#chalcocite

Malachite and Chalcocite in Matrix

Cuprine rarity

While the sulphates and carbonates of copper are often a vivid blue or green, the sulphides are most often a dull grey or black, like the Djurleite here, a very rare member of the Chalcocite group (seehttp://bit.ly/2HyAYqN). It forms as a secondary mineral, after alteration in the deposit by the mineralised waters of the Earth, that dissolve and reprecipitate it in a different crystal structure, often containing more copper than slag compared to the original primary minerals. This process is known as supergene enrichment, and often makes all the difference in the viability of a prospective deposit because cashflow can be front loaded and higher grade ore processed first and loans more swiftly repaid. Most often it is massive, and attractive crystals like this are very rare and fetch eye watering prices from the committed collector. It can only be distinguished from Chalcocite by laboratory tests such as x-ray powder diffraction to reveal their different crystal structures (shining the rays through the crystal produces a characteristic pattern of dots on a photographic plate behind the mineral) it was named after a Swedish chemist who first synthesised it in 1958 before natural examples were discovered in 1962 in Mexico. Crystals are either prisms or tabular slabs, often as pseudohexagonal twins like some of the ones seen here. The mineral is both brittle and soft, being a mere 2.5 on Mohs hardness scale. The 8.6 x 4.4 x 3.6 cm piece of green matrix with crystals was mined in Morocco and this 2016 find of a single pocket is considered as the best of species worldwide so far mined. Analysis has of course confirmed their true identity.

Loz

Image credit: Rob Lavinsky/iRocks.com

Tennantite, Chalcocite

Cu12As4S13, Cu2S

Locality:

Manto Cobriza Mine, Copiapó, Copiapó Province, Atacama Region, Chile

Field of View: 4 mm

Small, untarnished chalcocite crystals on a larger crystal with iridescent coating. The surrounding matrix is calcite with exceptionally lustrous tennantite-tetrahedrite.

Tony Peterson’s Photo

Tennantite is a copper arsenic sulfosalt mineral with an ideal formula Cu12As4S13. Due to variable substitution of the copper by iron and zinc the formula is Cu6[Cu4(Fe,Zn)2]As4S13. It is found in hydrothermal veins and contact metamorphic deposits in association with other Cu–Pb–Zn–Ag sulfides and sulfosalts.  

Chalcocite, Djurleite & Chalcopyrite