The coprecipitation of rare-earth elements with calcite and aragonite

Abstract

Apparent partition coefficients of rare-earth elements (REE's) between calcium carbonates and aqueous solution were experimentally examined. Calcite or aragonite crystals were synthesized by gradual release of CO2 gas from calcium bicarbonate solution doped with REE's at room temperature and pressure. The apparent partition coefficients for five calcite runs range from ∼ 2.5 to ∼ 10, and those for the aragonite runs fall in a range from ∼ 2.5 to ∼ 5. The mutual REE fractionation for a set of partition coefficients is relatively small. The pattern shapes of apparent partition coefficients for calcite obviously change with varying concentration level of the REE's doped in the initial solution. Such concentration dependence reveals that crystal structure-ionic radius control is not a major factor under the present experimental conditions, but other mechanisms such as the effect of REE complexation in the solution seem to be in operation.