MYLONITE (Gr. μυλών, a mill), in petrology, a rock which has been crushed and ground down by earth movement and at the same time rendered compact by pressure. Mylonites are fine-grained, sometimes even flinty, in appearance, and often banded in parallel fashion with stripes of varying composition. The great majority are quartzose rocks, such as quartzite and quartz-schist; but in almost any type of rock mylonitic structure may be developed. Gneisses of various kinds, hornblende-schists, chlorite-schists and limestones are not infrequently found in belts of mylonitic rock. The process of crushing by which mylonites are formed is known also as “granulitization” and “cataclasis,” and mylonites are often described as granulites, though the two terms are not strictly equivalent in all their applications. Mylonites occur in regions where there has been considerable metamorphism. Thrust planes and great reversed faults are often bounded by rocks which have all been crushed to fine slabby mylonites, that split readily along planes parallel to the direction in which movement has taken place. These “crush-belts” may be only a few feet or several hundred yards broad. The movements have probably taken place slowly without great rise of temperature, and hence the rocks have not recrystallized to any extent.
Crushing and movement on so extensive a scale are to be expected principally in regions consisting of rocks greatly folded and compressed. Hence mylonites are commonest in Archean regions, but may be found also in Carboniferous and later rocks where the necessary conditions have prevailed. Within a short space it is often possible to trace rocks from a normal to a highly mylonized condition, and to follow by means of the microscope all the stages of the process. A sandstone, grit, or fine quartzose conglomerate, for example, when it approaches a mylonitic zone begins to lose its clastic or pebbly structure. The rounded grains of quartz become cracked, especially near their edges, and are then surrounded by narrow borders, consisting of detached granules: this is due to the pebbles being pressed together and forced to pass one another as the rock yields to the pressures which overcome its rigidity. Then each quartz grain breaks up into a mosaic of little angular fragments; the rounded pebbles are flattened out and become lenticular or cake-shaped. Finally only a small oval patch of fine interlocking quartz grains is left to indicate the position of the pebble, and if the matrix is quartzose this gradually blends with it and a uniform fine-grained quartzose rock results. If felspar is present it may become crushed like quartz, but often tends to recrystallize as quartz and muscovite, the minute scales of white mica being parallel to the foliation or banding of the rock, and a finely granulitic or mylonitic quartz-schist is the product. In hornblendic rocks, such as epidiorite, amphibolite and hornblende-schist, the mineral composition may remain unchanged, but very often chlorite, carbonates and biotite develop, epidote and sphene being also frequent. Biotite- and muscovite-gneisses yield very perfect mylonites, in which the micas have parallel orientation, giving the rock a flat banding and marked schistosity (see Petrology, Pl. iv., fig. 6). When these mylonitic gneisses contain pink garnet (often with kyanite or sillimanite) they pass into normal granulites; limestones, if fossiliferous, become changed into finely crystalline masses, often fissile, sometimes with lenticular or augen structure. An interesting variety of mylonite, developed in granite-porphyry and gneiss, is fine, dark and almost vitreous in appearance, consisting mainly of very minute grains of quartz and felspar and resembling flint in appearance. These form threads and vein-like streaks ramifying through the normal rocks. Examples are furnished by the flinty-crushes of west Scotland and the “trap-shotten” gneisses of south India. (J. S. F.)