THE WORLD'S
MINERALS
BY
LEONARD J. SPENCER, M.A., F.G.S.
OF THE MINERAL DEPARTMENT, BRITISH MUSEUM
EDITOR OF THE MINERALOGICAL MAGAZINE
WITH FORTY COLORED PLATES AND TWENTY-ONE
DIAGRAMS
NEW YORK
FREDERICK A. STOKES COMPANY
PUBLISHERS
5
H//
Copyright, 1911, by
FREDERICK A. STOKES COMPANY
EA. ;t
SCIENCES
LIBRARY
/// rights reserved, including that of translation into foreign
languages, including the Scandinavian
GPOtOGICAL SCIENCES
PREFACE
THE text of this book on minerals is in the main
descriptive of the 116 species of the more common
simple minerals, which are illustrated by 163 figures
on the colored-plates. Descriptions of a few other
important species have been added. Technical terms
are explained in the preliminary chapters; their use,
however, has been avoided as far as possible, and an
attempt has been made-to present in popular language
an interesting and readable account of the several
kinds of minerals. Points of general interest are
touched upon, and attention is drawn to such of the
more prominent characters as will fielp the student
and collector of minerals to identify his own speci-
mens. Mention is also made of the various practical
applications of minerals, their importance as ores of
the metals, as precious stones, etc.
The forty colored-plates have been prepared under
the supervision of Dr. Hans Lenk, Professor of Min-
eralogy and Geology in the University of Erlangen,
and many of the pictures represent actual specimens
belonging to the mineral collection under his charge.
A correct idea of minerals cannot, however, be
conveyed by pictures and descriptions alone. It is
necessary to handle actual specimens, and if the
student possesses a small collection of his own, his
acquaintance with the minerals will be all the more
v
M160151
vi PREFACE
real. There are few objects more attractive and
readily preserved, from a collector's point of view,
than minerals. Specimens may be acquired for a
small sum from the mineral dealers in all large cities;
but if the student can visit mining districts and collect
and observe for himself, he will learn far more. A
knowledge of minerals is not only of interest in itself,
but it can often be turned to useful account.
L. J. S.
CONTENTS
CHAPTER PAGE
I. INTRODUCTION I
II. THE FORMS OF MINERALS .6
Cubic System ......... 9
Tetragonal System 19
Orthorhombic System 21
Monoclinic System 22
Anorthic System 23
Rhombohedral System . .. 23
Hexagonal System 25
The Habit of Crystals 25
Intergrowths of Crystals 27
Forms of Aggregation of Crystals 29
Amorphous Minerals 32
Pseudomorphs 32
III. THE PHYSICAL CHARACTERS OF MINERALS .... 34
Colors of Minerals -34
Transparency and Luster 37
Specific Gravity .38
The Fracture and Cleavage of Minerals .... 41
Hardness 43
Magnetic and Electrical Properties ... -44
Touch, Taste, and Smell ... .45
IV. THE CHEMICAL COMPOSITION AND CLASSIFICATION OF
MINERALS . . 46
V. THE NATIVE ELEMENTS * 53
Non-metallic Elements . . . -54
Semi-metallic Elements . 67
Metallic Elements 70
VI. THE SULPHIDES, ARSENIDES, AND SULPHUR-SALTS . . 78
VII. THE HALOIDS 101
VIII. THE OXIDES . . no
IX. THE CARBONATES . . .138
* For an enumeration of the several species of minerals described in Chapters V.-
XIV., see the List of Plates given on pages ix and x.
viii CONTENTS
CHAPTER PACK
X. THE SULPHATES, CHROMATES, MOLYBDATES, AND TUNG-
STATES . . . . . . . . . .156
XI. THE PHOSPHATES, ARSENATES, AND VANDADATES . . 170
XII. THE SILICATES . . . . . . . . .183
The Felspar Group . . . . , . , .186
The Amphibole Group , , . . . . . 195
The Pyroxene Group 202
The Sodalite Group ... . . . . .207
The Garnet Group . ... . . . . 213
The Mica Group 233
The Chlorite Group .239
The Zeolite Group . . . . . . 244
XIII. THE TlTANO-SILICATES, TlTANATES, AND NlOBATES . . 249
XIV. THE ORGANIC SUBSTANCES . . . . . , .252
INDEX 261
LIST OF PLATES
FACING
PAGE
Plate i. NON-METALLIC ELEMENTS: Diamond, Graphite, Sulphur 62
2. METALLIC ELEMENTS : Arsenic, Antimony, Bismuth,
Silver 70
3. Platinum, Gold, Copper . . 74
4. SULPHIDES : Stibnite, Realgar, Orpiment .... 80
5. Molybdenite, Zinc-Blende, Galena . . 84
6. SULPHIDES AND ARSENIDES: Niccolite, Cinnabar, Mis-
pickel .... 88
7. SULPHIDES : Marcasite, Iron-pyrites, Pyrrhotite . . 94
8. SULPHUR-SALTS, &c. : Copper-pyrites, Smaltite, Tetra-
hedrite, Pyrargyrite, Proustite 98
9. HALOIDS: Fluor-spar 102
10. HALOIDS AND OXIDES : Rock-salt, Atacamite, Opal . . 108
ii. OXIDES (Quarts Group} : Amethyst, Smoky-quartz,
Rock-crystal, Cat's-eye, Rose-quartz . .114
12. (Quartz Group) : Agate, Jasper, Hornstone . 116
13. Haematite 122
14. Magnetite, Corundum, Cassiterite, Zircon, Pitch-
blende, Limonite ...... 130
15. Manganite, Pyrolusite, Psilomelane . . . 134
16. CARBONATES: Calcite 138
17. Calamine, Chalybite, Rhodochrosite,
Cerusite 144
18. Aragonite, Cerusite 150
19. Chessylite, Malachite .... 154
20. SULPHATES: Barytes, Anglesite, Celestite . . . 160
21. Gypsum, Linarite 164
22. CHROMATES, TUNGSTATES, &c. : Crocoite, Wolframite,
Wulfenite, Scheelite . 168
23. PHOSPHATES, &c. : Apatite, Pyromorphite, Vanadinite,
Erythrite 174
24. PHOSPHATES: Wavellite, Lazulite, Cuprouranite, Vivi-
anite, Turquoise 178
25. SILICATES (Felspar Group) : Orthoclase, Labradorite,
Microcline, Anorthite .... 190
ix
LIST OF PLATES
PAGE
Plate 26. SILICATES : (Amphibole Group) : Hornblende, Actinolite,
Crocidolite, Nephrite .... 198
27. (Pyroxene Group) : Augite, Diopside, Bronzite,
28. Sodalite, Lapis-lazuli, Leucite, Beryl . . 208
Hypersthene, Wollastonite . . . 204
29. Garnet, Olivine, Idocrase . . . . 216
30. Topaz 220
31. Andalusite (Chiastolite), Kyanite . . .224
32. Epidote, Axinite, Prehnite . ... 226
33. Tourmaline, Staurolite 232
34. (Mica and Chlorite Groups') : Muscovite, Biotite,
Zinnwaldite, Lepidolite, Clinochlore . . 238
35. Serpentine, Meerschaum, Talc . . . 240
36. (Zeolite Group) ; Analcite, Chabazite . . 244
37- Natrolite, Stilbite . . 248
38. TITA NO-SILICATES, NIOBATES, &c. I Sphene, Columbite,
Perovskite . . 250
39. ORGANIC SUBSTANCES: Amber (enclosing insect), Asphal-
tum, Lignite .... 254
40. Coal, Anthracite . . . .258
FIGURES IN THE TEXT
PAGE
Fig. i. The Octahedron, with Axes of Reference 10
2. The Cube, with Axes of Reference 10
,, 3. Net for the construction of a Model of the Octahedron . n
4. Net for the construction of a Model of the Cube . . n
5. The Rhombic-dodecahedron, with Axes of Reference . 13
,. 6. Net for the construction of a Model of the Rhombic-
dodecahedron -13
,, 7-9. Crystals consisting of the Cube and the Octahedron in
Combination 15
,, 10, ii. Mis-shapen Octahedra 17
,, 12, 13. Mis-shapen Cubes ....... 17
14, 15. Tetragonal Bipyramids 19
16. Tetragonal Prism with Basal Planes .... 19
17. Rhombic Bipyramid 21
18. Rhombic Prism with Pinacoid 21
19. Acute Rhombohedron 24
20. Obtuse Rhombohedron ... .... 24
21. Hexagonal Prism with Basal Planes .... 24
xi
THE WORLD'S MINERALS
THE WORLD'S MINERALS
CHAPTER I
INTRODUCTION
THE natural objects found on the surface of our
earth, or within its crust at a moderate distance from
the surface, may be conveniently classified into three
great groups or kingdoms the animal, vegetable,
and mineral kingdoms. The two first embrace every-
thing organic that is to say, everything that is, or
has been, endowed with life; these are largely made
up of the chemical element carbon combined with
the elements (hydrogen and oxygen) of water. The
mineral kingdom, on the other hand, includes all in-
organic bodies that have not been produced by the
agency of life. Contrasted with materials of organic
origin, they show a much greater diversity in the
chemical elements that enter into their composition.
A little consideration will show that a compre-
hensive study of the mineral kingdom is not so sim-
ple a matter as might at first sight appear. About
a century ago this study was known as mineralogy,
but at the present day it is divided up into a number
of distinct, though still related, sciences. The geolo-
gist studies the wider questions relating to the forma-
tion and subsequent destruction of the rocks of the
1
/ . THE WORLD'S MINERALS
earth's crust; the petrologist determines the different
kinds of rocks and their relations to one another; the
palaeontologist studies the mineralized remains of ex-
tinct animals and plants found embedded in rocks;
finally, the mineralogist examines the various kinds
of simple or individual materials which enter into
the composition of the earth's crust, or, as we may
say, the A B C of the mineral kingdom. Some other
branches of inquiry, such as those relating to ore-
deposits, building, ornamental, and precious stones,
are also now coming to be studied as more or less
special subjects.
It is, of course, not always possible to draw a hard-
and-fast line between the animal, vegetable, and
mineral kingdoms. To take a simple example : am-
ber is usually treated of as a mineral, and as such
finds a place in the present volume; but it is regarded
by the botanist as a fossil resin, and it may be of in-
terest to the zoologist also on account of the insects
which it encloses. Again, pearls and coral, though
consisting of material chemically identical with the
minerals aragonite and calcite, and further ranking
with the precious stones, are clearly products of the
animal kingdom. In such cases there is a certain
amount of overlapping; but it is no disadvantage to
have the same objects studied from the different
points of view of the zoologist, botanist, and miner-
alogist. These, indeed, are not the only points of
view from which such objects may be regarded. The
man of commerce looks upon amber, for instance, as
valued at so much a pound, and he watches with
INTRODUCTION 3
interest the fluctuations in the supply and demand of
the raw material. The practical worker of amber
studies each piece with a view to making the most
of his available material in fashioning some object
of utility. Still again, we may have the legal aspect;
and only recently the law courts have been called
upon to decide what is amber and what is not amber,
and whether china-clay is or is not a mineral.
It would, indeed, be difficult to frame a definition
of a mineral which would be satisfactory from every
point of view. But as students of nature, it is desir-
able that we should endeavor to obtain a clear con-
ception of what we may call a simple mineral, or a
mineral species.
As examples of mineral substances met with in
everyday life, we may mention the following kinds
of stones: granite as used for curb-stones, basalt used
for macadam roads, marbles used for statuary and
the decoration of buildings ; many others will at once
present themselves to the reader.
If we examine closely a piece of granite, we at
once see that it is made up of more than one kind of
material; and if we crush it to a coarse powder, we
can pick out fragments of three kinds, which to the
unaided eye present quite different appearances.
These three kinds of material are called quartz, fel-
spar, and mica. If we take them separately and crush
them to a still finer powder, each kind will still
present exactly the same characters as before; and it
is not possible by mechanical means to resolve them
into material presenting other characters. It is true
4 THE WORLD'S MINERALS
that the chemist by analytical methods can still fur-
ther resolve them into two or more (and in the case
of mica as many as a dozen) chemical elements; but
by so doing he destroys the original nature of the
material and all the characters that appertain to it.
Taking now our next example, that of basalt: To
the unaided eye this appears to consist throughout of
only one kind of material, or, in other words, it is
apparently homogeneous. When, however, a slice is
cut so thin that it is transparent, and this is examined
under the microscope, it is at once apparent that here
also we have a mixture of materials. In the case of
marble, on the other hand, we find that this consists
entirely of a single kind of material, which is the min-
eral calcite; and in a white statuary marble the in-
dividual crystalline grains of calcite are visible to the
unaided eye.
Granite and basalt, therefore, though mineral sub-
stances, consist of mixtures of different kinds of min-
erals. Such mineral mixtures are known as rocks,
and the ultimate kinds of minerals of which they are
composed are known as simple minerals or mineral
species. The term rock may also, for convenience, be
applied to marble, since it occurs in nature in very
large masses under the same conditions as other rock-
masses; if it were found only in small masses it would
be merely regarded as a compact variety of the min-
eral species calcite.
In the following descriptions of the different kinds
of minerals that is, of the various mineral species
we shall find that each species possesses certain es-
INTRODUCTION
sential characters that are always the same, and that
these are characteristic of and peculiar to each kind
of matter or stuff (Stoff ', as so expressively used in
German).
Before, however, passing to the more detailed de-
scription of the species of minerals represented on the
colored plates, it will be necessary to give, at least
briefly, some account of the characters of minerals
in general (Chapters II-IV). Here, also, will be
given some explanation of the few technical terms
which are employed in the descriptive portion.
CHAPTER II
THE FORMS OF MINERALS
WHEN minerals grow freely in rock-cavities they as-
sume of their own accord certain external shapes.
These forms have the appearance of artificially-made
geometrical solids, being bounded by plane surfaces
which intersect in straight lines, and they are known
as crystals. Crystals are, however, not confined to the
mineral kingdom, for many other substances, both
inorganic and organic, have the power of shaping
themselves into such regular forms.
It is quite an easy experiment to cause the growth
of crystals of alum, common salt, saltpetre, sugar, etc.,
by simply dissolving these substances in water and
allowing the solution to evaporate. The best method
of proceeding is to add to boiling water as much of
the substance as will dissolve, and then to pour the
concentrated solution into a saucer; as the liquid cools
some of the material held in solution will separate out
in the form of bright, sparkling crystals on the bot-
tom of the saucer. Larger and better-shaped crystals
can be obtained by taking one of those grown in the
saucer and suspending it by a thread in a concentrated
solution of the same substance; the liquid is then put
aside in a room where the temperature is fairly con-
stant, and allowed to evaporate slowly for some days.
6
THE FORMS OF MINERALS 7
Experimenting in this way with different sub-
stances that are soluble in water or other liquids, we
may get together an interesting and instructive col-
lection of crystals. It will then be recognized that
the crystals of different substances are different in
shape, each substance, in fact, crystallizing in its own
fashion or form. Alum, for instance, gives rise to
octahedra, common salt to cubes, and sugar to tabular
forms of rectangular outline (as in the well-known
sugar-candy) . Amongst minerals, also, each kind as-
sumes a form of its own; and it is thus possible to
distinguish one kind of mineral from another from
a consideration of the shapes of their crystals.
The problem is, however, not always an easy one,
for there being an almost endless number of minerals
and other chemical compounds known to mineral-
ogists and to chemists, so there must be an almost end-
less variety in crystals. The problem is further com-
plicated by the fact that the crystals of one and the
same substance are not always exactly the same in
their forms, although these can be reduced to the
same type or fundamental form; the common mineral
calcite, for example, presents hundreds of different
forms of crystals.
It is the business of the crystallographer to study
the different kinds of crystals and endeavor to classify
the large mass of known facts respecting them. The
serious study of crystallography (the science of crys-
tals) dates from the end of the eighteenth century,
and during the early part of the nineteenth century
the fundamental laws of the science were slowly ar-
8 THE WORLD'S MINERALS
rived at by patient study. It has been found that all
crystals can be grouped into thirty-two classes, ac-
cording to the different degree of symmetry they
possess ; and that these classes fall into seven systems.
It is not necessary in this place to give an account
of the thirty-two classes of crystals, but the seven
systems are of prime importance. The latter may be
defined by the relative lengths and angles of inclina-
tion of a set of axes of reference, or crystallographic
axes, which we are to imagine drawn inside the crys-
tals. It will be convenient (for purposes of reference
alone) to give a summary of the seven systems of
crystals in this place, and later to explain the matter
more fully.
1. CUBIC SYSTEM, with three axes, all at right
angles to one another, and of equal lengths.
2. TETRAGONAL SYSTEM, with three axes, all at
right angles to one another, two being of equal lengths
arid the third either longer or shorter.
3. ORTHORHOMBIC SYSTEM, with three axes, all at
right angles to one another, and all of unequal
lengths.
4. MONOCLINIC SYSTEM, with two axes inclined to
one another, but both at right angles to a third axis,
the three axes being of unequal lengths.
5. ANORTHIC SYSTEM, with three axes, all inclined
at oblique angles, and all of unequal lengths.
6. RHOMBOHEDRAL SYSTEM, with three equal axes,
mutually inclined at the same angles, which are not
right angles.
7. HEXAGONAL SYSTEM, with three equal axes, in-
CRYSTALS THE CUBIC SYSTEM 9
clined to one another at 60 in one plane, and a fourth
axis of different length perpendicular to this plane.
CUBIC SYSTEM
It will be necessary, to begin with, to explain
briefly some of the fundamental principles of crystal-
lography, and for this reason our description of the
cubic system will be longer than that of any of the
other six systems. Consider a regular octahedron
(Fig. i) that is, a solid bounded by eight equilat-
eral triangles; by joining the opposite corners we
obtain within the solid three lines, AA', BB', and
CC', of equal lengths, which intersect at right angles
in the center O, of the crystal. These three equal
rectangular axes are the axes of reference, or crystal-
lographic axes, of the cubic system.
Again, taking a cube (Fig. 2) that is, a solid
bounded by six square planes at right angles to one
another in three pairs; by joining the centers of op-
posite faces we obtain the same set of three equal
rectangular axes, which are here parallel to the edges
of the cube.
The cube and the octahedron are thus referable to
the same set of axes. (In Figs, i and 2 the axes are
drawn of the same lengths for both solids.) But
this is not the only point they have in common ; they
also possess the same degree of symmetry. From Fig.
i it is easy to see that the octahedron can be cut along
each of the three planes ABA'B', ACA'C' BCB'C'
into two equal and similar halves. If one of these
10
THE WORLD'S MINERALS
halves be placed with its cut surface against a mirror,
the mirror-reflection will reproduce the missing half,
and the solid will appear complete. These planes,
ABA'B', etc., are known as planes of symmetry of the
solid. Now a comparison of Figs, i and 2 will show
that these three planes of symmetry are respectively
parallel to the three pairs of parallel faces of the
cube ; and planes passing through the points bearing
Fig. i The Octahedron,
with Axes of Reference.
Fig. 2 The Cube,
with Axes of Reference.
the same letters in Fig. 2 will also cut the cube into
two equal halves. In addition to these three prin-
cipal planes of symmetry of the cube and the octa-
hedron, there are in each solid six other planes of
symmetry; these can be made out by a little study
from the figures, or, better still, with the aid of
models.
Models are, indeed, indispensable aids to the study
of crystals, and some of the more simple forms should
be made by the reader himself if he wishes to gain
much insight into this most interesting subject. Fig.
SYMMETRY OF CRYSTALS
11
3 gives the surface of an octahedron unfolded, or de-
veloped, in a plane. A tracing of this may be pasted
on thin cardboard, cut out, and folded along the
dotted lines; the result will be an octahedron. In
the same way Fig. 4 will give a cube. When the
sheets are folded, the edges of the models may be
fixed with gummed paper.
With the aid of the models, it will readily be seen
that these solids possess not only nine planes of sym-
metry, but also a number of axes of symmetry.
Fig. 3 Net for the construction
of a Model of the Octahedron.
Fig. 4 Net for the construction
of a Model of the Cube.
Standing the octahedron up on one corner, C', and
rotating it about the vertical axis, CC', through a
quarter of a complete revolution that is, so that the
point A comes into the position of the point formerly
occupied by B we find that the relative position and
aspect of the solid is exactly the same as before; and,
further, this covering position is obtained four times
during a complete revolution. Such an axis of sym-
metry is called a tetrad axis of symmetry; and it will
be readily seen that, both in the octahedron and the
cube, there are three such axes. Both solids also
12 THE WORLD'S MINERALS
possess four axes of triad symmetry (rotation about
which gives three coincident positions during a com-
plete revolution), and six dyad axes (giving only
two coincident positions). In addition to these nine
planes of symmetry and thirteen axes of symmetry,
there is also a center of symmetry (O in Figs, i and
2), through which every point on the solid has a
corresponding point reflected at an equal distance on
the other side. With the help of his home-made
models the student should be able to make out to his
own satisfaction all these symmetrical relations of
the cube and the octahedron.
We thus see that the cube and the octahedron,
though so very different in form, are both referable
to the same system of crystals the cubic system. But
these are not the only forms of this system ; there are
many others.
Studying again Figs, i and 2 from a somewhat
different point of view, we see that each face of the
octahedron intersects all the three axes of reference
at equal distances from the center. The face ABC,
for example, cuts the axes at the equal distances OA,
OB, and OC. The position of this face, with refer-
ence to these axes, is therefore expressed by the sym-
bol (in) (one, one, one). On the other hand, the
faces of the cube each intersect only one axis (at the
distance OA, OB, or OC), and are each parallel to
the other two axes that is, intersect it at an infinite
distance. The position of a face of the cube with
reference to the axes is thus expressed by the symbol
(i oooo) (one, infinity, infinity), or, if we take the
THE FACES OF CRYSTALS
13
inverse of these, by the symbol (100) (one, naught,
naught).
Now let us see what happens if we take a face with
the symbol (no) that is, a face that intersects two
axes at equal (unit) lengths and is parallel to the
third. We shall find that twelve faces of this kind
can be built up around the cubic axes, and the solid
Fig. 5 The Rhombic Dode-
cahedron, with Axes of
Reference.
Fig. 6 Net for the construction of
a Model of the Rhombic-Dodecahe-
dron.*
we arrive at is known as the rhombic-dodecahedron
(Fig. 5), which is bounded by twelve equal rhomb-
shaped faces. Continuing our exercise in paper cut-
*A larger model, more convenient for study, may be made by draw-
ing this net on a larger scale ; but it is important that the angles of the
rhombs should be exactly 70^ and 109^, and that the edges be all
equal.
14 THE WORLD'S MINERALS
ting and folding, we shall find that a model (Fig. 6)
of this solid possesses all the symmetrical relations
of the cube and the octahedron.
Another variation in the intersection of faces on
the axes may be introduced by taking fractional
lengths on one or two of the axes. Thus a face may
intersect two of the axes at the unit length (OA),
and the third axis at one half (or one third, etc.) this
distance from the center. We shall then derive a
more complex form with twenty-four faces, for
which the symbol is (112) (or 113, etc.), which is
known as an icositetrahedron. Proceeding in this
way, we may obtain in the cubic system seven differ-
ent kinds of simple forms, the most complex being
bounded by forty-eight faces, all of which are sym-
metrically arranged as before.
Now in actual crystals these simple forms may ex-
ist either alone or in combination with each other.
It may thus happen that one and the same crystal
may be bounded by hundreds of small faces; but it
will be found that they all obey the definite laws of
whole or fractional intercepts on the axes, and that
all are arranged with regularity as required by the
planes and axes of symmetry noted above.
To give a simple illustration of this combination
of forms, we may suppose that the corners of the octa-
hedron are truncated, or chopped off, by faces paral-
lel to the planes AA'BB', AA'CC', and BB'CC'
that is, parallel to the faces of the cube; the result
will then be as shown in Fig. 7. Or, again, we may
truncate the eight corners of the cube by triangular
MALFORMATION OF CRYSTALS 15
faces parallel to the eight faces of the octahedron
(Fig. 9). By increasing the size of the small square
faces in Fig. 7, or by increasing the triangular faces
in Fig. 9, we arrive at the form shown in Fig. 8,
which is known as a cubo-octahedron. These three
crystals are therefore essentially the same, differing
only in the relative sizes of the faces of the cube and
octahedron. The ingenious reader will find instruc-
tion in making models of these crystals.
So far, we have considered crystals to be ideally
perfect in their development, but actual crystals
Fig. 7 F'g- 8 Fig. 9
Crystals consisting of the Cube and the Octahedron in combination.
as found in nature very frequently display some
irregularity or distortion. This irregularity is, how-
ever, more apparent than real, and depends only on
the relative sizes of the faces and the distances of
these from the center of the crystal. However much
a face may be displaced, it always remains parallel
to its true position; consequently the angle between
two adjacent faces is always the same. This law of
the constancy of angles is one of the fundamental
laws of crystallography.
Any apparent irregularities are due merely to ac-
cidents of growth. The crystal may have grown in
16 THE WORLD'S MINERALS
a cramped position, and material for its growth may
not have been supplied to it on all sides at the same
rate. Thus the crystals of alum which we caused
to grow in a saucer rested on the bottom, and there-
fore could not grow downwards, but only upwards
and sideways; as a result of this, many of these crys-
tals will be of a flattened shape (Fig. 10). When,
however, we grow an alum crystal freely suspended
in its mother-liquid, material for its growth is sup-
plied equally on all sides, and a crystal of geometric-
al regularity is formed.
As illustrations of this malformation of crystals,
we may give the somewhat extreme cases shown in
the accompanying figures (Figs. 10-13). In Fig. 10
we have an octahedron flattened parallel to one pair
of opposite and parallel faces ; here the material for
the growth of the crystal was supplied mainly at the
edges, or the crystal may have started its growth in a
narrow fissure in solid rock. In Fig. 1 1 the octahe-
dron has grown more in the direction of an edge
between two adjacent faces. Similarly, Figs. 12 and
13 represent distorted forms of the cube. A good
example of distortion is shown by the crystals of
sodalite in Plate 28. Fig. i; here six faces of the
rhombic-dodecahedron are elongated in the direction
of a triad axis to give a form resembling a hexagonal
prism.
Although not geometrically perfect, such crystals
as these are crystallographically perfect. A com-
parison of Figs. 10-13 with the perfectly shaped octa-
hedron and cube in Figs, i and 2 will show that all
MALFORMATION OF CRYSTALS 17
the edges between corresponding faces of these crys-
tals retain their true directions, while the angles be-
tween corresponding edges and faces are the same.
Fig. 10 Fig. ii
Misshapen Octahedra.
It might be said that these distorted forms are not
symmetrical with respect to the planes and axes of
symmetry mentioned above ; but the symmetrical re-
lations of the angles are preserved, and so is the
regularity in the internal structure of the crystal.
Fig. 12
Misshapen Cubes.
Fig. 13
An important result of this parallelism of the
edges of crystals is that their faces are arranged in
In other words, there is a girdle of faces, the
zones.
18 THE WORLD'S MINERALS
edges of intersection of which are parallel. This is
best shown by the nets of the cube (Fig. 4) and the
rhombic-dodecahedron (Fig. 6) ; it will be seen that
in the cube there is a continuous strip of four faces,
and in the rhombic-dodecahedron one of six faces.
Again, in Fig. 7, we see that four octahedron faces
and two cube faces lie in a zone, since their edges of
intersection are parallel; and on this crystal there are
six zones of this kind.
Turning now to the figures on the colored plates,
we shall find many examples from actual mineral
specimens of the forms of crystals belonging to the
cubic system, of which a brief sketch has been given
above.
Minerals that crystallize in the form of the octa-
hedron are illustrated by diamond (Plate i, Fig. i)
and magnetite (14, i) ; those in the cube by native
copper (3,5), iron-pyrites (7,2), fluor-spar (9, i and
2), rock-salt (10, i and 2), and perovskite (38, 4).
Combinations of the cube and the octahedron are
shown by galena (5, 3 and 4) and smaltite (8, 3).
The icositetrahedron with the symbol (112) is shown
by crystals of analcite (36, i) and leucite (28, 3),
and in combination with the rhombic-dodecahedron
by garnet (29, i and 2). Other cubic minerals rep-
resented on the colored plates are zinc-blende, tetra-
hedrite, and sodalite. The special hemihedral forms
of iron-pyrites and of zinc-blende and tetrahedrite
will be mentioned under these minerals.
TETRAGONAL SYSTEM
19
TETRAGONAL SYSTEM
Corresponding to the octahedron of the cubic sys-
tem, we have in the tetragonal system a solid also
bounded by eight equal triangles; but here the tri-
angles are isosceles and not equilateral, two of their
sides being equal and either longer or shorter than
Fig. 14 Fig. 15
Tetragonal Bipyramids.*
Fig. 16
Tetragonal Prism with
Basal Planes.
the third. This form is a double pyramid on a square
base, and is known as a tetragonal bipyramid (Figs.
14 and 15). In Fig. 14 the vertical axis joining the
upper and lower apices of the pyramid is longer than
the two (equal) horizontal axes, and we have a steep
or acute pyramid. In Fig. 15 the vertical axis is
shorter, and the pyramid is low or obtuse.
*In Fig. 14 the vertical axis has been made twice the length of tjie
equal horizontal axes ; and in Fig. 15 it is half the length of the latter.
20 THE WORLD'S MINERALS
Here, then, we see that there may be a variation in
the angle between the faces of the crystal. In the
cubic system the angles of the octahedron are, of
course, always the same whatever be the mineral
crystallizing in this form. In the tetragonal system,
on the other hand, the angle between the faces of the
pyramid will be different for different minerals, but
it will always be the same for the same kind of min-
eral. The particular value of this angle (from which
can be calculated the ratio or relative lengths of the
vertical and horizontal axes) is fixed for, and charac-
teristic of, each kind of mineral. Other pyramids
may, however, be present if they intersect the unit
vertical axis in simple multiple or sub-multiple dis-
tances; and in this way we may have a combination
of steep and low pyramids on the same crystal.
Suppose that the pyramid faces become so steeply
inclined that they intersect the vertical axis at an in-
finite distance, we should then have four faces
parallel to this axis, and so obtain a square prism.
Again, we may imagine the pyramid to become flat-
ter and flatter until finally its four upper faces and
its four lower faces coincide in one plane or pair of
parallel planes. We then have a form of the tetrag-
onal system known as the basal plane, or, taking the
two parallel planes together, as the basal pinacoid.
Neither of these forms the square prism and the
basal pinacoid completely encloses space, so that in
crystals they can exist only in combination with other
forms. Fig. 16, for instance, is a combination of
these two simple forms, the ends of the tetragonal
ORTHORHOMBIC SYSTEM 21
prism being closed by the pair of parallel faces of
the basal pinacoid.
Tetragonal crystals are represented on the colored
plates by scheelite (Plate 22, Fig. 4) in simple tetrag-
onal bipyramids; wulfenite (22, 3) and cuprou-
ranite (24, 3) in flat square plates with a large de-
velopment of the basal pinacoid. Combinations of
two tetragonal prisms with a pyramid are shown by
zircon (14, 4) and idocrase (29, 4), the latter show-
ing also the basal plane. Other tetragonal minerals
here represented are copper-pyrites (8, i and 2) and
cassiterite (14, 3).
ORTHORHOMBIC SYSTEM
Here, again, the primary form is a solid bounded
by eight equal triangles, but now the edges of the
c
Fig. 17 fte- 18
Rhombic Bipyramid. Rhombic Prism with Pinacoid.
triangles are all unequal. This form is called a
rhombic bipyramid (Fig. 17), since the three sec-
tions of it made by planes, each passing through four
22 THE WORLD'S MINERALS
of its corners, are all rhombs. These three planes
each cut the solid into two equal and similar halves,
and they are therefore planes of symmetry (cor-
responding to the planes ABA'B', ACA'C', and
BCB'C of the regular octahedron). In the ortho-
rhombic system these are the only possible planes of
symmetry. Other simple forms are rhombic prisms
and pinacoids. The prisms consist of four faces
parallel to one or other of the three axes, and they
have a rhomb-shaped cross-section. The three pina-
coids each consist of a pair of faces parallel to two
of the axes. Fig. 18 shows a rhombic prism parallel
to the axis BB', its ends being closed by the pinacoid
parallel to the axes AA' and CC'.
From the angles between the faces of the crystals
it is possible to calculate the relative lengths of the
three unequal axes AA', BB', and CC. These values
are different for different minerals, but constant for
crystals of the same mineral.
Many minerals crystallize in this system; on the
colored plates examples are shown of orthorhombic
crystals of sulphur, mispickel, marcasite, atacamite,
manganite, cerusite, aragonite, barytes, anglesite, cel-
estite, olivine, topaz, andalusite, staurolite, and col-
umbite.
MONOCLINIC SYSTEM
Here we have an oblique angle between two of
the three axes, and this system is consequently often
known as the oblique system. The crystals possess
only one plane of symmetry, as may be seen from the
RHOMBOHEDRAL SYSTEM 23
figures of gypsum (Plate 21, Fig. i )and augite (27,
i). The simple forms consist of no more than two
or four faces, which of themselves cannot enclose
space; so that all crystals of this system consist of
two or more simple forms in combination. The crys-
tals of lazulite shown in Plate 24, Fig. 2, consist of
a combination of two monoclinic hemi-pyramids;
and here, since the oblique angle does not differ
much from 90, the combined form has quite the ap-
pearance of a rhombic pyramid.
Other common minerals crystallizing in this sys-
tem are wolframite, orthoclase, hornblende, diopside,
epidote, muscovite, zinnwaldite, clinochlore, and
sphene, all of which are represented on the plates.
i
ANORTHIC SYSTEM
Here not only are all the axes of unequal lengths,
but they are all inclined at oblique angles. There
are no planes or axes of symmetry, but only a center
of symmetry. Each simple form, therefore, consists
only of a pair of parallel faces, and a complete crystal
must be bounded by at least three such forms.
Examples are given by microcline, anorthite, ky-
anite, and axinite.
RHOMBOHEDRAL SYSTEM
In this system the typical or primary form is a
rhombohedron that is, a solid bounded by six equal
rhomb-shaped faces. This form possesses a certain
24 THE WORLD'S MINERALS
resemblance to the cube. If we stand a cube up on
one corner with one of its four triad axes (p. 12) in a
vertical position, and imagine the solid to be drawn
out or compressed along this direction, we obtain a
rhombohedron. An acute rhombohedron so pro-
duced by the elongation of a cube is shown in Fig.
Fig. 19 Fig. 20 Fig. 21
Acute Rhombohedron. Obtuse Rhombohedron. Hexagonal Prism
with Basal Pinacoid.
19, and an obtuse rhombohedron formed by the com-
pression of a cube along the triad axis is shown in
Fig. 20. These forms exhibit a three-fold arrange-
ment of their faces about the vertical axis, which is
thus an axis of triad symmetry, corresponding to one
of the triad axes of the cube. There are also three
vertical planes of symmetry, each perpendicular to
a face of the rhombohedron, and coinciding with an
edge. All the edges of an ideally developed rhom-
bohedron are equal in length and are equally inclined
to the vertical axis; and these are the lines which in
HEXAGONAL SYSTEM 25
this system of crystals are taken as the axes of ref-
erence.
If we imagine a rhombohedron to be enormously
elongated in the direction of the vertical axis, its six
faces will eventually become parallel to this axis,
and a hexagonal prism will result. If, on the other
hand, the rhombohedron be compressed in the same
direction, the limiting form will consist of a pair of
faces perpendicular to the vertical axis; this form
is then known as the basal pinacoid. Fig. 21 shows
a combination of a hexagonal prism and the basal
pinacoid. Another simple form of importance in the
rhombohedral system is that known as the scaleno-
hedron; this is a solid bounded by twelve scalene tri-
angles, and still showing a three-fold arrangement
about the vertical axis.
Simple rhombohedra are shown by the minerals
calcite (Plate 16, Figs. 2 and 3), rhodochrosite (17,
3), chalybite (5, 3), and chabazite (36, 2) ; and in
combination with other forms by pyrargyrite, prous-
tite, quartz, haematite, corundum, and tourmaline.
HEXAGONAL SYSTEM
This system has many points in common with the
rhombohedral system, but instead of showing a three-
fold arrangement of faces about the vertical axis,
there is a six-fold arrangement. In the crystals of
apatite, pyromorphite, vanadinite, and beryl, illus-
trated on the colored plates, the only forms present
are the hexagonal prism and the basal pinacoid, ex-
26 THE WORLD'S MINERALS
actly as in Fig. 21 of the rhombohedral system. In
some crystals, however, especially those of beryl and
apatite, many more faces are sometimes present.
i
THE HABIT OF CRYSTALS
Apart from the actual degree of symmetry which
they possess and the system to which they belong,
crystals very often present a certain kind of develop-
ment or "habit," and this may often be a character-
istic feature of some minerals. Differences in habit
result from the unequal development of the different
forms or faces of the crystals. The prism faces, for
instance, may be greatly extended, and we then have
a columnar crystal, which is said to be prismatic in
habit, as in crystals of beryl (Plate 28, Fig. 4), ara-
gonite (28, 4), andalusite (31, i and 3), tourmaline
(33, i), etc. If the prisms are relatively longer and
thinner, the habit may be described as long-prismatic,
or rod-like; as in stibnite (4, 2), actinolite (26, 2),
epidote (32, i), etc. When the prisms are still thin-
ner, the habit becomes acicular or needle-like; as in
crocoite (22, i). In some crystals the prisms may be
so very long and slender that the habit is capillary,
or hair-like; this is an extremely characteristic habit
of the mineral millerite (sulphide of nickel), which
on this account is sometimes known as hair-pyrites.
If, on the other hand, the prism faces be short and
the basal plane largely developed, the crystal will be
plate-like, or tabular, in habit. Examples of this
are shown by barytes (Plate 20, Fig. i), wulfenite
INTERGROWTHS OF CRYSTALS 27
(22, 3), the micas (34, 1-3), and chlorites (34, 4).
This tabular habit is especially characteristic of the
micas, the crystals of which often have the form of
small scales. A pyramidal habit is usually shown by
native sulphur (1,3), scheelite (22, 4) , etc. Rock-salt
(10, i and 2) and fluor-spar (9, i and 2) are char-
acterized by their cubic habit, while diamond (i, i)
and magnetite (14, i) most frequently exhibit an
octahedral habit; octahedra of rock-salt and fluor-
spar and cubes of diamond and magnetite are, how-
ever, sometimes found.
INTERGROWTHS OF CRYSTALS
Intergrowths of two or more crystals are of fre-
quent occurrence. Two crystals which commenced
their growth at neighboring centers would in time
fill up the intervening space; they would then co-
alesce and hinder each other's growth. The two
individuals, having commenced their growth quite
independently, would naturally not be related to one
another in any definite manner, and their relative
positions would be quite accidental. Examples of
irregular intergrowths of this kind are to be seen
on almost all of the accompanying plates. Some-
times, however, there may be a parallel grouping of
the indivdual crystals; as shown, for example, by
barytes in Plate 20, Fig. 2, and celestite in Fig. 4 of
the same plate.
In certain cases, however, the two individuals are
grown together in a certain definite and regular man-
28 THE WORLD'S MINERALS
ner, as if one were the mirror-reflection of the other.
Such a grouping is called a twinned crystal, or a
twin, and the plane of reflection is called the twin-
plane. One portion of the twin can be brought into
a position identical with that of the other portion
by a rotation of half a revolution (180) about an
axis perpendicular to the twin-plane. Twinned crys-
tals usually show re-entrant angles between the faces
of the two individuals, and very often this is errone-
ously taken as an indication of the presence of twin-
ning; but it must be remembered that in any acci-
dental or parallel intergrowth of the two crystals
there would also be re-entrant angles between the
faces.
A few good examples of twinned crystals are
shown on the colored plates. The cross of staurolite
(Plate 33, Fig. 5) clearly shows the intergrowth of
two prismatic crystals almost at right angles to one
another. In Plate 21, Fig. 2, is shown a "swallow-
tail" twin of the gypsum. Plate 9, Fig. i, shows in-
terpenetrating twinned cubes of fluor-spar with the
corners of smaller cubes projecting from the faces
of larger cubes; and Plate 36, Fig. 2, very similar
groups of chabazite, but here the crystals are rhom-
bohedra with very nearly the shape of cubes. Knee-
shaped twins of cassiterite are shown in Plate 14, Fig.
3 ; and in marcasite (Plate 7, Fig. i ) five orthorhom-
bic crystals are twinned together to produce a pen-
tagonal form. The pseudo-hexagonal prisms of
aragonite (Plate 18, Fig. 3) are also the result of the
twinning together of several orthorhombic crystals.
AGGREGATION OF CRYSTALS 29
FORMS OF AGGREGATION OF CRYSTALS
Many minerals, instead of growing as single and
distinctly developed crystals, give rise to various,
more or less, accidental shapes and various kinds of
structure or texture, due to the crowding together
of a large number of crystal individuals. Although
this, of course, complicates the study of the crystals
themselves, yet these forms of aggregation are often
extremely characteristic of particular minerals.
When the material consists of a vast number of
minute crystalline individuals closely crowded to-
gether, and each individual is developed to approxi-
mately the same extent in all directions, we have a
granular crystalline structure. Here the crystalliza-
tion of the material had clearly started simultane-
ously at a large number of neighboring centers, and
all the intervening spaces soon became filled up,
forming a mass of crystalline grains, but without the
development of any crystal faces. Such a structure
is well shown by statuary marble and by loaf-sugar;
on fractured surfaces of these materials the extent
of each crystalline individual forming the mass is
readily seen when the bright cleavage surfaces are
examined with a magnifying-glass. If the crystal-
line grains are so small that they cannot be dis-
tinguished, except under the high powers of a mi-
croscope, the structure is described as compact, and
the mineral is said to be massive. Examples of this
are shown on the colored plates by jasper (Plate 12,
30 THE WORLD'S MINERALS
Figs. 2 and 3), hornstone (12, 4), pitchblende (14,
5), turquoise (24, 5), nephrite (26, 4), lapis-lazuli
(28, 2), serpentine (35, i), and meerschaum (35, 2).
Masses of these minerals will, of course, show no
external crystalline form; and if they present any
external surface, other than that of fracture, this will,
as a rule, be rounded, or nodular.
A different kind of structure results when the in-
dividual crystals of the aggregate are greatly elon-
gated in one direction. If the crystals are of an
appreciable size, the structure may be said to be
columnar, while if they are finer the structure is
described as fibrous. Now, we may have different
kinds of fibrous structure, according to the manner
in which the fibers are arranged in the aggregate.
They may have a parallel arrangement, as in ceru-
site (Plate 18, Fig. 4), gypsum (21, 3) and crocido-
lite (26, 3). When the fibers are very fine this
parallel fibrous structure gives rise to a silky luster
in the mineral, as is to be seen in the satin-spar
variety of gypsum and in crocidolite. Again, the
fibers may be matted together to produce a felt-like
aggregate; as in the mineral known as mountain-
leather, or mountain-cork. Or, again, they may ra-
diate from a center, producing star-like groups. The
last form of aggregation is especially common among
minerals, and may be of different degrees of coarse-
ness or fineness; for example, in tourmaline (Plate
33> Fi S- 3)> chessylite (19, i), stibnite (4, i), pyrolu-
site (15, 2), wavellite (24, i), natrolite (37, i and
2), etc.
