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.