Part 9

These differences are tabulated below:—

Muscovite = Biotite = Acidic mica. Basic mica. Non-ferruginous mica. Ferruginous mica. Potash mica. Magnesian mica. White mica. Black mica. Non-dichroic mica. Dichroic mica.

Feldspar Family.—Like mica, feldspar is the name of a family of minerals; and these are, geologically, the most important of all minerals. They are, above all others, the minerals of which rocks are made, and their abundance is well expressed in the name,—feldspar being simply the German for field-spar, implying that it is the common spar or mineral of the fields.

Chemically, the feldspars are silicates of aluminum and potassium, sodium or calcium. They crystallize in the monoclinic and triclinic systems; and all possess easy cleavage in two directions at right angles to each other, or nearly so. The general physical characters, including the cleavage, are well exhibited in the common species, orthoclase (specimen 22).

In hardness the feldspars range from 5 to 7, being usually near 6, and almost always distinctly softer than quartz. Sp. gr. varies from 2.5-2.75; lustre, from vitreous to pearly; color, from white and gray to red, brown, green, etc., but usually light. Streak, always white; rarely transparent. By exposure to the weather, feldspars gradually lose their alkalies and lime, become hydrated, and are changed to kaolin or common clay. A similar change takes place with the micas, augite, and hornblende; but these species, being usually rich in iron, make clays which are much darker colored than those derived from feldspars. The fact that the feldspars contain little or no iron undoubtedly explains their low specific gravity and light colors, as compared with the other minerals just named. The only common minerals for which the feldspars are liable to be mistaken are quartz and the carbonates. From the latter they are easily distinguished by their superior hardness and non-effervescence with acids; and from the former, by possessing distinct cleavage, by being rarely transparent, by being somewhat softer, and by changing to clay on exposure to the weather.

The feldspars of greatest geological interest are five in number, and may be classified chemically as follows:—

Orthoclase,—silicate of aluminum and potassium, or potash feldspar.

Albite,—silicate of aluminum and sodium, or soda feldspar.

Anorthite,—silicate of aluminum and calcium, or lime feldspar.

Oligoclase,—silicate of aluminum and sodium, and calcium, or soda-lime feldspar.

Labradorite,—silicate of aluminum and calcium, and sodium, or lime-soda feldspar.

This appears like a complex arrangement, but it can be simplified. Orthoclase crystallizes in the monoclinic system, and all the other feldspars in the triclinic system. With the exception of albite, which is a comparatively rare species, the triclinic feldspars all contain less silica than orthoclase; i.e., are more basic. This is shown by the subjoined table giving the average composition of each of the feldspars:—

SiO₂ Al₂O₃ K₂O Na₂O CaO Total. Orthoclase, 65 18 17 -- -- = 100 Albite, 68 20 -- 12 -- = 100 Oligoclase, 62 24 -- 9 5 = 100 Labradorite, 53 30 -- 4 13 = 100 Anorthite, 43 37 -- -- 20 = 100

As we should naturally expect, the triclinic feldspars occur usually with other basic minerals, while the monoclinic species, orthoclase, is acidic in its associations; furthermore, the triclinic feldspars are often intimately associated with each other, but are rarely important constituents of rocks containing much orthoclase. In other words, the distinction of orthoclase from the basic or triclinic feldspars is important and comparatively easy, while the distinction of the different basic feldspars from each other is both unimportant and difficult. Hence, in lithology, we find it best to put all these basic feldspars together, as if they were one species, under the name plagioclase, which refers to the oblique cleavage of all these feldspars, and contrasts with orthoclase, which refers to the right-angled cleavage of that species.

This statement of the relations of the feldspars is, of course, beyond the comprehension of many children, and yet it should be understood by the teacher who would lead the children to any but the most superficial views.

15. Orthoclase.—This is the common feldspar, and the most abundant of all minerals, being the principal constituent of granite, gneiss, and many other important rocks. The most characteristic colors are white, gray, pinkish, and flesh-red. Specimen 22.

