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.
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.