Isothermal equation of state and high-pressure phase transitions of synthetic meridianiite (MgSO ₄·11D ₂O) determined by neutron powder diffraction and quasielastic neutron spectroscopy

Abstract

We report high-pressure neutron powder diffraction measurements of the most hydrated phase in the MgSO ₄–H ₂O system, magnesium sulfate undecahydrate, including an analysis of the elastic strain tensor and observations concerning phase changes that occur at high pressure.

Abstract

We have collected neutron powder diffraction data from MgSO ₄·11D ₂O (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 V ₀ = 706.23 (8) Å ³, zero-pressure bulk modulus K ₀ = 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, (D ₂O) ₁₆. At high pressure we observed two different phase transitions. First, warming of MgSO ₄·11D ₂O 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 D ₂O 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 MgSO ₄. Second, at ∼ 0.9 GPa, 240 K, MgSO ₄·11D ₂O decomposed into high-pressure water ice phase VI and MgSO ₄·9D ₂O, a recently discovered phase that has hitherto only been formed at ambient pressure by quenching small droplets of MgSO ₄(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.