Structure, thermal expansion and incompressibility of MgSO(4)·9H(2)O, its relationship to meridianiite (MgSO(4)·11H(2)O) and possible natural occurrences

Since being discovered initially in mixed-cation systems, a method of forming end-member MgSO(4)·9H(2)O has been found. We have obtained powder diffraction data from protonated analogues (using X-rays) and deuterated analogues (using neutrons) of this compound over a range of temperatures and pressures. From these data we have determined the crystal structure, including all hydrogen positions, the thermal expansion over the range 9–260 K at ambient pressure, the incompressibility over the range 0–1.1 GPa at 240 K and studied the transitions to other stable and metastable phases. MgSO(4)·9D(2)O is monoclinic, space group P2(1)/c, Z = 4, with unit-cell parameters at 9 K, a = 6.72764 (6), b = 11.91154 (9), c = 14.6424 (1) Å, β = 95.2046 (7)° and V = 1168.55 (1) Å(3). The structure consists of two symmetry-inequivalent Mg(D(2)O)(6) octahedra on sites of [Image: see text] symmetry. These are directly joined by a water–water hydrogen bond to form chains of octahedra parallel with the b axis at a = 0. Three interstitial water molecules bridge the Mg(D(2)O)(6) octahedra to the SO(4) (2−) tetrahedral oxyanion. These tetrahedra sit at a ≃ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. The temperature dependences of the lattice parameters from 9 to 260 K have been fitted with a modified Einstein oscillator model, which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion coefficient, α(V), is substantially larger than that of either MgSO(4)·7D(2)O (epsomite) or MgSO(4)·11D(2)O (meridianiite), being ∼ 110 × 10(−6) K(−1) at 240 K. Fitting to a Murnaghan integrated linear equation of state gave a zero-pressure bulk modulus for MgSO(4)·9D(2)O at 240 K, K (0) = 19.5 (3) GPa, with the first pressure derivative of the bulk modulus, K′ = 3.8 (4). The bulk modulus is virtually identical to meridianiite and only ∼ 14% smaller than that of epsomite. Above ∼ 1 GPa at 240 K the bulk modulus begins to decrease with pressure; this elastic softening may indicate a phase transition at a pressure above ∼ 2 GPa. Synthesis of MgSO(4)·9H(2)O from cation-pure aqueous solutions requires quench-freezing of small droplets, a situation that may be relevant to spraying of MgSO(4)-rich cryomagmas into the surface environments of icy satellites in the outer solar system. However, serendipitously, we obtained a mixture of MgSO(4)·9H(2)O, mirabilite (Na(2)SO(4)·10H(2)O) and ice by simply leaving a bottle of mid-winter brine from Spotted Lake (Mg/Na ratio = 3), British Columbia, in a domestic freezer for a few hours. This suggests that MgSO(4)·9H(2)O can occur naturally – albeit on a transient basis – in certain terrestrial and extraterrestrial environments.