Structure and microstructure of gypsum and its relevance to Rietveld quantitative phase analyses

Single crystals of gypsum were studied in a diffractometer equipped with a CCD two-dimensional detector. The microstructure of the crystal gave wide poorly shaped spots showing sometimes curved streaks around the spots, which made the integration process very difficult, yielding a low quality structure. The crystal structure and microstructure of gypsum has been studied by high-resolution synchrotron powder diffraction of a ground single crystal. The intensities in the synchrotron powder pattern can be reliably fitted although the peak shape displays anisotropic peak broadening. The Rietveld results for gypsum were a = 6.522 91(3) angstrom, b = 15.197 63(9) angstrom, c = 6.522 91(3) angstrom, 6 = 118.479(1)degrees, V = 494.536(5) angstrom(3) and Z=4 (s.g. 12/c) with R-WP=5.39% and R-F = 1.64%. We have also studied the influence of the structural description used for gypsum in a synchrotron Rietveld quantitative phase analysis of a standard mixture containing 50 wt % of CaSO4 center dot 2H(2)0 and Al2O3. Finally, the effects of the type of preferred orientation correction for laboratory X-ray powder data are also discussed. (c) 2004 International Centre for Diffraction Data.