AGGREGATION OF CRYSTALS 31
Very often minerals with a radially fibrous struc-
ture present on their free surfaces a rounded form.
Many varieties of such external rounded forms may
be distinguished, and these are sometimes quite char-
acteristic of certain minerals. A globular or hemi-
spherical form is invariably shown by wavellite
(Plate 24, Fig. i ; here the hemispherical aggregates
have been broken across). Nodules of haematite are
very often kidney-shaped, or reniform (13, 2). In
the mineral prehnite the rounded forms are them-
selves often aggregated like a bunch of grapes, and
on this account this form of aggregation is described
as botryoidal. Mamillary or breast-like surfaces are
shown by malachite (19,4) and native arsenic (2, i).
Combined with a radially fibrous structure, there
may also be a concentric shelly structure, as shown
by malachite (19, 4) and aragonite (18, 2). In the
last instance the concentric shelly structure predom-
inates ; and a number of the pea-like forms are them-
selves aggregated to give a pisolitic structure.
Again, we may have a radial arrangement of fibers,
not about a point, but around a line or axis; and the
external form may then be stalactitic (e.g. psilomel-
ane, 15, 4), or coralloidal (e.g. aragonite, 18, i).
Such a coralloidal form is especially characteristic
of the variety of aragonite known as flos-ferri.
Many other forms of aggregates might be men-
tioned for example, mossy, leafy, wiry, (native
silver, Plate 2, Fig. 5), dendritic or tree-like (wollas-
tonite, 27, 6) . The almond-like form of agates arises
from the mineral filling a rock-cavity of this shape.
32 THE WORLD'S MINERALS
AMORPHOUS MINERALS
There are but few minerals that possess no indi-
cations of crystalline structure. These are of the
nature of glasses, or colloids; and the best example
is afforded by opal (Plate 16, Figs. 4 and 5). Such
minerals present a smooth and glassy fracture, and
usually occur as fillings in rock-cavities ; sometimes,
however, they may present rounded or botryoidal
external surfaces.
PSEUDOMORPHS
When a mineral, still in the earth's crust, is ex-
posed to conditions other than those that prevailed at
the time of its origin and growth, it may happen that
these will lead to its destruction. The crystals may
be re-dissolved, and their material carried away in
solution to be deposited elsewhere; or chemical
changes may take place and a new generation of
minerals be produced. There are thus in the inor-
ganic world, just as in the organic, periods of growth
and decay, and minerals are by no means so perma-
nent as might be imagined.
A crystal of iron-pyrites (sulphide of iron) em-
bedded in the solid rock may at last be attacked by
percolating surface waters containing oxygen in so-
lution; its sulphur would be removed as sulphuric
acid or as calcium sulphate, while the iron would
be oxidized and take up water, so giving rise to a
new growth of the mineral limonite (hydroxide of
PSEUDOMORPHS 33
iron). The space formerly occupied by the cube
of iron-pyrites is now taken up by limonite, a mineral
which never crystallizes as cubes, or indeed in any
other form. We have then a pseudomorph (or false
form) of limonite after iron-pyrites.
CHAPTER III
THE PHYSICAL CHARACTERS OF MINERALS
THE physical characters of minerals afford many
extremely interesting subjects for study, but here we
can touch on only a few of the more important points.
Of especial interest is the fact that in crystals many
of the physical characters vary with the direction
within the crystal. For instance, light travels
through a crystal with different velocities according
to the direction of transmission ; in other words, a ray
of light can travel more quickly along some paths
within a crystal than along others. The action of
crystals on light presents many problems of extreme
complexity, which can be studied only with the aid
of special instruments. Nevertheless, the color of
minerals and of their crystals is a character that first
attracts attention; so that this part, at least, of the
subject must be dealt with here.
COLORS OF MINERALS.
The colors of some minerals are so well known
that they are used as descriptive terms in defining
color; for instance, we speak of gold-yellow or gold-
en, silver-white, emerald-green, ruby-red, sapphire-
blue, etc. It would, indeed, be possible to draw up
a complete nomenclature of colors by comparison
34
COLORS OF MINERALS 35
with minerals, for these exhibit every possible range
of color. The recognition of minerals by their color
alone is, however, very uncertain, and a wide expe-
rience is necessary before such a knowledge can be
of much practical value.
Many species of minerals may themselves exhibit
a wide range of color, so that one and the same kind
of mineral may be very different in its appearance in
this respect. For instance, the popular idea of topaz
is a gem-stone of a sherry-yellow color; but when
quite pure and free from coloring matter this stone
is perfectly colorless and water-clear; or again, it
may be pink, bluish, or greenish. In the same way,
the mineral corundum is colorless when quite pure;
but other crystals may range through all the colors
of the rainbow red (ruby), orange, yellow (orien-
tal topaz), green (oriental emerald), blue (sap-
phire), and violet (oriental amethyst), and when
larger amounts of impurities are present we have
the black emery; all these are color- varieties of one
and the same kind of mineral. Quartz, fluor-spar, and
many other minerals show similar ranges of color.
In the instances just mentioned the crystals are
transparent and clear, and they owe their color to the
presence of very small amounts often mere traces
of coloring matter diffused through the substance it-
self in much the same way that a dye is dissolved in
water.
In other cases we may have a colorless crystal col-
ored by the inclusion of particles of other minerals,
which may be very minute, and present in such con-
36 THE WORLD'S MINERALS
siderable numbers as to make the crystal opaque. A
good instance of this is given by ferruginous quartz
(or Eisenkiesel) and jasper, which owe their yellow
and red colors to the presence of yellow and red
oxides of iron in relatively large amounts.
In the two cases so far mentioned the color shown
by the crystals is not the color of the pure mineral
which of itself is often white or colorless but is due
to the presence of some coloring matter. The true
color may be determined by crushing a fragment of
the mineral on a sheet of white paper, or by rubbing
a piece on unglazed porcelain. This is known as the
streak of the mineral, and the color of the streak is
a much more definite character of a mineral than
the color seen in bulk.
Other minerals possess a color of their own, and
for these the color of the streak is the same, or nearly
the same, as the color shown by the mineral in bulk.
The colors of gold and copper, for instance, are
inherent in these substances; it is, however, to be
remembered that native gold may be paler in color
when alloyed with much silver, and that native cop-
per is often black or green on the surface, owing to
alteration. Cinnabar (Plate 6, Figs. 2 and 3), orpi-
ment (4, 3), malachite (19, 3 and 4), and chessylite
(19, i and 2), again, are minerals with colors of their
own, respectively red, yellow, green, and blue, which
are the same also in the streaks. The different ores of
iron can be readily distinguished by the colors of
their streaks; that of magnetite is black, haematite
red, limonite brown, and chalybite white.
TRANSPARENCY AND LUSTER 37
The colors displayed by some minerals are of a
totally different nature ; they are not in the minerals
themselves, but are produced by the action of certain
structures in the mineral on white light. The differ-
ent colored rays of which white light is composed
are acted upon in such a way that certain colors are
eliminated, while others remain as flashing rainbow
colors. The interference-colors so produced by pre-
cious opal and labradorite are of the same nature as
those shown by a soap-bubble or by a film of oil
on water. Of the same nature also are the iridescent
colors shown by the tarnished surfaces of some min-
erals, or reflected from cracks in the interior of
crystals.
Still another color effect of crystals, which may be
mentioned here, is that known as pleochroism or
dichroism. Many colored crystals are of a different
color according to the direction through which they
are viewed. A crystal of the mineral cordierite (or
dichroite), for instance, is dark-blue, light-blue, or
straw-yellow, when viewed through in three direc-
tions at right angles. This character, which can
be most conveniently examined with a little instru-
ment called a dichroscope, affords a ready means of
distinguishing many kinds of colored gem-stones.
TRANSPARENCY AND LUSTER
Some minerals are always opaque, as, for example,
the metals and most of the metallic ores. Others
e.g. quartz may vary from perfect transparency to
38 THE WORLD'S MINERALS
complete opacity, an intermediate degree being de-
scribed as translucency ; but in such cases the speci-
mens which are opaque in the mass will show light
through the edges of thin splinters.
The luster of minerals varies not only in intensity,
but also in kind, and the kind of luster is often a very
characteristic feature of different minerals. The
metallic luster of metals and many metallic ores is,
for instance, quite different in character from the
glassy (or vitreous) luster of quartz ; and the peculiar
adamantine luster of diamond enables this mineral
to be recognized at a glance. The luster of other
minerals may be waxy, greasy, resinous, silky, etc.
The physical characters so far enumerated depend
on the action of crystals on light. On the other hand,
some minerals are themselves acted upon by light;
on exposure they may lose their color, transparency,
and luster. A striking instance of this is afforded
by the mineral proustite (Plate 8, Fig. 6), which is
ruby-red, transparent, and has a brilliant adamantine
luster; but on exposure to light it soon becomes black,
opaque, and dull. The transparent, aurora-red crys-
tals of realgar (Plate 4, Fig. 4) soon fall to a yellow
powder when exposed to light. In collections, such
minerals must, therefore, be protected from the light.
SPECIFIC GRAVITY
After color and luster, the character which next
attracts attention is that of heaviness. It is soon no-
ticed that some minerals are heavier than others, and
SPECIFIC GRAVITY
in this respect there are indeed very wide differences.
The extremes of specific gravity range from 1.05 in
amber to 23 in iridium ; that is, amber is only slightly
heavier than an equal volume of water, while irid-
ium is twenty- three times as heavy. The following
table illustrates how various common minerals differ
in their specific gravity.
Amber 1.05
Borax. 1.7
Sulphur 2.1
Rock-salt 2.15
Gypsum 2.3
Orthoclase. . . 2.56
Quartz 2.65
Calcite 2.72
Muscovite. . . 2.9
Fluor-spar. . . 3.2
Jadeite
Diamond...
Chessylite . .
Corundum. .
Barytes
Pyrolusite . .
Iron-pyrites.
Arsenic
Mispickel. . .
Cerusite. .
3.33 Cassiterite. . . 7.0
3.52 Galena 7-5
3.8 Cinnabar 8.1
4.0 Copper 8.85
4.5 Bismuth 9-75
48 Silver 10.6
5.0 Lead 11.4
5.7 Mercury 13.6
6.0 Platinum.... 17.0
6.5 Gold 19-0
The specific gravity can be determined with ac-
curacy, and its value expressed in definite numbers;
and being different for different minerals, it is an
extremely valuable character of determinative value.
With a little practise, it is possible by simply hand-
ling a specimen to gain some idea of the approximate
value of its specific gravity, and so to distinguish
between minerals which may be alike in general ap-
pearance.
The methods available for the accurate determi-
nation of this most important character are explained
in the text-books on physics; they depend on the prin-
ciple of determining the weight of water displaced
by the body, and so finding the ratio between the
weight of the body and the weight of an equal volume
40 THE WORLD'S MINERALS
of water. A specially quick and ready method, par-
ticularly suitable for determining the specific gravity
of minerals, is that given by the use of heavy liquids.
The liquid compound methylene iodide has a specific
gravity of 3.33 (i.e. it is more than three times heav-
ier than water), and is miscible in all proportions
with benzole (sp. gr. 0.89). A cheaper* heavy
liquid is bromoform, with the lower specific gravity
of 2.8.
If fragments of the minerals named in the above
table of specific gravities be dropped into methylene
iodide, fluor-spar and all those of lower specific
gravity will float on the surface of the liquid, while
diamond and all of higher specific gravity will sink
to the bottom; jadeite, with a specific gravity exactly
the same as that of the liquid, will, however, remain
suspended in the liquid, neither floating nor sinking.
By the addition of benzole to the methylene iodide
the specific gravity of the liquid may be reduced to
any desired amount; so that any of the lighter min-
erals will remain suspended.
Selecting a series of known minerals of known
specific gravity for use as indicators in the heavy
fluid, we shall, by comparison, be able to determine
the specific gravity of a fragment of an unknown
mineral, and this will be of great assistance in de-
termining the kind of mineral. This, of course, can
*Methylene iodide costs about 80 cents per ounce, and bromoform
about 20 cents per ounce. Methylene iodide darkens on exposure to
light, and it should, therefore, be kept covered; any depth of color it
may acquire can be removed by allowing bits of metallic copper to
remain in the liquid.
FRACTURE AND CLEAVAGE 41
only be done when the specific gravity of the mineral
is less than 3.33; but still, it is often of great help to
learn that the specific gravity of an unknown mineral
is greater than this. For instance, faceted gem-
stones of similar appearance can often be readily dis-
tinguished by simply dropping them into the heavy
liquid. Thus, if we wish to distinguish between
quartz (sp. gr. 2.65) and topaz (sp. gr. 3.5), it will
at once be seen that the former floats and the latter
sinks.
THE FRACTURE AND CLEAVAGE OF MINERALS
When minerals or crystals are broken their sur-
faces of fracture present many points of difference,
and these differences are important aids in distin-
guishing minerals of different kinds. The crystals
of some minerals possess a special kind of fracture
known as cleavage, it being possible to split or cleave
them along plane surfaces parallel to certain faces
of the crystal. Thus crystals of rock-salt or galena,
whether they be cubes or of any other form, can be
readily split parallel to the faces of the cube. Cubes
of fluor-spar cannot be split in these directions, but
only parallel to the faces of the octahedron. Dia-
mond also possesses a perfect octahedral cleavage,
while zinc-blende cleaves parallel to the faces of the
rhombic-dodecahedron. These cleavages may be de-
veloped parallel not only to one face of these simple
forms, but parallel to all; since the internal crystal-
line structure of the material possesses the same de-
gree of symmetry as the external form of the crystal.
42 THE WORLD'S MINERALS
Thus, since a cube is bounded by three pairs of paral-
lel equivalent faces, there must be in rock-salt and
galena three directions of cleavage at right angles to
one another. In the same way, fluor-spar and dia-
mond each have four directions of cleavage parallel
to the four pairs of parallel faces of the octahedron;
and in zinc-blende there are six.
In calcite there are three directions of perfect
cleavage parallel to the three pairs of parallel faces
of the primary rhombohedron. In crystals belonging
to crystal-systems other than the cubic, there may be
only one plane direction of cleavage, since there can
be no symmetrical repetition of this direction within
the crystal. Thus the micas possess one very perfect
cleavage parallel to the basal plane, and gypsum one
parallel to the single plane of symmetry.
The cleavage of a crystal is an expression of a
minimum of cohesion in a certain direction in the
material of the crystal; and it is important to remem-
ber that the crystal can be split up indefinitely along
its direction of cleavage. Taking, for example, a
crystal of mica, we can keep splitting off thin leaves
until the whole crystal is separated into flakes, and
these flakes can still further be subdivided with a
knife-edge along the same direction of cleavage.
With the harder minerals the best way of producing
the cleavage is to place a knife-edge on the crystal
parallel to the particular crystal-face, and to strike
the back of the knife with a sharp blow from a
hammer.
The cleavages of minerals differ not only in di-
HARDNESS 43
rection and the number of directions, but also in their
perfection. Some minerals cleave more readily than
others, and their surfaces of separation are much
smoother and brighter. In other minerals the cleav-
age may be so poor that it is scarcely noticeable.
Here the kind of fracture is often of importance.
Quartz and opal, for example, break with a smooth
and glassy conchoidal fracture, the surfaces being
rounded and having curved lines like the lines of
growth on the surface of a bivalve shell. Other
kinds of fracture are described as sub-conchoidal,
splintery (e.g. nephrite), hackly (e.g. copper), etc.
HARDNESS
By the hardness of a mineral is meant its capability
of scratching other substances. Some minerals are
so soft that they can be scratched by the finger-nail,
others can be scratched by a knife or file, and others
again are still harder. For expressing degrees of
hardness, mineralogists make use of the following
ten minerals as a scale of hardness, ranging succes-
sively from talc, the softest, to diamond, the hardest.
1. Talc 6. Felspar
2. Gypsum 7. Quartz
3. Calcite 8. Topaz
4. Fluor-spar 9. Corundum
5. Apatite 10. Diamond
Nos. i and 2 on the scale can be scratched by the
finger-nail; Nos. 1-6 by a knife, though No. 6 only
with difficulty. No. 6 will scratch ordinary win-
44 THE WORLD'S MINERALS
dow-glass, but not so easily as will No. 7. Tests
such as these will give a first rough idea of the hard-
ness of an unknown mineral. To determine its
hardness on the scale, we must find the scale-mineral
that it can only just scratch, and the lowest one that
it can be scratched by. Thus, if a mineral scratches
calcite about as easily as it can itself be scratched
by fluor-spar, its hardness may be expressed as 3^2.
In applying this test it is best to rub a sharp corner
of the scratching mineral across a smooth surface
of the mineral to be scratched; the powder produced
should be wiped off, and the surface carefully ex-
amined with a lens, to make sure whether a scratch
has really been produced or only powder rubbed off
the corner of the scratching mineral. In selecting a
smooth surface, such as a cleavage surface or a crys-
tal face, care must be taken that the specimen is not
damaged or a good crystal spoilt. Nothing looks
worse in a collection of minerals than a scratched
and damaged specimen. Earthy or loose aggregates
of crystals do not, of course, show by scratching the
true hardness of a mineral.
MAGNETIC AND ELECTRICAL PROPERTIES
Magnetite being the only strongly magnetic min-
eral, mention of this property may be deferred until
we come to a description of the mineral itself. Most
minerals, except metals and metallic ores (which are
good conductors), acquire a charge of electricity
when rubbed ; sulphur and amber so acquire a nega-
TOUCH, TASTE, AND SMELL 45
tive charge, and most gem-stones a positive charge.
Some few acquire an electrical charge when heated
(see tourmaline).
TOUCH, TASTE, AND SMELL
Some minerals are greasy or soapy to the touch
for example, talc, which for this reason is sometimes
called soapstone; others have a rough, harsh feel.
A few minerals which are soluble in water possess a
characteristic taste for example, salt and epsom-
salts. A few others have a characteristic smell for
example, the bituminous odor of asphaltum. Clays
when breathed upon have an earthy odor. Iron-
pyrites and mispickel, when struck, emit, respective-
ly, a sulphurous and a garlic-like odor.
CHAPTER IV
THE CHEMICAL COMPOSITION AND CLASSI-
FICATION OF MINERALS
MINERALS being chemical elements and compounds,
it is necessary in order properly to understand them
to have some knowledge of chemistry. Of the eighty
different kinds of elementary matter known to chem-
ists, all are found in minerals; and for the majority
of them, minerals are the only source. All the inor-
ganic products prepared by the manufacturing chem-
ist are derived directly from minerals. The more
common chemical elements (forty-one in number)
which are present as essential constituents of the
minerals described in this book are given in the fol-
lowing table, together with their chemical symbols*
and their atomic or combining weights. Several
other rarer elements may also be present in these
minerals; for example, pitchblende contains the ele-
ments radium, helium, and argon, and zinc-blende
often contains the elements cadmium, gallium, and
indium. It will be noticed that the abundant ele-
ment nitrogen is not represented in the table; the
explanation of this is that nitrates and ammonium
*When these are derived from the Latin name of the element this
name is added in parentheses.
46
CHEMICAL COMPOSITION
salts, being all soluble in water, are of rare occur-
rence as minerals.
Atomic
Symbol weight
Aluminium Al 27
Antimony (Stibium) Sb 120
Arsenic As 75
Barium Ba 137
Beryllium Be 9.1
Bismuth Bi 208
Boron B n
Calcium Ca 40
Carbon C 12
Chlorine Cl 35-5
Chromium Cr 52
Cobalt Co 59
Copper (Cuprum).. Cu 63.5
Fluorine F 19
Gold (Aurum) Au 197
Hydrogen H I
Iron (Ferrum) Fe 56
Lead (Plumbum) ... Pb 207
Lithium Li 7
Magnesium Mg 24
Manganese Mn 55
Atomic
Symbol weight
Mercury (Hydrargy-
rum) Hg 200
Molybdenum Mo 96
Niobium Nb 93.5
Nickel Ni 58.7
Oxygen O 16
Phosphorus P 31
Platinum Pt 195
Potassium (Kalium) K 39
Silicon Si 28
Silver (Argentum).. Ag 108
Sodium (Natrium). . Na 23
Strontium Sr 87.6
Sulphur S 32
Tin (Stannum) Sn 119
Titanium Ti 48
Tungsten (Wolfram) W 184
Uranium U , 238.5
Vanadium V 51.2
Zinc Zn 65.4
Zirconium Zr 90.6
Of the elements enumerated above only four
namely, oxygen, hydrogen, fluorine, and chlorine
are gases when in their free state; only one is liquid
namely, mercury; and all the others are solid at
the ordinary conditions of temperature and pressure.
The gases, together with carbon, boron, silicon, sul-
phur, and phosphorus, are classed as non-metallic
elements, all the others being metals.
The chemical compounds met with as minerals
consist, in most instances,* of a combination of one
or more metallic elements with one or more non-
*A notable exception is the common mineral quartz.
48 THE WORLD'S MINERALS
metallic elements. These are combined together in
definite proportions. For example, iron-pyrites con-
sists of a combination of one atom of iron with two
atoms of sulphur that is, 56 parts by weight of iron
with 32 X 2 = 64 parts by weight of sulphur. The
actual weights (grams or tons) is immaterial, since
the atomic weights or combining weights as given in
the above table are only ratios. The chemical sym-
bol, or formula, for iron-pyrites is then FeS 2 ; this not
only expresses that the compound is disulphide of
iron, but it also enables us to calculate the percentage
weight of each constituent namely, 46.6 per cent,
of iron and 53.4 per cent, of sulphur. Again, calcite
is a combination of 40 parts by weight of calcium,
with 12 parts of carbon, and 16X3 = 48 parts of
oxygen, the formula being CaCOs, which expressed
in words is carbonate of calcium.
In order fully to determine the nature of a mineral,
it is necessary to analyze it by the ordinary methods
of chemical analysis; and even for purposes of identi-
fication it is often necessary to apply some simple
chemical tests. Mineral carbonates, for example, can
always be recognized by the fact that they effervesce
in acids. Sulphur is readily tested for by heating the
powdered mineral with sodium carbonate before the
blowpipe on charcoal, and moistening the fused mass
with water on a silver coin; if sulphur is present a
characteristic black stain will be produced on the
silver. At the same time, if a heavy metal is present
in the mineral, a bead of the metal will be formed
on the charcoal.
CLASSIFICATION OF MINERALS 49
Two very important principles relating to the
chemistry of minerals are those of polymorphism
(including dimorphism) and isomorphism. Several
cases are known in which totally distinct minerals
are identical in chemical composition. For example,
diamond and graphite both consist simply of the ele-
ment carbon ; but they consist of carbon crystallized
in different forms, so that, although chemically alike,
they are quite distinct, or dimorphous, substances.
Again, calcium carbonate (CaCOs) crystallizes in
nature either as calcite or as aragonite, which also
are two quite distinct minerals. The most remark-
able case, however, is that of the compound titanium
dioxide, which is met with in nature as the three
minerals rutile, anatase, and brookite; these, though
chemically identical, are distinct in the form of their
crystals and in all their physical characters. Another
instance of trimorphism is given by the minerals ky-
anite, andalusite, and sillimanite (see Chapter XII).
On the other hand, we may have minerals of dif-
ferent chemical composition appearing in crystals
that are almost identical in form; such minerals are
said to be isomorphous. Of this there are numerous
examples amongst minerals, one of the best being
that given by the group of rhombohedral carbonates.
The carbonates of the chemically related elements
calcium, magnesium, iron, zinc, and manganese all
crystallize in rhombohedra with very nearly the same
angles between their faces. But not only this, they
are capable of so intimately growing together that
they may all help to build up one and the same crystal.
50 THE WORLD'S MINERALS
The tiny crystalline bricks of these different sub-
stances are so nearly alike in their shape that they
will all fit into the same structure. We then have
what is called a mixed crystal, consisting of two or
more of these carbonates mixed together in variable
proportions. In such cases the chemical composition
of the crystal cannot be fully expressed by a simple
chemical formula.
The classification adopted in the descriptive por-
tion of this book is a strictly chemical classification,
on the same lines as in the modern treatises on scien-
tific mineralogy. In the first group are placed the
chemical elements; in the next three the sulphides,
haloids, and oxides, respectively; and in succeeding
groups the large number of oxygen-salts, these being
subdivided according to the acid which enters into
combination with a metal to form the various salts,
such as carbonates, sulphates, phosphates, silicates,
etc.* Within each of these groups, minerals which
are isomorphous with one another are placed to-
gether. In an appendix a few organic substances,
which, strictly speaking, are not definite mineral
species, are brought together.
Other systems of classification are of course pos-
sible. For example, in the eighteenth century, before
the development of modern chemistry, natural his-
tory systems of classification were in vogue. These
took account only of the external characters of min-
erals, and naturally lead to very deceptive results;
*An enumeration of the minerals placed under each of these headings
will be found in the List of Plates, p. ix.
CLASSIFICATION OF MINERALS 51
since totally distinct minerals may often closely re-
semble one another in their general appearance and
color. Another method of classification, and one
which is often used at the present day, is to group
minerals according to the metals they contain. But
this system, though of convenience to the practical
miner, is not altogether satisfactory, since, unless
duplication is to occur, certain minerals must be arbi-
trarily allocated to one group or another.
As an example of such a classification, based on
economic lines, the following may be given. This
will, at the same time, give some idea of the practical
uses of the minerals described in the present volume.
A certain amount of duplication is here unavoidable ;
for example, mispickel appears with the ores of iron,
but being of more importance as an ore of arsenic,
it also finds a place under this metal. Linarite comes
under both copper and lead. Again, quartz appears
amongst the precious stones, the rock-forming min-
erals, and the abrasives. On the other hand, several
interesting, though commercially valueless, minerals,
which will be described farther on, do not fall into
this classification.
METALLIC ORES
GOLD: Native gold.
SILVER: Native silver, Pyrargyrite, Proustite, Tetrahedrite.
COPPER: Native copper, Copper-pyrites, Tetrahedrite, Atacamite, Ches-
sylite, Malachite, Linarite, Cuprouranite.
PLATINUM : Native platinum.
LEAD: Galena, Cerusite, Anglesite, Linarite, Crocoite, Wulfenite, Pyro-
morphite, Vanadinite.
ZINC: Zinc-blende, Calamine.
52 THE WORLD'S MINERALS
IRON: Iron-pyrites, Marcasite, Pyrrhotite, Mispickel, Haematite, Mag-
netite, Limonite, Chalybite, Vivianite.
MANGANESE: Manganite, Pyrolusite, Psilomelane, Rhodochrosite.
NICKEL : Niccolite.
COBALT: Smaltite, Erythrite.
MERCURY : Cinnabar.
TIN : Cassiterite.
TUNGSTEN: Wolframite, Scheelite.
URANIUM: Pitchblende, Cuprouranite.
VANADIUM : Vanadinite.
CHROMIUM : Crocoite.
MOLYBDENUM : Molybdenite, Wulfenite.
ARSENIC: Native arsenic, Realgar, Orpiment, Mispickel.
ANTIMONY: Native antimony, Stibnite.
BISMUTH : Native bismuth.
PRECIOUS STONES (GEM-MINERALS)
Diamond, Corundum, Opal, Quartz (Amethyst, Smoky-quartz, Rock-
crystal, Cat's-eye, Rose-quartz, Agate, Jasper), Zircon, Turquoise, Beryl,
Garnet, Olivine, Idocrase, Topaz, Epidote, Tourmaline, Amber.
ROCK-FORMING MINERALS
Quartz group, Felspar group, Amphibole group, Pyroxene group,
Sodalite, Leucite, Olivine, Idocrase, Mica and Chlorite groups, Serpen-
tine, Talc.
SPARRY MINERALS (SPARS)
Fluor-spar, Calcite (calc-spar), Aragonite, Barytes (heavy-spar),
Celestite, Gypsum.
ABRASIVES
Diamond, Corundum (emery), Quartz, Garnet.
SALTS (SOLUBLE IN WATER)
Rock-salt.
INFLAMMABLES
Organic substances (Amber, Asphaltum, Coal, etc.), Graphite, Dia-
mond, Sulphur.
CHAPTER V
THE NATIVE ELEMENTS
ALTHOUGH some eighty different kinds of elementary
matter have been identified by chemists in the ma-
terials of the earth's crust, yet only few of these are
found in a free state in nature. The gaseous ele-
ments, oxygen and nitrogen, which form the bulk of
our atmosphere, can scarcely be regarded as min-
erals; though the former, when in chemical combi-
nation, is present in a large number of minerals. The
chemical elements may be roughly divided into two
great groups the non-metallic and the metallic ele-
ments. Belonging to the former group we find as
minerals the elements carbon and sulphur; and in
the latter group we have the native metals copper,
silver, gold, and platinum, and the so-called semi-
metals arsenic, antimony, and bismuth.
Carbon, though abundant in the organic world,
and present also in chemical combination in lime-
stone rocks and the mineral carbonates, is of rare
occurrence as a native mineral. It is found crystal-
lized in two quite distinct forms, which as minerals
are known as diamond and graphite. The other
chemical elements met with as minerals are known
by their ordinary or chemical names, sometimes pre-
53
54 THE WORLD'S MINERALS
ceded by the word native: thus, we may speak of
sulphur or native sulphur, gold or native gold, etc.
NON-METALLIC ELEMENTS
DIAMOND
(Plate i, Fig. i). Diamond is perhaps the most
remarkable and interesting of all minerals, and for
this reason it will be treated in rather more detail
than the other species. One would little imagine that
such a brilliant, transparent, colorless, and at the
same time intensely hard gem is composed merely of
carbon, our usual idea of which is a black, opaque
substance, so soft that it soils the fingers.
When heated in air, or better still in oxygen, a dia-
mond burns with a small bluish flame, and gradually
disappears. The product of combustion is the gas
carbon dioxide, identical with that produced when
any other form of carbon burns in the air. Diamond
is, in fact, pure carbon ; but, conversely, pure carbon
is not necessarily diamond. We must have the car-
bon crystallized, and, moreover, the crystals must be
of particular form, for, as already mentioned,
graphite is also a crystallized form of carbon.
Crystals of diamond belong to the cubic system.
They most commonly present the form of the regular
octahedron that is, a solid bounded by eight equi-
lateral triangles (Text-Fig, i, p. 10). The crystal
seen partly embedded in the matrix in Plate i is of
this form. Less frequently the crystals have the form
DIAMOND 55
of the cube (Text-Fig. 2) or the rhombic-dodeca-
hedron (Text-Fig. 5). A peculiarity presented by
crystals of diamond is that their faces are usually
curved and their edges rounded, and sometimes this
rounding is so pronounced that the crystals are al-
most spherical in form. Again, the faces are fre-
quently beautifully marked with tiny triangular pits
or depressions.
The great hardness of diamond is one of its most
remarkable features. It is by far the hardest of all
known substances, whether natural or artificial.
There is nothing that it will not scratch with ease.
This was well known to the ancients, who called
the stone adamas, on account of its adamantine or
unconquerable nature; and it is from this word that
our name diamond is derived. The ancients believed
that a diamond could not be broken, but would rather
shatter the hammer and anvil. There is, however,
an important difference between hardness and fran-
gibility, and, as a matter of fact, diamond is quite
brittle.
Further, diamond has the peculiar property of
splitting with ease along certain directions in the
crystal. There are four such directions of easy split-
ting, or cleavage, which are parallel to the four pairs
of parallel faces of the octahedron. In these direc-
tions the fractured surfaces are perfectly plane and
smooth.
The specific gravity of diamond is 3.52 (that is,
a diamond is three and a half times heavier than an
equal volume of water), being considerably higher
56 THE WORLD'S MINERALS
than that of other forms of carbon ; the specific grav-
ity of graphite, for example, is only 2.2. We thus
see that in diamond the ultimate particles of carbon
must be very closely packed together; and it is this
close packing, according to a regular and symmetric-
al plan, that gives to diamond its particular crystal-
line form and its unique physical properties.
Of other physical characters of the diamond, some
mention must be made of its optical properties, or its
behavior with respect to light, since on these largely
depends its use as a gem. The index of refraction
is very high, and consequently a ray of light traveling
from air into diamond will be more strongly bent, or
refracted, than a ray passing from air into water or
ordinary glass. Further, the exact value of the index
or refraction depends on the color of the light, being
2.407 for red light, and 2.465 for violet light. A
prism of diamond, therefore, gives a much longer
colored spectrum than many other transparent sub-
stances; in other words, the dispersion, or dispersive
power, is high. It is owing to this high dispersive
power that a faceted diamond displays its brilliant
flashes of rainbow colors. The high refractive index
is the cause of the marked brilliancy, or fire, shown
by a faceted diamond. Connected also with the
high refractive index we have the characteristic ada-
mantine luster, which enables a diamond to be recog-
nized at sight by the experienced eye. On the rough
surfaces of the less clear stones this luster is almost
metallic in character, and, indeed, some crystals have
almost the appearance of metallic lead.
DIAMOND 57
In the color of diamond there is a considerable
range. Perfectly pure crystals are colorless and
transparent, or water-clear, and are described in the
trade as stones of the "first water." Other crystals
range from pale yellow to dark brown or even black,
while, less frequently, the color may be pronounced
shades of blue, green, or red. These colors are due
to the presence of mere traces of coloring matter in
the material of the diamond itself.
Diamonds are of quite local distribution, and in
Europe the only recorded occurrence is in the Ural
Mountains. The principal diamond-producing coun-
tries are South Africa, India, and Brazil; other oc-
currences of less importance are those of. Borneo,
New South Wales, British Guiana, and the United
States of North America. Although the gem has
been obtained from India ever since the time of the
Romans, it was not discovered in Brazil until the
year 1725, and in South Africa not until 1867. At
the present time the diamond-mining industry of
South Africa is in a very flourishing condition, and
far more diamonds have been found there than in
India and Brazil together for centuries.
The usual mode of occurrence is in the sands and
gravels of river-beds. By a simple process of wash-
ing in a shallow dish, called in Brazil a "batea," the
lighter materials are carried away in a stream of
water, leaving behind the diamond with the other
denser stones. In India and Brazil some of these
diamantiferous deposits are of great antiquity, and
the sand and pebbles are cemented together to form
58 THE WORLD'S MINERALS
a solid rock known as conglomerate. The materials
of these secondary deposits must clearly have been
derived by the weathering and breaking down of
pre-existing rocks; but in India and Brazil the
original rock which supplied the diamond has never
yet been discovered.
In South Africa, on the other hand, the diamonds,
which were first found amongst the gravels of the
Vaal River, were very soon traced to volcanic pipes
in the neighborhood; and when this discovery was
made the town of Kimberley very quickly sprang up
on the spot.
These volcanic pipes are of a rather special kind.
At the surface they have a more or less circular or
oval outline, with a diameter of 200 to 300 yards, and
they project only slightly above the general level of
the ground. Downwards they extend to unknown
depths, passing through horizontal strata of shale,
diabase, and quartzite. The material filling the
pipes is known locally as "blue ground," though
usually it is more of a dark greenish shade, as rep-
resented in Plate i. It is really a mixture of ma-
terials, and consists largely of the alteration products
of rocks rich in the mineral olivine. Chemically,
the blue ground is essentially a hydrated silicate of
magnesium. In structure the material is broken and
fragmentary, and it has no doubt been brought up
into the pipes from an underground reservoir of
molten rock by a series of steam explosions under
high pressure. There does not appear to have been
any active volcano discharging material at the earth's
DIAMOND 59
surface. The diamonds are found embedded in this
material; that they also were brought up into the
pipes by the same series of explosions is abundantly
proved by the fact that the crystals are often broken
and fragmentary. The blue ground is thus not the
original mother-rock of the diamond, and the exact
nature of this still remains unknown, though prob-
ably it was an olivine-rock.
The actual amount of diamond present in the blue
ground is relatively very small, only 2 to 5 millionths
per cent., and a specimen showing a crystal actually
embedded in the matrix (as in Plate i, representing
a specimen from Kimberley) is a rarity. Large
quantities of the rock have therefore to be extracted
by mining operations from the upper portions of
pipes by open workings, and at greater depths by a
regular system of underground mining. The ex-
cavated rock is spread out on extensive and specially
guarded "floors," where it is watered, harrowed, and
exposed to the weather for about a year. After this
treatment it is soft enough for crushing, and the
heavy minerals (diamond, garnet, ilmenite, etc.) are
separated by specially constructed washing machin-
ery. From this heavy residue the diamonds were
formerly picked out by hand, but now it is passed
over by a vibrating table with a greased surface; the
diamonds have the curious property of adhering to
the grease, while the other minerals slide off.
In the neighborhood of Kimberley, in Cape Col-
ony, there are several of these diamond-bearing pipes,
and there are others in Orange River Colony and in
60 THE WORLD'S MINERALS
the Transvaal. By far the largest pipe is one situated
about twenty miles W.N.W. of Pretoria, which was
discovered in 1902, and is worked as the Premier
mine. This pipe measures half a mile across, and it
is further remarkable in having produced the largest
known diamond, namely, the "Cullinan."
Another mode of occurrence of diamond remains
to be mentioned one which, though of no practical
importance, is of considerable scientific interest. Mi-
croscopic diamonds have been detected in a few of
the meteoric stones and irons which as shooting stars
have fallen to our earth from surrounding space.
This mode of occurrence has a bearing on the
artificial production of diamond. By dissolving car-
bon in molten iron and by cooling the mass rapidly
under an enormous pressure, microscopic diamonds
have resulted; and similar results have been ob-
tained by dissolving carbon in molten olivine.
As to the practical applications of diamond, its
use in jewelry is too well known to need more than
a passing mention. In the operation of fashioning
a faceted gem, the rough stone is first reduced to a
suitable size and shape by taking advantage of the
property of cleavage. The general form is then pro-
duced by rubbing two diamonds together a process
known as bruting; and the facets are finished by
grinding on a rapidly revolving disc or lap of iron,
the grinding material being diamond powder itself.
The form of cutting most usually adopted is the bril-
liant, and cut diamonds are consequently often known
in the trade simply as brilliants. For small, flat
DIAMOND 61
stones the less effective rose-cut is the form sometimes
employed.
Certain important technical applications of the
diamond depend on the great hardness of the mater-
ial, which, as already mentioned, far exceeds that of
all other substances. The glazier's diamond consists
of a crystal with a naturally rounded edge; sharp
cleavage splinters of diamond are used for writing
on glass. Diamond powder is much used by lapi-
daries and gem-cutters, it being, in fact, the only
material with which diamond can be ground and
polished. Extensive use is made of diamond for
rock-drills; the cloudy, rounded crystals known as
bort f and the black, granular variety called carbon-
ado, being used for this purpose. The stones are
embedded by pressure in the steel crown of the drill.
We cannot conclude this account of the mineral
diamond without reference to some of the more note-
worthy of the large and famous cut gems, about
which, indeed, it would be possible to write a whole
volume of romance.
Several large gems were seen in India by the
French traveler Tavernier about the middle of the
seventeenth century. Large Indian diamonds were,
however, known in Europe long before that time, for
Charles the Bold, Duke of Burgundy (1433-77),
was the possessor of the "Florentine" and the "Sancy"
diamonds. After many vicissitudes of fortune these
gems are now, one with the Austrian crown jewels,
and the other in the possession of an Indian prince;
they weigh 133^ and 53 *A carats, respectively.
62 THE WORLD'S MINERALS
The "Koh-i-noor" (a name meaning "Mountain
of Light") was taken in 1739 by Nadir Shah, the
Persian conqueror of the Mogul Empire, and in
1850 it was presented by the East India Company to
Queen Victoria. In its Indian form of cutting it
weighed 186^ carats, but this was reduced by re-
cutting in England to 106^ carats.
Most of the historic diamonds are of Indian origin,
only one or two coming from Brazil. But during
recent years much larger stones have been found in
comparatively large numbers in the South African
diamond-mines. These are, indeed, sometimes too
large for cutting into a single gem, and they have to
be divided by cleavage. This was the case with the
"Excelsior," a stone of 969^ carats, found in 1893
in the Jagersfontein mine, Orange River Colony;
and also the "Cullinan," which was found in 1905
in the Premier mine near Pretoria, Transvaal. The
"Cullinan" is by far the largest diamond yet dis-
covered, weighing in the rough no less than 3025 H
English carats (=621.2 grams), or nearly 22 oz.
avoirdupois. It was presented by the Transvaal gov-
ernment to King Edward VII. in 1907, and has since
been cut into nine large brilliants and ninety-six
small brilliants. The two largest brilliants, weigh-
ing 516^ and 309^ carats, are each far larger than
any other cut diamond.
NON-METALLIC ELEMENTS.
Plate 1
1, Diamond. 2, Graphite. 3, Sulphur.,
GRAPHITE 63
GRAPHITE
(Plate i, Fig. 2). Like diamond, this consists
simply of the chemical element carbon ; but although
these two minerals are identical in chemical com-
position, they exhibit the most marked differences
in their physical characters. Diamond is the hardest,
while graphite is the softest of minerals. Diamond
is colorless and perfectly transparent, while graphite
is black and opaque; diamond is a non-conductor of
electricity, and graphite is a good conductor. There
is also a very marked difference in the specific grav-
ity, that of diamond being 3.52, while that of
graphite is only 2.2. Again, while diamond is nearly
always found as well-developed octahedral crystals,
graphite is only rarely found as small and indistinct,
six-sided scales. More usually the material is very
imperfectly crystallized, taking the form of scaly,
lamellar, or fibrous masses. These striking differ-
ences between diamond and graphite are to be at-
tributed to the different, but in each case orderly,
grouping of the ultimate particles of carbon that
is, to a difference in the crystalline structure of the
material.
Scales of graphite may be readily divided into still
thinner leaves; in other words, they possess a perfect
cleavage parallel to their large surface. The ma-
terial is iron-black in color, with a metallic luster.
It is greasy to the touch, and soils everything it comes
in contact with. On this depends, 6f course, its well-
64 THE WORLD'S MINERALS
known use for making pencils (the so-called "lead,
pencils," which contain no lead!) ; and, indeed, the
name graphite is derived from the Greek word
meaning "to write." Plumbago is another name for
this mineral.
The better qualities of material, such as are used
for making pencils, contain a certain amount of im-
purity, which remains behind as ash when the graph-
ite is burnt. Inferior qualities, containing still more
impurity, are used for making stove-polish, foundry
facings, crucibles, etc. The purified material, con-
sisting of fine scales floated off in water, is employed
as a dry lubricant.
Graphite occurs mainly in the older crystalline
rocks, in which it forms veins or irregular masses;
and it is mined principally in Ceylon, Siberia, the
State of New York, and Canada. The famous Bor-
rowdale mine, near Keswick, in Cumberland, was
worked since the beginning of the seventeenth cen-
tury, and possibly earlier; but this mine is now
exhausted. At the present time a certain amount of
commercial graphite is prepared artificially in the
electric furnace.
