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GRANULITE (Lat. granulum, a little gr...
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See also:GRANULITE (See also:Lat. granulum, a little See also:grain) , a name used by petrographers to designate two distinct classes of rocks. According to the terminology of the See also:French school it signifies a See also:granite in which both kinds of See also:mica (See also:muscovite and See also:biotite) occur, and corresponds to the See also:German Granit, or to the See also:English " muscovite biotite granite." This application has not been accepted generally. To the German petrologists " granulite " means a more or less banded See also:fine-grained metamorphic See also:rock, consisting mainly of See also:quartz and See also:felspar in very small irregular crystals, and containing usually also a See also:fair number of See also:minute rounded See also:pale-red garnets. Among English and See also:American geologists the See also:term is generally employed in this sense. The granulites are very closely, allied to the gneisses, as they consist of nearly the same minerals, but they are finer grained, have usually less perfect foliation, are more frequently garnetiferous, and have some See also:special features of microscopic structure. In the rocks of this See also:group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a See also:mosaic closely fitted together. The individual crystals have never perfect See also:form, and indeed rarely any traces of it. In some granulites they interlock, with irregular See also:borders; in others they have been See also:drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between the larger. This is especially true of the quartz and felspar which are the predominant minerals; mica always appears as See also:flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be See also:present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary schistosity, and they may be aggregated into bands—in which See also:case the granulites are indistinguishable from certain varieties of See also:gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the See also:eye as See also:pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals. The felspar of the granulites is mostly See also:orthoclase or cryptoperthite; See also:microcline, See also:oligoclase and See also:albite are also See also:common. Basic felspars occur only rarely. Among See also:accessory minerals, in addition to See also:apatite, See also:zircon, and See also:iron oxides, the following may be mentioned: See also:hornblende (not common), riebeckite (rare), See also:epidote and See also:zoisite, See also:calcite, See also:sphene, See also:andalusite, See also:sillimanite, kyanite, hercynite (a See also:green See also:spinel), See also:rutile, orthite and See also:tourmaline. Though occasionally we may find larger grains of felspar, quartz or epidote, it is more characteristic of these rocks that all the minerals are in small, nearly See also:uniform, imperfectly shaped individuals. On See also:account of the minuteness with which it has been described and the important controversies on points of theoretical See also:geology which have arisen regarding it, the granulite See also:district of See also:Saxony (around See also:Rosswein, Penig, &c.) may be considered the typical region for rocks of this group. It should be remembered that though granulites are probably the commonest rocks of this See also:country, they are mingled with granites, gneisses, gabbros, amphibolites, mica See also:schists and many other petrographical types. All of these rocks show more or less See also:metamorphism either of a thermal See also:character or due to pressure and crushing. The granites pass into gneiss and'granulite; the gabbros into flaser See also:gabbro and See also:amphibolite; the slates often contain andalusite or chiastolite, and show transitions to mica schists. At one See also:time these rocks were regarded as Archean gneisses of a special type. Johannes Georg See also:Lehmann propounded the See also:hypothesis that their present See also:state was due principally to crushing acting on them in a solid See also:condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them together into coherent rock. It is now believed, however, that they are comparatively See also:recent and include sedimentary rocks, partly of Palaeozoic See also:age, and intrusive masses which may be nearly massive or may have gneissose, flaser or granulitic structures. These have been See also:developed largely by the injection of semi-consolidated highly viscous intrusions, and the varieties of texture are See also:original or were produced very shortly after the See also:crystallization of the rocks. Meanwhile, however, Lehmann's advocacy of See also:post-consolidation crushing as a See also:factor in the development of granulites has been so successful that the terms granulitization and granulitic structures are widely employed to indicate the results of dynamometamorphism acting on rocks at a See also:period See also:long after their solidification. The Saxon granulites are apparently for the most See also:part igneous and correspond in See also:composition to granites and porphyries. There are, however, many granulites which undoubtedly were originally sediments (arkoses, grits and sandstones). A large part of the See also:highlands of See also:Scotland consists of paragranulites of this See also:kind, which have received the group name of " Moine gneisses." Along with the typical See also:acid granulites above described, in Saxony, See also:India, Scotland and other countries there occur dark-coloured basic granulites (" See also:trap granulites "). These are fine-grained rocks, not usually banded, nearly See also:black in See also:colour with small red spots of See also:garnet. Their essential minerals are See also:pyroxene, See also:plagioclase and garnet: chemically they resemble the gabbros. Green See also:augite and See also:hypersthene form a considerable part of these rocks, they may contain also biotite, hornblende and quartz. Around the garnets there is often a radial grouping of small grains of pyroxene and hornblende in a clear See also:matrix of felspar: these " centric " structures are frequent in granu-lites. The rocks of this group accompany gabbro and See also:serpentine, but the exact conditions under which they are formed and the significance of their structures is not very clearly understood. (J. S. Additional information and CommentsThere are no comments yet for this article.
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