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The crystal structure of gypsum-II determined by single-crystal synchrotron X-ray diffraction data

The crystal structure of gypsum-II determined by single-crystal synchrotron X-ray diffraction data

American Mineralogist 95.4

The crystal structure of gypsum-II, a polymorph of CaSO4 .2H2 O stable above 4 GPa, has been solved using single-crystal synchrotron X-ray diffraction data collected at 5.35 and 6.74 GPa. Gypsum-II is monoclinic, space group P21 /n, with lattice parameters a=5.865(12), b=15.045(14), c=5.478(12) Aa, beta =115.3(2) degrees , and V = 437.0(14) Aa3 at 5.35 GPa, and a=5.776(2), b=15.017(2), c=5.473(2) Aa, beta =114.98(4) degrees , and V=430.3(2) Aa3 at 6.74 GPa. The crystal structure has been refined to R1 =3.7 (5.35 GPa) and 3.9% (6.74 GPa). It closely resembles that of gypsum at room pressure with a stacking of CaO8 and SO4 polyhedra along the b-axis to form layers. With increasing pressure, a continuous increase in distortion of the SO4 tetrahedron and a strong change in the bonding style of the water molecules are observed. The mechanism of phase transformations previously hypothesized in gypsum, on the basis of high-pressure spectroscopic data, is here clarified for the polymorph stable between 4-8 GPa.

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Accession: 037177787

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DOI: 10.2138/am.2010.3452

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