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 . µiµgrns, 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 32f 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 . KaµauXos, curved, on account of the remarkable curvature of the faces of the crystals .

(L . J .