STRUCTURAL REFINEMENT OF MONOCLINIC LOW TRIDYMITE MC WITH DIFFRACTIONINTENSITIES OF A TWIN CRYSTAL

The crystal structure of the monoclinic low tridymite MC, a polymorph of SiO2, has been re-refined using the unpublished as-measured intensi ties of a twinned crystal collected by Kato and Nukui [Acta Crystallog r. B32 (1976) 2486-2491]. Crystal data: space group Cc, a = 18.494(8) Angstrom, b = 4.991(2) Angstrom, c = 25.832(8) Angstrom, beta = 117.75 (2)degrees, z = 48. R = 0.071, R-w = 0.071 based on 6453 F-o's for 325 variable parameters. The Si-O distances are between 1.575(6) Angstrom and 1.620(7) Angstrom; mean 1.598 Angstrom. Except for an extreme val ue of 178.8(6)degrees, the Si-O-Si angles range from 142.9(4)degrees t o 156.3(6)degrees; mean 148.6 degrees. The structure was refined also as a commensurate modulated structure based on the high-tridymite stru cture.