CHLORITE, a group of green micaceous minerals which are hydrous silicates of aluminium, magnesium and ferrous iron. The name was given by A. G. Werner in 1Q98, from XX wpLTCs, "a green stone." Several species and many rather ill-defined va rieties have been described, but they are difficult to recognize. Like the micas, the chlorites (or "hydromicas") are monoclinic in crystallization and have a perfect cleavage parallel to the flat face of the scales and plates. The cleavage is, however, not quite so prominent as in the micas, and the cleavage flakes though pliable are not elastic. The chlorites usually occur as sof t (H. = 2-3) scaly aggregates of a dark-green colour. They vary in specific gravity between 2.6 and 3.o, according to the amount of iron present. Well-developed crystals are met with only in the species clinochlore and penninite ; those of the former are six sided plates and are optically biaxial, whilst those of the latter have the form of acute rhombohedra and are usually optically uniaxial. The species prochlorite and corundophilite also occur as more or less distinct six-sided plates. These four better crystal lized species were grouped together by G. Tschermak as ortho chlorites, the finely scaly and indistinctly fibrous forms being grouped by the same author as leptochlorites.
Chemically, the chlorites are distinguished from the micas by the presence of a considerable amount of water (about 13%) and by not containing alkalis; from the soft, scaly, mineral talc they differ in containing aluminium (about 2o%) as an essential constituent. The magnesia (up to 36%) is often in part replaced by ferrous oxide (up to 30%), and the alumina to a lesser extent by ferric oxide ; alumina may also be partly replaced by chromic oxide, as in the rose-red varieties kammererite and kotschubeite.
The chlorites usually occur as alteration products of other minerals, such as pyroxene, amphibole, biotite, garnet, etc., often occurring as pseudomorphs after these, or as earthy material filling cavities in igneous rocks composed of these minerals. Many altered igneous rocks owe their green colour to the presence of secondary chlorite. Chlorite is also an important constituent of many schistose rocks and phyllites, and of chlorite-schist it is the only essential constituent. (L. J. S.)