Structure of magnesium hydroxide sulfate [2MgSO(4).Mg(OH)(2)] and solid solution in magnesium hydroxide sulfate hydrate and caminite

Magnesium hydroxide sulfate [2MgSO(4).Mg(OH)(2); MHS] is tetragonal with a = 7.454(1), c = 12.885(2) Angstrom, V = 716.0 Angstrom(3), space group P4(3)2(1)2, Z = 4 and D-x 2.774 g cm(-3). The structure (single-crystal X-ray, R = 0.025, wR = 0.023) comprises spiral (43) Single chains of corner-shared Mg(2) octahedra cross-linked by SO4 tetrahedra and face-shared Mg(1) octahedra. A linear ternary group of face-shared Mg octahedra [Mg(2)-Mg(1)-Mg(2)] alternates with an unoccupied octahedral position in rows along [<(1)over bar 10>]. A crystal of MHS was grown hydrothermally (0.15 Cpa, 673 K) from gold-bearing MgS-O-H fluid. The MHS structure, with Mg(1) octahedra sharing two octahedral faces with Mg(2) octahedra, revises the structure assumed for the complex solid solution magnesium hydroxide sulfate hydrate and the related ocean-floor mineral caminite [2MgSO(4).xMg(OH)(2).(2 - 2x)H2O; 0.5 less than or equal to x < 1.0]. The substitution reaction appears to be Mg(1)(2+) reversible arrow 2H(+). The H-substituted MHS structure is distinguished from that of kieserite (MgSO4.H2O), which has straight single chains of corner-shared Mg octahedra.