Sulfide Mineral

any of various minerals comprising a class and consisting of sulfur compounds of metals. Sulfide minerals comprise (by weight) about 0.15 percent of the earth’s crust; more than 200 sulfide mineral varieties are known. The natural selenides, tellurides, and arsenides, as well as the antimonides and bismuth minerals, are close to the sulfide minerals.

The main species-forming elements of sulfide minerals are Pb, Cu, Sb, As, Ag, Bi, Fe, Co, and Ni, which are the components of dozens of mineral species. Only three to five compounds each are formed by Zn, Cd, Mn, Ge, Sn, Tl, Mo, and Hg, but among them are such common and industrially important minerals as sphalerite (ZnS), molybdenite (MoS₂), and cinnabar (HgS). Among the elements encountered as isomorphic admixtures in sulfide minerals are Au, Ag, Ga, Ge, In, Tl, and Re. In addition to simple sulfide minerals, which are derivatives of hydrogen sulfide, H₂S (for example, Ag₂S and PbS), there are also persulfides, which are derivatives of H₂S₂ (for example, FeS₂), complex sulfides (for example, Pb₅Sb₄S₁₁), sulfosalts with [AsS]^3– and [SbS₃]^3– anions (for example, Ag₃SbS₃), and double sulfides (for example, chalcopyrite, CuFeS₂).

In the modern classification of sulfide minerals, which is based on data from crystal chemistry, subclasses are distinguished within the major chemical types and groups corresponding to crystal structures within these subclasses. The most important sulfide minerals include the following:

(1) Subclass of coordination minerals: galena (PbS) group, sphalerite (ZnS) group, pyrrhotite (Fe_1–xS) group, pent-landite [(Fe,Ni)₉S₈] group, cubanite (CuFe₂S₃) group, chalcopyrite (CuFeS₂) group, and bornite (Cu₅FeS₄) group.

(2) Subclass of framework minerals: argentite (Ag₂S) group and the gray ores.

(3) Subclass of ring minerals: realgar (AsS) group.

(4) Subclass of insular minerals: pyrite (FeS₂) group, cobaltite-arsenopyrite (Co AsS – Fe AsS) group.

(5) Subclass of chained minerals: antimonite (Sb₂S₃) group, millerite (NiS) group, and cinnabar (HgS) group.

(6) Subclass of layered minerals: molybdenite (MoS₂) group, orpiment (As₂S₃) group, and covellite (CuS) group.

The natural sulfide minerals have mainly covalent bonding. Most of them are semiconductors (those with pyrite, marcasite, and arsenopyrite structures), but compounds with metallic conductivity, as well as insulators and superconductors, also exist. (Several disulfides are among the superconductors.) A number of sulfide minerals—for example, antimonite—are ferroelectric. On the basis of magnetic properties, a distinction is made between diamagnetic, paramagnetic, ferromagnetic, and antiferromagnetic sulfide minerals.

Most sulfide minerals are optically opaque, and they often have high reflectivity. Their hardness on Mohs’ scale usually ranges from 2 to 4; the hardness of layered minerals, such as molybdenite and covellite, is only 1–2, whereas persulfides, such as pyrite, have a hardness of up to 5–7. The density of sulfide minerals is greater than 4,000 kg/m³.

The vast majority of sulfide minerals are components of hydrothermal sulfide ores. Some sulfides of Fe, Ni, Cu, and Pt are associated with processes of magma formation in ultrabasic rocks. Sulfide minerals may be of sedimentary origin, or they may be exogenous, having been deposited from surface solutions under the action of H₂S—for example, in coal-bearing strata and in oxidation zones of sulfide deposits. Upon oxidation on the earth’s surface, sulfide minerals change readily into sulfates and then into hydroxides, carbonates, and the salts of other oxy acids, and, less often, into such native elements as copper and silver. Many sulfide minerals are important ore minerals.