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| |Fo| - |FC| |/Σ|Fo | 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.

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