIG. 567. Muscovite. Blowpipe Tests. Fusible on thin edges (at 5) and whitens. In the closed tube it gives a little water. Insoluble in acids and not decomposed by hot concentrated H 2 S0 4 . Distinguishing Features. Muscovite is easily recognized by its elastic cleavage, and flakes with perfect cleavage in one direction. Pink muscovite is distinguished from lepidolite by the easy fusibility of the latter. Uses. Muscovite is used princi- pally as an insulator for electrical apparatus, but has numerous other uses. India is the principal pro- ducer of muscovite. Occurrence. 1. In granite peg- matites and granite aplites. 2. In schists and gneisses, often the main constituent of the mica schists. 3. In granites. Granite is about the only igneous rock in which muscovite occurs as an original constituent. 4. In sandstones and sands as a detrital mineral. Sericite, H 2 KAl 3 (SiO 4 )3 Form. Sericite is probably dimorphous with muscovite. It occurs in scaly masses and very rarely is distinct crystals which are like muscovite in form and habit but very minute. Cleavage, perhaps in one direction. H. = 1J to 2. Sp. gr. 2.8 . Color, white, gray, or pale green. Optical Properties. The optical constants are near those for muscovite, rc 7 (1.597) n a (1.560) = 0.037. Fragments are cleavage flakes or shreds with low first-order interference colors. The shreds show change of relief when examined in clove oil under the microscope in polarized light (use lower Nicol only). Chemical Composition. Acid potassium aluminum orthosili- cate; formula probably the same as muscovite, SILICATES 395 (H 2 = 4.5 per cent.). The potassium is partially replaced by sodium. Blowpipe Tests. Fusible on thin edges (at 5). In the closed tube it yields water. Insoluble in acids. Distinguishing Features. Sericite is recognized by its pecu- liar silky luster. It is usually called talc by miners. Uses. It is a possible source of potassium salts. Extensive deposits of sericite schists are found in Georgia. Occurrence. 1. As a product of hydro thermal alteration in ore deposits. It is, according to the researches of the author, a comparatively late mineral formed after the hypogene sulfids. 2. In schists and gneisses forming the sericite schists and sericite gneisses. Lepidolite, LiKAl 2 (OH,F)(SiO 3 ) 3 Form. Lepidolite usually occurs in scaly masses, rarely in crystals. Crystals are pseudohexagonal like those of muscovite, but are much smaller. Cleavage, very perfect in one direction (parallel to 001). H. = 2^ to 3>i Sp. gr. 2.84 . Color, pale to deep lilac. Optical Properties. n Y (1.605) - n(1.560) = 0.045. Cleav- age flakes give low first-order interference colors, and in conver- gent light a negative, biaxial interference figure with large axial angle (2E = 60 to 80). Chemical . Composition. Lithium, potassium aluminum fluo- silicate, LiKAl 2 ^OH,F)(Si0 3 ) 3 . Blowpipe Tests. Easily fusible (at 2) with intumescence to a white glass, coloring the flame purple. In the closed tube on intense ignition it gives water which has an acid reaction due to the HF formed. Partially decomposed by HC1. After fusion (alone, not with Na 2 CO 3 ) it gelatinizes with HC1. 396 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Lepidolite is usually recognized by its lilac color and micaceous character. It is distinguished from the other micas by its easy fusibility and by the lithium flame. Uses. Lepidolite is a source of lithium salts. Occurrence. 1. In granite pegmatites and surrounding gran- ites associated with tourmaline, albite, muscovite, spodumene, amblygonite, etc. Pala, San Diego county, California. BIOTITE, (H,K) 2 (Mg,Fe)Al 2 (SiO 4 ) 3 Form. Biotite occurs in embedded crystals, in disseminated scales, and in lamellar masses. Crystals are pseudohexagonal like those of muscovite. Cleavage, very perfect in one direction (parallel to 001). H. = 2H to 3. Sp. gr. 2.90 . Color, black or dark brown. Thin sheets are translucent. Optical Properties. ^(1.638) - w a (1.579) = 0.044. Cleavage flakes are almost dark between crossed nicols and in convergent light give a negative biaxial interference figure with a small axial angle (2E = 0-20) which is sometimes practically uniaxial. Chemical Composition. Acid potassium magnesium and iron aluminum orthoclase, (H,K)2(Mg,Fe) 2 Al2(Si0 4 )3. Ferric iron re- places part of the aluminum. Blowpipe Tests. Fusible on edges (at 5) and turns white. In the closed tube it gives a little water (2 to 4 per cent.) on intense ignition. Decomposed by hot concentrated H 2 SO4. Distinguishing Features. Biotite is distinguished from the other micas by its dark color. Occurrence. 1. In many kinds of igneous rocks, but espe- cially prominent in granites, and also in certain dike rocks known as lamprophyres. 2. In schists and gneisses, sometimes as the dominant mineral, and often associated with muscovite. 3. In high-temperature veins. SILICATES 397 Phlogopite, H 2 KMg3Al(SiO 4 )3 Form. Like the other micas, phlogopite occurs in crystals, in disseminated scales, and in lamellar masses. The crystals are pseudohexagonal, but are often prismatic in habit as well as tabular. Cleavage, very perfect in one direction. H. = 2K to 3. Sp. gr. 2.80 . Color, varies from pale brown to dark brown. Optical Properties. w 7 (1.606) -- n a (1.562) = 0.044. Cleav- age flakes give very low first-order interference colors and, in con- vergent light, a negative biaxial interference figure with a small axial angle (2E - 5 to 20). Chemical Composition. Acid potassium magnesium aluminum orthosilicate, H 2 KMg 3 Al(Si0 4 )3. It also contains iron and fluorin. Blowpipe Tests. Fusible (at 5) on thin edges and whitens. In the closed tube it gives a little water on intense ignition. Easily decomposed by concentrated H 2 S04. Distinguishing Features. Phlogopite is darker than musco- vite and lighter colored than biotite. Its occurrence in crystal- line limestone is characteristic. Uses. Phlogopite is used principally as an insulator in elec- trical apparatus. Canada is the only large producer of phologo- pite. Occurrence. 1, In crystalline limestones associated with spinel, graphite, chondrodite, etc. 2. In veins with apatite, calcite, and diopside. CHLORITE, 1 H 8 (Mg,Fe) 5 Al 2 (SiO 6 )3 Form. Chlorite crystals are monoclinic but pseudohexagonal in habit and resemble crystals of the micas. The mineral also occurs in disseminated flakes and in scaly masses. 1 Chlorite is really the name of a group of minerals, but on account of the difficulty of distinguishing them they are all included under one heading. 398 INTRODUCTION TO THE STUDY OF MINERALS Cleavage, perfect in one direction. H. = 2 to 2>^. Sp. gr. 2.8 . Color, green of various tints and shades, varying from almost white to almost black. Cleavage laminae are flexible, but not elastic. Optical Properties. W T (1.596) - w(l.585) = 0.011. Frag- ments are irregular, green in color with faint pleochroism, and very low (often Berlin blue) first-order interference colors. Cleavage flakes in convergent light give an interference figure which is either uniaxial or biaxial with a small axial angle (2E = 0-60). Chemical Composition. Acid magnesium and iron aluminum silicate. The composition varies for different chlorites; for one of them (clinochlore) the formula is H 8 (Mg,Fe) 5 Al 2 (Si0 6 )3. In some varieties chromium and ferric iron partly replace the aluminum. Blowpipe Tests. Fusible with difficulty (at 5>). In the closed tube gives about 12 per cent, of water at a high temperature. Decomposed by H 2 S04. Distinguishing Features. Chlorite is characterized by its perfect cleavage, absence of elasticity in the cleavage flakes, and green color. Occurrence. 1. A secondary mineral in igneous rocks, formed by the alteration of such silicates as biotite, hornblende, augite, etc. 2. In schists, often forming independent rock masses, the chlorite schists. These are formed from original basic igneous rocks. ANTIGORITE, H 4 Mg3Si 2 O9 Form. Antigorite has never been found in crystals, though it often occurs pseudomorphous after other crystallized minerals. It is usually compact or granular massive, but also occurs in fibrous, columnar, and lamellar forms. It is the main constituent SILICATES 399 of serpentine, which is properly considered a rock rather than a mineral. H. = 3 to 4. Sp. gr. 2.5 . Color, green of various tints and shades, from greenish-white to greenish-black. It is also often yellow, brown, or red, and the color is not apt to be uniform, but is often in spots and streaks. Optical Properties. n Y (1.571) - n a (1.560) = 0.011. Frag- ments are irregular, or prismatic with parallel extinction and positive elongation. The interference colors are low first-order. The irregular fragments show aggregate structure between crossed nicols. Chemical Composition. Acid magnesium silicate, H4Mg 3 Si 2 O 9 ; (H 2 O = 12.9 per cent.). Part of the magnesium is usually replaced by ferrous iron. Some analyses show a little aluminum and some a little calcium. These constituents are largely due to residual pyroxenes. Analyses of Antigorite MgO FeO CaO A1 2 8 Fe 2 O 3 SiO 2 H 2 O Misc. 42.6 0.1 0.1 0.3 42.0 14.7 36.5 1.9 5.1 42.9 13.2 NiO = 0.6 41.2 2.4 41.3 14.5 36.8 7.2 2.6 41.6 12.7 Cr 2 O 3 = tr. Blowpipe Tests. In the closed tube it gives water at a high temperature. Decomposed by HC1 with the separation of non-gelatinous silica. Distinguishing Features. Antigorite is distinguished by its green color, moderate hardness (4), and its mottled, veined, or compact massive structure. Uses. Serpentine is used as an ornamental stone. Occurrence. 1. An alteration product of olivine and to a less 400 INTRODUCTION TO THE STUDY OF MINERALS extent of bronzite, forming the metamorphic rock serpentine from original peridotite. 2. A secondary mineral in seams and veins and on the border of serpentine rocks. 3. An alteration product of diopside and forsterite in crystalline limestones; these form the rocks known as ophicalcites. Chrysotile, H 4 Mg3Si 2 O 9 Form. Chrysotile occurs in seams and always has a fibrous structure. H. = 2J. Sp. gr. 2.2 + . Color, green to golden yellow. Optical Properties. w T (1.55) n a (1.54) = 0.01. Fragments are needles with parallel extinction and positive elongation. Chemical Composition. Chrysotile is dimorphous with anti- gorite and has the same chemical composition, H4Mg 3 Si 2 O 9 . (H 2 O= 12.9 pfcr cent.). Ferrous iron may replace part of the magnesium. Blowpipe Tests. Fusible (at 6) in minute splinters. Decomposed by HC1 with the separation of fibers of silica. Distinguishing Features. Chrysotile is distinguished from antigorite by its fibrous structure and by the difference in optical properties. From other fibrous minerals it is usually distin- guished by its association in serpentine rocks. The parallel extinction and high water content distinguish it from tremolite asbestos. Uses. Chrysotile is one of the minerals included under the term abstestos, which is so extensively used as a refractory. Occurrence. 1. In seams and veins in serpentine rocks, associated with antigorite. Thetford, Ontario. TALC, H 2 Mg 3 (SiO 3 ) 4 Form. Talc is found in scales, in foliated, compact, or fibrous masses. Distinct crystals are exceedingly rare. SILICATES 401 "*' " v Cleavage, perfect in one direction. H. = usually 1, but sometimes 4. Sp. gr. 2.7 + . Color, white, gray, or pale green. Luster, pearly. Optical Properties. w 7 (1.59) - n(1.54) = 0.05. Cleavage flakes give a negative biaxial interference figure with a small axial angle (2i! = 10-20). ' Chemical Composition. Acid magnesium metasilicate, H 2 - Mg 3 (SiO 3 ) 4 ; (H 2 O= 4.8 per cent.). It usually contains iron and aluminum in small quantities. Blowpipe Tests. Fusible (at 5J^) on thin edges. In the closed tube gives water on intense ignition. Heated intensely with cobalt nitrate solution, white varieties give a faint pink color. Not decomposed by acids. Distinguishing Features. Talc is characterized by its soapy feel. It can be distinguished from pyrophyllite by blowpipe or chemical tests. Uses. Talc is used for soap, French chalk, talcum powder, and in the form of soapstone as a refractory material. A fibrous variety (agalite) is used in the manufacture of paper. The United States is the principal producer of talc and soapstone. In this country New York, Virginia, and Vermont lead. Occurrence. 1. A secondary mineral occurring as an altera- tion product of various silicates such as antigorite, enstatite, and tremolite (including actinolite). 2. In schists often forming the rock masses known as talc schists and soapstones. Pyrophyllite, HAl(SiO 3 ) 2 Form. In radiated forms and compact masses, but not in distinct crystals. H. = \y^. Sp. gr. 2.8 . Color, white, yellow, gray, brown. Luster, pearly. Optical Properties. w 7 |1.59) - n(1.57) = 0.02. Fragments 26 402 INTRODUCTION TO THE STUDY OF MINERALS are prismatic with parallel extinction and positive elongation. The interference colors are low first-order. Chemical Composition. Acid aluminum metasilicate, HA1- (SiO 3 ) 2 ; (H 2 O = 5.0 per cent.). Blowpipe Tests. Infusible, often exfoliates. In the closed tube it gives water on intense ignition. Heated with cobalt nitrate solution it becomes deep blue. Partially decomposed by H 2 SO4. Distinguishing Features. Pyrophyllite is so much like talc in appearance that it is usually necessary to make the cobalt nitrate test for aluminum in order to prove its identity. Occurrence. 1. In schistose metamorphic rocks. 2. In igneous rocks as a product of hydrotherrnal alteration. Chondrodite, Mg 5 (F,OH) 2 (SiO 4 )2 Form. Chondrodite occurs in disseminated crystals and grains, but is also sometimes massive. Crystals are monoclinic, equidimensional, and rather complex. An interesting feature of the crystals is that the angle (the angle between the a- and c- axes) is 90. H. = 6 to 6K- Sp. gr. 3.20 . Color, red, orange, yellow, brown. Optical Properties. n Y (1.639) - n a (1.607) = 0.032. Frag- ments are irregular and colorless, or yellow with slight pleo- chroism. The interference colors are bright. Chemical Composition. Basic magnesium orthosilicate, Mg 5 - (F,OH) 2 (SiO 4 ) 2 or Mg(F,OH) 2 -2Mg 2 SiO 4 . Iron replaces part of the magnesium and hydroxyl part of the fluorin. Blowpipe Tests. Infusible. In the closed tube it gives a little water (about 1.3 per cent.) at a high temperature. In the closed tube with NaPO 3 it etches the inside of the tube. Gelatinizes with HC1. Distinguishing Features. Chondrodite may often be recog- nized by its characteristic occurrence in crystalline limestones. SILICATES 403 From garnet it may be distinguished by its low specific gravity and by optical tests. Occurrence. 1. In crystalline limestones with phlogopite, spinel, etc. Tilly Foster mine, Brewster, N. Y. Kaolinite, H 4 Al 2 Si 2 O 9 Form. Kaolinite is sometimes found in minute pseudo-hexa- gonal (monoclinic) crystals of tabular habit. Figure 568 repre- sents crystals found by the author at Argentine, Kansas, in a dolomitic limestone. The usual occurrence of kaolinite is in clay-like masses. H. = 2to2>i Sp.gr. 2.6+. Color, white, grayish, yellowish, etc. Luster, pearly to dull. Optical Properties. w y ( 1.567) - n a (1.561) = 0.006. Fragments are irregu- lar and show aggregate structure beween crossed nicols. Chemical Composition. Acid aluminum silicate, H4Al 2 Si 2 O 9 ; (H 2 O = 14.0 per cent.). Iron is often present in small amounts and thus it grades into nontronite (H4Fe 2 Si 2 9 ). Blowpipe Tests. Infusible if pure. Heated with cobalt nitrate solution it becomes deep blue. In the closed tube it gives water. Insoluble in acids. Distinguishing Features. Soft scaly masses of minute crystals with pearly luster are characteristic of kaolinite. It resembles halloysite, sericite, and alunite and as a rule can only be dis- tinguished by optical tests. Uses. Kaolin, a mixture of kaolinite and other aluminum silicates with more or less quartz, feldspar, etc., is used in the manufacture of porcelain, china, and pottery. Occurrence. 1. A secondary mineral formed from the feld- spars and other silicates, probably by the action of meteoric water. FIG. 568. Kaolinite crystals (x 500). 404 INTRODUCTION TO THE STUDY OF MINERALS Halloysite, H 4 AlSi 2 O 9 (H 2 O) a! Form. Halloysite is the amorphous equivalent of kaolinite. It occurs in a massive form and occasionally shows colloform structure in cavities. H. = 1 to 2. Sp. gr. 2.2 . Color, white, gray, yellowish, reddish, etc. Optical Properties, n = 1.55 + . Fragments are irregular and dark between crossed nicols. Chemical Composition. Acid aluminum silicate with a variable amount of adsorbed (or dissolved) water. H 4 Al 2 Si 2 - O 9 (H 2 O). (Total water = 15.0 to 25.0 per cent.). Blowpipe Tests. Infusible if pure. It turns blue when ignited with cobalt nitrate solution. Yields abundant water in the closed tube. Decomposed by HC1. Distinguishing Features. Halloysite is distinguished from kaolinite by optical tests and by its higher water content. Uses. Halloysite is the principal constituent of some clays. Occurrence. 1. In sedimentary beds. Lawrence county, Indiana. 2. In veins in decomposed igneous rocks. Stone Mountain, Georgia. Garnierite, H 2 (Ni,Mg)SiO 4 H 2 O Form. Garnierite occurs in earthy masses and has never been found in distinct crystals, though the polarizing microscope proves it to be crystalline. H. = 2 to 3. Sp.gr. 2.5 . Color, bright green to pale green. Optical Properties, n about 1.59. Fragments are irregular, greenish in color, and show aggregate structure in polarized light. Chemical Composition. A hydrous acid nickel and magnesium orthosilicate, H 2 (Ni,Mg)SiO 4 'H 2 O. (Ni = 10 to 35 per cent.). SILICATES 405 Blowpipe Tests. Infusible. Heated on charcoal it becomes magnetic. In the closed tube it blackens and yields water. The borax bead is violet when hot. Partially decomposed by HC1. Distinguishing Features. Garnierite is usually distinguished by its apple green color and earthy appearance. Uses. Next to pentlandite, garnierite is the chief ore of nickel. The French colony of New Caledonia is the only important locality. Occurrence. 1. A secondary mineral associated with serpen- tinized peridotites (it is probably an alteration product of nickel- bearing olivine). Riddles, Oregon. CHRYSOCOLLA, CuSiO 3 2H 2 O Form. 'Chrysocolla is a cryptocrystalline mineral occurring in seams and in incrustations which sometimes have a colloform surface. Its amorphous equivalent is a mineral called cornuite. H. = 2 to 4. Sp. gr. 2.1 . Color, bluish-green or greenish-blue. Optical Properties. n T (1.57) - n a (1.46) = 0.11. Fragments are irregular, and usually show aggregate structure in polarized light. Chemical Composition. Hydrous copper metasilicate, CuSiO 3 . 2H 2 O; Cu = 36.1 per cent., H 2 O = 20.5 per cent.). Blowpipe Tests. Infusible. Colors the flame green. In the closed tube it blackens and gives water. Decomposed by HC1 without gelatinization. Distinguishing Features. Chrysocolla resembles turquois but is distinguished from it by its inferior hardness. Uses. Chrysocolla is one of the so-called oxidized ores of copper. Occurrence. 1. A secondary mineral often associated with malachite, azurite, and cuprite, and usually found in the upper workings of mines. Gila county, Arizona. 406 INTRODUCTION TO THE STUDY OF MINERALS Glauconite, KFe //r (SiO 3 )2 (H 2 O) X Form. Glauconite occurs in disseminated grains or in loosely cemented, sandy deposits called "greensand. " The grains often have the form of foraminiferal shells. H. = 2. Sp. gr. 2.3 . Color, green to dark green. Optical Properties. n y (1.628) - n a (1.610) = 0.018 Frag- ments are green with aggregate polarization. Chemical Composition. Potassium ferric silicate with variable amounts of water; probable formula, KFe /// (Si03)2'(H 2 O)x;(K 2 O = 5 to 8 per cent.). Aluminum replaces part of the iron. Sod- ium, ferrous iron, and magnesium are usually present, probably as a replacement of the potassium. Blowpipe Tests. Easily fusible before the blowpipe to a black magnetic glass. In the closed tube yields water (6 to 10 per cent.). Practically insoluble in HC1. Distinguishing Features. The little rounded green pellets are characteristic. It resembles some varieties of chlorite but has higher double refraction. Uses. As a fertilizer. On account of the high potash con- tent it may in the future be used for potassium salts. Occurrence. 1. In sandstones, sands, clays, and limestones. It is marine in origin and is forming in the ocean at a depth of about 100 fathoms. Beds of "greensand " occur in the Cretaceons of New Jersey. Apophyllite, (H,K) 2 Ca(SiO 3 ) 2 H 2 O Form. Usually occurs in distinct crystals in cavities and along seams. Apophyllite crystallizes in tetragonal crystals of varying habit. Usual forms: a{100(, 2/{310}, p{lll}, c{001|. Interfacial angles: cp(001 : 111) = 60 32'; ap(100 : 111) = 52 0'; pp(lll : ill) = 76 0'; ay(10Q :310)= 18 26'. (Figs. 569 572). Cleavage, perfect in one direction parallel to { 001} . H. = 4>ito5. Sp. gr. 2.3 . SILICATES 407 Color, colorless or white. Luster of (001) face, pearly; of other faces, vitreous. Optical Properties. n 7 (1.535) - n a (1.533) = 0.002. Frag- ments are square or rectangular, and are either dark between crossed nicols or have low first-order interference colors. Cleav- age flakes give a positive uniaxial interference figure in conver- gent light. Chemical Composition. A hydrous acid calcium metasilicate, (H,K) 2 Ca(SiO3)2'H 2 O. A little potassium replaces part of the hydrogen and some analyses show a little fluorin. FIG. 569. FIG. 570. FIG. 571. FIGS. 569-572. Apophyllite. FIG. 572. Blowpipe Tests. Easily fusible (at 2) with exfoliation to a white enamel. In the closed tube it yields water (about 16 per cent.). Decomposed by HC1 with the separation of non-gelatinous silica. Distinguishing Features. The tetragonal crystals with per- fect basal cleavage are highly characteristic of apophyllite. Occurrence 1. In cavities of basic igneous rocks associated with zeolites. West Paterson, New Jersey. ZEOLITE GROUP Under the zeolites are included a number of well crystallized hydrous silicates of aluminum with calcium and the alkalies, 408 INTRODUCTION TO THE STUDY OF MINERALS which are chemically similar to the feldspars except for the water of crystallization. They are characterized by low specific gravity (2 to 2.5) and moderate hardness (3 to 5J^). They are all decomposed by HC1 with the separation of slimy or gelatinous silica and are easily fusible (at 2 to 3) with intumes- cence, hence the name zeolite from the Greek word meaning to boil. The zeolites are usually found as secondary minerals in cavities of such basic igneous rocks as basalts and diabases. Table Mt. at Golden, Colorado, and Bergen Hill, New Jersey, are promi- nent localities for zeolites. Heulandite, H 4 CaAl 2 (SiO 3 )6 3H 2 O Form. Heulandite crystallizes in the monoclinic system. Usual forms :b{ 010}, c{001(, {201}, s{201},ra{ 110]_. Angles: ct- <. (001:201) = 63 40'; cs(001:201) = 66 0'; 6w(010:110) = 68 2'. The habit is usu- ally thick tabular parallel to {010}. The unsymmetrical outline of Fig. 573 is characteristic. Cleavage, perfect in one direction parallel to {010}. H. = 3>^ to 4. Sp. gr. 2.2 . Color, colorless, white, pale brown, reddish. Luster pearly on the (010) face. Optical Properties. n 7 (1.505) - n a (1.498) = 0.007. Fragments are plates with low first-order interference colors. Cleavage flakes give a positive biaxial inter- ference figure in convergent light. Chemical Composition. Hydrous acid calcium metasilicate, H 4 CaAl 2 (SiO 3 ) 6 -3H 2 O; (H 2 = 14.8 per cent.). The calcium is usually partly replaced by small amounts of sodium, potassium, and strontium. Brewsterite is a similar isomorphous mineral with the calcium largely replaced by strontium and barium. Blowpipe Tests. Easily fusible (at 3) with exfoliation to a white enamel. In the closed tube it gives water. SILICATES 409 Decomposed by HC1 with the separation of non-gelatinous silica. Distinguishing Features. The monoclinic crystals with pearly luster and unsymmetrical outlines are characteristic. Occurrence. 