Experimental Hydrothermal Dedolomitization: ABSTRACT

Abstract

Hydrothermal replacement of dolomite by calcite has been examined through experimental reactions carried out in teflon-lined stainless steel bombs, at temperatures ranging from 50° to 200°C (122° to 392°F), at End_Page 456------------------------------ equilibrium vapor pressure, and for durations of days to months. Solution compositions were varied to examine the effects of ionic strength, Mg2+/Ca2+ ratio, and SO42- concentration on the direction and/or rate of reaction. The starting solid for these experiments was a stoichiometric and well-ordered synthetic dolomite; several experiments were also seeded with solid reagent-grade calcium carbonate (calcite). In pure CaCl2 solution (0.1 M) replacement was complete in less than one week at temperatures of 100°C (212°F) and greater. Replacement was minor after several weeks at 50°C (122°F). Longer term experiments at 50° are still underway. A series of experiments established that dedolomitization only occurs at Mg2+/Ca2+ ratios of less than 1/10 (total ionic strength = 0.1) at all temperatures from 100° to 200°C (212° to 392°F). Because this reaction proceeds far faster than the reverse reaction (dolomitization), we believe this result establishes the most precise calcite/dolomite phase boundary to date. The presence of SO42- was found to strongly inhibit the dedolomitization reaction, even a concentrations far less than in seawater. Seeding the reaction with a small amount of calcite had little effect on the rate of reaction, suggesting that nucleation is not rate-limiting in dedolomitization. Finally, increasing ionic strength from 0.01 up to 0.5 increased the rate of reaction. End_of_Article - Last_Page 457------------