A re-examination of thortveitite

Four different natural crystals of thortveitite (Sc,Y)₂Si₂O₇ have been analyzed by electron microprobe, and the structure has been determined from single-crystal data using MoATa radiation up to at least θ = 40°. The form factor of the metal has been derived from the chemical analysis of each crystal. The final disagreement factor R = Σ| |F_o| - |F_C| |/Σ|F_o | is around 0.02; the standard deviations of the bond distances are between 0.001 and 0.002 Å, 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 Si₂O₇ 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.