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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...

The neutron diffraction data of the rare-earth sesquicarbides with the Pu2C3-type structure (D5c type), La2C3, Ce2C3, Pr2C3, and Tb2C3, were analyzed using the complete-matrix least-squares method. These sesquicarbides contain the C2 groups with the C–C distances, 1.236±0.009, 1.276±0.005, 1.239±0.003, and 1.240±0.005 A, respectively, for La2C3, Ce2C3, Pr2C3, and Tb2C3. These values are, except for Ce2C3, significantly longer than the C–C distance in CaC2, 1.191 A, but are shorter than the average C–C distance in the rare-earth dicarbides, 1.278 A. The paramagnetic scattering analyses show that all metal atoms in these sesquicarbides are in their trivalent Hund ground state, except Ce in Ce2C3, whose possible valency is about 3.4. The screening constants for the 4f hydrogenic radial wave functions of Pr and Tb as determined from the diffuse scattering analyses of their sesquicarbides are 40 and 43, respectively. The valency and metallic bonds in these sesquicarbides are briefly discussed.