Site preference of rare earth elements in fluorapatite

Abstract

Crystals ofLa-, Gd-, and Dy-bearing fluorapatite [La-FAp, Gd-FAp, Dy-FAp; CalO-X-2YNayREEx+y (P1_xSixO.)6A2, with x = 0.24-0.29, Y = 0,32-0.36; P6/m] have been synthesized hydrothermally, and their structures refined at room temperature with single-crystal X-ray intensities to R = 0.015-0.018. Na is essentially restricted to the Cal position in La-FAp, Gd-FAp, and Dy-FAp, in contrast to Nd-FAp, which was synthesized under slightly different conditions and has appreciable Na in Ca2 as well. Site occupancies for REE in Cal and Ca2, respectively, are 0.023(1) and 0.093(1) in LaFAp, 0.038(1) and 0.111(1) in Nd-FAp, 0.038(0) and 0.077(0) in Gd-FAp, and 0.039(1) and 0.060(1) in DyFAp. The REE site occupancy ratio (REE-Ca2 to REE-Cal) appears to decrease monotonically for REE3+cations through the 4f transition-metal series. With this assumption, site occupancy ratios (REE-Ca2 to REE-Cal) for some other REE in natural apatite are estimated to be: La 4.04, Ce 3.67, Pr 3.30, Nd 2.92, Sm 2.47, Eu 2.25, Gd 2.03, Dy 1.54, Y 1.29, Er 1.05. These single-REEsite occupancyratios may not be transferrable to natural apatite, The Cal and Ca2 site occupancies are generallyconsistent with site preferences deduced from bond-valence calculations, which show that the substitutions for Ca lead to equalization of Cal and Ca2 bond valences. Also, the REE site occupancy ratio correlates inversely with F bond valence.