Realgar (red) and calcite (grayish-whitish) from Nevada, USA. (field of view ~3.5 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 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.

Realgar and orpiment are both arsenic sulfides. Realgar is an intensely reddish-orangish arsenic sulfide (AsS), while orpiment is a bright yellow-colored arsenic sulfide (As2S3). They are always associated with each other. Arsenic is a rare element in Earth’s crust, but because As has very few uses in modern society, it has practically no value. Orpiment & realgar have a nonmetallic luster and are fairly soft (H=1 for yellow orpiment & H=2 for reddish-orange realgar). The two minerals are fairly insoluble, but they do volatilize readily. When heated, they release a garlic smell (arsenic). Realgar tends to alter to orpiment when exposed at Earth's surface.

One of the prettiest specimens I’ve ever seen is this specimen from the Getchell Mine of northern Nevada, USA. The whitish gray crystals are calcite (CaCO3 - calcium carbonate). The red crystals are realgar (AsS; a.k.a. As4S4 - arsenic sulfide). The small specks of yellowish and orangish areas are pararealgar, which has the same chemical formula as realgar. Realgar is unstable when exposed to light, so the orangish-yellowish pararealgar you see in this specimen used to be red realgar.

Geology - Cambrian-aged Preble Formation limestone, contact metamorphosed into marble by the Cretaceous-aged Osgood Granodiorite Stock. During the late Middle Eocene, at 39 million years ago, the rock was subjected to Carlin-type mineralization by fluids moving along the Getchell Fault (a major normal fault formed during Basin & Range extensional tectonics). The mineralization event precipitated calcite and realgar on the marble (plus a teeny-tiny amount of disseminated gold, but not enough to make this sample a gold ore).

Locality: 4950-194 stope of the Getchell Mine, northern end of the Osgood Mountains, eastern Humboldt County, northern Nevada, USA

Photo gallery of realgar: www.mindat.org/gallery.php?min=3375

Photo gallery of orpiment: