Incorporation of Rare Earths and Yttrium in Calcite: A Critical Re-evaluation

Abstract

The reported partition coefficients of REE with calcite are reviewed and critically discussed. In some of the reported experimental sets, REE concentrations are found to be supersaturated with respect to individual REE₂(CO₃)₃ but never to REE(OH)₃. Although the solutions are unsaturated with respect to individual REY carbonates, REY including Y are incorporated in calcite surfaces, where they are overgrown by calcite. Charge balances may be obtained by building {REY–Na-(CO₃)₂}_n or by exchange of 2Ca²⁺ against REY³⁺?+?blank space calcite lattice. These surface compounds may either be homogeneously distributed or clustered. Both the size and frequency of clusters increase with [REY]/[Ca] or [ΣREY³⁺]/[Ca²⁺] in solution. If these surface precipitates are removed into solutions saturated with respect to ΣREE₂(CO₃)₃, they start growing in the aqueous phase. In this case, the apparent D_REY and k_REY values decrease with increasing REY concentrations in solution. In previous studies, only the individual distribution coefficients are reported not considering that the entirety of REY determines their behavior in partitioning. Given enough time, these surface clusters equilibrate with the aqueous phase before being overgrown by calcite. In the double logarithmic plots of {REY}/{Ca} versus [REY]/[Ca] or [REY³⁺]/[Ca²⁺], two relationships evolve characterizing the REY distribution in marine calcite and experimental calcites grown in Mg²⁺-free solutions. The double logarithmic plots of partition coefficients of REY³⁺_i in calcite grown from seawater show a slope exceeding unity, whereas those from fluids without Mg²⁺ depict slopes less than unity being both in contrast to the Henderson–Kracek rule.