English: Native copper pseudomorph after aragonite (1.7 cm across at its widest) from the Tertiary of Bolivia.

Pseudomorphs are minerals that have replaced other minerals, while retaining the crystal shape of the original mineral. This is a hexagonal crystal of native copper (Cu). The original crystal was pseudohexagonal, cyclically twinned aragonite (CaCO3 - calcium carbonate). The very dark red is cuprite, the result of oxidation of the native copper. The greenish areas are malachite (Cu2CO3(OH)2 - copper hydroxy-carbonate).

This interesting specimen came from Bolivia's Corocoro Copper Mining District, but more specific provenance info. was not accompanying. It may be from the Umacoya Vein, a cupriferous horizon in coarse-grained siliciclastics of the Vetas Formation (Lower Miocene). Another possibility is that it came from the Ramos Formation (Eocene). Copper mineralization in the Corocoro Mining District is associated with the Corocoro Fault.

Locality: in or near the town of Corocoro, Corocoro Mining District, southern La Paz Department, western Bolivia.