Botryoidal, having grape-like surfaces.
Stalactitic, forming stalactites or pendant columns.
Amygdaloidal or Concretionary, forming separate globular masses in the
enclosing rock.
Dendritic, branching or arborescent.
3. _Hardness._—By the hardness of a mineral we mean the resistance which
it offers to abrasion. But hardness is a purely relative term, calcite,
for example, being hard compared with talc, but very soft compared with
quartz. Hence mineralogists have found it necessary to select certain
minerals to be used as a standard of comparison for all others, and
known as the _scale of hardness_. These are arranged at nearly equal
intervals all the way from the softest mineral to the hardest, as
follows:—
_Scale of Hardness._
(1) Talc.
(2) Gypsum.
(3) Calcite.
(4) Fluorite.
(5) Apatite.
(6) Orthoclase.
(7) Quartz.
(8) Topaz or Beryl.
(9) Corundum.
(10) Diamond.
If a mineral scratches calcite and is scratched by fluorite, we say its
hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is
scratched by orthoclase, its hardness is 6; and so on. There are very
few minerals harder than quartz, and hence the first seven members of
the scale are sufficient for all ordinary purposes; and these are all
included in the series of specimens accompanying this Guide.
Although it is desirable to be acquainted with the scale of hardness,
and to understand how to use it, still the student will learn, after a
little practice, that almost as good results may be obtained much more
conveniently by the use of his thumb-nail and a good knife-blade or
file. Talc and gypsum are easily scratched with the nail; calcite and
fluorite yield easily to the knife or file, apatite with more
difficulty; while orthoclase is near the limit of the hardness of
ordinary steel, and quartz is entirely beyond it.
4. _Specific Gravity._—The specific gravity of a mineral, by which we
mean its weight as compared with the weight of an equal volume of water,
is determined by weighing it first in air and then in water, and
dividing the weight in air by the difference of the two weights.
Minerals exhibit a wide range in specific gravity; from petroleum, which
floats on water, to gold, which is nearly twenty times heavier than
water. Although this is one of the most important properties of
minerals, yet, being more difficult to measure than hardness, it is less
valuable as an aid in distinguishing species. One can with practice,
however, estimate the density of a mineral pretty closely by lifting it
in the hand.
5. _Lustre._—Of all the properties of minerals depending on their
relations to light the most important is lustre, by which we mean the
quality of the light reflected by a mineral as determined by the
character or minute structure of its surface. Two kinds of lustre, the
_metallic_ and _vitreous_, are of especial importance; in fact all other
kinds are merely varieties of these.
The metallic lustre is the lustre of all true metals, as copper and tin,
and characterizes nearly all minerals in which metallic elements
predominate. The vitreous lustre is best exemplified in glass, but
belongs to most minerals composed chiefly of non-metallic elements.
Metallic minerals are always opaque, but vitreous minerals are often
transparent.
Other kinds of lustre are the _adamantine_ (the lustre of diamond),
_resinous_, _pearly_, and _silky_. When a mineral has no lustre, like
chalk, it is said to be dull.
It should be made clear to children that lustre and color are entirely
distinct and independent. Thus, iron, copper, gold, silver, and lead are
all metallic; while white or colorless quartz, black tourmaline, green
beryl, red garnet, etc., are all vitreous. Generally speaking, any color
may occur with any lustre.
6. _Color and Streak._—The colors of minerals are of two
kinds,—_essential_ and _non-essential_. By the essential color in any
case we mean the color of the mineral itself in its purest state. The
non-essential colors, on the other hand, are chiefly the colors of the
impurities contained in the minerals.
Metallic minerals, which are always opaque, usually have essential
colors; but vitreous minerals, which are always more or less
transparent, often have non-essential colors. The explanation is this:
In opaque minerals we can only see the impurities immediately on the
surface, and these are, as a rule, not enough to affect its color; but
in diaphanous minerals we look _into_ the specimen and see impurities
below the surface, and thus bring into view, in many cases, sufficient
impurity so that its color drowns that of the mineral.
