Chapter 4 Chemistry of Dolomite Formation

Abstract

Chemical experiments under atmospheric conditions so far have not yielded any unequivocal answers on the stability of dolomite. The possibility that nesquehonite or hydromagnesite might be the stable magnesium carbonate at 25°C and 1 atm. in the system MgCO 3 -CO 2 -H 2 O has added further complications. Unless further experimentation proves the contrary, the possibility exists that the stability of dolomite in the system CaCO 3 -MgCO 3 -CO 2 -H 2 O at 25°C and 1 atm. is related not only to temperature and pressure, but also to the partial pressure of CO 2 . The geologic occurrence of magnesium-bearing carbonates in Recent sediments is somewhat puzzling. The common occurrence of dolomite in ancient carbonate rocks, however, indicates that dolomite, rather than a mineral pair, is the stable phase under the low temperature and pressure conditions of carbonate diagenesis. The possibility that the calcite-hydromagnesite pair might represent a stable assemblage at 25°C and 1 atm. and extremely low pco 2 in the system CaCO 3 - MgCO 3 -CO 2 -H 2 O has been suggested by the theoretical considerations and by experimental data. This tentative conclusion is not ruled out by the field evidence. Experimental evidence on the solubility of dolomite is controversial. The composition of the ground waters in dolomites is such that the writer believes the highest reported values ( K d × 10 −17 ) are more nearly correct than the lower values. This interpretation is not accepted by those who question whether equilibrium has been established or even approximated between a ground water and the solid carbonate phases of its host rocks. The controversy on the solubility of dolomite will probably not be resolved until dolomite is synthesized under controlled atmospheric conditions. Inasmuch as the solubility of dolomite is not known, the question whether any natural water (such as normal marine sea water) is saturated with dolomite cannot be satisfactorily answered. Experimental results on the composition of solution at dolomite-calcite-solution equilibrium differ radically, and deductions from such results have led to controversies. Nevertheless, all experimental results as well as deductions on the basis of ground water composition studies suggest that the K dz value is less than 1 at room temperatures and atmos-pheric or near-surface pressures. Paradoxically, sea water with a magnesium/calcium-concentration ratio of 5.3 is apparently not dolomitizing. The reason is not clear, although alternative explanatidns have been suggested.