SULPHUR
(Plate i, Fig. 3). This mineral frequently occurs
beautifully crystallized, and the crystals are trans-
parent and of a bright primrose-yellow color. Pretty
specimens like that represented in Plate i are not at
all uncommon, and with their bright and attractive
color they form conspicuous objects in any collection
SULPHUR 65
of minerals. In this picture we see several crystals
with sharply defined planes and edges scattered over
the surface of a matrix consisting of minutely crys-
tallized calcite. The characteristic form of crystals
is clearly shown; they are rhombic (four-faced)
pyramids, truncated by a rhomb-shaped face at their
summits. The system of crystallization is ortho-
rhombic, and the crystals have three planes of sym-
metry.
The crystal faces are always very bright and
smooth, with a luster like that of resin. There is no
cleavage, and the fractured surfaces are typically
conchoidal. The material is quite soft (H. 2),
though very brittle; and it is not at all heavy (sp.
gr. = 2.07). It is a bad conductor of heat; and for
this reason a crystal of sulphur when held in the
warm hand emits a crackling noise and becomes
fissured. Good specimens must, therefore, be han-
dled with care.
The chemical properties of sulphur are well
known to students of chemistry. When heated, the
material readily melts to a dark-brown, viscous
liquid, and in the air it burns with a small blue
flame, producing a most penetrating and suffocating
odor. The product of combustion is the gas sulphur
dioxide (SO 2 ), the sulphur having entered into
chemical combination with the oxygen of the air.
Sulphur is dissolved by the liquid carbon disulphide,
and as the solution slowly evaporates it deposits bril-
liant rhombic pyramids of the same form as the
natural crystals.
66 THE WORLD'S MINERALS
Like the element carbon, sulphur also exists in
more than one crystalline modification. When mol-
ten sulphur solidifies, it takes the form of long, spear-
shaped crystals, which belong to the monoclinic sys-
tem, and have the lower specific gravity of 1.86.
These crystals are, however, not stable at the ordinary
temperature. After a short time cloudy patches of
a pale yellow color make their appearance in the
amber-yellow crystals; these gradually spread, and
finally the crystals fall to powder. Under the micro-
scope this powder is seen to consist of minute ortho-
rhombic crystals. There has thus been a spontaneous
conversion of the unstable monoclinic sulphur to the
more stable orthorhombic sulphur; and for this rea-
son the monoclinic modification of sulphur is un-
known as a natural mineral. On the other hand,
the two crystalline modifications of carbon are both
stable at the ordinary temperature, and they are con-
sequently both found in nature as minerals.
Sulphur is found in all volcanic districts, it being
a common product of sublimation from the sulphur-
ous gases of volcanoes. This sublimed sulphur is,
however, never distinctly crystallized; it forms
powdery and earthy, or sometimes more compact and
stalactitic, masses. The beautifully crystallized speci-
mens of sulphur are found in crevices in beds of clay
and marl, together with crystals of gypsum, calcite,
aragonite, and celestite.
The best-known locality for such crystals is in the
neighborhood of Girgenti, in Sicily a district far
removed from Mount Etna and the materials here
SULPHUR 67
are not of volcanic origin. The specimen represented
in Plate I is from this locality. In this district there
are numerous sulphur-mines. The native sulphur is
separated from its matrix by fusion, and the crude
sulphur so obtained is further purified by sublima-
tion, giving "flowers of sulphur." This is then fused
and cast into sticks to give the roll-sulphur of
commerce.
Sulphur has many important technical applica-
tions. It is used in medicine; in the manufacture of
matches, gun-powder, and fireworks; for vulcanizing
india-rubber; in the preparation of sulphuric acid
(oil of vitriol) , ultramarine, and many other sulphur
compounds. Much of the oil of vitriol of commerce
is now prepared from iron-pyrites, but this is not so
pure as that obtained from sulphur. The sulphur
dioxide produced by burning sulphur in the air is
extensively used for disinfecting and bleaching pur-
poses, and in the boiling of wood-pulp employed in
the manufacture of paper.
In chemical combination sulphur is present in sev-
eral kinds of minerals. The sulphides are compounds
of sulphur with a metal, and the sulphates contain
oxygen in addition to sulphur and a metal.
SEMI-METALLIC ELEMENTS
In this class we have the native elements arsenic,
antimony, and bismuth, which, though possessed of
a metallic luster and high specific gravity, approach
the non-metals in some of their chemical properties.
68 THE WORLD'S MINERALS
They are closely related to each other chemically,
being isomorphous, and form a progressive series
with arsenic at the non-metallic and bismuth at the
metallic end of the series. The progressive nature
of this series is well shown by the specific gravities,
which are 5.7, 6.7, and 9.8, respectively, in arsenic,
antimony, and bismuth. As minerals they are of
comparatively rare occurrence, and for commercial
purposes these elements are more often extracted
from their compounds, which occur in nature mostly
as sulphides.
ARSENIC
(Plate 2, Fig. i). Native arsenic is usually found
as masses with a rounded (mamillated) surface and
an internal shelly structure. The characteristic form
of surface is represented in the picture. Freshly
fractured surfaces are tin-white, with a metallic lus-
ter; but on exposure to the air these very soon tarnish,
and become dull and dark grey.
This mineral occurs in metalliferous veins, to-
gether with ores of silver and cobalt. It is found in
the Harz Mountains and in Saxony. Globular
masses of radiating crystals are found embedded in
clay at Akatani, in Japan.
ANTIMONY
(Plate 2, Fig. 2). Native antimony is found as
granular or lamellar masses, which have a tin-white
color and metallic luster. There is a perfect cleav-
BISMUTH 69
age in one direction. It occurs in metalliferous veins,
usually with ores of silver, at but few localities for
example, in Sweden and Borneo.
BISMUTH
(Plate 2, Fig. 3). Native bismuth is met with as
reticulated or feathery forms, and also as granular
masses. It usually shows a yellowish tarnish, but a
freshly fractured surface is silver-white, with a char-
acteristic tinge of red. It is found in metalliferous
veins in Cornwall, Saxony, Bolivia and Queensland.
Distinctly developed crystals of bismuth, though
rare in nature, are readily prepared artificially by
fusion. The metal is melted in a crucible, and after
it has cooled for a little while the crust is broken and
the still molten portion poured out. The interior of
the crucible is then seen to be completely lined with
sharply formed crystals, which have sunken faces
and a brilliantly colored iridescent tarnish on their
surface. Very pretty groups of crystals are obtained
in this way. These crystals have the appearance of
cubes, but in reality are rhombohedra, the faces be-
ing not quite at right angles to one another. Fur-
ther, there being only one direction of cleavage (per-
pendicular to the principal axis), only two of the
opposite corners of the apparent cubes can be trun-
cated by cleavage.
A certain quantity of commercial bismuth is ob-
tained from native bismuth, especially in Bolivia,
and the remainder is extracted from the sulphide
70 THE WORLD'S MINERALS
(bismuthinite, Bi 2 S 8 ). It is largely used for making
readily fusible alloys, which find an application in
the safety-plugs of boilers and fire-extinguishers.
Salts of bismuth are extensively used in medicine for
relieving indigestion.
METALLIC ELEMENTS
SILVER
(Plate 2, Figs. 4 and 5). The useful metal silver
occurs native as a mineral, but as a source of the
metal the various ores of silver are of much greater
importance, while a considerable amount is extracted
from ores of lead, which always contain a small pro-
portion of silver.
Native silver is found principally in the upper
levels of silver-mines, it having resulted by the sec-
ondary alteration, or weathering, of various silver-
bearing minerals. It is met with in cavities in the
ore as fine, twisted, wire-like forms. This form is
extremely characteristic of native silver; the wires
are usually finer than represented in the picture
(Fig. 5). Under these conditions it is found in
Cornwall and Saxony, and more abundantly in the
silver-mines of Mexico and Chile.
Crystals of native silver are quite rare. They be-
long to the cubic system, and usually have the form
of distorted cubes (Fig. 4). Most of the crystallized
specimens to be seen in mineral collections have come
from the silver-mines at Konigsberg, in Norway, this
METALLIC ELEMENTS.
Plate 2.
4
1, Arsenic. 2, Antimony. 3, Bismuth. 4, ^ Silver:
SILVER PLATINUM 71
being the locality of the specimen represented in
Fig. 4. Masses of native silver weighing as much
as 5 cwt. have been found in these mines.
The specific gravity of native silver ranges from
10 to n, this variation being due to the presence
of small amounts of gold or copper alloyed with
the silver.
PLATINUM
(Plate 3, Fig. i). This metal was first brought
to Europe in the year 1735 from Colombia, South
America. Being a white metal, resembling silver in
appearance, it was called platina in Spanish, plata
being the Spanish name for silver. It differs from
silver, however, in its very high specific gravity,
which ranges from 14 to 19 in the native metal (this
variation being due to the presence of impurities,
principally iron), and is as high as 21.5 in the chem-
ically pure metal. With the exception of iridium
and osmium, it is the heaviest of metals, and, indeed,
of all kinds of matter. It also differs from silver in
being fusible only with great difficulty, and in being
very resistant to acids.
These latter properties make the metal invaluable
for the construction of certain kinds of chemical ap-
paratus, such as crucibles, basins, wire, and foil. The
metal is of rare occurrence, and owing to the limited
supply the price is constantly rising. Formerly the
metal was coined in Russia, but now its value is
greater than that of gold.
Native platinum is found as grains and small nug-
72 THE WORLD'S MINERALS
gets (Plate 3, Fig. i) in the beds of streams. Par-
ticles of chromite (chrome iron-ore) are sometimes
seen attached to the nuggets, proving that the ma-
terial has been derived from the olivine-rocks which
outcrop in the neighborhood. Practically the whole
of the platinum used commercially comes from the
Ural Mountains.
The only source of platinum is the native metal,
but the element is also known to occur in the tiny
crystals of the rare mineral sperrylite, an arsenide of
platinum, found in Canada.
GOLD
(Plate 3, Figs. 2-4). Though of sparing occur-
rence, gold is a very widely distributed mineral, and
it is found in the native state in almost every country
of the world. Being found as grains and nuggets in
the beds of streams and rivers, it must have attracted
the attention of man at a very early period, and gold
ornaments have been unearthed from the burial-
places of prehistoric man.
Well-shaped crystals of gold are small and very
rare; they have the form of the regular octahedron
or cube, usually with rounded edges. A small group
of gold crystals is represented in Plate 3, Fig. 2.
Thin plates of gold (so-called leaf-gold), bearing
delicate crystalline markings on their surfaces, are
rather more frequent, and examples from Transyl-
vania in Hungary, and Colombia in South America,
are well known. Alluvial gold, as found in the beds
GOLD 73
of streams and rivers, has the form of scales, grains,
and rounded nuggets; while in gold-quartz, the
precious metal takes the form of veins and irregular
patches (Figs. 3 and 4). The largest mass of native
gold on record was a nugget called the "Welcome
Stranger," found in Victoria, Australia, in the year
1869; it weighed 156 Ib. avoirdupois, and was val-
ued at 9534.
Native gold is never quite pure, but is always
alloyed with from i to 38 per cent, of silver. The
greater the amount of silver present, the lower is the
specific gravity (which ranges from 15 to 19), the
paler the yellow color, and the greater the hardness.
Pure gold is as soft as lead too soft, indeed, for
use in coinage and jewelry. Its hardness and dur-
ability are increased by alloying it with copper, or
sometimes with silver. The amount of gold actually
present in such an alloy is expressed as so many carats
that is, as so many parts in 24. Thus, 22-carat gold
(as in the English gold coinage) means that in each
24 parts of metal 22 parts are of gold and 2 of base
metal; and in i2-carat gold only half is pure gold.
Gold is separated from the sand and gravel of
alluvial deposits by a simple process of washing,
whereby the lighter materials are carried away in a
stream of water, leaving the heavier gold behind.
The finer particles are collected from this heavy
residue by the addition of mercury, with which gold
readily forms an amalgam. On heating the amal-
gam the mercury is volatilized, leaving the gold.
Being easily worked, such incoherent deposits are al-
74 THE WORLD'S MINERALS
ways the first to be attacked, and in many countries
they are now exhausted. Recourse must then be had
to the solid rocks, the working of which is a far
more laborious operation. The gold-bearing rock
taken from the mine is crushed to powder under
heavy stamps, and the gold is dissolved by a dilute
solution of potassium cyanide a process known as
the cyanide process. The gold is deposited from
this solution by means of an electric current.
The mother-rock, or matrix, of gold is very often
a white quartz (Plate 3, Figs. 3 and 4) , which occurs
as veins, traversing the older crystalline rocks. In
the rich goldfields of the Witwatersrand, or the
"Rand," in the Transvaal, the matrix is, however, a
hard, compact conglomerate, known locally as "ban-
ket," consisting of pebbles of quartz cemented to-
gether by a siliceous material. In this rock the gold
is so finely divided that only rarely is it visible to the
unaided eye. Gold is also found in metalliferous
veins, together with iron-pyrites, ores of silver, etc.
In some of these veins the gold exists not only in the
native state, but also in chemical combination with
tellurium. These tellurides of gold are of consider-
able importance as a source of gold at Kalgoorlie,
in Western Australia, and at Cripple Creek, in
Colorado.
When these solid gold-bearing rocks are broken
down by the slow action of weathering agencies and
their materials transported by running water, many
of the accompanying minerals are destroyed; but the
quartz and the gold, being indestructible, accumu-
METALLIC ELEMENTS.
Plate 3.
1, Platinum. 24, Gold. 5, 6, Coppef.r,: ; V; \ ; ;
GOLD COPPER 75
late in the beds of streams and rivers, forming the
alluvial deposits already mentioned. In this way the
work of extracting gold from the solid rock is per-
formed by nature herself.
In the British Isles, small nuggets, the largest
weighing 27 oz., have been found in Cornwall, Scot-
land, and County Wicklow. During the sixteenth
century there were quite extensive gold-washings in
the Leadhills district in Lanarkshire; while at the
present day a thousand or more ounces of gold are
annually extracted from the quartz-veins in the
neighborhood of Dolgelly, in Merionethshire.
The principal gold-producing countries are the
Transvaal, Australia, California and Alaska. In the
year 1907 the production for the whole world
amounted to 604 tons of fine gold, and of this amount
365 tons was produced in the British Empire.
COPPER
(Plate 3, Figs. 5 and 6). Like gold and silver,
native copper crystallizes in the cubic system, but
distinctly formed crystals are here also of rare oc-
currence. Well-shaped cubes, such as represented in
Fig. 5, are quite exceptional, and even the complex
twinned crystals shown in Fig. 6 are but rarely met
with. Usually the metal has the form of thin plates,
filling narrow crevices in rocks of various kinds, such
as sandstone, slate, or igneous rocks; and these plates
frequently have a delicate frond-like or dendritic
structure. Mossy aggregates are also common, par-
76 THE WORLD'S MINERALS
ticularly in the upper portions of veins of copper-
ore, where the material has resulted by the decom-
position of various copper-bearing minerals.
On its surface, native copper is usually dull and
tarnished, with a dark chocolate-brown color, or
sometimes green (owing to the surface alteration of
the material to green carbonate of copper) ; and only
on a fresh fracture is the characteristic copper-red
color, with bright metallic luster, to be seen. The
native metal is usually almost chemically pure cop-
per, and its specific gravity does not vary much
from 8.9.
Small specimens of native copper are found in
most copper-mines; but as a source of metal this
mineral is of far less importance than copper-pyrites
and other compounds of copper. In the State of
Michigan, on the south shore of Lake Superior, it
exists in large quantities and is extensively mined;
but owing to the toughness of the metal, the mining
operations are attended with some difficulty. One
mass of pure copper found at this locality was esti-
mated to weigh as much as 420 tons. In the old
copper-mines in the Lizard district of Cornwall,
masses of copper up to 3 tons in weight were found
embedded in the serpentine rock.
IRON
We cannot conclude our description of the native
elements without adding a brief mention of native
iron. Although iron, in the form of its various
IRON 77
chemical compounds, is of such abundance amongst
the materials of the earth's crust, yet native iron is
of the greatest rarity. This is readily explained by
the fact that metallic iron is readily altered by oxida-
tion when exposed to the weather. Small grains
of metallic iron have been observed embedded in
basaltic rocks at a few places, and in Greenland large
masses have been found in basalt. But of special
interest are the masses of metallic iron (invariably,
however, alloyed with about 9 per cent, of nickel)
which occasionally fall to our earth from outside
space. Such a mass of meteoric iron, weighing j%.
lb., fell at Rowton, in Shropshire, on April 20, 1876;
and a mass weighing as much as 50 tons has been
found in Mexico.
CHAPTER VI
THE SULPHIDES, ARSENIDES, AND
SULPHUR-SALTS
IN this class we have the sulphides and arsenides
of the heavy metals, and also a few rarer minerals,
in which both sulphur and arsenic (or antimony)
enter into chemical combination with a heavy metal.
Most of the minerals to be here described are of
importance to the miner as ores. They are usually
heavy and opaque, with a metallic luster, thus some-
what resembling metals in their appearance; but
some are quite transparent and brightly colored.
They occur, for the most part, in mineral-veins, or
lodes, which intersect the older rocks of the earth's
crust. The different kinds are frequently found to-
gether in the same vein, and they are also much inter-
mixed with various sparry minerals. It is, there-
fore, part of the work of the miner, after he has
raised the ore to the surface, to separate the different
kinds of minerals, in order to prepare a product, or
products, which can be dealt with by the smelter for
the extraction of valuable metal.
STIBNITE
(Plate 4, Figs, i and 2). This is a sulphide of
antimony, with the chemical formula Sb 2 S 3 . It is
78
STIBNITE 79
also often known by the name antimonite or anti-
mony-glance, and it is the most important of the ores
of antimony.
It is usually found as fibrous or lamellar masses,
which break with shining flat surfaces, owing to the
existence of a perfect cleavage in one direction in
the crystals. A very characteristic feature of stib*
nite is that these bright cleavage surfaces are marked
by fine striations in a direction at right angles to
their length. Sometimes the fibrous or acicular crys-
tals, which build up the massive material, have a
radial or stellate arrangement, as represented in Fig. i.
Crystals of stibnite belong to the orthorhombic
system, and have the form of long prisms, varying
from the thickness of a needle to an inch or more
across. They are usually much grooved and fur-
rowed in the direction of their length (Fig. 2) ; and
the perfect cleavage is also parallel to the length of
the crystals. Magnificent groups of prismatic crys-
tals from Japan are known; and very good specimens
are abundant at Felsobanya, in Hungary, this being
the locality of the specimen represented in Fig. 2,
and also of that shown in Plate 20, Fig. i, where
the needles of stibnite are associated with crystals of
barytes.
The color of the mineral is steel-grey and the
luster metallic, but on exposure to light the surface
gradually becomes dulled with a bluish or blackish
tarnish. The mineral is quite soft (hardness = 2),
and the crystals are easily damaged, unless handled
with care.
80 THE WORLD'S MINERALS
Metallic antimony, which for the most part is
obtained from the mineral stibnite, is much used for
making alloys. For example, type-metal as used in
printing is an alloy of four parts of lead with one of
antimony. Salts of antimony find an application in
medicine, and as pigments. Stibnite is used in the
East for darkening the eyebrows, and it was also used
by the ancients for the same purpose.
REALGAR
(Plate 4, Figs. 3 and 4). Two distinct sulphides
of arsenic occur in nature as minerals; they differ
not only in the proportions in which the arsenic and
sulphur are chemically combined, but also in their
external characters, the difference in color being par-
ticularly striking. These two minerals the bright
red realgar and the bright yellow orpiment fre-
quently occur together, as is shown in the two pictures
on Plate 4.
In realgar, each chemical molecule consists of one
atom of arsenic and one atom of sulphur, the formula
being AsS. This compound is found in nature as
small, monoclinic crystals, with an aurora-red color,
and often with perfect transparency (Fig. 4). On
exposure to light, these crystals undergo a remark-
able change; after some time, in place of a brilliant
crystal, we find a heap of yellow powder. This
change is due to an absorption of oxygen from the
air, with the production of the yellow sulphide (As 2 -
S 3 ) and white arsenious oxide (As 2 O 3 ). The latter
SULPHIDES.
Plate 4.
1, 2, Stibnite. 3, 4, Realgar and OrpimcnV
ORPIMENT MOLYBDENITE 81
is soluble in water, and, like all soluble arsenic com-
pounds, it is extremely poisonous. It is, therefore,
necessary to keep specimens of this mineral in the
dark, otherwise they soon become spoilt.
Both the specimens represented on Plate 4 are
from Hungary. In Fig. 3 the realgar is massive,
and shows no crystalline form.
ORPIMENT
(Plate 4, Figs. 3 and 4). In the other native sul-
phide of arsenic, two atoms of arsenic are combined
with three atoms of sulphur to form the chemical
molecule, the formula here being As 2 S 3 , which is
analogous to the formula of stibnite, with arsenic
in place of antimony. The mineral also shows a re-
lation to stibnite in its crystalline structure and per-
fect cleavage in one direction. It usually occurs as
lamellar masses, distinctly developed crystals being
extremely rare. The color is bright lemon-yellow;
and the bright, smooth cleavage surfaces have a
pearly appearance. The name orpiment is a corrup-
tion of the Latin name auripigmentum, which means
"gold paint." The pigment known as "king's yellow"
is, however, now prepared artificially.
MOLYBDENITE
(Plate 5, Fig: i). This mineral is the disulphide
of the rare metal molybdenum, the chemical formula
being MoS 2 . It is found as platy or lamellar masses,
82 THE WORLD'S MINERALS
and sometimes as six-sided plates or scales; but dis-
tinctly formed crystals are rare. It is opaque, with
a metallic luster and a lead-grey color. The material
is very soft (hardness i) and greasy to the touch,
and it readily marks paper. This mineral presents,
in fact, a most striking resemblance to graphite (p.
63), with which it is sometimes confused. There is,
however, a slight difference in color, which is notice-
able when the two minerals are compared side by
side, molybdenite showing a bluish tinge. In the
specific gravity there is a very wide difference, the
value for molybdenite being 4.7, and for graphite
only 2.2. This affords a ready means of distinguish-
ing the two minerals, and the difference in weight
will be at once noticed if pure specimens of equal
size are handled. When, however, a small amount
of the mineral is embedded in a large piece of matrix,
the specific gravity cannot be judged by merely
handling the specimen. We may then detach a small
scale of the mineral and drop it into a heavy liquid
(such as bromoform, sp. gr. 2.8 ; or methylene iodide,
sp. gr. 3.3) ; if the flake be molybdenite it will sink,
while if it be graphite it will float on the surface
of the liquid. The chemical behavior also serves to
distinguish the two minerals; when heated before
the blowpipe, molybdenite gives a strong smell of
burning sulphur.
Molybdenite is found as scales embedded in vari-
ous crystalline rocks, such as gneiss and granite; and
it is also met with in some metalliferous veins. It is
mined in Norway, Canada, and New South Wales.
ZINC-BLENDE 83
The mineral is used to a small extent for the prep-
aration of the molybdenum compounds used as re-
agents in the chemical laboratory. Within recent
years it has been employed in the manufacture of
steel ; the addition of molybdenum gives a very tough
and hard steel especially suitable for tools.
ZINC-BLENDE
(Plate 5, Fig. 2). The sulphide of zinc (ZnS),
known as zinc-blende, or simply as blende, is the
most abundant of zinc-bearing minerals, and at the
same time the most important ore of this metal.
Crystals belong to the tetrahedral division of the
cubic system, and their characteristic form is well
shown in the picture (Fig. 2). Each of the three
crystals here represented on the rocky matrix con-
sists of a tetrahedron, the edges of which are trun-
cated by narrow faces of the cube, and the corners are
truncated by small faces of another (so-called nega-
tive) tetrahedron; in addition there are six small tri-
angular faces symmetrically grouped round each of
the corners. Although crystals of zinc-blende are of
quite common occurrence, it is not often that their
form can be so easily made out as in the example just
mentioned. More usually the crystals are much
distorted and the faces rounded, while frequent
twinning still further adds to their complexity. A
very important crystallographic character is the ex-
istence of perfect cleavages in six directions parallel
to the faces of the rhombic-dodecahedron; the angles
84 THE WORLD'S MINERALS
between adjacent cleavage surfaces is, therefore,
120 or 90.
The faces of crystals and the cleavage surfaces are
often very smooth and bright, and they display a
characteristic resinous to adamantine luster. The
color and transparency range between wide limits
from almost colorless and transparent to jet-black and
opaque. Most commonly the mineral is dark yellow
or brown and translucent, as represented in the pic-
ture. This mineral is thus very variable in its ap-
pearance, and this, together with the usually obscure
crystalline form, makes its identification sometimes
a matter of difficulty. Indeed, the old German word
blende means "to deceive," and the scientific name
sphalerite, derived from the Greek, has the same sig-
nification. The miners' names, "black jack" and
"false lead," also indicate that the mineral was often
mistaken for lead-ore.
As a help towards identifying the mineral, it
should be scratched across a bright cleavage surface
with a knife (having a hardness of rather less than 4
it readily yields), when a brown powder will be ob-
tained. This, in conjunction with the perfect cleav-
ages and the adamantine luster, will often serve to
distinguish zinc-blende from other minerals it may
resemble in appearance. But as a confirmatory test,
the mineral should also be examined before the
blowpipe. For this purpose a small quantity of the
powdered mineral is mixed with sodium carbonate
and heated on charcoal in a reducing flame; the
sublimate so obtained on the charcoal is yellow when
SULPHIDES.
Plate 5.
I, Molybdenite. 2, Zink-blende or Sphalerite. 3, 4, Galena.
ZINC-BLENDE 85
hot and white when cold, and if it be moistened with
cobalt nitrate solution and again heated it becomes
bright green, indicating the presence of zinc.
When chemically pure, zinc-blende contains 67
per cent, of zinc, but frequently iron is also present
to the extent of i or 2 per cent., or even as much as
1 8 per cent. It is owing to the presence of this
variable amount of iron that the mineral varies so
widely in color and transparency. The specific
gravity of 4.0 is much lower than that of the lead-ore
(galena, sp. gr. 7.5), with which zinc-blende is so
often associated; and advantage is taken of this dif-
ference in the methods employed for the separation
of the two minerals from the ore.
Besides occurring as crystals, zinc-blende is abun-
dant as granular or compact masses, but in such
masses the bright cleavage surfaces may still be
recognized.
Zinc-blende occurs in metalliferous veins, together
with galena, calcite, fluor-spar, etc., and these veins
often traverse limestone rocks. Good crystallized
specimens are found at many localities. The speci-
men represented in the picture (Fig. 2) is from the
Binnenthal, in Switzerland, where small crystals
usually occur singly in cavities of a white dolomite.
As an ore of zinc this mineral is mined in Cum-
berland, Wales, Germany, Missouri, and many other
localities. Metallic zinc is largely used for gal-
vanizing iron, making alloys (brass, etc.), and in the
construction of galvanic batteries. Its salts are used
in dyeing and in medicine.
86 THE WORLD'S MINERALS
GALENA
(Plate 5, Figs. 3 and 4). This is sulphide of lead
(PbS), and a very important ore, containing 86.6
per cent, of the metal. Curiously, however, it is also
of importance as an ore of silver, since galena almost
invariably contains a minute proportion of silver,
averaging about 0.03 per cent, or about 10 oz. of
silver to the ton of galena. When the ore is smelted,
this small amount of silver alloys with the metallic
lead; and before the lead is sold for plumber's work
the silver is extracted from it by a special process.
Large amounts of silver have been obtained from the
lead-roofing of old buildings by this modern process.
Good crystals of galena are quite common. They
belong to the cubic system, and usually have the form
of the cube, or of the cube combined with the octa-
hedron. In the crystals represented on Plate 5, the
predominating form is the octahedron, the corners
of which are truncated by small faces of the cube.
In Fig. 3 there are also narrow faces of an icositet-
rahedron (p. 14) replacing the edges between the
cube and the octahedron ; and in Fig. 4 the edges of
the octahedron are truncated by narrow faces of the
rhombic-dodecahedron.
The crystals can be split, or cleaved, with great
ease parallel to the faces of the cube, and the surfaces
of fracture are perfectly bright and smooth. These
perfect cleayages in three directions at right angles
are also shown by massive galena, on which no ex-
GALENA NICCOLITE 87
ternal crystal faces are present; and any lump of
galena-ore when broken shows three sets of bright
steps, while isolated fragments have quite the ap-
pearance of dice. This important character, taken
in conjunction with the lead-grey color and bright
metallic luster, makes galena easily recognizable at
sight. The softness (hardness = 2^) and heaviness
(sp. gr. 7.5) are also characters of importance. The
mineral is opaque, and its streak is lead-grey.
Galena commonly occurs in metalliferous veins
intersecting limestone rocks, and is often associated
with zinc-blende, quartz, calcite, fluor-spar, etc. It
is of wide distribution, and is mined in many parts
of the world; for instance, in Derbyshire, County
Durham, Lanarkshire, Spain, Germany, and the
United States. The two specimens represented in
Plate 5 are from Neudorf , in the Harz Mountains ;
here the crystals are associated with small rhombo-
hedra. of chalybite (iron carbonate) on a base of
quartz.
NICCOLITE
(Plate 6, Fig. i). A name very often misapplied
to this mineral is copper-nickel, from the old Ger-
man name Kupfernickel; but, although the mineral
has a very characteristic copper-red color, it contains
no copper. It is, in fact, an arsenide of nickel
(NiAs). Crystals are very rare, and the mineral is
usually found as compact masses. It is brittle and
breaks with an irregular fracture. This character,
together w r ith its greater hardness (5^2) and black
88 THE WORLD'S MINERALS
streak, is sufficient to distinguish it from native cop-
per, which in color it so closely resembles.
Niccolite is found, together with ores of silver,
cobalt, and copper, at several places in Germany,
and it has also been met with in Cornwall and Scot-
land. The specimen figured is from the copper-
mining district of Mansfeld, in Prussia. Here the
mineral was formerly used for the extraction of
nickel, and also as a source of arsenic; but most of
the nickel of commerce is now obtained from pyrrho-
tite (p. 94) and a silicate of nickel.
CINNABAR
(Plate 6, Figs. 2 and 3). This is sulphide of
mercury (HgS), and is of importance as being the
only ore of mercury, or quicksilver. This remark-
able metal, which at the ordinary temperature is a
liquid 1^/2 times as heavy as water, is also found in
the native state, though only in small amount, so that
it is not a mineral of any economic importance. It
is often to be seen on specimens of cinnabar, to the
easy decomposition of which mineral it owes its
origin. The silver-white spots on the upper portion
of Fig. 3 represent small liquid globules of native
mercury.
Crystals of cinnabar are not very common, and are
mostly quite small. Large, well-shaped rhombo-
hedra have, however, recently come from the cin-
nabar-mines in central China; these crystals usually
consist of two interpenetrating rhombohedra inter-
SULPHIDES & ARSENIDES.
Plate 6.
a
tv,
* ' **//
1, Niccolite. 2, 3, Cinnabar. 4, Mispickel or Arsenopyrite.
CINNABAR MISPICKEL 89
grown in twinned position. The crystals have per-
fect cleavages parallel to the six faces of the hex-
agonal prism. They are often transparent, with a
deep-red color and brilliant luster. On the darker
crystals the luster is quite metallic in character, es-
pecially when the full blaze of reflected light is ob-
served from one of the crystal faces (Fig. 2).
The crystals are quite soft (hardness = 2), and
when scratched they yield a scarlet powder. This is
the characteristic color of the mineral in its granular
or earthy condition as commonly seen in the ore. In
Fig. 2 there is shown a single crystal of cinnabar
on massive cinnabar; and in Fig. 3 the massive min-
eral is scattered through the rocky matrix, which
contains also yellow specks of iron-pyrites. Some-
times the ore is dark brown or black, owing to the
admixture of clay or carbonaceous material.
Veins of mercury-ore are rather sporadic in their
distribution, and mines are worked at but few places,
notably Almaden in Spain, Idria in Carniola, and
New Almaden in California.
MISPICKEL
(Plate 6, Fig. 4). This mineral, which is also
known as arsenopyrite, or arsenical pyrites, is a com-
pound of iron, arsenic, and sulphur in equal atomic
proportions, the formula being FeAsS. It is thus,
at the same time, a sulphide and an arsenide, and
may be described as a sulph-arsenide of iron.
Crystals of mispickel belong to the orthorhombic
90 THE WORLD'S MINERALS
system. Their characteristic rhombic form is clearly
shown in the picture; here we have a short rhombic
(four-sided) prism terminated at either end by a
pair of dome faces like the roof of a house. Usually
the prism faces are longer, and the crystals are then
prismatic in habit. Columnar and granular aggre-
gates of massive mispickel are also of common
occurrence.
The mineral is tin-white, with a metallic luster,
but it often shows a bluish (Fig. 4), yellowish, or
blackish tarnish. The specific gravity is 6; and the
hardness is also 6, so that the mineral can be scratched
with a knife only with difficulty.
Mispickel occurs in metalliferous veins, with ores
of silver, copper, or tin. It is abundant in Cornwall
and Devon, and at Freiberg, in Saxony. The speci-
men represented in Fig. 4 is from the latter locality;
the large crystals rest with minutely crystallized
chalybite on a bed of quartz.
This mineral is of importance in being the prin-
cipal source of the extremely poisonous arsenious
oxide (As 2 O 3 ), or white arsenic, which is obtained
by simply roasting the ore in a current of air.
A variety of mispickel, in which a part of the iron
is replaced by an equivalent amount of cobalt (5 to
10 per cent), is called danaite, in honor of the fa-
mous American mineralogist, whose several books
have long been standard works in this branch of
science.
MARCASITE
MARCASITE
(Plate 7, Fig. i). This is disulphide of iron
(FeS 2 ), like the more common mineral iron-pyrites
to be next described. These two minerals, although
identical in chemical composition, are quite distinct
in their crystalline form and physical characters. We
have here another example of dimorphism, just as
with the two crystalline forms of carbon (p. 63).
Marcasite also shows a relation to mispickel, the
mineral last considered, in that the type of crystal-
lization is the same and the angles between the faces
almost identical. Further, if we consider one of the
atoms of sulphur in the marcasite formula to be re-
placed by one atom of arsenic, we arrive at the
formula of mispickel. The two minerals mispickel
and marcasite are, therefore, said to be isomorphous,
meaning, in Greek, of the same form.
The form of marcasite crystals is, however, very
often obscured by twinning, and we have complex
groups of crystals to which the names cockscomb-
pyrites and spear-pyrites are applied. The five-
sided, crystal shown in Fig. i really consists of five
crystals united together by twinning; the striated
faces correspond to the similarly striated dome faces
of mispickel.
The color of marcasite is pale brass-yellow, with a
metallic luster, but it is often obscured By surface
tarnish and alteration. This mineral is, indeed, par-
ticularly liable to alteration by weathering, the sul-
phur being removed as sulphuric acid and the iron
92 THE WORLD'S MINERALS
remaining behind as hydrated oxide of iron, which
still preserves the original form of the crystals. Thus
the brassy-looking marcasite comes to be replaced
by the rusty brown limonite. The specific gravity
(4.8) of marcasite is slightly less than that of iron-
pyrites, and the hardness is much the same in the two
minerals. The color of marcasite is usually rather
paler than that of iron-pyrites.
When distinctly formed crystals are not to be ob-
served, as in the commonly occurring nodular masses
with internal radiating structure, it is often extremely
difficult, and in some cases impossible, to distinguish
between marcasite and iron-pyrites.
Though not an uncommon mineral, marcasite is
far less abundant and less widely distributed than is
iron-pyrites. It is found in metalliferous veins in
Derbyshire and Cornwall, and in beds of brown coal
in Bohemia. The specimen represented in Plate
7 is from Dover, in England, and shows one larger
and several smaller crystals embedded in a matrix of
chalk. When found in sufficiently large amounts,
marcasite is used for the same purposes as iron-
pyrites.
IRON-PYRITES
(Plate 7, Figs. 2 and 3). As already mentioned
this is disulphide of iron (FeS 2 ), identical in chem-
ical composition with marcasite. The names pyrites
and pyrite are also in common use for this species.
It is one of the most widely distributed of minerals,
being not only abundant in metalliferous veins, but
IRON-PYRITES 93
found also as crystals embedded in rocks of all
kinds, while sometimes it forms enormous bedded
deposits.
Crystals are very common, and they usually have
the form of small cubes (Fig. 2). On a critical in-
spection, it will be seen that these cubes differ from
the cubes in which most other minerals crystallize;
their faces are each striated parallel to only one edge,
and the striations on adjacent faces are at right angles
to one another. The system of crystallization, though
cubic, is of a lower (pentagonal-dodecahedral) type
of symmetry. The pentagonal-dodecahedron is also
a common form of crystals of iron-pyrites (Fig. 3) ;
this solid is bounded by twelve five-sided faces
(which are, however, not regular pentagons). The
faces of this form are also striated on the same plan
in three directions at right angles namely, parallel
to the cubic axes of the crystal, as is clearly shown
in Fig. 3.
The color both of the crystals and of compact
masses is pale brass-yellow, with a bright metallic
luster; but the powder or streak of the mineral is
dark greenish-black or brownish-black. The min-
eral shows no cleavage; it is brittle and breaks with
an irregular fracture. The specific gravity is 5, and
the hardness 6. It can scarcely be scratched with a
knife; and when struck with steel it gives sparks,
due to the burning of the sulphur in the detached
fragments. Iron-pyrites was formerly used in tinder-
boxes, and firearms, and, indeed, the Greek word
pyrites means "fire stone." These characters at once
94 THE WORLD'S MINERALS
serve to distinguish iron-pyrites from native gold,
for which it may possibly be mistaken.
Like marcasite, iron-pyrites is readily altered by
weathering; and pseudomorphs of limonite (hy-
drated oxide of iron) with the form of crystals of
iron-pyrites are not at all uncommon.
Iron-pyrites often contain small amounts of copper,
gold, and other metals. The large deposits which
are extensively worked at Rio Tinto in the south of
Spain, in Norway, and in the Harz Mountains, con-
tain on the average 3 per cent, of copper; but this
may be accounted for by the admixture of copper-
pyrites with the iron-pyrites. Such ores are roasted
in a current of air to yield sulphur dioxide for the
manufacture of sulphuric acid (oil of vitriol), and
the residue is then treated for the extraction of the
copper and traces of gold.
Good crystallized specimens of iron-pyrites are
found at many places, amongst which may be men-
tioned the iron (magnetite) mines of Traversella, in
Piedmont, Italy, and the iron (haematite) mines of
Rio, in the island of Elba. The specimen represented
in Plate 7, Fig. 3, is from the latter locality, and
shows the crystals of iron-pyrites on haematite; that
in Fig. 2 is from Cornwall.
PYRRHOTITE
(Plate 7, Fig. 4) . Pyrrhotite, or magnetic pyrites,
is another sulphide of iron, but one approximating
in composition to the monosulphide FeS. Hexagonal
SULPHIDES.
Plate 7.
1, Marcasite. 2, 3, Iron-pyrites. 4, Pyrrhotkei
PYRRHOTITE COPPER-PYRITES 95
crystals of platy habit are rare, and usually only
compact masses (Fig. 4) are found. The color is
bronze-yellow tarnishing to brown, while the metallic
luster is usually dull. The mineral is rather softer
(hardness = 4) than iron-pyrites, and it also differs
from this in its magnetic character, small fragments
being attracted by a magnet.
Pyrrhotite occurs as grains, embedded in certain
igneous rocks, such as gabbro, and sometimes it is
concentrated towards the margins of the rock-mass,
forming workable deposits of ore. It is also found
in metalliferous veins. The ore sometimes contains
a small amount of nickel (3 to 5 per cent.), as in the
large deposits at Sudbury in Ontario, and in Norway,
where it is largely worked for the nickel. The speci-
men shown in Plate 7 is from Bodenmais, in Bavaria.
COPPER-PYRITES
(Plate 8, Figs, i and 2). Copper-pyrites, or
chalcopyrite, is a sulphide of iron and copper, with
the formula CuFeS 2 , and contains, when pure, 34^
per cent, of copper and 30^2 per cent, of iron. It is
the most abundant of copper minerals, and at the
same time the most important ore of copper.
Crystals of copper-pyrites belong to the tetragonal
system; they are usually small and indistinct, and it
is not often that their form can be easily deciphered;
often the form approximates to an octahedron (Fig.
2) or tetrahedron. Compact masses are much more
common.
96 THE WORLD'S MINERALS
This mineral closely resembles iron-pyrites in
general appearance; it differs, however, in its
darker brass-yellow color and in its lower degree
of hardness (H. = 4). Copper-pyrites can readily
be scratched with a knife, giving a dark, greenish-
black streak, while iron-pyrites is scratched only
with difficulty. In case of doubt, a confirmatory test
should be made by dissolving a little of the powdered
mineral in nitric acid, when the presence of copper
will be indicated by the green color of the solution.
On the addition of a large quantity of ammonia to
this solution the green color changes to blue, and
the iron is thrown down as a bulky reddish-brown
precipitate.
The surface of massive copper-pyrites very often
displays brilliant iridescent colors, such as intense
red, or golden yellow, which, combined with the
metallic luster, give the mineral a very striking ap-
pearance. On this account it is called "peacock-ore"
by the miners. These iridescent colors are due to
an extremely thin film of some alteration product on
the surface of the copper-pyrites.
This mineral is found in metalliferous veins, and
especially those near the junction of granite and
slates; associated minerals are zinc-blende, quartz,
chalybite, etc. It also forms masses in igneous rocks,
as in Tuscany; or beds in sedimentary rocks, as in
Thuringia. The crystallized specimens shown in
Plate 8 are from Cornwall (Fig. i, on quartz), and
Freiberg, Saxony (Fig. 2).
SMALTITE 97
SMALTITE
(Plate 8, Fig. 3). This important ore of cobalt
is an arsenide of cobalt, CoAs 2 , but it also contains
variable amounts of iron and nickel.
The crystals are cubic, but are rare, and never
very sharply developed. In Fig. 3 is shown a con-
fused group of cubes, with rounded surfaces, and
the corners truncated by small triangular faces of
the octahedron. Granular and compact masses are
more usual. The color is steel-grey, with a metallic
luster.
The presence of cobalt in this mineral is very often
betrayed by the occurrence of crimson specks and
patches of earthy erythrite, or cobalt-bloom, on the
surface of the specimens. This is a hydrated arsenate
of cobalt, formed by the weathering of the smaltite.
Similarly, on the surface of chloanthite (NiAs 2 ) a
mineral closely allied to and very similar in appear-
ance to smaltite, but differing from it in containing
nickel in place of cobalt there may very often be
detected patches of earthy, apple-green annabergite,
or nickel-bloom. Indeed, the presence of these
alteration products affords a ready means of recog-
nizing these minerals.