To prove this we have only to take any mineral (serpentine is a good
example in our series) having a non-essential color, and make it opaque
by pulverizing it or abrading its surface, when the non-essential color,
the color of the impurity, immediately disappears; just as water, yellow
with suspended clay, becomes white when whipped into foam, and thus made
opaque.
What we understand by the _streak_ of a mineral is its essential color,
the color of its powder; and it is so called because the powder is most
readily observed by scratching the surface of the mineral, and thereby
pulverizing a minute portion of it. The streak and hardness are thus
determined at the same time. The streak of soft minerals is easily
determined by rubbing them on any white surface of suitable hardness, as
paper, porcelain, or Arkansas stone.
ESSENTIAL AND ACCESSORY MINERALS.—Lithologists, regarding minerals as
constituents of rocks, divide them into two great classes: the
_essential_ and the _accessory_. The essential constituents of a rock
are those minerals which are essential to the definition of the rock.
For example, we cannot properly define granite without naming quartz and
orthoclase; hence these are essential constituents of granite; and if
either of these minerals were removed from granite it would not be
granite any longer, but some other rock. But other minerals, like
tourmaline and garnet, may be indifferently present or absent; it is
granite still; hence they are merely accidental or accessory
constituents. They determine the different _varieties_ of granite, while
the essential minerals make the _species_.
This classification, of course, is not absolute, for in many cases the
same mineral forms an essential constituent of one rock and an accessory
constituent of another. Thus, quartz is essential in granite, but
accessory in diorite.
PRINCIPAL MINERALS CONSTITUTING ROCKS.—Having studied in a general way
the more important characteristics of minerals, brief descriptions of
the chief rock-forming species are next in order. We will notice first
and principally those minerals occurring chiefly as _essential_
constituents of rocks.
1. Graphite.—Essentially pure carbon, though often mixed with a little
iron oxide. Crystallizes in hexagonal system, but usually foliated,
granular, or massive. Hardness, 1-2, being easily scratched with the
nail. Sp. gr., 2.1-2.3. Lustre, metallic; an exception to the rule that
acidic elements have non-metallic or vitreous lustres. Streak, black and
shining (see pencil-mark on white paper). Color, iron-black. Slippery or
greasy feel. Every _black-lead_ pencil is a specimen of graphite.
Specimen 9.
The different kinds of mineral coal are, geologically, as we have seen,
closely related to graphite, but they are such familiar substances that
they need not be described here.
2. Halite (common salt).—Chloride of sodium: chlorine, 60.7; sodium,
39.3; = 100. Isometric system, usually forming cubes. Hardness, 2.5, a
little harder than the nail. Sp. gr., 2.1-2.6. Lustre, vitreous. Streak
and color both white, and hence color is essential. Often transparent.
Soluble; taste, purely saline. In specific gravity and lustre it is a
good example of a mineral in which an acidic element predominates.
Specimen 11.
3. Limonite.—Hydrous sesquioxide of iron: oxygen, 25; iron, 60; water,
15; = 100. Usually amorphous; occurring in stalactitic and botryoidal
forms, having a fibrous structure; and also concretionary, massive, and
earthy (yellow ochre). Hardness, 5-5.5. Sp. gr., 3.6-4. Lustre, vitreous
or silky, inclining to metallic, and sometimes dull. Color, various
shades of black, brown, and yellow. Streak, ochre-yellow; hence color
partly non-essential. Specimen 12.
4. Hematite.—Sesquioxide of iron: oxygen, 30; iron, 70; = 100. Hexagonal
system, in distinct crystals, but usually lamellar, granular, or
compact,—columnar, botryoidal, and stalactitic forms being common.
Hardness, 5.5-6.5; good crystals are harder than steel. Sp. gr.,
4.5-5.3. Lustre, metallic, sometimes dull. Color, iron-black, but red
when earthy or pulverized (red ochre). Streak, red, and color,
therefore, mainly non-essential; sometimes attracted by the magnet.