16. Plagioclase.—Like orthoclase, these species may be of almost any color; yet these two great divisions of the feldspars are usually contrasted in this respect. Thus, bluish and grayish colors are most common with plagioclase, and white or reddish colors with orthoclase. Specimen 23 is labradorite, and, in every respect, a typical example of plagioclase. On certain faces and cleavage-surfaces of the plagioclase crystals we may often observe a series of straight parallel lines or bands which are often very fine,—fifty to a hundred in a single crystal. These striæ are due to the mode of twinning, and are of especial importance, since, while they are very characteristic of plagioclase, they never occur in orthoclase. As stated, these twinning striæ in plagioclase are often visible to the naked eye; and when they are not, they may usually be revealed by examining a thin section under the microscope with polarized light. Plagioclase decays much more rapidly when exposed to the weather than orthoclase. This point becomes perfectly clear when we compare weathered ledges of diabase (or any trap-rock, see specimen 2) and granite; for plagioclase is the principal constituent of the former rock, and orthoclase of the latter.

Hydrous Silicates.—Many silicates contain water, and some of these are of great geological importance. What has been stated on a preceding page concerning the softness and lightness of hydrated minerals is especially applicable here; for all the geologically important hydrous silicates are distinctly softer and lighter than anhydrous minerals of otherwise similar composition. Furthermore, they usually have an unctuous or slippery feel; and, with one exception (kaolin), are of a green or greenish color.

17. Kaolinite (Kaolin).—Hydrous silicate of aluminum: silica (SiO₂), 46; alumina (Al₂O₃), 40; and water (H₂O), 14; = 100. Orthorhombic system, in rhombic or hexagonal scales or plates, but usually earthy or clay-like. Hardness, 1-2.5; sp. gr., 2.4-2.65. The pure mineral is white; but it is usually colored by impurities, the principal of which are iron oxides and carbonaceous matter. Kaolin is the most abundant of all the hydrous silicates, and it is the basis and often the sole constituent of common clay,—a very common mineral, but rarely pure. We have already (p. 11) noticed the mode of origin of kaolin or clay. It results from the decomposition of various aluminous silicate minerals, especially the feldspars. Under the combined influence of carbon dioxide and moisture, feldspars give up their potassium, sodium, and calcium, and take on water, and the result is kaolin. This mineral is believed to be always a decomposition product. Perhaps the best, or at least the most convenient, test for kaolin is the argillaceous odor, the odor of moistened clay. Specimen 24.

18. Talc.—Hydrous silicate of magnesium: silica (SiO₂), 63 (acidic); magnesia (MgO), 32; water (H₂O), 5; = 100. Orthorhombic system, but rarely in distinct crystals. Cleavage in one direction very perfect; the cleavage lamellæ are flexible, but not elastic, as in mica. Hardness, 1; see scale. Sp. gr., 2.55-2.8. Lustre, pearly. Color, apple-green to white; and streak, white. The feel is very smooth and greasy; and, in connection with the color and foliation, affords the best means of distinguishing talc from allied minerals. Talc sometimes results from the alteration of augite, hornblende, and other minerals, but it is not always nor usually an alteration product.

19. Serpentine.—Hydrous silicate of magnesium: silica (SiO₂), 44 (basic); magnesia (MgO), 44; water (H₂O), 12; = 100. Essentially amorphous. Hardness, 2.5-4; sp. gr., 2.5-2.65. Lustre, greasy, waxy, or earthy. Color, various shades of green and usually darker than talc, but streak always white. Feel, smooth, sometimes greasy. Distinguished from talc by its hardness, compactness, and darker green. Sometimes results from the alteration of olivine and other magnesian minerals, but usually we are to regard it as an original mineral. Specimen 25.