Smaltite occurs in metalliferous veins, together
with ores of silver; it is found at several places in
Germany, notably at Schneeberg, in Saxony, this be-
ing the locality of the specimen represented in Plate
8. Recently, large quantities of the mineral have
98 THE WORLD'S MINERALS
been found at Cobalt City, near Lake Temiskaming,
in Ontario. This ore of cobalt is principally used
for the preparation of smalt (hence the name smalt-
ite), or cobalt-blue, which is employed as a pigment
and for coloring glass and porcelain.
TETRAHEDRITE
(Plate 8, Fig. 4). Other names for this mineral
are grey-copper-ore and fahlore (German, Fahlerz) .
Ideally, it is a compound of copper, antimony, and
sulphur that is, a sulph-antimonite of copper, with
the formula Cu 3 SbS 3 . Actually, however, the copper
may be in part replaced by silver, iron, zinc, mer-
cury, etc., and the antimony by arsenic. In certain
mining districts it is an important ore of copper; but
when silver is also present (sometimes to the extent
of 30 per cent), it is of far greater importance as
an ore of silver, and many of the rich Bolivian silver-
ores are of this type.
Its crystals belong to the tetrahedral division of
the cubic system, the name tetrahedrite referring, in
fact, to their characteristic tetrahedral habit (Fig. 4) .
In this picture the predominating form of the two
crystals, shown attached to the matrix, is a three-
faced tetrahedron; the acute corners are each re-
placed by three small faces of the rhombic-dodeca-
hedron, and the edges are truncated by narrow faces
of a deltoid-dodecahedron.
The color is iron-black or steel-grey, and the luster
metallic. Sometimes, however, the crystals have a
Plate 8
6
1, 2, Chalcopyrite. 3, Smaltite. 4, Tetrahedrite. 5. Pyrargyrite 6, Proustite.
TETRAHEDRITE PYRARGYRITE 99
thin coating of copper-pyrites on the surface, having
then the appearance of a brassy-yellow mineral. Fine
crystals of this kind, often with a brilliant iridescent
tarnish on their surface, were formerly found in
Herodsfoot mine, Cornwall. Other localities for
well-crystallized specimens are in the Harz Moun-
tains (Fig. 4), and Kapnik, in Hungary.
PYRARGYRITE
(Plate 8, Fig. 5). This mineral is also known
as dark-red silver-ore, or as ruby-silver; and con-
taining 60 per cent, of silver, it is a rich ore of this
metal. It is a compound of silver, antimony, and
sulphur that is, a sulph-antimonite of silver, with
the formula Ag 3 SbS 3 .
The crystals are rhombohedral, and usually consist
of a hexagonal prism, terminated by a flat rhombo-
hedron (as shown in Fig. 6 for proustite) ; in Fig. 5
the hexagonal prism is terminated by large faces of a
scalenohedron, with small faces of the rhombohedron
at the apex, the form here being the same as in calcite
(Plate 1 6, Fig. i).
The color seen on the surface of the crystals is
greyish-black, but small crystals and fragments are
deep ruby- red by transmitted light; and the same
ruby-red color is also to be seen on portions of the
crystals that are bruised or fractured. The color of
the streak, or of the powdered mineral, is purplish-
red.
Pyrargyrite is found in veins of silver-ore at An-
100 THE WORLD'S MINERALS
drcasberg, in the Harz Mountains; at Freiberg, in
Saxony; and at Guanajuato, in Mexico.
PROUSTITE
(Plate 8, Fig. 6). This is another ruby silver-ore,
known as light-red silver-ore, which differs from the
last in containing arsenic in place of antimony, the
chemical formula being Ag 3 AsS 3 . The pure mineral
contains 65^ per cent, of silver. The crystals are
of the same type as those of pyrargyrite, though the
scalenohedral habit (Fig. 5) is more characteristic
of proustite than the prismatic habit (Fig. 6). The
crystals are transparent, with a magnificent ruby-red
color; but on exposure to light they very soon blacken
and become opaque. For this reason specimens must
be kept in the dark. The color of the streak is scarlet.
This difference in the color of the streak affords a
ready means of distinguishing pyrargyrite and
proustite, which are often confused. Indeed, it seems
probable that Figs. 5 and 6 of Plate 8 have been
accidentally interchanged, as suggested by the habit
of the crystals.
Magnificent groups of transparent ruby-red crys-
tals of proustite are found in the silver-mines at
Chanarcillo, in Chile, and good specimens are also
known from Freiberg, in Saxony.
CHAPTER VII
THE HALOIDS
THE chemical elements fluorine, chlorine, bromine,
and iodine are known collectively as halogens, and
the compounds, or salts, formed by their union with
a metal are known as haloids. This word is derived
from the Greek name for salt that is, common salt or
rock-salt, which may be taken as the type of this
group.
Only a small number of minerals fall into this
group, and most of them are quite rare and not suit-
able for illustration in color. To the descriptions of
the three species figured in Plates 8 and 9, we shall
add a brief mention of one or two others which
present certain points of interest.
In their general appearance and external charac-
ters the different minerals of this group have little in
common, though they are all transparent to a greater
or less degree. In color they exhibit a wide range,
and their practical applications are equally diverse.
FLUOR-SPAR
(Plate 9, Figs, i and 2). Fluor, or fluorite, is a
beautiful mineral, presenting many points of interest.
It is of fairly wide distribution, and in certain dis-
101
102 THE;, WORLD'S MINERALS
tricts it is found in enormous quantities in association
with lead-ore.
Chemically, it is a fluoride of calcium (CaF 2 ),
being a compound of the extremely active gas fluo-
rine with the metal calcium. It is insoluble in water,
and is very resistant to most chemical reagents; but
when warmed with sulphuric acid, it is decomposed
with the liberation of hydrofluoric acid gas. This
gas readily attacks glass and other silicates, and on
this action depends its use for etching glass. Fluor-
spar fuses at a red-heat, and on this account it is
employed as a flux in the smelting of ores. For this
reason, the mineral takes its name from the Latin
fluo f I flow.
Good crystallized specimens are common. The
crystals belong to the cubic system, and most fre-
quently have the form of perfect cubes (Plate 9) . On
specimens from certain localities the edges of the
cube are sometimes each bevelled by two narrow
faces of a form known as a four-faced cube; or
again, the corners may be each replaced by six small
faces of a six-faced octahedron; but these modifica-
tions of the cube are comparatively rare. Another,
but far less common form, is the simple octahedron,
as in the beautiful pink octahedra of the fluor-spar
from Chamounix and a few other places in the Alps.
Very frequently the crystals are twinned; there
being an intergrowth or interpenetration of two
cubes, with the result that the corners of one cube
project from the faces of the other. The two cubes
are related to one another in a perfectly regular
HALOIDS.
Plate 9.
1, 2, Fluorite or Fluor-spar.
FLUOR-SPAR 103
geometrical manner; one of the diagonals joining
opposite corners of the cube is common to the two
cubes ; and if one of the cubes be rotated about this
diagonal as axis through half a complete revolution,
it will come into a position coincident with the sec-
ond cube. This peculiar twin intergrowth is an
extremely characteristic feature of fluor-spar.
Another very important character of this mineral
is its perfect octahedral cleavage that is, the crystals
may be split with ease along four plane directions
parallel to the faces of the octahedron. For this rea-
son specimens of fluor-spar are very liable to be
damaged by having the projecting corners of the
cubes knocked off. Thus, in Plate 9, Fig. i, some
of the corners of the cubes are replaced by small
triangular cleavage surfaces. In the same specimen
may be seen cleavage cracks in the interior of the
crystals, with the cracks intersecting the cube faces
parallel to the diagonal of the latter.
The faces of the crystals and of the cleavage sur-
faces are usually very bright and smooth, with a
luster like that of glass. Fluor-spar can be readily
scratched with a knife; and it is very brittle. Its
specific gravity is 3.2.
The range of colors shown by fluor-spar is very
extensive, and, indeed, no other mineral affords so
good an example of how color may vary in one and
the same species. When quite pure, fluor-spar is
perfectly colorless and transparent. The mineral
may also show delicate or intense shades of blue,
purple (Fig. i), green (Fig. 2), pink, yellow, or
104 THE WORLD'S MINERALS
sometimes even black. Frequently also the same crys-
tal may be of different colors in layers parallel to the
cube faces. The well-known "blue-John" of Derby-
shire is a dark purplish, fibrous variety of fluor-spar.
These colors are due to the presence of mere traces
of coloring matter, so small in amount that the exact
nature of the pigment is still a matter for discussion.
The fact that in many cases the color is destroyed by
heat the fluor-spar becoming quite colorless sug-
gests that the coloring matter is a hydrocarbon of
some kind. Certain specimens are changed in color
on exposure to sunlight; for example, green may be
changed to purple.
A remarkable property exhibited by some crystals
of fluor-spar particularly those from the north of
England is that they show one color by transmitted
light, and another color by reflected light. The
crystals like those represented in Fig. i, if held be-
tween the observer's eye and the window, appear
pale green; but if the eye be between the specimen
and the window, a rich velvety, plum-blue color is
to be seen on the surfaces of the crystals. This phe-
nomenon, having been first recognized in fluor-spar,
is known as fluorescence. Another interesting prop-
erty, of a somewhat similar optical nature, is that
of phosphorescence. When fluor-spar is heated to a
temperature below red-heat it glows with a soft
greenish light, like that of the glow-worm.
Fluor-spar occurs in nature under a variety of con-
ditions. It is found in granite, and in veins of tin-ore,
in Cornwall; in gneiss in the Alps; in limestone, and
FLUOR-SPAR ROCK-SALT 105
in veins of lead-ore, in Derbyshire and the north of
England; and rarely in the lava of Vesuvius.
The best crystallized specimens are those found
abundantly in the lead-mines of the north of Eng-
land, in Cumberland, Northumberland, and more
particularly in County Durham. In Weardale
(County Durham) large cavities completely lined
with beautiful crystals, measuring sometimes as much
as twelve inches or even more along the edges of
the cubes, are not uncommonly met with. The two
specimens represented in Plate 9 are from Weardale;
in Fig. i the crystals are partly encrusted with chaly-
bite, and in Fig. 2 they rest on a base of iron-stone.
Some of the practical uses of fluor-spar have al-
ready been briefly mentioned. Large quantities
thousands of tons annually are mined in Derbyshire
and the north of England for use as a flux in smelting
ores. The mineral is also used for the preparation
of hydrofluoric acid and for etching glass, and in the
manufacture of opaline glass. Perfectly colorless and
transparent crystals find an application in the con-
struction of lenses for microscopes ; while the Derby-
shire "blue-John" is carved into vases and other small
ornaments. The finer colored crystals have occasion-
ally been cut as gem-stones, but the low degree of
hardness of the mineral renders it unsuitable for use
in jewelry.
ROCK-SALT
(Plate 10, Figs, i and 2). Salt, or common salt,
is a mineral of great importance in everyday life.
106 THE WORLD'S MINERALS
To the mineralogist it is known as rock-salt, or halite
(from the Greek name for salt) . The reddish-brown
lumps of salt given to cattle to lick is rock-salt in the
condition in which it is found in the earth. This may
be readily purified by dissolving it in water and al-
lowing the earthy impurities to settle; on evaporating
the clear solution, the pure white table-salt or cook-
ing-salt is obtained. It may be remarked in passing
that very few minerals are soluble in water; the
only one described in this book being the one under
discussion.
Salt is deposited by the evaporation of sea-water
in land-barred gulfs and from the salt water of in-
land lakes, such as the Dead Sea and the Great Salt
Lake of Utah. Such deposits may become covered
up by layers of mud, and so protected and preserved.
In this way beds of rock-salt, sometimes of enormous
thickness, have been formed in past epochs of the
earth's history; and these are now found and mined
in the sedimentary rocks of various geological pe-
riods. The beds of rock-salt for which Cheshire is
famous are interstratified with the red marls and
standstones of Triassic age, while many of those in
the United States are of considerably greater an-
tiquity.
The rock-salt occurring in these beds is massive,
and may be quite pure, being then white or colorless,
or it may be somewhat intermixed with clay and red
oxide of iron, when it is reddish-brown in color.
Cavities when present in this massive material are
lined with crystals of rock-salt. Good crystals are,
ROCK-SALT 107
however, not common ; the best come from Strassfurt,
in Prussia (Plate 10, Fig. i), and from Wieliczka, in
Poland (Fig. 2).
Crystals of rock-salt belong to the cubic system,
and almost invariably they have the form of simple
cubes (Plate 10, Figs, i and 2). These crystals are
thus of exactly the same form as those of fluor-spar,
but there is a very important difference between them
in their cleavage. While in fluor-spar the perfect
cleavages are parallel to the faces of the octahedron,
in rock-salt they are parallel to the faces of the cube.
A crystal of rock-salt may therefore be easily broken
up into a number of smaller cubes, and this sub-
division can be repeated indefinitely.
The crystals are usually perfectly colorless and
transparent (Fig. 2), but occasionally they are of a
deep blue color. The cause of this blue color is very
mysterious, for the color disappears when the salt
is dissolved in water or when it is heated; further,
a similar color is induced in colorless rock-salt by
heating it in the vapor of metallic sodium.
Beds of rock-salt are sometimes worked by the
ordinary methods of mining, but more frequently
water is admitted through bore-holes into the salt-
bearing strata and the solution of salt pumped up.
Large quantities of salt are also yielded by natural
salt springs and by the solar evaporation of sea-water
in salt-pans. The famous mines of Wieliczka, near
Cracow, which have been worked for the last 600
years, form a large subterranean town carved in the
thick bed of solid rock-salt.
108 THE WORLD'S MINERALS
Some other modes of occurrence of rock-salt are
of interest, though of no practical importance. The
mineral is sometimes to be found as an encrustation
on Vesuvian lava; and in the extremely minute cav-
ities in the quartz of many granites, cubes of rock-
salt immersed in a liquid may be seen under the
higher powers of the microscope.
ATACAMITE
(Plate 10, Fig. 3). This is a rare copper mineral,
though at one or two places it is found in sufficient
abundance to be mined as an ore of copper. It is a
combination of chloride of copper and hydroxide of
copper, with the somewhat complex formula CuCU.-
3Cu(OH) 2 . Its crystals are of a rich, deep-green
color, with a brilliant luster; they have the form of
rhombic prisms, with pyramidal terminations. Small
but brilliant crystals are found in the Desert of Ata-
cama (hence the name atacamite) and other parts
of Chile; while large crystals are abundant at Wal-
laroo, in South Australia. The specimen represented
in the figure is from the latter locality.
Atacamite was put to a curious use before the
days of blotting-paper, the powdered mineral being
sprinkled over letters to dry up the ink. A sample
of the "writing sand" from Atacama was presented
to the British Museum collection by the Abbe
Rochon in the year 1790.
HALOIDS : OXID
Plate 10.
1, 2, Halite. 3, Atacamite. 4, 5, OpaK
CERARGYRITE CRYOLITE 109
CERARGYRITE
Or Horn-silver. Under this term are included
several minerals which are compounds of silver with
chlorine, bromine, and iodine. In Chile, Nevada,
and New South Wales they are found in considerable
quantity and are of importance as ores of silver.
These minerals present a horn-like appearance, and
like horn they can be cut with a knife. Their pale
yellow or greenish color quickly darkens on exposure
to light. Their cubic crystals are small and obscure,
and not suitable for illustration.
CRYOLITE
This is the only other haloid that we need mention
in this place. It is a fluoride of sodium and alu-
minium (Na 3 AlF 6 ) , and is of importance as an ore of
metallic aluminium. It is a white or colorless min-
eral, with somewhat the appearance of ice, and for
this reason it is known as ice-spar (the name cryolite
having the same meaning in Greek) . It is extensive-
ly mined at one spot in Greenland, and is employed
in the manufacture of opal-glass and iron-enamel.
Formerly metallic aluminium was extracted exclu-
sively from cryolite, but now bauxite, together with
a small proportion of cryolite, is employed.
CHAPTER VIII
THE OXIDES
THIS group of oxides, or compounds of oxygen with
another chemical element, includes several important
minerals, some of which are used as precious stones
and others as ores of the metals.
QUARTZ
(Plates ii and 12). Quartz is the most abundant
and widely distributed of all minerals. The sand
of the seashore consists almost entirely of grains of
quartz; and when these are cemented together to
form solid rock, we have the abundantly occurring
sandstone. Various other kinds of rocks, such as
granite and gneiss, also consist in part of quartz, and,
indeed, the mineral is the most important of the rock-
forming minerals. In the neighborhood of London,
and other places in the southeast of England, prac-
tically the only kind of stone to be found is flint,
which is a compact variety of quartz.
Chemically, quartz is the dioxide of the non-
metallic element silicon; this oxide is usually known
simply as silica, and its formula is SiO 2 .
Not only is quartz the most abundant of minerals,
110
QUARTZ 111
but it is one that appears in a greater variety of forms
or guises than any other mineral. It may be either
perfectly colorless and transparent like glass, or black
and opaque like jet. Or, combined with varying
degrees of transparency, it may be of all shades of
color from one end of the spectrum to the other
red, orange, yellow, green, rarely blue, and violet.
Again, in the character of its luster, it may be either
glassy or waxy, or quite dull and earthy. Further,
it may be crystallized in beautiful forms, or it may
be compact with no obvious crystalline structure.
Owing to this great diversity in its external ap-
pearance, a considerable number of varieties of
quartz are recognized and distinguished by special
names for example, rock-crystal, amethyst, jasper,
carnelian, agate, onyx, flint, hornstone, and many
others. These several varieties may be conveniently
divided into two great groups namely, crystallized
quartz and crypto-crystalline (i.e. minutely crystal-
line) quartz, which are illustrated on Plates 11 and
12 respectively.
Quartz is thus a mineral not always easy to recog-
nize. Characters of the first importance which help
in its determination are ( i ) the absence of cleavage,
(2) the hardness, and (3) the specific gravity. The
degree of hardness (No. 7 on the scale) is such that
the mineral cannot be scratched with a knife, and
when an attempt is made to scratch it a metallic mark
is left on the stone. It is, further, a comparatively
light mineral (sp. gr. 2.65) , being only about two and
a half times as heavy as an equal volume of water.
112 THE WORLD'S MINERALS
If, therefore, we have a stone, consisting to all ap-
pearances of the same kind of material throughout,
which is not heavy, cannot be scratched with a knife,
and shows no trace of cleavage, we are fairly safe
in pronouncing that it consists of the very common
mineral quartz. By way of confirmation, we may
determine whether a fragment of the stone remains
suspended together with a known crystal of quartz
in a heavy liquid (such as a mixture of methylene
iodide and benzine; see p. 40) of suitable density.
If, however, the specimen is crystallized, there
need not be the slightest hesitation in the determina-
tion, for crystals of quartz possess an extremely char-
acteristic form. As clearly shown in Plate n, Figs.
1-3, they consist of a hexagonal, or a regular six-
sided, prism capped by a six-sided pyramid. In Fig.
2 the six pyramid faces are of equal size, but in Figs.
i and 3 they are alternately larger and smaller, with
a three-fold arrangement. Additional faces are
sometimes, though rarely, present on crystals of
quartz. On each of the three alternate corners be-
tween the prism and pyramid there may occasionally
be a small rhomboidal face (Fig. 3), and sometimes
also a small trapezoidal face (Fig. 2) between the
latter and a prism face. In Fig. 2, this trapezoidal
face is placed on the right-hand side of a prism face,
and the crystal is, therefore, a right-handed crystal.
We may also have left-handed crystals of quartz.
Two such crystals, a right-handed and a left-handed,
may be exactly alike, except that one is the mirror-
reflection of the other, and like the right and left
QUARTZ 113
hands they cannot be brought into coincident po-
sitions.
A specially characteristic feature of crystals of
quartz, and one which helps in the recognition of the
mineral, is that the prism faces are always more or
less deeply striated, or grooved, horizontally; that is,
they are marked by the fine lines perpendicular to
the edge of the prism. In many other minerals which
crystallize in prismatic forms e.g. beryl and topaz
the prism faces are more usually striated parallel to
the prism edges.
To proceed still further with the crystallography
of quartz about which, indeed, a whole volume
might be written it may be remarked that not only
are the shapes of the crystals and the striations char-
acteristic and peculiar to the mineral, but also the
magnitude of the solid angles between the faces are
equally characteristic and constant for the species. It
was determined, so long ago as 1669, by Nicolaus
Steno, a Danish physician, that the angle between
adjacent prism faces is always exactly 120, and that
the angle between a prism face and a pyramid face is
about 142 (more exactly 141 47' of arc). The
angle of slope* of the pyramid faces to the horizontal
is thus very nearly 52, which curiously is the same as
that of the Pyramids of Egypt. Again, at the top
of the crystal the angle between alternate pyramid
faces is 94 14', or nearly a right angle; so that, when
three alternate faces are largely developed at the ex-
*In Plate ix, Fig. I, the crystals are not drawn quite correctly in
this respect, the pyramid being rather too low.
114 THE WORLD'S MINERALS
pense of the other three, the crystal may assume a
cube-like aspect.
The shapes of the faces themselves are likewise
characteristic, and the plane angles between their
edges are also fixed in magnitude. The prism faces
are rectangular in outline, with angles of 90, and
the striations on them are parallel to one pair of
edges. The pyramid faces when equal in size (as in
Fig. 2) have the shape of acute isosceles triangles,
with angles of 70 at the base and 40 at the apex, and
with the sides half as long again as the base. It will
be found useful to bear these figures in mind. For
quartz frequently occurs minutely crystallized in
cavities and crevices, forming so-called "drusy" sur-
faces; and if such material be examined with a mag-
nifying-glass, the characteristic shapes of the prism
and pyramid faces can usually be distinguished when
the tiny facets catch the light.
There being no cleavage, the fracture of quartz is
typically conchoidal, and on the rounded, shell-like
surfaces of fracture the luster is bright and vitreous
in character. A fractured surface of quartz presents
exactly the appearance of broken glass. Amethyst
shows on its fractured surface a very peculiar and
characteristic appearance; it is marked with minute
ripples or "thumb-marks," just like those formed
when the thumb is pressed against some plastic sub-
stance.
Quartz when perfectly pure is quite transparent
and colorless, this being the variety known as rock-
crystal. The specimen represented in Plate n, Fig.
OXIDES (Quartz group).
Plate 11.
5
1, Amethyst. 2, Smoky-quartz. 3, Quartz. 4, Cat's-e5yei : ^ Rtofe-quarti,
QUARTZ 115
3, is from the Swiss Alps. Such specimens were be-
lieved by the ancients to be ice, frozen so hard on the
highest peaks of the Alps that it could not be again
thawed. It is, indeed, owing to this belief that rock-
crystal received its name, the word krystallos meaning
in Greek "clear ice." In the seventeenth century
this name came to be extended to the other plane-
faced bodies which we now know collectively as
crystals.
The distinction between other varieties of crystal-
lized quartz depends solely on differences in color.
Clear yellow crystals are known as citrine, occidental
topaz, or false topaz; and those of a more pronounced
brown color, as smoky-quartz (Plate n, Fig. 2; rep-
resenting a specimen from Switzerland) or cairn-
gorm, after the locality in Scotland where such crys-
tals are found. Amethyst (Fig. i; from Brazil) is
violet, or purple, crystallized quartz. Rose-quartz
(Fig. 5; from Rabenstein, in Bavaria) is of a fine
rose-red color; this variety is, however, found only as
broken masses, without crystal faces.
Another variety known as cat's-eye (Plate n, Fig.
4; from Ceylon) owes its special appearance to the
enclosure in the crystallized quartz of large numbers
of very fine fibers of asbestos arranged parallel to
each other; or sometimes the asbestos may have been
dissolved out of the stone, leaving hollow canals.
Such stones, especially when cut and polished with
a convex surface (as represented in the picture), ex-
hibit a milky band of reflected light, and, as the stone
is moved about, this band travels across the surface.
116 THE WORLD'S MINERALS
A fancied resemblance to the eye of a cat gives these
stones their name of catVeye, while the special op-
tical effect is known as chatoyancy. Strictly, how-
ever, they should be called quartz cat's-eye, for the
same effect may be shown by other minerals for
example, chrysoberyl, tourmaline, and gypsum
which may accidentally possess an internal fibrous
structure. Quartz cat's-eyes are usually pale yellow-
ish or greenish in color, but those from the Asbestos
Mountains, on the Orange River, in South Africa,
display a magnificent golden-yellow sheen. The
latter are known by the special name of tiger-eye, and
often, erroneously, as crocidolite (compare the de-
scription of Plate 26, Fig. 3).
Coming now to the several varieties of crypto-
crystalline or compact quartz, these consist not of
single crystals, but of aggregates of vast numbers
of minute crystalline individuals, or interlocking
grains so closely crowded together that they have
had no opportunity to develop crystal faces at their
boundaries. The exact mode of aggregation may of
course be varied, giving rise to a corresponding num-
ber of varieties distinguished by special names.
Jasper consists of such an aggregate, the minute
grains of quartz being here much intermixed with
clayey material and with red or yellow oxides and
hydroxides of iron. These impurities give rise to the
different colors (red, yellow, brown, green, blue),
which may be present singly or together in the same
piece of stone. When the arrangement of colors is
in parallel bands, we have the variety called riband-
OXIDES (Quartz group).
Plate 12.
1, Agate. 2, 3, Jasper. 4, Hornstone.
QUARTZ 117
jasper. In the red jasper shown in Plate 12, Fig. 3,
the coloration is nearly uniform, while in the ball-
jasper in Fig. 2 (from Freiberg, in Baden) there is
a concentric banding of red and yellow. Hornstone
(Fig. 4) is of much the same character, but with
much less admixed impurity; it presents a certain
amount of translucency, with dull greyish or brown-
ish colors, so that it somewhat resembles horn in
appearance. Its fracture is splintery.
Chalcedony is a rather special variety of crypto-
crystalline quartz, in which the structure, as seen on
a fractured surface, is minutely fibrous. On the sur-
face it presents rounded (mamillated, botryoidal, and
stalactitic) forms, with a semi-transparent appear-
ance, and a luster like that of wax. Here, again, we
have several varieties depending on differences in
color and the arrangement of differently colored
portions. The color of common chalcedony is white
or creamy. When it is bright orange-red, we have
the well-known carnelian; and when brown, the sard.
Pale apple-green chalcedony is known as chrysoprase,
and that of a dark leek-green color as plasma. An
interesting variety of a dark-green color, marked
with bright red spots, like drops of blood, is called
bloodstone, or heliotrope. When it is banded and of
different colors, the bands may be more or less con-
centric, as in agate (Plate 12, Fig. i),* or straight, as
in onyx. Again, we may have in chalcedony enclo-
*In this figure the banding in the outer portion is not so clearly
defined as may usually be seen in an actual specimen. On the specimens
shown in Figs. 1-3 the surfaces have been ground flat, and polished.
118 THE WORLD'S MINERALS
sures of coloring matter, bearing in their form some
resemblance to moss and other plant-like, or den-
dritic, forms, as in moss-agate and mocha-stone.
As to the practical uses to which quartz is applied,
these are extremely numerous. In the form of sand-
stone, it is much used for building and paving. Pure
quartz-sand, the so-called silver-sand, is extensively
used in the manufacture of glass, and also for abrasive
purposes (sand-paper) and for scouring, being often
mixed with soap. Clear rock-crystal is employed for
the construction of prisms and lenses for optical
purposes; it is the "Brazilian pebble" of the spec-
tacle-makers, though quartz is much less used now
than formerly for spectacle-lenses. Recently fused
quartz, or silica-glass, has been much used for the
construction of various pieces of apparatus for the
chemical laboratory.
Several varieties, especially those which are clear
or of a fine color, are extensively used in jewelry as
semi-precious stones. Of these, amethyst is the most
valuable; others whose names are familiar are car-
nelian, chrysoprase, agate, moss-agate, etc. Onyx is
engraved as cameos, and small ornaments of various
kinds are carved in agate. Rock-crystal was carved
by the ancients to form valuable crystal vases and
bowls; while at the present day spheres of rock-
crystal (or, perhaps, often of glass) find a ready sale
to the so-called crystal-gazers. Jasper, being found
in larger masses, is used as polished slabs for the
decoration of both the inside and the outside of
buildings.
OPAL 119
OPAL
(Plate 10, Figs. 4 and 5). Opal is one of the very
few minerals that exhibit no trace of crystalline form.
Not even under the microscope can any indication of
an internal crystalline structure be detected. The
mineral is amorphous, like glass; it is, in fact, merely
a solidified jelly, and has been formed by the drying
up of masses of gelatinous hydrated silica. A small
but variable proportion of water (3 to 10 per cent.)
is still retained by the solid. Chemically, opal has
the composition of quartz, with the addition of water.
Opal usually occurs as a filling in cavities and fis-
sures in rocks of various kinds, and it is also found in
metalliferous veins. Since it usually completely fills
the cavity in which it is found (Plate 10, Fig. 5), it
only rarely has an opportunity of developing any
external boundaries of its own; and these, when
present, are always rounded (botryoidal or stalac-
titic) surfaces. The fractured surfaces of the mineral
are curved or conchoidal, like those of broken glass
(Fig- 5)-
In its external appearance opal is very variable;
and to gain some idea of its general character the
student should inspect a series of specimens in a col-
lection of minerals. It may be perfectly colorless
and transparent, as in the variety known as hyalite,
or Muller's glass; but more often it is semi-trans-
parent to opaque, and of various colors. Frequently
it displays a kind of milky appearance, or opal-
120 THE WORLD'S MINERALS
escence. Further, the character of the luster may
vary from glassy to waxy (wax-opal) or pitchy
(pitch-opal). The mineral is not very hard; it can
be scratched by quartz, but it will itself scratch glass.
The specific gravity (1.9-2.3) is appreciably lower
than that of quartz.
Several varieties of opal are distinguished by spe-
cial names, the more important being common opal
(Plate 10, Fig. 4) and precious opal (Fig. 5). The
precious opal, with its gorgeous colors flashing and
changing with every movement of the stone, is fa-
miliar to everybody. These magnificent colors are
not, however, inherent in the substance of the opal
itself; for if a precious opal be held up between
the eye and the window, so that the rays of light
pass through the stone, only a pale yellowish or
milky color is to be seen. The play of rainbow colors
seen by reflected light is entirely an optical effect,
and is due to the interference of the rays of light at
the surfaces of fissures or internal films. Such colors
are, of course, only to be seen in white light; in the
yellow or red light of a photographer's dark room
a precious opal will show no play of colors. Certain
specimens exhibit a play of colors only when they
are immersed in water and the liquid has penetrated
into the pores of the stone.
Localities at which common opal is found are very
numerous (the specimen represented in Plate 10, Fig.
4, is from Hungary) ; but precious opal is of much
more restricted occurrence. Before the discovery of
the rich opal-fields of Australia (New South Wales
OPAL HAEMATITE 121
and Queensland) practically all the precious opal
used in jewelry came from Hungary, and it was then
much more valuable. In Hungary the mother-rock
of the opal is a volcanic rock known as trachyte; in
New South Wales it is sandstone, and in Queensland
a hard siliceous iron-stone (Fig. 5).
A peculiar variety of opal is the siliceous sinter,
or geyserite, which is deposited in a great variety of
fantastic forms, and as an encrustation on vegetable
matter, from the water of the hot springs of Iceland,
the Yellowstone National Park in the United States,
and in New Zealand. Somewhat similar in charac-
ter is the powdery, white material known as diato-
mite, or infusorial earth (also called tripolite and
Kieselguhr), which consists of the siliceous skeletons
of vast numbers of diatoms. These microscopic or-
ganisms inhabit both fresh water and salt water, and
their remains accumulate on the floor of the ocean
and of lakes, forming sometimes thick deposits. This
material is largely used for polishing; it is highly
absorbent, and when mixed with nitro-glycerine
forms the explosive known as dynamite.
HAEMATITE
(Plate 13, Figs. 1-4). This important ore of iron
is an oxide (the sesquioxide) of iron, Fe 2 O 3 , known
to chemists as ferric oxide. It contains, when pure,
70 per cent, of iron. When crystallized, it is black
or steel-grey, with a brilliant metallic luster, and is
then known as iron-glance , or specular iron-ore; but
122 THE WORLD'S MINERALS
when massive it is dull red, and is then commonly
called red iron-ore. These two varieties thus differ
considerably in their external appearance; but it will
be found that when scratched with a knife or crushed
they yield a characteristic dark red powder. The
resemblance of this color to that of dried blood is the
reason for the name hamatite, which in Greek means
"bloodstone."
Crystals of haematite belong to the rhombohedral
system, but they vary considerably in habit. Usually,
however, the crystals are more or less tabular, as in
Figs, i and 4, and sometimes they have the form of
quite thin plates or scales, as in the so-called mica-
ceous iron-ore.
The massive varieties often exhibit an internal
radiated structure (Figs. 2 and 3), and when natural
external surfaces are present these are rounded or
nodular in form hence the name kidney iron-ore
(Fig. 2). Such masses may be quite soft and earthy,
being then of a brighter red color; or they may be
harder and more compact, with a darker color. The
fibrous masses are, indeed, sometimes almost as hard
and black as the crystals themselves; and as they can
be split up along the fibers into thin rods, such ma-
terial is know as pencil-ore.
Large deposits of crystallized haematite (specular
iron-ore) occur in the island of Elba, where they
have been extensively worked as an ore ever since
the time of the Romans. The specimen shown in
Plate 13, Fig. i, is from these iron-mines. Kidney
iron-ore is raised in large quantities from the several
OXIDES.
Plate 13.
14, Haematite.
HEMATITE MAGNETITE 123
mines in north Lancashire and west Cumberland
(Figs. 2 and 3), where the ore fills large cavities in
limestone. In these mining districts the red color of
the haematite is everywhere in evidence. The crystals
represented in Fig. 4 are not from a mining district;
they are found singly, and in small numbers, attached
to the walls of crevices in mica-schist in the Binn
valley in Switzerland. These crystals contain a con-
siderable amount of titanium dioxide, in addition to
iron oxide, and they are sometimes regarded as
belonging to the closely allied mineral ilmenite.
Haematite is also found in some volcanic districts
(Vesuvius and Madeira) as thin, very brilliant
plates, encrusting the surface of the lava.
The chief use of haematite is, of course, as an ore
of iron. The powdered mineral is also used for pol-
ishing (jeweler's rouge), and in the manufacture
of paints and red pencils. The hard, compact pencil-
ore is cut and polished as pin-stones and ring-stones
for use in jewelry; it takes a very brilliant polish,
and is of a deep black color. The same material,
when polished, is also used by jewelers and book-
binders for burnishing tools.
MAGNETITE
(Plate 14, Fig. i). Magnetite, or magnetic iron-
ore, is another important ore of iron, which also con-
sists of oxide of iron. Here, however, it is the oxide
Fe 3 O 4 , containing rather more iron (72.4 per cent),
and correspondingly less oxygen, than haematite.
124 THE WORLD'S MINERALS
Chemically, it may be regarded as a combination of
ferric oxide (Fe 2 O 3 ) and ferrous oxide (FeO).
In its strongly magnetic character, magnetite is
unique amongst minerals, no other even approaching
it in this respect. Fragments of the mineral are
readily attracted by a magnet; and a freely suspended
magnetic needle may be turned completely round by
holding near to it a piece of the mineral. Further,
certain specimens are not only magnetic, but mag-
netic with polarity; one corner or end of the speci-
men which attracts one pole of the magnetic needle
will repel the other pole; and if the specimen be
freely suspended it will, like a magnetic needle, set
itself in a north and south position under the influ-
ence of the earth's magnetism. Such specimens are
the well-known lodestones, or natural magnets.
Crystals of magnetite are not uncommon, but the
mineral is more usually found as compact or granu-
lar masses. These are dull, black, and heavy, with
no very characteristic appearance; but their identity
with magnetite may be at once determined simply
by holding them near a magnetic needle. The streak,
or powder, of the mineral is always black.
The crystals have the form of the regular octahe-
dron, or sometimes of the rhombic-dodecahedron.
In the picture (Fig. i) are shown several lustrous
black octahedra on the surface of mica-schist. This
specimen is from the Binn valley in Switzerland,
where crystals of magnetite are found under exactly
the same conditions as crystals of haematite (Plate
13, Fig. 4). Excellent crystals of magnetite are also
MAGNETITE SPINEL 125
found in the iron-mines at Traversella, in Piedmont,
and Nordmark, in Sweden.
As an ore of iron, magnetite is of considerable im-
portance, most of the Swedish ores being of this
class, and it is also extensively mined in the Urals
and in New York.
SPINEL
Magnetite is a member of the spinel group of
minerals, all of which crystallize in the cubic system,
usually in octahedral forms; and further, they are
analogous in chemical constitution, though not in
actual composition. Taking the formula of magne-
tite, Fe 3 (X or FeO.Fe 2 O 3 , if we replace the ferrous
oxide by magnesium oxide and the ferric oxide by
aluminium oxide, we arrive at the formula of spinel,
MgO.AloOs. Similarly, chromite, or chrome-iron-
ore, another member of this group, consists of ferrous
oxide with chromic oxide, FeO.Cr 2 O 3 ; while an ore
of zinc known as gahnite has the analogous formula
ZnO.Al 2 O 3 . All these, and several other minerals,
belong to the spinel group.
Spinel itself is of interest in being one of the
precious stones used in jewelry. It is found in the
gem-gravels of Ceylon as small, red, octahedral
crystals; and when cut and polished it presents a
striking resemblance to the true ruby, being, in fact,
known as spinel-ruby, or balas-ruby. It may, how-
ever, be readily distinguished from the true ruby by
its optical characters, specific gravity, and hardness.
126 THE WORLD'S MINERALS
CORUNDUM
(Plate 14, Fig. 2). The oxide of the light and
useful metal aluminium is known to the chemist as
alumina (A1 2 O 3 ), to the mineralogist as corundum,
to the jeweler as ruby and sapphire, and to the lapi-
dary as emery. All these substances, or rather vari-
eties, of the mineral corundum, though differing so
widely in their external and general appearance, are
identical in their essential characters that is, chem-
ical composition, crystalline structure, hardness,
specific gravity, and other physical and optical char-
acters. The differences they exhibit depend solely in
the presence of smaller or greater amounts of ad-
mixed coloring matter or impurities. A convincing
proof of this is that the same stone, even when faceted
as a gem, may show bands of different colors.
When quite pure, crystallized corundum is per-
fectly colorless and transparent. But when small
amounts of various inorganic coloring materials are
present the crystals may be brilliantly colored, or,
we may say, dyed, and yet retain their transparency.
The colors of such transparent stones of gem-quality,
or precious corundum, may be red, orange, yellow,
green, blue, or violet all the colors of the rainbow.
These differently colored stones are unfortunately
designated by special names in jewelry, much to the
confusion of the uninitiated. It would, indeed, be
just as reasonable to call silk ribbon by different
names according to the color it has been dyed.
CORUNDUM 127
When the crystals are cloudy, or opaque, due to
the presence of fissures and the enclosure of foreign
materials, the mineral is known as common corun-
dum, as distinct from precious corundum. When
it is granular and mixed with a considerable amount
of foreign matter, especially magnetite, we have the
variety known as emery.
These two more abundant forms of corundum are
of considerable importance for abrasive purposes,
being used for the making of corundum-wheels and
emery-cloth, or as powder for grinding precious
stones and in lapidaries' work generally. This ap-
plication of corundum depends, of course, on its
great hardness (No. 9 on the scale). With the ex-
ception of diamond, corundum is the hardest of all
minerals. Recently, however, its place as an abrasive
agent has been taken by the still harder artificial
product carborundum, a silicide of carbon.
Crystals of corundum belong to the rhombohedral
system. The crystal represented on the matrix of
crystalline limestone in Plate 14, Fig. 2, consists of
a hexagonal prism truncated at right angles by the
base, and with small triangular faces of a rhombo-
hedron on three of its corners. A more usual form
for crystals of sapphire is, however, a steeply in-
clined six-sided pyramid.
The specific gravity (4.0) of corundum is con-
siderably greater than that of metallic aluminium
(2.5), and, with the exception of zircon and some
varieties of garnet, it is the heaviest of the precious
stones. Why the combination of the light metal
128 THE WORLD'S MINERALS
aluminium with the gas oxygen should produce a
stone of such density is difficult to understand.
An interesting optical effect is exhibited by the
varieties known as star-sapphire and star-ruby. When
cut and polished with a convex surface, these show
a six-rayed star of reflected light, which travels over
the surface as the stone is moved about, just as does
the single band in the catVeye, mentioned on p. 115.
Corundum occurs embedded in granite, gneiss,
serpentine, or crystalline limestone. It is not a widely
distributed mineral, though abundant at certain lo-
calities. It is mined on a large scale for abrasive
purposes in Ontario and North Carolina; and there
have long been important emery-mines in Asia Minor
and the island of Naxos. Rubies of the best quality
are those found in a crystalline limestone, or marble,
at Mogok, in Upper Burmah. The best sapphires
are from the gem-gravels of Ceylon, and stones of
good gem-quality are also found in Australia and the
United States.
Of special interest are the variously colored cor-
undums of gem-quality, which within the last few
years have been made artificially in large quantities
by a cheap and simple process. These stones possess
all the characters of natural crystallized corundum,
differing from it only in their mode of origin, and
when cut and polished they furnish handsome gems.
CASSITERITE 129
CASSITERITE
(Plate 14, Fig. 3). Cassiterite, or tin-stone, is
practically the only ore of tin, other minerals con-
taining this metal being of quite rare occurrence.
It is the dioxide, SnO 2 , and contains 78.6 per cent,
of metallic tin.
It is a very heavy mineral (sp. gr. 7), usually of a
dark-brown color, though this may vary from almost
white to black. Most frequently it is met with as
compact masses, or as grains disseminated through
the rock, but crystals are not uncommon. Sometimes
it has a fibrous structure resembling that of wood, be-
ing then known as 'wood-tin.
The crystals are tetragonal, and are, as a rule,
rather small. Their form is that of a square prism
terminated by a square pyramid. These forms are,
however, not always easily made out, and in the pic-
ture (Fig. 3) they are not very obvious. Here, how-
ever, a still more characteristic feature of crystals of
cassiterite is represented; in the lower left-hand
corner of the picture are seen two twinned crystals,
with their very characteristic re-entrant angles. The
faces of the crystals are very brilliant, with a luster
like that of diamond.
As above mentioned, the most characteristic feature
of the mineral is its heaviness, but the only certain
way of determining a specimen devoid of crystals is
to test for the presence of tin. This may be readily
done by heating the powdered mineral, mixed with
130 THE WORLD'S MINERALS
sodium carbonate, on charcoal in the reducing flame
of the blowpipe, when bright beads of metallic tin
are obtained.
Cassiterite is not a mineral of wide distribution,
but in certain districts it is met with in considerable
abundance. Veins of tin-ore usually occur in the im-
mediate neighborhood of granite, as in the long-
known tin-mining district of Cornwall. With the
weathering and breaking-down of the tin-bearing
rocks, the heavy, indestructible tin-stone collects in
the beds of rivers, and is then known as stream-tin.