1. A secondary mineral in cavities of basic igneous rocks, associated with. other zeolites. Stilbite,H 4 rCa,Na 2 )Al 2 (Si0 3 )6-4H 2 Form. Stilbite usually occurs in indistinct crystals or in sheaf-like aggregates. Crystals are monoclinic but are pseudo- orthorhombic by twinning. The symmetrical outline of Fig. 574 is typical of stilbite. Cleavage, in one direction fairly good. H. = 3Mto4. Sp. gr. 2.1 . Color, white, yellow, brown. Optical Properties. n y ( 1.500) - n tt (1.494) = 0.006. Fragments are prismatic with parallel extinction and negative elongation. The interference colors are upper first-order. Chemical Composition. Hydrous acid calcium and sodium aluminum metasilicate, H4(Ca,Na 2 )Al 2 (SiO 3 )6-4H 2 O; (H 2 = 17.2 FIG. 574. stilbite. per cent, if Ca:Na = 6:1). Blowpipe Tests. Easily fusible (at 3) with exfoliation to a white enamel. In the closed tube it yields water. Decomposed by HC1 with the separation of non-gelatinous silica. Distinguishing Features. The symmetrical outline and sheaf- like grouping of the crystals is characteristic. The cleavage is not as perfect as that of heulandite. Occurrence. 1. A secondary mineral in cavities and seams of igneous rocks, especially basalts and diabases. 2. In miarolitic cavities of granites and pegmatites as the last mineral to be formed. 410 INTRODUCTION TO THE STUDY OF MINERALS Chabazite, (Ca,Na 2 )Al 2 (SiO 3 )6-6H 2 O Form. Chabazite practically always occurs in distinct cube- like rhombohedral crystals (10Tl:Il01) = 85 14' (Fig. 575). Penetration twins with 6 as the twin-axis are common. H. = 4^. Sp. gr. 2.1 . Color, white, colorless, pink, red. Optical Properties. n T Q.488) - n a (1.485) = 0.003. Frag- ments are nearly square rhombs or are irregular. The inter- ference colors are low first-order. Chemical Composition. Hydrous calcium-sodium aluminum metasilicate, (Ca,Na 2 )Al 2 (SiO 3 )4-6H 2 O. A little po- tassium is usually present. Blowpipe Tests. Fuses (at 3) with in- tumescence to a white glass. In the closed tube it yields water (about 21 per cent.). Decomposed by HC1 with the separation of non-gelatinous silica. FIG. 575. Chabazite. Distinguishing Features. The cube- like rhombohedral crystals are charac- teristic. It is distinguished from calcite by the absence of perfect cleavage as well as by optical tests. Occurrence. 1. A secondary mineral in cavities and seams of igneous rocks associated with the other zeolites. Analcite, NaAl(SiO 3 ) 2 H 2 O Form. Analcite occurs in attached crystals or in druses lining cavities and seams. It is isometric in crystallization; the only common form is the trapezohedron, {211}, the same form that is common on garnet (Fig. 576). H. = 5to5H- Sp.gr. 2.25 . Color, colorless or white. Optical Properties, n = 1.487. Isotropic. Fragments are irregular and dark between crossed nicols. SILICATES 411 FIG. 576. Analcite. Chemical Composition. Hydrous sodium aluminum metasili- cate, NaAl(SiO) 3 ) 2 -H 2 O; (H 2 O = 8.2 per cent.). Blowpipe Tests. Fusible at 3^ to a colorless glass. In the closed tube it yields water. Decomposed by HC1 with the separation of gelatinous silica. Distinguishing Features. Analcite is similar in form to leucite and garnet; from these it is distinguished by its mode of occurrence. Occurrence. 1. As secondary mineral in seams and cavities of basic igneous rocks. 2. As Jate magmatic mineral in certain diabases and basalts. Natrolite, Na 2 Al 2 Si 3 Oi -2H 2 O Form. This mineral occurs in divergent crystal groups or in fibrous masses. Crystals are orthorhombic but apparently tetra- gonal (110 : HO = 88 45'). The habit is long pris- matic or acicular, terminated by the low bipyramid {111}. Figure 577 represents a typical natrolite crystal. The presence of the side pinacoid b proves it to be orthorhombic. H. = 5. Sp.gr. 2.25 . Color, colorless or white. Optical Properties. n T (1.488)-n(1.475) = 0.013. Fragments are prismatic or acicular with parallel ex- tinction, positive elongation, and bright interference colors. Chemical Composition. Hydrous sodium alu- minum silicate, Na 2 Al 2 Si 3 Oio'2H 2 0; (H 2 O = 9.5 per cent.). Blowpipe Tests. Easily fusible (at 2J) to a color- less glass giving a yellow flame. In the closed tube it yields water. Decomposed by HC1 and on evaporation the solution gelatinizes. FIG. 577. Natrolite. 412 INTRODUCTION TO THE STUDY OF MINERALS Distinguishing Features. Natrolite is distinguished from the other zeolites and from aragonite by the square cross-sec- tion of the crystals. Occurrence. 1. A secondary mineral occurring in cavities of basalts and diabases. Titanite, CaTiSiO 5 Form. Titanite or sphene occurs in attached crystals, and in disseminated crystals and grains. Crystals are monoclinic of varied habit, and are usually acute rhombic in cross-section. The envelope-shaped form of Fig. 578 is typical. Usual forms: c{001}^ rafllO}, rzjlll}. Interfacial angles: mm Q10 :IlO) = 66 29'; nn (111: ill) = 43 49'; cm (001:110) = 65 30'; en (001:111) = 38 16'. Cleavage. There is sometimes prominent parting in two directions at angles of 54. H. = 5 to 5K- Sp. gr. 3.45 . Color, varying tints and shades of yellow and brown. Luster, adamantine or subadamantine. FIG 578 Ptical Properties. n T ^2.00) - n(1.88) = 0.12. Titanite. Fragments are irregular and slightly pleochroic with very high-order interference colors. Chemical Composition. Calcium titano-silicate, CaTiSiO 5 , a salt of H 2 Si 2 O5 in which one atom of silicon is replaced by one of titanium. Iron is usually present in small amounts. Blowpipe Tests. Fusible (at 4) to a colored glass. It gives a violet NaPO 3 bead in R.F. Partially soluble in HC1. Distinguishing Features. Titanite is often distinguished by the acute-angled crystals which are envelope-shaped. Occurrence. 1. A common and widely distributed accessory constituent of plutonic igneous rocks (granites, granodiorites, dioriteSj syenites). 2. In clefts and seams or disseminated through metamorphic rocks, probably often formed from titaniferous pyroxenes. SILICATES 413 MINERALOIDS A mineral may be defined as a naturally occurring, homo- geneous, inorganic substance of definite or fairly definite chemical composition. Now there are some homogeneous substances found in the earth's outer shell which do not fulfill the conditions of the above definition, yet they deserve the attention of the student of mineralogy. Among these substances are the glasses and the hydrocarbons. The glasses are inorganic but are too indefinite in chemical composition to" be considered minerals, while the hydrocarbons, though occasionally of de- finite composition, are organic. These two classes of substances may be treated in an appendix under the term mineraloid. Glass Form. Amorphous, usually massive and structureless, but sometimes vesicular, spheroidal (perlitic), or banded. H. = 5 to 7. Sp. gr. = 2.2 to 2.7. Color, colorless, gray, black; sometimes green, brown, or red. Translucent to transparent on thin edges. Luster. Vitreous to resinous, dull if devitrified. Optical Properties. n= 1.48 to 1.67 (increases in general with decrease of SiO 2 ). Usually isotropic but may be doubly refract- ing due to strain, especially the perlitic varieties. Chemical Composition. Variable, contains SiO 2 , A1 2 O 3 , Fe 2 O 3 , FeO, MgO, CaO, Na 2 0, and K 2 in amounts comparable to Table of Analyses of Natural Glass 02 q < 6 A 3 fe % S 3 o q a fc q C? O w Misc. Rhyolitic obsidian 75.8 12.4 0.2 1.3 0.3 0.8 4.0 4.6 0.4 Rhyolitic pitchstone 71.6 13.1 0.7 0.3 0.1 0.7 3.8 4.1 5.5 Andesitic perlite 65.1 15.7 2.2 1.9 1.4 3.6 2.9 4.0 2.4 61 2 18 1 3 4 5 4 1 9 6 5 5 9 5 MnO = 0.4 53 8 13 5 3 7 4 6 5 10.3 3.2 0.6 0.6 TiO = 0.2 Basaltic obsidan 50.7 12.0 3.4 8.1 7.3 12.4 2.7 0.2 0.5 Ti0 2 = 1.7 414 INTRODUCTION TO THE STUDY OF MINERALS those found in various types of igneous rocks ranging from the high silica content of rhyolite to the relatively low silica content of basalt. Water is low in obsidian, but comparatively high in pitchstone. Blowpipe Tests. The fusibility of volcanic glass is character- istic. It is fusible to a vesicular enamel, but this enamel on further heating is infusible, which is due to the fact that the water is driven off. In the closed tube it gives more or less water (0.5 to 5. per cent.). Insoluble in acids. Tests for silica and the metals can be obtained by making a sodium carbonate fusion (see p. 49). The alkalies must be determined in a separate sample by fusion with NH 4 C1 and CaCO 3 . Distinguishing Features. Opal is about the only mineral ordinarily confused with volcanic glass. These two can easily be distinguished by their fusibility. (Opal is infusible.) The index of refraction of glass, though variable, is always higher than that of opal. Occurrence. 1. Volcanic glass is an igneous rock occurring in surface flows or as selvage on lavas and occasionally in dikes (pitchstones). It has been formed by the rapid cooling of the molten magma, and as a consequence some of the water of the original magma is usually retained. Hydrocarbons The naturally occurring hydrocarbons vary from natural gas [largely methane (CH 4 ) with variable amounts of ethane, (C 2 - H 6 ), carbon dioxid, nitrogen, argon, neon, and helium] through liquid petroleum (largely hydrocarbons of the methane or paraffin series with the general formula C n H 2n +2) and viscous maltha to solid hydrocarbons which may be divided into four fairly well-defined groups, viz., resins, waxes, asphaltum, and coal. Resins. The resins are oxygenated hydrocarbons. They are amorphous and have a resinous luster. The specific gravity SILICATES 415 is slightly above unity (1.00 to 1.25). They burn or melt easily and are more or less easily soluble in alcohol, ether, and turpentine. Of the various fossil resins amber is the best known on account of its well-known uses. It varies from a pale yellow to deep brown and has a specific gravity of about 1.05. The best amber is found along the Baltic coast of Prussia. It has been formed by an extinct species of pine. Other fossil resins include gum copal from Africa and kauri gum from New Zealand used in varnishes. There are also many local names used for various resins. Mineral waxes. Ozocerite or ozokerite is the best known representative of this group of hydrocarbons which are natural paraffins with impurities. Ozocerite is the name applied to a soft brown mineral wax from Galicia, also found in southern Utah. It is soluble in ether and has a specific gravity of 0.85 to 0.95. Refined ozocerite is used in the manufacture of candles, ointments, and as an insulator for electrical apparatus. Asphaltum or asphalt is a general name for a great variety of black, solid, more or less oxygenated hydrocarbons. They include besides the well-known Trinidad Lake asphalt, other varieties which have received special names such as albertite, gilsonite, grahamite, manjak, wurtzilite, and many others of local importance. Each of these has special characters of its own, but they are all similar with a hardness of 1 to 2J^, specific gravity of 1.0 to 1.8. They melt easily and burn with a dis- agreeable odor. They are more or less soluble in alcohol, ether, turpentine, carbon bisulfid, and chloroform. The relative solubility in these various solvents is the best method of dis- tinguishing the various kinds of asphal turns. Asphalturn occurs in veins usually and rarely in lake deposits as on Trinidad Island. Asphaltum and semi-solid hydrocarbons also occur as impregnation of sandstones or limestones. These bituminous sandstones and limestones have been used for paving in some parts of the United States. 