1911 Encyclopædia Britannica/Mimetite

by Leonard James Spencer

MIMETITE, a mineral consisting of lead chloro-arsenate, (PbCl)Pb₄(AsO₄)₃, crystallizing in the hexagonal system and closely resembling pyromorphite (q.v.) in appearance and general characters. The arsenic is usually partly replaced by equivalent amounts of phosphorus, and there may thus be a gradual passage from mimetite to pyromorphite. The two species can, as a rule, only be distinguished by chemical analysis, and because of their close resemblance the less frequently occurring chloro-arsenate was named mimetite or mimetesite, from Gr. μιμητής, imitator. Crystals of pyromorphite though usually optically uniaxial are sometimes biaxial, but in mimetite this anomalous character is almost always present; a cross-section of a hexagonal prism of mimetite shows a division into six optically biaxial sectors or a complex lamellated structure. In colour mimetite is usually yellow or brown, rarely white or colourless; the lustre is resinous to adamantine. The hardness is 3 1/2, and the specific gravity 7·0–7·25. Like pyromorphite, mimetite is found in the upper parts of veins of lead ore, where it has been formed by the oxidation of galena and mispickel. When found in large amount it is of importance as an ore of lead. The best crystallized specimens are those from Johanngeorgenstadt in Saxony and Wheal Unity in Cornwall. It was formerly found in considerable amount at Dry Gill in Cumberland, as six-sided barrel-shaped crystals of a brownish-red or orange-yellow colour and containing a considerable proportion of phosphoric acid; this variety has been called campylite, from Gr. καμπύλος, curved, on account of the remarkable curvature of the faces of the crystals.  (L. J. S.)