Alluvial deposits of stream-tin are largely worked
in the Malay Peninsula and in Australia. Another
district which was formerly of considerable im-
portance is in the mountains between Saxony and
Bohemia; the specimen represented in Plate 14 is
from Schlaggenwald, Bohemia.
ZIRCON
(Plate 14, Fig. 4). This mineral is a double
oxide of zirconium and silicon, with the formula
ZrO 2 .SiO 2 . Writing this formula in the form ZrSiCh,
the composition is represented as a silicate of zir-
conium. Owing, however, to the remarkable simi-
larity of the crystalline form of zircon to that of
cassiterite, it seems more natural to class this mineral
with the oxides rather than with the silicates.
Zircon is found only as crystals, though these may
sometimes be much rounded and water-worn, es-
pecially when found in gem-gravels. The crystals
OXIDES.
Plate 14.
1, Magnetite. 2, Corundum. 3, Cassiterite. 4, Zircon. 5, Pitchblende or Uraninite,
6, Limonite.
ZIRCON 131
are tetragonal, consisting of a square prism capped
by a square pyramid. In the picture (Fig. 4) we see
two such crystals partly embedded in felspar from
the Ilmen Mountains, in the southern Urals; one
crystal, seen in side view, shows the edge of the large
square prism truncated by narrow planes (only one
is visible in the picture) of a second square prism;
the other crystal, seen in end view, shows only the
four faces of the square pyramid.
The color of zircon is somewhat variable; brown,
as shown in the picture, is the f most frequent color
of the crystals. Reddish-brown stones of gem-quality
are known as hyacinth, and green and yellow stones
are sometimes called jargoon. Owing to these bright
colors and to its high degree of brilliancy and hard-
ness, zircon, when cut and polished, makes a very
effective gem-stone; and when colorless it may even
be mistaken for diamond. Stones of a reddish-brown
color may be completely decolorized by heat, and are
sometimes sold as diamonds.
A very remarkable feature presented by zircon is
the wide range of its specific gravity, which in dif-
ferent specimens may vary from 4.0 to 4.7. Further,
certain stones when heated change, not only in color,
but also in specific gravity.
Zircon is an almost invariable constituent of gran-
ite and certain other rocks of igneous origin ; but, as
a rule, the crystals are extremely minute, and present
in only relatively small amount. When such rocks
are broken down by weathering, the zircon, being
indestructible, is carried away by running water with
132 THE WORLD'S MINERALS
the debris. For this reason microscopic crystals of
zircon may be detected in almost all sands, gravels,
and sandstones. The larger pebbles and water-worn
crystals of zircon so abundant in the gem-gravels of
Ceylon have in the same way been derived from the
granitic rocks of the island.
PITCHBLENDE
(Plate 14, Fig. 5). Pitchblende is an ugly black
mineral, which in color, luster, and fracture bears
some resemblance to pitch. When, however, a speci-
men is handled, it will at once be noticed that it is
far heavier than pitch far heavier, indeed, than
most other materials. Its specific gravity ranges from
8.0 to 9.7, the former value being greater than the
specific gravity of iron, and the latter greater than
that of copper.
In spite of its unattractive appearance, pitchblende
has within recent years come to be a mineral of ex-
traordinary interest by reason of its remarkable radio-
active properties. The invisible rays continually
being emitted by a specimen of pitchblende are ca-
pable of acting on a photographic plate; so that it is
possible with its aid to photograph the outlines of
objects in the dark. The rays may also cause certain
substances (especially artificially prepared hexagonal
zinc sulphide) to become luminous, or to phosphor-
esce, in the dark. Again, they possess the power of
making the air a conductor of electricity; so that
if a piece of pitchblende be placed near a charged
PITCHBLENDE LIMONITE 133
gold-leaf electroscope, the charge of electricity rapid-
ly escapes, and the gold-leaves fall together.
These remarkable properties were first discovered
in uranium compounds by the late Professor Henri
Becquerel in 1896; and the observation that they are
strongly marked in pitchblende led Madame Curie
to the isolation of the particular elements radium
and polonium to which these special effects are due.
The amount of radium contained in pitchblende
the richest ore of radium is, however, extremely
minute, not more than one part in five million. Sev-
eral tons of the mineral have to be treated by a long
series of complex chemical operations to obtain even
a very small amount of a radium compound.
Chemically, pitchblende is an oxide of uranium,
and on this account it is known to mineralogists as
uraninite. Oxides of some other rare metals are often
also present in variable amount.
The most productive mines of pitchblende are
those at Joachimsthal, in Bohemia, where it occurs
in metalliferous veins, together with ores of silver
and cobalt. It is also found in some of the Cornish
mines. At these places it is always massive; but in
the felspar quarries in the south of Norway it is
found as small cubic crystals (known by the special
names broggerite and cleveite) embedded in the
felspar.
LIMONITE
(Plate 14, Fig. 6). On account of its usual rusty-
brown color this important ore of iron is often known
134 THE WORLD'S MINERALS
as brown iron-ore. Certain masses when presenting a
rounded (nodular or stalactitic) surface may, how-
ever, be black and shining; but when such masses
are broken open, or scratched with a knife, the char-
acteristic brown color is at once rendered evident.
These more compact masses usually exhibit an in-
ternal fibrous structure, with a banded arrangement
perpendicular to the fibers (Fig. 6). Crystals of
limonite are quite unknown. Earthy or powdery
masses are the most frequent.
Limonite is a hydrated oxide of iron, with the
chemical formula 2Fe 2 O 3 3H 2 O that is, the same as
haematite, with the addition of water. It is the final
product of weathering of all iron-bearing minerals.
As a yellow, powdery, or slimy substance, it is de-
posited by the waters of chalybeate springs; and ma-
terial of a similar nature is formed in marshes and
on the beds of certain lakes. The latter constitutes
the Swedish lake-ore, or bog-iron-ore; and when it
has been all collected from a particular spot a fresh
deposit is ready for working after a lapse of a num-
ber of years. Limonite is the most important iron-
ore in Germany, and large quantities are also mined
in the north of Spain. Besides being used as an ore
of iron, the purer, powdery variety, or yellow-ochre,
is used as a pigment
MANGANESE OXIDES
(Plate 15). The three species manganite, pyrolu-
site, and psilomelane, represented on Plate 15, may
OXIDES.
Plate 15
ii*S
m,
F
1, Manganite. 2, 3, Pyrolusite. 4, Psilomelane.
MANGANESE OXIDES 135
conveniently be treated together, for they closely re-
semble one another in appearance, and are, as a rule,
not readily to be distinguished; further, they occur
together, and are all mined and put to the same uses.
In color they are all black, usually with a semi-
metallic luster; their streak, or powder, is also black,
and some varieties are so soft that they soil the fingers
when touched. The black streak is, indeed, the most
characteristic and constant feature of these minerals;
if we have a soft, black mineral which gives a black
streak we may be pretty certain that it is one of these
oxides of manganese.
The only species which occurs distinctly crystal-
lized is manganlte (Fig. i ) . The crystals are usually
prismatic in form, and at a few localities very fine
specimens are to be found, but elsewhere they are
rare. The picture shows bundles of prismatic crystals
on barytes, and represents a specimen from Ilfeld, in
the Harz Mountains, Germany. This species is the
hydrated oxide, Mn 2 O 3 .H 2 O.
Another species, pyrolusite (Figs. 2 and 3), is per-
haps the most abundant of these oxides of manganese.
It usually forms radiating aggregates of platy or
needle-like crystals, as represented in the pictures.
In chemical composition it approximates to the di-
oxide, MnO 2 .
A third species, psilomelane (Fig. 4), is never
found crystallized, but usually as nodular or stalac-
titic masses with rounded surfaces. The stalactites
have sometimes an internal radiated structure, as
shown in the picture (where they have been broken
136 THE WORLD'S MINERALS
across; three of the natural rounded ends are also
shown). The surface of these masses is often very
smooth and shining, and the material is then quite
hard. In the variety known as wad, the material is,
however, quite soft, and sometimes so porous that it
floats on water. Psilomelane and wad are very vari-
able in chemical composition; though consisting
largely of manganese dioxide, they also contain
oxides of barium, potassium, cobalt, etc.
These black manganese minerals owe their origin
to the alteration by weathering of other minerals
containing manganese, and in some places they form
large deposits. Such deposits are mined principally
in Brazil, Russia, and India. These minerals have
long been used for the manufacture of chlorine and
bleaching powder and for decolorizing glass. More
recently, large quantities have been used for prepar-
ing the spiegeleisen and ferro-manganese employed
in the manufacture of iron.
CUPRITE
The only remaining oxide which need be men-
tioned in this place is the suboxide of copper (or
cuprous oxide) Cu 2 O. It is sometimes found beauti-
fully crystallized in cubes or octahedra, and the
crystals, being very brilliant, transparent, and of a
rich ruby-red color, are very attractive in appearance.
On this account the mineral is also known as ruby-
copper, or red copper-ore. In a peculiar variety
known as chalcotrichite (or hair-copper), the cubes
CUPRITE 137
are enormously elongated in the direction of one of
their edges, so resembling fine hairs; and when, as
is usually the case, large numbers of these hair-like
crystals are closely aggregated or matted together,
the material has the appearance of crimson plush.
Another variety is earthy in character and inter-
mixed with limonite, and from its appearance it is
known as tile-ore. In some copper-mining districts
cuprite is of importance as an ore.
CHAPTER IX
THE CARBONATES
FROM the oxides considered in the last chapter, we
pass to the very large series of oxygen-salts, and the
first group of this series is that of the carbonates.
These consist of carbon and oxygen in combination
with one or other of the metals; that is, they are
salts of carbonic acid (H 2 CO 3 ), which acid in its
gaseous form is the well-known carbon dioxide
(CO 2 ), produced when all organic substances are
burnt in the air. In addition to these elements, some
minerals of this group may contain the elements of
water, being hydrated carbonates; while others may
contain an excess of the metal in the form of hydrox-
ide, these being hydrated basic carbonates.
When a carbonate is heated to redness, the carbon
dioxide it contains is expelled, leaving the metallic
oxide; this takes place when limestone (CaCOs) is
burnt to give quicklime (CaO). Or the carbon di-
oxide may be expelled simply by placing the car-
bonate in acid, when the gas that is liberated bubbles
through the liquid. This property that carbonates
have of effervescing in contact with acid affords a
simple test by which they can always readily be de-
tected. It is only necessary to place a drop of acid
(dilute hydrochloric acid is the best) on a mineral
138
CARBONATES
Plate 16.
13, Calcite.
CALCITE 139
to decide whether or not it is a carbonate. In certain
cases, however, it may be necessary to place the
powdered mineral in warm acid before the effer-
vescence is produced.
Most carbonates, when in a pure state, are color-
less and transparent. They are all fairly soft, and
can be scratched with a knife. Most of them fall into
well-defined isomorphous groups.
CALCITE
(Plate 1 6, Figs. 1-3). Next to quartz, calcite or
calc-spar is the most abundant of minerals, and it also
appears in a great variety of forms or guises. The
common and plentiful rocks of the limestone class
(chalk, oolite, marble) consist entirely, or almost en-
tirely, of compact calcite. These rocks have in most
cases been formed by the accumulation of the cal-
careous remains of various marine organisms, such
as foraminifera, corals, molluscs, etc., which, like
calcite itself, are composed chemically of calcium
carbonate (CaCOs).
Calcite is remarkable for the frequency with which
it is found in a well-crystallized condition; indeed,
calcite crystals are the commonest of all kinds of
crystals. Further, they display a greater variety of
forms than is met with in any other mineral. We
may have plate-like crystals, which are sometimes as
thin as a sheet of paper; or the crystals may be pris-
matic in habit, with either a hexagonal or a triangular
cross-section, and they may sometimes be long and as
140 THE WORLD'S MINERALS
slender as needles. Or, again, we may have rhombo-
hedra, either flat or steep, or various scalenohedral
forms. Calcite of the several forms just enumerated
is often known by several trivial names, such as
paper-spar, cannon-spar, rhomb-spar, nail-head-spar,
and dog-tooth-spar. Combinations of these simple
forms give rise to an almost endless variety of forms
for crystals of calcite. In Plate 16, Fig. 3, the form
of the crystals is that of the primitive rhombohedron ;
in Fig. 2, it is another more acute rhombohedron;
and in Fig. i, a combination of a hexagonal prism, a
scalenohedron, and an obtuse or flat rhombohedron.
Although differing so widely in form and appear-
ance, all these crystals belong to one common type.
They are each symmetrical with respect to three
planes of symmetry, with a three-fold arrangement
about the vertical axis ; and the angles between sim-
ilar faces are always the same.
A very important property of crystals of calcite is
that of splitting, or cleaving, with great ease in three
directions parallel to the faces of the primitive rhom-
bohedron. Any crystal of calcite, whatever be its
external form, may be broken up by a hammer into
a number of small rhombohedra, with smooth and
bright faces. This cleavage rhombohedron is of the
same form as the primitive rhombohedron (Fig. 3),
with identical angles (105 5', or 74 55') between
adjacent faces. The plane angles on the rhomb-
shaped faces are 102 and 78.
This perfect cleavage of calcite is of the first im-
portance as an aid to the recognition of the mineral.
CALCITE 141
Other characters of importance in this direction are
the low hardness (No. 3 on the scale) and low
specific gravity (2.72) ; the mineral can be readily
scratched with a knife. Another test that may be
readily applied is given by the fact that the mineral
effervesces freely with cold, dilute acid. A minute
fragment of calcite, when placed in a drop of hy-
drochloric acid on a microscope slide, quickly dis-
solves with effervescence; on adding a drop of sul-
phuric acid to this solution, and allowing the liquid
to evaporate, star-like groups of crystals of gypsum
are formed, and these present a very characteristic
appearance under the microscope.
Crystals of calcite vary not only in their geometric-
al form, but they may also vary considerably in their
degree of transparency and color. Most frequently
they are of a pale yellowish or reddish color, owing
to the enclosure of hydrated oxides of iron, and are
only semi-transparent. When the material is quite
pure it is perfectly colorless and transparent. The
purest crystallized material of the best quality comes
from one spot in Iceland, and is known as Iceland-
spar. This is much used for making optical prisms
for polarizing apparatus.
The use just mentioned depends on a very peculiar
property possessed by crystals of calcite namely,
that of double refraction, a property which is pos-
sessed by many other minerals, though rarely to such
a high degree. If a cleavage-rhomb of Iceland-spar
be placed over an object (such as a dot or a cross
drawn on a sheet of paper) , the object will be shown
142 THE WORLD'S MINERALS
double through the spar, there being seen two dots
or two crosses. A ray of light on entering a crystal
of calcite is split into two rays, which travel along
different paths through the crystal. For this reason
Iceland-spar is also known as doubly-refracting spar.
Beautiful crystallized specimens of calcite are to
be found at very many places. One of the best-known
localities is in the west Cumberland iron-mining dis-
trict in the neighborhood of Egremont. Here large
fissures or cavities in the haematite iron-ore and in the
adjacent limestone are completely lined with num-
berless sparkling crystals, which are often quite
transparent and tipped with delicate shades of red.
Enormous crystals of water-clear calcite have been
found in cavities in lava in Iceland; and magnificent
crystals of a rich wine-yellow or wine-red color are
met with in the lead and zinc mines near Joplin, in
Missouri. Many other well-known localities might
easily be mentioned. The specimen represented in
Plate 1 6, Fig. i, is from Egremont, in Cumberland,
the crystals here shown being on a matrix of iron-ore.
The specimen in Fig. 2 is from Fontainebleau, in
France; the closely aggregated groups of rhombo-
hedral crystals from this locality are remarkable in
enclosing a large amount of sand.
The well-known stalactites, found hanging from
the roofs of limestone caves, consist of calcite. These
have been formed by the deposition of calcium car-
bonate from the water which drips from the roof of
the caves.
Calcite may be taken as the type of a group of
CALCITE CALAMINE 143
minerals known as the rhombohedral carbonates.
These all crystallize in rhombohedra, which differ
only slightly from the primitive rhombohedron of
calcite, there being a difference of only a few degrees
in the angles between the faces. They also all pos-
sess the same perfect cleavages parallel to the faces
of this rhombohedron. The several members of the
group, though consisting of carbonates of different
metals, are yet analogous in their chemical consti-
tution, for the metals they contain are closely related.
They have the general formula R"CO 3 , where R"
may stand for calcium, magnesium, iron, zinc, or
manganese. We have here an excellent example of
an isomorphous group of minerals.
Three of the minerals of this group, which are
represented in Plate 17, Figs.i-3, will each be
specially described below. Others, which we may
mention by name in this place, are magnesite, or
magnesium carbonate (MgCOs), and dolomite
(CaCOs.MgCOs), in which both calcium and
magnesium are present, this species being midway
between calcite and magnesite.
CALAMINE
(Plate 17, Fig. i). This is the carbonate of zinc
(ZnCOs) belonging to the group of rhombohedral
carbonates. Distinctly developed rhombohedral crys-
tals are, however, quite rare, and are always small.
More usually the mineral forms masses with con-
centric shelly layers and a nodular surface, or it is
144 THE WORLD'S MINERALS
frequently earthy or compact. In color it is often
creamy or yellowish, though sometimes bluish or
greenish (Fig. i). It is a mineral presenting no
very distinctive external features, and it is frequently
mistaken for other minerals. The only safe way of
determining it is to test for the presence of carbonate
and of zinc, the latter by the method already men-
tioned under zinc-blende.
Calamine occurs in metalliferous veins, together
with zinc-blende, and it frequently forms extensive
bedded deposits in limestones. It contains 64.8 per
cent, of zinc, and is an important ore of this metal.
CHALYBITE
(Plate 17, Fig. 2). The rhombohedral carbonate
of iron (FeCOs), known as chalybite or spathic iron-
ore, or sometimes as siderite, is often found as small,
well-developed crystals of various forms, the rhombo-
hedron being the most frequent. Brown crystals of
this kind are shown, in association with galena, in
Plate 5, Figs. 3 and 4; and a brown encrustation of
chalybite, on fluor-spar, is shown in Plate 9, Fig. i.
More often the mineral occurs as granular masses
showing numerous bright cleavages on the fractured
surfaces; sometimes such masses may be very coarse-
ly crystalline, with large cleavage surfaces. Fre-
quently the material is compact, with a more or less
pronounced radiated structure (Plate 17, Fig. 2).
It is then often mixed with various impurities, such
as clay and carbonaceous matter, as in the varieties
CARBONATES.
Plate 17.
1, Calamine. 2, Siderite. 3, Rhodochrosite. 4, Corusske. **
CHALYBITE RHODOCHROSITE 145
known as clay iron-stone and black band iron-stone,
which are of great importance as ores of iron.
The crystallized varieties of chalybite are more
usually found in metalliferous veins for instance, in
Cornwall and in the Harz Mountains; while the
granular and compact varieties form beds in sedi-
mentary rocks. In the coal-measures of England
and South Wales we have the abundantly -occurring
clay iron-stone and black band iron-stone. The im-
portant iron-ores of the Cleveland district of York-
shire consist of oolitic chalybite, forming beds in the
Lias formation. When chemically pure, chalybite
contains 48.2 per cent, of iron, but usually, owing to
the presence of admixed impurities, the ore does not
contain so high a percentage as this.
RHODOCHROSITE
(Plate 17, Fig. 3). The carbonate of manganese
(MnCOa) known as rhodochrosite, or manganese-
spar, is very different in its appearance from the
manganese minerals which we have previously con-
sidered (p. 134). Instead of being black and dirty,
it is of a beautiful rose-red color, and it is found as
clean-looking rhombohedral crystals (Fig. 3). These
crystals possess the perfect rhombohedral cleavage
characteristic of this group of carbonates, and con-
sequently they sometimes exhibit a more or less
marked pearly appearance on their surfaces. More
frequently, however, the faces of crystals are curved,
sometimes so pronouncedly that the crystals some-
146 THE WORLD'S MINERALS
what resemble a saddle in form. The mineral also
occurs as granular masses, with a globular surface,
being then sometimes known as raspberry-spar, as
suggested by its appearance.
Crystallized rhodochrosite is found in cavities in
limonite at Horhausen, in Rhenish Prussia, and in
metalliferous veins at Kapnik, in Hungary. The
best crystals are found with iron-pyrites at various
places in Colorado (Fig. 3). The massive mineral
forms extensive beds in limestone in the Pyrenees,
where it is mined as an ore of manganese. This
ore cannot, however, be used for the same purposes
(bleaching, etc.) as the black ores of manganese, since
it contains no dioxide; it is used only in the manu-
facture of iron.
ARAGONITE
(Plate 1 8, Figs. 1-3). In chemical composition
aragonite is the same as calcite, being calcium car-
bonate (CaCOs). Nevertheless, we have here two
quite distinct minerals, which differ not only in their
crystalline form, but also in all their physical char-
acters. We have, in fact, the same chemical com-
pound crystallizing in two totally distinct modifica-
tions, just as carbon crystallizes either as diamond
or as graphite (see p. 63), and as sulphur forms
either orthorhombic or monoclinic crystals (p. 66).
In other words, calcite and aragonite are dimorphous
forms of calcium carbonate.
In its general appearance aragonite is, however,
not unlike calcite, the material being usually color-
ARAGONITE 147
less, white, or pale-colored, either transparent or
translucent, and with a glassy luster. And, indeed,
when the material is very minutely crystallized it is
not always quite easy to distinguish between calcite
and aragonite.
Distinctly formed crystals of aragonite are com-
paratively rare; their system of crystallization is
orthorhombic, and they are usually prismatic in their
development. The orthorhombic form is, however,
very often obscured by twinning, and frequently three
crystals are twinned together to produce a form
closely resembling a hexagonal prism, as shown in
Plate 1 8, Fig. 3. Twinned crystals of this kind are
found in Aragon, Spain hence the name aragonite
for this species; in the sulphur-mines near Girgenti,
in Sicily; at Herrengrund, in Hungary (Fig. 3) ; and
in the copper-mines at Corocoro, in Bolivia, where
the crystals are often replaced by native copper, with
their original form still preserved.
More frequently, crystals of aragonite have the
form of very steep spires, spikes, or fine needles,
which are usually aggregated in divergent groups
or in delicate feathery forms. This acicular, or
needle-like, habit of the crystals is very character-
istic of aragonite; but it is not correct to suppose (as
is frequently done) that all needle-like crystals of
calcium carbonate are aragonite, for we may also
have acicular crystals of calcite.
We have already seen that when a crystal of calcite
is broken, the surfaces of fracture are quite plane and
smooth, since the mineral possesses perfect cleavages
148 THE WORLD'S MINERALS
parallel to the faces of the primary rhombohedron.
In aragonite, on the other hand, there is no cleavage,
and the fractured surfaces of the crystals are curved
(or subconchoidal), with a glassy or almost resinous
appearance. This presence or absence of cleavage is
of prime importance, and affords a very ready means
of distinguishing between crystals of calcite and
aragonite. If we take a thin, prismatic or needle-
shaped crystal (which, having been previously tested
with acid, we know to be calcium carbonate) and
snip off the end, we can at once say whether the min-
eral is calcite or aragonite; if it be calcite, it will
break along a smooth surface, set obliquely to the
length of the prism; while if it be aragonite, there is
no plane surface of fracture.
Most frequently, however, aragonite is met with
in a very minutely crystallized condition, forming
finely fibrous compact masses of various shapes with
rounded outlines. These rounded shapes may be
nodular or coralloidal, and they are often of con-
siderable complexity and beauty. A coralloidal (i.e.
coral-like) form is represented in Fig. i, this being
the so-called flos-ferri (i.e. flower of iron), which is
found in the iron-mines at Eisenerz, in Styria. These
snow-white, branching forms are often extremely
beautiful, and they present quite the appearance
of certain organic structures. When one of these
branches is broken across it will be seen to have an
internal radiated structure, due to the very close
aggregation of numberless fine, needle-like crystals,
arranged perpendicular to the axis of the branch.
ARAGONITE 149
Another well-known form of aragonite is the so-
called pisolite, or pea-stone, represented in Fig. 2,
which consists of concentric shells of material, form-
ing small balls about the size of peas; numbers of
these balls are closely packed together, and the spaces
between them filled with material of the same kind,
to form large, compact masses. The several layers
of the balls can be readily peeled off like the coats of
an onion ; and each layer when broken across shows
a finely fibrous structure, with the fibers arranged
perpendicular to the surfaces of the shells. Pisolitic
aragonite is being deposited at the present day by the
hot springs at Carlsbad, in Bohemia, the waters of
which carry calcium carbonate in solution. Many
other waters, especially in limestone districts, contain
calcium carbonate in solution; but at the ordinary
temperature this is deposited as calcite and not as
aragonite. The specimen represented in Fig. 2 is
from Carlsbad.
The finely fibrous forms of aragonite just described
are much more frequent than distinctly formed crys-
tals ; in calcite, on the other hand, crystals are of more
common occurrence. We must not, however, assume
that all fibrous forms of calcium carbonate are ara-
gonite, for they are also met with in calcite. Indeed,
most stalagmitic or onyx marbles consist of calcite,
and not of aragonite, as often stated.
Aragonite is both rather harder ( hardness = 3^-
4) and denser (sp. gr. 2.93) than calcite; but in finely
fibrous and perhaps somewhat cellular masses these
differences may not be appreciable. A doubtful
150 THE WORLD'S MINERALS
specimen may, however, often be determined by
floating a minute fragment in a heavy liquid together
with known fragments of calcite and aragonite. The
differences in the optical characters of the two min-
erals are also of assistance for purposes of determi-
nation.
Both calcite and aragonite effervesce freely in cold,
dilute hydrochloric acid; but although these two
minerals are identical in chemical composition, it
does not follow that they will always show the same
chemical reactions, for they possess differences in
their molecular structure. For instance, if the finely
powdered material be added to a dilute solution of
cobalt nitrate and -this be boiled for a few minutes,
aragonite becomes lilac-red in color, while calcite
remains white.
CERUSITE
(Plate 17, Fig. 4; Plate 18, Fig. 4). Cerusite,
or white-lead-ore, is carbonate of lead (PbCOs),
crystallizing in the orthorhombic system. The crys-
tals are closely related in form, angles, and twinning
to crystals of aragonite, so that these two minerals,
together with some others (namely, witherite, or
barium carbonate, and strontianite, or strontium car-
bonate), belong to the same isomorphous group. Of
this group of orthorhombic carbonates, aragonite
may be taken as the type, just as calcite was taken as
the type of the isomorphous group of rhombohedral
carbonates (p. 143). Now we have already seen that
calcite and aragonite are dimorphous (p. 146), so we
CARBONATES.
Plate 18.
^/Swr/^roSs
1 3, Aragonite (Flos-ferri). 4, Cenjssit?:
CERUSITE 151
have here an excellent example of two parallel groups
of isomorphous minerals, forming all together what
is known as an isodimorphous series.
Simple crystals of cerusite, such as represented in
Plate 17, Fig. 4, are of comparatively rare occur-
rence. Almost invariably the crystals are twinned,
and this twinning is indeed a very characteristic fea-
ture of the mineral. A six-rayed arrangement of
platy crystals is very often to be recognized. Fibrous
aggregates of needle-like crystals (Plate 18, Fig. 4)
and granular and compact forms are also frequent.
Besides the twinning, cerusite possesses several
other characteristic features which enable it to be
readily identified. One of these is the adamantine
luster on the bright crystal faces; and another is the
high specific gravity (6.5) , which is remarkably high
for a colorless, transparent mineral. If we have a
white, very heavy, and fairly soft (H. = 3/^) min-
eral which effervesces with acid, we may be sure that
we are dealing with cerusite. The high specific
gravity is of course due to the presence of lead, and
beads of this metal are readily obtained by heating
the mineral on charcoal before the blowpipe.
Cerusite occurs in the upper oxidized zones of
metalliferous veins, having resulted by the action of
carbonated waters on galena. In Plate 18, Fig. 4,
the white fibrous cerusite is associated with brown
limonite, the latter being a product of alteration of
iron-pyrites; we have here two secondary minerals
produced by the alteration of sulphide minerals in a
metalliferous vein.
152 THE WORLD'S MINERALS
When met with in large quantities cerusite is an
important ore of lead, containing, when pure, 83.5
per cent, of the metal. It is found in upper levels of
most lead-mines, and is thus a fairly common min-
eral. Very fine specimens have been found at Broken
Hill in New South Wales and at Broken Hill in
Northwestern Rhodesia. The unusually fine crystal-
lized specimen represented in Plate 17, Fig. 4, is
from Badenweiler, in Baden; and that in Plate 18,
Fig. 4, is from Monte Vecchio, in Sardinia.
CHESSYLITE
(Plate 19, Figs, i and 2). In Plate 19 are rep-
resented two minerals, each consisting of copper car-
bonate combined with an excess of copper oxide and
some water; that is, they are hydrated basic copper
carbonates. They thus contain the same chemical
elements; but these are combined together in dif-
ferent proportions, and we have, in fact, two quite
distinct chemical compounds. The formula of ches-
sylite is 2CuCO 3 .Cu(OH) 2 , or 3CuO.2CO 2 .H 2 O;
while that of malachite is CuCO 3 .Cu(OH) 2 , or
2CuO.CO 2 .Hi5O, there being rather more copper ox-
ide and water in the latter. When pure, the two
minerals contain respectively 55.2 and 57.4 per cent,
of metallic copper.
Although there is only a slight difference in the
chemical composition of these two minerals, yet they
present a striking difference in their external appear-
ance. Chessylite is of a bright blue color, while
CHESSYLITE 153
malachite is emerald-green. They are both soft min-
erals (H. = 3/^-4) , and can be readily scratched with
a knife; and being carbonates they both effervesce
with acid, giving a green copper solution. Further,
they frequently occur together, having been produced
by the alteration, due to the action of carbonated
waters, of other copper-bearing minerals, particu-
larly copper-pyrites.
Chessylite receives its name from the locality
Chessy, near Lyons, in France whence the best
crystallized specimens are obtained. Another name
also in common use for this species is azurite, in
allusion to the characteristic azure-blue color of the
mineral. It crystallizes in the monoclinic system,
but although it nearly always occurs as crystals, these
do not, as a rule, present any very characteristic
form. In Fig. i are shown divergent groups of small
prismatic crystals, and in Fig. 2 small, bright crystals
are thickly clustered on the surface of the matrix;
both these specimens are from Chessy. Good crystal-
lized specimens of chessylite are also found in the
Copper Queen mine and in other copper-mines in
Arizona, and at Broken Hill in New South Wales.
Although crystals are the most frequent, this min-
eral is also found as powdery or earthy masses of a
bright sky-blue color, the color here being less dark
than in the crystals. The streak, or powder, of the
crystals is also sky-blue.
As already mentioned, chessylite frequently occurs
in association with malachite, and in Figs, i and 2 are
to be seen patches of green malachite with the blue
154 THE WORLD'S MINERALS
chessylite. In Arizona the two minerals are some-
times banded together in compact masses, and speci-
mens of this kind, when cut and polished, find a
limited application in cheap jewelry.
MALACHITE
(Plate 19, Figs. 3 and 4). While chessylite is
usually found as crystals, on the other hand crystals
of malachite are of rare occurrence. These are never
very distinctly developed, usually having the form of
fine needles grouped together in silky tufts. Ordi-
narily the mineral occurs as compact masses (Fig. 3) ,
which may sometimes present rounded nodular
(mamillary) surfaces (Fig. 4). These nodular masses
when broken open or when cut and polished (as in
Fig. 4) exhibit a concentric arrangement of lighter
and darker green bands, together with a more or less
distinct radially fibrous structure.
Malachite is a much more common mineral than
chessylite, and it is the more stable of the two. This
is proved by the fact that we often find crystals of
chessylite altered to malachite that is, pseudo-
morphs of malachite after chessylite. The green
stains to be observed on the stones and ore from
all copper-mines consist of malachite, this having
been formed by the action of weathering agents en
the other copper-bearing minerals of the ore. The
green patina on bronze is also due to the surface al-
teration to malachite of the copper contained in the
bronze; and on ancient bronze implements which
CARBONATES.
Plate 19.
V
1, 2, Azurite. 3, 4, Malachite;
MALACHITE 155
have long been buried in the soil, nodular concretions
of malachite are sometimes to be seen.
Large masses of compact malachite, sometimes
weighing several tons, are found in the copper-mines
of the Ural Mountains (Fig. 4), at Burra-Burra in
South Australia, and in Arizona. Such material,
especially that from Russia, is cut and polished for
ornamental purposes, being made into vases and other
small articles, or used for inlaying in table-tops and
columns. The material is easily worked and may
be turned in the lathe, and it takes a very good polish.
When found in sufficiently large quantities, mala-
chite is also of importance as an ore of copper.
CHAPTER X
THE SULPHATES, CHROMATES, MOLYB-
DATES, AND TUNGSTATES
IN this group of oxygen-salts are brought together
those minerals which contain in their acid portion
the chemical elements sulphur, chromium, molyb-
denum, or tungsten, all of which, with the exception
of sulphur, are themselves metals. These are closely
related elements, falling in the sixth group of the
chemists' periodic classification. Their salts can all
be expressed by a similar chemical formula, which
may be written generally as R"MO 4 when R" is the
metal and MO 4 the acid portion, M standing for one
or other of the four elements named above. Some
of the salts may contain in addition water of crystal-
lization.
The sulphates are the most important and numer-
ous members of this group. These are combinations
of a metal with the well-known sulphuric acid, or
oil of vitriol. Thus, the mineral anglesite is lead
sulphate, and the general formula given above here
becomes PbSO 4 . This formula is similar to that of
galena (PbS) with the addition of four atoms of
oxygen, and it is interesting to note that anglesite is
formed in nature by the weathering and oxidation of
galena.
156
BARYTES 157
All the minerals here considered, with the excep-
tion of gypsum (which contains water of crystal-
lization), are very heavy. They are often trans-
parent and well crystallized, and are sometimes
brightly colored. Most of them are of economic
importance.
BARYTES
(Plate 20, Figs, i and 2). This is the sulphate of
barium (BaSCh). It is a common mineral, and one
which is very often found as large, well-developed
crystals. The crystals belong to the orthorhombic
system, and they vary considerably in their habit,
being tabular (Fig. i) or prismatic (Fig. 2). The
rhomb-shaped tabular crystals shown in Fig. i are
bounded by the large basal plane, perpendicular to
which are narrow faces of a rhombic prism forming
the edges of the plates. The angles between the
prism faces are 78^ and ioij^, these being also
the plane angles of the rhomb forming the basal
plane. The crystal shown in Fig. 2 is bounded by
the same faces, with the addition of a macrodome.
Here the crystal is elongated in the direction of one
of the horizontal crystallographic axes the macro-
axis, which in the picture is set up vertically. The
two faces at the top of the crystal (one of them at the
back, and therefore not visible in the picture), which
slope away on either side from the upper line or
ridge, are the faces of the rhombic prism, with an
angle of 78^. The long, narrow face in the front,
and extending to the summit of the crystal, is the
158 THE WORLD'S MINERALS
basal plane; and the two long faces on either side of
this belong to a macrodome. At the lower end of
this crystal are to be seen a number of smaller crys-
tals grown in parallel position with the main crystal.
A very important character of barytes is its cleav-
age, there being three directions in which the crystals
may be readily split. The best cleavage is parallel
to the basal plane, and on this surface the crystals
frequently exhibit a pearly appearance, owing to the
presence of cleavage cracks within the crystal. The
two other rather less perfect cleavages are parallel
to the two pairs of parallel prism faces; in the pic-
tures, especially in Fig. 2, these cleavages are shown
as cracks, running across the crystal parallel to the
prism faces. Any crystal of barytes can be easily
broken along these three cleavages into rhomb-shaped
plates, similar to the crystals in Fig. i. We have
already seen that calcite also possesses three direc-
tions of perfect cleavage; but there is an important
difference between the cleavage of calcite and barytes.
In calcite the angle between any two of the cleav-
ages is always the same; while in barytes the prism
cleavages enclose an angle of 78^, and these are
each at 90 (i.e. at right angles) to the basal cleavage.
Another character of importance is the high spe-
cific gravity (4.5), and on this account the mineral
is commonly known as heavy-spar. The hardness is
about the same as that of calcite; so that the mineral
can be readily scratched with a knife.
Barytes is often white or colorless, or various
shades of yellow or brown; and the crystals may be
BARYTES 159
quite transparent, or only translucent to opaque. In
addition to crystals, platy or granular masses, show-
ing larger or smaller cleavage surfaces when broken
across, are of frequent occurrence.
In their general appearance, cleavage masses of
white barytes are not unlike calcite; but the two
minerals may be readily distinguished by their
weight, by the angles between their cleavages, and
by the fact that barytes does not effervesce with acids.
Barytes is usually found in veins, either alone or
more often in association with metalliferous ores, es-
pecially ores of lead, and it is thus a common mineral
in many mining districts. The specimen represented
in Plate 20, Fig. i, showing crystals of barytes resting
on and partly penetrated by needles of stibnite (anti-
mony-ore), is from the mines at Felsobanya, in
Hungary. The one shown in Fig. 2 is from the
iron (red haematite) mines at Frizington, near
Whitehaven, in Cumberland, where large and
beautifully crystallized specimens are abundant.
Barytes is used commercially for the manufac-
ture of white paint, for which purpose it is often
mixed with white lead. It is also used for giving
weight and finish to certain kinds of paper. Much
of the material mined under the name of barytes is
really the more valuable mineral ivitherite, or car-
bonate of barium, and not the sulphate. This, being
soluble in acids, is more readily converted into vari-
ous barium compounds, which, amongst other uses,
are employed in sugar-refining and for making rat
poison.
160 THE WORLD'S MINERALS
CELESTITE
(Plate 20, Fig. 4). This is strontium sulphate
(SrSCX). In the form, angles, and cleavages of its
orthorhombic crystals it presents the greatest similar-
ity to barytes. These two minerals are, in fact, iso-
morphous ; and belonging to the same group we also
have the next mineral to be considered namely,
anglesite, or lead sulphate.
In Fig. 4 we have a parallel grouping of crystals
of prismatic habit. These crystals are terminated at
their upper ends by the prism faces, parallel to which
are perfect cleavages, enclosing an angle of 76
(corresponding to the angle of 78^ in barytes).
The vertical faces in the picture belong to a brachy-
dome, and the small triangular faces on the corners
belong to a macrodome.
A characteristic, though not constant, feature of
celestite is a faint bluish shade of color (not greenish
as in picture). It is on this account that the mineral
receives its name, although the color is never a deep
sky-blue. White and yellowish crystals are also
common.
Celestite is rather lighter (sp. gr. 4.0) than barytes,
but its hardness is about the same. The best methods
of distinguishing between these two minerals is af-
forded by the colors they impart to the non-luminous
flame of a Bunsen-burner. A fragment of the min-
eral, moistened with hydrochloric acid, is supported
on platinum wire in the flame, when celestite gives
SULPHATES.
Plate 20.
1, 2, Barytes. 3, Anglesite. 4, Celestite.
CELESTITE ANGLESITE 161
an intense crimson color and barytes a pale yellowish-
green, these being the characteristic flame colorations
of all strontium and barium compounds respectively.
On this property depends the use of celestite for
producing the red fire of pyrotechnic displays. An-
other use for the strontium compounds obtained from
celestite is in sugar-refining.
Fine crystallized specimens of celestite are abun-
dant in the red marls of Triassic age in the neighbor-
hood of Bristol. Good crystals are also common in
the sulphur-mines near Girgenti, in Sicily (Fig. 4) ;
and very large bluish crystals are found in a
limestone cave on Strontian Island in Lake Erie.
ANGLESITE
(Plate 20, Fig. 3). Sulphate of lead (PbSCX) is
another orthorhombic mineral, belonging to the same
isomorphous group with barytes and celestite. Its
crystals are usually small and complex in form, and
they are not always readily determined by mere in-
spection. The three crystals on the matrix in Fig. 3
are unusually perfect. Often the crystals are color-
less and transparent, with a very brilliant and
adamantine luster. Being a lead mineral, it is very
heavy (sp. gr. 6.3).
Anglesite is found in the upper weather portions
of lead-bearing veins, where it has been formed by
the alteration of galena. In the old mine on Parys
Mountain, in the island of Anglesey, minute crystals
were found in abundance, encrusting the surface of
162 THE WORLD'S MINERALS
cellular limonite, the latter being also a mineral of
secondary origin. It is from this locality that the
mineral receives its name. Good crystals have also
been found in the lead-mines of Derbyshire, at
Monteponi in Sardinia ( Fig. 3 ) , Broken Hill in New
South Wales, and in Tasmania.
GYPSUM
(Plate 21, Figs. 1-3). This is a common mineral
of considerable practical importance. It is composed
of calcium sulphate, with two molecules of water of
crystallization, the formula being CaSO 4 .2H 2 O.
Bearing this composition in mind, it is of interest
to trace the chemical processes by which gypsum is
formed in nature. By the weathering of iron-pyrites,
a mineral which as scattered grains and crystals oc-
curs in rocks of almost every kind, sulphuric acid or
iron sulphate is produced, and these compounds, be-
ing soluble in water, may be transported from their
place of origin. Now if such solutions come into
contact with limestone rocks or marls containing
some calcium carbonate, a reaction takes place, the
calcium of the rock combining with the sulphuric
acid to form calcium sulphate. This new compound,
being only slightly soluble in water, may be deposited
as crystals of gypsum; but if a large excess of water
is present, the whole of the calcium sulphate may be
carried away in solution. In the latter case a hard
water results, and one of which the hardness is per-
manent, since it cannot be removed by boiling or by
GYPSUM 163
the addition of lime, as may be done with a water
rendered hard by the presence of calcium carbonate.
Water containing much calcium sulphate in solution
may under certain conditions collect in lakes, and
with the gradual evaporation of the water gypsum
may be deposited on the bed of the lake. By such a
process vast beds of gypsum have been deposited in
inland seas in past geological epochs, and it is such
beds of gypsum that are quarried at the present day.
The formation of gypsum can readily be demon-
strated experimentally. A fragment of calcite is dis-
solved in a drop of hydrochloric acid placed on a
microscope slide, and to the solution of calcium
chloride so obtained a drop of dilute sulphuric acid,
or a fragment of any soluble sulphate, is added. The
drop is then allowed to evaporate, when, under the
microscope, crystals of gypsum may be observed in
the process of growth. These crystals have the form
of slender needles, with oblique terminations (as in
Plate 21, Fig. i), and they arrange themselves in
pretty star-like groups.
Well-shaped crystals of gypsum are of common oc-
currence in nature, and when found embedded in
clay; as is very frequently the case, they are bounded
on all sides by crystal-faces. The crystals represented
in Plate 21, Fig. i, being attached to the matrix, show
faces at one end only. Gypsum crystallizes in the
monoclinic system, and the crystals possess only one
plane of symmetry. The most usual form is as shown
in Fig. i ; here the two larger crystals are tabular in
habit parallel to the plane of symmetry, while the
164 THE WORLD'S MINERALS
smaller crystals of the group are prismatic in habit.