416 INTRODUCTION TO THE STUDY OF MINERALS The oil shales found so abundantly in western Colorado should also be mentioned here. On distillation they yield gas, crude oil, and ammonia. The crude oil on refining furnishes gasoline, burning oils, and paraffin wax. Coal. Finally we have the coals which vary from lignite through subbituminous, bituminous, and semianthracite to anthracite. The chemical compounds in coal are for the most part unknown. Coal consists largely of carbon, hydrogen, and oxygen, with small amounts of nitrogen and sulfur. Analyses are usually given in a proximate form: moisture, volatile matter, fixed carbon, and ash. The specific gravity of coal varies from about 1.2 to 1.7. The hardness reaches 2 to 2^ in anthracite. Coal is distinguished from other hydrocarbons by the fact that it is practically insoluble in the organic solvents (ether, alcohol, etc.). Coal occurs in beds and so must be ranked as a rock as well as a mineraloid. Anthracite may be said to constitute a kind of metamorphic coal. Coal is known to be of vegetable origin. All gradations have been traced from peat through lignite into various members of the coal series. There is, however, disagreement as to the details of the formation of coal. Salts of Organic Acids. The discussion of mineraloids would be incomplete without a brief reference to the salts of certain organic acids. One of these, calcium oxalate, occurs in plant tissues and is also found in coal beds in monoclinic crystals which have received the name whewellite. They have the composition CaC 2 O 4 -H 2 O. PART III THE OCCURRENCE, ASSOCIATION, AND ORIGIN OF MINERALS A. GENERAL PRINCIPLES The determination of the properties of a mineral does not end its investigation. There still remains to be determined the problem of its role in nature. What is its relation to associated minerals and how has it been formed? This is to some extent an independent subject, for with the possible exception of amorphous minerals of colloidal origin, the essential properties of a mineral are not dependent on its previous source or history. The facts of occurrence and association are important from the scientific standpoint and also from the economic standpoint in case the mineral or one of its associates is of commercial value. A great many of the subjects considered in the following pages are treated under the head petrography, the science that deals with rocks especially from the descriptive side. A broader treat- ment of the whole subject of mineral occurrences, including mineral deposits as well as rocks is attempted. 1. ASSOCIATION OF MINERALS The minerals of rocks and other mineral deposits occur to- gether in more or less definite association one with another. Many of the associations are so characteristic that the expe- rienced mineralogist makes use of the facts in determining minerals. Franklinite, for example, is practically always asso- ciated with willemite and zincite (ZnO). Lepidolite, tourmaline, microcline, albite, spodumene, and beryl are characteristic of granite pegmatites. Nepheline occurs with the feldspars in igneous rocks and is never found with quartz. The zeolites 27 417 418 INTRODUCTION TO THE STUDY OF MINERALS occur largely with datolite, apophyllite, prehnite, calcite, chalce- dony, and quartz as secondary minerals in basalts and related rocks. Chondrodite is almost invariably found with phlogopite and spinel in metamorphic limestones. The term paragenesis is used as a general term for the association of minerals with special reference to their occurrence and origin. 2. ORDER OF SUCCESSION Not only the association but also the order of succession of minerals is often characteristic. In many copper ore deposits, for example, the order in which the minerals have been formed is as follows: (1) pyrite, (2) chalcopyrite, (3) bornite, (4) chalcocite. In most ore deposits the ore-minerals have been formed after quartz and the silicates, although some of the silicates, among them sericite, chlorite, tremolite, and antigorite, are apparently formed after the ore-minerals. The order of succession of the minerals of a deposit combined with the geology gives valuable clues to the history of the deposit. 3. PROCESSES OF MINERAL FORMATION Minerals may be formed in various ways. Some have been formed from water solution (veins, spring deposits, secondary minerals in cavities) either by concentration of solutions or by chemical reactions. Some have been formed by separation from a molten magma (minerals of the igneous rocks) . The magma is to be looked upon as a solution of certain compounds in others, for the minerals separate out in the order of solubility rather than of fusibility. Others have been formed by organisms. Still others have been formed by the chemical readjustment incident to metamorphism. A few minerals are the result of exhalations of gases in volcanic regions. 4. SYNTHESIS OF MINERALS Besides filling up gaps in isomorphous groups and furnishing better material for study, the synthesis of a mineral often gives a THE ORIGIN OF MINERALS 419 clue to its origin in nature. Most minerals have been produced artificially, but a few, such as tourmaline, have never been pro- duced except in Nature's laboratory itself. The methods of mineral synthesis differ greatly; the apparatus varies from a test-tube to the electric furnace. A general method is that of the sealed tube. A hard glass tube containing the proper substances is sealed up and heated in a bomb-furnace for several hours or days, or even weeks if necessary. Water vapor under pressure plays an important part in reaction, as it often seems to in nature. An example of this method is the pro- duction of artificial covellite (CuS) by heating powdered sphaler- ite (ZnS) in a water solution of copper sulfate. An atmosphere of CO 2 is used to prevent oxidation. After a few hours a blue- black powder (CuS) appears. The reaction is ZnS + CuSC>4 = CuS + ZnSO 4 . This experiment was performed by the author in an attempt to explain covellite pseudomorphs after sphalerite found by him in the Joplin district. In such experiments geologic time is compensated for by increased temperature and pressure. Some of the " basic" igneous rocks are easily reproduced, and such minerals as olivine, pyroxene, leucite, and plagioclase crystal- lize out from a molten mass of the proper constituents. At- tempts to reproduce the "acid" igneous rocks on the other hand are not successful, for the magma solidifies as a glass. The lack of gases which were present in the natural magma accounts for the failure. French mineralogists and chemists have been especially active in mineral synthesis. Moissan produced diamond by dissolving carbon in molten iron and the plunging the mass into water. Artificial rubies, sapphires, and other colored varieties of corun- dum are now made in Paris on a commercial scale. These were first successfully produced by Verneuil in 1904. Except for very minute bubbles they have exactly the same physical prop- erties as the natural gems and are distinguished from them with great difficulty. Artificial stones resembling the emerald are easy to produce but they are really glass and not true artificial beryl. 420 INTRODUCTION TO THE STUDY OF MINERALS Intermediate between the naturally occurring minerals and the so-called artificial minerals are the mineral substances formed on mine-tools, prehistoric implements, old coins, etc. Man has unintentionally furnished part of the material, but has not di- rected the conditions of the experiments, hence the term accidental synthesis. The author has identified cuprite, copper, malachite, azurite, and cerussite on buried Chinese coins of the seventh century found at Kiukiang, China. In the old Roman baths at the hot springs of Bourbonne-les-Bains, France, bronze coins thrown in the spring as votive offerings were found by Daubree to be incrusted with such minerals as chalcocite, chalcopyrite, bornite, and tetrahedrite, and the conduits leading to the baths were lined with zeolites. 6. ALTERATION AND REPLACEMENT OF MINERALS A mineral formed under one set of conditions may be unstable under another set of conditions. . This accounts for the observed replacement of one mineral by another. Pyrite, for example, is unstable under oxidizing conditions and so in the oxidized zone we usually find it more or less altered to limonite, turyite, or some- times to iron sulfates. If copper solutions are present it may be altered to chalcocite. Replacements in which there is a chemical relation between the original mineral and the replacing mineral are called alterations. The more common chemical changes involved in alteration are oxidation (sulfids to sulfates, sulfids to oxids, arsenids to arsenates, etc.), reduction (sulfates to sulfids, sulfids to metals, oxids to metals), carbonation (sulfids to car- bonates), and hydra tion (anhydrous salts to hydrous salts). There are also more complex changes, some of which have received special names. The following may be mentioned as prominent examples of alterations: pyrite to limonite; galena to cerussite, usually through the intermediate stage of anglesite ; sphalerite to smithsonite; bornite to chalcocite; copper to cuprite; calcite to smithsonite; olivine to antigorite (serpentinization) ; pyroxene to actinolite (uralitization) ; and feldspars to sericite (sericitiza- tion). THE ORIGIN OF MINERALS 421 One of the best evidences of alteration and replacement is the occurrence of pseudomorphs. A pseudomorph is one mineral with the form of another, a false form as the name indicates. Thus limonite, an amorphous mineral, is often found in cubes. The explanation is that the cubes were originally pyrite and were altered to limonite by oxidation and hydration. Such a specimen is said to be a limonite pseudomorph after pyrite. Four general classes of pseudomorphs are recognized : 1. Alteration pseudomorphs with either a loss, gain, or inter- change of some constituent. Examples of the three cases re- spectively: copper after azurite; malachite after cuprite ;cerussite after galena. 2. Paramorphic pseudomorphs or par amor phs. A pseudomorph of one polymorphous mineral after another is called a paramorph. Example: calcite after aragonite. 3. Substitution pseudomorphs. An interchange of substance not involving alteration. Example: chalcedony after calcite. 4. Incrustation pseudomorphs. If one mineral incrusts another and then the original mineral is dissolved there remains a cavity which may afterward be filled by still another mineral. Ex- ample: quartz after fluorite. The replacement of a fossil by a mineral is called a petrifaction. The more common minerals occurring as petrifactions are calcite, chalcedony, opal, quartz, limonite, pyrite, and cellophane, and rarely such minerals as barite and sphalerite. Fossil wood is usually preserved as opal and chalcedony, occasionally as quartz and rarely as calcite or dolomite. In New Mexico cuprified wood now made up of hematite, pyrite, bornite, chalcocite, and melaconite (amorphous CuO) is com- mon in certain regions. The cell-structure of the wood is often preserved and sometimes the wood may be identified. Fossil bone, as the author has recently discovered, is made up almost entirely of the mineral cellophane. The structure of the bone is usually perfectly preserved. The original bone consists of calcium carbonophosphate and an organic substance called 422 INTRODUCTION TO THE STUDY OF MINERALS ossein, one of the proteins. The fossilization of the bone con- sists of the elimination of the organic substance (some of it usually remains behind as a pigment) and an enrichment of the inorganic portion. Replacement may take place on an extensive scale and in this way some of our prominent types of ore-deposits have been formed. Whole rock formations may be replaced by solutions. Many phosphorites, for example, have been formed by the action of phosphatic solutions upon limestones. Limestones may also be replaced by silica, usually in the form of chalcedony. B. MINERAL OCCURRENCES INCLUDING ROCKS No satisfactory classification of the various mineral occurrences can be made on account of the multiplicity of factors to be taken into account. For convenience, rocks and mineral deposits are treated under the following heads. 