Mineralogy of Sulfides - 2017

Mining & Minerals - Evaluations / Mineral Exploration Activities - General

Vaughan and Corkhill reported in 2017 that metal sulfides are the most important group of ore minerals. In this paper, the authors review what is known about their compositions, crystal structures, phase relations and parageneses. Much less is known about their surface chemistry, their biogeochemistry, or the formation and behavior of ‘nanoparticle’ sulfides, whether formed abiotically or biogenically.

Sulfide minerals are compounds in which sulfur is combined as an anion with a metal (or semi-metal) cation or cations. The definition is commonly widened to include minerals in which the anion is As or Sb, sometimes together with S, and to include Se and Te minerals. The sulfosalts are a special group of the sulfide minerals that have a general formula A_mT_nX_p and in which the common elements are A = Ag, Cu, Pb; T = As, Sb, Bi; X = S. They generally contain pyramidal TS₃ groups in their structures. Several hundred sulfide minerals are known, but only five are sufficiently abundant accessory minerals to have been categorized as ‘rock forming’. These five are pyrite, pyrrhotite, galena, sphalerite and chalcopyrite, and it is the iron sulfides (pyrite and pyrrhotite) which are dominant. The very fine particulate iron sulfides found in reducing environments beneath the surfaces of some sediments and soils are also important. Formerly known as ‘amorphous iron sulfides’, they are now known to be mackinawite (tetragonal FeS) and, to a lesser extent, greigite (Fe₃S₄). Both mackinawite and greigite are metastable compared to pyrite and pyrrhotite.

Above all, the sulfides are the most important group of ore minerals because they are responsible for the concentration of a wide range of metals as mineable deposits. They are also potential sources of pollution, be it of the air, surface waters, or soils. Air pollution may arise both from the smelting of sulfide ores and from the burning of coal, which contains sulfur mainly as sulfide impurities. The breakdown of sulfides exposed by weathering at the Earth’s surface generates sulfuric acid, as well as releasing potentially toxic metals into waters and soils. This form of pollution may arise from mine wastes (acid mine drainage) or sulfide-containing natural rocks (acid rock drainage).

The literature on sulfide minerals is extensive, with a number of overview textbooks and monographs. Comprehensive reviews can be found in Ribbe (1974), Vaughan and Craig (1978), and, most recently, in Vaughan (2006). The present article provides a brief overview of the compositions and crystal structures of the major sulfide minerals, aspects of their chemistries (bulk and surface) and their occurrence. In addition to the sulfides mentioned above, pentlandite [(Fe,Ni)₉S₈] and its alteration product, violarite (FeNi₂S₄), are important as the major ore minerals of nickel; bornite (Cu₅FeS₄) and chalcocite (Cu₂S) as major copper minerals; and molybdenite (MoS₂) is the primary source of molybdenum. Tetrahedrite (Cu₁₂Sb₄S₁₃₎ is notable because of the large range of metals, silver in particular, which can substitute at percent levels for copper or antimony in its structure. In contrast, arsenopyrite (FeAsS) is the major natural source of arsenic, an extremely toxic pollutant.