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.