Specimen 13.
Hematite has the same composition as limonite, minus the water; and by
comparing the hardness and specific gravity of these two minerals we see
that they are a good illustration of the principle that hydrous minerals
are softer and lighter than anhydrous minerals of analogous composition.
Limonite and hematite are two great natural coloring agents, and almost
all yellow, brown, and red colors in rocks and soils are due to their
presence.
5. Magnetite.—Protoxide and sesquioxide of iron: oxygen, 27.6; iron,
72.4; = 100. Isometric system, usually in octahedrons or dodecahedrons.
Most abundant variety is coarsely to finely granular, sometimes
dendritic. Hardness, 5.5-6.5, same as hematite. Sp. gr., 4.9-5.2.
Lustre, metallic. Color and streak, iron-black, and hence color
essential. Strongly magnetic; some specimens have distinct polarity, and
are called loadstones. Specimen 14.
The three iron-oxides just described—limonite, hematite, and
magnetite—are all important ores of iron, and form a well-marked natural
series. Thus limonite is never, hematite is usually, and magnetite is
always, crystalline. Again, limonite with 60 per cent. of iron is never
magnetic, hematite with 70 per cent. is sometimes magnetic, while
magnetite with 72.4 per cent. is always magnetic. As the iron increases
so does the magnetism. We have here an excellent illustration of the
principle that the properties of the elements can be traced in those
minerals in which they predominate. Iron is the only strongly magnetic
element: magnetite contains more iron than any other mineral, and it is
the only strongly magnetic mineral.
These three iron-ores are easily distinguished from each other by the
color of their powders or streak,—limonite yellow, hematite red, and
magnetite black,—and from all other common minerals by their high
specific gravity.
6. Quartz.—Oxide of silicon or silica: oxygen, 53.33; silicon, 46.67; =
100. Hexagonal system. The most common form is a hexagonal prism
terminated by a hexagonal pyramid. Also coarsely and finely granular to
perfectly compact, like flint; the compact or cryptocrystalline
varieties often assuming botryoidal, stalactitic, and concretionary
forms. It has no cleavage, but usually breaks with an irregular,
conchoidal fracture like glass. Hardness, 7, being No. 7 of the scale;
scratches glass easily. Sp. gr., 2.5-2.8. Lustre, vitreous. Pure quartz
is colorless or white, but by admixture of impurities it may be of
almost any color. Streak always white or light colored. Quartz is
usually, as in specimen 15, transparent and glassy, but may be
translucent or opaque. It is almost absolutely infusible and insoluble.
The varieties of quartz are very numerous, but they may be arranged in
two great groups:—
1. _Phenocrystalline_ or _vitreous_ varieties, including rock-crystal,
amethyst, rose quartz, yellow quartz, smoky quartz, milky quartz,
ferruginous quartz, etc.
2. _Cryptocrystalline_ or _compact_ varieties, including chalcedony,
carnelian, agate, onyx, jasper, flint, chert, etc. Only three varieties,
however, are of any great geological importance; these are: common
glassy quartz (spec. 15), flint (spec. 16), and chert.
Quartz is one of the most important constituents of the earth’s crust,
and it is also the hardest and most durable of all common minerals. We
have already observed (p. 12) that it is entirely unaltered by exposure
to the weather; _i.e._, it cannot be decomposed; and, being very hard,
the same mechanical wear which, assisted by more or less chemical
decomposition, reduces softer minerals to an impalpable powder or clay,
must leave the quartz chiefly in the form of sand and gravel. This
agrees with our observation that sand (spec. 30), especially, is usually
merely pulverized quartz.
_Opal_ is a mineral closely allied to quartz, and may be mentioned in
this connection. It is of similar composition, but contains from 5 to 20
per cent. of water, and is decidedly softer and lighter. Hardness,
5.5-6.5; sp. gr., 1.9-2.3.
7. Gypsum.—Hydrous sulphate of calcium: sulphur trioxide (SO₃), 46.5;
lime (CaO), 32.6; water (H₂O), 20.9; = 100. Monoclinic system. Often in
distinct rhombic crystals; also foliated, fibrous, and finely granular.
