Reinvestigation of the structure of low cristobalite

Twinned crystals of low cristobalite from Ellora, Hyderabad, India were found to have space group P4121 (or P4321) and cell dimensions a = 4.978 and c = 6.948A. A three-dimensional full-matrix least-squares refinement based upon 110 integrated intensities which were corrected for the effects of twinning, confirmed NIEUWENKAMP's original structure and reduced the final R value to 4.4%. The refined set of four positional parameters are; u = .30004 ::I::.00033, x = .23976::1:: .00089,y = .10324::1:: .00085,z = .17844::1:: .00052. These refined parameters lead to Si-O distances of 1.601 ::I::.004 and 1.608 ::I::.004 A.