14. Nearly all the minerals we know of contain oxygen as a necessary
ingredient, there being only a very few minerals in which that gas does
not occur in chemical union with other elements. Three of these
minerals, _sulphur_, the _diamond_, and _graphite_, consist of simple
substances, and are of great commercial importance, but none of them is
of so frequent occurrence, as a rock constituent, as the minerals
presently to be described. _Sulphur_ occurs sometimes in thin beds, but
more frequently in small nests and nodules, &c. in other rocks, or in
joints, and fissures, and veins. It is frequently found in volcanic
districts. The _diamond_, which consists of pure _carbon_, is generally
met with in alluvial deposits, but sometimes, also, in a curious
flexible sandstone, called _itacolumite_. _Graphite_ is another form of
carbon. It occurs both in a crystalline and amorphous form, the latter,
or non-crystalline kind, being extensively used for lead-pencils.
_Rock-salt_ is a _chloride of sodium_, and appears sometimes in masses
of a hundred feet and more in thickness. Another mineral which contains
no oxygen is the well-known _fluor-spar_. It occurs chiefly in veins,
and is often associated with ores. With these, and a few other
exceptions, all the minerals hitherto discovered contain oxygen as an
essential element; and so large is the proportion of this gas which
enters into union with other elements to constitute the various minerals
of which the rocks are composed, that it forms at least one-half of all
the ponderable matter near the earth's surface. When the student learns
that there are probably no fewer than six or seven hundred different
minerals, he will understand how impossible it is to do more in a short
geological treatise than point out a few of the most commonly occurring
ones. And, indeed, a knowledge of the chief rock-forming minerals, which
are few in number, is all that is absolutely requisite for geological
purposes. Some of these we accordingly proceed to name.[B]
[B] It is needless to describe the minerals minutely here. The
student can only learn to distinguish the different species
by carefully examining actual specimens.
15. _Quartz._--This mineral has already been partially described. It is
the most abundant of all the rock-forming minerals, and occurs in three
forms: (1) _crystallised quartz_ or _rock crystal_; (2) _chalcedony_,
both of which are composed of silica--that is, silicon and oxygen; and
(3) _hydrated quartz_--that is, silica with the addition of water.
_Hematite._--This is an oxide of iron. It occurs in mammillary rounded
masses, with a fibrous structure, and a dull metallic lustre.
_Magnetite_ or magnetic iron ore, _specular iron_, and _limonite_ are
also oxides of iron. _Hematite_ shews a red streak when scratched with a
knife, which distinguishes it from magnetite.
_Iron pyrites._--This is a sulphide of iron of very common occurrence.
Its crystalline form is cubical. When broken, it emits a sulphurous
smell. The brass-yellow coloured cubes so often seen in roofing-slates
are familiar examples of the mode of its occurrence. But it is also
frequently found in masses having a crystalline surface.
16. SULPHATES.--Only two sulphates may be noticed--namely, _gypsum_,
which is a sulphate of lime, with its varieties, _selenite_,
_satin-spar_, and _alabaster_; and _barytes_, a sulphate of baryta.
_Barytes_ scratches easily with the knife, and from its great specific
gravity is often called _heavy-spar_. Gypsum is softer than barytes.
CARBONATES.--Two of these only need be mentioned: _calcite_ or
_calc-spar_, a carbonate of lime, which scratches with the knife, and
effervesces readily with dilute hydrochloric acid; and _arragonite_,
also a carbonate of lime, but denser than calcite.
SILICATES.--These are by far the most abundantly occurring minerals. The
species are also exceedingly numerous, but we may note here only a few
of the more important. They are composed of silica and various bases,
such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or
_pyroxene_ is a black or greenish-black mineral, found, either as
crystals, which are generally small, or as rounded grains and angular
fragments, in basaltic and volcanic rocks. It never occurs in granite
rocks. It is brittle, and has a vitreous or resinous lustre. There are a
number of varieties or sub-species of augite. _Hornblende_, like augite,
also includes a great many minerals. When the crystals are small, it is
often difficult to distinguish hornblende from augite. Common hornblende
occurs crystallised or massive, and is dark green or black, with a
vitreous lustre. It is generally sub-translucent. It usually
crystallises in igneous rocks which contain much quartz or silica; while
augite, on the other hand, crystallises in igneous rocks which are of a
more basic character--that is to say, rocks in which silica is not so
abundantly present. _Felspar_ is a generic term which embraces a number
of species, such as _orthoclase_ or _potash-felspar_, _albite_ or
_soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is
white, red or pink, and gray. It is one of the ordinary constituents of
granite, and enters into the composition of many rocks. _Albite_ is
usually white. It often occurs as a constituent of granite, not
unfrequently being associated in the same rock with pink felspar or
orthoclase. In syenite and greenstone it occurs more commonly than
orthoclase. _Anorthite_ occurs in white translucent or transparent
crystals. It is not so common a constituent of rocks as either of the
other felspars just referred to. _Mica_: this term includes several
minerals, which all agree in being highly cleavable into thin elastic
flakes or laminae, which have a glistening metallic lustre. Mica is one
of the common constituents of granite. _Talc_ is a silvery white,
grayish, pale or dark-green coloured mineral, with a pearly lustre. It
splits readily into thin flakes, which are flexible, but not elastic,
and may be readily scratched with the nail. It is unctuous and greasy to
the touch. It occurs in beds (_talc-slate_), and is often met with in
districts occupied by metamorphic crystalline rocks. _Serpentine_ is
generally of a green colour, but brown, red, and variously mottled
varieties occur. It has a dull lustre, and is soft, and easily cut; it
is tough, however, and takes on a good polish. It forms rock-masses in
some places. The finer varieties are called _noble serpentine_.
_Chlorite_ is another soft, easily scratched mineral, generally of a
dark-green colour. It has a pearly lustre. Sometimes it occurs in beds
(_chlorite-slate_), and is often found coating the walls of fissures in
certain rocks. It has a somewhat greasy feel. The three last-mentioned
minerals--talc, serpentine, and chlorite--are all silicates of magnesia.
_Zeolites_ is a term which comprises a number of minerals of varying
chemical composition, all of which tend to form a jelly when treated
with acids. When heated by the blow-pipe they bubble up, owing to the
escape of water; hence their name _zeolites_, from _zeo_, I boil, and
_lithos_, a stone. The zeolites occur very commonly in cavities in
igneous rocks, and also in mineral veins.
Having now mentioned the chief rock-forming minerals, we proceed to a
brief description of some of the more typical representatives of the
five great classes of rocks referred to at page 8.