Common Minerals and Rocks

by William O. Crosby

The most characteristic accessory constituents of norite, besides those already mentioned, are magnetite and chrysolite; though garnet, serpentine, and pyrite often occur. In texture, this rock varies from compact to very coarsely crystalline. The specimen of labradorite (No. 23), from the norite of Labrador, affords some idea of the coarseness of the crystallization in much of this rock. It is not a common rock, except in certain regions, the best known of which in eastern North America are the coast of Labrador, various points in Canada north of the St. Lawrence, and the eastern border of the Adirondack Mountains. In hand-specimens, norite rarely appears stratified; but in the solid ledges the stratification is often as distinct as could be desired.

Many lithologists call the rocks here designated norite gabbro, and class them all in the eruptive division as essentially a coarse variety of diabase. In a similar manner, diorite and syenite are denied a place in the sedimentary series. But the stratified plagioclase rocks seem to have as strong a claim to recognition as gneiss.

We turn now to the important and interesting division of the non-feldspathic rocks or schists.

Mica Schist.—This is, next to gneiss, the most abundant rock in New England. Specimen 43 is a typical example, and from it we can readily learn what mica schist is. A glance suffices to show that it is chiefly composed of mica, but not entirely; for, on carefully examining the edges of the specimen, we cannot fail to see thin layers of hard, glassy quartz interwoven with the mica. The quartz layers are short and overlapping, and we have here a good illustration of the schistose texture; this is, in fact, a typical schist.

Mica schist usually consists, as in this instance, of mica and quartz; but it may be composed of mica alone; and sometimes kaolin or clay takes the place of the quartz, forming argillaceous mica schist. The mica in the latter is usually in very fine scales and rather inconspicuous, and the rock often passes into ordinary clay slate. Similarly, when the mica becomes deficient in the quartzose mica schist, a passage into ordinary quartzite is the result. A little feldspar is sometimes present in the rock, which thus passes into micaceous gneiss. Specimen 43 contains several crystals of red garnet, giving the variety garnetiferous mica schist. There is no other rock that contains such a large variety of beautiful accessory minerals as mica schist; and for the mineralogist it is one of the most attractive rocks. Few rocks are more distinctly stratified; and the stratification can usually be observed in hand-specimens. The mica in these rocks may be either muscovite or biotite, or both; but the former is most common. No rock shows a greater variation in the percentage of silica which it contains than mica schist, as we pass from varieties which are nearly all quartz to those which are nearly all mica.

Closely related to mica schist is the rock now known as hydromica schist, in which the ordinary anhydrous micas are replaced by hydromica. It is distinguished from mica schist by being somewhat softer, less harsh to the touch, and less lustrous. It is to be regarded usually as an incipient mica schist, which has not yet become anhydrous; though it may sometimes be just the reverse; viz.: an old mica schist which has become hydrated through the action of meteoric waters. It contains fewer accessory minerals than mica schist.

Hornblende Schist.—This is a stratified aggregate of hornblende and quartz. The quartz is granular and in thin layers, as in mica schist; but the micaceous structure is wanting, and consequently the rock does not cleave readily in the direction of the bedding. The hornblende is mostly finely crystalline, but sometimes occurs in large, bladed crystals. Garnet and some other minerals are of common occurrence in the rock; but it is not rich in accessories like mica schist. The chief difficulty in recognizing this rock consists in determining whether the white mineral is all quartz or partly feldspar. In the latter case, of course, it becomes a hornblendic gneiss.

Amphibolite (Hornblende Rock).—This is the name applied to a rock having hornblende as its sole essential constituent. Hornblende schist sometimes passes into amphibolite, through the absence of quartz; and so does diorite, when the feldspar is deficient or wanting. Specimen 20, though small, is a typical example of this rock. The physical and chemical characteristics are essentially the same as for the mineral hornblende. The texture varies from coarsely to finely crystalline. The crystals are usually short and thick, and lie in all directions in the rock, which is thus very massive, the schistose texture being entirely wanting, and the stratification rarely showing in small masses. Biotite is a common accessory in amphibolite, and garnet and magnetite frequently occur.

By the substitution of augite for hornblende, in the description of amphibolite, we get the much rarer, but otherwise very similar, rock, pyroxenite.

Talc Schist (Steatite or Soapstone).—Although not abundant, this is a useful and familiar rock. The composition is implied in the name; and by comparing it with the specimen of talc (No. 58) we can readily see that they are essentially identical. Typical talc schist is pure talc; but the talc is often mixed with more or less quartz or feldspar; and mica, chlorite, hornblende, garnet, and other minerals are of common occurrence.