File:Chalcocite (Lubin Copper Mine, Poland) 3.jpg

English: Chalcocite from Poland. (~5.3 centimeters across at its widest)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5700 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.

Chalcocite is a significant copper ore mineral having the formula Cu2S - copper sulfide. It has a metallic luster, a dark gray to black color and streak, no obvious cleavage, is moderately heavy for its size, and is moderately soft (H = 2.5 to 3). Chalcocite is found with other copper minerals such as chalcopyrite. It usually forms in the near-surface, secondary enrichment zone at and below the water table, in areas having copper mineralization. It also occurs as a primary mineral in some hydrothermal veins.

This chalcocite sample comes from a copper mine in Poland. Mining targets a copper sulfide-rich horizon known as the "Kupferschiefer" (= "copper shale"). The unit is a thin (less than 1 meter thick), black shale horizon in the Permian of many parts of northern Europe - for example, Germany, Poland, and parts of Britain. The horizon is estimated to be present at the surface or in the subsurface over an area of at least 20,000 square kilometers.

Reported metallic minerals include chalcocite, chalcopyrite (CuFeS2 - copper iron sulfide), bornite (Cu5FeS4 - copper iron sulfide), pyrite (FeS2 - iron sulfide), galena (PbS - lead sulfide), sphalerite (ZnS - zinc sulfide), tetrahedrite (Cu12Sb4S13 - copper antimony sulfide), and others. Minor amounts of precious metals, such as gold and platinum-group elements, are also known.

The origin of the Kupferschiefer's mineralization has been explained by several hypotheses in the literature. Traditionally, this stratabound copper sulfide deposit was interpreted as having formed by metal sulfide precipitation on an ancient Permian seafloor in stagnant water with reducing conditions.

Subsequent investigations have demonstrated that metal-rich fluids have gone through the Kupferschiefer, plus some overlying and underlying rocks, and precipitated various sulfide minerals. Two pulses of sulfide mineralization have been identified: at around 149 Ma (Late Jurassic) and 53 Ma (Eocene). Suggested causative events for the mineralization are the breakup of Pangaea during the Mesozoic and the closure of the Tethys Sea during the early Tertiary (see Borg et al., 2012).

Stratigraphic context: unrecorded/undisclosed

Locality: Lubin Copper Mine, southwestern Poland

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Photo gallery of chalcocite: www.mindat.org/gallery.php?min=962

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Info. at: de-m-wikipedia-org.translate.goog/wiki/Kupferschiefer?_x_...

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Partly synthesized from: Guilbert & Park (1986) - The Geology of Ore Deposits. 985 pp.

Borg et al. (2012) - An overview of the European Kupferschiefer Deposits. Society of Economic Geologists Special Publication 16: 455-486.