Hardness, 1.5-2; the hardest varieties being No. 2 of the scale of
hardness. Sp. gr., 2.3. Lustre, pearly, vitreous, or dull. Color and
streak usually white or gray. The principal varieties of gypsum are
(_a_) _selenite_, which includes all distinctly crystallized or
transparent gypsum; (_b_) _fibrous gypsum_ or _satin-spar_; (_c_)
_alabaster_, fine-grained, light-colored, and translucent. Gypsum is
easily distinguished from all common minerals resembling it by its
softness and the fact that it is not affected by acids. Specimen 17.
8. Calcite.—Carbonate of calcium: carbon dioxide (CO₂), 44; lime (CaO),
56; = 100. Hexagonal system, usually in rhombohedrons, scalenohedrons,
or hexagonal prisms. Cleavage rhombohedral and highly perfect (specimen
18). Also fibrous and compact to coarsely granular, in stalactitic,
concretionary, and other forms. Hardness, 2.5-3.5, usually 3 (see scale
of hardness). Sp. gr., 2.5-2.75. Lustre, vitreous. Color and streak
usually white. Transparent crystallized calcite is known as
_Iceland-spar_, and is remarkable for its strong double refraction. When
finely fibrous it makes a _satin-spar_ similar to gypsum. Geologically
speaking, calcite is a mineral of the first importance, being the sole
essential constituent of all limestones. It is readily distinguished
from allied species by its perfect rhombohedral cleavage; by its
softness, being easily scratched with a knife; and above all by its
lively effervescence with acids, for it is the _only common_ mineral
effervescing _freely_ with _cold dilute_ acid. To apply this test it is
only necessary to touch the specimen with a drop of dilute chlorohydric
acid. The effervescence, of course, is due to the escape of the carbon
dioxide in a gaseous form. Specimen 18.
9. Dolomite.—Carbonate of calcium and magnesium: carbonate of calcium
(CaCO₃), 54.35; carbonate of magnesium (MgCO₃), 45.65; = 100. Hexagonal
system, being nearly isomorphous with calcite. Rhombohedral cleavage
perfect. Hardness, 3.5-4; sp. gr., 2.8-2.9, being harder and heavier
than calcite. Lustre, color, and streak same as for calcite, from which
it is most easily distinguished by its non-effervescence or only feeble
effervescence with cold dilute acid, though effervescing freely with
strong or hot acid. Spec. 19.
10. Siderite.—Carbonate of iron: carbon dioxide (CO₂), 37.9; protoxide
of iron (FeO), 62.1; = 100. Crystallization and cleavage essentially the
same as for calcite and dolomite. Hardness, 3.5-4.5, and sp. gr.,
3.7-3.9. Lustre, vitreous. Color, white, gray, and brown. Streak, white.
With acid, siderite behaves like dolomite. It is distinguished from both
calcite and dolomite by its high specific gravity, which is easily
explained by the fact that it is largely composed of the heavy element,
iron.
With one exception, the fifteen minerals which we have yet to study
belong to the class of silicates, which includes more than one-fourth of
the known species of minerals, and, omitting quartz and calcite, all of
the really important rock-constituents. The silicate minerals may be
very conveniently divided into two great groups, the _basic_ and
_acidic_. This is not a sharp division; on the contrary, there is a
perfectly gradual passage from one group to the other; and yet this is,
for geological purposes at least, a very natural classification. The
dividing line falls in the neighborhood of 60 per cent. of silica;
_i.e._, all species containing this proportion of silica or _less_ are
classed as basic, since in them the basic elements predominate; while
those containing _more_ than 60 per cent. of silica are classed as
acidic, because their characteristics are determined chiefly by the acid
element or silica. The principal bases occurring in the silicates, named
in the order of their relative importance, are aluminum, magnesium,
calcium, iron, sodium, and potassium; and of these, magnesium, calcium,
iron, and usually sodium, are especially characteristic of basic
species.
