SITE PREFERENCE OF ND IN FLUORAPATITE [CA-10(PO4)(6)F-2]

A Nd-bearing fluorapatite [Ca10-x-2yNayNdx+y(P1-xSixO4)(6)A(2), with x approximate to 0.32, y approximate to 0.51, A(2) = F-1.63(OH)(0.37); P6(3)/m], has been synthesized hydrothermally and its crystal structure refined at room temperature with single-crystal X-ray intensities to R = 0.024. The F content is consistent with the conditions of synthesis and the Ca(2)-A bond length. Site occupancies of substituents are 0.041(1) for Nd and 0.077(8) for Na in the Ca(1) position and 0.110(1) for Nd and 0.034(8) for Na in the Ca(2) position, with Nd determined by direct refinement and Na by iteration. The Nd and Na occupancies are consistent with site preferences deduced from bond valence calculations. The bonding requirements of the A (or O) anion exert a considerable influence on the site preference of cations in apatites. (C) 1994 Academic Press, Inc.