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CHLORITE

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Originally appearing in Volume V06, Page 257 of the 1911 Encyclopedia Britannica.
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CHLORITE , a See also:

group of See also:green micaceous minerals which are hydrous silicates of See also:aluminium, See also:magnesium and ferrous See also:iron. The name was given by A. G. ;See also:Werner in 1798, from xXwpIric, " a green See also:stone." Several See also:species and many rather See also:ill-defined varieties have been described, but they are difficult to recognize. Like the micas, the chlorites (or " hydromicas ") are See also:monoclinic in See also:crystallization and have a perfect cleavage parallel to the See also:flat See also: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 See also:salt (H=2–3) scaly aggregates of a dark-green See also:colour. They vary in specific gravity between 2.6 and 3•o, according to the amount of iron See also:present. Well-See also: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 See also: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 See also:

water (about 13%) and by not containing alkalis; from the soft, scaly, See also:mineral See also:talc they differ in containing aluminium (about 20%) as an essential constituent. The See also:magnesia (up to 36%) is often in See also:part replaced by ferrous See also: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 See also:rose-red varieties kammererite and kotschubeite. The See also:composition of both clinochlore and penninite is approximately expressed by the See also:formula H8(Mg,Fe)6Al2Si3O1s, and the formulae of See also: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 See also:serpentine See also: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 See also:Dana are daphnite, cronstedtite, thuringite, stilpnomelane, strigovite, diabantite, aphrosiderite, delessite and rumpfite. The chlorites usually occur as alteration products of other minerals, such as See also:pyroxene, See also:amphibole, See also:biotite, See also: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 See also:Zlatoust, in the Urals, and at the See also:Ala valley near See also:Turin, See also:Piedmont; also as large plates at See also:West See also:Chester in See also:Pennsylvania and at other See also:American localities. Crystals of penninite are found in serpentine at See also:Zermatt in See also:Switzerland and' in the green See also:schists of the Zillerthal in See also: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 See also:phlogopite.

The name is from the Latin vermiculor, " to breed See also:

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

End of Article: CHLORITE

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