High-Pressure Behavior of an Iron-Bearing Mineral Relevant to Icy, Sulfate-Rich Worlds

Abstract:

Hydrated sulfates have become increasingly relevant in determining the interior compositions of icy satellites in the Solar System. Observations of Europa's icy surface indicate that it is a complex mixture of hydrates, including sulfates, assumed to be composed of H₂O and chondritic material, and spectral observations of Ganymede indicate the presence of hydrated minerals on its icy surface. Spectral similarities between measurements of Europa's surface and laboratory experiments identify magnesium hydrated sulfate-H₂O mixtures as a dominant phase, and strongly support its existence at depth. Magnesium sulfates observed on Earth and Mars easily incorporate iron in their structure, for example forming the kieserite (MgSO₄·H₂O) – szomolnokite (FeSO₄·H₂O) solid solution. Characterizing the properties of these end member compositions is important to fully understand the complex mixture of natural compositions that could exist in icy, sulfate-rich interiors.

Investigation of the high-pressure behavior of szomolnokite provides a pathway to understand the effect that iron has in hydrous, sulfate-rich environments (e.g. Perez et al. 2019, Minerals; Meusburger et al. 2019, J. Solid State Chem.). This work presents the results of diamond anvil cell experiments conducted at beamline 12.2.2 at the Advanced Light Source of Lawrence Berkeley National Laboratory (ALS). We conducted high-pressure X-ray diffraction experiments at the ALS on a powdered sample of synthetic szomolnokite using a helium pressure transmitting medium up to pressures of 83 GPa. Our results indicate two structural phase transitions occurring between 5-7 GPa and 13-17 GPa. Elastic properties were determined from 3rd order Birch-Murnaghan equations of state for the three distinct polymorphs. Comparison between our results and reported elastic properties for the Mg-endmember kieserite and other hydrated sulfates aim to quantify the effect of iron within a hydrous environment under compression. These results can inform future interior structure models of icy, sulfate-rich worlds.