Mica-schist is a rock composed essentially of quartz and mica, usually either muscovite or biotite. The mica is the prominent mineral, occurring in irregular leaves and in foliated masses. The mica plates all lie with their cleavage planes parallel to each other and give to the rock a striking laminated or "schistose" structure. Next to the gneisses, the mica-schists are the most common metamorphic rocks. Mica-schist is a rock which is closely related on the one hand to gneiss, and on the other to quartzite. It is not only a very common companion of gneisses, in regions of metamorphic rocks, but in many places gneiss grades into mica-schist, so that no definite line can be drawn between them. It has also many other analogies with gneiss, some of which will be presently mentioned. Of that great class of rocks known as schists, it is, excluding gneiss, if the latter be reckoned among them, the most widely distributed and important.
Composition Minerals and Texture: The essential minerals of mica-schist are quartz and mica, and it is especially the latter which gives the rock its particular character. Different varieties of mica occur; the most common is a silvery white muscovite; biotite of a dark color is common, while the soda-bearing mica paragonite is rare. Muscovite and biotite occur alone, and also in combination, as in gneiss. The micas are in irregular leaves or tablets, without crystal boundaries, or in leafy or foliated aggregates; biotite and muscovite are found intergrown, and often so that they have a common cleavage. The micas lie with their cleavage planes in the direction of schistosity, and it is this which produces the extraordinary fissile character of the rock. They are also very often curved, bent, or twisted, as may be easily seen by the reflections from their cleavage surface. The cleavage of the mica is so marked that the surface of chief fracture, or the schistose plane of the rock, appears completely coated by it, and it may produce the impression that it is the only mineral present; to see the quartz, the other essential component, the cross fracture should be examined with the lens. The quartz forms irregular grains, or aggregates of grains, and these are sometimes arranged in small lenses, and sometimes in thin layers, concordant with the layers of mica.
Mica-schists, while they are very often composed of these two minerals alone, also very commonly carry crystals, often of large size, of other minerals. The most common of these is a dark red garnet, sometimes sparsely, but generally thickly, sprinkled through the rock, and varying in size from that of coarse shot to that of a plum. These garnets are often in the form of simple, rounded nodules, but in most cases they show more or less distinct crystal form, and sometimes they are beautifully crystallized in the shapes mentioned in the description of this mineral. This garnetiferous variety of mica-schist is a very common metamorphic rock; in New England it is widely distributed among the boulders of the glacial drift. Other minerals which occur in mica-schist, in a manner similar to garnet, are staurolite, often with garnet, kyanite, epidote, andalusite, and hornblende. These sometimes are in large and well-formed crystals, which, especially staurolite, andalusite, and kyanite, are not infrequently colored dark, by included carbonaceous matter. Graphite occurs in some mica-schists in quantity sufficient to produce a distinct variety. Graphite is such a strong coloring matter, that a relatively small amount will cause the rock to appear as if almost entirely composed of it; in consequence unsuccessful attempts have been made in places to exploit such schists for graphite.
Hornblende, when it occurs, is in dark-colored prisms; by its increase in amount transitions into amphibolite or hornblende schist are formed. Kyanite, andalusite, and staurolite occur in prismatic crystals, which may attain a length of several inches. Their formation is contemporaneous with the metamorphism of the rock, and they produce a pseudo-porphyritic texture. Another variety of mica-schist is one which contains more or less calcite mingled with the quartz; it is readily detected by its effervescence with acids. This variety is especially apt to contain accessory garnet, epidote, hornblende, etc. The parallel texture of the rock is its especial feature, and its ready fissility is produced by the mica. If the components are in thin, parallel layers, the surface of rock cleavage is smooth and flat; if the lenticular arrangement of the quartz is prominent, the surface is uneven or lumpy. Frequently the surfaces of schistosity are bent, folded and crumpled, showing pressures and shearing secondary to its production.
Chemical Composition: As in the gneisses, the chemical composition of these rocks is too variable a feature to be of specific value. This comes from the natural variability in the composition of the sediments from which they are formed. In addition, not many of these rocks have been chemically investigated, and some of the older analyses have been very poorly executed. It is clear, however, that they must contain silica, alumina, and potash, to form the quartz and mica, and also magnesia and iron, if biotite is present. The excess of magnesia over lime, taken with the high silica, is a character foreign to igneous rocks, and is clearly indicative of sedimentary origin. They are probably formed mostly by the metamorphism of feldspathic sandstones.
The color of these rocks varies from light gray through yellow and brown tones to very dark, depending on the ratio of light to dark mica present. Carbonaceous is also present in some types. Some pure mica schist are silvery or light gray. The hardness and firmness of the rock depend on the proportion of mica; the more this is present, the softer and more easily cleavable it is. For this reason they are of little or no value for practical purposes. Inclusions of various kinds occur in mica-schists as in gneiss, thus veins and lenticular masses of quartz, deposited from solution in cracks and cavities opened by movement and foldings of the rocks, are common. They also contain in places lenticular masses of other schists, which may vary from very small to huge dimensions. And sometimes they are penetrated by seams and patches of granite, felsite, and pegmatite as the result of granitic injections. With respect to the alteration of mica-schist, the varieties composed of muscovite are chiefly mechanically disintegrated by the action of weathering without much chemical change. The muscovite resists alteration energetically, and the gravelly or sandy soils formed, are in consequence filled with its sparkling flakes. Where much biotite is present it alters easily; the rocks turn yellow or brown, lose their luster, and eventually much limonite is separated out.
