CHRYSOCOLLA

This article appears in Volume V06, Page 320 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: CHR-CLI

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

APPEARANCE (from Lat. apparere, to appear)

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

OXIDE

(CuO), for example, varying in different analyses from 17 to 67%; the

formula

is usually given as CuSiO3+2H2O. The hardness (24) and specific gravity (2.02.8) 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

SILICA

, &c.) are also commonly present, and several varieties have been distinguished by

special

SPECIAL

names: thus dillenburgite, from Dillenburg in Nassau, contains copper carbonate; demidoffite and cyanochalcite contain copper phosphate; and pilarite contains alumina (perhaps as

allophane

ALLOPHANE

). The

mineral

MINERAL

occurs in the upper parts of veins of copper ores,and has resulted from their alteration by the action of waters containing

silica

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

ISLAND (O.E. ieg =isle, +land')

of Cyprus, may have been identical with the

mineral

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

APPEARANCE (from Lat. apparere, to appear)

. In thin sections it is translucent and optically isotropic, and

recent

RECENT

examinations seem to prove that it is a homogeneous mineral and not a mechanical mixture of chrysocolla and limonite. (L. J. S.)

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