Aragonite was converted to calcite in dilute CaCl2 fluid at temperatures ranging from 50 to 100°C. Surface areas of aragonite and calcite seed crystals were varied by over an order of magnitude to permit independent assessment of calcite nucleation and growth processes. Aragonite conversion rates were measured using isotopic attenuation of dissolved 44Ca, which was added to the fluid at the beginning of each experiment. Measured conversion rates were found to be constant with respect to time and proportional to the initial surface area of aragonite. Rates were independent of the surface area of calcite seed crystals owing to heterogeneous nucleation of calcite on aragonite during experiments. The data imply that calcite nucleates on aragonite surfaces until the level of saturation with respect to calcite reaches a critical threshold value where further nucleation is precluded. Thereafter, conversion to calcite occurs at a steady state rate consistent with aragonite dissolution at a fixed level of saturation. Aragonite converts to calcite under these conditions and in dilute fluids at rates of approximately 10 and 100 microns/yr at 25 and 100°C, respectively.
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R. Knurr is thanked for performing many of the analysis presented in this paper. D. Doctor is also thanked for early discussions and for his participation in preliminary experimental work related to this study. We are grateful to P. Brady and an anonymous reviewer for their thoughtful suggestions to improve the manuscript. This work was supported by NSF grant EAR 94-18719.