14.3: Sulfide Minerals

Sulfides and related minerals vary greatly in atomic arrangement. In this book, we divide them into tetrahedral sulfides, octahedral sulfides, and other sulfides. In tetrahedral sulfides, all metal atoms are in 4-fold coordination. In octahedral sulfides, all metal atoms are in 6-fold coordination. And, in the third category, metal coordination is unusual, for example 5-fold coordination, or mixed. Bonding is generally some combination of ionic and covalent, but some sulfides have significant metallic character.

Many sulfides share common properties, sometimes making distinguishing them problematic. For example, common metallic sulfides include pyrite, chalcopyrite, molybdenite, galena, acanthite, chalcocite, bornite, pyrrhotite, millerite, pentlandite, stibnite, marcasite, cobaltite, arsenopyrite, tetrahedrite, argentite, and niccolite. Of these, pyrite, chalcopyrite, pyrrhotite, millerite, and pentlandinte have brass to yellow to gold colors and are occasionally confused. Molybdenite, galena, acanthite, chalcocite, stibnite, tetrahedrite, and enargite have gray to black colors and so, too, are sometimes difficult to distinguish.

Sulfide minerals have variable compositions and a wide variety of complex solid solutions are possible, especially at high temperature. At low temperature, however, many sulfide minerals exhibit exsolution resulting from unmixing to form more stable phases.

In this book, we group the sulfide and (the much less common) arsenide minerals together because they share many properties. In these minerals, sulfur and arsenic are nearly closest packed with metals between. The simplest sulfide minerals, such as galena (PbS), have atomic arrangements similar to the arrangement in halite – symmetrical arrangements of metal atoms alternating with S, resulting in cubic or hexagonal crystals.