Once of the most important copper-ore minerals, chalcocite occurs as a secondary mineral in or near the oxidized zone of copper sulfide deposits. Under surface conditions, the primary copper sulfides are oxidized, the soluble sulfates formed move downward reacting to form chalcocite, and thus, enriching the ore in copper. The water table is the lower limit of the zone of oxidation and here a "chalcocite blanket" may form.

Mineral Photos courtesy of R.Weller/Cochise College