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High-pressure behavior of gypsum: A single-crystal X-ray study

¹ Department of Earth Sciences, University of Perugia, Perugia Italy
² GeoForschungsZentrum, Potsdam, Germany

Correspondence: ^* E-mail: comodip{at}unipg.it

High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 ± 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded K_T0 = 44(3) GPa and (K_T/P)₀ = 3.3(3), where K_T0 and ( K_T/P)₀ are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were β_0a^EoS = 6.1(1) and β_0c^EoS = 5.6(1) 10^–3 GPa^–1. The value of β_0b ^EoS was 6.2(8) 10^–3 GPa^–1 fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic (β_0a: β_0b: β_0c = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca- and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that SO₄ volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)–1.66(9) ų; 1.4733–1.48 Å]. The SO₄ tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) Å and the larger increases from 1.4735(9) to 1.51(2) Å. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) ų at ambient P to 24.7(1) ų at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) 10^–3 GPa^–1, about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) Å for OW-H1···O2, and from 2.883(2) to 2.69(3) Å for OW-H2···O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the ₁ sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa.

Key Words: Gypsum • high pressure • single-crystal X-ray diffraction • phase transition

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