The faces present belong to clino-pinacoid (a pair
of faces both parallel to the plane of symmetry), a
rhombic prism (the two narrow faces placed ver-
tically in the crystal forming the center of the group) ,
and a pyramid or dome (the two narrow faces placed
obliquely at the top of the crystal).
Crystals of 'gypsum are often twinned, two crystals
being grown together in such a position that they are
symmetrical about a plane which truncates the front
edge of the prism in Fig. i. Such a twinned crystal
is shown in Fig. 2.
Cleavage is a very important character of gypsum;
the crystals can be split with great ease into thin
leaves parallel to the plane of symmetry or clino-
pinacoid. On this surface the luster is often pearly
in character, and there are frequently to be seen
brightly colored bands of the same nature as New-
ton's rings. The crystals also possess a characteristic
fibrous cleavage parallel to the pair of dome faces,
this cleavage being often shown by silky bands run-
ning obliquely across the surface of the perfect cleav-
age first mentioned.
Crystals of gypsum are 7 usually colorless and
transparent, and only rarely do they show the yellow-
ish colors often seen in massive gypsum. The name
selenite, sometimes used for this species, is more
correctly applied to the transparent crystals; this
name refers to the somewhat fanciful resemblance
of the luster of the crystals to moonlight.
Another character of importance is the low degree
Plate 21.
1 3, Gypsum. 4, Linarite.
GYPSUM 165
of hardness (H. 2). Gypsum is one of the few
common, crystallized minerals that can be scratched
with the finger-nail. The specific gravity (2.3) is
also low. Other light and very soft minerals with a
perfect cleavage, which may perhaps be mistaken
for gypsum, are mica and talc. Cleavage flakes of
gypsum, though flexible, are not elastic like mica, nor
greasy to the touch like talc ; and, further, they always
show, when bent, the secondary fibrous cleavage.
Besides occurring as crystals, gypsum is frequently
found as granular masses, forming enormous beds.
Such material when fine-grained and compact is the
well-known alabaster, which, owing to the softness
of the mineral, particularly lends itself to carving.
Fibrous masses are also common as a filling of veins,
the fine fibers being arranged perpendicularly to the
walls of the vein (Fig. 3). Such material (satin-
spar), when cut and polished, with a rounded sur-
face, displays a satiny luster like catVeye, and it is
used for making beads and other small ornaments.
Good crystallized specimens of gypsum are found
at very many places, and detached crystals are to be
found embedded in most clays. The finest crystals
are those from the salt-mines at Bex, in Switzer-
land r and the sulphur-mines near Girgenti, in Sicily.
Large numbers of enormous crystals, a yard in length,
have been found lining a cave in Utah. Extensive
beds of massive gypsum are quarried in the neigh-
borhood of Paris, in Nottinghamshire and Stafford-
shire, and many other places. The material is heated
in kilns to drive off a portion of the water of
166 THE WORLD'S MINERALS
crystallization, and thereby converted into the well-
known plaster of Paris.
LINARITE
(Plate 21, Fig. 4). Linarite is a very attractive
though quite rare mineral, and one which is found
at only a few localities. It is a basic sulphate of lead
and copper of complex composition, the brilliant
sky-blue color being due to the presence of the latter
element. The crystals are small, and have a bright
luster as well as a bright color; in general appear-
ance they are not unlike crystals of chessylite. The
specimen represented in Fig. 4 is from the old lead
and copper mines of Roughten Gill, near Keswick,
in Cumberland. Specimens have also been found
at Leadhills, in Scotland, but, curiously, none at
Linares, in Spain, from which place the mineral
receives its name.
CROCOITE
(Plate 22, Fig. i). Chromate of lead (PbCrO*)
occurs in nature as beautiful crystals with a bright
hyacinth-red color and brilliant luster, which in gen-
eral appearance are not unlike crystals of the artificial
salt potassium bichromate. Unfortunately the crys-
tals lose their luster and become dull on exposure
to light, and in collections it is therefore necessary
to keep them under cover. The crystals belong to
the monoclinic system, and are usually prismatic in
habit. In the picture (Fig. i) are shown several
CROCOITE WOLFRAMITE 167
prismatic crystals irregularly grouped on the surface
of a piece of yellowish quartz. This specimen is
from Beresov, in the Ural Mountains, where the min-
eral is found, together with galena, in veins of
gold-bearing quartz. Magnificent groups of pris-
matic crystals several inches in length have recently
been found in a lead-mine near Dundas, in Tasmania.
Crocoite is a mineral of secondary origin, having
been formed by the alteration of galena.
WOLFRAMITE
(Plate 22, Fig. 2). This mineral is a tungstate
of iron and manganese, the two isomorphous mole-
cules FeWCX and MnWO* being mixed together in
variable proportions, so that the formula may be
written as (Fe,Mn)WO 4 . Here W is the chemical
symbol of the metal tungsten, or wolfram. The
mineral is very heavy (sp. gr. 7.1 to 7.5, varying with
the proportion of iron and manganese), and opaque,
with a dark brownish-black or pitch-black color and
a sub-metallic luster.
Crystals of wolframite are rarely met with, while
fine isolated crystals, such as that shown in Fig. 2,
are quite exceptional. They are monoclinic, and
possess a perfect cleavage in one direction parallel
to the plane of symmetry. This perfect cleavage is
always to be seen in the more commonly occurring
granular or columnar masses, such as are often found
embedded in quartz.
Wolframite often occurs in association with tin-
168 THE WORLD'S MINERALS
stone (cassiterite) for instance, in Cornwall, and at
Zinnwald in Bohemia, the crystal in Fig. 2 being
from the latter locality. Being both very heavy
minerals, their separation is rather a troublesome
operation for the tin-miner.
Sodium tungstate, prepared by fusing wolframite
with soda, is employed in dyeing and for rendering
fabrics non-inflammable. Metallic tungsten is used
for the manufacture of the very hard and tough tung-
sten-steel ; and quite recently, in the form of fine wire,
it has been used for the filaments of the so-called
"osram" electric lamps.
WULFENITE
(Plate 22, Fig. 3). This name is not to be con-
fused with the name of the mineral last described.
It was given in honor of the eighteenth-century Aus-
trian mineralogist Wulfen; while wolfram is an old
German mining term meaning "wolf froth." Wul-
fenite is a molybdate of lead (PbMoCX), crystal-
lizing in the tetragonal system. The crystals almost
always have the form of square plates with bevelled
edges, and sometimes the corners also are replaced by
narrow pyramidal faces (Fig. 3). The crystals are
often of a yellowish or greyish color, but sometimes
they are bright orange-yellow or orange-red, with a
brilliant luster. The mineral is one of secondary
origin, having been formed by the alteration of ga-
lena in veins of lead-ore. The earliest known local-
ity, and one that has produced a large number of
CHROMATES, TUNGSTATES, &c.
Plate 22.
1, Crocoite. 2, Wolframite. 3, Wulfenite. 4, Scheelite.
WULFENITE SCHEELITE 169
good specimens, is Bleiberg (meaning, in German,
"lead mountain"), in Carinthia; while very fine
groups of crystals of richer colors have been found
in Arizona (Fig. 3) and Utah.
SCHEELITE
(Plate 22, Fig. 4). It was in this mineral that
the Swedish chemist Scheele discovered, in the year
1781, the element tungsten. The mineral itself had
previously been known as tungsten, which means, in
Swedish, "heavy stone," and it was afterwards re-
named scheelite in honor of Scheele. Chemically, it
is a tungstate of calcium (CaWO 4 ), and it is iso-
morphous with the molybdate of lead, wulfenite.
Its crystals are tetragonal; but instead of being tabu-
lar in habit like crystals of wulfenite, they are
pyramidal, having the form of square pyramids. The
crystals are white, greyish, or yellowish in color, and
sometimes almost transparent. To all appearance
they might be expected to be quite light, but when
a crystal of scheelite is handled it will be found to be
surprisingly heavy (sp. gr. 6.0).
Scheelite often occurs in association with wolfram-
ite and tin-stone, and it is also met with in granitic
veins, sometimes together with gold. Good crystals
have been found at Tavistock, in Devonshire, and at
Carrock Fell, in Cumberland. The picture (Fig. 4;
from the tin-mines at Zinnwald, Bohemia) shows a
number of small crystals of the characteristic form
and color scattered over the surface of a matrix of
quartz.
CHAPTER XI
THE PHOSPHATES, ARSENATES, AND
VANADATES
THE phosphates, which are by far the most impor-
tant members of this group, are salts of phosphoric
acid, the chemical formula of which is HsPO*. The
non-metallic and highly inflammable element phos-
phorus is here combined with oxygen and hydrogen,
and when this hydrogen is replaced by a metal we
have a salt called a phosphate.
Phosphates are of great importance in the economy
of nature, for they enter into the composition of
plants and animals, and bones are composed largely
of calcium phosphate together with calcium car-
bonate. These phosphates are all indirectly of min-
eral origin, having been extracted from the soil by
growing vegetation. On this account the mineral
apatite, to be presently described, is largely used in
the manufacture of artificial manures, or fertilizers.
Another phosphate namely, turquoise is a valu-
able precious stone; while still another (pyromor-
phite) is an ore of lead. We thus see that the mineral
phosphates are of considerable importance. Many
of the numerous species, of which no mention need
be made in this book, are of quite rare occurrence,
and of interest only to the mineralogist and crystal-
170
APATITE 171
lographer. In addition to phosphate, several of the
minerals of this group contain water of crystalliza-
tion, or some other constituent.
Closely related to the phosphates, and strictly
isomorphous with them, we have the arsenates, anti-
monates, and vanadates. In these salts the place of
phosphorus is taken by the chemically related ele-
ments arsenic, antimony, or vanadium, which, though
metallic elements, here play the part of acids. The
chemical formula of all the salts is of the same type;
for instance, arsenic acid is H 3 AsO 4 , and vanadic
acid is H 3 VO 4 .
APATITE
(Plate 23, Fig. i). The name apatite means, in
Greek, "to deceive," because this mineral is often
mistaken for other species; indeed, some eminent
mineralogists have described as new minerals ma-
terial which afterwards proved to be merely apatite.
The name also suggests a certain class of humor; the
announcement a few years ago of an American
dealer ran: "Minerals for presents: send your friend
an 'apatite' for his Christmas dinner."
In its chemical composition apatite is some-
what complex; it is essentially a phosphate of
calcium, but in combination with this we have a
small proportion of calcium fluoride, or calcium
chloride, and the formula becomes 3Ca 3 (PO 4 ) 2 .CaF 2 ,
or (CaF)Ca 4 (PO 4 ) 3 . This is fluor-apatite, and the
formula of the corresponding compound chlor-apa-
tite is 3Ca 3 (PO4) 2 .CaCl 2 . We thus have two va-
172 THE WORLD'S MINERALS
rieties of apatite, but these can only be distinguished
by chemical tests.
Apatite is often found beautifully crystallized.
The crystals have the form of a hexagonal prism,
terminated at each end by six-sided basal planes
placed at right angles to the six faces of the prism.
The prism may be either short (Fig. i ) or long (as in
Fig. 3, of pyromorphite), and the habit of crystals,
therefore, either tabular or prismatic. Occasionally
the edges between the prism and the base are re-
placed by narrow faces of hexagonal pyramids, and
the corners by other small facets. The faces on the
corners are arranged in a peculiar manner, which is
very characteristic of crystals of apatite; they are
present on one side, not both sides, of each edge of
the prism, and the crystals are therefore said to be
hemihedral.
Crystals of apatite may be colorless and transparent
or white and opaque; often they are of a greenish
or brownish shade of color, or sometimes they are
sky-blue or violet (Fig. i). The crystals possess no
cleavage, and their fracture is sub-conchoidal. In
addition to. crystals, compact and earthy masses are
of abundant occurrence. This form of the mineral
is known as phosphorite, or rock-phosphate, and be-
ing found in large beds, it is extensively mined for the
manufacture of the compound called superphosphate
of lime, which, being soluble in water, can be directly
assimilated by plants.
The hardness of apatite is No. 5 on the scale; the
crystallized material can be scratched with a knife,
APATITE 173
though not very easily; earthy masses may, however,
be quite soft. This degree of hardness affords an
easy means of distinguishing crystals of apatite from
the hexagonal crystals of other minerals (beryl, tour-
maline, quartz), which may somewhat resemble
apatite in appearance. The specific gravity (3.2)
also serves for distinguishing apatite. But in case
of doubt the only sure way is to test the material
chemically for phosphoric acid and calcium.
Apatite is a mineral of wide distribution. It is
present in small amount as minute crystals in all
igneous and crystalline rocks ; and in veins traversing
granite and gneiss it is sometimes found as beautiful
crystals. It also occurs in metalliferous veins, es-
pecially in those containing tin-ore. The specimen
represented in Fig. i is from a tin-mine at Greifen-
stein, near Ehrenfriedersdorf, in Saxony; the bril-
liant and transparent violet crystals here encrust
crevices in an altered granitic rock known as greisen.
The extensive apatite deposits of southern Norway
occur in connection with a rock called gabbro; while
those of Canada are in crystalline limestone. Many
of the rock-phosphates are of recent formation, and
have been derived by the action of bird guano on
limestone, often a coral limestone on small islands
frequented by large flocks of birds.
PYROMORPHITE
(Plate 23, Figs. 2 and 3). This ore of lead is an-
alogous to, and isomorphous with, apatite, the form-
174 THE WORLD'S MINERALS
ula being 3Pb 3 (PO 4 ) 2 .PbCl 2 , or (PbCl)Pb 4 (PO 4 )s
that is, just the same as the formula of chlor-apatite,
with lead in place of calcium. The crystals are also
extremely similar, though in pyromorphite, faces
other than the hexagonal prism and the base are only
rarely present. In the group of crystals shown in
Fig. 3 this characteristic form is plainly to be seen;
while in Fig. 2 the prisms are longer and more closely
aggregated. Only rarely do the crystals show any
degree of transparency; but they are usually prettily
colored, being wax-yellow, oraage-y^llpw, brown
(Fig. 3), or bright grass-green (Fig. 2). On this
account the mineral is variously known to miners as
"brown lead-ore," "green lead-ore," or "variegated
lead-ore." Since it contains a large amount of
lead (76^ per cent.), the mineral is very heavy
(sp. gr. 7.0).
Pyromorphite occurs in veins of lead-ore, where
it has been formed by the alteration of galena; the
phosphate solutions producing the change have per-
haps in some cases been derived from the surface
soil or from guano deposits. Good crystallized speci-
mens have been found in Cornwall; the specimen
represented in Fig. 2 is from Hofsgrund, in Baden,
and that in Fig. 3 is from the Friedrichssegen mine,
near Ems, in Nassau.
Another ore of lead very similar in appearance
to pyromorphite is known as mimetite, this being the
corresponding arsenic compound with the formula
3Pb 3 ( AsO 4 ) 2 .PbCl 2 . The crystals are very like those
of pyromorphite, but are often much curved, having
PHOSPHATES, &c.
Plate 23.
1, Apatite. 2, 3, Pyromorphite. 4, Vanadinite. ; 5,\ 6, ;
VANADINITE ERYTHRITE 175
the appearance of barrels or small balls. Abundance
of crystals of this kind (known by the name of
campylite) are to be found among the old mine
refuse in Dry Gill, a deep valley on the side of Car-
rock Fell, in Cumberland.
VANADINITE
(Plate 23, Fig. 4). This is still another mineral
of th . same isomorphous group of minerals, with the
analogous formula 3Pb 3 (VO4) 2 .PbCl 2 , the phosphor-
us of pyromorphite being here replaced by the rare
metal vanadium. This mineral was formerly found
as brown crystals and small warty masses at Wan-
lockhead, in Scotland; and more recently very bril-
liant ruby- red crystals have come from Arizona (Fig.
4). In the picture there are to be seen several small
hexagonal prisms, with six-sided planes at their ends,
scattered over the surface of the matrix. Like pyro-
morphite and mimetite, this mineral is of secondary
origin in veins of lead-ore.
ERYTHRITE
(Plate 23, Figs. 5 and 6). We pass now to an-
other isomorphous group of minerals known as the
vivianite group, for the several members of which the
formula may be written generally as R"3Q 2 O 8 .8H 2 O,
where the metal R" may be iron, cobalt, nickel, zinc,
or magnesium, and Q stands for either phosphorus
or arsenic. The type mineral (vivianite) of this
176 THE WORLD'S MINERALS
group will be described presently; the one to
be now considered is erythrite, the hydrated arsenate
of cobalt, in which the general formula becomes
Co 3 As 2 O 8 .8H 2 O.
The name erythrite, meaning "red" in Greek, re-
fers to the characteristic crimson or peach-blossom-
red color of the mineral. The mineral could not
be confused with the kind of sugar of the same name,
though the identity in name is certainly confusing.
Another name often applied to the mineral is cobalt-
bloom.
Crystals of erythrite are small and comparatively
rare. They are monoclinic and have the form of
needles or small blades, and are usually arranged in
radiating tufts or star-like groups. They possess a
highly perfect cleavage in one direction parallel to
the plane of symmetry, and on this surface the luster
is often beautifully pearly. In Fig. 6 is shown a
number of crimson crystals scattered over the surface
of white quartz.
Much more frequent than crystals are earthy or
powdery masses of the same beautiful color, which
are to be seen as encrustations on most specimens of
cobalt-ore. The mineral, in fact, owes its origin to
the weathering of cobalt arsenide (smaltite, p. 97).
Occurring mixed in this manner with cobalt-ore, it is
used for the manufacture of cobalt-blue, so that from
a mineral of a bright crimson color there is obtained
a beautiful sky-blue product. The two specimens
represented in Figs. 5 and 6 are both from Schnee-
berg, in Saxony.
ANNABERGITE WAVELLITE 177
Analogous to cobalt-bloom we have the corre-
sponding nickel compound, Ni 3 As2O8.8H 2 O, known
aj annabergite, or nickel-bloom. This is of a char-
acteristic pale apple-green color, and in the same
way it is produced by the weathering of nickel arsen-
ide, or niccolite (p. 87) ; a small quantity of nickel-
bloom is shown on the specimen of niccolite in Plate
6, Fig. i.
WAVELLITE
(Plate 24, Fig. i). This is a hydrated basic
phosphate of aluminium, with the formula aAlPCh.
A1(OH) 3 .4^H 2 O. Distinct crystals are extremely
rare, and almost always this mineral is found as hemi-
spherical or globular forms attached to the surface of
slaty rocks. When these nodules are broken across
they show in the interior a beautiful radiated or star-
like structure, due to the close grouping of acicular
crystals around a center. This form and structure,
together with the greenish or greenish-yellow color,
is indeed a very characteristic feature of the mineral.
Wavellite, though abundant at certain localities,
is not met with at many spots. It was first found
by Dr. W. Wavel (after whom it was named), at
the end of the eighteenth century, in crevices of a
black slaty rock near Barnstaple, in Devonshire. It
has also been found in Ireland, and at Magnet Cove,
Arkansas, the specimen in Fig. i being from the
latter locality.
178 THE WORLD'S MINERALS
LAZULITE
(Plate 24, Fig. 2). This also is a phosphate
of aluminium, but one which contains, in addi-
tion, some iron and magnesium, the formula being
2AlPO 4 .(Fe,Mg) (OH) 2 . As the name implies, this
mineral is of an azure-blue color, and so also is the
mineral, lazurite. It is therefore necessary to dis-
tinguish carefully between these two deep-blue min-
erals; lazurite will be described farther on amongst
the silicates.
Lazulite is found as sharp, clean-cut crystals em-
bedded in quartz. These crystals have very much
the appearance of square tetragonal pyramids (Fig.
2), but in reality they are monoclinic, with only one
plane of symmetry. It is a comparatively rare min-
eral, found at only a few localities. The best speci-
mens are from Werfen, in Salzburg, and Graves Mt,
in Lincoln Co., Georgia. The specimen figured is
from the American locality, and shows the crystals,
with their characteristic form and color, embedded
in quartz.
CUPROURANITE
(Plate 24, Fig. 3). Cuprouranite, or torbernite,
is the typical representative of an isomorphous group
of minerals known as the "uranium micas," so called
because they contain uranium and form thin, platy
crystals, with a perfect cleavage like the micas. They
are all phosphates, or arsenates of uranium with
PHOSPHATES.
Plate 24.
1, Wavellite. 2, Lazulite. 3, Torbernite. 4> Vivianite. 5, Turquoise.
CUPROURANITE 179
copper or calcium, and water of crystallization. Like
all compounds of uranium, they are radio-active.
Cuprouranite is the hydrated phosphate of
uranium and copper, with the complex formula
CuO.2UO 2 .P 2 O 5 .8H 2 O. Its crystals belong to the
tetragonal system, and have the form of thin, square
plates, with a perfect micaceous cleavage parallel to
the surface of the plate. On this surface the luster
is consequently pearly in character. The color is
always a bright grass-green (much more vivid than
represented in Fig. 3) The crystals are quite soft,
being only slightly harder than gypsum.
Very beautiful crystallized specimens have been
found near the surface in some of the Cornish mines
namely, near Calstock, Grampound Road, and
Redruth. The crystals are deposited on limonite (as
in Fig. 3, from Cornwall), and they evidently have
been formed by the alteration of the pitchblende and
copper ores found deeper in the same mines.
Another of the uranium micas is the mineral cal-
couranite, or autunite, which has the same chemical
composition as cuprouranite, except that calcium
takes the place of copper. This crystallizes in thin,
square plates of a sulphur-yellow color, sometimes
with a greenish tinge. It is found at Autun in France
(hence the name autunite}, at Sabugal in Portugal,
and some other places, often as a coating on the
surface of crevices in weathered granite. In Portu-
gal it has quite recently been mined as an ore of
radium, one gram of which is extracted from 600
tons of the crude ore that is, one part.in 600 million.
180 THE WORLD'S MINERALS
VlVIANITE
(Plate 24, Fig. 4) . It has already been mentioned
that vivianite is the type member of an isomorphous
group of minerals, to which erythrite and annaber-
gite also belong. In this group vivianite is the
hydrated phosphate of iron (ferrous iron), with
the formula Fe 3 (PO 4 ) 2 .8HoO. Its crystals are mono-
clinic, usually with a prismatic or blade-like habit;
they have a very perfect cleavage parallel to the
plane of symmetry, and on this surface the luster is
pearly. The crystals are very soft, and are easily
bent and distorted. The characteristic color is deep
indigo-blue, though sometimes it is greenish-blue.
When, however, a rock-cavity containing crystals of
vivianite is freshly broken open, the crystals are col-
orless and transparent, but on exposure to air and
light they very quickly become blue, this change be-
ing due to a partial alteration of the ferrous iron to
ferric iron. For this reason the crystals to be seen
in collections are always blue, and probably but few
mineralogists have had an opportunity of seeing the
colorless crystals.
This specimen represented in Fig. 4 shows bundles
of prismatic crystals grown in a cavity in massive
pyrrhotite which in part has been altered to limonite.
Many fine specimens of this kind were found years
ago in a mine called Wheal Jane, near Truro, in
Cornwall. Groups of larger crystals have been more
recently found at Leadville, in Colorado.
VIVIANITE TURQUOISE 181
Crystallized vivianite is, however, of comparative-
ly rare occurrence; but in the form of a pale-blue
powder, known as blue iron-earth, this mineral is of
wide distribution, being found in practically all peat-
bogs and deposits of bog iron-ore, and especially on
bones and horns buried in such bogs. It is also
sometimes found encrusting the roots of plants when
these are embedded in a ferruginous clay.
TURQUOISE
(Plate 24, Fig. 5). As a precious stone turquoise
is known to everybody, but probably few are aware
that in chemical composition it is a hydrated phos-
phate of aluminium colored by copper and iron
compounds. The exact composition is, however,
rather doubtful, but the proportions are different
from those of wavellite (p. 177) and the several
other hydrated phosphates of aluminium that occur
as minerals. In turquoise there is usually about 5
per cent, of copper oxide.
Although showing under the microscope a minute-
ly crystalline and granular structure, turquoise is
never found as crystals, but only as nodular masses,
or more usually as a compact mineral, rilling crevices
in rocks, thus resembling opal in its mode of occur-
rence. It is an opaque mineral, which, when cut,
takes a good polish, and its use as a precious stone
depends on its pure sky-blue color and soft; waxy
luster. For use in jewelry it is always cut with a
convex surface, and when mounted with a surround
182 THE WORLD'S MINERALS
of small diamonds it is very effective. Very clear
imitations, so far as color goes, are made in glass,
but these do not show quite the true luster of the
genuine stone, and they have a transparent, glassy
appearance, which is especially seen when a frag-
ment is broken off. The natural mineral is often of
a greenish color; but such stones, though at one time
in favor, are now of very little value.
The best turquoises come from near Meshed, in
Persia, where they occur as a filling in crevices in a
weathered rock of igneous origin. Here the mineral
has been mined for hundreds of years, and being ex-
ported to the west through Turkey, it came to be
known as turquoise. In the Sinai Peninsula, tur-
quoise-mines were worked by the ancient Egyptians,
and the stone was employed by them as a material for
carving scarabs. Ancient mines, worked in prehis-
toric times, are situated in New Mexico and other
western states of North America, and at the present
day very fine turquoises are obtained from this re-
gion; the color of some of the American stones is,
however, rather liable to fade on exposure to light.
CHAPTER XII
THE SILICATES
THE group of silicates is by far the largest, and at
the same time the most complex, in our chemical
classification of minerals. Although a good number
of species are here described, yet there are a great
many more known to mineralogists, several of which
are, however, only of scientific interest. The abun-
dance and complexity of silicon compounds in the
inorganic world is in a way analogous to the vast
number of complex carbon compounds in the organic
world. It is interesting to note that these two non-
metallic elements, carbon and silicon, occupy ad-
jacent positions in the same group of the chemists'
periodic classification of the elements ; in other words,
these elements are closely related in their chemical
properties. The ultimate product, resulting by the
destruction of organic compounds, is the common
gas, carbon dioxide; and, similarly, the ultimate
product produced by the weathering and disinte-
gration of silicon compounds is silicon dioxide, which
has been already described as the common minerals
quartz and opal.
Silicon dioxide, or silica, when combined with
water, gives an acid known as silicic acid, which can
be prepared in the laboratory as a gelatinous sub-
183
184 THE WORLD'S MINERALS
stance. The proportions of water and silica may vary :
thus in meta-silicic acid (H 2 O + SiO 2 = H 2 SiO 3 )
we have equal molecular proportions one molecule
of water combined with one molecule of silica ; while
in ortho-silicic acid (2H 2 O + SiO 2 = H 4 SiO 4 ) they
are in the ratio of two to one. Several other hypo-
thetical silicic acids have to be assumed to explain
the chemical composition of various silicates which
occur in nature as minerals. The formula used to
express the chemical composition of these minerals
are often very complex, and in some cases not yet
completely determined by mineral chemists. It
would, therefore, be out of place in this book to enter
into much detail in this direction, and we must con-
tent ourselves by stating the principal elements that
are present in the more complex minerals.
The simple silicates of the alkali metals for ex-
ample, sodium silicate and potassium silicate (known
as water-glass] are soluble in water. But all the
naturally occurring silicates are insoluble in water,
and most of them are unattacked by acids (except
hydrofluoric acid) ; further, they can be fused only
with great difficulty. On the other hand, the com-
pounds of carbon are more readily dealt with and
can be prepared in the laboratory; so that by a va-
riety of reactions and replacements the organic chem-
ist is able to draw conclusions as to the chemical con-
stitution of such compounds. The mineral chemist
is, however, placed at a disadvantage by reason of
the intractable nature of the material he has to deal
with. For instance, the usual way of decomposing
THE SILICATES 185
a silicate for analysis is to fuse the finely powdered
mineral with sodium carbonate in a platinum crucible
over the intense flame of a blowpipe. Mineral sili-
cates can be prepared artificially only with great dif-
ficulty ac very high temperatures, and even when the
experiments extend over a period of several days only
crystals of microscopic dimensions are obtained. The
element of time is here a matter of consequence; and
in nature's laboratory, where high temperatures and
pressures are available for long periods, very finely
crystallized products result indeed, amongst the
silicates we find some of the most beautiful of crystal-
lizations.
It is not surprising, therefore, that but little knowl-
edge has been acquired respecting the chemical con-
stitution of naturally occurring silicates. And for
this reason no really satisfactory classification of the
whole group has yet been devised, different authors
using different systems of classification. Neverthe-
less, certain closely related species fall naturally
together into a number of isomorphous groups. In
the following pages some of these groups will be
considered, while some other species must be dealt
with singly in no particular order.
The silicates are of prime importance as rock-
forming minerals, since they form the bulk of the
rocks of the earth's crust. All the rocks of igneous
and metamorphic origin are composed almost en-
tirely of silicates, sometimes with the addition of
quartz; and it is only in certain rocks of sedimen-
tary origin, such as limestone and the pure quartz-
186 THE WORLD'S MINERALS
sandstones, that they are absent. The silicates
are also of importance from other points of view.
Several species are used as precious stones or for
ornamental purposes; while others have important
technical applications.
THE FELSPAR GROUP
This is a very important group of rock-forming
minerals, which are analogous in chemical compo-
sition and similar in crystalline form; that is, they
form an isomorphous group. They are silicates of
aluminium, together with either potassium, sodium,
or calcium ; so that, chemically, we have three kinds
of felspar namely, potash-felspar, soda-felspar, and
lime-felspar. Again, although very similar to one
another in the form of their crystals, yet these belong
to two different systems, the monoclinic and the
triclinic. We may, therefore, divide the felspars into
monoclinic felspars and triclinic felspars.
A character of special importance, and one com-
mon to all kinds of felspar, is that of cleavage. The
crystals possess a perfect cleavage parallel to the basal
pinacoid, this being called the basal cleavage; and a
second, rather less perfect cleavage, parallel to the
brachy-pinacoid, called the brachy-pinacoidal cleav-
age. Now in monoclinic felspar the second cleavage
is parallel to the single plane of symmetry, which is
perpendicular to the basal plane. The two directions
of cleavage are here, therefore, at right angles to one
another, and this kind of felspar is consequently
THE FELSPAR GROUP 187
called orthoclase, meaning, in Greek, "splitting at
right angles." In the triclinic felspars, on the other
hand, the two cleavages are not quite at right angles,
but inclined at an angle varying in the different spe-
cies from 86 24' (albite) to 85 50' (anorthite) ;
these are therefore referred to collectively as plagio-
clase, which means "splitting obliquely." In addi-
tion to these we have another kind of triclinic felspar
known as microcline, so called because the angle be-
tween the cleavages differs only very slightly from a
right angle, being about 89 30'.
The various kinds of felspar may now be tabulated
as follows :
Chemical System of
Formula Crystallization
Orthoclase, or potash-felspar. . KAlSi 3 O 8 Monoclinic
Microcline, or potash- felspar. . KAlSi 3 O 8 Triclinic
Albite, or soda-felspar NaAlSi 3 O' 8 Triclinic
Anorthite, or lime-felspar CaAlzSiaOs Triclinic
By the mixing together of albite and anorthite in
indefinite proportions, we have other members of the
plagioclase series, which are known as oligoclase,
andesine, and labradorite; these are intermediate be-
tween albite and anorthite, not only in chemical
composition, but in all their properties.
Twinned crystals are of very frequent occurrence
in all the felspars, and several types of twinning are
known. The plagioclase felspars are invariably
twinned according to the "albite law," one portion
of a twinned crystal being a reflection of the other
portion across a plane the twin-plane parallel to
the brachy-pinacoid. In one and the same crystal
188 THE WORLD'S MINERALS
this twinning is repeated many times, so that the
whole crystal consists of a pile of twin-lamellae. This
repeated lamellar twinning gives rise to a very char-
acteristic appearance, to be seen when basal cleavage
flakes, or thin sections of plagioclase crystals, are ex-
amined in polarized light under the microscope.
Further, it gives rise to a system of fine lines on the
basal plane and also on the basal cleavage; these lines
or striations being parallel to the edge between the
basal plane and the brachy-pinacoid. On this account
the plagioclase felspars are known also as "striated
felspars" ; and in hand-specimens they can always be
recognized by this character.
In addition to similarity of crystalline form and
cleavage, the felspars all possess the same degree of
hardness (No. 6 on the scale). They may be color-
less and transparent, but more often are white or
pinkish and opaque. They are not heavy, the specific
v gravity ranging with the chemical composition from
2.55 (orthoclase) 102.75 (anorthite). If, therefore,
we have a white, opaque mineral with two good
cleavages, nearly or quite at right angles, which is not
heavy, and can only just be scratched with a knife,
we may be pretty sure that the mineral in question is
felspar.
There are extremely few kinds of igneous rocks
and gneisses which do not contain a large proportion
of one or other of the felspars; so that these minerals
are abundant and of wide distribution. By the
weathering and breaking down of these rocks the
felspars are decomposed, their alkalis being assim-
FELSPAR: ORTHOCLASE 189
ilated by plants or carried away in solution; while
the alumina and silica combine with water to form
hydrated aluminium silicate. The latter, together
with other mineral particles, may be removed by run-
ning water and deposited as beds of clay, such as is
used for the making of bricks and pottery. A purer
form of white clay, known as china-clay, or kaolin,
also results from the decomposition under special con-
ditions of the felspar of granite. We thus see that,
indirectly at least, the felspars are of considerable
economic importance.
ORTHOCLASE
(Plate 25, Figs, i and 2). This is potash-felspar,
crystallizing in the monoclinic system. It is the most
abundant of the felspars, and the one most commonly
found as large, well-formed crystals. For this rea-
son it is sometimes called "common felspar." It is
an important constituent of all granites and syenites,
of the lavas known as rhyolite and trachyte, and of
several other kinds of igneous rocks, more especially
those which also contain some quartz. It is also
abundant in gneisses; and sometimes it is present in
the sandstones and grits which have been formed of
the debris of such rocks. The pegmatite-veins as-
sociated with granitic masses consist largely of coarse-
ly crystallized orthoclase. Cavities in granite, and
more especially in pegmatite, are often lined with
well-formed crystals of orthoclase, together with
crystals of topaz, tourmaline, and other minerals.
190 THE WORLD'S MINERALS
In Plate 25, Fig. i, is shown a group of orthoclase
crystals, and in Fig. 2 a single crystal, taken from a
cavity in the granite at Alabashka, near Mursinka,
in the Urals. In both specimens the reddish felspar
is associated with smoky-quartz; and in Fig. 2 there
is a regular interpenetration of the two minerals. The
crystals here represented are bounded by a rhombic
prism, the clino-pinacoid (on the right in Fig. 2),
and the basal plane and an ortho-pinacoid at the top.
The crystals from granite have frequently a dull,
chalky or stony appearance; but certain crystals have
quite the appearance of glass, and are thus known as
glassy felspar, or ice-spar, or more commonly as sani-
dine. Crystals of sanidine are found embedded in
the trachytic lavas of the Rhenish district. Clear
crystals of the variety called adularia are found lining
crevices in the gneissic rocks of the Swiss Alps. Still
another variety of orthoclase is that known as moon-
stone, so called because of the moon-like reflection
or opalescence, seen on its surface. Such stones are
found plentifully in Ceylon, and are cut and polished,
with a rounded surface, for use in jewelry. Rarely
to be seen in orthoclase are the brilliant blue, green,
or yellow sheens, or colored reflections, of the same
nature as those characteristic of labradorite;
material of this kind forms a large part of the augite-
syenite of Laurvik and Fredriksvarn, in southern
Norway, a rock which, when cut into slabs and pol-
ished, is extensively used for ornamental purposes.
Large quantities of orthoclase are quarried from
the pegmatite-veins of southern Norway, the material
SILICATES (Felspar group).
Plate 25.
1, 2, Orthoclase. 3, Labradorite. 4, Microcline. 5, Anorthite.
FELSPAR: MICROCLINE 191
being used in the manufacture of porcelain, more
especially for the production of the glaze.
MICROCLINE
(Plate 25, Fig. 4). This also is potash-felspar,
and is identical, chemically, with orthoclase. Crystal-
lographically, however, it is triclinic, while ortho-
clase is monoclinic; so that these two species are to
be considered as dimorphic forms of potash-felspar.
In their external appearance, however, crystals of
microcline cannot be distinguished from crystals of
orthoclase, and indeed the only method of distin-
guishing the two is by means of their optical char-
acters as observed under the polarizing microscope.
Basal cleavage flakes or thin sections of microcline
show in polarized light a very characteristic cross-
hatched structure, due to repeated lamellar twinning
in two directions. Orthoclase and microcline are thus
only to be distinguished by special optical tests, and
the difference between them seems to be a matter of
only slight importance, and especially so when we
bear in mind that they occur in nature under ex-
actly the same conditions as constituents of granite
and pegmatite. Indeed, many mineralogists assert
that they are in reality identical, the cross-hatching
being on so minute a scale in orthoclase that it is not
visible under the higher powers of the polarizing
microscope. This ultra-microscopic twinning would
account for the observed differences in the optical
characters of orthoclase and microcline.
192 THE WORLD'S MINERALS
A beautiful green variety of microcline deserves
special mention. This is known as amazon-stone
(Fig. 4) ; it is found as large, well-shaped crystals
in cavities in granite at a few places, the specimen
represented in the picture being from Pike's Peak,
in Colorado. Here we have a parallel intergrowth
of two crystals, each bounded by the rhombic prism,
clino-pinacoid, and at the top by the basal plane and
an ortho-pinacoid. Amazon-stone is sometimes cut
and polished for the construction of various small
ornamental objects.
ALBITE
Mention has already been made of the plagioclase
felspars, which form a continuous isomorphous se-
ries, ranging from albite to anorthite. The soda-
felspar called albite, which forms one end of this
series, is of importance as a rock-making mineral,
and it is also of interest to collectors as being found
in pretty groups of white crystals. Such crystals are
found, together with clear crystals of quartz (rock-
crystal), attached to the walls of crevices in the
gneissic rocks of the Alps and in the slates at Tintagel,
in Cornwall. The name albite refers to the white
color of the crystals.
OLIGOCLASE
Oligoclase is an intermediate member of the pla-
gioclase series of felspars, consisting of a mixture of
three to six parts of albite with one part of anor-
FELSPAR: LABRADORITE 193
thite. It is to this species that the beautiful variety
known as sun-stone, or avanturine felspar, belongs.
This shows on its surface a very pretty metallic
spangled reflection, due to the enclosure in the stone
of numerous small scales of haematite. The best
specimens such as are cut and polished for use
in jew r elry are from Tvedstrand, in the south of
Norway.
LABRADORITE
(Plate 25, Fig. 3). Labradorite, or labrador-spar,
is another intermediate member of the plagioclase
group of felspars, and consists of a mixture of one
part of albite with three to six parts of anorthite. It
is an important constituent of the more basic igneous
rocks, such as basalt and gabbro. In these rocks it
is present as smaller or larger embedded crystalline
grains; crystals bounded by faces being almost un-
known. On the coast of Labrador it forms, with
hypersthene (Plate 27, Fig. 5), a rock called norite,
which is here so very coarsely grained that the mas-
sive pieces of labradorite measure as much as a foot
across. In this region pebbles and boulders of lab-
radorite are widely scattered over the surface, the
material having been dislodged from the solid rock
by the action of ice. These stones are of a dull,
greyish-black color and opaque; but when a wet peb-
ble is held in the hand and turned slowly round, at
a certain position, it suddenly flashes with a bril-
liantly colored metallic reflection. The color is an
intense red, yellow, green, or blue, resembling the
194 THE WORLD'S MINERALS
brilliant metallic colors of a peacock's feather or
the wings of some tropical butterflies. When the
stone is turned away from this position, however
slightly, the color totally disappears. This curious
and pretty effect must have been noticed long ago by
the Esquimaux; but it was not until 1775 that speci-
mens reached Europe, when they were brought by the
Moravian missionaries.
The brilliancy of the reflection is much increased
when the stone is cut with a flat surface and polished ;
but even then it is only in one definite position of
the stone relative to the eye of the observer that the
color is seen.
These colored reflections of labradorite are due to
the enclosure in the felspar substance of vast numbers
of microscopic plates of other minerals all ar-
ranged parallel to a certain crystallographic direc-
tion. The substance itself is not colored, and the
colors are produced by the splitting up and reflection
of a portion of the white light when its rays strike
the parallel series of minute plates at a particular
inclination.
This special optical effect is thus quite an acci-
dental character of labradorite, and it is, in fact,
shown only by the labradorite from certain localities,
more particularly from Labrador (Fig. 3), and less
vividly in some specimens from Russia. Labradorite
is a common constituent of certain igneous rocks in
the British Isles; but none of this material displays
the beautiful interference-colors.
FELSPAR: ANORTHITE 195
ANORTHITE
(Plate 25, Fig. 5). Anorthite, or lime-felspar,
finds its place at the other end of the plagioclase
series. The name refers to the fact that none of the
angles on the crystals are right angles, the crystals
belonging to the triclinic or anorthic system. It is
an important constituent of some of the more basic
rocks of igneous origin, such as basalt and gabbro,
and it has also been detected in some meteoric stones.
In igneous rocks it is present only as dull, irregular
grains or imperfect crystals. Beautifully clear and
glassy crystals, with a profusion of small, brilliant
faces, are, however, found in ejected blocks of meta-
morphic limestone amongst the lavas of Monte Som-
ma, the ancient portion of Mount Vesuvius. These
blocks had been ejected long before the sudden and
disastrous eruption of 79 A. D. which built up the
present cone. Good but opaque crystals, of a pinkish
color, are also found in a metamorphic limestone on
the Pesmeda-Alp in Fleims valley, in southern Tyrol ;
a very fine crystal from this locality is represented
in Fig. 5.
THE AMPHIBOLE GROUP
This is another group of isomorphous minerals,
which are of importance as constituents of silicate
rocks, both those of igneous and of metamorphic
origin. Their chemical composition, though varying
between wide limits, can always be expressed by
196 THE WORLD'S MINERALS
the general formula R"SiO 3 that is, as salts of meta-
silicic acid (see p. 184). In this formula, R" usually
represents calcium, magnesium, and ferrous iron, but
sometimes manganese may be present, or aluminium
or ferric iron in conjunction with sodium. The
actual formulae may thus be extremely complex.
All the members of the group (with a few ex-
ceptions, which need not be mentioned in this book)
crystallize in the monoclinic system, and the crystals
are usually prismatic in habit. This prismatic habit
is often so pronounced that the crystals have the form
of fine fibers, needles, or even hairs. Of importance
is the presence of two perfect cleavages parallel to
the faces of the rhombic prism, the angle of which
is about 124.
HORNBLENDE
(Plate 26, Fig. i). Common hornblende occurs
as an essential constituent of a variety of rocks for
example, granite (hornblende-granite), syenite, di-
orite; certain volcanic lavas, such as some basalts and
andesites; gneisses, schists (hornblende-schist), and
metamorphic limestones.
Usually it forms bladed masses embedded in the
rock, on the broken surfaces of which the bright
prismatic cleavages of the hornblende are conspic-
uous. In color it is usually dark green or black, but
sometimes brown. The most perfectly developed
crystals are those found in limestones which have
been baked by close contact with a molten mass of
igneous rock. Good crystals are also found, together
HORNBLENDE TREMOLITE 197
with other crystallized silicates, in the magnetite-
mines of Arendal, in Norway (Fig. i).
