ON THE FORMS AND PROPERTIES OF MINERALS
Rocks
All we know of the earth by direct observation is confined to less than
four miles depth; though by projecting downward the layers of rock that
come to the surface, we may fairly assume a knowledge of the structure
down to six or eight miles depth. This outer portion is often referred
to as the “crust of the earth,” but the idea that the deeper portions
are molten is no longer held. This outer portion is made of rocks, and a
rock may be defined as, _a mass of material, loose or solid, which makes
up an integral part of the earth_, as granite, limestone, or sand. The
rocks (except glassy igneous ones) are aggregates of one or more
minerals; either in their original form like the quartz, feldspar and
mica of granite, or in a secondary grouping, resulting from the units
having been dislodged from their primary position and regrouped a second
time, as in sandstone or clay.
Minerals
Since the rocks are aggregates of minerals, it is best to take up the
minerals first. A mineral may be defined as _a natural inorganic
substance of definite chemical composition_. It is usually solid,
generally has crystalline structure, and may or may not be bounded by
crystal faces. _A crystal is a mineral, bounded by symmetrically grouped
faces, which have definite relationships to a set of imaginary lines
called axes._ There are between 1100 and 1200 minerals, of which 30 are
so frequently present, and so dominant in making up the rocks, that they
are termed _rock-forming minerals_. About 150 more occur frequently
enough so that they can be termed common minerals, and one may expect to
find a fairly large proportion of them. Some of these are abundant in
one part of the country and rare in others, but this book is written to
cover the United States, and so all those which have a fair abundance
are included, though some will only be found in the west and others
mostly in the east. Then there are some more minerals which are really
rare, but which are cherished because of their beauty of color, and are
used as gems. These are mentioned, and many of the gems are simply clear
and beautiful examples of minerals, which in dark or cloudy forms are
much more common. If one finds any of these rare minerals which are not
mentioned in this book, he must turn to one of the larger mineralogies
mentioned in the literature list to determine them.
Crystal Structure
A crystal is a mass of molecules, all of the same composition. A
molecule in its turn is made up of atoms, and each atom is a unit mass
of an element. Thus the calcite molecule is made up of one unit or atom
of calcium, one of carbon, and three of oxygen (CaCO₃). These atoms are
held together by an attraction, and make a molecule, and for the study
of minerals the molecule is the unit. The mineral, calcite, is a mass of
molecules all like the one above, and each molecule so small as to be
invisible even with the aid of the most powerful microscope. When
calcite is in crystal form, the molecules, like ranks of soldiers, are
arranged each in its place, each at a definite distance from the other.
While each molecule may vibrate or wiggle within certain limits it does
not leave its place. (The comparison with soldiers is a good one for the
molecules of one layer, but it must be remembered that in a crystal
there are also like spacings and ranks up and down as well.) As long as
the molecules remain in fixed ranks, up and down, forward and back, and
sideways, the crystal is perfect. Calcite may be heated until it melts
and becomes liquid. Then the molecules leave their definite arrangement
and move about in all sorts of directions, like the soldiers after ranks
have broken. So long as the molecules are thus free to move about but
keep together, the substance is a liquid. There are cases when the
molecules in this disorder take fixed positions without falling into
ranks. Such minerals are non-crystalline and usually appear glassy. If
still greater heat is applied to the mineral in liquid form, a point is
reached (the vapor point), above which the molecules go flying away from
each (like soldiers in a panic), each seeking to get as far from the
other as possible, so only a container will prevent their dissipation.
When in this condition a mineral is gaseous. When cooled, the reverse
order obtains. The molecules of gas gather into a miscellaneous mob or
liquid: and if this is further cooled (but not too suddenly), they fall
into ranks and make a crystal. This may be illustrated with water. When
above 212° F. it is steam (molecules wildly dissipated); when between
212° and 32° it is water (molecules close to each other, but milling
like a herd of cattle); and when below 32° it is ice, the molecules
ranged in perfect order, rank on rank.
Crystal Systems
With all the possible forms that crystals can and do take, there are six
systems of arrangement. First there is the case where ranks, files, and
vertical rows are all equal, and now to be scientific, instead of
talking about ranks, files, etc., we use the term axes to express these
ideas; the files or arrangements from front to back, being called the _a
axis_, the ranks, or side to side arrangement the _b axis_, and the
vertical arrangement the _c axis_. (See Plate 1.) These axes are
imaginary lines, but they represent real forces.
Isometric system
When the axes are all equal and at right angles to each other, a crystal
is said to be in the isometric system. The cube is the basal form and
each side is known as a face. The ends of the axes come to the middle of
the cube faces. The essential feature of this system is that whatever
happens to one axis must happen to all, which is another way of saying
that all the axes are equal. If we think of the cube as having the
corners cut off, we would have a new face on each of the eight corners,
in addition to the six cube faces. Then if each of these new faces were
enlarged until they met and obliterated the cube faces, an eight-sided
figure, the octahedron, would result. In this the axes would ran to the
corners. Another modification of the cube would be to bevel each of its
twelve edges, making twelve new faces in addition to the six cube faces.
If we think of these new faces being developed until they meet and
obliterate the cube faces, there will result a twelve-sided figure, the
dodecahedron. And the 24 edges of the dodecahedron could be beveled to
make a 24-sided figure, and so on. Of course in Nature the corners are
not cut, nor the edges beveled, but as a result of the interaction of
the forces expressed by the axes and the distribution of the molecules,
the molecules arrange themselves in a cube, octahedron, dodecahedron or
combination of these basal forms.
Crystal formation
Crystals are formed in liquids as they cool or evaporate and can no
longer hold the minerals in solution. Crystals start about a center or
nucleus, and molecule by molecule, the orderly arrangement is increased
and the crystal grows, there being no size which is characteristic. If
free in the liquid the crystal grows perfectly on all sides, but if
crystals are growing side by side, there comes a time when they
interfere with each other. Then the free faces continue to grow and the
orderly internal arrangement is maintained, though externally there is
interference.
Tetragonal system
In the second or tetragonal system one axis (the c axis) is different
from the other two, but all three are still at right angles with each
other. This is saying scientifically that the lines of force are greater
or less in one direction than in the other two, but they act at right
angles to each other. The a and the b axes are equal and anything that
happens to one of these two must happen to the other, but need not
happen to the c axis. Thinking of the molecules that arrange themselves
under this system of forces, it is clear that the simplest form will be
a square prism, _i.e._, front to back, and from side to side the numbers
of molecules will be equal, but up and down there will be a greater or
lesser number. If the eight corners of this prism were cut, and these
corner faces increased in size until they met, the resulting octahedron
would be longer (or shorter) from top to bottom than from side to side
or front to back, but the measurement from front to back would be equal
to the one from side to side. In this system we may have the vertical
edges of the prism beveled, and not have to bevel the horizontal ones,
or we may bevel the horizontal edges and not the vertical ones. There is
no dodecahedron in this system or in any other system than the
isometric. The forms in this tetrahedral system are really a combination
of the four sides of the square prism with such modifications as equally
affect them all, with two ends which may be flat, or pyramidal, or
modified pyramidal faces.
Orthorhombic system
The third system has all three axes unequal, but all three are still at
right angles with each other. This is saying that the lines of force in
the crystals are all at right angles to each other but of unequal value.
The faces in this case are all in pairs. What happens at one end of an
axis must happen at the opposite end, but does not need to happen at the
ends of any of the other axes. We are dealing with pairs of faces (one
at either end of an axis), and if three such pairs are combined in the
simplest manner, the resulting figure will be a rectangular prism. If we
cut the eight corners of this prism and enlarge the faces until they
meet, the result is an octahedron, in which the distance from top to
bottom, from side to side, or from front to back is not the same in any
two cases. (See Plate 2.) In this system if a face is made by beveling
one edge of the prism there must be a corresponding face on the edge
diagonally opposite, but there does not have to be one on any of the
other edges. However if a corner is cut, that face affects all the axes
and so all the corners must be cut. A great many crystals occur in this
system, and some of them which are prismatic in shape may give trouble,
for it is not uncommon for the vertical edges of the prism to be so
beveled, that two of the original prism faces are obliterated, and the
two remaining faces added to the four new faces make a six-sided prism,
which at first glance seems to belong to the hexagonal system. (See
Plate 3, fig. 3.) Close examination however will show that, instead of
all the prism faces being alike, as would be necessary for the hexagonal
system, they are really in pairs, and one pair at least will be
distinguished in some way, such as being striated, pitted, or duller.
