HYDROGEN BONDING AND PHASE TRANSITIONS OF HYDROUS SULFATES EPSOMITE AND MELANTERITE AT THE PRESSURES AND TEMPERATURES OF GALILEAN SATELLITES

Magnesium hydrous sulfates (MgSO₄·nH₂O) have been identified in chondritic meteorites, on the surface of Mars, and likely form from the coexistence of water and sulfates in the interiors of Ganymede and Europa. Although liquid water at the surface of Solar System bodies remains elusive, hydrous minerals including hydrated Mg-, Fe-, and Ca- sulfates can provide key insights into the hydrologic history of terrestrial bodies. Epsomite (MgSO₄·7H₂O) contains 51 wt% water and is one of the most hydrated magnesium sulfates. Yet, despite the important role epsomite may play in the hydrogen cycle of icy satellites, little is known about the hydrogen bonding and dehydration of epsomite at the pressure and temperature conditions relevant to the interiors of icy satellites. Additionally, there is disagreement in the literature regarding the phase diagram of epsomite at high pressures. In this study, we combined elevated pressures produced by diamond anvil cells with cryogenic and external heating techniques to systematically probe the chemical bonding in epsomite at a range of temperatures and pressures using Raman and Fourier transform infrared (FTIR) spectroscopies. These measurements, performed at beamline 22-IR-1 of the National Synchrotron Light Source-II, enable us to probe competing temperature and pressure effects and to re-evaluate the phase diagram of magnesium hydrate sulfates. Based on significant changes in the ν₁ mode of the sulfate ions (982 cm⁻¹) and the ν₁ and ν₃ stretching modes of H₂O in the 3200-3450 cm⁻¹ region, our preliminary results suggest two distinct phase transitions in epsomite at room temperature at approximately 0.8 and 2.1 GPa. As a corollary, we probed the high-pressure chemical bonding of melanterite (FeSO₄·7H₂O), a hydrous iron(II) sulfate which is the chemical analogue of epsomite. Our preliminary results are consistent with phase transitions in melanterite at 2.4 and 8.2 GPa. By re-evaluating the stability and chemical bonding of epsomite and melanterite under a range of temperatures and pressure conditions, we hope to better assess the extent to which these phases contribute to the storage and cycling of hydrogen in the interiors of icy satellites.