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...

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In order to obtain either the complete diffraction diagram or the distribution of interlayer distances from the observed diffraction intensities in the calculation of X-ray diffraction effects of interstratified clay minerals a knowledge is required of the structure factor of the layers. This represents the contribution of the layer structure to the variation of scattered amplitude in the [00/] direction in reciprocal space, and is a continuous function for random structures. The layer structure per/~,2 is given by (MacEwan, Ruiz Amil & Brown, 1961): Fz = Z NJ'~ exp(2rcir* Z~) , S where N~ is the number of atoms of type s per layer per/~z, fi is the atomic scattering factor of atoms s, Z~ is the coordinate (/~) in a direction perpendicular to layers of atoms s and r* (=2 sin 0/~) is the reciprocal spacing coordinate (/~) measured along the [00/] direction. The calculations for interstratified minerals require a knowledge of IFd 2= F~Fz* where Fz* is the complex conjugate of Fz. Where the

Konyaite [Na2Mg(SO4)2·5H2O] was identified for the first time in Australia in salt efflorescences from a Tertiary marine deposit below exposed basalt flows at a quarry near Geelong, Victoria. Other soluble salts in the efflorescence were epsomite (MgSO4·7H2O), gypsum (CaSO4·2H2O), and halite (NaCl). Heating at 110°C converted the konyaite to a mixture of bloedite [Na2Mg(SO4)2·4H2O], loeweite [Na4Mg2(SO4)4·5H2O], and thenardite (Na2SO4). Recrystallization at 23°C of the efflorescence dissolved in varying amounts of water indicated that epsomite and thenardite form from dilute solutions, whereas increasing amounts of konyaite recrystallize from the more concentrated solutions under faster evaporative conditions. The salts are transported to the basalt quarry site in river seepage or ground water from the neighboring marine deposits and are left on the exposed surfaces by evaporation.