By Bob Farrar
Galena is the chief ore of lead, and is thus of great economic importance. Appropriately, its name comes from the Greek galene, for“lead ore.” At first thought, lead ore might seem to be a rather mundane mineral. However, it also forms interesting and attractive crystals, and so is also a favorite among mineral collectors. Galena consists of lead sulfide, PbS. It may also contain trace amounts of silver; such material is known as “argentiferous” galena. It crystallizes in the cubic system. Crystals are most commonly cubes, but other habits, such as octahedrons and combinations also occur. Rarely, it forms “spinel-law twins”, in which two octahedral crystals are joined at the base. In galena, spinel twins usually appear as tabular six-sided crystals, sometimes with triangular faces. As would be expected for lead sulfide, galena is very heavy, with a specific gravity of 7.4 to 7.6. It is typically metallic gray in color. Other physical properties include perfect cubic cleavage and a hardness of 2.5 to 2.7. Crystal form, specific gravity, and cleavage are usually sufficient to distinguish galena.