Orpiment mass (yellow) with a little realgar (red) from near Manhattan, Nevada, USA. (6.8 cm across at its widest)

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.

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

Photo gallery of orpiment: