574 MINERALS/Sulphides Sulphides D J Vaughan, University of Manchester, Manchester, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The sulphide minerals are the major source of world supplies of a very wide range of metals and are the most important group of ore minerals In addition to their concentrated occurrence in ore deposits and areas of mineralization, a limited number of sulphide minerals are found as accessory minerals in rocks However, only pyrite, pyrrhotite, galena, sphalerite, chalcopyrite, and possibly the binary copper sulphides of the chalcocite family could be classed as rock-forming minerals Pyrite (FeS2) is by far the most abundant sulphide mineral The very fine particle iron sulphides found in reducing environments beneath the surfaces of Recent sediments and soils, although transient species, are also volumetrically important Formerly referred to as amorphous iron sulphide, these phases are now known to be largely composed of fine-particle mackinawite (tetragonal FeS) and, in some cases, the sulphospinel mineral greigite (Fe3S4) Both phases are metastable in relation to pyrite and pyrrhotite As well as being important sources of metals, sulphide minerals are potential sources of pollution In particular, the release of sulphur through the weathering of sulphides in natural rocks or in mine wastes generates sulphuric acid, resulting in acid rock drainage or acid mine drainage, and a significant proportion of the sulphurous fumes generated by combustion of coal (or by the smelting of sulphide ores) originates from sulphide minerals; these fumes react to form acid rain A more positive role may have been played by sulphide minerals in the emergence of life on Earth; although speculative, such theories have been developed following the discoveries of ocean-floor hydrothermal systems generating large volumes of sulphide minerals and associated with novel life forms and ecosystems The mineralogy, mineral chemistry, and geological occurrences of sulphide minerals have been well researched, particularly because of their economic and environmental importance The crystal structures, chemical compositions, physical properties, thermochemistry, and phase relations of the major sulphides were well established by the latter part of the twentieth century, as were the macroscopic and microscopic characteristics of sulphide minerals in natural occurrences Recent decades have seen major advances in the understanding of the chemical bonding in sulphide minerals and in the surface chemistry of sulphides An understanding of the genesis of sulphide-ore deposits has developed through systematic geological, petrographic, compositional, and isotopic studies of ores and host rocks In this article, the compositions, crystal structures, physical properties, and certain aspects of mineral chemistry and geological occurrence of the major sulphide minerals will be reviewed, with suggestions given for further reading Compositions and Structures Several of the common sulphide minerals were among the first minerals to be studied by X-ray crystallography, and since that time the structures of nearly all the mineralogically significant sulphides have been determined It is possible to categorize the mineral sulphides into a series of groups based on major structure types, or having key structural features in common, as shown in Table In many cases, these structures are the classic structures of crystalline solids, such as the rock-salt structure of the galena group (Figure 1A), the sphalerite and wurtzite forms of zinc sulphide (Figures 1B and 1C), or the nickel arsenide structure (Figure 1E) The disulphides are characterized by the presence of dianion (S–S, S–As, As–As, etc.) units in the structure: as well as the pyrite structure, in which FeS6 octahedral units share corners along the c-axis direction, there is the marcasite form of FeS2, in which octahedra share edges to form chains along the c-axis (Figure 1D) The structures of loellingite (FeAs2) and arsenopyrite (FeAsS) are variants of the marcasite structure that have shorter and alternate long and short, respectively, metal–metal distances across the shared octahedral edge (see Figure 1D) A few sulphides such as molybdenite, covellite (Figure 1F), and mackinawite have layer structures, and a small number exhibit structures that are best characterized as containing rings or chains of linked atoms (e.g realgar, AsS) A diverse group of sulphides, the metal-excess group, is composed of an unusual set of structures that is well illustrated by the important example of the mineral pentlandite ((Ni,Fe)9S8; Figure 1G) As can be seen from Table 1, in many of these groups a number of minerals share the actual structure type, but there are also, commonly, other minerals that have structures based on these ‘parent’ structures and that can be thought of as being derived from them The relationship between a derivative structure and the parent structure may involve