A RE-EXAMINATION OF THORTVEITITE

Four diflerent natural crystals of thortveitite (Sc,Y)2Si2O7, have been analyzed by electron microprobe, and the structure has been determined from single-crystal data using MoKa radiation up to at least d : 40". The form factor of the metal has been derived from the chemical analysis of each crystal. The final disagreement factor R : > | 14 | - 14 | l/>lF"l is around 0.02; the standardd eviationso f the bond distancesa re between0 .001 and 0.002 A, and those of the angles are around 0.1". The Si-O-Si angle is indeed 180.; the crystal symmetry is C2/m, and there is no evidence for disorder involving the SirO, group; the analytical data indicate extensive substitution of Y and REE for Sc, as well as nonnegligible amounts of Zr and Hf; the charge balance is maintained by multiple substitution of Ca, Mg, and Mn in the metal position and of Al for Si.