The crystal here represented has the form of a six-
sided prism, terminated by three rhomb-shaped faces ;
and it has very much the appearance of a hexagonal
prism with a rhombohedral termination (compare,
for example, the rhombohedral crystal of tourmaline
shown in Plate 33, Fig. 2). The six-sided prism
is here, however, a combination of the four faces of
a rhombic prism, with the angle of 124 (i.e. not very
far from 120), and two faces of the clino-pinacoid
parallel to the clino-axis or plane of symmetry (at
the sides of the figure) . The two rhomb-shaped faces
at the top are pyramid faces, and the lower one be-
longs to an ortho-pinacoid (perpendicular to the
plane of symmetry). Such a crystal can be cleaved
parallel to the faces of the rhombic prism only, and
not parallel to the faces of the clino-pinacoid; where-
as if the form were truly hexagonal it could be
cleaved equally well parallel to all the prism faces.
TREMOLITE
Tremolite, or white amphibole, is, chemically, the
simplest member of the group, its composition being
expressed by the formula CaMg 3 (SiO 3 )4. It is found
as aggregates of bladed crystals embedded in white
crystalline limestone, or marble, at Tremola, in
Switzerland, and in several other regions where
metamorphic rocks occur.
198 THE WORLD'S MINERALS
ACTINOLITE
(Plate 26, Fig. 2). Here we have some of the
magnesium in the formula just given for tremolite
replaced by a small amount of ferrous iron, and it is
to this constituent that the mineral owes its charac-
teristic bright green color. As bladed or needle-like
crystals, it is a common constituent of many schistose
rocks, and in some cases it makes up the bulk of such
rocks. In Fig. 2, representing a specimen from the
Zillerthal, Tyrol, dark green blades of actinolite
penetrate in all directions a pale green talc-schist.
The name actinolite is a Greek form of the old Ger-
man name Strahlstein, meaning "ray-stone," in allu-
sion to the ray-like form of the crystals.
ASBESTOS
Asbestos is identical, both chemically and crystal-
lographically, with tremolite and actinolite, the
whiter varieties, with little or no iron, being near to
tremolite. Here the crystals, instead of being bladed
or acicular, are thread-like, being enormously elon-
gated prismatic crystals, so fine that they are flexible.
The difference is thus merely one of texture. The
fibres sometimes grow like hairs from a rock surface,
or they may form bundles in the crevices of horn-
blende-schists and other rocks of metamorphic origin.
This mineral has some curious properties and uses.
It may be combed, spun, and woven into cloth, or
SILICATES (Amphibolc group).
Plate 26.
1, Hornblende. 2, Actinolite. 3, Crocidolite. 4, Nephrite.
ASBESTOS NEPHRITE 199
made into string and lamp-wicks. Being unaffected
by fire (the name asbestos means, in Greek, "un-
quenchable"), and at the same time a non-conductor
of heat, it is used in a variety of forms as a fire-proof
and insulating material for example, theatre cur-
tains, firemen's clothes, linings for iron safes, packing
for the pistons of steam-engines, coverings for steam
and hot-water pipes and cold-storage plants, etc.
Napkins made of asbestos may be cleansed by placing
them on a bright fire.
The best qualities of asbestos, with fibers up to six
feet in length, come from Lombardy and Piedmont,
in the north of Italy. The greater part of the asbestos
used commercially is not the amphibole-asbestos here
described, but a fibrous variety of the mineral ser-
pentine, known as chrysotlle or serpentine-asbestos,
to be mentioned farther on. Still another finely
fibrous, asbestos-like mineral is the blue asbestos, or
crocidolite, to be mentioned presently.
NEPHRITE
(Plate 26, Fig. 4). This, again, is a variety of
amphibole, which, like asbestos, is identical in its
essential characters with tremolite and actinolite.
The structure is also finely fibrous ; but here the fibers
are short, and so closely compacted and matted to-
gether that they are only recognizable when thin
sections of the mineral are examined under the micro-
scope. This peculiarity of structure gives to the stone
an extraordinary degree of toughness, so much so that
200 THE WORLD'S MINERALS
it is extremely difficult to break a pebble or boulder
of nephrite by blows from a hammer. The fracture
is splintery; and the hardness, as determined by
scratching, is not more than 6 on the scale. Being
very compact and fairly hard, the mineral takes a
very good polish; and on the polished surface the
luster is somewhat greasy in character. The color
is usually green of various shades, depending on the
amount of iron present; but it may sometimes be
white if iron is absent.
In general character and appearance nephrite is
absolutely indistinguishable from the mineral ]adeite,
a member of the pyroxene group (p. 202). Both
these minerals are included under the general term
jade, and both are employed for the elaborate carv-
ings so highly prized by the Chinese. They differ
widely, however, in their essential characters, notably
in their chemical composition, nephrite being a sili-
cate of calcium and magnesium, with often a little
iron; while jadeite is a silicate of sodium and alu-
minium. The easiest method of distinguishing the
two kinds is by their specific gravity, which may be
determined by weighing the carved ornaments in air
and in water. The specific gravity of nephrite is
3.0 (or up to 3.1, with darker green varieties), while
that of jadeite is 3.33. Nephrite, therefore, floats
in methylene iodide; while jadeite neither sinks nor
floats, but remains suspended in the liquid.
Prehistoric celts of nephrite have been found in
many parts of Europe, particularly in the ancient
lake-dwellings in Switzerland. Axes and other
NEPHRITE CROCIDOLITE 201
weapons fashioned in nephrite, as well as curious
idols, with eyes of inlaid mother-of-pearl, were found
in the possession of the Maoris when New Zealand
was discovered by Tasman in 1642. In China and
India the use of this mineral for carvings dates back
many centuries, the nephrite quarries in the Kuen-
Lun Mountains having been worked by the Chinese
for two thousand years. Several other localities pro-
ducing nephrite are known in central Asia, but usual-
ly the material is found as water-worn pebbles in the
beds of rivers and streams. Much of the material
now cut in Europe for pendants and other small
personal ornaments comes from New Zealand, and is
generally known as New Zealand greenstone. This is
of a darker shade of green (Fig. 4) than most of the
Asiatic nephrite.
The name nephrite is from the Latin Lapis neph-
riticus, or "kidney-stone," a name so given because
this stone was worn by the ancients as a charm in the
belief that it prevented kidney disease.
CROCIDOLITE
(Plate 26, Fig. 3). This is a dark blue, fibrous
variety of amphibole, differing considerably in chem-
ical composition from the preceding varieties. It is
a silicate of sodium and iron, and has long been
familiar through specimens from the Asbestos Moun-
tains, near the Orange River, in South Africa, where
it occurs as veins in jasper-schists. The veins are
filled from wall to wall by a closely compacted mass
202 THE WORLD'S MINERALS
of very fine fibers, which, when the stone is rubbed,
separate out into a soft, woolly mass. It was this
character that suggested the name crocidolite, from
the Greek name for wool. The mineral is mined in
the Asbestos Mountains, and is put to the same com-
mercial uses as the other kinds of asbestos.
In part of the specimen represented in the picture
(Fig. 3) the characteristic blue color of the mineral
gives place to a golden-yellow. This change in color
is the result of a chemical alteration of the mineral,
the iron it contains as ferrous silicate being oxidized
and hydrated with the formation of limonite. At the
same time this alteration is accompanied by the sepa-
ration of free silica as quartz, which assumes the
fibrous structure of the original mineral. We then
have a pseudomorph of quartz and limonite after
crocidolite; and this is the manner in which the beau-
tiful gem-stone called tiger-eye (p. 116) has had its
origin. Unfortunately the name crocidolite is incor-
rectly applied in the trade to this pseudo-crocidolite.
THE PYROXENE GROUP
This is another important group of rock-forming
minerals, the members of which are in many respects
very much like those of the amphibole group.
They have the same type of chemical formula,
R"SiO 3 , and are also very much of the same type in
crystallization; but with this important difference,
that the angle between the prismatic cleavages is here
nearly 93, instead of 124 as in the amphiboles.
THE PYROXENE GROUP 203
There are also important differences in the optical
characters of the crystals.
This rhombic prism, with angles approximating to
right angles, is usually combined with two pinacoids,
respectively parallel and perpendicular to the single
plane of symmetry; so that we have a nearly regular
eight-sided prism (Plate 27, Figs. 1-3), much like
the eight-sided tetragonal prism of zircon (shown
in Plate 14, Fig. 4). The crystals of the pyroxenes
shown in Plate 27 belong, however, to the monoclinic
system, with only one plane of symmetry, as may be
seen from the pictures. In Fig. i this eight-sided
prism is terminated by a pair of obliquely placed
pyramid planes; and in Fig. 2 there are two such
pairs of pyramid planes, together with a small plane
at the top of the crystals which is perpendicular to
the plane of symmetry. Although these Crystals (of
augite and diopside) are monoclinic, there are other
members (enstatite, bronzite, and hypersthene) of
the pyroxene group which belong to the orthorhom-
bic system; but these only rarely occur as distinctly
formed crystals.
The name pyroxene has rather a curious history: it
means, in Greek, "a stranger to fire," and was given
in 1796 by the celebrated French mineralogist, the
Abbe Haiiy, in the belief that the crystals he so
named had been accidentally caught up in the vol-
canic lavas in which they were found. As a matter
of fact, as we now know, the pyroxenes are typically
minerals of igneous origin, having crystallized out
from molten rock-magmas.
204 THE WORLD'S MINERALS
AUGITE
(Plate 27, Fig. i). This variety of pyroxene is
of abundant occurrence as an essential constituent of
basalts, gabbros, and other dark-colored (i.e. basic)
rocks of igneous origin. In the lavas of several vol-
canoes, many of them now extinct, it is found as well-
formed crystals embedded in the rock, as shown in
Fig. i, representing a fragment of lava from Bo-
hemia. When the enclosing rock is decomposed and
softened by weathering, the crystals of augite fall
out, and are often found loose in the soil. These
crystals are of a black or very dark green color, with
dull surfaces.
DIOPSIDE
(Plate 27, Figs. 2 and 3). This is another mono-
clinic pyroxene, differing from augite in being less
complex in chemical composition. Its formula may
be written as CaMg(SiO 3 ) 2 , but very often a small
amount of ferrous iron takes the place of an equiva-
lent amount of magnesium. As the iron varies in
amount in the isomorphous mixture, there is a cor-
responding variation in the color of the crystals, from
almost white to a dark green, the crystals shown
in Figs. 2 and 3 being of an intermediate rich green
color. This variety of pyroxene is usually found as
well-formed crystals in limestones which have been
subjected to the baking action of molten rock-masses.
The lustrous crystals from Ala, in Piedmont, rep-
SILICATES (Pyroxene group).
Plate 27.
1, Augite. 2, 3, Diopside. 4, Enstatite. 5, Hypersthene. 6, Wollastonite.
BRONZITE HYPERSTHENE 205
resented in Figs. 2 and 3, are found, together with
bright crystals of garnet (hessonite), in veins travers-
ing serpentine. These clear, green crystals are some-
times cut in Turin as gem-stones.
BRONZITE
(Plate 27, Fig. 4). The orthorhombic pyroxenes
form a series ranging in chemical composition from
magnesium meta-silicate (MgSiOs) to iron meta-
silicate (FeSiOs) , which mix together isomorphously
in all proportions. When but little iron is present we
have the mineral enstatite, which is sometimes found
as transparent green crystals, suitable for cutting as
gems. Bronzite is an intermediate member of this
series, with a composition that may be expressed by
the formula (Mg,Fe)SiO 3 . As represented in the
picture (Fig. 4), it occurs as masses composed of a
confused aggregate of crystalline individuals, with a
fibrous structure, and exhibiting a bronze-yellow
color with a metallic sheen or luster (hence the name
bronzite). These characters are brought out to ad-
vantage when the stone is cut and polished, and it is
therefore sometimes used for ornamental purposes.
HYPERSTHENE
(Plate 27, Fig. 5) . This is another member of the
series of orthorhombic pyroxenes, differing from
bronzite in containing less magnesium and more iron,
so that the formula becomes (Fe,Mg)SiO 3 . There
206 THE WORLD'S MINERALS
being here more iron in the mineral, its color is
deeper, being dark brown or brownish-green. In
certain specimens the mineral exhibits a very pro-
nounced metallic reflection, and on this account it
is cut and polished as an ornamental stone. This
character is best shown by specimens from Labrador,
where the mineral forms, together with labradorite
(p. 193), a coarse-grained igneous rock called norite.
WOLLASTONITE
(Plate 27, Fig. 6). Named after the English
chemist and mineralogist, W. H. Wollaston (1766-
1828) , this mineral of the pyroxene group was earlier
known as tabular-spar, on account of the plate-like
shape of its rarely occurring crystals. These are
white in color, and consist of calcium meta-silicate,
CaSiOa. The mineral is usually found in metamor-
phic limestones, in which it occasionally forms den-
dritic growths, as in the black limestone near Pirna,
in Saxony (Fig. 6).
There are a few other minerals of the pyroxene
group which, although not represented on the plates,
are of some importance, and also they supply gem-
stones of a fine color. Of these, jadeite, a silicate of
sodium and aluminium, has already been mentioned
under the mineral nephrite (p. 199), which it so
closely resembles in appearance. Another is spodu-
mene, a silicate of lithium and aluminium, usually
found as ash-grey crystals (hence the name), but
sometimes as transparent crystals of a beautiful em-
THE SODALITE GROUP 207
erald-green or violet color; the green variety is
known as hiddenite, and the violet, or lilac, as kunz-
ite. A third species is rhodonite, a silicate of man-
ganese (MnSiOs), so named because of its charac-
teristic rose-red color. This is found as small, bright
crystals, or as larger, suitable for cutting into slabs.
THE SODALITE GROUP
This group includes a few less common minerals,
which are sometimes of importance as constituents of
certain kinds of igneous rocks. They are complex in
chemical composition, and present the peculiarity of
containing chloride, sulphate, or sulphide in com-
bination with the silicate portion. In crystallization
they are all cubic; but distinctly formed crystals are
of rare occurrence.
SODALITE
(Plate 28, Fig. i). This is a silicate and chloride
of sodium and aluminium, the name sodallte refer-
ring to the presence of sodium, or soda. It is found
as clear, glassy crystals in the ejected bombs of Monte
Somma, the ancient portion of Vesuvius. The re-
markably fine crystals represented in the picture
(Fig. i) are distorted rhombic-dodecahedra, being
elongated in the direction of one of the triad axes,
and so presenting the appearance of hexagonal prisms
terminated by rhombohedral planes.
In another form, of very different appearance, the
mineral occurs as compact masses of a bright sky-blue
208 THE WORLD'S MINERALS
color (very like Fig. 2 on the same plate). These
masses are often of considerable size, and form a
constituent of sodalite-syenite at Miask in the Ural
Mountains, at Litchfield in Massachusetts, and at
Bancroft in Hastings Co., Ontario. At the last-
named place the mineral is quarried, and slabs are
polished for ornamental purposes. The deposits
were being developed at the time of the visit of the
Prince and Princess of Wales to Canada, and on
account of the interest taken by the Princess (now
Queen Mary) in the stone, it came to be known in
the trade as Princess Blue.
LAPIS-LAZULI
(Plate 28, Fig. 2). Another mineral belonging to
the sodalite group, and known to mineralogists as
lazurlte (not to be confused with lazulite, p. 178),
resembles sodalite in composition, except that sulphur
takes the place of chlorine. This intensely blue min-
eral enters largely into the composition of lapis-
lazuli, which itself is really a fine-grained mixture of
minerals, or, in other words, a rock, being of the
nature of a crystalline limestone impregnated with
lazurite, sodalite, iron-pyrites, etc. On this account,
and depending on the amount of lazurite present,
the depth of color shown by lapis-lazuli is somewhat
variable, being usually a paler blue in specimens
from Lake Baikal (Fig. 2) and Chile, and a richer
and deeper blue in the more prized specimens from
Badakshan, in central Asia. The stone is spotted and
SILICATES.
Plate 28.
1, Sodalite. 2, Lapis-lazuli. 3, Leucite. 4, Beryl.
LAPIS-LAZULI LEUCITE 209
veined with white or yellowish (iron-stained) calcite,
and speckled with grains of iron-pyrites. The deep-
blue ground set with bright, brassy specks of iron-
pyrites suggests a comparison with the blue sky
bedecked with stars.
As is well known, lapis-lazuli is much used for
small ornaments, such as beads, crosses, etc., and for
inlaying in boxes and table-tops. Being an opaque
stone, its beauty depends solely on its rich blue color.
When the stone is crushed to powder and the- pure
blue material is separated by sedimentation in water,
the pigment known as ultramarine is obtained. For-
merly, when lapis-lazuli was the only source of ultra-
marine, this was very expensive; but now it is manu-
factured on a large scale by heating in crucibles a
mixture of china-clay (a hydrated silicate of alu-
minium) , sodium carbonate, charcoal, and sulphur.
LEUCITE
(Plate 28, Fig. 3). Well-formed crystals of this
mineral are not uncommon in the lavas of Vesuvius
and of the extinct volcanoes near Rome and in the
Eifel. They have a rounded appearance, looking
like so many angular peas embedded in the rock.
They are bounded by twenty-four trapezoidal faces,
exactly like the crystals of analcite shown in Plate
36, Fig. i. This form is the icositetrahedron of the
cubic system, the symbol being (112) (see p. 14).
210 THE WORLD'S MINERALS
Crystals of leucite were long thought to belong to
the cubic system; but when they were examined in
thin sections under the microscope, it was seen that
their internal structure and optical characters did not
conform with cubic symmetry. They really consist
of a complex lamellar intergrowth of twinned ortho-
rhombic crystals, with the external form of a cubic
crystal. When, however, the crystals are heated to a
temperature of 714 C., these peculiarities of internal
structure disappear, and the crystals are then truly
cubic; but on their cooling again the structure reap-
pears. We must, therefore, assume that when the
crystals were formed in the red-hot lava they grew as
cubic crystals, and that as they cooled the cubic sub-
stance became transformed into an orthorhombic sub-
stance. We thus have here a peculiar case of di-
morphism, analogous to the dimorphism of diamond
and graphite (pp. 49 and 50), but one in which the
cubic form can only exist at a high temperature;
when cooled it changes spontaneously into the second
form.
Chemically, leucite is a silicate of aluminium and
potassium, with the formula KAlSi 2 Oe, thus contain-
ing the same elements as potash-felspar, but combined
in different proportions. It is found, sometimes to-
gether with potash-felspar, in certain igneous rocks
rich in potash. The name leucite means "white
stone" ; but this is scarcely an appropriate name, since
the crystals are usually of a dark grey color, as shown
by the specimen from Vesuvius (Fig. 3).
BERYL 211
BERYL
(Plate 28, Fig. 4). This mineral is a silicate of
the metals beryllium and aluminium, and when quite
pure and free from coloring matter and flaws, it is
colorless and clear like glass. Such crystals are, how-
ever, quite uncommon; more usually they are dull
and cloudy and of a pale greenish or yellowish color,
this color being due to the presence of traces of iron.
Sometimes they are transparent and of a rich grass-
green color, and we then have the variety known as
emerald, which on account of its rarity in clear crys-
tals of a good color is one of the most valuable of
gem-stones. The color here is probably due to a
small amount of chromium oxide. When the color
of the clear crystals is pale yellowish-green, bluish-
green, or sea-green, we have the gem-variety known
as aquamarine. Other crystals are of a yellow
color, approaching golden-yellow. Recently very
fine beryls of a rich pink color have been found in
Madagascar and California.
Beryl crystallizes in the hexagonal system, usually
in very simple form with only the hexagonal prism
and the basal plane (Fig. 4, and Text-Fig. 21, p. 24) ;
but sometimes, especially in the variety aquamarine,
there is a rich development of pyramidal faces on the
edges and corners between the prism and the basal
plane. The habit of the crystals is almost invariably
prismatic, and the prism faces are striated in the
direction of their length. Enormous crystals, weigh-
212 THE WORLD'S MINERALS
ing as much as one or two tons, have been found at
Grafton and Acworth in New Hampshire, and large
crystals are also found in southern Norway. The
specific gravity is 2.7, only slightly higher than that
of quartz ; so that beryl is one of the lightest of gem-
stones. The hardness of 7^ is between that of quartz
and topaz.
Beryl is of common occurrence in some granites,
more particularly in the coarsely crystallized veins
of pegmatite, which traverse granitic masses. Very
good transparent crystals of a blue color have been
found in the granite of the Mourne Mountains in
County Down, and opaque crystals in the granite of
Counties Dublin and Donegal and in Banffshire.
The crystals embedded in white quartz in Fig. 4 are
from Bodenmais, in Bavaria. The emerald variety
is found in mica-schist in the Ural Mountains, at
Habachthal in Salzburg, and at Jebel Zabara in
Upper Egypt. At the last-named locality the so-
called Cleopatra's emerald-mines were worked in
1650 B. C. f and the stones cut as beads and scarabs by
the ancient Egyptians. The best emeralds, however,
all come from Muzo in Colombia, South America,
where they occur, with calcite and black limestone,
in crevices in clay-slate. Many of the crystals are
much fissured, and stones perfectly free from flaws
are extremely rare. The majority of the emeralds so
much admired by Indian princes are doubtless of
South American origin, and many of them perhaps
formed part of the spoil taken from the Peruvians by
the Spanish conquerors in the sixteenth century.
THE GARNET GROUP 213
Aquamarines of gem-quality are mostly from Brazil,
the Ural Mountains, and Transbaikalia in Siberia.
THE GARNET GROUP
The garnets afford an excellent example of an
isomorphous group of minerals. They are all alike
in their crystalline form, belonging to the cubic sys-
tem, and they all have the same type of chemical
formula. This formula may be expressed generally
as an ortho-silicate, RTsR'"*SuOi, where R" stands
for calcium, ferrous iron, magnesium, or manganese;
and R'" stands for aluminium, ferric iron, or chro-
mium. We may thus have the following kinds of
garnet, which, it will be seen, vary widely in their
actual chemical composition :
Chemical Mineralogical
Chemical Name Formula Name
Calcium-iron-garnet Ca 3 Fe2Si 8 Oi2 Andradite
Calcium-chromium-garnet Ca 3 Cr 2 Si3Oi 2 Uvarovite
Calcium-aluminium-garnet CasAUSisOu Grossularite
Iron-aluminium-garnet FeaAUSisOw Almandine
Magnesium-aluminium-garnet MgsALSisO^ Pyrope
Manganese-aluminium-garnet MnaALSiaOw Spessartite
Although these may be taken as the types, it is only
rarely that actual crystals correspond exactly with
the compounds above stated. What actually hap-
pens is that two or more of these compounds help
together to build up one and the same crystal, as has
already been explained under isomorphism (p. 49).
In applying the mineralogical names, we imply that
214 THE WORLD'S MINERALS
the corresponding chemical type predominates in the
particular crystal.
Corresponding with these wide differences in
chemical composition, there must of necessity be wide
differences in color (white, black, green, red, yellow,
but not blue) , specific gravity (3.4-4.3) , and mode of
occurrence. Amongst them we have gem-stones of
red, brown, yellow, and green colors; while common
garnet is used as an abrasive agent in the form of
garnet-paper (sometimes sold under the name of
emery-paper). The hardness is about the same as
that of quartz, varying from 6^2 to j l /2. Still an-
other use for garnets is for gravelling garden-walks;
the small, rejected material, after picking out stones
large enough for cutting as gems, being so used in the
garnet-mining district of Bohemia.
Crystals of garnet are always developed to an equal
extent in all directions; that is, they present neither a
prismatic nor a platy habit, but what may perhaps be
described as a granular habit. Most crystals found
embedded in rocks have, in fact, the form of rounded
grains. It is no doubt on account of this characteristic
granular form that the mineral has received its name.
The most common crystal form is the rhombic-dode-
cahedron, which for this reason is sometimes called
the garnetohedron. The icositetrahedron, with the
symbol (112), as in the crystals of leucite (p. 209)
and analcite, is also a common form of garnet crystals.
In a combination of these two forms the faces of the
icositetrahedron truncate the edges of the rhombic-
dodecahedron (Plate 29, Figs, i and 2).
HESSONITE ALMANDINE 215
HESSONITE
(Plate 29, Fig. i). Hessonite, or cinnamon-
stone, is essentially a calcium-aluminium-garnet; but
it always contains, in addition, small amounts of fer-
rous and ferric iron, manganese, and magnesium; so
that it is a mixture of five of the garnet substances.
The crystals are often transparent, and of a warm
reddish-brown, honey-yellow, or hyacinth-red color,
and when cut with facets they afford pretty gems.
Beautiful crystals occur, in association with clear,
green crystals of diopside, in veins in serpentine at
Ala, in Piedmont. The crystals on limestone shown
in Fig. i are from Sweden. Material of the best
gem-quality is found as water-worn pebbles in the
gem-gravels of the cinnamon island of Ceylon; the
name cinnamon-stone is, however, in allusion to the
cinnamon-color of the mineral.
ALMANDINE
(Plate 29, Fig. 2). Almandine is essentially the
iron-aluminium-garnet, but, like all the garnets, it
contains variable amounts of the other types in its
mixed crystals. Its color is a deep rich red, often
with a tinge of violet. This garnet is usually cut and
polished in rounded convex forms (en cabochon),
when it often passes under the name carbuncle. Such
stones are cut in large numbers at Jaipur, in India,
and these Indian-cut gems have much the appearance
216 THE WORLD'S MINERALS
of jujubes. Almandine is found at many localities,
occurring usually in mica-schists and gneisses. The
fine crystal embedded in mica-schist shown in Fig. 2
is from Fort Wrangell, in Alaska.
PYROPE
Pyrope, or magnesium-aluminium-garnet, is one of
the best-known varieties, and the one most extensively
cut as a gem-stone. It is of a fiery-red color, and
passes under a variety of names, according to the lo-
cality at which it is found. The best known of these
is Bohemian garnet, from the district in northern
Bohemia, where an important garnet mining and cut-
ting industry has been established for several cen-
turies. Cape ruby is only another name for pyrope
from the diamond-mines of South Africa. Elie ruby
is a pyrope from Elie, in Fifeshire, and Arizona ruby
from Arizona. Intermediate, in chemical composi-
tion, between pyrope and almandine is the beautiful
gem-stone called rhodolite, from North Carolina, the
color of which is a peculiar and delicate rhododen-
dron-pink.
ANDRADITE
Andradite, the common calcium-iron-garnet, is
usually of a dark brown or black color; but certain
specimens are yellowish-green or emerald-green, and
perfectly transparent. The latter occur as rounded
nodules in the serpentine-rocks of the Urals, or as
pebbles in the neighboring gold-washings. They cut
SILICATES.
Plate 29.
1, 2, Garnet. 3, Olivine. 4, Idocrase.
GROSSULARITE UVAROVITE 217
into very effective and brilliant gems, and go under
the names of demantoid, or Uralian emerald; but in
the trade they are often erroneously called olivine.
Being softer (H. = 6^2) than the other varieties of
garnet, they, however, do not wear well when
mounted in rings.
GROSSULARITE
When chemically pure, calcium-aluminium-garnet
is colorless; but the small water-clear crystals are
quite exceptional. Usually the crystals are of a
greenish-yellow color hence the name grossularite,
which, in Latin, means "gooseberry-stone." Beauti-
ful rose-pink crystals are found embedded in a white
marble at Xalostoc, in Mexico; and with its splashes
of pale-green epidote, this garnet-bearing rock has
quite a pretty effect when cut into slabs and polished.
UVAROVITE
Uvarovite, or calcium-chromium-garnet, is of rare
occurrence as small, brilliant crystals of a bright
emerald-green color. These are, however, too small,
and wanting in transparency, for cutting as gems.
This variety of garnet occurs in association with
deposits of chrome-iron-ore.
218 THE WORLD'S MINERALS
OLIVINE
(Plate 29, Fig. 3). This is a member of another
isomorphous group of minerals, in which magnesium
and iron replace each other in the ortho-silicate for-
mula R" 2 SiC>4. The pure magnesium ortho-silicate,
Mg 2 SiO 4 , is known as forsterite, and the pure iron
ortho-silicate, Fe 2 SiO 4 , as fayalite, the intermediate
members of the series with variable proportions of
magnesium and iron being known as olivine. We thus
have here a similar case to that shown by the mag-
nesium and iron meta-silicates in the group of ortho-
rhombic pyroxenes. With the entry of some other
metals, the olivine group may also extend in other di-
rections, and several minerals of this group are known.
In the gem-varieties of olivine, and also in most
rock-forming olivines, the amount of magnesium is
largely in excess of the iron; and the color of such
varieties is yellowish-green or olive-green. When
more iron, and less magnesium, is present the color
may be brown. Crystals are of rare occurrence, the
mineral being usually present as irregular grains or
granular masses embedded in certain rocks, of which
it forms an important constituent. These rocks are
dark basic rocks of the basalt, gabbro, and peridotite
families. Some of the peridotites are composed al-
most entirely of olivine for instance, the rock called
dunite, from the Dun Mountain in New Zealand.
Such rocks are very liable to alteration when exposed
to weathering processes; and by the absorption of
OLIVINE IDOCRASE 219
water the magnesium silicate passes into the hydrated
magnesium silicate serpentine. It is in this way that
the large rock-masses of serpentine have originated.
An interesting occurrence of olivine is as a constitu-
ent of meteoric stones, and as grains in some meteoric
irons.
The bright crystals shown in Fig. 3, in parallel
grouping, in the cavity of a dark volcanic rock are
from Monte Somma, Vesuvius ; but such perfect crys-
tals are quite exceptional. They are bounded by
three pairs of pinacoids at right angles to one another,
three rhombic prisms, and a rhombic pyramid (the
small triangular faces on the corners).
Precious olivine that is, clear, transparent ma-
terial suitable for jewelry is also known by the
names peridot and chrysolite. Its specific gravity is
3.3; and the hardness being only 6%, the stone is
rather liable to get scratched when worn in rings.
Practically all the material of gem-quality comes
from the small island of St. John in the Red Sea,
where it is found as crystals in cavities in an altered
dunite. The gem-mining here is a monopoly of the
Khedive of Egypt, and is jealously guarded. This
material contains a small amount of nickel, to which
possibly the rich leaf -green color is partly due. Gem-
material is also found in Arizona and New Mexico.
IDOCRASE
(Plate 29, Fig. 4). Although this mineral is very
simple in its crystalline form, it is extremely complex
220 THE WORLD'S MINERALS
in its chemical composition. It is essentially a sili-
cate of calcium and aluminium, but many other ele-
ments are also present. Crystals are common, and are
usually well developed. Those represented in Fig.
4 consist of a combination of two square (tetragonal)
prisms, a tetragonal pyramid, and the small, square
basal plane at the top. The brown crystals here de-
picted are from Monte Somma, Vesuvius, where
they occur plentifully in the ejected blocks of the old
volcano. From this well-known occurrence the min-
eral is often known as vesuvianite. Crystals of a
bright-green color are found at Ala, in Piedmont, and
these are occasionally cut as gems at Turin. At other
localities the mineral is usually found in metamor-
phic limestones. Large blocks of massive idocrase of
a rich green color and with a marked degree of trans-
lucency have recently been found in California, this
material being cut and polished for a variety of small
ornaments under the name of californite.
TOPAZ
(Plate 30, Figs, i and 2). This well-known gem-
stone presents a wide range of colors. Frequently it
is perfectly colorless and water-clear, and pebbles of
such material are known to the Brazilians as pingos
d'agua (drops of water). The popular idea of topaz
is, however, a stone of a sherry-yellow color, this
being the common Brazilian topaz formerly much
used in jewelry; but when this stone is heated it
changes in color to a rose-pink, being then known as
SILICATES.
Plate 30.
1, 2, Topaz.
TOPAZ 221
burnt topaz. Other stones, which also may be from
Brazil, are of delicate blue, bluish-green, or smoke-
brown colors.
Topaz is a fluo-silicate of aluminium with the
chemical formula (AlF) 2 SiO 4 ; but the fluorine is
often partly replaced by the elements of water in the
form of hydroxyl (OH). The mineral is usually
met with as well-formed crystals, but sometimes it is
found as water-worn crystals and pebbles. These are
orthorhombic, with a prismatic habit, and the prism
faces are striated in the direction of their length.
The prism invariably consists of a combination of
two vertical rhombic prisms, terminated by horizon-
tal prisms or domes, pyramids, and frequently also by
the basal plane. The two crystals in Fig. 2 are each
bounded by the two vertical prisms, two horizontal
prisms, two pyramids, and the base. In Fig. i the
regularity of the crystal is somewhat interrupted by
parallel grouping; but the forms here are the same as
before, except that only one horizontal prism (the
large triangular face to the front) is present; on the
left are four faces of two pyramids, but on the right
the two faces of only one pyramid.
A very important character of crystals of topaz is
the perfect cleavage in one direction parallel to the
basal plane. The crystals are usually grown attached
at one end to the matrix, as in Fig. 2, and when they
are detached from this they break away with a
smooth plane surface along the cleavage; this is
shown by the even underside of the detached crystal
in Fig. i.
222 THE WORLD'S MINERALS
The specific gravity of topaz is about the same as
that of diamond namely, 3.5; but the hardness
(H. = 8) and brilliancy of luster are much lower.
The crystals are often perfectly transparent, with
bright faces; but large crystals are frequently opaque
and dull. For instance, a Norwegian crystal, two feet
in length and weighing 137 lb., shown in the mineral
gallery of the British Museum, is quite opaque and
rough. Another noteworthy specimen to be seen in
the same collection of minerals is a water-worn peb-
ble of clear, colorless topaz, weighing very nearly
13 lb. Years ago this block had been used as a door-
step at a shop in Fleet Street in London. By the man
in the street it would no doubt be regarded as a lump
of glass and not worth carrying away; but the critical
eye of a mineralogist noticed, that on two opposite
sides of the block there are plane and smooth surfaces
of fracture, these being, of course, due to the perfect
basal cleavage so characteristic of topaz.
Topaz occurs in nature in veins of tin-ore (cas-
siterite), and under these conditions small colorless
crystals are found in some of the Cornish tin-mines,
and in the stream-tin deposits of New South Wales
and Japan. The larger crystals occur in crystal-lined
cavities in granites and pegmatites. That shown in
Fig. i is from the granitic rocks at Alabashka, near
Ekaterinburg, in the Ural Mountains. Other well-
known Russian occurrences are in the Ilmen Moun-
tains and the Sanarka River (here as red crystals in
the gold-washings) in the southern Urals, and in
Transbaikalia in Siberia. One of the localities in
TOPAZ ANDALUSITE 223
Transbaikalia rejoices in the name of the Borsch-
chovochnoi Mountains ; some very fine crystals from
there are preserved in the British Museum collection ;
but, as they lose their delicate brown color on ex-
posure to light, they are kept under cover. The
crystals shown in Fig. 2 are from the Thomas Range
in Utah, where they occur in cavities in a volcanic
rock called rhyolite. Much of the gem-material of
various colors comes from Minas Geraes, in Brazil.
ANDALUSITE
(Plate 31, Figs. 1-3). This is a silicate of alu-
minium, Al 2 SiO 5 , consisting of a combination of one
molecule of alumina (AUOs) with one of silica
(SiO 2 ). It is, however, not the only mineral with
this chemical composition; there are two others
namely, kyanite (Fig. 4) and sillimanite. These
three mineral species are thus trimorphous forms of
the same chemical compound, differing from one an-
other in crystalline form and physical characters.
Andalusite forms simple rhombic prisms, with an
angle not far from 90, which are terminated at right
angles to their length by the basal pinacoid (Fig. 3) ;
they belong to the orthorhombic system. Usually the
crystals are opaque and of a dull grey color; and
they are often coated on their surface with a film
of mica (Fig. 3) , which has resulted from their alter-
ation. The crystals shown in Fig. 3 are embedded in
a vein of white quartz traversing mica-schist, and are
from the Lisens-Alp, in the Tyrol. Specimens were
224 THE WORLD'S MINERALS
first collected at the end of the eighteenth century
in Andalusia, and it is from this locality that the
mineral takes its name. The same mineral is also
found as transparent pebbles in the gem-gravels of
Minas Geraes, Brazil. These are remarkable for
their very strong pleochroism: when looked through
in different directions they exhibit different colors
olive-green and blood-red. When faceted as gems,
such stones present a very striking effect
A peculiar variety of andalusite is that known as
chiastolite, or cross-stone. This contains regularly
arranged enclosures of black carbonaceous matter,
which has been caught up by the crystals during their
growth. When these crystals are cut in slices per-
pendicularly to the length of the prism, the cross-
sections show a pattern, either a black cross on a
dirty white or yellowish ground (Fig. i) or a white
cross on a black ground (Fig. 2), depending on the
relative amounts of the black enclosures. The crys-
tals represented in the two pictures have been so cut
and polished. Polished slices are often mounted and
worn as charms. These crystals occur embedded in
clay-slates near the contact of these rocks with a mass
of granite, and they have been formed by the baking
action of the molten igneous mass when it was in-
truded into the slates. The specimens represented in
Figs, i and 2 are from Lancaster, in Massachusetts,
but similar specimens are found at several other
places. Small crystals can be collected in abundance
from the slates surrounding the granite of Skiddaw,
in Cumberland.
SILICATES.
Plate 31.
1-3, Andalusite (12, var. Chiastolite). 4, Kyanite.
KYANITE 225
KYANITE
(Plate 31, Fig. 4). Though identical with an-
dalusite in chemical composition, this mineral is
quite different in appearance. It occurs as bladed
crystals of a sky-blue color; and it is named kyanite,
or cyanite, on account of this characteristic color.
The crystals belong to the anorthic system, but they
only rarely show any other form than the very charac-
teristic blades. They have a perfect cleavage parallel
to the surface of the blades, and running transversely
across this cleavage surface are fine lines due to the
presence of secondary twinning (shown at the top in
Fig. 4), just as in crystals of stibnite (p. 79). This
mineral is of special interest to mineralogists by rea-
son of its peculiarities of hardness. On the bladed
cleavage surface in a direction perpendicular to the
length the hardness is 7, but in the direction of the
length of the crystal it is only 5. This surface can
thus be scratched by quartz in one direction, but not
in the other.
The bladed crystal in a matrix of white mica-schist
represented in Fig. 4 is from Monte Campione, in
the St. Gotthard district, in Switzerland. Good
specimens are also found at Botriphnie, in Banffshire,
and at Carrowstrasna, in County Donegal. Clearer
material of a deep blue color is occasionally cut as a
gem-stone.
226 THE WORLD'S MINERALS
EPIDOTE
(Plate 32, Fig. i). This is a complex silicate of
aluminium, iron, and calcium, with a small propor-
tion of water; but owing to isomorphous mixing the
exact composition is somewhat variable. Crystals are
monoclinic, with the peculiarity that they are usually
elongated in the direction perpendicular to the single
plane of symmetry. Only rarely can the forms of
the crystals be made out on inspection; they are
usually rod-like or needle-like, and arranged in con-
fused or divergent bundles (Fig. i). They possess
a perfect cleavage in one plane direction parallel to
their length. One of the most characteristic features
of common epidote is its peculiar shade of yellowish-
green color, which is compared to that of the pis-
tachio-nut, and on this account the mineral is some-
times known as pistacite. Sometimes, however, the
mineral is brown in color. Epidote occurs in schists
and other metamorphic rocks, and has been formed
by the alteration of various minerals, such as felspar,
hornblende, etc. A rock composed entirely of epi-
dote and quartz is called an epidote-schist, and it is
in the crevices of such rocks that the finest crystals of
epidote are found, such as those from the Knappen-
wand, in Untersulzbachthal, Salzburg (Fig. i). The
green crystals from this locality are occasionally cut
as gem-stones.
SILICATES.
Plate 32.
1, Epidote. 2, Axinite. 3, Prehriite 1 . :
AXINITE PREHNITE 227
AXINITE
(Plate 32, Fig. 2). Axinite takes its name from
the characteristic axe-like shape of its anorthic crys-
tals, all the faces of which are obliquely inclined to
one another. There being only a center of symmetry,
each simple form on the crystals consists only of a
pair of parallel faces; but it will be noticed from the
picture that these are arranged in zones around the
crystal, with series of parallel edges. The charac-
teristic color is clove-brown, and the crystals are
sometimes glassy and transparent, being then oc-
casionally cut as gem-stones. Chemically, the mineral
is a silicate of aluminium, calcium, manganese, and
iron, with some boron and a little water. It is of
metamorphic origin, and is usually found in crevices
in hornblende-schist or in diabase. The best crystals
are from Bourg d'Oisans, in Dep. Isere, France (Fig.
2) ; but good specimens are also found at several
places in Cornwall.
PREHNITE
(Plate 32, Fig. 3). This is a silicate of alumin-
ium and calcium, containing a small proportion of
water. It crystallizes in the orthorhombic system,
but distinctly formed crystals are extremely rare.
The individual crystals are closely aggregated in
radiating forms, with rounded external surfaces; the
botryoidal forms ( like a bunch of grapes) shown in
Fig. 3 being especially characteristic of the mineral
228 THE WORLD'S MINERALS
when taken in conjunction with the pale green color.
Prehnite is a mineral of secondary origin, occurring
in cavities of basic igneous rocks, such as basalt and
diabase, and often in association with the zeolites,
with which, indeed, it is sometimes classified. Fine
specimens are found in the ancient volcanic rocks at
several places in the neighborhood of Edinburgh and
Glasgow, and in those of Paterson, in New Jersey
(Fig. 3). The mineral is also found in the Lake
Superior copper-mines, and, together with axinite, at
Bourg d'Oisans, in the French Alps. Large masses
were long ago met with at Cradock, in Cape Colony,
the mineral having been first found there towards the
end of the eighteenth century by Colonel Prehn, the
governor of the Dutch colony of the Cape of Good
Hope, and it was consequently named in honor of
him.
TOURMALINE
(Plate 33, Figs. 1-3). Owing to isomorphous re-
placements, this mineral varies so widely in chemical
composition that we are really dealing with a group
of isomorphous minerals rather than a single min-
eral. In addition to silicon, aluminium, and boron,
it contains smaller and variable amounts of iron, mag-
nesium, lithium, sodium, water, fluorine, etc. It is,
therefore, theoretically possible to distinguish as
chemical varieties iron-tourmaline, magnesium-tour-
maline, and lithium-tourmaline; but such a classifi-
cation is of little practical value, since these differ-
ences in chemical composition are not associated with
TOURMALINE 229
any marked difference in physical character which
would enable us to distinguish readily between one
variety and another. To make a chemical analysis of
tourmaline involves a long and tedious series of op-
erations; and if only a small gem-stone were available
for examination the whole of the material might be
used up in arriving at a decision as to its nature. It
has, however, long been the custom to distinguish
differently colored gem-varieties of tourmaline by
special names: the red as rubellite, blue as indicolite,
green as Brazilian emerald, brown as dravite, color-
less as achroite, while black tourmaline is commonly
known as schorl.
