Chalcocite

Chalcocite


Composition: Cu2S
Crystal System : Monoclinic
Colours: Grey, silver-grey, grey-black
Lustre : Metallic
Hardness (H) : 2.5-3.0
Specific Gravity (S.G) : 5.5-5.8

Distribution

Abundant. An important ore mineral throughout the Tsumeb deposit.

Occurrence

  • Hypogene
  • Supergene
  • 1st Oxidation Zone
  • 2nd Oxidation Zone
  • 3rd Oxidation Zone

Notable Finds

Solid masses of chalcocite to 20 cubic metres occurred in the first oxidation zone. Acicular crystals (unusual habit) were found in the second oxidation zone. Large and well-formed crystals were found in the third oxidation zone, with perfect crystals to 40 mm found between 44 and 47 levels, some of them associated with stolzite and nickeloan siderite.

Paragenetic and General Notes

Chalcocite occurs mainly as a component of massive sulphide ores; well-formed crystals are comparatively rare. They occur in a diverse range of habits, however, ranging from tabular, through prismatic, to acicular. Twinning on {011} - the so-called V-twins, which commonly repeat to form pseudo-hexagonal plates - is common.

Most, if not all "chalcocite" at Tsumeb is an intimate mixture of at least two different species - chalcocite and djurleite; less commonly, digenite and roxbyite may also be involved. Chalcocite forms veins in the primary ore and gangue and is therefore late in the hypogene sequence (Keller, 1977). While chalcocite occurs in the primary (hypogene) assemblage at Tsumeb, much of the chalcocite, and probably all of the crystallised specimen material, is of supergene origin.

Chalcocite is reported to form pseudomorphs after the following minerals: enargite (rare); tennantite (rare).

The following minerals are reported to form pseudomorphs after chalcocite:  betekhtinite (rare); conichalcicte (rare); ludlockite (rare).