1. IGNEOUS ROCKS General Discussion The igneous rocks include all rocks that are the result of solidification of molten material on or in the earth's crust or outer shell. They are of especial importance because of the fact that they are the original source of the other two great groups of rocks, the sedimentary and the metamorphic. Igneous rocks are characterized by the presence of certain minerals which are practically absent from other rocks, by the massive appearance or absence of stratification and foliation, and also by the absence of fossils. The prominent features of igneous rocks may be discussed under the following heads: (1) chemical composition, (2) mineral composition, (3) texture, (4) structure, and (5) mode of occur- rence. These are the factors used in the description and classifi- cation of igneous rocks. THE ORIGIN OF MINERALS 423 Chemical Composition. Chemical analyses of igneous rocks are always recorded in the form of oxids. The nine principal oxids are: silica (SiO 2 ), alumina (A1 2 O 3 ), ferric oxid (Fe 2 3 ), ferrous oxid (FeO), mag- nesia (MgO), lime (CaO), soda (Na 2 O), potassa (K 2 O), and water (H 2 O). Practically all -igneous rocks also contain small amounts of titania (TiO 2 ), carbon dioxid (CO 2 ), baryta (BaO), manganous oxid (MnO), and phosphoric anhydrid (P 2 O 5 ). The oxids are in chemical combination and do not exist free except in a few cases, the most prominent of which is free silica in the form of quartz. Among the various constituents silica predominates. It varies from a maximum of about 75 per cent, in granites and rhyolites to a minimum of about 40 per cent, in peridotites. Igneous rocks high in silica are persilicic (the so-called acid rocks), those low in silica, subsilicic (the so-called basic rocks), and those of intermediate silica content, mediosilicic. Granites and rhyolites, for example, are persilicic rocks, basalts, olivine gabbros, and peridotites, subsilicic rocks. Alumina in general is fairly constant, but is very low in peridotites and high in syenites. The iron oxids, magnesia, and lime are low in persilicic rocks and high in the subsilicic rocks. The alkalies (soda and potassa), on the other hand, are relatively high in persilicic rocks and almost lacking in the peridotites. The following list of analyses will serve to show the range in chemical composition of the more common types of igneous rocks. i s 4 o e8 5 MH O d 9 g .2 a .O 2 5 CO O 49 O f 1 | o O 6 o I w I I | II J w g j H o ^ c Cu 2 (OH): Cua(OH). o PH CC FeCOa Ca(M g , I MnCOa 6 o PH I O u 1 o 6 o d u w O M g4 (OH 2 I 3 CO g s ? W s ^ g -a ss 2 .S HH fc -^ kj O <3 d 8 - & ^ < rH 03 w | B Mineral MALACHITE Azurite COLLOPHAN SIDERITE DOLOMITE Rhodochrosite CERUSSITE SMITHSONn Witherite Strontianite CALCITE Aragonite DOLOMITE Magnesite Hydromagnesi Dahllite a-| .s A) 09 o |i| 1 45 * M 3 IS g 3 O 1 _ 1 V e a 1 M 55 1 1 Wt i O v ' QJ "-I I 1 1 1 d 1 1 fl o 6 a I 1 1 H M -5 G *~^ H O 1 .s ' 1 O PH I QQ I 6 O '5 6 ' 2 . QR 1 S 1 ^ n pi ^ S -S ce P^ rl S w 9 - 2 g 1. | Fusibility Is Fusible witl difficulty. Infusible 1 3 3 s 3 1 a Fusible witl difficulty. ^J ^-H 1 2 s c . .^ a) Ij : .a 2 g g "2 OJ. J 3 & II ats m -C C/2 r*~ gll ais glt^-3 "8 s 'o 494 INTRODUCTION TO THE STUDY OF MINERALS O F^ ffi B S a QQ O 9 3 a -o o - 'I O O o3 Pi ^1 03 ."S O 2 fcJD g ? s i,- 02 Ji||i 111 s i '5c; fl O 05 ' 8 2 S si IS O T3 <5.Sl So * CO 01 s 00 1C PH CO CO 8 1 co CO CO CO 3 q ^ d Q w 6 o 1 a CO i 3 "3 I .2 Kainite Chalcanthi GYPSUM Brochantit 1 ANHYDR: Anglesite BARITE Celestite Alunite w G W q W q M q w i i 2 1 1 , 00 H bi G O w s | i s o 03 PQ V 6 g h 3 +> '5 3 co rt >H S a G 1 o * 1 | B 43 G a 3 | 1 O a 0) 3 I a 83 T3 > JS i^3 \N \C4 \N \e \N rQ L3 1 .s a T! 2 |J5a^ ^ ! o> i 0) ^ ~ '2 o -o ** 1 A Willemite CALAMINE H 7- H | Forsterite Chondrodite Willemite 1 a ! 0) I 0> 1 o 5 S w 11 a) 1 a o to 1 S 5 Labradorite V 3 S ANTIGORIT to 1 JD CHRYSOCO Garnierite aunp d * TI < Z o'n 1 d ^ t 2 - co 1 1 enoffc.t.lowt O ffi JS 1 i SO a a e a "S a < a 9 s a distinguished or opt. tests. ^ a -d S _c c PH ti bis pi -. 1 i a i > 3 s- _5 \ 9 e a S 9 X 9 S 3 fl "5 ^ 1 c3 O ^ a 3 | 03 2 "S 1 1 1 11 O S3 K 3 ^ o /; S ^o o X l o ^ O S a; ^ ^- - S * y. ^C) "o ^ 9 t "S Fusibility , M * - -r ,o -isnjuj ot e. eo eo eo CO eo - - ss> -t 1 >c ~ -isnju i Q C, 3 9 ~ 1 o 1 i o i 'g 1 rt jj a S 1 "S 1 9 S rt 3 g a s "o 8 1 "3 4* | 8 ^ i 4 s C S 1 3 5 "o 3 -n g s ts 3 << o ^ 3 O Q ^ w > ^ ^ o o * ** - N NM Nw x ?" \n N -\ -\ -'. -' rj - -\ ^j\ - DETERMINATION OF MINERALS 497 t* $ i i I . eS BQ I i c I '-: i S r. 1 I i i 1 s 1 5 i r - - ^ 9 w J - ^ O - = 6 6 - | 9 I O r ^ J* ^ x f H | I ^ S KA18iOi *0!HIVM i .S < S fe > ^- 5 r f s." I 6 1 X i 1 5 6 s = 6 i 1 FeAls(OH 4- I HAl(8iO S -.- s g z. 1 MUSCOVIT1 Sericite BIOT1T* ORTHOCLA Adularla Microcline j = I C Anthophyllite i CHLORITE 1 Enatatite f >- M Kaolinite QUARTZ CHALCEDO 8 C H H Staurolite Andalusite Silliinanite PyrophylHte 1 N 5 >-. m --. M 33 s c 1 = : a | S A 9 6 ~ -S s * e 5 | I - s 1 a 1 Contains Fe. Always contai Practically no Practically no Oreen flame w May contain ] & o Fe variable. Little or no F -z r o o 3 No water in c ! | r 1 S 1 i - 5 5 Q i r 1 i i- o 15 ,- .- I 7- *C t* pK ,:, t T- 1 498 INTRODUCTION TO THE STUDY OF MINERALS pd 01 co o> if) 1C CO 00 o be (N '1 P^ co CO CO CO CO CO CO a .2 q | a 1 g o 8 . V i i' ^3 a 1 q d 43 ^ s . o y Q s p^ a ft g s o ^ 'S. 00 g i 1 6 o 6 ft VM o CO 6 CO O CO CO rO PH CO PH 1 i IB S W * "3 % 4> fc ft ft^ 1 APATITE COLLOPH. DahUite Pyromorphi Mimetite Vivianite Turquois 02 S S ^ S 13 S 'ft! 711 1 02 ^ I e d a 0* a o a *' 5 S 18| c W o e a i 1 o 5 mirror i ^ ! g a * x> 1 6 o fe. PH CO M XI PH 1 H 0) G ^ CHROM Vanadinii Carnotite Wulfenite CO "co o 8 H O o c ^J G o o O 3 .s 3 X! S i Xi 1 3 \N X. 1 1 ": (N IN G G G S S Xi O CL( bo 6 15 PH >> gl ! f I QQ ,2 o '^ 0) 1 i i g CO I a .2 1 a S o O J? 1 O M 1 U 5 1 G H V 'R H V 1 M ^ I i W CO O o m i o W 03 OH i-i o 1 O S -M o 9 w w Intense ye; .S w o Abundant Soluble in '. 1 \IN t-K NJ X 1 " . ^ ^ PN flj i 1 1 "3 a M 9 .2 ^. "a *3 2 5s fe S .S * B * ^ c EH - |1 500 INTRODUCTION TO THE STUDY OF MINERALS (D oT o | I o oT H -g o 'C P o W 08 PH rj o ^ ll l - 2 P JH -O 1 S i i S c5|w cc t- M N n N ^ co cc M i i co C"J ro I X -o oo x 00 IN CC Kl IN pj cc o M c 9 8 g K ^ B 9 a X H ^ ^ o GO T i S 6 1 ! B I 1 H 2 Fe 2 O 4 (H 1 Fe 2 0s-H 2 q 3 i c? O | MnO 2 (H 2 O ^ aT DC d ^ B c3 1 Ca 2 B 6 Oiro: < s ^ M U 1 1 CaW0 4 9 ^ 3 < ^ 5 < i < S tc 'A C O H Mineral Stibiconite HEMATITE S >, 3 3 Z 3 " J .- O PSILOMELAr 'Jo aj "o B 01 1 CT3 t-l 1 3 X S a 0) "o O "o 5 D 1 SPHALERITE I Scheelite CORUNDUM 1 s. CO 1 5 ii rj 8 3 PQ i! i o 3 I Q a _ o' as B cJ u B ,_ 1 "d a O B c 9 o y, p C B 3 a 9 B c> a 9 No H 2 O in c.t. "2 a 9 B X! ^ C) S B 3 hJ e a 9 B s< ^5 9 s c 1 1 PC a 1 s "o 1 1 2 a CO 6 22 B o I | i 5 g 00 B 5 it 1 | a C B o c O B o /, c a O B a ^ 21 S 5 z M a ~ ~ 5 1 "rt ^ r ! ^Q anjg C ^ .S ,5 rS j2 NM vs ^ t M _C _: V(S ?i CM -V. 3 *rj ' -, ^ i \ -^ S * ~<\ -<\ " "53 1 3 *2 ^^ ale t "a PM I if | | 1 "o II s 3 g ^ 03 S ll S'S' S g g? ~? ^-o 'S^ s 3 *3 &S~ p CO its c a cj -| fe'u . ^.S -tf.5 P ^*T3 f**' fe T3 3 >> j ^ "3 en al pj 1 11 S.o O a> a 3 O"" 1 O o a 3 ffl ft ll 3 g 3 g T 5 ^ THE DETERMINATION OF MINERALS 501 _g prt O H g QQ ^ !'I "7 e | | 1^ ii?l . "^^ M g 00 ^ ^ S 2 o H ^ OQ ^ H-l HH _. --. 1 II 8l i o >c ^ r*i 3 O s; 00 ^1 Oi r\^ co i c^ oo s|s * ^1 ^ CO i ib s 1 8 1 Tfl ^ r c .2 ^ ^ 1 - I 1 O OQ N . ~7. x / ^ QQ , y3 QQ CO 45 o> fe 1 3 '1 X x 5 V i M i tx q tac < OQ r. CJ 7? X o *?. 'f- SJ Is r tra I QQ .> 2. DQ 3 O o "1 2 < V 1 H H J H S _ s *J 2 JQ ^ a OJ ARSENOPY 0) a STIBNITE TETRAHED Jamesonite Pyrargyrite Stephanite 1 >, H H H 5 I b a x Pentlandite O .-1 J s rt M 3 < U g 3 < O o o H-) a Covellite Bismuthinite Argentite Calaverite Molybdenite m 1 1 G "S a 1 5 p ^0 f 1 c 1 1 G 1 Ni test. Co(NO 3 ) 3 3 ^ g S5 | sublima 3 OQ O T; 1 5 ! ;ton o G I 6 G x d 3 U , for Cu. 1 ^ "3 a -tJ 1 1 j oJ 1 ^ 3 ^ 3 o J cJ o g ? g g :3 tx if fcj .5 fl o 1 / ^ 1 C ^ 1 "S o H UJ !S3 E -< < < CD OQ /< o X H N H 55 QQ s tf >, 1 '2 (N ^ ^ -X n ^ ,, _ CQ CO eo >\ M X c^ C et SM --, \p c7 ^ IN ^ 2 3 fe 3 ^} >> Js^ S3 | 6 > V s w tx | '8 a 3 g.2 111 'ft* S "w if o u 3 3 T3 fe ^ "** EC "^ 00 **"* h'i O o ^> ll^ 111 pf! e I 1 ' "- 1 a> ^ 1*| la '> B O a g S flT3 c ~i a O aS Jsy? o 0^ o 3 s 502 INTRODUCTION TO THE STUDY OF MINERALS cc d h 22 >* > ^ s J i 1 g - s ^1 1 CO a * 2 .0 is _0 "3 s~ is o FH ^5 1 Jx I "M 1 1 Strongly mag 1 >. c s 03 p c 3 i a; 1 S 1 0) 1 S c Reduction co Reduction co /; | St is 1 s~ 1 3 ! X 1 3 6 PH > s i ^ "c o ^ 1 B 1 | -M 3 3 3 T*< - : ' 3 3 3 3 Em us X o >c 'C in i 3 -H >H 1 CO N 1 N 5 1 i < * IQ i I *: M i g 7 2| I 5 c 3 . j] 1 "I C 1l i i PH'* tf- \ __>, ll g 1 a 9 1 3 > 1| 1 e *! 3 2 3 ft o a 1 3 3 8 THE DETERMINATION OF MINERALS 503 BLANK FORM FOR REPORTING MINERALS Date No Name Form Cleavage Luster and color Hardness Streak Spec. Grav Other characters Associates Mineral suspected Optical characters of crushed fragments Fusibility Flame coloration Closed tube Open tube On charcoal alone On charcoal with Na 2 CO s Borax bead NaPO* bead Solubility Wet tests Miscellaneous tests Summary of important characters Mineral * The space in the upper right-hand corner is for a sketch of crystals or crushed fragments. INDEX AND GLOSSARY A n . Symbol used for an axis of n-fold symmetry. Abrasives. Corundum, diamond, diatomite, emery, garnet, quartz. Absorption, 201 scheme, 204 Accessory minerals of igneous rocks, 424 Acicular. Needle-shaped. "Acid" igneous rocks. The same as persilicic rocks, 423 Acids, 8 Acid salt, 10 Actinolite. Ferriferous tremolite. Acute bisectrix, 193 Adamantine luster, 154 Adjustments of the polarizing micro- scope, 173 Adularia, 346 Agate. A banded or variegated variety of chalcedony. Agglomerate, 439 Aggregate polarization. The intri- cate structure of microcrystal- line substances between crossed nicols. Alabaster. Translucent massive gypsum. Formerly used for onyx marble. Alaskite, 429 Albite, 350 twinning, 349 Allotriomorphic. The same as an- hedral. Almandite, 371 Alpha (a). (1) The angle between the b- and c-axes of reference in the triclinic system. (2) The direction of the fastest ray in anisotropic crystals. Alteration of minerals, 420 Alumina. Aluminum oxid (A1 2 O 3 ). Aluminates, 277 Aluminum minerals. Alunite, and- alusite, cliachite, corundum cryolite, gibbsite, halloysite kaolinite, kyanite, sillimannite, topaz, turquois, and many silicates, tests for, 40 Alundum. Artificial A1 2 O 3 used as an abrasive. Alunite, 335 Amazon-stone. The green variety of microcline. Amber, 415 Amethyst. The purple variety of quartz used as a gem. Amorphous condition, 54 Amphibole. A group of silicates including anthophyllite, tremo- lite (actinolite), hornblende, and glaucophane, 363 Amygdaloidal, 426 Analcite, 410 Analyzer, 173 Andalusite, 381 505 506 INDEX AND GLOSSARY Andesine, 352 Andesite, 433 Andradite, 371 Angle, axial, 192 critical, 161 extinction, 185 interfacial, 56 Anglesite, 329 Anhedral, 54 ANHYDRITE mineral, 330 rock, 446 Anisotropic, 190 Ankerite. Ferriferous dolomite. Anomalous optical properties, 206 Anorthite, 354 Anthophyllite, 364 Anthracite, 416 ANTIGORITE, 398 Antimony glance. Synonym of stibnite. minerals, Jamesonite, polyba- site, pyrargyrite, stephanite, stibnite, stibiconite, tetra- hedrite. tests for, 41 ocher, Synonym of stibiconite. >n. Symbol used for composite symmetry with respect to an axis of ?i-fold symmetry and a plane normal thereto. APATITE, 313 Apophyllite, 406 Apparatus, blowpipe, 20 Aquamarine. A transparent sea- green variety of beryl. Aragonite, 302 Arborescent. Branching like a tree. Argentite, 228 Arkose, 441 Arsenates, 310 Arsenic minerals. Arsenopyrite, enargite, mimetite, smaltite. tests for, 41 ARSENOPYRITE, 240 Asbestos. Fibrous varieties of tre- molite, anthophyllite, or chrysotile. Asbolite. A soft earthy cobalt- bearing variety of psilomelane. Asphaltum, 415 Association of minerals, 417 Asterism. A six-rayed star effect produced by symmetrically ar- ranged inclusions in certain minerals. Asymmetric class. The crystal class devoid of symmetry. Atomic weights, table of, 6 Auganite, 434 porphyry, 428 Augite, 361 Augitite, 436 Automorphic. The same as euhedral. Aventurine. A spangled appear- ance produced by inclusions of hematite, goethite, or mica. Axes of reference, 73 of symmetry, 60 optic, 191 Axial angle, 192 colors, 202 cross, 85 elements. A collective name used for the axial ratio and the angles between the axes of reference, plane, 192 ratio, 75 Axinite, 391 Azurite, 307 B Balance, specific gravity, 149 BARITE, 327 INDEX AND GLOSSARY 507 Barium minerals. Barite, witherite. tests for, 42 Barytes. Synonym of barite. Basal pinacoid. The pinacoid {001} or j 0001 | Basalt, 435 porphyry, 428 Basaltic hornblende. A deep brown variety of hornblende found in volcanic rocks. Bases, 9 "Basic" igneous rocks. The same as subsilicic rocks, 423 salt, 10 Bastite. Lamellar antigorite pseu- domorphous after pyroxene. Batholith, 426 Bauxite, 442 Baveno twin, 345 Bead tests, 31-32 Becke, Austrian mineralogist (1855- Becke test, 165 Berlin blue. An anomalous inter- ference color of the first-order. Bertrand lens. Beryl, 368 Beryllium minerals. Beryl, tests for, 42 Beta (0). (1) The angle between the a- and c- axes of reference in the monoclinic and triclinic systems. (2) The direction nor- mal to the plane of a and 7 in biaxial crystals. Biaxial, 191. Bibliography XV-XVII1. BIOTITE, 396 Bipyramid, 69 Birefringence. The strength of the double refraction. Bisectrix, 193 Bisilicates. The same as meta- silicates R n SiO 3 = RO.SiO 2 , 343 Bismuth