CLINTONITE, a group of micaceous minerals known as the "brittle micas." Like the micas and chlorites, they are monoclinic 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 6 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 afterwards as margarite, from , a pearl. It is a characteristic associate of corundum, of which it is frequently an alteration product (facts which suggested the synonymous names corundellite and emerylite), and is found in the emery deposits of Asia Minor and the Grecian Archipelago, and with corundum at several localities in the United States.
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 (H2CaMg4Si3O12) and an aluminate (H2CaMgAl6O12). Seybertite (the original clintonite) occurs as reddish-brown to copper-red, brittle, foliated masses in metamorphic limestone at Amity, New York; brandisite as yellowish-green hexagonal prisms in metamorphic limestone in the Fassathal, Tirol; xanthophyllite as yellow folia and as distinct crystals (waluewite) in chloritic schists in the Urals.
Chloritoid has the formula H2(Fe,Mg)Al2SiO7. It forms tabular crystals and scales, with indistinct hexagonal outlines, which are often curved or bent and aggregated in rosettes. The colour is dark grey or green; a characteristic feature is the pleochroism, the pleochroic colours varying from yellowish-green to indigo-blue. Hardness, 6; specific gravity, 3.4-3.6. It occurs as isolated scales scattered through schistose rocks and phyllites of dynamo-metamorphic origin. The ottrelites of the phyllites and ottrelite-schists of Ottrez and other localities in the Belgian Ardennes is a manganiferous variety of chloritoid, but owing to enclosed impurities the analyses differ widely from those of typical chloritoid.
(L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)