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 1798, from xXwpIric, " a green stone." Several species and many rather ill-defined varieties 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 salt (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 crystallized species are grouped together by G . Tschermak as orthochlorites, 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 20%) as an essential constituent . The magnesia (up to 36%) is often in part replaced by ferrous oxide (up to 3o%), 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 composition of both clinochlore and penninite is approximately expressed by the formula H8(Mg,Fe)6Al2Si3O1s, and the formulae of pro-chlorite and corundophilite are H40(Mg,Fe)23Al,4Si13090 and H20(Mg,Fe)11Al8Si6046 respectively . The variation in composition of these orthochlorites is explained by G . Tschermak by assuming them to be isomorphous mixtures of H4Mg3Si2O9 (the serpentine molecule) and H4Mg3Al2SiO9 (which is approximately the composition of the chlorite amesite) . The leptochlorites are still more complex, and the intermixture of other fundamental molecules has to be assumed; the species recognized by Dana are daphnite, cronstedtite, thuringite, stilpnomelane, strigovite, diabantite, aphrosiderite, delessite and rumpfite . The chlorites usually occur as alteration products of other minerals, such as pyroxene, amphibole, biotite, garnet, &c., 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 . Well-crystallized specimens of the species clinochlore are found with crystals of garnet in cavities in chlorite-schist at Achmatovsk near Zlatoust, in the Urals, and at the Ala valley near Turin, Piedmont; also as large plates at West Chester in Pennsylvania and at other American localities . Crystals of penninite are found in serpentine at Zermatt in Switzerland and' in the green schists of the Zillerthal in Tirol . Closely allied to the chlorites is another group of micaceous minerals known as the vermiculites, which have resulted by the alteration of the micas, particularly biotite and phlogopite .

The name is from the Latin vermiculor, " to breed

worms," because when heated before the blowpipe these minerals ex-foliate into long worm-like threads . They have the same chemical constituents as the chlorites, but the composition is variable and indefinite, varying with that of the original mineral and the extent of its alteration . Several indistinct varieties have been named, the most important of which is jeff ersonite . (L . J .