flux. The same as iodid flux, 23 minerals. Bismuthinite. tests for, 42 Bismuthinite, 226 Bisphenoid, rhombic, 71 tetragonal, 71 Blackband. An impure carbon- aceous siderite. Blackjack. A miner's name for ferriferous sphalerite. Black opal. A dark-colored pre- cious opal. Bladed, 128 Blende. Synonym of sphalerite. Blowpipe, 20 apparatus, 20 reagents, 22 tests, 25 Bluestone. Synonym of chalcan- thite. Bog iron ore. Synonym of limonite. Bog manganese. Synonym of wad, an impure earthy variety of psilomelane. Bone-ash (reagent), 23 Bone turquois. An aluminous variety of cellophane formed by the replacement of fossil teeth. Borates, 310 Borax (reagent), 22 bead tests, 31 Boric acid flux, 23 BORNITE, 245 Boron minerals. Axinite, colema- nite, datolite, tourmaline, ule- xite. tests for, 42 508 INDEX AND GLOSSARY Bort. Diamond in the form of aggregates without distinct cleavage. Botryoidal, 128 Bravais. French crystallographer and physicist (1811-63). Brazilian twin of quartz. Inver- sions twins of a right-handed crystal with a left-handed one. Breccia, 441 Breithaupt, German mineralogist (1791-1873). Brittle silver ore. Synonym of stephanite. Brochantite, 332 Bronze mica. Synonym of phlogo- pite. Bronzite. Ferriferous variety of enstatite. Brucite, 288 Brush, American mineralogist (1831- 1912). C. Symbol used to indicate that a crystal has a center of symmetry. Cabochon. The rounded form of cut gem-stones used especially for opal. Cairngorm. Smoky quartz. CALAMINE, 376 Calaverite, 242 Calcareous ooze, 444 tufa, 448 Calc-spar. Synonym of calcite. CALCITE, 291 Calcium minerals. Anhydrite, apa- tite, aragonite, calcite, clinozoi- site, colemanite, cellophane, dahllite, dolomite, fluorite, gyp- sum, scheelite, wollastonite, ulexite, and many silicates. tests for, 43 Cane sugar, 80 Capillary. Very fine hair- like crystals. Carbonaceous rocks, 444 Carbonado. A black tough opaque variety of diamond used in diamond drills. Carbonates, 290 tests for, 43 Carborundum. An artificial silicon carbid (SiC) extensively used as an abrasive. Carbuncle. Garnet cut in cabochon form. Carlsbad twin, 345 Carnallite, 257 Carnelian. A clear red chalcedony used as a semi-precious stone. Carnotite, 321 CASSITERITE, 272 Cat's eye. Gems which exhibit a peculiar reflection because of their fibrous structure. Celestite, 328 Center of symmetry, 62 Cerargyrite, 253 CERUSSITE, 305 Ceylonite. An iron-bearing variety of spinel. Chabazite, 410 Chalcanthite, 334 CHALCEDONY, 261 CHALCOCITE, 230 CHALCOPYRITE, 244 Chalcotrichite. A capillary variety of cuprite. Chalk, 433 Chalybite. Synonym of siderite. Charcoal. Use of, as a blowpipe support, 28 INDEX AND GLOSSARY 509 Chatoyant. The peculiar optical effect produced by cat's eye. Chemical composition, 5 compounds, 58 properties, 5 tests, 25 types, 10 Chert, 449 Chessylite. Synonym of azurite, from a locality, Chessy in France. Chiastolite. A variety of andalusite with symmetrically arranged carbonaceous impurities. Chile saltpeter. The same as nitratine. Chlorids, 251 Chlorin, tests for, 44 CHLORITE, 397 Chondrodite, 402 Chondrules. Rounded nodules with excentric fibrous struc- ture characteristic of certain meteorites. CHROMITE, 281 Chromium minerals. Chromite. tests for, 44 CHRYSOCOLLA, 405 Chrysolite. Synonym of olivine. Chrysoprase. An apple-green vari- ety of chalcedony used as a semi-precious stone. Chrysotile, 400 CINNABAR, 234 Citric acid (reagent), 23 Classification of crystals, 78 of minerals, 212 of ores, 455 of rocks, 422 Clastic. Made up of broken frag- ments of preexisting rocks. Clay, 442 Cleavage, 129 CLIACHITE, 287 Clinographic drawing or projection,. 85 Clinozoisite, 386 Closed forms, 72 tube tests, 28 Coal, 444 Cobalt minerals. Smaltite. tests for, 44 Coefficients, 75 Colemanite, 323 Colloform. The rounded, more or less spherical forms assumed by amorphous and metacolloidal minerals in open spaces. Colloids, 17 COLLOPHANE, 319 Collophanite. Synonym of cello- phane. Color of minerals, 154 Colors, interference, 175 Columnar, 128 Columbite, 312 Columbium. Synonym of niobium, one of the chemical elements. Combination of forms, 55 Complementary forms, 72 Composite crystals, 123 symmetry, 62 Composition-face of a twin crystal, 124 Conchoidal fracture, 131 Concretion, 128 Conglomerate, 441 Congruent forms, 72 Conoscope. A polariscope for con- vergent light. Contact goniometer, 57 metamorphism, 453 twins, 124 COPPER, 221 510 INDEX AND GLOSSARY Copper glance. Synonym of chal- cocite. pyrites. Synonym of chalcopy- rite. Copper minerals. Azurite, bornite, brochantite, chalcanthite, chal- cocite, chalcopyrite, chryso- colla, copper, covellite, cuprite, enargite, malachite, tetrahe- drite. ores of: bornite, chalcocite, chalcopyrite, copper, tests for, 45 Cornu, Austrian mineralogist (1882 -1909). Cornuite. An amorphous copper silicate, corresponding to crys- talline chrysocolla. CORUNDUM, 268 Corundum syenite, 431 Covellite, 234 Critical angle, 161 Cristobalite, 264 Crossed nicols, 173 Cryolite, 256 Cryptocrystalline. Apparently amorphous, but made up of very fine interlocking crystals (for example, chalcedony). Crystal, definition of, 54 drawing, 83 classes, 78 systems, 81 Crystals, classification of, 78 Crystalline aggregates, 128 Crystalline limestone, 452 Crystallites. Minute hair- or fern- like forms found in volcanic glass. They are supposed to represent incipient crystals. Crystallographic axes. The same as axes of reference. Crystallography. The science relat- ing to crystals in all aspects, often erroneously used for geo- metrical crystallography. Cube, 90 Cupellation, silver, 34 Cuprite, 267 Curve of hardness, 154 Cyanite. Variant of kyanite, 382 Cyclic twin, 124 D Dacite, 432 Dahllite, 315 Dana, American geologist and min- eralogist (1813-1895). Datolite, 391 Decrepitate. To fly to pieces when .heated. Dedolomitization, 453 Deltohedron, 94 Dendritic. Branching like a tree. Dense igneous rocks, 427 Determination of minerals, 463 Devitrification. The gradual change of glass to crystalline aggregates. Diabase, 435 Diallage. A lamellar variety of diopside. Diamond, 213 Diatomaceous earth. A synonym of diatomite. Diatomite, 444 Dichroic, 202 Dichroscope, 201 Dihexagonal bipyramid class, 102 pyramidal class, 80 Dike, 426 Dimorphism. The particular case of polymorphism in which there are two polymorphs. Diopside, 360 INDEX AND GLOSSARY 511 Diorite, 433 Diploid, 96 Diploidal class, 95 Dispersion. The divergence of the optical constants for different parts of the spectrum. Disseminated. Scattered through a rock or vein in small quantities. Disthene. Synonym of kyanite. Ditetragonal bipyramidal class, 98 prism, 69 pyramidal class, 80 Ditrigonal bipyramidal class, 80 prism, 69 pyramidal class, 107 Dodecahedron, 90 Dodecants. The twelve divisions into which space is divided by the four axes of reference of the hexagonal system. Dog-tooth spar. A variety of sharp-pointed calcite crystals. DOLOMITE, 296 Dolomitic limestone, 449 Dolomitization, 449 Domatic class. The crystal class with a single plane of symmetry, 80 Dome, 69 Double refraction, 169 Drusy. Apparently sprinkled over with minute crystals. Dry-bone. A local synonym of smithsonite (Wis.) or cerussite (Mo.) Drawing of crystals, 83 Dunite, 436 Eclogite. A metamorphic rock with garnet and either a pyroxene or an amphibole. Edge of a crystal, 55 Effervescence. The bubbling caused by the evolution of gas such as CO 2 . Efflorescent. Gives up 'its water of crystallization on standing. Elaeolite. A synonym of nepheline or nephelite. Elastic. An elastic mineral springs back when bent in contrast to a flexible mineral which remains bent. Electrum. A naturally occurring alloy of gold or silver with over 20 per cent, of silver. Elements, axial. A collective name used for the axial ratio and the angles between the axes of reference. list of chemical, 6 occurrence of chemical, 6 of symmetry, 63 Ellipse, optic, 189 Ellipsoid, optic, 189 Emerald. The clear green variety of beryl used as a gem. Emery. A mixture of corundum with magnetite. Enantiomorphous, 72 Enargite, 250 Enstatite, 358 EPIDOTE, 384 Equant. The same as equidimen- sional. Erubescite. A synonym of bornite. Essonite (Hessonite). A variety of grossularite garnet used as a gem. Etch-figures, 66 Ether, 156 Euhedral, 54 Exfoliate. To swell up or spread out like the leaves of a book. 512 INDEX AND GLOSSARY Extinction, 184 angle, 185 direction, 184 Extraordinary ray, 169 Extrusive (igneous) rocks, 426 F Fabric, 425 Face, crystal, 55 Face color. The color of a crystal when viewed in a certain direc- tion. -symbol, 76 Facet, 55 Fahlore. A synonym of tetra- hedrite. Faster ray, 187 Fedorov, a Russian crystallographer. Feldspars, 344 Feldspathoids, 354 Felsitic. The fine grained texture of volcanic rocks without phenocrysts. Ferberite. The iron end-member of the wolframite series. Ferromagnesian, 424 Fertilizers, minerals used as: carnal- lite, collophane, kainite, iiitra- tine, sylvite. Fire-opal. Opal with fire-like reflections. Flame, oxidizing, 25 reducing, 26 tests, 26 Flexible. See elastic. Flint. A massive chalcedony rock, practically the same as chert. Fluorids, 251 Fluorin, tests for, 45 Fluorescence, 255 FLUORITE, 254 Fluor spar. A synonym of fluorite. Foliated. Made up of flat plates. Fool's gold. A synonym of pyrite. Form, 68 closed, 72 general, 79 limit, 79 open, 72 -symbol, 76 Formula, determination of, of a mineral, 11 weight. The sum of the atomic weights of the atoms of a chemical compound. Forsterite, 374 Fowlerite. A zinc-bearing variety of rhodonite. Fracture, 131 Franklinite, 280 Freibergite. A silver-bearing tetra- hedrite. French chalk. A variety of talc used by tailors. Friable. Capable of being pulver- ized by rubbing between the fingers. Friedel, G., French crystallo- grapher and mineralogist. (1865 ), Fusibility, scale of, 34 G Gabbro, 434 GALENA, 228 Galenite. A synonym of galena. Gamma (7). A Greek letter used (1) for the angle between the a- and 6-axes of reference in the triclinic system and (2) for the direction of the slowest ray in anisotropic crystals. Gangue. The non-metallic part of a vein, minerals, 456 GARNET, 371 INDEX AND GLOSSARY 513 Garnierite, 404 Gel, 17 Gelatinization, 495 Gems. Diamond, emerald, ruby, sapphire, etc. General form, 79 Geode, 128 * Geometrical crystallography, 54 Geyserite. A variety of opal formed in hot springs. Gibbsite, 286 Gilsonite, 415 Glance. An abbreviation of copper glance (chalcocite). Glass, 413 Glauconite, 406 Glaucophane, 367 Glide-plane of symmetry, 138 Gliding, 131 Gneiss, 451 Goethite, 284 GOLD, 218 Gold minerals. Calaverite, gold. tests for, 45 Goniometer, contact, 57 reflection, 57 Gossan, 459 Grained igneous rocks, 427 Granite, 429 porphyry, 430 Granitic or granitoid. The texture of even-grained igneous rocks such as granite. Granodiorite, 432 Graphic determination of indices and axial ratios: 97, 101, 111, 115, 120, 123. granite. An intergrowth of quartz and orthoclase or microcline. texture. The texture like that of graphic granite, 33 GRAPHITE, 216 Gray copper. A synonym of tetra- hedrite. Graywacke, 441 Greisen. A quartz-muscovite rock formed by high-temperature hydrothermal solutions. Greensand, 441 Greenstone. Grossularite, 371 Groth, German crystallographer (1843 ). Groundmass, 425 Gypsite, 446 GYPSUM (mineral), 332 (rock), 445 wedge. A thin wedge-shaped slice of selenite. Gyroid. A 24-faced form with the symmetry 6A2-4A3-3A4. Gyroidal class, 80 H Habit, crystal. The general shape of a crystal determined by growth in certain directions. Hackly, 131 HALITE, 251 Halloysite, 404 Haloids, 251 Hardness, scale of, 153 Hausmannite, 282 Haiiy, R. J. French mineralogist and physicist. The founder of the science of geometrical crys- tallography (1743-1822). Heavy liquids, 150 Heavy spar. A synonym of barite. HEMATITE, 270 Hematite, brown. A synonym of limonite. 