ALBERT

Description / Table of Contents: The importance of sulfide minerals in ores has long been, and continues to be, a major reason for the interest of mineralogists and geochemists in these materials. Determining the fundamental chemistry of sulfides is key to understanding their conditions of formation and, hence, the geological processes by which certain ore deposits have formed. This, in turn, may inform the strategies used in exploration for such deposits and their subsequent exploitation. In this context, knowledge of structures, stabilities, phase relations and transformations, together with the relevant thermodynamic and kinetic data, is critical. As with many geochemical systems, much can also be learned from isotopic studies. The practical contributions of mineralogists and geochemists to sulfide studies extend beyond areas related to geological applications. The mining of sulfide ores, to satisfy ever increasing world demand for metals, now involves extracting very large volumes of rock that contains a few percent at most (and commonly less than one percent) of the metal being mined. This is true of relatively low value metals such as copper; for the precious metals commonly occurring as sulfides, or associated with them, the mineable concentrations (grades) are very much lower. The "as-mined" ores therefore require extensive processing in order to produce a concentrate with a much higher percentage content of the metal being extracted. Such mineral processing (beneficiation) involves crushing and grinding of the ores to a very fine grain size in order to liberate the valuable metal-bearing (sulfide) minerals which can then be concentrated. In some cases, the metalliferous (sulfide) minerals may have specific electrical or magnetic properties that can be exploited to enable separation and, hence, concentration. More commonly, froth flotation is used, whereby the surfaces of particles of a particular mineral phase are rendered water repellent by the addition of chemical reagents and hence are attracted to air bubbles pulsed through a mineral particle-water-reagent pulp. An understanding of the surface chemistry and surface reactivity of sulfide minerals is central to this major industrial process and, of course, knowledge of electrical and magnetic properties is very important in cases where those particular properties can be utilized. In the years since the publication of the first ever Reviews in Mineralogy volume (1974, at that time called MSA "Short Course Notes") which was entitled Sulfide Mineralogy, sulfides have become a focus of research interest for reasons centering on at least two other areas in addition to their key role in ore deposit studies and mineral processing technology. It is in these two new areas that much of the research on sulfides has been concentrated in recent years. The first of these areas relates to the capacity of sulfides to react with natural waters and acidify them; the resulting Acid Rock Drainage (ARD), or Acid Mine Drainage (AMD) where the sulfides are the waste products of mining, has the capacity to damage or destroy vegetation, fish and other aquatic life forms. These acid waters may also accelerate the dissolution of associated minerals containing potentially toxic elements (e.g., As, Pb, Cd, Hg, etc.) and these may, in turn, cause environmental damage. The much greater public awareness of the need to prevent or control AMD and toxic metal pollution has led to regulation and legislation in many parts of the world, and to the funding of research programs aimed at a greater understanding of the factors controlling the breakdown of sulfide minerals. The second reason for even greater research interest in sulfide minerals arose initially from the discoveries of active hydrothermal systems in the deep oceans. The presence of life forms that have chemical rather than photosynthetic metabolisms, and that occur in association with newly-forming sulfides, has encouraged research on the potential of sulfide surfaces in catalyzing the reactions leading to assembling of the complex molecules needed for life on Earth. These developments have been associated with a great upsurge of interest in the interactions between microbes and minerals, and in the role that minerals can play in biological systems. In the rapidly growing field of geomicrobiology, metal sulfides are of major interest. This interest is related to a variety of processes including, for example, those where bacteria interact with sulfides as part of their metabolic activity and cause chemical changes such as oxidation or reduction, or those in which biogenic sulfide minerals perform a specific function, such as that of navigation in magnetotactic bacteria. The development of research in areas such as geomicrobiology and environmental mineralogy and geochemistry, is also leading to a greater appreciation of the role of sulfides (particularly the iron sulfides) in the geochemical cycling of the elements at or near the surface of the Earth. For example, the iron sulfides precipitated in the reducing environments beneath the surface of modern sediments in many estuarine areas may play a key role in the trapping of toxic metals and other pollutants. In our understanding of "Earth Systems," geochemical processes involving metal sulfides are an important part of the story. The main objective of the present text is to provide an up-to-date review of sulfide mineralogy and geochemistry. The emphasis is, therefore, on such topics as crystal structure and classification, electrical and magnetic properties, spectroscopic studies, chemical bonding, high and low temperature phase relations, thermochemistry, and stable isotope systematics. In the context of this book, emphasis is on metal sulfides sensu stricto where only the compounds of sulfur with one or more metals are considered. Where it is appropriate for comparison, there is brief discussion of the selenide or telluride analogs of the metal sulfides. When discussing crystal structures and structural relationships, the sulfosalt minerals as well as the sulfides are considered in some detail (see Chapter 2; also for definition of the term "sulfosalt"). However, in other chapters there is only limited discussion of sulfosalts, in part because there is little information available beyond knowledge of chemical composition and crystal structure.