English: Greenockite from Arkansas, USA. (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, 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 5800 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.

Greenockite is a scarce cadium sulfide mineral, CdS. It has a nonmetallic luster, a yellow to red color, an orangish-yellowish to maroon streak, and is somewhat soft (H = 3 to 3.5). It can occur as discrete crystals or as earthy, powdery coatings.

In the mining districts of America's Mississippi Valley, sphalerite weathers into bright yellow powder in the supergene zone (= above the water table) - the powder can be a mix of metal sulfides that includes cadmium (for example (Zn,Fe,Cd)S).

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Photo gallery of greenockite:

www.mindat.org/gallery.php?min=1746