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Mica-Schist

1911 Encyclopedia Britannica

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In petrology, a rock composed essentially of mica and quartz, and having a thin parallel-banded or foliated structure, with lamellae rich in mica alternating with others which are principally quartz. They split readily along the micaceous films, and have smooth or slightly uneven surfaces covered with lustrous plates of muscovite or biotite; the quartzose lamellae are often visible only when the specimens are looked at edgewise. Mica-schists are very common in regions of Archean rocks accompanying gneisses, crystalline limestones and other schists. Some have a flat banding yielding smooth slabs; others are crumpled or contorted with undulating foliation. Occasionally the quartz forms elliptical lenticles or "eyes." In some cases mica composes nearly the whole of the rock, in others quartz preponderates so that they approach quartz-schists and quartzites.

The mica may be muscovite or biotite; both are often present, while paragonite and green fuchsite or chrome-mica are not so common. In addition to quartz there may be a small amount of feldspar, usually albite. A great number of accessory minerals are known in mica-schists, and when these are conspicuous or important they may be regarded as constituting special varieties receiving distinctive names. Garnet, in rounded red crystals, not uncommonly idiomorphic, is the most frequent. Brown staurolite, pinkish andalusite, and grey or blue kyanite occur in some kinds of micaschist, separately or together. The white mica-schist of the St Gothard contains kyanite and staurolite. Graphite (or graphitoid) is also a very frequent ingredient of these rocks, giving them a leaden grey colour and causing them to soil the fingers when handled. In some mica-schists there is much calcite (calc-mica-schists); and hornblende, scapolite and augite are often seen in rocks of this sort. Tourmaline occurs, sometimes in large black prisms but more commonly in minute crystals visible only in microscopic sections. Rutile in tiny prisms, ilmenite and hematite in black or brown scales, zircon, apatite, granules of epidote or zoisite chlorite, chloritoid and pyrites occur with more or less frequency in the rocks of this group.

Mica-schists are in nearly all cases sedimentary rocks which have been recrystallized and have obtained a schistose structure during the process. This can be proved by their chemical composition, which is very much the same as that of clays, shales and slates. In some districts it is possible to trace every gradation from a slate to a mica-schist, the intermediate stages being represented by phyllites (q.v.) which consist of quartz, muscovite and chlorite, and are neither so crystalline nor so well foliated as the schists. In a few places, e.g. Bergen in Norway, fossils have been found in mica schists. The association of quartzites and quartz-schists, graphiteschists and crystalline limestones with mica-schists in the field is explained by the fact that all these rocks are altered sediments, viz. sandstones, carbonaceous shales and limestones.

Under the microscope the appearance presented by mica-schists differs according to whether the rock is cut parallel to or across the planes of foliation. In the latter case thin alternating bands composed of black or brown mica, and of quartz, cross the field of view (see Petrology, Plate 4, fig. 8). The mica scales have their cleavages and their flat sides parallel; the quartz occurs in rounded, elliptical or irregular grains, with usually a small admixture of feldspar (albite, oligoclase, orthoclase); apatite and iron oxides are rarely absent from these rocks. If garnet is present it may form large well-shaped crystals containing innumerable enclosures of quartz, biotite and iron ores; in some cases the garnets are cracked as if they had been broken by the pressures to which the rock had been subjected. Often the garnets are surrounded by small "eyes" of quartz, and they may be embedded in green chlorite, which is probably a secondary or decomposition product. Some mica-schists are rich in iron oxides and pass into haematite-schists (itabirites). When graphite occurs in mica-schists its crystals are small flat plates perfectly opaque even in the thinnest sections.

Like all metamorphic rocks, mica-schists are principally found in Archean areas; the great majority of them are of pre-Cambrian age. There are, however, in the Alps, Himalayas, &c., many rocks of this sort which are believed to be secondary or even tertiary; the evidence for this is not in all cases satisfactory, as of course the fossils, which if preserved would be sufficient to prove it, are nearly always destroyed by the metamorphism. Mica-schists are rarely of economic value, being too fissile for building-stones and too brittle for roofing-slates. They are of wide-spread distribution in the Scottish Highlands, Norway and Sweden, Bohemia, Saxony, Brittany, the Alps, many parts of North America, &c. (J. S. F.)