Isothermal equation of state and high-pressure phase transitions of synthetic meridianiite (MgSO4·11D2O) determined by neutron powder diffraction and quasielastic neutron spectroscopy
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- 作者：A. Dominic Fortes ; Felix Fernandez-Alonso ; Matthew Tucker and Ian G. Wood
- 关键词：meridianiite ; undecahydrate ; enneahydrate ; equation of state ; neutron diffraction ; quasielastic neutron scattering
- 刊名：Acta Crystallographica Section B
- 出版年：2017
- 出版时间：February 2017
- 年：2017
- 卷：73
- 期：1
- 页码：33-46
- 全文大小：2200K
- ISSN：2052-5206
文摘
We have collected neutron powder diffraction data from MgSO4·11D2O (the deuterated analogue of meridianiite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. Our measurements, made using the PEARL/HiPr and OSIRIS instruments at the ISIS neutron spallation source, covered the range 0.1 < m>P m> < 800 MPa and 150 < m>T m> < 280 K. The refined unit-cell volumes as a function of m>P m> and m>T m> are parameterized in the form of a Murnaghan integrated linear equation of state having a zero-pressure volume m>V m>0 = 706.23 (8) Å3, zero-pressure bulk modulus m>K m>0 = 19.9 (4) GPa and its first pressure derivative, m>K m>′ = 9 (1). The structure's compressibility is highly anisotropic, as expected, with the three principal directions of the unit-strain tensor having compressibilities of 9.6 × 10−3, 3.4 × 10−2 and 3.4 × 10−3 GPa−1, the most compressible direction being perpendicular to the long axis of a discrete hexadecameric water cluster, (D2O)16. At high pressure we observed two different phase transitions. First, warming of MgSO4·11D2O at 545 MPa resulted in a change in the diffraction pattern at 275 K consistent with partial (peritectic) melting; quasielastic neutron spectra collected simultaneously evince the onset of the reorientational motion of D2O molecules with characteristic time-scales of 20–30 ps, longer than those found in bulk liquid water at the same temperature and commensurate with the lifetime of solvent-separated ion pairs in aqueous MgSO4. Second, at ∼ 0.9 GPa, 240 K, MgSO4·11D2O decomposed into high-pressure water ice phase VI and MgSO4·9D2O, a recently discovered phase that has hitherto only been formed at ambient pressure by quenching small droplets of MgSO4(aq) in liquid nitrogen. The fate of the high-pressure enneahydrate on further compression and warming is not clear from the neutron diffraction data, but its occurrence indicates that it may also be a rock-forming mineral in the deep mantles of large icy satellites.