Iron is the heaviest base; but all the bases, except sodium and
potassium, are heavier than the acid—silica; consequently basic minerals
must be, as a rule, heavier than acidic minerals. And since basic
minerals contain more iron than acidic, they must be darker colored. In
general, we say, _dark, heavy_ silicates are _basic_, and _vice versa_.
All this is of especial importance because in the rocks nature keeps
these two classes separate in a great degree.
11. Amphibole.—Silicate of aluminum, magnesium, calcium, iron, and
sodium. The bases occur in very various proportions, forming many
varieties; but the only variety of especial geological interest is
_hornblende_, the average percentage composition of which is as follows:
silica (SiO₂), 50; alumina (Al₂O₃), 10; magnesia (MgO), 18; lime (CaO),
12; iron oxide (FeO and Fe₂O₃), 8; and soda (Na₂O), 2; = 100. Monoclinic
system: usually in rhombic or six-sided prisms which may be short and
thick, but are more often acicular or bladed. Hardness, 5-6; sp. gr.,
2.9-3.4. Lustre, vitreous; color, black and greenish black; and streak
similar to color, but much paler. Compare with quartz, and observe the
strong contrast in color possible with minerals having the same lustre.
Specimen 20.
12. Pyroxene.—Like amphibole, this species embraces many varieties, and
these exhibit a wide range in composition; but of these _augite_ alone
is an important rock-constituent. Hence in lithology we practically
substitute for amphibole and pyroxene, hornblende, and augite
respectively.
Augite is very similar in composition to hornblende, but contains
usually more lime and less alumina and alkali. Physically, too, these
minerals are almost identical, crystallizing in the same system and in
very similar forms, and agreeing in hardness, color, lustre, and streak.
Augite is heavier than hornblende, sp. gr., 3.2-3.5. A certain prismatic
angle, which in augite is 87°5´, is 124°30´ in hornblende. Slender,
bladed crystals are more common with hornblende than augite. When
examined in thin sections with the polarizer, augite does not afford the
phenomenon of dichroism, which is strongly marked in hornblende.
However, as these minerals commonly occur in the rocks, in small and
imperfect crystals, these distinctions can only be observed in thin
sections under the microscope; so that, as regards the naked eye, they
are practically indistinguishable.
It might appear at first that the distinction of minerals so nearly
identical is not an important matter; but nature has decreed otherwise.
Augite and hornblende are typical examples of basic minerals; but augite
is, both in its composition and associations, the more basic of the two.
In proof of this we need only to know that it very rarely occurs in the
same rock with quartz, while hornblende is found very commonly in that
association. Quartz in a rock means an excess of acid or silica, and
almost necessarily implies the absence of highly basic minerals. In
other words, hornblende is often, and augite very rarely, found in
connection with acidic minerals; and it is this difference of
association chiefly that makes their distinction essential to the proper
recognition of rocks; while at the same time it affords an easy, though
of course not absolutely certain, means of determining whether the black
constituent of any particular rock is hornblende or augite.
_Mica Family._—Mica is not the name of a single mineral, but of a whole
family of minerals, including some half-dozen species. Only two,
however,—muscovite and biotite,—are sufficiently abundant to engage our
attention. These are complex, basic silicates of aluminum, magnesium,
iron, potassium, and sodium. The crystallization of biotite is
hexagonal, and of muscovite monoclinic; but both occur commonly in flat
six-sided forms. Undoubtedly the most important and striking
characteristic of the whole mica family is the remarkably perfect
cleavage parallel with the basal planes of the crystals, and the
wonderful _thinness_, and above all the _elasticity_, of the cleavage
lamellæ. The cleavage contrasts the micas with all other common
minerals, and makes their certain identification one of the easiest
things in lithology. The micas are soft minerals, the hardness ranging
from 2 to 3, and being usually easily scratched with the nail. Sp. gr.
varies from 2.7-3.1. Lustre, pearly; and streak, white or uncolored.