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Open AccessArticle

A Synchrotron Mössbauer Spectroscopy Study of a Hydrated Iron-Sulfate at High Pressures

Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
*
Authors to whom correspondence should be addressed.
Now at Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA.
Now at Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
§
Now at École Normale Supérieure de Lyon, Laboratoire de Géologie, CNRS, UMR 5276, 69342 Lyon, France.
Minerals 2020, 10(2), 146; https://doi.org/10.3390/min10020146
Received: 21 December 2019 / Revised: 4 February 2020 / Accepted: 4 February 2020 / Published: 8 February 2020
(This article belongs to the Special Issue Mineral Physics—In Memory of Orson Anderson)
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. View Full-Text
Keywords: iron-sulfates; hydrous phases; szomolnokite; high-pressure; synchrotron Mössbauer spectroscopy; spin-transition iron-sulfates; hydrous phases; szomolnokite; high-pressure; synchrotron Mössbauer spectroscopy; spin-transition
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Perez, T.; Finkelstein, G.J.; Pardo, O.; Solomatova, N.V.; Jackson, J.M. A Synchrotron Mössbauer Spectroscopy Study of a Hydrated Iron-Sulfate at High Pressures. Minerals 2020, 10, 146.

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