Structure, thermal expansion and incompressibility of MgSO₄·9H₂O, its relationship to meridianiite (MgSO₄·11H₂O) and possible natural occurrences

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• 作者：A. Dominic Fortes ; Kevin S. Knight and Ian G. Wood
• 关键词：enneahydrate ; meridianiite ; undecahydrate ; thermal expansion ; equation of state ; neutron diffraction ; DFT ; elastic constants
• 刊名：Acta Crystallographica Section B
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：73
• 期：1
• 页码：47-64
• 全文大小：2308K
• ISSN：2052-5206

文摘

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) ų. The structure consists of two symmetry-inequivalent Mg(D₂O)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(D₂O)₆ octahedra to the SO₄²⁻ 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 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, K₀ = 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.