Abstract
Szomolnokite is a monohydrated ferrous iron sulfate mineral, FeSO4·H2O, where the ferrous iron atoms are in octahedral coordination with four corners shared with SO4 and two with H2O groups. While somewhat rare on Earth, szomolnokite has been detected on the surface of Mars along with several other hydrated sulfates and is suggested to occur near the surface of Venus. Previous measurements have characterized the local environment of the iron atoms in szomolnokite using Mössbauer spectroscopy at a range of temperatures and 1 bar. Our study represents a step towards understanding the electronic environment of iron in szomolnokite under compression at 300 K. Using a hydrostatic helium pressure-transmitting medium, we explored the pressure dependence of iron’s site-specific behavior in a synthetic szomolnokite powdered sample up to 95 GPa with time-domain synchrotron Mössbauer spectroscopy. At 1 bar, the Mössbauer spectrum is well described by two Fe2+-like sites and no ferric iron, consistent with select conventional Mössbauer spectra evaluations. At pressures below 19 GPa, steep gradients in the hyperfine parameters are most likely due to a structural phase transition. At 19 GPa, a fourth site is required to explain the time spectrum with increasing fractions of a low quadrupole splitting site, which could indicate the onset of another transition. Above 19 GPa we present three different models, including those with a high- to low-spin transition, that provide reasonable scenarios of electronic environment changes of the iron in szomolnokite with pressure. We summarize the complex range of Fe2+ spin transition characteristics at high-pressures by comparing szomolnokite with previous studies on ferrous-iron bearing phases.
Highlights
Sulfate minerals have been studied mainly in the context of surface processes such as evaporitic deposits and hydrothermal systems [1] or in the context of mine tailings and wastes [2].Sulfate salts play important roles in the cycling of metals and sulfates in terrestrial systems [3], and are thought to play important roles on the surface weathering processes on other planetary bodies
The shift were determined using time-domain spectroscopy (SMS) spectra were fitted with version 2.2.0 of the CONUSS software [21], which uses a leastsquare algorithm to fit iron’s hyperfine parameters in szomolnokite, as well as material properties
In order to keep the number of fitting parameters at a minimum, the quantitative constraints for szomolnokite
Summary
Sulfate minerals have been studied mainly in the context of surface processes such as evaporitic deposits and hydrothermal systems [1] or in the context of mine tailings and wastes [2]. Sulfate salts play important roles in the cycling of metals and sulfates in terrestrial systems [3], and are thought to play important roles on the surface weathering processes on other planetary bodies. Chou et al [3] performed experiments on the stability of a variety of hydrated and anhydrous sulfates in Martian reaction environments. They suggest that hydrated sulfates could play important roles in the hydrologic cycle of Mars. Barsukov et al [10] suggested that barium and strontium sulfates could possibly be stable in the crust in Venus, it is not clear if they would be formed sub-surface as a result of dehydration processes
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