PHYLLITE (Gr. <j>v\\ov, a leaf, probably because they yield leaf-like plates, owing to their fissility), in petrology, a group of rocks which are in practically all cases metamorphosed argillaceous sediments, consisting essentially of quartz, chlorite and muscovite, and possessing a well-marked parallel arrangement or schistosity. They form an intermediate term in the series of altered clays or shaly deposits between clay-slates and mica-schists. The clay-slates have a very similar mineral constitution to the phyllites, but are finer grained and are distinguished also by a very much better cleavage. In the phyllites also white mica (muscovite or sericite) is more abundant as a rule than in slate, and its crystalline plates are larger; the abundance of mica gives these rocks a glossy sheen on the smooth planes of fissility. Many of the best Welsh slates are rich in small scales of white mica, which polarize brightly between crossed nicols. The Cornish slates are still more micaceous and rather coarser grained, so that they might be called mica-slates or even phyllites.
A microscopical section of a typical phyllite shows green chlorite and colourless mica both in irregular plates disposed in parallel order, with a greater or smaller amount of quartz which forms small lenticular grains elongated parallel to the foliation. Grains of iron oxide (magnetite and haematite) and black graphitic dust are very commonly present. Feldspar is absent or scarce, but some phyllites are characterized by the development of small rounded grains of albite, often in considerable numbers. The minute needles of rutile, so often seen in clay-slates, are not often met with in phyllites, but this mineral forms small prisms which may be intergrown with black magnetite; at other times it occurs as networks of sagenite. Other phyllites contain carbonates (usually calcite but sometimes dolomite) in flat or spindle-shaped crystals, which often give evidence of crushing. Very tiny blue needles of Tourmaline are by no means rare in phyllites, though readily overlooked. Garnet occurs sometimes, a good example of garnetiferous phyllite being furnished by the whetstones of the Ardennes, in which there are many small isotropic crystals of magnesian garnet Hornblende, often in branching feathery crystals, is a less frequent accessory. In some phyllites a mineral of the chloritoid group makes 'its 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 rock, so that they seem to have developed after the movements and pressures which gave rise to the foliation had ceased.
The structural 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 building materials. They are exceedingly common in all parts of the world where metamorphic rocks occur; as in the Scottish Highlands, Cornwall, Anglesey, north-west Ireland, the Ardennes, the Harz Mountains, .Saxony, the Alps, Norway, the Appalachians, the Great Lakes district in America, etc. (J. S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)