Millerite in a geode from Kentucky, 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.

Millerite is an odd, scarce nickel sulfide mineral (NiS). It tends to form radiating clusters or tufts of long, hairlike needles. Some geodes in the Mississippian limestone outcrop belts of America’s Upper Mississippi Valley & Ohio Valley have millerite inside. Millerite crystals are very slender hexagonal needles with a metallic luster and a brassy-gold color.

Locality: Hall's Gap, central Lincoln County, central Kentucky, USA

Photo gallery of millerite: