THE STRUCTURE OF NaCa2LuSi2O7F2, A SYNTHETIC PHASE OF THE CUSPIDINE GROUP

MICHAEL E. FLEET
Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7

YUANMING PAN
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0

Abstract

Crystals of a new cuspidine-group phase NaCa2LuSi2O7F2 have been synthesized at 690 C and 0.12 GPa P(H2O), and the basic cuspidine- type structure confirmed by X-ray structure refinement using reflection intensities from a twinned crystal [P21/a; a 11.024(5), b 10.303(3), c 7.391(2) Å, 109.40(3) ]. The site occupancies of the four X positions are: Ca(1): 0.155 Ca, 0.845 Na; Ca(2): 0.845 Ca, 0.155 Na; Ca(3): 0.155 Ca, 0.845 Lu; Ca(4): 0.845 Ca, 0.155 Lu. Lutetium is restricted to the Ca(3) and Ca(4) positions and Na to Ca(1) and Ca(2), in agreement with the predicted site-preference from bond-valence sums for the Ca positions in cuspidine (Ca4Si2O7F2). However, unlike the behavior of high-field-strength cations that are restricted to the smaller Ca(3) position in the mineral representatives of the cuspidine group, Lu does enter Ca(4) in synthetic Lu-cuspidine. This is because Lu3+ is closer in size to Ca2+; when Lu occupies Ca(4), linkage of the diorthosilicate group to two edges of the Ca(4)O6F polyhedron displaces the bridging oxygen and straightens the bond angle involving the bridging oxygen.

Keywords: cuspidine group, lutetium calc-silicate, bond valence, structure refinement, site preference.