Common Minerals and Rocks

by William O. Crosby

Rock-Salt.—This interesting and useful rock, as we have already learned, is deposited in a purely chemical way, and only in drying-up portions of the sea, like the Dead Sea, Great Salt Lake, etc. In some parts of Europe there are beds of solid rock-salt over a hundred feet thick.

Phosphate Rock.—Although not specially abundant or attractive, this rock is of great economic interest and importance on account of its extensive use as a fertilizer. Under the general head of phosphate rock are included: (1) the typical guano, which is the consolidated excrement of certain marine birds inhabiting in great numbers small coral islands in the dry or rainless regions of the tropics; (2) the underlying coral rock, which is often changed to phosphate rock through the percolation of the rain-water falling on the guano; (3) accumulations of the bones and coprolites of the higher animals; (4) phosphatic limestones from which the carbonate of lime has been largely dissolved away, leaving the more insoluble phosphate of lime.

(5) Metamorphic Group (stratified silicates).—All the chemically and organically formed rocks which we have studied up to this point are simple, i.e., they consist each of only one essential mineral; but most of the rocks in this great group of silicates are mixed, or consist each of several essential minerals. Quartz is the only important constituent of these rocks which is not, strictly speaking, a silicate, but in a certain sense it is also not an exception, since it may always be regarded as an excess of acid in the rock.

This group of stratified rocks composed of silicate minerals is of exceptional importance, first, on account of the large number of species which it includes, and, second, on account of the vast abundance of some of the species. These are, above all others, the rocks of which the earth’s crust is composed. With unimportant exceptions, all the rocks of this group are crystalline; and they constitute the principal part of what is generally included under the term metamorphic rocks—a general name for all stratified rocks which have been so acted upon by heat, pressure, or chemical forces as to make them crystalline. Although the crystalline limestone, dolomite, iron-ores, etc., show us that metamorphic rocks are not wanting in the other groups.

As already explained, the metamorphic or crystalline stratified rocks are usually older than the corresponding uncrystalline rocks; but a point of greater importance here is this: the development in the silicate rocks of crystalline characters has usually made it impossible to determine the method of their deposition, whether mechanical or chemical. In a few cases, as with the rock greensand, we know that the deposition is chemical; while it is equally certain that such common silicate rocks as gneiss, mica schist, and many others, often result from the crystallization of ordinary mechanical sediments, like sandstone and conglomerate. We classify all these rocks as of chemical origin, however, without considering the mode of their deposition, because the subsequent crystallization is itself essentially a chemical process; and that justifies us in saying that these rocks are made what they now are chiefly by the action of chemical forces. Whatever they were originally, they have become, through their crystallization, rocks having a definite mineral composition which can be classified chemically.

Some of the details of the classification of this group, as shown in the table, require explanation. In studying the silicate minerals it was stated to be important to recognize two classes—the acidic and the basic—the dividing line falling in the neighborhood of 60 per cent. of silica. This division is important simply because Nature has in a great degree kept the acidic and basic minerals separate in the rocks; and few things in lithology are more important than the distinction of the silicate rocks in which acidic minerals predominate from those in which basic minerals predominate. The amount of silica which any rock of this group contains is shown at a glance by the chart. The vertical broken lines, with the figures at the top, indicate the proportion of silica, which increases from 30 per cent. on the right to 85 per cent. on the left; so that the percentage of silica which a rock contains determines its position, the acidic species being on the left, and the basic on the right. As most of these rocks are composed of two or more minerals mixed in very various proportions, there is usually a wide range in the percentage of silica which the same species may contain; and this is expressed in each case by the length of the dotted line under the name of the rock. Thus, in syenite, the silica ranges from 55 per cent. to 65 per cent. The horizontal line in the chart separates the gneisses, containing feldspar as an essential constituent, from the schists, in which feldspar is wanting, except as an accessory constituent. We will take up the gneisses first.

Gneiss.—This is the most important of all rocks. It forms not far from one-half of New England, and a very large proportion of the earth’s crust. The name (pronounced same as nice) is known to have originated among the Saxon miners, but its precise derivation is lost in obscurity. To find out what this very important rock is, we will consult specimen 41. The first glance shows us that it is not, like the rocks we have just been studying, composed of a single mineral, but of several minerals, the most conspicuous of which is the pink feldspar—orthoclase. This we recognize as a feldspar: (1) by its hardness, which is a little less than that of quartz, and distinguishes it from calcite, a mineral having the general appearance of feldspar; (2) by its color, which separates it from hornblende and augite; and (3) by its cleavage, which distinguishes it easily from quartz. Finally, we know it is orthoclase, and not plagioclase, by its general aspect, and by its association with an abundance of quartz, which is the next most important constituent of the rock. The quartz is less abundant than the orthoclase, and more easily overlooked, yet anyone familiar with the mineral will not fail to recognize it. It forms small, irregular, glassy grains, entirely devoid of cleavage, and scratching glass easily. On weathered surfaces of the rock the orthoclase becomes soft and chalky, while the quartz remains clear and hard, and then the two minerals are very easily distinguished. Besides these, there are numerous black, thin, glistening scales, which we can easily prove to be elastic, and recognize as mica.

In most books on the subject, these three minerals—orthoclase, quartz, and mica—are set down as the normal or essential constituents of gneiss. But it is now recognized by the best lithologists that we may have true gneiss without any mica; or we may have hornblende in the place of mica. Quartz and orthoclase are the only essential constituents of gneiss; and when these alone are present, we have the variety known as binary gneiss. The addition to these essential constituents of mica, gives micaceous gneiss; and of hornblende, hornblendic gneiss. Of these three principal varieties, the micaceous gneiss is by far the most common and important. The mica may be either the white species, muscovite, or the black species, biotite; but it is usually the former.

Orthoclase is the predominant constituent in all typical gneiss, usually forming at least one-half of the rock. The orthoclase may, however, be replaced to a greater or less extent by albite, or even by oligoclase. But we frequently see the term gneiss carelessly, or ignorantly, applied to rocks which are destitute of feldspar, though having the general aspect of gneiss.

Augite rarely occurs in gneiss; and hence, when we observe a gneiss containing a black mineral which we know is either augite or hornblende, it is pretty safe to call it the latter.