File:Sphalerite (Elmwood Mine, Smith County, Tennessee, USA) 3 (30544848301).jpg

Sphalerite from Tennessee, USA. (field of view ~7.0 cm across)

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 about 5400 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.

Sphalerite is a somewhat common zinc sulfide mineral (ZnS). It has a metallic to submetallic to resinous to adamantine luster. Many metals can substitute for the zinc, such as iron, cadmium, and manganese. Sphalerite almost always has some iron in it, so a better chemical formula would be (Zn,Fe)S. Sphalerite has a wide color range, depending principally on iron content. Pure to almost pure sphalerite is whitish to greenish. With increasing iron content, sphalerite becomes yellowish to brownish to blackish. One variety of sphalerite has a strikingly intense dark red color (ruby sphalerite). It's streak color also varies with iron content from whitish to pale yellowish to brownish. Sphalerite is also distinctive in being moderately heavy for its size and having six different planes of cleavage.

Sphalerite is the most important zinc ore mineral. Zinc produced from sphalerite is used for many purposes, including mixing with copper to produce brass, rust protection of iron & steel, and for making modern American pennies (although the cost of making each zinc penny is >1¢).

The crystalline sphalerite plate shown above is from the well-known Central Tennessee Zinc District (a.k.a. Central Tennessee Ba-F-Pb-Zn District). In this mining district, sphalerite mineralization is hosted in shallow marine dolostones of the upper Knox Group (Lower Ordovician) - probably from dissolution collapse brecciated and karstified dolomitized limestones of the Mascot Dolomite. The Central Tennessee Zinc District is a Mississippi Valley-type deposit in the Nashville Dome. It formed by mineralization from heavy metal-rich basinal brines that migrated from the adjacent Illinois Basin or Appalachian Basin. Published research indicates that mineralization occurred during the Late Paleozoic (Mississippian to Permian) and was associated with the Allegheny Orogeny, a tectonic collision event between Africa and North America. Main stage calcite in the Central Tennessee Mining District dates to 260±42 Ma (= Late Permian).

The small patches of yellowish and reddish seen above are also sphalerite.

Locality: Elmwood Mine, south/southeast of the town of Carthage, south-central Smith County, north-central Tennessee, USA

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