Monoclinic system
The fourth system has all the axes unequal, the a axis and the b axis at
right angles to each other, but the c axis is inclined to the a axis,
meeting it at some other than a right angle. The monoclinic system is
like the orthorhombic system except that it leans, or is askew, in one
direction. The result is that the faces at the ends of the b axis are
rhombohedral, while the others are rectangular. As in the foregoing
system, the faces are in pairs at opposite ends of the axes; and as in
the orthorhombic system, a face may occur on one edge and only have to
be repeated on the edge diagonally opposite. The simplest form in this
system will be made by combining the three pairs of faces at the
opposite ends of the axes, which gives a prism, which is rectangular in
cross section, but leans backward (or forward) if placed on end. As in
all the systems, if a corner is cut, all must be cut; and if these
corner faces are extended to meet each other, an octahedron results, in
which, as in the prism, no two axes are equal. If this octahedron is
properly orientated (_i.e._ with the a and b axes horizontal), it will
lean forward or backward. Many minerals belong to this system; and, as
in the orthorhombic system, it is not uncommon to have the vertical
edges so beveled that two of the prism faces are obliterated, and the
remaining two prism faces with the four new faces make a six-sided
prism, which seems hexagonal. (See plate 3, figure 3.) However, such a
pseudo-hexagonal prism may be recognized by at least one pair of the
faces having distinguishing marks (striæ, pits, or dullness), instead of
all being just alike.
Triclinic system
The fifth or triclinic system has all the axes unequal, and no two of
them intersect at right angles. As in the two preceding systems the
faces occur in pairs at the opposite ends of the axes. This is the most
difficult system in which to orientate a crystal, but fortunately only a
few crystals occur in this system, such as the feldspars.
Hexagonal system
Lastly there is a group of crystals which have four axes, one vertical,
and three in the horizontal plane which intersect each other at angles
of 60°, all these three being equal to each other, but different from
the vertical axis. The simplest form in this system is the six-sided
prism. If one corner of this prism is cut all must be, and if these
corner faces are extended to meet each other, a double-six-sided pyramid
results. In this system if one of the vertical edges of the prism is
beveled, all must be, but the horizontal edges need not be; or the
horizontal edges may be beveled and the vertical ones not. The ends as
they are related to the c axis may be developed independently of the
prism, and so the prism may be simply truncated by a flat end, or have
pyramids on either end.
Hemihedral forms
In this system it is quite common to have forms which result from the
development of each alternate face of either the prism or the double
pyramid. In the case of the prism, if every alternate face is developed
(and the others omitted) a three-sided prism results, as in tourmaline.
In the case of the double pyramid if the three alternate faces above are
united with the three alternate faces below, a six-sided figure is
formed, which is known as the rhombohedron, as all the faces are
rhombohedral in out-line and all equal. These forms in which only half
the faces are developed are known as hemihedral forms. The same sort of
thing may happen in the isometric system in the case of the octahedron,
and also in the case of the octahedron of other systems. When half the
faces of the octahedron are developed, two above unite with two below
and make a four-sided figure, known as a tetrahedron. (See plate 10.)
While tetrahedrons may occur in any of the first five systems they are
not common outside the isometric system.
Twinning
Another modification of the simple forms which will be met occasionally
is twinning. By this is meant two crystals growing together as though
placed side by side on some one of the faces, and then revolved until
the two axes which would normally be parallel are at some definite angle
with each other, 60°, or 180° which is commoner. The surface of contact
between the two crystals is called the _composition face_, and as no
more material can be added on that face the crystals continue to grow
developing the other faces, and we find faces in contact with each other
which should be at the opposite end or other side of the crystals. This
contact of faces which should not come in contact, and the presence of
reentrant angles are indications of twinning. In some minerals the
twinning may be repeated time and again, and if the twinning is on one
of the end faces a branching structure results, as in frost and snow
crystals, or the multiple twinning may be of crystals growing side by
side when the final form will approximate a series of thin sheets placed
side by side as in some feldspars. The peculiar forms characteristic of
individual minerals are taken up under the respective minerals.
Other important properties of minerals are hardness, cleavage, specific
gravity, streak, luster, and color.
Hardness
Hardness may be defined as the mineral’s resistance to abrasion or
scratching. It is measured by comparing a mineral with Moh’s scale, a
set of ten minerals arranged in the order of increasing hardness, as
follows:
1 talc
2 gypsum
3 calcite
4 fluorite
5 apatite
6 feldspar
7 quartz
8 topaz
9 corundum
10 diamond
A set for measuring hardness may be purchased from any dealer in mineral
supplies. For rough determination, as in the field, the following
objects have the hardness indicated; the finger nail 2¼, a penny 3, a
knife blade about 5.5, and glass not over 6. In testing, a mineral is
harder than the one it will scratch, and softer than the one by which it
is scratched. For instance, if a mineral will scratch calcite and is
scratched by fluorite, it is between 3 and 4 in hardness, say 3.5. When
two samples mutually scratch each other they are of equal hardness. Care
must be used in determining hardness, especially with the harder
minerals; for often, when testing a mineral, the softer one will leave a
streak of powder on the harder one, which is not a scratch. One should
always rub the mark to make sure it is really a groove made by
scratching.
Cleavage
Cleavage is the tendency, characteristic of most minerals, and due to
the arrangement of their molecules, to cleave or break along definite
planes. The cleavage of any mineral is not irregular or indefinite, but
characteristic for each mineral, and always parallel to possible or
actual faces on the crystal, and always so described. For instance
galena has three cleavages, all equally good, and parallel to the cube
faces; so it is said to have cubic cleavage. In the same way fluorite
has octahedral cleavage, and calcite rhombic cleavage. In some minerals
cleavage is well developed in one plane, and less developed in other
planes, or it may be lacking altogether. The varying degrees of
perfection by which a mineral cleaves are expressed as, perfect or
imperfect, distinct or indistinct, good or poor, etc.
Specific gravity
The specific gravity of a mineral is its weight compared with the weight
of an equal volume of water, and is therefore the expression of how many
times as heavy as water the mineral is. For instance the specific
gravity of pyrite is 5.1, which is saying it is 5.1 times as heavy as
water. In a pure mineral the specific gravity is constant, and an
important factor in making final determinations. As ordinarily obtained,
a piece of pure mineral is weighed in air, which value may be called x.
It is then immersed in water and again weighed, and this value is called
y. The difference between the weight in air and that in water is the
weight of an equal volume of water. Then we have the following formula:
specific gravity = (x)/(x-y).
Various balances have been devised for making these measurements, but
any balance which will weigh small objects accurately, may be adapted to
specific gravity work, by hanging a small pan under the regular weighing
pan. When using this balance, care is taken to see that the lower pan is
always submerged in water, even while the mineral is being weighed in
air, so that when weighed in water in the lower pan, the weight of this
lower pan has already been considered.
Streak
By streak is meant the color of the mineral when powdered. For some
minerals, especially metallic ores, it is of great importance, for it
remains constant, though the color of the surface of the mineral changes
materially. It is most readily determined by rubbing a corner of the
mineral on a piece of unglazed porcelain. Small plates, known as “streak
plates” are made for this purpose.
Luster
The luster of a mineral is the appearance of its surface by reflected
light, and it is an important aid in determining many minerals. Two
types of luster are recognized; metallic, the luster of metals, most
sulphides and some oxides, all of which are opaque on their thin edges;
and non-metallic, the luster of minerals which are more or less
transparent on their thin edges, and most of which are light colored.
The common non-metallic lusters are; vitreous, the luster of glass;
resinous, the appearance of resin; greasy, oily appearance; pearly, the
appearance of mother-of-pearl; silky, like silk due to the fibrous
structure; adamantine, brilliant like a diamond; and dull, as is chalk.
Color
When used with caution color is of the utmost importance in determining
minerals, especially in making rapid determinations. In metallic
minerals it is constant and dependable; but in the non-metallic minerals
it may vary, due to the presence of small amounts of impurities which
act as pigments. Color depends on chemical composition, and when not
influenced by impurities is termed _natural_; but when the color is due
to some inclosed impurity it is termed _exotic_. In this latter case
caution must be used in making determinations. Many minerals are
primarily colorless, but take on exotic colors as a result of the
presence of small quantities of impurities; for instance, pure corundum
is colorless, but with a trace of iron oxide present becomes red, and is
called the ruby, or with a trace of cobalt becomes blue and is called
sapphire.