Conflicting results for the deformation properties of forsterite, Mg2SiO4

Deformation properties of forsterite have been deduced simultaneously from X-ray diffraction data affected by extinction in Bonn-Pittsburgh (B), and in Groningen (G). For the G crystals, GI and GII, extinction is anisotropic and considerably larger than for crystal B. Measurements were made with Mo radiation for B, and with Mo and Ag radiation for GI and GII. As the Becker & Coppens [Acta Cryst. (1974), A30, 129-147, 148-153; Acta Cryst. (1975), A31, 417-425] extinction model is not exact, the deformation properties had to be filtered from the data with refinement models. The flexible B model Ice's and populations for single exponential functions (SEF's) refined for l=0-4] and the more rigid G model (SEF populations refined for l = 0-3 and a for l= 0; further a's and n's fixed at standard values) yield different results. Refinement of a makes the majority of the SEF's notably diffuse, presumably due to correlation with incorrect extinction corrections. The order of the deformation potentials at the Mg(1) and Mg(2) sites is reversed for B and G. Maxima on the Si-O bonds, which are polarized towards O, are smaller for G (0.20-0.25 e A -3) than for B (0.25-0.45 e/~-3). Although each of the two sets of deformation properties looks acceptable by itself, the present comparison shows that neither of them may be sufficiently close to the truth. The diffraction data are available on request from the Electron Density Data Bank (Professor H. Burzlaff, Institut ffir Angewandte Physik, Bismarckstrasse 10, D-8520 Erlangen, Federal Republic of Germany). Details of the measurements are described in the paper.