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) Å, b=15.197 63(9) Å, c=6.522 91(3) Å, β=118.479(1)°, V=494.536(5) Å 3 and Z=4 (s.g. I2/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 CaSO 4 ⋅2H 2 O and Al 2 O 3 . Finally, the effects of the type of preferred orientation correction for laboratory X-ray powder data are also discussed.