Isothermal equation of state and high-pressure phase transitions of synthetic meridianiite (MgSO₄·11D₂O) determined by neutron powder diffraction and quasielastic neutron spectroscopy

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 < P < 800 MPa and 150 < T < 280 K. The refined unit-cell volumes as a function of P and T are parameterized in the form of a Murnaghan integrated linear equation of state having a zero-pressure volume V₀ = 706.23 (8) ų, zero-pressure bulk modulus K₀ = 19.9 (4) GPa and its first pressure derivative, K′ = 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.