514 INDEX AND GLOSSARY Hemihedral, 83 Hemimorphic. Crystals in which the two ends are differently terminated. Hemimorphite. A synonym of calamine. Hessel. German mineralogist (1796-1872). Heulandite, 408 Hexagonal bipyramidal class, 80 pyramidal class, 80 scalenohedral class, 104 system, 102 trapezohedral class, 80 Hexahedron. Synonym of a cube. Hexoctahedral class, 90 Hexoctahedron, 91 Hextetrahedral class, 93 Hextetrahedron, 94 Hiddenite. An emerald-green vari- ety of spodumene used as a gem. Hintze, German mineralogist (1851- 1916). Holohedral, 83 Holosymmetric, 83 HORNBLENDE, 365 Hornblendite. An igneous rock con- sisting essentially of hornblende. Hornfels, 454 Horn silver. A synonym of cerargyrite. Horse-flesh ore. A synonym of bornite. Hiibnerite. The manganese end- member of the wolframite group. Hue, 155 Hyacinth. A variety of zircon used as a gem. Hyalite. A clear, colorless, collo- form variety of opal. Hydrargillite. A synonym of gibbsite. Hydrion. The element H in an acid salt. Hydrocarbons, 414 tests for, 44 Hydrochloric acid (reagent), 23 Hydrogel, 18 Hydromagnesite, 308 ' Hydrothermal metamorphism, 454 Hydrous salts, 11 Hydroxids, 284 Hydroxyl. The radical or ion (OH). Hypersthene, 359 Hypogene, 461 Ice (mineral), 266 (rock), 449 Iceland spar. The clear trans- parent variety of cleavable calcite. Ideal form. A crystal form or combination in which like faces are of the same shape and size. Idocrase. A synonym of vesu- vianite. Igneous rocks, 422 Ilmenite, 310 Imitative forms, 128 Index of refraction, 159 Indicatrix. A synonym of the optic ellipsoid. Indices, Miller, 75 of refraction, 159 Indigo copper. A synonym of covellite. Injected rocks, 426 Intercepts, 73 Interfacial angle, 56 Interference, 175 colors, 175 of light, 175 INDEX AND GLOSSARY 515 Interference figures, 193 color chart, 180, 181, 183 Intergrowth. An interlocking ar- rangement of two substances produced by simultaneous crys- tallization. Internal structure of crystals, 132 Intrusive, 426 Intumesce, 34 Inversion, 62 twin, 124 lodid flux, 23 Iridescence. A rainbow effect pro- duced by the interference of light in thin surface films. Iron, 223 minerals. Arsenopyrite, bor- nite, chalcopyrite, chromite, columbite, franklinite, goethite, hematite, ilmenite, iron, jaro- site, limonite, magnetite, mar- casite, pyrite, pyrrhotite, side- rite, turyite, vivianite, wolfra- mite, and some silicates, tests for, 46 pyrites. A synonym of pyrite. Isodimorphism, 290 Isometric system, 89 Isomorphism, 14 Isomorphous mixtures, 16 Isotropic, 190 Jade. A tough compact green or greenish-white ornamental stone consisting either of an amphibole (nephrite) or of a pyroxene (jadeite). Jamesonite, 245 Jargon. A pale-colored variety of zircon used as a gem. Jarosite, 336 Jasper. A red to yellow or brown variety of chalcedony colored by iron oxids. Jolly balance. A specific gravity balance in which use is made of a spiral brass wire. K Kainite, 331 Kaolin. Impure kaolinite. Kaolinite, 403 Kaolinization. The process by which kaolinite is formed. Kidney ore. A reniform variety of hematite from England. Kunzite. A transparent lilac- colored variety of spodumene used as a gem. Kyanite, 382 Labradorite, 352 Laccolith, 426 Lamellar, 128 Lampadite. A cupriferous variety of psilomelane. Lapilli, 439 Lapis lazuli. A mixture of lazurite, calcite, diopside, etc. Latite, 432 Law of constancy of interfacial angles, 64 of rational indices, 74 of rational symmetric inter- cepts, 74 Lazurite, 357 Lead minerals. Anglesite, cerus- site, galena, Jamesonite, mime- tite, pyromorphite, vanadinite, wulfenite. tests for, 47 516 INDEX AND GLOSSARY Left-handed quartz crystal, 260 Lenticular. Lens-shaped. Lepidolite, 395 Leucite, 355 Light, convergent, 193 nature of, 156 polarized, 107 Limburgite, 436 Lime. Calcium oxid (CaO), often incorrectly used for calcium carbonate. Limestone, crystalline, 452 oolitic, 448 sedimentary, 442 Limit form, 79 LIMONITE, 286 Lindgren, American geologist (1860- ). Linear projection, 88 Lithia. Lithium oxid (Li 2 O). Lithium minerals. Lepidolite, spo- dumene. tests for, 47 Lodestone. A variety of magnetite which acts as a magnet. Luster, 154 M Made. Synonym of a twin-crystal. Macroscopic. The same as megascopic. Magma, 424 Magnesia. Magnesium oxid (MgO) Magnesian limestone. Limestones containing magnesium, but in quantities insufficient for dolo- mite. Magnesite, 298 Magnesium minerals. Antigorite, anthophyllite, brucite, carnal- lite, chondrodite, chrysotile, dolomite, enstatite, forsterite, hydromagnesite, kainite, mag- nesite, olivine, spinel, talc, tremolite, and other silicates, tests for, 47 Magnetic iron ore. A synonym of magnetite. pyrites. A synonym of pyr- rhotite. MAGNETITE, 279 MALACHITE, 307 Malleable. Capable of being ham- mered out flat. Maltha, 414 Mammillary, 128 Manebach twin, 345 Manganese minerals. Franklinite, hausmannite, manganite, pyro- lusite, psilomelane, rhodochro- site, rhodonite, tests for, 47 Manganite, 285 Marble. Any limestone that will take a good polish, but also used by some for a metamorphic limestone. Marcasite, 239 Martite. A pseudomorph of hema- tite (or turyite) after magnetite. Massive. Without definite form or structure. Measurement, of axial angle, 199 of crystals, 56 of index of refraction, 161 of interfacial angles, 56 Mechanically formed sedimentary rocks, 439 Mediosilicic igneous rocks, 423 Megascopic. Capable of being seen with the unaided eye in contrast with microscopic. Melanite. A titaniferous variety of andradite. INDEX AND GLOSSARY 517 Menaccanite. A synonym of ilmenite. Mercury minerals. Cinnabar, tests for, 47 Metacolloids. Microcrystalline sub- stances of colloidal origin. Metamorphic rocks, 450 Metamorphism, 450 Metasilicates, 343 Metasomatic replacement. The re- placement of a rock mass by solutions. Methylene iodid, 150 Miarolitic. A term applied to the structure of injected rocks con- taining cavities, lined with euhedral crystals. Micaceous, 128 Mica group, 392 plate, 188 Microchemical gypsum test, 43 Microcline, 347 Microcosmic salt. The same as salt of phosphorus or acid sodium ammonium phosphate used in blowpipe analysis. Microlites. Minute crystals found in volcanic glasses. Microperthite. Perthite on a small scale. Microscope, polarizing, 172 Miller, W. H., English crystallo- grapher and mineralogist (1801 -1880). Miller indices, 75 Mimetic twinning. The tendency of twinning to raise apparently the grade of symmetry and thus imitate other crystals. Mimetite, 316 Mineral, definition of, 1 Mineraloid, definition of, 413 Mispickel. A synonym of arseno- pyrite. Molecular compounds, 10-11 Molybdates, 338 Molybdenite, 227 Molybdenum minerals. Molybden- ite, wulfenite. tests for, 48 Monochromatic light, 158 Monoclinic system, 115 Monzonite, 431 Moonstone. A variety of adularia with a pearly reflection, used as a gem. Morganite. A pink variety of beryl, used as a gem. Morphological properties, 54 Morphology, crystal. That portion of crystallography which deals with the form and internal structure of crystals. Mountain cork j Varieties of leather. ( tremolite. Mundic. A local name for pyrite or marcasite. MUSCOVITE, 393 N n. A symbol used for the index of refraction. Native copper, 221 elements, 213 gold, 218 iron, 223 platinum, 222 silver, 220 Natrolite, 411 Negative crystal. A cavity within a crystal which has the character- istic shape of the crystal itself. elongation, 189 optically, 190 518 INDEX AND GLOSSARY Nepheline, 355 Nepheline syenite, 431 Nephelite. A synonym of nepheline. Nephelite syenite. A synonym of nepheline syenite. Nickel minerals. Garnierite, pentlandite. tests for, 48 Nicol. The same as a Nicol prism. Nicol prism, 171 Niobates, 312 Niobium minerals. Columbite. tests for, 48 Niter. Potassium nitrate, KNOs. Nitrates, 310 tests for, 48 Nitratine, 322 Nitric acid (reagent), 23 Nodular, 128 Non-metallic luster. Any luster but metallic. Nontronite. The iron analogue of kaolinite. Normal salts, 10 Norite, 434 Notation of crystal faces, 73 Oblique extinction, 184 Obsidian, 430 Obtuse bisectrix, 193 Obtuse rhombohedron, 105 Occurrence of minerals, 417 Ocher. A clay colored by iron oxids. Octahedron, 90 Ocular of microscope. The same as the eye-piece. Oligoclase, 351 Oil shale, 416 OLIVINE, 373 Olivine gabbro, 435 diabase. A diabase containing olivine. Omphacite. A bright green variety of diopside characteristic of eclogites. Onyx. A variety of chalcedony with sharply contrasted bands of colors. Onyx marble. A banded variety of calcite or aragonite formed by water solutions. Onyx, Mexican. The same as onyx marble. Ooids. The individual spheres of an oolitic rock. Oolite. A rock made up of minute spheres formed by concretion- ary action. Oolitic limestone, 448 structure. The structure of an oolite. OPAL, 262 Opalescence. The peculiar milky appearance often seen in opal. Opalized wood. A replacement of wood by opal. Open form, 72 Open tube tests, 27 Ophicalcite, 455 Optical anomalies, 206 character, 190 constants, 192 orientation, 207 properties, 156 tests, 204 Optic axes, 191 axis. The c-axis of reference in the tetragonal and hexa- gonal systems. 'ellipse, 189 ellipsoid, 189 INDEX AND GLOSSARY 519 Optic normal, 191 Optics, crystal. Optical crystal- lography or the branch of crys- tallography that deals with the transmission of light in crystals. Order of succession, 418 Ordinary ray, 169 Ores, 456 Organically-derived sedimentary rocks, 442 Oriental amethyst. A purple vari- ety of corundum, emerald. A green variety of corundum, ruby. The true ruby, a variety of corundum, sapphire. The true sapphire, a variety of corundum, topaz. A yellow variety of corundum. Orientation of a crystal. The plac- ing of a crystal in its conven- tional position with the c-axis vertical and the o-axis pointing toward the observer, optical, 207 Origin of minerals, 417 Ortho-axis. The fr-axis in the mono- clinic system. Orthographic drawing or projection, 84 Orthorhombic system, 111 ORTHOCLASE, 344 Orthosilicates, 343 Oxidizing flame, 25 Oxids, 258 Ozocerite, 415 P. Symbol used for plane of symmetry. Paragenesis, 418 Parallel growth, 123 Parameters. The relative intercepts of the unit face (1 1 1) on the axes of reference. Paramorph, 424 Parting, 131 Path difference, 176 Peacock copper. Tarnished chal- copyrite. Pearl-spar. A synonym of dolomite. Pearly luster, 154 Pediad class. The asymmetric crys- tal class, in which the general form is a pedion. Pedion, 69 Pegmatites, 437 Penetration twin, 124 Penfield. American mineralogist (1856-1906). Pentagonal dodecahedron. A syn- onym of pyritohedron. Pentlandite, 233 Percussion-figure, 392 Pericline. A variety of albite elon- gated in the direction of the 6-axis. Pericline twin-law. The twin-law for plagioclase feldspars in which the 6-axis is the twin axis, 349 Peridot. A gem variety of olivine. Peridotite, 436 Perlite, 430 Persilicic igneous rocks, 423 Perthite. An intergrowth of micro- cline and albite, 348 Petrifaction. The replacement of fossils by mineral substances, 421 Petrographic microscope, 172 Petrography. The science which treats of rocks, especially from the descriptive side. 520 INDEX AND GLOSSARY Petroleum, 414 Petrology. The study of rocks from a broad geological stand- point. Phantom crystal. A crystal in which an earlier stage of growth is marked by a color difference or a row of inclusions. Phase, 158 Phenocryst, 425 Phlogopite, 397 Phonolite, 432 Phosphates, 310 tests for, 49 Phosphate rock. A synonym of phosphorite. Phosphorite, 450 Physical properties, 147 Picotite. A chromium-bearing va- riety of spinel. Piezoelectric. The property of de- veloping electricity by changes in pressure exerted upon a crystal. Pinacoid, 69 Pinacoidal class, 121 Pisolitic. Made up of spheres the size of buck-shot or larger which have been formed by concre- tionary action. Pistazite. A synonym of epidote. Pitchblende, 324 Pitchstone, 430 PLAGIOCLASE, 348 Plaster tablets, 21 tests on, 30 Platinum, 222 tests for, 49 Pleochroism, 201 Pleonaste. An iron-bearing variety of spinel. Plumose. Feather-like. Plutonic rocks. Deep-seated igne- ous rocks as contrasted with the volcanic or surface igneous rocks. Point-systems, 138 Polar axis. An axis of symmetry which has different faces at opposite ends. edges. Edges that intersect the c-axis. Polariscope, 193 Polarized light, 167 Polarizer, 173 Polarizing microscope, 172 Polybasite, 249 Polymorphism, 18 Polysilicates, 343 Polysynthetic twin, 236 Porodine, 17 Porphyritic, 425 Porphyry. An igneous rock with a porphyritic texture. Positive elongation, 189 optically, 190 Potash feldspar. Either orthoclase or microcline. Potassa. Potassium oxid (K 2 O). Potassium minerals. Adularia, alu- nite, biotite, carnallite, jarosite, kainite, lepidolite,leucite, micro- cline, muscovite, orthoclase, syl- vite. tests for, 49 Precious opal. A variety of opal with a