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Redetermination of magnesian calcite solubilities indicates that the constants currently in use overestimate their solubility by 300%. The relative chemical stability of aragonite and magnesian calcite plays a fundamental role in controlling the behavior of these metastable minerals during the diagenetic transformation to stable low-magnesian calcite limestone via dissolution and reprecipitation reactions. Temporal evolution of rock and groundwater geochemistry, as well as development of limestone fabrics, depends on the relative chemical reactivity of carbonate grains during dissolution as well as their receptiveness to cementation in both marine and meteoric diagenetic systems.
Following the laboratory determination of magnesian calcite stabilities by L. N. Plummer and F. T. Mackenzie in 1974, it has been accepted that magnesian calcites containing greater than 7.5 mole % MgCO3 are more unstable than aragonite, and become progressively more unstable with increasing mole % MgCO3. For example, 18 mole % magnesian calcite was thought to be seven times more soluble than aragonite.
The solubilities of several biogenic magnesian calcites (echinoid, 12 mole % MgCO3; red algae, 18 mole % MgCO3) have been redetermined. The new data for the stability of the magnesian calcites indicate that previously determined solubilities for magnesian calcites were significant overestimates. Using the new stability constants from the present study, 12.5 mole % magnesian calcite is equivalent in stability to aragonite, and an 18 mole % magnesian calcite is only twice as soluble as aragonite. Thus, the solubility of magnesian calcites must be reduced by 300% from existing values in the literature.
It is believed that the discrepancy with previous work on magnesian calcite stability is due to sample preparation procedure. In the present study, care was taken both to eliminate submicron fine particles by ultrasonic cleaning and to reduce the crystal strain induced by crushing. Experiments show samples not so treated yield larger magnesian calcite solubilities because of enhanced reactivity and solubility of adhering submicron particles and strained crystal surfaces.
The new stability relations have significant implications for the sequence and pathways of diagenetic reactions. Mineralogic stability and available surface area are the main chemical controls on the rate of diagenetic alteration. Given the reduced solubility offset between aragonite and magnesian calcite, aragonites having more complex microstructures and, hence, more reactive surface area, dissolve more rapidly than coexisting high-magnesian calcites.
The lower solubilities for the magnesian calcites sharply reduce the potential chemical region in ground waters where the incongruent dissolution of magnesian calcite is possible. Since the specific nature of dissolution is closely related to the rate and style of cementation, these findings prompt the reevaluation of possible pathways which may be taken during early diagenesis.
More work remains to be done to quantify the precise effects of mineralogy and grain microstructure into a complete predictive model for patterns of cementation, but it is clear that the reduced mineralogic stability offset between aragonite and magnesian calcite makes microstructural detail and ground-water geochemistry prime controls over the reaction paths followed during the early diagenetic stabilization of shallow-water carbonate deposits.
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