The structure of monoclinic low-tridymite, a polymorph of SiO2, was determined by the X-ray method with a synthetic crystal twinned about [301]. Low-tridymite crystallizes in the space group Cc with the unit-cell dimensions a=18"494 (8), b=4"991 (2), c=25-832 (8) A and fl=117.75 (2)°; Z--48. The mean Si-O distance is 1.595 or 1.610 .~ if corrected for thermal motion of the atoms. The Si-O-Si angles range from 143-2 to 156.7 °, except for an extremely large angle of 176.4°; the mean value is

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... 4 °. The final R and weighted Rt values based on 4117 Fo's collected on a diffractometer are 0-085 and 0"055 respectively.