1911 Encyclopædia Britannica/Olivenite
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 "woodcopper." 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 descloizite (q.v.). (L. J. S.)