Chrysocolla

(mineralogy) CuSiO₃·2H₂O A silicate mineral ordinarily occurring in impure cryptocrystalline crusts and masses with conchoidal fracture; a minor ore of copper; luster is vitreous, and color is normally emerald green to greenish-blue.

A silicate mineral, composition CuSiO₃ · 2H₂O. Small acicular crystals have been observed, but it ordinarily occurs in impure cryptocrystalline crusts and masses with conchoidal fracture. The hardness varies from 2 to 4 on Mohs scale, and the specific gravity varies from 2.0 to 2.4. The luster is vitreous and it is normally green to greenish-blue, but may be brown to black when impure. Chrysocolla is a secondary mineral occurring in the oxidized zones of copper deposits, where it is associated with malachite, azurite, native copper, and cuprite. It is a minor ore of copper.

Environment

In the oxidized zone of copper deposits, mainly in arid climates.

Microcrystalline, usually in solid vein-filling or botryoidal masses, from dull to glassy and opal-like in appearance. Convincingly crystal-like needles were only found at one locality (Mackay, Idaho).

Sky blue to greenish blue and green, often streaked with black. Luster glassy, dull, or earthy; hardness 2-4; specific gravity 2.0-2.4; fracture conchoidal. Physically unstable, sectile to brittle.

Basic copper silicate (45.2% CuO, 34.3% SiO ₂ , 20.5% H ₂ O).

Tongue usually clings to specimen. Blackens and gives water in closed tube. Practically infusible, but decomposed by hydrochloric acid with a separation of silica. Desiccated material, glassy or matte and long in a collection, tends to take up water and crumble when immersed, so never wash. Breaks apart violently in a flame.

Only likely to be confused with much harder turquoise (6, so not scratched by a knife and it does not stick to the tongue) and with chrysocolla-impregnated chalcedony which has, of course, the quartz hardness of 7.

Chrysocolla occurs very widely in the Southwest in copper deposits. Blue chrysocolla-impregnated quartz, covered by sparkling little crystals of white quartz, from the Globe Mine, Gila Co., Arizona, and Mexico are frequent in collections. Elsewhere, as in Superior, Arizona, the chrysocolla is spread through the chalcedony to color a hard and attractive chrysoprase-like gem material (which amateur lapidaries loosely tend to call chrysocolla, too). Fine massive specimens were found, in the early stages, in most of the w. U.S. copper mines. Still found in abundance in Africa and Chile. Russia and England (Cornwall and Cumberland) once produced good specimens. Glassy-luster specimens are uncommon.

It is an ore of copper when associated with other secondary copper minerals. In the West it often occurs impregnating and tinting otherwise worthless rock with associated copper minerals, as in Israel (Elat stone), and used like turquoise for jewelry. True chrysocolla, free of quartz, is fragile and likely to crack as it loses water in dry cabinet environments.

Chrysocolla
Chrysocolla from Nevada, USA
General
Category	Silicate mineral
Chemical formula	(Cu,Al)2H2Si2O5(OH)4·nH2O
Identification
Color	Blue, blue-green, green
Crystal habit	Massive, nodular, botryoidal
Crystal system	Orthorhombic
Cleavage	none
Fracture	Brittle to sectile
Mohs scale hardness	2.5 - 3.5
Luster	Vitreous to dull
Streak	white to a blue-green color
Diaphaneity	Translucent to opaque
Specific gravity	1.9 - 2.4
Optical properties	Uniaxial (+)
Refractive index	nω = 1.460 nε = 1.570
Birefringence	+0.110

Chrysocolla (hydrated copper silicate) is a mineral, (Cu,Al)₂H₂Si₂O₅(OH)₄·nH₂O. It is of secondary origin and forms in the oxidation zones of copper ore bodies. Associated minerals are quartz, limonite, azurite, malachite, cuprite, and other secondary copper minerals.

Chrysocolla has an attractive blue-green colour and is a minor ore of copper, having a hardness of 2.5 to 3.5. It is also used as an ornamental stone. It is typically found as glassy botryoidal or rounded masses and crusts, or vein fillings. Because of its light color, it is sometimes confused with turquoise. Commonly it occurs only as pourous crusts unsuitable for gem use, but high quality, gem grade chrysocolla can be translucent and is highly prized.

Chrysocolla

Chrysocolla

The name comes from the Greek chrysos, "gold", and kolla, "glue", in allusion to the name of the material used to solder gold, and was first used by Theophrastus in 315 BCE.

Notable occurrences include Israel, Democratic Republic of Congo, Chile, Cornwall in England, and Arizona, Utah, New Mexico and Pennsylvania in the United States.

References

• Handbook of Mineralogy, 2001, Mineral Data Pub.
• Mindat
• Webmineral data

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