play of colors and used as a gem. stones. Diamond, emerald, ruby, and sapphire. Other stones used as gems are called semi-precious. Prehnite, 387 Primary, 460 INDEX AND GLOSSARY 521 Primitive form. A crystal form from which other forms may be derived. Principal axis. The c-axis of refer- ence (A 3 , A 4 , or A 6 ) in the tetragonal and hexagonal sys- tems. Prism, 69 Prismatic class, 116 habit, 469 Projection, clinographic, 85 linear, 88 orthographic, 84 Proustite. The arsenic analogue of pyrargyrite. Pseudohexagonal. Orthorhombic or monoclinic crystals which simulate crystals of the hexa- gonal system. Pseudomorph, 421 PSILOMELANE, 289 Pumice, 430 Pycnometer, 150 Pyramid, 69 Pyramidal habit, 469 Pyrargyrite, 246 PYRITE, 236 Pyrites. A synonym of pyrite. Pyritohedron, 95 Pyroclastic rocks, 438 Pyroelectric, 389 Pyrognostic tests. Blowpipe tests, 25 Pyrolusite, 274 Pyromorphite, 315 Pyrope, 371 Pyrophyllite, 401 PYROXENE, 359 group, 357 Pyroxenite, 436 PYRRHOTITE, 235 Q Qualitative scheme, 39 Quarter-undulation mica plate, 188 QUARTZ, 258 wedge, 199 Quartzite, 452 Quartz monzonite. A quartz- bear- ing monzonite. porphyry. An altered, devitri- fied rhyolite or rhyolite por- phyry. R R. A general symbol standing for some metal. Radical. A group of chemical ele- ments which act as a unit such as NH 4 or SO 4 . Rare-earth metals. Cerium, erbium, lanthanum, neodidymium, pras- codidymium, thorium, and yttrium. Rational indices, 74 Ray of light, 158 Reagents, 22-25 Reduction color tests, 33 Reducing flame, 26 Reflection goniometer, 57 total, 161 twins, 123 Refraction of light, 159 index of, 159 indices of, table of, 207-209 Refractometer, 163 Regional metamorphism, 451 Regular system. The same as the isometric system. Relief, 167 Reniform, 128 522 INDEX AND GLOSSARY Replacement, 420 Residual minerals, 440 Resinous. The luster of resin. Resins, 414 Reticulate. Made up of a net-work. Rhodochrosite, 300 Rhodonite, 362 Rhombic bipyramid, 69 bipyramidal class, 112 bisphenoid, 71 bisphenoidal class, 80 dodecahedron, 90 prism, 69 prismatic class. The same as the prismatic class. pyramid, 69 pyramidal class, 80 section of plagioclase crystals, 350 Rhombohedral carbonates, 290-301 class, 80 subsystem, 102 Rhombohedron, 71 Rhyolite, 430 porphyry, 430 Right-handed quartz crystal, 260 Rock crystal. The clear trans- parent variety of quartz used for ornamental purposes and in optical and piezoelectric work. Rocks, 422 , igneous, 422 , metamorphic, 450 , sedimentary, 439 Rock-forming minerals, 424 Rock-salt, 446 Rotation twins, 123 Rotatory-reflection. The simul- taneous operations of reflection and rotation. Rubellite. The pink to red variety of tourmaline. Ruby. The transparent red variety of corundum. copper. A synonym of cuprite. silver. A group name for pyr- argyrite and proustite. Rutile, 273 S Saline residues, 447 Salt. (1) Synonym of halite. (2) compounds formed by the union of bases with acids. Salt of phosphorus. Hydrous acid sodium ammonium phosphate used in bead tests. Salts, acid, 10 basic, 10 normal, 10 Sand, 440 Sandstone, 441 Sanidine. A clear transparent vari- ety of orthoclase found in vol- canic igneous rocks. Sapphire. The transparent blue variety of corundum. Sard. A brownish-red variety of chalcedony used as a gem. Sardonyx. Agate with red and white bands. Satin-spar. A fibrous variety of gypsum. Scalar, 147 Scalenohedral, hexagonal, class, 104 tetragonal, class, 80 Scalenohedron. There are two kinds of scalenohedrons but when no qualifying term is used the hexagonal scalenohedron is meant. Scalenohedron, hexagonal, 71 tetragonal, 71 INDEX AND GLOSSARY 523 Scale of fusibility, 34 hardness, 153 Scapolite, 377 Scheelite, 339 Schistose. With the characters of a schist. Schists, 451 Schoenflies, German mathematician (1853 ). Schorl. An old name for tourmaline. Sclerometer, 154 Screw-axis of symmetry, 138 Secondary, 460 enrichment, 460 Sectile. Capable of being cut by a knife but not flattened out by a hammer. Sedimentary rocks, 439 Selenite. The cleavable variety of gypsum. Selenite plate. A plate of selenite showing the sensitive tint (q.v.}. Semi-opal. Common opal as dis- tinguished from precious opal and fire opal. Sensitive tint. The purple inter- ference color between red of the first-order and blue of the second-order, 185 Sericite, 394 Serpentine (rock), 455 minerals of. Antigorite, chrysotile. Serpentinization. The alteration of peridotite to serpentine. Shade, 155 Shale, 442 SIDERITE, 299 Silica minerals, stability relations of, 265 Silicates, 341 Siliceous sinter, 449 Sill, 426 Sillimanite, 383 SILVER, 220 assay, 34 cupellation, 34 glance. A synonym of argentite. minerals of. Argentite, cerar- gyrite, polybasite, pyrargy- rite, silver, stephanite. tests for, 50 Skeleton crystals. More or less hollow crystals formed by rapid crystallization. Slate, 451 Slower ray, 187 Smaltite, 238 SMITHSONITE, 301 Smoky quartz. A variety of quartz with a brown pigment. Snow, 266 Soapstone. A massive metamor- phic rock consisting essentially of talc. Soda. Sodium oxid (Na 2 O). This term is often incorrectly used for sodium carbonate. Soda-lime feldspar. A synonym for plagioclase. Sodalite, 356 Soda niter. A synonym of nitratine. Sodium carbonate bead tests, 33 metaphosphate bead tests, 32 minerals. Albite, analcite, cry- olite, glaucophane, halite, lazurite, .natrolite, nepheline, nitratine, sodalite, ulexite, scapolite. tests for, 50 Sohncke, German mathematician (1842 ). Solid solutions, 16 524 INDEX AND GLOSSARY Solubility of minerals, 38 Solutions, solid, 16 Space-groups, 139 Space-lattice, 132 Specific gravity, 148 Specific gravity balance, 149 Specular iron-ore. A synonym of hematite. SPHALERITE, 231 Spherulites, 128 Sphene. A synonym of titanite. Sphenoid, 69 Sphenoidal class, 80 Spinel, 278 Spodumene, 370 Stalactitic, 128 Staurolite, 387 Steatite. The same as soapstone. Stellate. With a radiate star effect. Steno. A Danish scientist, who first discovered the law of constancy of interfacial angles (1638- 1687). Stephanite, 248 Stibiconite, 275 STIBNITE, 225 Stilbite, 409 Streak, 155 Streak-plate, 155 Stream-tin. A variety of cassiterite found in placer deposits. Striations, oscillatory. Striations produced by alternate develop- ment of two adjacent forms, twinning, 125 Strontianite, 304 Strontium, minerals of. Celestite, strontianite. tests for, 51 Structure, internal, of crystals, 132 of minerals, 128 of igneous rocks, 425 Sub-. A prefix indicating a lower quality or degree than the normal. Subhedral, 54 Subjacent, 426 Sublimates on charcoal, 29 in closed tube, 28 in open tube, 27 on plaster, 30 Submetallic. With a luster inter- mediate between metallic and adamantine. Subsectile. Imperfectly sectile like chalcocite. Subsilicates. Silicates of the type n RO-SiO 2 where n > 2, 343 Subsilicic, 423 Succession, order of, of minerals, 418 Sulfantimonates, 243 Sulfantimonites, 243 Sulfarsenates, 243 Sulfarsenites, 243 Sulfates, 326 tests for, 51 Sulfids, 225 tests for, 51 Sulfo-acids, 9 Sulfoferrites, 243 Sulfo-salts, 243 SULFUR, 217 Sulfuric acid (reagent), 23 Sunstone. A variety of oligoclase with a spangled appearance due to inclusions of hematite or goethite. Supergene, 461 enrichment, 460 Surficial. Deposits found on the immediate surface of the earth. Syenite, 431 porphyry, 431 Sylvite, 253 INDEX AND GLOSSARY 525 Symbols of crystal faces, 74 Symmetry, 60 Symmetry, axis of, 60 center of, 62 composite 62 plane of, 61 Symmetrical extinction, 184 Synthesis of minerals, 418 Systems, crystal, 81 Table, of the 32 crystal classes, 80 of atomic weights, 6-7 Tables, determinative, 466 indices of refraction, 207-209 specific gravity, 150-152 Tabular habit, 468 TALC, 400 Talc schist, 451 Tarnish. A thin surface film caused by oxidation. Tellurids, 242 Tellurium, minerals of. Calaverite. tests for, 52 Test-plates, 188 Tetartohedral, 83 Tetartoid. The twelve-faced gen- eral form of the tetartoidal class. Tetartoidal class, 80 Tetragonal bipyramidal class, 80 bisphenoidal class, 80 pyramidal class, 80 scalenohedral class, 80 system, 97 trapezohedral class, 80 TETRAHEDRITE, 247 Tetrahedron, 94 Tetrahexahedron, 91 Texture of igneous rocks, 424 Thin-section. A paper-thin slice of a mineral or rock, used for microscopic examination and optical tests. Tin minerals. Cassiterite. tests for, 52 Tin-stone. A synonym of cas- siterite. Tin-stone veins, 457 Tint, 155 Titanite, 412 Titanium minerals. Ilmenite, rutile, titanite. tests for, 52 Topaz, 380 Total reflection, 161 Touchstone. A black variety of chalcedony used for testing the purity of gold and other metals. The color and solubility of the streak are tested. TOURMALINE, 388 Trachyte, 431 Trachyte porphyry Translation, 136 Trap. A dense black igneous rock, usually basalt or diabase. Trapezohedron, 91 hexagonal, 71 tetragonal, 71 trigonal, 71 Travertine, 448 Tremolite, 364 Trichroic, 202 Triclinic system, 121 Tridymite, 263 Trigonal bipyramid, 70 bipyramidal class, 80 prism, 68 pyramid, 70 pyramidal class, 80 trapezohedron, 71 trapezohedral class, 109 Trimorphism. The particular case 526 INDEX AND GLOSSARY of polymorphism in which there are three polymorphs. Trisoctahedron, 91 Tristetrahedron, 94 Troostite. A manganese-bearing variety of willemite. Truncation. The modification of an edge of a crystal by a face that makes equal angles with adja- cent faces. Tufa, calcareous, 448 Tuff, 439 Turgite. The same as turyite. Tungstates, 338 Tungsten minerals. Scheelite, wol- framite, tests for, 53 Turkey-fat ore. A yellow variety of smithsonite containing CdS in solid solution. Turmeric paper. Paper saturated with a solution of turmeric and used for testing borates and zirconium. Turquois, 320 Turyite, 271 Twin-axis, 123 -crystals, 123 -law, 124 -plane, 123 -seam, 125 Twinning striations, 125 Type symbols, 75 Types chemical, 10 U Ulexite, 324 Ultrabasic rocks, 437 Uniaxial, 190 Unisilicates. The same as ortho- silicates. R n 2 SiO 4 = 2R"OSiO 2 , 343 Unit bipyramid. The form {ill) or {1011}. face, 75 forms. The forms (llOj, (101), (Oil), and fill), prism. The form jllO). pyramid. The form Jill) or lion), rhombohedron. The rhombo- hedron, (lOll). series of bipyramids, 113 Uralite. An actinolite pseudo- morph after, or alteration of, pyroxene. Uralitization. The alteration in- volved in the formation of uralite. Uraninite. The crystalline equiva- lent of pitchblende. Uranium minerals. Carnotite, pitch- blende, tests for, 53 Valencianite. A synonym of adularia. Vanadates, 317, 321 Vanadinite, 317 Vanadium minerals. Carnotite vanadinite. tests for, 53 Variations in the chemical composi- tion of minerals, 4 Vectorial, 147 Veins, 455 minerals of, 456 Verd antique. A serpentine rock veined or mottled with car- bonates. Vertices of a crystal, 55 Vesicular, 425 Vesuvianite, 378 INDEX AND GLOSSARY 527 Vibration directions, 184 plane of, 167 Vicinal faces. Crystal faces with high indices which adjoin or replace faces with lower indices. Vitreous luster, 154 Vitrophyre, 430 Vivianite, 318 Volcanic ash, 439 bombs, 439 breccia. Practically the same as an agglomerate. dust, 439 emanations; 437 rocks, 426 tuff, 439 Vug. A cavity in a vein lined with crystals. W Water of constitution, 11 of crystallization, 11 tests for, 46 Wave-front, 158 -length, 158 -motion, 156 Weathering, 440 Wedge, gypsum, 206 quartz, 199 Weiss. German crystallographer (1780-1856). We'ss symbols, 75 Wernerite. The most prominent member of the scapolite group. Whewellite. Naturally occurring hydrous calcium oxalate, 416 Willemite, 375 Witherite, 305 Wolframite, 338 Wollaston. An English chemist who invented the reflection goniometer (1766 - 1828). Wollastonite, 369 Wood-opal. The same as opalized wood. Wood, silicified. Wood replaced by chalcedony, opal, or quartz. Wulfenite, 340 X-rays and crystal structure, 132 X-ray spectrometer, 141 ZEOLITE (group), 407 Zinc blende. A synonym of sphaler- ite. minerals. Calamine, franklin- ite, smithsonite, sphalerite, willemite. tests for, 53 Zircon, 379 Zirconium minerals. Zircon, tests for, 53 Zone of crystal faces, 56 Zone of oxidation, 459 Zone of secondary enrichment, 460 FOURTEEN DAY USE RETURN TO DESK FROM WHICH BORROWED LOAN DEPT. This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. LD 21-100m-2,'55 (Bl39s22)476 General Library University of California Berkeley YE 1 6 / /b 57 1 6 UNIVERSITY OF CALIFORNIA LIBRARY