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See also:PHYLLITE (Gr. 40)XAov, a See also:leaf, probably because they yield leaf-like plates, owing to their fissility)
, in See also:petrology, a See also:group of rocks which are in practically all cases metamorphosed argillaceous sediments, consisting essentially of See also:quartz, See also:chlorite and See also:muscovite, and possessing a well-marked parallel arrangement or schistosity
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They See also:form an intermediate See also:term in the See also:series of altered See also:clays or shaly deposits between See also:clay-slates and See also:mica-See also:schists
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The clay-slates have a very similar See also:mineral constitution to the phyllites, but are finer grained and are distinguished also by a very much better cleavage
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In the phyllites also See also: See also:Garnet occurs some-times, a See also:good example of garnetiferous phyllite being furnished by the whetstones of the See also:Ardennes, in which there are many small isotropic crystals of magnesian garnet . See also:Hornblende, often in branching feathery crystals, is a less frequent See also:accessory . In some phyllites a mineral of the chloritoid group makes its See also:appearance; this may be ottrelite, sismondine or other varieties of chloritoid, and occurs in large sub-hexagonal plates showing complex twinning, and lying across the foliation planes of the See also:rock, so that they seem to have See also:developed after the movements and pressures which gave rise to the foliation had ceased . The structural See also:variations presented by the phyllites are comparatively few . The most finely crystalline specimens have generally the most perfect parallel arrangement of their constituents . The foliation is generally flat or linear, but in some rocks is undulose or crumpled . From the imperfection of their cleavage phyllites are rarely suitable for roofing materials; their softness renders them valueless as road stones, but they are not uncommonly employed as inferior See also:building materials . They are exceedingly See also:common in all parts of the See also:world where metamorphic rocks occur; as in the Scottish See also:Highlands, See also:Cornwall, See also:Anglesey, See also:north-See also:west See also:Ireland, the Ardennes, the Harz Mountains, iSaxony, the See also:Alps, See also:Norway, the Appalachians, the See also:Great Lakes See also:district in See also:America, &c . (J . S . |
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