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Structure determination of the 2.5 hydrate MgSO₄ phase by simulated annealing

¹ Department of Geological Sciences, Indiana University, 1001 East 10^th Street, Bloomington, Indiana 47405, U.S.A.
² Chesapeake Energy Corporation, 6100 N. Western Avenue, Oklahoma City, Oklahoma 73118, U.S.A.

Correspondence: ^* E-mail: bish{at}indiana.edu

The crystal structure of the 2.5 hydrate MgSO₄ phase was determined by simulated annealing from laboratory X-ray powder diffraction data measured from 2–140 °2 using CuK radiation. The 2.5 hydrate is monoclinic, space group C2/c, with unit-cell parameters a = 18.8636(4) Å, b = 12.3391(2) Å, c = 8.9957(2) Å, β = 94.568(2)°, V = 2087.1(6) ų, and Z = 16. The model was refined using fundamental-parameters Rietveld refinement, converging to R_wp = 8.89%, R_p = 6.61%, R_exp = 3.33%, R_Bragg = 3.95%, and ² = 2.67. The refined structure is consistent with a formula of 2.5 H₂O. Bond-valence calculations for the refined model show that the structure is chemically sensible. In the refined structure, [Mg(O,H₂O)₆] octahedra and [SO₄] tetrahedra build up 2-D double-sheet slabs by sharing vertex O atoms, which are held together by inter-slab H-bonds involving (SO₄)^2– groups and H₂O molecules coordinated with Mg²⁺ cations to form the layer structure of the 2.5 hydrate phase.

Key Words: MgSO₄·2.5H₂O • MgSO₄·2.4H₂O • crystal structure • simulated annealing • structure determination • powder diffraction • Rietveld refinement