GRANULITE (Lat. granulum, a little grain), a name used by petrographers to designate two distinct classes of rocks. Ac cording to the terminology of the French school it signifies a granite in which both kinds of mica (muscovite and biotite) occur, and corresponds to the German Granit, or to the English "musco vite—biotite—granite." This application has not been accepted generally. To the German petrologists "granulite" means a more or less banded fine-grained metamorphic rock, consisting mainly of quartz and felspar in very small irregular crystals, and con taining usually also a fair number of minute rounded pale-red garnets. Among English and American geologists the 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 special features of microscopic structure. In the rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a mosaic closely fitted together. The individual crystals have never perfect form, and indeed rarely any traces of it. In some granulites they interlock, with irregular borders; in others they have been 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. Both muscovite and biotite may be 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 case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible as 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 orthoclase or micro perthite; microcline, oligoclase and albite are also common. Basic felspars occur only rarely. Among accessory minerals, in addition to apatite, zircon and iron oxides, the following may be men tioned : hornblende (not common), riebeckite (rare), epidote and zoisite, calcite, sphene, andalusite, sillimanite, kyanite, hercynite (a green spinel), rutile, orthite and tourmaline.
On account of the minuteness with which it has been described and the important controversies on points of theoretical geology which have arisen regarding it, the granulite district of Saxony (around Rosswein, Penig, etc.) may be considered the typical region for rocks of this group. Very similar granulites occur in Austria near Krems (in the Wald-Viertel). J. G. Lehmann pro pounded the hypothesis that the present state of the Saxon gran ulites was due principally to crushing acting on them in a solid condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them to gether into coherent rock. It is now believed, however, that they are comparatively recent and include sedimentary rocks, partly of Palaeozoic age, and intrusive masses which may be nearly massive or may have gneissose, veined or granulitic structures. These have been developed largely by the injection of semi-con solidated highly viscous intrusions, and the varieties of texture are original or were produced very shortly after the crystalliza tion of the rocks.
The Saxon and Austrian granulites are apparently for the most part igneous and correspond in composition to granites and por phyries. There are, however, many granulites which undoubtedly were originally sediments (arkoses, grits and sandstones). A large part of the highlands of Scotland consists of paragranulites of this kind, which have received the group name of "Moine gneisses." Along with the typical acid granulites above described, in Saxony, India, Scotland and other countries there occur dark coloured basic granulites ("trap granulites"). These are fine grained rocks, not usually banded, nearly black in colour with small red spots of garnet. Their essential minerals are pyroxene, plagioclase and garnet; chemically they resemble the gabbros. Green augite and 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 matrix of felspar; these "centric" structures are frequent in granulites.