ROCK-FORMING MINERALS.
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]
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 laminæ, 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 zeō, 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].