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With the use of three-dimensional X-ray diffraction data the crystal structure of gypsum has been refined by least-squares methods. Initial parameters used were those found by Atoji & Rundle [J. Chem. Phys. (1958), 29, 1306] from neutron-diffraction data. Significant changes were made in the x and z parameters of the oxygen atoms of the sulphate ion and the oxygen atom of the water molecule. Bond lengths around the sulphate ion show equal sulphur-to-oxygen distances of mean value 1.459 A,, but one pair of oxygen-to-oxygen distances of 2"323 A and the corresponding pair of oxygen-sulphuroxygen bond angles of 105.5 ° are significantly different from the others, which have mean values of 2.411 A and 111.5 °, respectively. As a result the symmetry of the sulphate ion is lowered from a tetrahedron to a sphenoid. The new oxygen parameters indicate that there are two distinctly different hydrogen-bond lengths of 2.816 and 2"896/~ rather than one only of mean value 2.820 A, as found by Atoji & Rundle. A re-working of the data of Atoji & Rundle produced refinement beyond that which they reported and clearly indicated the two different hydrogen-bond lengths (difference 0.065 _+ 0.018 A) shown by the X-ray work. The re-refining of the neutron data also confirmed the lowering of the symmetry of the sulphate ion, although this did not show up in Atoji & Rundle's refinement. On the basis of the new parameters for the hydrogen atoms from the re-worked neutron data and the parameters for the water oxygen found from the X-ray data, the two O-H distances in the water molecule are calculated as 0.962 and 0.944 A, but the difference is not statistically significant. The details of the refined structure are in agreement with conclusions drawn by other workers from spectroscopic data. The anisotropic thermal parameters are discussed with respect to the structure. IntroductionThe crystal structure of gypsum, CaSO4.2H20 was determined by Wooster (1936) from two-dimensional X-ray diffraction data with Fourier methods, which gave an R index of 13.6% on observed data. It was further refined by Atoji & Rundle (1958) from Okl, hkO and hkh neutron-diffraction data by means of both Fourier and least-squares refinement methods to an R index of 15.5% on observed data. The neutron-diffraction work permitted the fixing of the positions of the hydrogen atoms and indicated small but significant changes in the positions of all of the oxygen atoms from those established by Wooster. Denne & Jones (1969) re-examined the neutron-diffraction data of Atoji & Rundle by an occupation-factor approach and found that the distribution of the hydrogen atoms was centrosymmetric.By modern standards the crystal structure of gypsum is only poorly refined and the present study was undertaken to refine the structure of this important industrial and building material by using three-dimensional X-ray data collected on a modern semi-automatic counter diffractometer. The study has shown (Cole & Lancucki, 1972a, b) that the symmetry of the SO4 ion in gypsum is lower tha...

X-ray, differential thermal analysis, and dehydration methods have been used to study the magnesium oxychlorides 3Mg(OH)2.MgCl2.8H2O and 5Mg(OH)2.MgCl2.8H2O formed at temperatures below 100 �C, and 2Mg(OH), . MgCl2.5H2O and 9Mg(OH)2.MgCl2.6H2O formed at temperatures at and above 100 'C. Lower hydrates of the 3-form exist with 5 and 4 molecules of water, results which differ from those of Feitknecht and Held (1944), Feitknecht (1953), and Wehner (1933). It 1s not clear from the new results whether the lower hydrates for the 5-form hare 5 and 3 or 4 and 3 molecules of water, but the latter seems the more likely ; the results of Feitknecht and Held suggested the former. For the 2- and 9-forms a lower hydrate with 2 molecules of water exists. In addition all forms have an anhydrous phase with the crystal structure of Mg(OH)2. The stability of all phases under natural atmospheric conditions is discussed.

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