94 SEDIMENTARY ROCKS/Evaporites Morrow DW (1999) Regional subsurface dolomitiza tion: models and constraints Geoscience Canada 25: 57 70 Nordeng SH and Sibley DF (1994) Dolomite stoichiometry and Ostwald’s step rule Geochimica et Cosmochimica Acta 58: 191 196 Pray LC and Murray RC (eds.) (1965) Dolomitization and Limestone Diagenesis A Symposium Society of Economic Paleontologists and Mineralogists Special Publication 13 Town: Society of Economic Paleontolo gists and Mineralogists Purser B, Tucker M, and Zenger D (eds.) (1994) Dolomites A Volume in Honour of Dolomieu: Special Publi cation 21 Town: International Association of Sedi mentologists Sibley DF and Gregg JM (1987) Classification of dolomite rock textures Journal of Sedimentary Petrology 57: 967 975 Van Tuyl FM (1914) The Origin of Dolomite Annual Report 1914, vol XXV, pp 257 421 Town: Iowa Geological Survey Wright WR (2001) Dolomitization, fluid flow and miner lization of the Lower Carboniferous rocks of the Irish Midlands and Dublin Basin Unpub Ph.D thesis, Univer sity College Dublin, Belfield, Ireland, 407 p Zenger DH, Dunham JB, and Ethington RL (eds.) (1980) Concepts and Models of Dolomitization Society of Eco nomic Paleontologists and Mineralogists Special Publica tion 28 Town: Society of Economic Paleontologists and Mineralogists Evaporites A C Kendall, University of East Anglia, Norwich, UK ß 2005, Published by Elsevier Ltd Deposits Produced by the Evaporation of Seawater Seawater is considered to be the major or the only feedstock capable of generating large bodies of evaporite All deposits of potash salts are associated with large basin-central evaporites and, consequently, are believed by most to have been formed by the evaporation of seawater The problem with this marine origin is that the chemical and mineralogical characters of most potash deposits depart significantly from those that would be expected from simple seawater evaporation If the marine origin is correct, then other processes must have been involved to cause the differences Seawater becomes progressively more concentrated as it evaporates until it is supersaturated with respect to a particular mineral phase, which then precipitates Precipitation of a salt preferentially extracts chemical components from the seawater-derived brine, altering its overall composition Initially, calcium combines with bicarbonate, but, after seawater has been concentrated approximately 3.5 times, gypsum (CaSO4 Á 2H2O) saturation is reached and calcium and sulphate are extracted from the brine Seawater contains abundant sulphate, and, after the greater part of the calcium has been extracted (as carbonates and as gypsum), fully two-thirds of the original sulphate remains in the brine At this stage, and in all subsequent stages of evaporation, a marine-derived brine will be impoverished in calcium and should contain abundant sulphate The next mineral to precipitate by continued seawater evaporation is halite (NaCl), and this extracts sodium and chloride from the brine At about 60 times seawater concentration, the sulphate remaining in the brine should begin to be removed in the form of various magnesium sulphate minerals Only after considerable amounts of sulphate have been eliminated from the brine will the next mineral – carnallite (hydrated magnesium and potassium chloride) – precipitate Finally, at the last stages of concentration, the mineral bischoffite (MgCl2) is precipitated Typical Composition of Evaporite Deposits Only about 10% of all potash-bearing evaporites contain significant quantities of magnesium sulphate, which would be expected from simple seawater evaporation Of these 10%, all differ from direct seawater precipitation sequences in having different magnesium sulphate minerals in different amounts from those expected These differences can be explained in two ways First, most magnesium sulphates precipitated during experimental seawater evaporation are highly unstable hydrous phases These alter to less hydrous minerals upon burial Second, during evaporation, the concentrated brines may react with previously precipitated calcium sulphate to form the mineral polyhalite This reaction removes sulphate and potassium from the brine, so changing its composition Further evaporation of this modified brine is capable of generating the mineral sequences found in 10% of potash salt deposits The majority of potash salts, however, differ substantially from those expected to result from seawater