An adequate geochemical model of dolomitization requires a knowledge of the stability of dolomite. In theory, the equüibrium constant for the dolomitization reaction can be obtained by a variety of ways: (1) experimental study of equihbrium interaction between calcite, dolomite, and water, (2) calculation from thermodynamic data, and (3) study of the interaction of calcite, dolomite, and water in natural systems. Experimental efforts to determine the calcium-magnesium activity ratio in solutions which have equilibrated with both calcite and dolomite or to determine the ion activity product for dolomite at temperatures less than 200°C have been unsuccessful. The rate of reaction of dolomite in dilute solutions at less than 100°C appears to be too slow to offer any hope of obtaining equihbrium constants for ordered stoichiometric dolomite by direct experimental methods.
Calculation of equihbrium constants from thermodynamic data also involves difficulties. There is a serious disparity in the KIAP dolomite calculated from the thermodynamic properties of “fully-ordered” dolomite reported by Helgeson and others (1978) and from data on well-ordered dolomite reported by Robie and others (1978). The paper by Helgeson and others indicates that order-disorder in dolomite has a very significant effect on its thermodynamic properties and it is possible that the disparity in properties reported for dolomite is related to this factor.
The most promising approach to the problem appears to be through field studies on coexisting calcite, dolomite, and water. Johnson and Pyktowicz (1978) have recently published stoichiometric association constants for NaCl°, KCl°, MgCl+, and CaCl+. Their data indicates that cations in seawater are paired with chloride to a greater extent than with sulfate and bicarbonate and that approximately fifty percent of calcium and magnesium in seawater is paired with chloride. These results must be verified before data on carbonate interaction with chloride-rich water can be used as a basis for detemining the stability of dolomite. Rock-water systems best suited for deterrnining the stability of dolomite will probably be at depths of 1,000 to 5,000 m. The rocks should contain calcite and well-ordered, stoichiometric, iron-free dolomite. The pore water should be as dilute as possible and should contain at Least 10 ppm hydrogen sulfide to minimize the effect of iron on carbonate-water interaction.
Figures & Tables
Special Publication 28 has its roots in the 22nd Annual Research Symposium of SEPM entitled Concepts and Models of Dolomitization – Their Intricacies and Significance held on April 3,1979 in Houston, Texas as part of the joint annual meetings of AAPG and SEPM. The purpose of that symposium was to express the state-of-the-art of the study of the elusive process(es) of dolomitization. Most of the contributions in this volume are concerned with apparent early, nearsurface dolomitization, either by hypersaline brines, by the marine-meteoric mixing model or some variant thereof, or by both mechanisms where more than one phase or kind of dolomite exists, or where the origin of a particular dolomite is uncertain. Other models and aspects of dolomitization are treated here as well.