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.