Neutron Diffraction Study of Gypsum, CaSO42H2O

Abstract

Journal of The American Ceramic Society-Discussions and Notes 343the formation of the dehydration phases produced during the thermal dehydration of naturally occurring single crystals of gypsum. A modified transmission Laue method was used in these X-ray studies. The radiation source was a General Electric XRD-3 unit. A thin crystal of gypsum was mounted on a removable holder so that the bo axis of the monoclinic, Terpstra unit cell (a0 = 5.67 A, bo = 15.15 A, co = 6.28 A, and 6 = 113' 50')' was parallel to the incident beam of unfiltered Cu KCY radiation and perpendicular to the plane of the X-ray film. Initially, the Laue pattern of the gypsum was recorded. The crystal was then partly dehydrated at 14OoC, in a dry air atmosphere, and a second photograph was taken. This procedure was repeated several times until the crystal was completely converted to the hemihydrate. The sample was then heated at about 500°C until only r-CaS04, anhydrite, remained and a final X-ray pattern was taken.Selected X-ray patterns recorded in this study ( Fig. 1) clearly show that the polycrystalline dehydration phases are not randomly oriented relative to the parent crystal. The symmetry of the incomplete Debye rings is indicative of "fiber texture."The patterns in Fig. 1 are marked with the plane indices associated with the segmented Debye rings of both the hexagonal hemihydrate and orthorhombic anhydrite systems. Analysis of the partial dehydration of gypsum ( Fig. 1(B)) indicates that the hemihydrate crystallites develop with a texture axis (direction of preferred orientation) approximately perpendicular to the ( 100) planes and parallel to the (010) planes of the original crystal. The crystallites tend to be aligned with their co axes parallel to this texture axis but with random orientation of a0 axes. Analysis of the oriented growth of anhydrite ( Fig. l ( D ) ) indicates that the a. axes of the anhydrite crystallites developed parallel to the [OOl] axis of the gypsum crystal. Thus, the texture axes of the two dehydration products, hemihydrate and anhydrite, are mutually perpendicular but both are parallel to (010) planes of the dihydrate.The oriented growth of the dehydration products is understandable in terms of the structure of the dihydrate. The calcium sulfate and water molecules are arranged in alternating layers parallel to the (010) planes. The rather weak bonding of the water molecules as indicated by a hydrogen bond distance of 2.82 A2 gives rise to remarkably easy cleavage properties parallel to the bo planes of gypsum. After complete conversion of the Fig. 1. X-ray transmission patterns of CaS04.2 HzO and its dehydration products. (A) Dihydrate single crystal, ( B ) and (C) dihydrate partly converted to hemihydrate, and ( D ) anhydrite. Solid lines denote preferred orientation and plane indices are as marked.dihydrate to the hemihydrate or anhydrite the samples retained their original shapes although there was a general decrease in strength. The most pronounced weakening was in the forces between the calcium sulfate...

A refinement of the crystal structure of gypsum CaSO4.2H2O

“…225 (6) 16 (I) 204 (5) 3 (2) 149 (5) -7 (1) with the previous values found by Atoji & Rundle (1958). The thermal parameters arising from the anisotropic refinement of the same 2193 reflexions are given in Table 2.…”

“…In the crystal structure of gypsum (Wooster, 1936;Atoji & Rundle, 1958) molecule from those given in previous studies. As a result a clearer picture has emerged of the details of the sulphate ion and of the hydrogen-bond distances in gypsum.…”

“…A least-squares refinement was made on the counterdiffractometer data using the Busing, Martin & Levy (5) 3 (2) 149 (5) -7 (1) with the previous values found by Atoji & Rundle (1958). The thermal parameters arising from the anisotropic refinement of the same 2193 reflexions are given in Table 2.…”

“…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.…”

Laser-assisted removal of dark cement crusts from mineral gypsum (selenite) architectural elements of peripheral monuments at Knossos

“…However, the interlayer interactions are based on much weaker hydrogen bonds (Wooster, 1938). The calcium sulphate/water layers are arranged parallel to the (010) plane, a fact that is reflected on the perfect cleavage of the stone at that plane (Posnjak, 1938(Posnjak, , 1940Atoji and Rundle, 1958). Therefore, gypsum can be characterized as a phyllitic mineral.…”