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Structure, thermal expansion and incompressibility of MgSO 4 ·9H 2 O, its relationship to meridianiite (MgSO 4 ·11H 2 O) and possible natural occurrences
Author(s) -
Fortes A. Dominic,
Knight Kevin S.,
Wood Ian G.
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520616018266
Subject(s) - octahedron , crystallography , chemistry , oxyanion , monoclinic crystal system , molecule , crystal structure , metastability , hydrogen bond , tetrahedron , hydrogen , neutron diffraction , atmospheric temperature range , protonation , physics , thermodynamics , ion , biochemistry , organic chemistry , catalysis
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 P 2 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 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.