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Encyclopedia Britannica - Main :: MEC-MIC |
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MICAH , in the Bible, a man of the hill-country of Ephraim whose history enters into that of the foundation of the Israelite sanctuary at Dan (Judges xvii. seq.). He had stolen from his mother eleven hundred pieces of silver (for the number cf. Judges xvi. 5), and when she uttered a curse upon the unknown thief he restored the money and she consecrated it to Yahweh. A carved image was made and set up in his private temple together with an ephod
district
house
MICA-SCHIST,. 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. Occasion-ally 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
special
epidote
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 (q.v.) 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
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.)End of Article: MICAH If you wish, you can link directly to this article.
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