20. Chlorite.—This is, properly, the name of a group of highly basic minerals of very variable composition, but they are all essentially hydrous silicates of aluminum, magnesium, and iron; and the average composition of the most abundant species, prochlorite, is as follows: silica (SiO₂), 30; alumina (Al₂O₃), 18; magnesia (MgO), 15; protoxide of iron (FeO), 26; and water (H₂O), 11; = 100. The chlorites crystallize in several different systems, but in all there is a highly perfect cleavage in one direction, giving, as in talc, a foliated structure with flexible but inelastic laminæ. The cleavage scales, however, are sometimes minute, and the structure massive or granular. Hardness of prochlorite, 1-2; between talc and serpentine. Sp. gr., 2.78-2.96. All the chlorites have a pearly to vitreous lustre. Color usually some shade of green; in prochlorite a dark or blackish green, darker than serpentine, as that is darker than talc. Streak, a lighter, whitish green. Less unctuous than talc, but more so than serpentine. The chlorites are produced very commonly, but not generally, by the alteration of basic anhydrous silicates, like augite and hornblende. Specimen 26.

21. Hydro-mica.—This, too, is properly the name of a group of minerals; but for geological purposes they may be regarded as one species. Taking a general view of the composition, these are simply the anhydrous or ordinary micas, which we have already studied, with from 5 to 10 per cent. of water added. In crystallization and structure they are essentially mica-like. Although not distinctly softer than the common micas, they are lighter, always more unctuous and slippery, and usually of a greenish color. The micaceous structure with elastic laminæ serves to distinguish the hydro-micas from other hydrous silicates.

22. Glauconite.—Hydrous silicate of aluminum, iron, and potassium: silica (SiO₂), 50; alumina (Al₂O₃), protoxide of iron (FeO), and potash (K₂O), together, 41; and water (H₂O), 9; = 100. Amorphous, forming rounded and generally loose grains, which often have a microscopic organic nucleus. It is dull and earthy, like chalk, and always soft, green, and light, but not particularly unctuous. Glauconite is the principal, often the sole, constituent of the rock greensand, which occurs abundantly in the newer geological formations, and is now forming in the deep water of the Gulf of Mexico and along our Atlantic sea-board. Specimen 27.

This completes our list of minerals occurring chiefly as essential constituents of rocks; and following are three of the more common and important minerals occurring chiefly as accessory, rarely as essential, rock-constituents.

23. Chrysolite (Olivine).—Silicate of magnesium and iron: silica (SiO₂), 41; magnesia (MgO), 51; protoxide of iron (Fe₂O₃), 8; = 100. Orthorhombic system; but usually in irregular glassy grains. Hardness, 6-7. Sp. gr., 3.3-3.5. Lustre, vitreous; color, usually some shade of green; and streak, white. Chrysolite sometimes closely resembles quartz, but its green color usually suffices to distinguish it. It is a common constituent of basalt and allied rocks. By absorption of water it is changed into serpentine and talc. See examples in specimen.

24. Garnet.—The composition of this mineral is extremely variable; but the most important variety is a basic silicate of aluminum and iron: silica (SiO₂), 37; alumina (Al₂O₃), 20; and protoxide of iron (FeO), 43; = 100. Isometric system, usually in distinct crystals, twelve-sided (dodecahedrons) and twenty-four-sided (trapezohedrons) forms being most common. Hardness, 6.5-7.5; average as hard as quartz. Sp. gr., 3.15-4.3; compare with the high percentage of iron. Lustre, vitreous; colors, various, usually some shade of red or brown; and streak, white. Some varieties contain iron enough to make them magnetic. Garnet is easily distinguished by its form, color, and hardness from all other minerals. It is a common but not an abundant mineral, occurring most frequently in gneiss, mica schist, and other stratified crystalline rocks. See examples in specimen.

25. Pyrite.—Sulphide of iron: sulphur, 53.3; iron, 46.7; = 100. Isometric system, occurring usually in distinct crystals, the cube and the twelve-sided form known as the pyritohedron being the most common. Hardness, 6-6.5, striking fire with steel. Sp. gr., 4.8-5.2; heavy because rich in iron. Lustre, metallic and splendent. Color, pale, brass-yellow, and streak, greenish or brownish. Pyrite is sometimes mistaken for gold, but it is not malleable; while its color, hardness, and specific gravity, combined, easily distinguish it from all common minerals. As an accessory rock-constituent, pyrite occurs usually in isolated cubes or pyritohedrons. Specimen 10.