The Elastic, Electronic, and Structural Properties of Hydrous, Sulfur-Bearing Minerals in Planetary Environments: from the Surface to Deep Interiors

Citation

Pardo, Olivia Sabine (2023) The Elastic, Electronic, and Structural Properties of Hydrous, Sulfur-Bearing Minerals in Planetary Environments: from the Surface to Deep Interiors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yarz-tr12. https://resolver.caltech.edu/CaltechTHESIS:06112023-090146049

Abstract

In this thesis, a comprehensive investigation of the hydrous iron endmember sulfate szomolnokite (FeSO₄·H₂O) has been conducted using a suite of complementary techniques to measure its structural, elastic, electronic, and vibrational properties under extreme conditions. Through X-ray diffraction (XRD), nuclear resonant inelastic X-ray scattering (NRIXS), synchrotron Mössbauer spectroscopy (SMS), and synchrotron Fourier transform infrared spectroscopy (FTIR) in the diamond anvil cell, the material properties of szomolnokite have been characterized under high pressures and low temperatures relevant to hydrous, sulfur-rich planetary environments. XRD measurements presented in this work have revealed two structural phase transitions occurring at pressures between 5.0 and 6.6 GPa and between 12.7 and 16.8 GPa, with the latter phase stable up to 80 GPa. The elastic parameters of each phase have been determined by fitting third-order Birch-Murnaghan equations of state. I compare our results with elastic parameters of other relevant sulfate phases, highlighting the importance of reporting and comparing these parameters at the pressures where the phases are stable. Using NRIXS and SMS, the lattice vibrational response and the effects on the iron electronic environments during the structural transitions are examined. The NRIXS and SMS data reveal distinct features and pressure-dependent behaviors that characterize alterations in both iron-site specific and bulk lattice properties associated with the phase transitions, including lattice softening and decreased iron-coordination environment symmetry. Utilizing both the NRIXS and XRD results, I discuss how the presence of sulfates in the ice-rich crusts of planetary bodies could affect tidal loading observations. Synchrotron FTIR measurements demonstrate that structurally bound H₂O is retained within the unit cell during the structural transitions and upon subsequent decompression, confirming the retention of water up to 23 GPa and temperatures as low as 20 K and indicating the reversibility of both structural transitions. Supported by our quantum mechanics molecular dynamics simulations, the existence of two vibrationally unique water sites in szomolnokite’s crystal structure is proposed to explain the experimentally observed H2O-related features. I develop a spectral diagnostic for observing the high pressure structural transformations at ambient and low temperatures. The measured partial phonon density of states, predicted vibrational density of states, and measured FTIR spectrum are compared. Drawing from the insights gained, we emphasize the advantages of employing complementary experimental and computational techniques and discuss future research directions that can further enhance our knowledge of hydrous, sulfur-rich planetary environments.

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