Isobaric Crystallization of Aqueous Magnesium Chloride Solutions at Icy Moon Conditions

Subsurface oceans inside icy moons are expected to contain dissolved salts whose crystallization controls the mineralogy of their hydrospheres and potentially their surfaces. Here, we use in situ neutron diffraction to investigate the isobaric crystallization of a nearly saturated MgCl2/D2O solution between ambient pressure and 0.4 GPa. At ambient pressure, substantial supercooling leads primarily to MgCl2·12D2O, whereas increasing the pressure to only 0.1 GPa fundamentally changes the crystallization pathway, yielding the recently discovered MgCl2·10D2O-II as the dominant hydrate throughout the 0.1–0.4 GPa range. The simple isobaric route enables reliable preparation of decahydrate-II, opening the door to future studies, including spectroscopic characterization relevant to its remote detection on icy moons. We further determine a new thermal equation of state and reveal a curious contrast between its responses to pressure and temperature: the most compressible crystallographic direction exhibits the smallest thermal expansion. These findings demonstrate the importance of pressure for understanding salt-hydrate crystallization and icy-moon mineralogy.