Part 8

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.

The distinguishing features of muscovite and biotite are as follows:—

13. Muscovite.—Contains 47 per cent. of silica, 3 per cent. of sesquioxide of iron, and 10 per cent. of alkalies, chiefly potash; and the characteristic colors are white, gray, and, more rarely, brown and yellow. Non-dichroic. Usually found in association with acidic minerals. The mica used in the arts is muscovite. Specimen 21.

14. Biotite.—Contains only 36 per cent. of silica, 20 per cent. of oxide of iron, and 17 per cent. of magnesia; colors, deep black to green. Strongly dichroic. Commonly occurs with other basic minerals. Compare color with per cent. of iron.