Polymorphism of Mg-monohydrate sulfate kieserite under pressure and its occurrence on giant icy jovian satellites

Abstract

The high-pressure behaviour of hydrated magnesium sulfate kieserite, MgSO4⋅H2O, has been investigated on isothermal compression at T = 295 K up to 8.3 GPa hydrostatic pressure. The crystal properties of synthetic endmember single crystals were investigated using a high-pressure diamond anvil cell by means of in-situ X-ray diffraction and vibrational spectroscopy methods. The experimental study reveals a second-order phase transition from the monoclinic (C2/c) α-phase to a triclinic (P1¯) β-form at a transition pressure of 2.72 GPa. Elastic properties as determined from precise lattice parameters yield static elasticities as described by third-order Birch-Murnaghan equations of state with V0 = 355.5(4) ų, K0 = 48.1(5) GPa, K’ = 8.1(6) for the low-pressure polymorph (α-MgSO4⋅H2O), and V0 = 355.8(1.8) ų, K0 = 49.3(5.5) GPa, K’ = 4.8(1.0) for the high-pressure polymorph (β-MgSO4⋅H2O). The nature of the phase transition and its reversibility on pressure release make it seem unlikely that the β-polymorph can be recovered at surface conditions on any icy satellite, although in the context of impact events it is proposed to exist, but only on a limited time scale before re-transforming to α-MgSO4⋅H2O. With respect to the icy mantles of Ganymede and Callisto, the depth profile of Ganymede following the established thermal gradients suggest a stability field only for α-MgSO4⋅H2O being relevant to the presumable conditions in the icy mantle. In contrast, the depth profile for Callisto, as corresponding to maximum pressures of approximately 5 GPa, crosses the α-to-β-transition boundary and make the high-pressure polymorph a promising candidate rock-forming mineral for the deep icy mantle of the outermost Galilean moon. In particular the material parameters reported for the α and β form of MgSO4⋅H2O are fundamental to compute the icy mantle dynamics and accurately determine the radial density structure in models of Ganymede and Callisto.