SITE PREFERENCE OF RARE-EARTH ELEMENTS IN FLUORAPATITE - BINARY (LREE-SUBSTITUTED CRYSTALS(HREE))

Crystals of binary (LREE+HREE)-bearing fluorapatite [La, Gd-FAp, Ce, D y-FAp, Pr, Er-FAp, Eu, Lu-FAp; Ca10-x-2yNayREEx+y(P1-xSixO4)(6)A(2), w ith x = 0.12 - 0.20, y = 0.26 - 0.42; P6(3)/m] have been grown from H2 O-rich phosphate-fluoride melts, and their structures refined at room temperature with single-crystal X-ray intensities to R = 0.017 - 0.022 . These binary-REE-substituted fluorapatite samples have REE site-occu pancy ratios (REE-Ca2/REE-Cal) of 2.32, and 2.32, 2.03, 1.71, respecti vely, which are 0.47 - 0.16 smaller than corresponding ratios calculat ed using data for reference single-REE-substituted fluorapatite. Discr epancies in intracrystalline partitioning between multiple-REE-substit uted apatites and single-REE-substituted fluorapatite decrease with a decrease in REE concentration, becoming negligible at 0.2 - 0.3 total REE cations pfu in synthetic binary-REE-substituted fluorapatite and a t trace abundances of REE in natural apatites. However, quantitative t ransference of laboratory REE site preferences to natural apatites is frustrated by the compositional complexity in nature. In the synthetic fluorapatite, there is a profound change in the spatial accommodation of REE in the apatite structure at about the position of Nd in the 4f transition-metal series, corresponding to the peak in the experimenta l uptake curve. Discrepancies in intracrystalline partitioning between binary-REE- and single-REE-substituted fluorapatite are attributed to non-ideal mixing of LREE and HREE that results in contraction of the Cal coordination sphere.