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 5200 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.

Geode info. from the Field Museum of Natural History (Chicago, Illinois, USA):

"Geodes are hollow, subspherical bodies, ranging from an inch or two to a foot or more in diameter. Most geodes occur in limestones, rarely in shales. They have an outer chalcedonic silica layer which is separated from the enclosing limestone matrix by a thin clay film. The inner surface of the chalcedonic layer is usually lined with inward projecting quartz crystals, though in many geodes drusy coatings of calcite and dolomite occur commonly. Of less common occurrence, are crystals of magnetite, pyrite, sphalerite, and a few other such minor and rarer constituents.

The mode of origin of geodes in sedimentary rocks is but imperfectly understood. That geodes originate in an initial cavity, such as the unfilled space within a fossil, is well recognized, but whether such a cavity is a necessary prerequisite is open to question; geodes may originate in cavities formed by solution.

Many geodes show evidence of expansion, apparently resulting from pressure. A notable example of this singular phenomenon of expansion of the growing geodes is the "exploding bomb" structure. "

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

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