Abstract
Since being discovered initially in mixed-cation systems, a method of forming end-member MgSO4·9H2O 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. MgSO4·9D2O is monoclinic, space group P21/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) Å3. The structure consists of two symmetry-inequivalent Mg(D2O)6 octahedra on sites of \bar 1 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(D2O)6 octahedra to the SO4 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, αV, is substantially larger than that of either MgSO4·7D2O (epsomite) or MgSO4·11D2O (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 MgSO4·9D2O 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 MgSO4·9H2O from cation-pure aqueous solutions requires quench-freezing of small droplets, a situation that may be relevant to spraying of MgSO4-rich cryomagmas into the surface environments of icy satellites in the outer solar system. However, serendipitously, we obtained a mixture of MgSO4·9H2O, mirabilite (Na2SO4·10H2O) 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 MgSO4·9H2O can occur naturally – albeit on a transient basis – in certain terrestrial and extraterrestrial environments.
Highlights
The unidentified phase did not correspond to any of the highpressure phases of MgSO4Á7D2O we had encountered in other high-pressure experiments (Gromnitskaya et al, 2013) and one possible explanation was the occurrence of an intermediate hydration state that became stable only under pressure
The work paid dividends in terms of characterizing the extent to which various divalent metal cations can substitute for Mg2+ in the meridianiite structure and their effect on the unit-cell parameters (Fortes, Browning & Wood, 2012a,b), and revealed crystals isotypic with meridianiite formed from MgCrO4 and MgSeO4 (Fortes & Wood, 2012: Fortes, 2015)
In early studies of icerich enneahydrate samples, we found that the MS9 would transform irreversibly to the more stable undecahydrate, MS11, on time scales of hours at 250 K, presumably by reaction with water vapour
Summary
Our attention was originally drawn to this gap between n = 7 and n = 11 by high-pressure studies of MgSO4Á11H2O where we observed an apparent decomposition at $ 0.9 GPa, 240 K to a mixture of ice VI and what was – at the time – an unknown hydrate (Fortes et al, 2017). In our experiments we initially adopted chemical substitution as a crude proxy for hydrostatic stress, the expectation being that this would create analogue structures of the phase observed at high pressure. (Mg1 À xNix)SO4 for 0.3 < x < 0.8] by rapid quenching of aqueous solutions in liquid nitrogen. These included a monoclinic enneahydrate for all of the aforementioned cations and a triclinic octahydrate for Ni2+. The discovery of MgSO4Á9H2O resolved a long-standing question over the origin of weak parasitic peaks observed in our first neutron powder diffraction measurement on MgSO4Á11D2O, which did not appear to match any other known hydrate or plausible contaminant (Fortes et al, 2008, and Fig. S1 of the supporting information)
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