CHRYSOCOLLA  , a hydrous

copper silicate occurring as a decomposition product of copper ores . It is never found as crystals, but always as encrusting and botryoidal masses with 'a microcrystalline structure . It is green or bluish-green in colour, and often has the appearance of opal or enamel, being translucent and having a conchoidal fracture with vitreous lustre; some-times it is earthy in texture . Not being a definite crystallized substance; it varies widely in chemical composition, the copper oxide (CuO), for example, varying in different analyses from 17 to 67%; the formula is usually given as CuSiO3+2H2O . The hardness (2–4) and specific gravity (2.0–2.8) are also variable . It has recently been suggested that the material may really be a mixture of more than one hydrous copper silicate, since differences in the microcrystalline structure of the different concentric layers of which the masses are built up may be detected . Various impurities (silica, &c.) are also commonly present, and several varieties have been distinguished by special names: thus dillenburgite, from Dillenburg in Nassau, contains copper carbonate; demidoffite and cyanochalcite contain copper phosphate; and pilarite contains alumina (perhaps as allophane) . The mineral occurs in the upper parts of veins of copper ores,and has resulted from their alteration by the action of waters containing silica in solution . Pseudomorphs of chrysocolla after various copper minerals (e.g. cuprite) are not uncommon . It is found in most copper mines . The name chrysocolla (from xpveor, gold, and xoXXa, glue) was applied by Theophrastus and other ancient writers to materials used in soldering gold, one of which, from the island of Cyprus, may have been identical with the mineral now known by this name . Borax, which is used for this purpose, has also been called chrysocolla .

A mineral known as pitchy copper-ore (Ger . Kupferpecherz), and of some importance as an ore of copper, is usually classed as a variety of chrysocolla containing much admixed

limonite . It is dark brown to black in colour, with a dull to glassy or resinous lustre, and resembles pitch in appearance . In thin sections it is translucent and optically isotropic, and recent examinations seem to prove that it is a homogeneous mineral and not a mechanical mixture of chrysocolla and limonite . (L . J .