The most striking feature of these rocks as determined in the field of optical mineralogy is their foliation, a structure which, superficially, may resemble bedding. However, a microscopic investigation with the aid of polarizing microscope should always enable the student to make the distinction. This arrangement of the constituent minerals in layers, however, is not always exhibited, and is often inconspicuous. Some of the rocks are holocrystalline, and are only to be distinguished from igneous rocks by their foliation, which, it should be remembered, is often more conspicuous in hand specimen than in section.
Although many minerals are found both in igneous and metamorphic rocks, some are found almost invariably only in the latter group, and are therefore very useful when present. Such minerals are chiastolite, sillimanite, kyanite, cordierite, tremolite, actinolite, glaucophane, forsterite, talc, and zoisite. Garnets also, particularly those containing lime, are much more common in metamorphic than in igneous rocks. Other minerals found commonly, but not exclusively, in metamorphic rocks are quartz, feldspar, micas, chlorite, diopside, epidote, and calcite.
All stages of crystallization are found in metamorphic rocks, the particular stage reached being reached dependent on the nature of the original rock and the amount of change it has undergone. There are several important metamorphic rocks determined in the field of optical mineralogy. These are the gneisses, schists, granulites, and crystalline limestones. Gneisses are characterized by their holocrystalline nature, and, as a rule, by a coarse foliation and abundance of feldspar. The varieties receive the name of granite-gneiss, diorite-gneiss, etc., according to the plutonic igneous rock they most closely resemble. A special structure-variety is that known as augen-gneiss, from the eye-like fragments of a conspicuous mineral, usually feldspar. Schists are conspicuously foliated, holocrystalline rocks, from which feldspar is absent or not abundant, and in which the folia are thinner than in gneisses. They are named according to some mineral which is abundant, as mica-schist, hornblende-schist, chlorite-schist, talc-schist, quartz-schists, etc. Some rocks which were originally argillaceous sediments, and in which metamorphosis has not been carried to any great extent, are chiastolite slate, phyllite, and spotted slate. Granulites are holocrystalline, inconspicuously foliated, and consisting of more or less rounded mineral grains, forming a mosaic. The rock is named from some conspicuous constituent, as pyroxene-granulite, kyanite-granulite, etc. Crystalline limestones consist to a greater or less extent of crystalline calcite. The rocks were originally more or less impure limestones, and the existing rock is the result of the re-arrangement of the constituents. Especially instructive cases are those in which the rock consists of calcium and magnesium silicates, calcite, and magnesium oxides. They are the result of the metamorphism of an impure dolomite, the process being known as dedolomisation.
In the endeavor to determine the nature of the rock from which a metamorphic rock was derived, bear in mind that the changes effected have been changes merely of arrangement of constituents, not changes in the composition of the rock as a whole. It is therefore of considerable importance that one must be familiar with the composition of the more common minerals. Such knowledge will help him materially in his task of harmonizing his isolated scraps of knowledge regarding rocks, a task which all beginners should undertake. However, exceptions to the general rule are found in the case of the rocks which contain such minerals as tourmaline, topaz, cassiterite, and some micas. These minerals, together with many others, most of them characterized by the presence of fluorine or boron, usually owe their origin to a process of pneumatolysis, a process which necessarily involves transport of material from one rock to another in the form of vapor.
