The coprecipitation of Sr 2+ with calcite at 25°C and 1 atm

Abstract

The incorporation of Sr 2+ into calcite at earth surface aqueous conditions is affected by the absolute concentration of Sr 2+, the presence of Ba 2+ and NaCl in the solution, and the rate of precipitation. At solution ratios (molar) of Sr 2+ to Ca 2+ in the low 10 −3 range, which yield calcites with several hundred ppm Sr 2+, K sr calcite typically assumes a value between 0.10 and 0.20. Above these concentrations the value of k Sr calcite drops to approximately 0.06. Furthermore, if minor amounts of Ba 2+ or large amounts of Na + (0.48 M) are added to a dilute Sr 2+ solution, a value around 0.06 for k Sr calcite is found. This “strontium concentration effect” and the associated “competitive cation effect” suggest that small amounts of Sr 2+ may be incorporated into a limited number of nonlattice sites in calcite. Incorporation of Sr 2+ into these sites, presumably defects, noticeably affects k Sr calcite only at low Sr 2+ concentrations and in the absence of competition from other large cations. An increase in k Sr calcite with rate of precipitation, qualitatively similar to that found in other studies, was observed only when precipitation times were decreased from days to hours. For many geologic settings a partition coefficient for Sr 2+ into calcite of 0.06 appears appropriate, but there are situations—very low Sr 2+ concentrations, the presence of Mg 2+, and fast precipitation rates—in which a larger value might better approximate natural partitioning.