Specimen #1225

Name: Chalcocite

Type: Micromount

Locality: Tincroft Mine, Carn Brea and Tincroft United Mine, Carn Brea area, Camborne - Redruth - St Day District, Cornwall, England, United Kingdom

Favorite? N

Fluorescent? N

Size:

Weight:

Acquired: 2009-01-24

Source: Ian Heales Collection

Label: N

Comment: Chalcocite is an important copper mineral ore. It has been mined for centuries and is one of the most profitable copper ores. The reasons for this is its high copper content (67% atomic ratio and nearly 80% by weight) and the ease at which copper can be separated from sulfur. It is not however the primary ore of copper due to its scarcity. Although the richest chalcocite deposits have probably been mined out, it is still being mined and will almost certainly always be mined in the future.

Chalcocite occurs as a secondary mineral in many ore bodies in a zone called the supergene enrichment zone. Called a secondary enrichment mineral, although also a primary mineral as well, chalcocite commonly forms from the alteration of primary copper minerals that are attacked above the water table by oxygen. The oxygenated copper fluids descend to the water table where a reaction with primary ores results in the copper being reduced back to a sulfide, most commonly chalcocite. Ore bodies will have a layer of chalcocite which corresponds to the present or a past water table level and this layer is called a "chalcocite blanket". The chalcocite blanket is richer in copper than the upper oxidized portion of the ore body and usually richer than the primary unaltered ores below. The chalcocite blanket represents a real gold mine, or should that be copper mine, to the copper prospectors.

Fine crystals of chalcocite are quite uncommon and are much sought after. The now depleted mines at Cornwall, England and Bristol, Connecticut produced the most famous clusters of wonderfully formed chalcocite crystals. Some new localities with well formed crystals are promising, but so far the specimens from those old mines are the only good chalcocite crystals available on the market. The heavily striated pseudohexagonal tabular crystals are real classics for the mineral collector and often command an equally classic price.

Minerals: Chalcocite (Cu₂S)

There is one picture on file

M01225-01.jpg:

Chalcocite, Tincroft Mine, Carn Brea and Tincroft United Mine, Carn Brea area, Camborne - Redruth - St Day District, Cornwall, England, UK - FOV: 7.4 x 3.6 mm (9 stacked images)