Well-shaped crystals, belonging to the rhombo-
hedral system, are not uncommon, but often the min-
eral tends to form radiating aggregates of fine
needles. An interesting feature of the crystals is their
hemimorphic development, the two ends being pro-
vided with different crystal-forms. This is shown by
the long prismatic crystal in Fig. i, there being an
obtuse triangular pyramid at the top and a more
acute triangular pyramid at the lower end. The
shape of the cross-section of the prism is also a very
characteristic feature of the mineral. The prism is
sometimes simply hexagonal, as shown in Fig. 2, but
more often this hexagonal prism is combined with a
trigonal (three-faced) prism; in Fig. i the long,
narrow faces are those of the hexagonal prism, and
the wide face seen to the front is one of the three
faces of the trigonal prism. Now in most crystals of
tourmaline the prism faces are so deeply striated and
230 THE WORLD'S MINERALS
grooved parallel to their length (an indication of
this is seen in Fig. 2) that the nine (6 + 3) faces
round into one another, and we then have a cross-
section of the shape of a triangle, with curving con-
vex sides. This character alone is usually sufficient
to enable us to identify a mineral as tourmaline.
This hemimorphic or two-ended character of crys-
tals of tourmaline is an outward expression of a pe-
culiarity in the internal structure of the crystalline
material, there being a polarity in the arrangment of
the crystalline particles building up the crystal. As
a consequence of this, crystals of tourmaline behave
in a peculiar manner when they are subjected to
changes of temperature. If a crystal be warmed it
develops a charge of positive electricity at one of its
ends, and a charge of negative electricity at the other;
while if it be cooled these charges are reversed. The
presence of these electrical charges can be demon-
strated in a very pretty and conclusive manner by the
following simple experiment. A mixture of red-lead
and flowers of sulphur is dusted through a fine sieve
over a cooling crystal of tourmaline, when it will be
seen that one end of the crystal becomes red and the
other yellow. The reason of this is that the sulphur
is attracted to the positively charged end, and the
red-lead to the negatively charged end of the crystal.
This pyro-electric property of tourmaline was first
noticed by the Dutch in 1703. Some crystals of the
mineral which had been brought from Ceylon had
fallen amongst the hot ashes of a peat fire, and it was
found that the ashes adhered to the crystals. The
TOURMALINE 231
mineral was consequently called aschtrekker, mean-
ing, in Dutch, "ash-drawer." It is interesting to note
also that the name tourmaline dates from the same
period, being a corruption of the Cingalese word
turamali.
Another interesting property of tourmaline is one
depending on its very strong dichroism. One of the
two rays into which a single ray of light is split on
entering the crystal is almost entirely absorbed, es-
pecially in the darker colored crystals; a plate of
tourmaline, cut parallel to the prism edges, may
therefore be used for producing plane-polarized
light. Two such plates when placed in crossed po-
sition over one another almost completely cut out the
light; while one by itself or the two placed together
in parallel position are transparent. On this depends
the use of the little piece of apparatus known as tour-
maline tongs. In some crystals the absorption is so
great that a section cut parallel to the basal plane will
be quite opaque, while a much thicker section cut
parallel to the prism edges may transmit light. The
lapidary must take into account this absorptive action
of tourmaline, and he must cut a faceted stone in such
a direction from the crystal that the large table facet
at the front shall be parallel to the direction of the
prism edges.
As already mentioned, the color of tourmaline
[Varies widely; but we may also have a very wide
range of color in one and the same crystal, there often
being zones of color, either perpendicular (Fig. 2)
or parallel to the length of the prism. Some crystals
232 THE WORLD'S MINERALS
from the island of Elba, of a pale pink color, are
tipped with jet-black, and are consequently described
as "negro-heads." The luster of tourmaline is glassy,
and the crystals possess no cleavage. The specific
gravity varies from 3.0 to 3.2, and the hardness
(H. = 7^2 ) is rather greater than that of quartz.
Tourmaline occurs in granites, mica-schists,
gneisses, and crystalline limestones; and mixed with
quartz it sometimes forms large rock-masses. It is a
constant associate of cassiterite in veins of tin-ore,
black tourmaline being a very common mineral in
the Cornish tin-mines. Most of the gem-varieties
are found as crystals in cavities in pegmatite-veins
traversing granite, and they are obtained by quarry-
ing these rocks in California, Madagascar, and the
Ural Mountains. The black crystals embedded in
white quartz shown in Fig. i are from Horlberg, near
Bodenmais, in Bavaria; the crystal in Fig. 2 is from
Haddam, in Connecticut; and the radiating groups
of pink tourmaline (rubellite), embedded in pale
lilac-colored lepidolite (Fig. 3), from Pala, in San
Diego Co., California.
STAUROLITE
(Plate 33, Figs. 4 and 5). Like the name chiasto-
lite (p. 224), the name staurolite also means "cross-
stone." Here, however, instead of showing a colored
cross inside the crystals, the cross-shaped form is pro-
duced by the regular intergrowth of two crystals in
twinned position (Fig. 5). The crystals are ortho-
SILICATES.
Plate 33.
13, Tourmaline. 4, 5, Staurolite.
7 , I J f r.
THE MICA GROUP 233
rhombic, and are bounded by a rhombic prism and
the basal pinacoid. They are of a dark brown color,
and usually dull and opaque. Chemically, the mineral
is an extremely complex silicate of aluminium, iron,
etc. It is found as embedded crystals in mica-schist,
as shown in the specimen (Fig. 4) from Brittany.
The isolated twinned crystal (Fig. 5) is from Fannin
Co., Georgia.
THE MICA GROUP
The micas are not only of considerable importance
as rock-forming minerals, but they also find many
useful applications. As essential constituents of
igneous and crystalline rocks of many kinds they
have an extremely wide distribution. Granite, for
instance, is a rock composed of quartz, felspar, and
mica; and mica-schist is a foliated rock, consisting
of quartz and mica. When these rocks are broken
down by the action of weathering agents, and their
materials transported by running water to be de-
posited in lakes or seas and so form new rocks, the
mica, though broken up mechanically into fine scales,
resists any chemical actions, and it consequently en-
ters into the composition of the newly formed bedded
rocks. If we examine a sandstone with a magnifying
lens we shall see on the surface of the bedding planes,
along which the stone splits, numerous shining sil-
very scales of mica. In some sandstones these scales
of mica are very conspicuous; while in finer rocks,
such as clays, they are very minute.
234 THE WORLD'S MINERALS
The micas almost always crystallize as platy or
scale-like crystals, and when these have been free to
develop at their edges they always have a six-sided
outline (Plate 34, Figs, i and 3), with angles differ-
ing only very slightly from 1 20. To all appearances,
therefore, the crystals are hexagonal; but exact meas-
urements of the angles between the faces of perfectly
developed crystals, and an examination of their op-
tical characters, prove that, in reality, they belong to
the monoclinic system.
A very important feature of these crystals is the
possession of a perfect cleavage in one plane direction
parallel to their large, flat surface. In this direction
the crystals can be split up indefinitely into the thin-
nest of leaves, with perfectly smooth and bright
surfaces. At the mica-mines the large crystals are
called books, and these are split into leaves in pre-
paring the material for the market. Owing to the
presence of this highly perfect cleavage, the flat sur-
faces of mica display a pronounced pearly or silvery
luster, and also very frequently colored bands of the
same nature as Newton's rings. Being such a very
characteristic feature of mica, a cleavage of this high
degree of perfection is spoken of as a micaceous
cleavage, and as such is very often applied as
a descriptive term in connection with certain other
minerals.
The thin cleavage flakes split from a crystal of
mica can be readily bent, and when released they
spring back to their original position; that is, they
are both flexible and elastic. The material is not
THE MICA GROUP 235
hard (H. = 2>, about), and the smooth cleavage
surfaces can be scratched with the finger-nail, though
not so readily as can talc and gypsum. When a sheet
of mica is struck a smart blow with a sharp-pointed
instrument, a six-rayed star of fracture, called a per-
cussion figure, is produced.
In their chemical composition the micas present a
certain analogy to tourmaline; and it is an interesting
fact that, under certain conditions in the earth's crust,
tourmaline becomes altered to mica. The same chem-
ical elements are present, with the exception of boron.
The micas are essentially silicates of aluminium, to-
gether with water and alkali metals, and sometimes
magnesium, iron, and fluorine. As with tourmaline,
so here we may distinguish several chemical varieties
namely, potassium-mica, sodium-mica, lithium-
mica, magnesium-mica, and iron-mica. These differ-
ences in chemical composition (amongst the micas,
much more so than in tourmaline) are accompanied
by marked differences in physical characters, espe-
cially with respect to the optical properties; and it is
thus possible, and also advantageous, to distinguish
the different chemical varieties by special names.
The most important of these are mentioned below;
others are paragonite, or sodium-mica, which occurs
as very fine, snow-white scales in certain mica-schists
(for example, as the matrix of kyanite in Plate 31,
Fig. 4) ; and lepidomelane, or iron-mica, which is
jet-black in color.
236 THE WORLD'S MINERALS
MUSCOVITE
(Plate 34, Fig. i). This is the potassium-mica;
it is the most abundant, and at the same time the
chemically simplest, member of the group. Its com-
position is expressed by the formula HUKA^SiO^a.
The hydrogen here plays the part of a metal in the
silicate formula; it is expelled as water only when
the mineral is heated to a very high temperature.
The larger crystals are usually of a dark greyish
shade of color, but cleavage flakes are often quite
colorless and transparent. This transparency of the
cleavage sheets has a connection with the name mus-
covite, which is merely the mineralogical form of the
old and popular name Muscovy glass, clear sheets of
this mica having formerly been used instead of glass
for windows in Russia. A variety known as fuchsite
is bright green, this color being due to the presence
of chromium.
As small scales, muscovite is of abundant occur-
rence in all countries; but it is only in the pegmatite-
veins of certain localities that large crystals and sheets
are found. These are mined in India, the United
States, and Brazil. The crystal embedded in white
quartz shown in Fig. i is from the felspar-mines near
Kragero, in southern Norway.
Of the many useful applications to which mus-
covite as well as some other kinds of mica is put,
the following may be indicated: for the windows of
stoves and lanterns, the peep-holes of furnaces, and
PHLOGOPITE BIOTITE 237
the chimneys of incandescent gas-burners. When
used for these purposes the material is often popu-
larly called talc; but talc is quite a distinct mineral,
to be described presently. Powdered mica is used
for producing a spangled effect on toys, stage scenery,
wall-paper, etc., and as a lubricant. At the present
day, sheets of mica are extensively used in the con-
struction of electrical apparatus and machinery.
PHLOGOPITE
Phlogopite, or magnesium-mica, contains the ele-
ments magnesium and fluorine in addition to those
present in muscovite. It much resembles muscovite
in appearance, but is often of a yellowish or brownish
shade of color, and has a more pronounced silvery
appearance. It differs also in its mode of occurrence,
being usually found in crystalline limestones. The
mica extensively mined in Canada and Ceylon is of
this variety. It is used for the same purposes as is
muscovite.
BIOTITE
(Plate 34, Fig. 2). This differs from phlogopite
in containing some iron in addition to magnesium; it
is consequently darker in color, being deep brown or
black. Owing to this color it is of less commercial
value. It is a common constituent of some granites
and gneisses. In a kind of granite known as two mica
granite, the scales of light-colored mica are musco-
vite, and the dark-colored scales are biotite. Less
238 THE WORLD'S MINERALS
frequently it is found as large sheets; the cleavage
flake of irregular outline shown in Fig. 2 is from
Transbaikalia, in Siberia. The mineral takes its
name from the celebrated French physicist and as-
tronomer, J. B. Biot (1774-1862).
LEPIDOLITE
(Plate 34, Fig. 5). Lepidolite, or lithium-mica,
is of a characteristic lilac or peach-blossom color. It
usually occurs as compact scaly masses, to which the
cleavages give a spangled effect. Such material, es-
pecially that found near Rozena, in Moravia, is some-
times cut and polished for making small ornamental
boxes, etc. Lepidolite invariably occurs in associ-
ation with pink tourmaline (rubellite), and in Plate
33> Fig. 3, it is shown as the matrix of this mineral.
The specimen there represented is from Pala, in San
Diego Co., California, where lepidolite is extensively
mined. The mineral is used for the extraction of
salts of lithium, largely employed in the manufac-
ture of lithia-water.
ZlNNWALDITE
(Plate 34, Fig. 3). Like lepidolite, this mica also
contains lithium and fluorine, but in addition some
iron. In appearance it is not unlike biotite. It is
found as crystals never of large size in veins of
tin-ore; the specimen represented in Fig. 3 being
from the tin-mines at Zinnwald, in Bohemia.
SILICATES (Mica & Chlorite groups).
Plate 34.
1, Muscovite. 2, Biotite. 3, Zinnwaldite. 4, Clinochlore. 5, Lepidolite.
THE CHLORITE GROUP 239
THE CHLORITE GROUP
The chlorites, or hydro-micas, closely resemble the
micas in the form of their scaly crystals and their
micaceous cleavage. The cleavage is, however, not
quite so perfectly developed, and the cleavage flakes,
though flexible, are not elastic. These minerals also
differ chemically from the micas in containing more
water and no alkali metals. They are characteris-
tically of a green color, the name chlorite meaning,
in Greek, "green stone."
These minerals have usually been produced by the
alteration of other minerals, such as biotite, pyroxene,
etc., and the green color frequently shown by altered
igneous rocks is due to the presence of chlorites
amongst the products of alteration. They also occur
in some schistose rocks, and in chlorite-schist (in
Plate 38, Fig. 4, as the matrix of perovskite) as an
essential constituent. Many varieties are distin-
guished, but as a rule these are only found as com-
pact masses, with a fine, scaly structure. The only
distinctly crystallized varieties are clinochlore and
penninite.
CLINOCHLORE
(Plate 34, Fig. 4). This member of the chlorite
group is a hydrated silicate of aluminium and mag-
nesium, with variable amounts of iron. It is found
as green, platy crystals at West Chester, in Pennsyl-
vania (Fig. 4).
240 THE WORLD'S MINERALS
SERPENTINE
(Plate 35, Fig. i). The three minerals repre-
sented on Plate 35 are all hydrated silicates of
magnesium, but, their elements being combined in
different proportions, they are quite distinct chemical
compounds. Their respective formulae are:
Serpentine .........................
Meerschaum ....................... H4Mg 2 Si 3 Oio
Talc ..................... . ........ H,Mg,SuO,,
A small proportion of the magnesium may be re-
placed by an equivalent amount of iron, this being
the case more especially in serpentine, which usually
contains 2 or 3 per cent, of ferrous oxide. The water
contained in these minerals is expelled only at a red-
heat, so that it is present as water of constitution.
Serpentine never occurs as crystals, but, neverthe-
less, it varies considerably in its crystalline structure.
Usually it is quite massive and compact (Fig. i), but
sometimes it is lamellar or fibrous ; the lamellar va-
riety is known as antigorite, and the fibrous as chrys-
otlle. The mineral is usually green, of a lighter
or darker shade, but is sometimes yellowish or red.
A very characteristic feature is the irregular distri-
bution of the color, with mottling and veining like
that on the skin of a serpent hence the name serpen-
tine. Specimens of a good color, and with a certain
degree of translucency, are referred to as noble or
precious serpentine. Such material, and also the
SILICATES.
Plate 35.
1, Serpentine or Sepiolite. 2, Meerschaum. 3, Talc.
SERPENTINE 241
common variety, is extensively cut and polished for
ornamental purposes, being frequently used in quite
large slabs. In its hardness (H. about 3), and the de-
gree of polish it takes, it resembles marble.
As already mentioned (p. 219), rock-masses of
serpentine have originated by the alteration in the
earth's crust of rocks composed largely of the mineral
olivine. Large masses of serpentine-rock are met
with in many districts; for example, it forms the
larger part of the Lizard peninsula in Cornwall.
Few visitors return from Lizard Town without
bringing back some of the small ornamental objects
carved in serpentine. The well-known green Con-
nemara marble is a crystalline limestone containing
much admixed serpentine. The specimen represented
in Fig. i is from Reichenstein, in Prussian Silesia.
The fibrous variety, chrysotile, occurs as veins in
the massive serpentine-rock. The fibers extend from
wall to wall of the veins and lie close together. In
mass this material presents a very pretty, silky luster,
with a bright yellowish-green color ; but when rubbed
between the fingers it separates into white cottony
fibers. The veins vary from mere threads to three or
four inches across, and they form a network through
the rock. Serpentine is used for the same purposes as
amphibole-asbestos, and is extensively mined in Can-
ada. In the trade it is known simply as asbestos, but to
avoid confusion it is best referred to as serpentine-
asbestos, since the different asbestiform minerals
differ so widely in their chemical composition and
other essential characters.
242 THE WORLD'S MINERALS
MEERSCHAUM
(Plate 35, Fig. 2). The chemical formula of this
hydrated silicate of magnesium is given above on p.
240. It is a compact mineral, with a fine earthy
texture, and so porous and light that, when dry, it
floats on water. The name meerschaum, in fact,
means, in German, "sea-froth." A mineralogical
term sometimes employed is sepiolite, which com-
pares the porous nature of the mineral to that of a
cuttle-fish bone. Owing to its porosity, and conse-
quently the avidity with which it absorbs water, the
mineral adheres strongly to the tongue, even in quite
a painful manner (the waxed and prepared material
used for tobacco-pipes does not, of course, do this).
The material is quite soft, and can be indented by
the finger-nail.
Meerschaum occurs in association with serpentine-
rocks; but the material exported in large quantities
from Asia Minor is found as small nodules in clayey
deposits on the plain of Eski-Shehr, where it is dug
from numerous shallow pits. To prepare the ma-
terial for the market it is scraped free from dirt into
curious rounded forms, dried in the air, and the sur-
face waxed. Most of the material is exported through
Vienna, and used for the manufacture of tobacco-
pipes. Meerschaum of inferior quality forms ex-
tensive beds at Vallecas, in Spain, affording a light
but valuable building-stone.
TALC 243
TALC.
(Plate 35, Fig. 3). The third of these hydrated
magnesium silicates (see chemical formula on p. 240)
differs from the two preceding in being sometimes
crystallized ; never, however, as distinct crystals, but
only as foliated or lamellar masses. These possess
a perfect micaceous cleavage parallel to their sur-
faces, on which the luster is silvery and pearly. Such
platy and scaly masses have a certain resemblance
to mica, but the cleavage flakes are more easily bent,
and when released they do not spring back to their
original position; that is, they are flexible, but not
elastic like mica. Further, the material is greasy or
soapy to the touch ; and, being the softest of minerals
(hardness No. i on the scale), it is very easily
scratched with the finger-nail. The color is always
of a delicate shade of apple-green with a silvery ap-
pearance (Fig. 3).
More frequently, the mineral occurs as compact
masses with an earthy fracture, and a white or greyish
color. This variety is known as steatite, and also
popularly as soap-stone, French-chalk, and pot-stone,
the name talc being often restricted to the foliated
or micaceous variety. Steatite finds many useful ap-
plications. It is used by tailors for marking cloth;
for the jets of gas-burners; for furnace-linings, sink-
stones, etc. In the powdered form, it enters largely
into the composition of toilet-powders, and is em-
ployed for preserving rubber, in paper-making, as a
244 THE WORLD'S MINERALS
dry lubricant, and for adulterating soap. Being soft
and easily cut with a knife, it is a favorite material
for the grotesque figures carved by the Chinese.
Talc is a very common mineral; it occurs in a>
sociation with serpentine-rocks, and also as a constitu-
ent of some schistose rocks for example, talc-schist
(shown in Plate 26, Fig. 2, as the matrix of the
needles of actinolite). Extensive beds of steatite are
quarried at several places in the United States, but
the best qualities come from the north of Italy.
THE ZEOLITE GROUP
The several interesting minerals belonging to this
group are hydrated silicates of aluminium, with
either sodium or calcium, or both of these. They do
not form an isomorphous group, as do the minerals
of the groups so far considered, for they crystallize
in several different systems. Water is present in
relatively large amount (10 to 20 per cent.), and as
this is only loosely held by the mineral it is regarded
as water of crystallization. When heated before the
blowpipe these minerals readily fuse, and the whole
mass boils up violently, owing to the rapid expulsion
of the water. It is on this account that these minerals
receive the name zeolite, which means, in Greek,
"boiling stone."
The zeolites are minerals of secondary origin, and
as such occur in the steam-cavities of certain volcanic
rocks, more particularly those of the basalt family.
Their light color, contrasted with the dark color of
ILICATES (Zeolite group).
Plate 36.
1, Analcite. 2, Chabazite.
THE ZEOLITE GROUP 245
the rock in which they are imprisoned, makes them
conspicuous objects. The matrix of volcanic rock is
shown in all the pictures in Plates 36 and 37. Fur-
ther, they are, as a rule, beautifully crystallized with
brilliant and glassy crystal-faces, and often delicate
coloring. Some of the most attractive specimens in
collections of minerals are to be seen in the showy
groups of crystallized zeolites. Being of abundant
occurrence in the volcanic rocks of certain districts,
good specimens can be readily obtained by the ama-
teur collector. The best-known localities are in the
Middle Lowlands and the Western Isles of Scotland,
the north of Ireland, the Faroe Islands, Iceland, and
the Midland Mountains in the north of Bohemia;
and there are many other important localities.
Apart from the point of view of the collector, the
zeolites present many points of scientific interest. But
as far as economic uses go they are valueless, except
that a few massive varieties are used to a very limited
extent as second-rate gem-stones. Nevertheless, it
would appear that the zeolites play an important
part in the economy of nature. In a very minute state
of division they have been proved to be present in
soils; and it is probable that the soluble forms of the
alkalis necessary for the life of plants are here held
as constituents of zeolites. The formation of zeolites,
resulting from the decomposition of felspar and other
mineral particles in the soil, prevents the immediate
and total removal of the alkalis in a still more soluble
form.
Besides the four species of zeolites figured on the
246 THE WORLD'S MINERALS
accompanying plates (36 and 37), many others are
known to mineralogists. Harmotome is of interest in
containing barium; bre on the mineralogist's
scale, being rather greater than that of gypsum and
alabaster. Chemically, it consists of carbon, hydro-
gen, and oxygen in approximately the proportions
given by the formula CioHieO. As shown by the
action of various solvents (turpentine, alcohol, etc.),
it really consists of a mixture of resins; and, in addi-
tion, it contains an organic acid known as succinic
acid, the presence of which is an important character
of the true Baltic amber (succinite).
Being a bad conductor of electricity, amber be-
comes electrified when rubbed on cloth, and it will
then attract to itself small bits of paper and other
light objects. It is from the Greek name elektron for
amber that our word electricity is, on this account,
derived.
The uses of amber are well known. It is cut and
polished as beads and other personal ornaments, and
for the mouth-pieces of tobacco-pipes, cigar- and
cigarette-holders. Small fragments and chips when
heated under hydraulic pressure become welded to-
gether, and the pressed amber, or ambroid, so ob-
tained is put to the same purposes as the natural
pieces.
ASPHALTUM
(Plate 39, Fig. 2). Asphalt, bitumen, or mineral-
pitch is a jet-black substance, breaking with a smooth
256 THE WORLD'S MINERALS
and bright conchoidal fracture (Fig. 2). It varies,
however, considerably in its characters, and is really
an indefinite mixture of hydrocarbons, which are in
part oxygenated. The specimen represented in the
picture is from the famous pitch-lake on the island
of Trinidad. This lake, or rather cake, of asphaltum
has a circumference of one and a half miles, and a
depth of eighteen to seventy-eight feet. It is said
that formerly the material was liquid and warm to-
wards the center, but now it is solid throughout. A
viscous bitumen oozes from the ground round the
Dead Sea, while in the lake itself there occasionally
rise to the surface considerable masses of bituminous
matter. As this dries on exposure to the sun it sets
as asphaltum. This material was used by the ancients
for pitching their boats, and the Greek name Lake
Asphaltites, meaning "bituminous lake," for the
Dead Sea, is the origin of our name, asphaltum.
Asphaltum impregnates many rocks of sedimentary
origin; but it also sometimes occurs in small amounts
in igneous and crystalline rocks and in some mineral-
veins. In the latter case the substance can scarcely
be of organic origin. In some of the Derbyshire
lead-mines there is found a peculiar form of as-
phaltum known as elaterite, or elastic bitumen, which
is soft and elastic like india-rubber.
LIGNITE
(Plate 39, Fig. 3). This is merely fossil wood,
still plainly showing the woody structure (Fig. 3).
LIGNITE COAL 257
It has thus little claim to rank as a mineral in our
meaning of the term. The degree of the f ossilization
may, of course, vary between wide limits, for lignite
is only an intermediate stage in the formation of coal.
Extensive beds of lignite are found in strata of the
more recent (Tertiary) geological periods for ex-
ample, at Bovey Tracey, in Devonshire. The speci-
men represented in the picture is from Germany, in
which country lignite, or brown coal, is of consider-
able importance as a fuel.
In strata of Liassic age near Whitby, in Yorkshire,
there is found a still more fossilized variey, approach-
ing coal in character; this is black and compact,
though the exterior form of the masses is that of
tree-trunks. This is the material well known as jet,
which is extensively cut and polished for a variety
of small ornaments.
COAL
(Plate 40, Fig. i). It would be out of place here
to give an adequate account of the characters, occur-
rence, and origin of this important substance, on
which the industrial prosperity of the present age so
largely depends. It is found as beds or seams, vary-
ing in thickness from a fraction of an inch to thirty
feet or more, interbedded with sandstones and shales
in the rocks of various geological periods, but mainly
in those of the period to which the name carbon-
iferous is appropriately applied. At this remote
period of the world's history there were extensive
swamps, with luxurious growths of vegetation vege-
258 THE WORLD'S MINERALS
tation of quite a different character from that of the
present day. Thick accumulations of vegetable mat-
ter were formed in much the same way that peat
forms at the present day; and these became covered
by the sedimentation of clay and sand, and so pre-
served. With a slow and continual sinking of the
area, owing to movements of the earth's crust, a great
thickness of alternate layers of sand, clay, and vege-
table remains was piled up. Under this enormous
pressure, and with various chemical reactions in the
presence of water, acting for millions of years, the
sands have become converted into sandstones, the
clays into shales, and the vegetable remains into coal.
Trunks and roots of fossil trees are frequently found
in the rocks of the coal-measures; but in the coal
itself the material has been so altered by fossilization
that little organized structure remains, and, as a rule,
can only be detected when thin sections of the ma-
terial are examined under the microscope.
The reader will no doubt have noticed that some
kinds of coal break into small cube-like blocks, with
smooth and bright surfaces. This fracture is a result
of jointing in the material, and is altogether distinct
from the cleavage of crystals, since, although the
blocks are sometimes quite small, the sub-division
cannot be repeated indefinitely.
Often, also, it may be noticed that such surfaces
have a thin, brassy coating, looking like gold. This
is the mineral iron-pyrites, which is sometimes pres-
ent as small nodules, causing the coal to detonate and
fly about when placed in the fire.
ORGANIC SUBSTANCES.
Plate 40.
1, Coal. 2, Anthracite,
ANTHRACITE 259
ANTHRACITE
(Plate 40, Fig. 2). This is a variety of coal in
which the process of fossilization has proceeded still
further, and no trace of organized structure remains.
The volatile material is much reduced in amount,
and the material consists largely (up to 95 per cent.)
of pure carbon. Anthracite is brittle, and breaks
with a smooth and bright conchoidal fracture very
like pitch, as may be seen from a comparison of the
two pictures on Plates 39 and 40. The surfaces of
anthracite sometimes display a brilliant iridescent
tarnish. The chief sources of supply are the coal-
fields of South Wales and Pennsylvania.
INDEX
Abrasives, 52.
Achroite, 229.
Acicular crystals, 26.
Actinolite, 198.
Acute pyramid, 19.
rhombohedron, 24.
Adamantine luster, 38, 56.
Adamas, 55.
Adularia, 190.
Agate, 117.
Aggregation of crystals, 29.
Alabaster, 165.
Albite, 192.
twin-law, 187.
Alluvial deposits, 57, 73, 130.
Almandine, 213, 215.
Alum crystals, 6, 16.
Aluminium, 126, 127.
oxide, 125.
phosphates, 177, 178, 181.
silicates, 223, &c.
Amalgam, 73.
Amazon-stone, 192.
Amber, 2, 253.
Ambroid, 255.
Amethyst, 115.
oriental (corundum), 35.
Amorphous minerals, 32.
Amphibole group, 195.
Amphibole-asbestos, 199.
Analcite, 246.
Analysis, chemical, 48.
Anatase, 49.
Andalusite, 223.
Andradite, 213, 216.
Angles of crystals, 15, 20, 113.
261
Angles, re-entrant, 28.
Anglesite, 161.
Annabergite, 177, 97.
Anorthic system, 8, 23.
Anorthite, 195.
Anthracite, 259.
Antigorite, 240.
Antimonite, 79.
Antimony-glance, 79.
, native, 68.
ores, 52.
sulphide, 78.
Apatite, 171.
Apophyllite, 246.
Aquamarine, 211.
Aragonite, 146.
Arizona " ruby " (garnet), 216.
Arsenates, 170.
Arsenic, native, 58.
ores, 52.
sulphides, 80, 81.
, white, 90.
Arsenical pyrites, 89.
Arsenides, 78.
Arsenopyrite, 89.
Artificial corundum (ruby, &c.),
128.
diamond, 60.
Asbestiform minerals, 241.
Asbestos, 1 08, 202, 241.
Aschtrekker, 231.
Asphalt, 255.
Asphaltum, 255.
Atacamite, 108.
Augite, 204.
Augite-syenite, 190.
262
INDEX
Auripigmentum, 81.
Australian opal, 120.
Autunite, 179.
Avanturine felspar, 193.
Axes, crystallographic, 9.
of reference, 9.
of symmetry, u.
, twin-, 28.
Axinite, 227.
Azurite, 153.
Balas-ruby, 125.
Ball- jasper, 117.
Baltic amber, 253.
Banket, 74.
Barium sulphate, 157.
Barytes, 157.
Basal pinacoid, 20, 25.
plane, 20.
Basalt, 3.
Batea, 57.
Bauxite, 109.
Beryl, 211.
Biotite, 237.
Bipyramid, 19, 21.
Bismuth, native, 69.
Bismuthinite, 70.
Bitumen, 255.
Black-band iron-stone, 145.
Black-jack, 84.
Black lead, 64.
Blende, 83.
Bloodstone, 117, 122.
Blue asbestos, 202.
ground, 53.
Blue-iron-earth, 181.
Blue-John, 104, 105.
Bog-iron-ore, 134.
Bohemian garnet, 216.
Bone amber, 254.
Books of mica, 234.
Bort, 61.
Botryoidal forms, 31, 227.
Brachy-dome, 160.
Brazilian emerald (tourmaline),
229.
pebble, 118.
Brewsterite, 246.
Brilliant form of cutting, 60.
Brittleness, 55.
Broggerite, 133.
Bromoform, 40.
Bronzite, 205.
Brookite, 49.
Brown coal, 257.
Brown-iron-ore, 134.
Brown-lead-ore, 174.
Bruting, 60.
Burnt topaz, 221.
Cabochon, 215.
Cairngorm, 115.
Calamine, 143.
Calcite, 139.
Calcium carbonates, 139, 146.
fluoride, 102.
phosphate, 171.
silicate, 206.
sulphate, 162.
titanate, 250.
tungstate, 170.
Calcouranite, 179.
Calc-spar, 139.
Californite, 220.
Cameos, 118.
Campylite, 175.
Cannon-spar, 140.
Cape ruby (garnet), 216.
Capillary crystals, 26.
Carat, 61, 73-
Carbon, 53.
compounds, 252.
Carbonado, 61.
Carbonates, 138.
Carbonic acid, 138.
Carborundum, 127.
Carbuncle, 215.
Carnelian, 117.
INDEX
263
Cassiterite, 129.
Cat's-eye, 115.
Celestite, 160.
Celts, 202.
Center of symmetry, 12, 23.
Cerargyrite, 109.
Cerusite, 150.
Chabazite, 246.
Chalcedony, 117.
Chalcopyrite, 95.
Chalcotrichite, 136.
Chalk, 139.
Chalybite, 144.
Characters of minerals, 5.
Chatoyancy, 116.
Chemical analysis, 48.
characters, 46.
compounds, 47.
elements, I, 46, 53.
formula, 48.
Chessylite, 152.
Chiastolite, 224.
China-clay, 189.
Chloanthite, 97.
Chlor-apatite, 171.
Chlorides, 101.
Chlorite group, 239.
Chromates, 156.
Chrome-iron-ore, 125.
Chromite, 125.
Chrysolite, 219.
Chrysoprase, 117.
Chrysotile, 240.
Cinnabar, 88.
Cinnamon-stone, 215.
Citrine, 115.
Classification of minerals, 50, 185.
Clay, 189.
Clay-iron-stone, 145.
Cleavage, 41.
Cleopatra's emerald mines, 212.
Cleveite, 133.
Clinochlore, 239.
Clino-pinacoid, 164.
Coal, 257.
Cobalt arsenide, 97.
arsenate, 176.
ores, 52.
Cobalt-bloom, 97, 176.
Cobalt-blue, 98.
Cockscomb-pyrites, 91.
Cohesion of Crystals, 42.
Colloids, 32.
Color, range of, 35.
, zones of, 231.
Color-varieties, 35.
Coloring matters, 35.
Colors of corundum, 126.
of fluor-spar, 103.
of gem-stones, 126.
of minerals, 34.
, interference-, 37, 120, 194.
, iridescent, 37, 96.
Columbite, 251.
Columnar crystals, 25.
structure, 30.
Combination of forms, 14, 20.
Common felspar, 189.
hornblende, 196.
opal, 120.
Compact aggregates, 30, 116.
Concentric shelly structure, 31.
Conchoidal fracture, 43.
Connemara marble, 241.
Constancy of angles, 15.
Copal-resin, 253.
Copper carbonates, 152, 154.
, native, 75.
ores, 51.
oxide, 136.
oxychloride, 108.
sulph-antimonite, 98.
sulphide, 95.
uranium phosphate, 179.
Copper-nickel, 87.
Copper-pyrites, 95.
Coral, 2.
Coxalloidal forms, 31.
264
INDEX
Cordierite, 37.
Cornelian = carnelian, 1 17.
Corundum, 35, 126.
Crocidolite, 199, 201.
(quartz), 116.
Crocoite, 166.
Cross-hatched structure, 191.
Cross-stone, 224, 232.
Cryolite, 109.
Crypto-crystalline, III, 116.
Crystal balls, 119.
models, 10.
systems, 7.
Crystalline grains, 29.
individuals, 29.
Crystallization, 6, 29.
, water of, 244.
Crystallographic axes, 8.
Crystallography, 7.
Crystals, 6.
, aggregation of, 29.
, growth of, 6.
, habit of, 26.
, intergrowths of, 27.
, malformation of, 16.
, shapes of, 7.
Cube, 9, 18.
Cubic cleavage, 42.
habit, 27.
system, 8, 9, 19.
Cubo-octahedron, 15.
Cullinan diamond, 62.
Cuprite, 136.
Cuprouranite, 178.
Cutting of gem-stones, 60.
Cyanide process, 74.
Cyanite, 225.
Danaite, 90.
Demantoid, 217.
Dendritic forms, 31, 118.
Density, 38.
Destruction of crystals, 32. .
Diamond, 53.
Diatomite, 121.
Dichroism, 37, 231.
Dichroite, 37.
Dichroscope, 37.
Dimorphism, 49, 91, 146, 191, 210,
231.
Diopside, 202.
Directional characters, 34.
Dispersive power, 56.
Distortion of crystals, 16.
Dog-tooth-spar, 140.
Dolomite, 143.
Dome faces, 90.
Double refraction, 142.
Doubly-refracting spar, 142.
Dravite, 229.
Drusy surfaces, 114.
Dunite, 218.
Dyad axis, 12.
Dyes of minerals, 35.
Earthy odor, 45.
Edges, parallelism of, 17.
Eisenkiesel, 36.
Elastic bitumen, 256.
Elaterite, 256.
Electrical properties, 44.
Elektron, 255.
Elements, chemical, 46, 53.
Elie " ruby " (garnet), 216.
Emerald, 211.
, Brazilian (tourmaline), 229.
, oriental (corundum), 35.
, Uralian (garnet), 217.
Emery, 127.
En cabochon, 215.
Enstatite, 205.
Epidote, 226.
Erythrite, 175, 97.
Essonite = hessonite, 215.
Excelsior diamond, 62.
External characters, 50.
Faces of crystals, 9.
INDEX
265
Faces, symbols of, 13.
Faceted gems, 60.
Fahlerz, 98.
Fahlore, 98.
False lead, 84.
topaz, 115.
Fayalite, 218.
Felspar group, 186.
Felt-like aggregates, 30.
Fertilizers, 170.
Fibers, crystal, 30.
Fibrous structure, 30.
Fire of diamond, 56.
First water diamond, 57.
Flint, no.
Florentine diamond, 61.
Flos-ferri, 148.
Flowers of sulphur, 67.
Fluor, 101.
Fluor-apatite, 171.
Fluorescence, 104.
Fluorides, 101.
Fluorite, 101.
Fluor-spar, 101.
Flux, 1 02.
Forms, crystal, 14.
of aggregation, 29.
of cutting for gems, 60.
of minerals, 6.
Formula, chemical, 48.
Forsterite, 218.
Fossil resin, 253.
wood, 256.
Fracture, 41.
Frangibility, 55.
French-chalk, 243.
Fuchsite, 236.
Gahnite, 125.
Galena, 86.
Garnet group, 213.
Garnetohedron, 214.
Gaseous elements, 47, 53.
Gem-stones, 37, 41, 52.
Gems, famous diamonds, 61.
Geology, i.
Geometrical regularity, 16.
solids, 6.
Geyserite, 121.
Glassy felspar, 190.
luster, 38.
minerals, 32.
Glazier's diamond, 61.
Globular forms, 31.
Gold, native, 72.
quartz, 74.
tellurides, 74.
Gooseberry-stone, 218.
Granite, 3, 189.
Granular habit, 214.
structure, 29.
Graphite, 63.
Greasy feel, 45.
luster, 38.
Green-lead-ore, 174.
Greenstone, New Zealand, 201.
Grey-copper-ore, 98.
Grossularite, 213, 218.
Growth of crystals, 6, 16, 27, 32.
Gypsum, 162.
Habit of crystals, 26.
Hackly fracture, 43.
Haematite, 121.
Hair-copper, 136.
Hair-like crystals, 26.
Hair-pyrites, 26.
Halite, 106.
Halogens, 101.
Haloids, 101.
Hardness, 43.
of diamond, 55, 61.
of kyanite, 225.
Harmotome, 246.
Heaviness, 38.
Heavy liquids, 39.
spar, 158.
Heliotrope, 117.
266
INDEX
Hematite = haematite, 121.
Hemimorphic crystals, 229.
Hemi-pyramid, 23.
Hemispherical forms, 31.
Hessonite, 215.
Hexagonal prism, 25.
system, 8, 25.
Hiddenite, 207.
Hornblende, 196.
Horn-silver, 109.
Hornstone, 117.
Hungarian opal, 121.
Hyacinth, 131.
Hyalite, 119.
Hydrocarbons, 252.
Hydro-micas, 239.
Hypersthene, 205.
Iceland-spar, 141.
Ice-spar, 109, 190.
Icositetrahedron, 14, 18, 209.
Idocrase, 219.
Ilmenite, 123.
Index of refraction, 56.
Indian diamonds, 61.
Indicolite, 229.
Inflammables, 52.
Infusorial earth, 121.
Inorganic bodies, i, 46.
Intercepts on axes, 13.
Interference-colors, 37, 120, 194.
Intergrowths of crystals, 27.
Iridescent colors, 37, 96.
Iron carbonate, 144.
hydroxide, 134.
, native, 76.
niobate, 251.
ores, 36, 51.
oxides, 121, 123.
phosphate, 180.
silicates, 202, 205, 218, &c.
sulph-arsenide, 89.
sulphides, 91, 92, 94.
tungstate, 167.
Iron-glance, 121.
Iron-mica, 235.
Iron-pyrites, 92, 32, 48.
in coal, 258.
Irregular intergrowths, 27.
Irregularity of crystals, 15.
Isodimorphism, 151.
Isomorphism, 49.
Isomorphous series, 143, 185, 213,
&c.
Jade, 200.
Jadeite, 206.
Jargoon, 131.
Jasper, 116.
Jet, 257-
Jeweler's rouge, 123.
Kaolin, 189.
Kauri-gum, 253.
Kidney-iron-ore, 122.
Kidney-stone, 201.
Kieselguhr, 121.
Kinds of minerals, 3.
King's yellow, 81.
Knee-shaped twins, 28.
Koh-i-noor diamond, 62.
Krystallos, 115.
Kunzite, 207.
Kupfernickel, 87.
Kyanite, 225.
Labradorite, 193.
Labrador-spar, 193.
Lake-ore, 134.
Lamellar twinning, 188.
Lapis-lazuli, 208.
nephriticus, 201.
Laumontite, 246.
Lazulite, 178.
Lazurite, 208.
Lead arsenate, 173.
carbonate, 150.
chromate, 166.
INDEX
267
Lead molybdate, 168.
ores, 51.
phosphate, 174.
sulphate, 161.
sulphide, 86.
vanadate, 175.
Leaf-gold, 72.
Leafy forms, 31.
Left-handed crystals, 112.
Lepidolite, 238.
Lepidomelane, 235.
Leucite, 209.
Light and crystals, 34, 38.
, polarized, 231.
Lignite, 256.
Lime-felspar, 186, 195.
Limestone, 138, 208.
Limonite, 32, 133.
Linarite, 166.
Liquids, heavy, 39.
Lithium-mica, 238.
Lodestone, 124.
Luster, 37.
Macro-axis, 157.
Macro-dome, 157.
Magnesite, 143.
Magnesium carbonate, 143.
silicates, 205, 218, 240, &c.
Magnesium-mica, 237.
Magnetic iron-ore, 123.
properties, 44, 124.
pyrites, 94.
Magnetite, 123.
Magnets, natural, 124.
Malachite, 154.
Malformation of crystals, 16.
Mamillary forms, 31.
Manganese carbonate, 145. i
ores, 51.
oxides, 134. >
silicate, 207. *
tungstate, 167. *
Manganese-spar, 145.
Manganite, 134. '
Marcasite, 91.
Massive minerals, 29.
Matter, kinds of, 5.
Meerschaum, 242.
Mercury, native, 88.
sulphide, 88.
Metallic elements, 47, 54.
luster, 38.
ores, 51.
Metals, 42, 47.
Meta-silicic acid, 184.
Meteoric iron, 77.
Meteorites, 219.
diamond in, 60.
Methylene iodide, 40.
Mica group, 233.
Mica-schist, 234.
Micaceous cleavage, 42, 234.
iron-ore, 122.
Microcline, 187, 191.
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