Varieties and Occurrence: The varieties composed chiefly of muscovite, or with associated garnet, are the most usual kinds, and are found all over the world as common rocks in metamorphic regions, and are generally associated with gneisses. They cover large areas in New England and extend southward to Georgia. Biotitic varieties are also very commonly found with them. Staurolitic mica-schist occurs in many places in New England, and in Maryland, and elsewhere along the Piedmont plateau; it is found in Scotland and various localities in Europe, in Brazil and elsewhere. Kyanite-mica-schist occurs in various places in New England; a variety in which the mica is paragonite comes from the St. Gothard region in the Alps, and is seen in mineral collections on account of the beautiful crystals of kyanite it contains; the common kind with muscovite is found in many places. Hornblendic mica-schists occur as included lenticular masses, often of large dimensions, in various places, in the ordinary mica-schists. Graphitic mica-schist is found in Connecticut and other places in New England, various localities in Germany, Norway, etc. Andalusite-mica schist occurs in the White Mountains in New England, in Scotland, Spain, Germany, etc. An interesting variety is the conglomerate-mica-schist, in which the rock contains pebbles of quartz, granite, and other rocks which are very apt to be flattened, lenticular, or drawn out by pressure and shearing. It is closely related to the conglomerate-gneiss previously described and has had a similar origin. Such rocks occur in Massachusetts, in Vermont, Scotland, Sweden, etc.
Transitions and Relation to other Rocks: The gneisses formed from sediments and the mica-schists have both been made from similar rocks; from feldspathic sandstones, shales and conglomerates. In the mica-schists the feldspar has been converted into mica; in the gneisses it has mostly persisted or been recrystallized. It is not intended in this statement to affirm that this is the only origin for mica-schists, only the most usual one; they may have been formed in some cases from quartzose-feldspathic igneous rocks, though positive evidence on this point is wanting. In this connection what is said elsewhere of phyllites should be consulted. On the whole it would seem most probable that the gneisses have been formed most often from the conglomerates and coarser-grained sandstones, the mica-schists from the finer-grained ones, and from the shales, though many exceptions must occur. It is therefore easy to understand that many mica-schists contain more or less of feldspar grains among those of quartz, which are difficult to detect without the aid of microscopic investigation. These may increase in amount until the rock passes over into a gneiss, and no hard and fast line can be drawn between them, as previously stated.
The decision as to whether a given rock should be classed as a gneiss or mica-schist is often a very difficult thing to make on purely megascopic grounds; in general if the amount of mica is large, and little or no feldspar can be detected with the lens, it is best to classify it as a mica-schist; if the amount of mica is small and feldspar can be seen, to define it as a gneiss. On the other hand, in proportion as the original sandstones were more and more purely composed of quartz grains there would be less and less of mica made, and in this way formal transitions into quartz schist and quartzite are produced in their metamorphic representatives. We thus see that gneiss, mica-schist, and quartzite form a graded series whose divisional lines must be purely arbitrary. Again, as the rocks become finer and finer in grain and in texture, the mica-schists pass into micaceous slates and so on into slates, and this becomes more marked if the amount of carbonaceous matter increases, as it tends to mask the mica. The divisional line thus becomes an arbitrary one in this case also.
Various Schists. There are various other kinds of schistose rocks, which are chiefly derived by the metamorphism of the ferromagnesian igneous rocks. The most important types are talc-schist, chlorite schist, amphibolite or hornblende-schist. They each are characterized, as their names indicate, by the preponderance of some metamorphic ferromagnesian mineral.
AMPHIBOLITE OR
HORNBLENDE-SCHIST:
The amphibolites are a large group of metamorphic rocks whose
distinguishing characters are, that they consist partly or largely of
hornblende, and that they possess a more or less pronounced schistose
structure. There are a number of varieties in the group, depending on the
kind of hornblende present, and on the minerals associated with it, so that
it is difficult to give a general description which will cover all cases. It
is best therefore to describe the most common kind first, and then give a
brief mention of some of the less common varieties. Common hornblende-schists
or amphibolites are rocks which vary in color from green to black; the green
is of varying tones, clear light green, gray-green, yellowish green to dark
green, greenish black to black; the darker colors are more common. The color
is given by the hornblende, though in a considerable degree, in some cases,
it is influenced by admixed chlorite. The grain of the rocks varies from
coarse to fine, the latter being more common. When coarse, the hornblende,
which is almost always present in slender prisms or blades and rarely in
grains, is easily recognized by the eye from its form and bright, good
cleavage.
TALC-SCHIST:
Talc-schist is a rock of pronounced schistose cleavage and
character, in which talc is the predominant mineral. The talc is present in
fine scales to coarse foliated aggregates. Other minerals also occur in
different varieties of the rock, such as quartz in grains, lenses, and
veins; or
magnetite and
chromite in black specks and grains; hornblende, usually in
white or green prisms, or crystals of enstatite; chlorite mingled with the
talc, etc. The color is usually light, white to pale green, or yellowish, or
gray; sometimes dark gray or greenish. The rock is soft and the talc gives
it a greasy feeling, and often a pearly or tallowy appearance on the
cleavage surface. In addition to its micaceous appearance and soft greasy
feel, the talc is easily told by its infusibility before the blowpipe, and
its insolubility in acids. The rock cleavage is sometimes thinly fissile,
sometimes thicker, and sometimes cleavage is nearly wanting, the rock is
more nearly massive, is compact, and has a lard-like or wax-like aspect, and
approaches soapstone in character. Chemically, these rocks consist mostly of
silica and magnesia with small amounts of water and other oxides.
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