Distribution of trace elements between phases in the Carbonate–Phosphate System with Fluorine at 500 MPa

Abstract

The distributions of Ti, Zr, Nb, La, Sm, Yb, and Y were experimentally studied between phosphate and carbonate melts with various Ca and Na concentrations and equilibrium solid phases: apatite (in 12 experiments) and buchwaldite, CaNaPO4 (in two experiments). The experiments were carried out in an internally heated pressure vessel at a pressure of 500 MPa in the CaCO3 + NaPO3 + NaF system with variable proportions of phosphate and carbonate, and with the addition of H2O, H2C2O4 and mixture of trace-element oxides. The temperatures in runs V-1-8 were conducted as 1000 °C for melting for 24 h, then for crystallization at 650 °C for 7 days, and quenching; the temperatures in runs VI-1-5 - 1000 °C for 1 day and immediate quenching. The proportion of phosphate and carbonate in the starting mixtures was expressed as χ(P2O5) (mol. %), where χP2O5=nP2O5nP2O5+nCaO×100%. The experimental products were analyzed by electron probe microanalysis. The following two types of quenched melts were obtained depending on χ(P2O5): (i) calcite-rich melt with 20 mol% Na2O at χ(P2O5) < 30% and (ii) sodic carbonate–phosphate melt with a low CaO concentration at χ(P2O5) > 30%. The solubilities of ZrO2, TiO2, and Nb2O5 in the calcite-rich quenched melts at 650 °C were low and limited by the crystallization of Zr, Ti, and Nb oxides. At the temperatures 1000 °C these oxides are dissolved in the melts, and their concentrations in the melts increased with increasing χ(P2O5). The TiO2 and ZrO2 distribution coefficients between apatite and melt are 0.06–0.2 and 0.4–1.0, respectively. The REE distribution coefficients between apatite and coexisting melt rich in Ca carbonate increase with increasing χ(P2O5) from 1.3 to 3.3 for La2O3, from 0.4 to 3.4 for Sm2O3, from 1.4 to 1.9 for Yb2O3, and from 0.8 to 2.9 for Y2O3. The REE distribution coefficients between apatite and the sodic carbonate–phosphate melt increase by a few orders of magnitude with increasing χ(P2O5), particularly significantly for La2O3 and Sm2O3, to 110 and 75, respectively, and to >32 and 20 for Yb2O3 and Y2O3. The TiO2 and ZrO2 distribution coefficients between buchwaldite and melt do not depend on χ(P2O5) and are 0.1 and 1.05, respectively. The REE distribution coefficients between buchwaldite and melt coexisting with it in the experiments are 1.5 times lower than the distribution coefficients between apatite and melt and increase from ∼1.1 for all REE at χ(P2O5) = 25.8%, to 3.6, 2.9, 3.2, and 2.0, respectively, for La2O3, Sm2O3, Yb2O3 и Y2O3 at χ(P2O5) = 41.7%.