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 structur e refined at room temperature with single-crystal X-ray intensities to R = 0.024. The F content is consistent with the conditions of synthes is 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 b y direct refinement and Na by iteration. The Nd and Na occupancies are consistent with site preferences deduced from bond valence calculatio ns. The bonding requirements of the A (or O) anion exert a considerabl e influence on the site preference of cations in apatites. (C) 1994 Ac ademic Press, Inc.