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Dolomitization of geologically significant volumes of limestone by subsurface solutions requires: (1) a favorable thermodynamic drive, (2) an adequate supply of reacting solution, and (3) a reaction rate sufficient for dolomitization within a (geologically) reasonable length of time. The most important factors governing the thermodynamic drive, for a solution of specified composition, are (a) the temperature and (b) the composition and particle size of both the reactants and the products.
Data on the composition of calcite and dolomite coexisting with oil-field water of known composition and temperature provide an estimate of the equilibrium calcium-magnesium activity ratios in aqueous solutions in the presence of the common varieties of calcite and dolomite. Mass-transfer calculations, based on such data, suggest that most dolostone probably formed from concentrated sea water and/or at elevated temperatures. Factors affecting reaction rates are numerous and complex. Experimental work suggests that the conversion of 1µm reagent calcite to dolomite at 150°C may involve the formation of magnesite and protodolomite (Ca60Mg40) as intermediate steps. Ferrous iron in limited amounts relative to magnesium accelerates this reaction. Larger amoun s of ferrous iron retard the reaction because much of the magnesium becomes incorporated into a relatively inert, ferroan magnesite.
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