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Bismuthinite from Colorado, USA. (public display, University of Wyoming Geological Museum, Laramie, Wyoming, USA)

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 4900 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.

Bismuthinite is a scarce bismuth sulfide mineral (Bi2S3). It's similar to stibnite (www.flickr.com/photos/jsjgeology/sets/72157654303222578) in its physical properties, and fine-grained specimens are difficult to distinguish visually. Bismuthinite has a metallic luster, silver color, gray streak, one perfect cleavage, is fairly soft (H = 2 to 2.5), and has a high specific gravity (it's heavy for its size). Nicely crystalline specimens often display radiating masses of long, needle-shaped crystals. Finer-grained and massive bismuthinite also occur. Crystals of bismuthinite are known to be slightly flexible, and the crystal faces typically have fine striations.

Bismuthinite is a key ore mineral for the element bismuth (Bi). It principally occurs in hydrothermal vein deposits, some pegmatites, and some volcanic exhalation deposits.

Photo gallery of bismuthinite: