MINERALS/Sulphates 573 Occurrence of Gypsum Gypsum occurs mainly as sedimentary deposits associated with limestones and shales and in evaporite deposits Gypsum can form by the direct evaporation of brine, by hydration of anhydrite and by oxidation of sulphides Gypsum is also one of the products of the reaction of acid sulphate solutions with carbonate rocks Seawater contains about 3.5% dissolved material, of which 80% is sodium chloride and about 4% is dissolved calcium sulphate The experimental evaporation of seawater results in the crystal deposition sequence: calcium carbonate – calcium sulphate – sodium chloride – sulphates/chlorides of magnesium – sodium bromide/potassium chloride Gypsum also occurs in soils either as disseminated crystals or in horizons beneath calcrete layers Percolating waters, which in dry seasons are drawn to the surface by capillary action, can evaporate and crystallize gypsum, sometimes in the form described as ‘desert roses’ The calcium sulphate formed in evaporates is sometimes gypsum, sometimes anhydrite, and often both minerals occur together Some geological evidence suggests that the original material of certain gypsum beds was anhydrite; this is supported by the fact that some gypsum beds grade into anhydrite at depth Along with jarosite and goethite, gypsum is one of the products formed by the oxidation of iron sulphide (pyrite) Gypsum is also found in volcanic regions along with native sulphur where it forms by the reaction of sulphurbearing fluids and gases with calcium-bearing rocks Natural bassanite is rare, and nearly always forms as a product of alteration of gypsum It has been found along with gypsum in volcanic craters, and in some soils and peat deposits Other Sulphates The most abundant barium mineral in the Earth’s crust is barite, BaSO4, and this is also the least soluble sulphate mineral It commonly occurs in hydrothermal metalliferous veins, but also occurs in cementations and nodules in sedimentary rocks Because of its high density (4.5 g cm 3) barite is widely used as a component of drilling muds in the petroleum industry, or in other dense fluid media, and also as a filler or extender in the manufacture of paper, plaster, rubber and plastics There is a complete solid solution series between BaSO4 (barite) and SrSO4 (celestine), although natural compositions are usually near to end-member compositions, reflecting either the Ba-rich or Sr-rich geochemical environments PbSO4 (anglesite) is isostructural with BaSO4 (barite), so a complete solid solution between these two end members would also be expected, and some natural samples occur that are intermediate in composition At room temperature, about 6% Ca2ỵ can be accommodated in the BaSO4 structure About 8% Ba2ỵ replacement of Ca2ỵ is the limit of solid solution tolerated by the CaSO4 (anhydrite) structure The oxidation of pyrite (FeS2), especially when mediated by the action of bacteria, frequently results in the formation of powdery assemblages of iron sulphates This alteration commonly occurs during the storage of pyrite-bearing materials under humid conditions Sulphates produced in this way include melanterite (FeSO4 Á 7H2O), rozenite (FeSO4 4H2O), szomolnokite (FeSO4 H2O) and rhomboclase (Fe3ỵ(SO4)2 (H3Oỵ) 3H2O) Several hundred compounds that contain essential sulphate (SO4) groups are known to occur in nature; these include sulphates, sulphate hydrates, sulphate hydroxy hydrates, sulphate/chlorides, and sulphate/arsenate/phosphate/vanadates of alkali- and transition-metals See Also Minerals: Definition and Classification; Sulphides Sedimentary Rocks: Evaporites Further Reading Alpers CN, Jambor JL, and Nordstrom DK (eds.) (2000) Sulfate Minerals Crystallography, Geochemistry and Environmental Significance Reviews in Mineralogy and Geochemistry, vol 40 Mineralogical Society of America Bragg WL (1937) The Atomic Structure of Minerals New York: Cornell University Press, and London: Oxford University Press Chang LLY, Howie RA, and Zussman J (1996) Rock Forming Minerals, vol 5B, Sulphates, Carbonates, Phos phates and Halides London: Longman Jerz JK and Rimstidt JD (2003) Efflorescent iron sulfate minerals: paragenesis, relative stability, and environmen tal impact American Mineralogist 88: 1919 1932 Pedersen BF and Semmingsen D (1982) Neutron diffraction refinement of the structure of gypsum, CaSO4 Á 2H2O Acta Crystallographica B38: 1074 1077 Wooster WA (1936) On the crystal structure of gypsum CaSO4 Á 2H2O Zeitschrift fur Kristallographie 94: 375 396