OLIVENITE  , a

mineral consisting of basic copper arsenate with the formula Cu2(OH)AsO4• It crystallizes in the ortho- rhombic system, and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces . More usually, however, it occurs as globular aggregates of acicular crystals, these fibrous forms often having a velvety lustre: sometimes it is lamellar in structure, or soft and earthy . A characteristic feature, and one to which the name alludes (German, Olivenerz, of A . G . Werner, 1789), is the olive-green colour, which varies in shade from blackish-green in the crystals to almost white in the finely fibrous variety known as " wood-copper." The hardness is 3, and the sp. gr . 4'3 . The mineral was formerly found in some abundance, associated with limonite and quartz, in the upper workings in the copper mines of the St Day district in Cornwall; also near Redruth, and in the Tintic district in Utah . It is a mineral of secondary origin, having been formed by the alteration of copper ores and mispickel . The arsenic of olivenite is sometimes partly replaced by a small amount of phosphorus, and in the species libethenite we have the corresponding basic copper phosphate Cu2(OH)PO4 . This is found as small dark green crystals resembling olivenite at Libethen in Hungary, and in small amount also in Cornwall . Other members of this isomorphous group of minerals are adamite, Zn2(OH)AsO4, and descloiz?te (q.v.) . (L .

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