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Encyclopedia Britannica - Main :: CLI-COM |
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CLINTONITE , a group of micaceous minerals known as the " brittle micas." Like the micas and chlorites, they are mono-clinic in crystallization and have a perfect cleavage parallel to the flat surface of the plates or scales, but differ markedly from these in the brittleness of the laminae; they are also considerably harder, the hardness of chloritoid being as high as 61 on Mohs' scale. They differ chemically from the micas in containing less silica and no alkalis, and from the chlorites in containing much less water; in many respects they are intermediate between the micas and chlorites. The following species are distinguished: Margarite is a basic calcium aluminium silicate, H2CaAl4Si2O12, and is classed by some authors as a lime-mica. It forms white pearly scales, and was at first known as pearl-mica and after-wards as margarite, from apyapirns, a pearl. It is a characteristic associate of corundum
Minor and the Grecian Archipelago, and with corundum
Seybertite, Brandisite and Xanthophyllite are closely allied species consisting of basic magnesium, calcium and aluminium silicate, and have been regarded as isomorphous mixtures of a silicate (H2CaM$4Si3O12) and an aluminate (H2CaMgA16O12). Seybertite (the original
York
Chloritoid has the formula
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