Iron isotope exchange and fractionation between jarosite and aqueous Fe(II)

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dc.contributor.authorWhitworth, AJen_AU
dc.contributor.authorBrand, HEAen_AU
dc.contributor.authorFrierdich, AJen_AU
dc.date.accessioned2021-10-29T03:47:16Zen_AU
dc.date.available2021-10-29T03:47:16Zen_AU
dc.date.issued2020-11-05en_AU
dc.date.statistics2021-10-25en_AU
dc.description.abstractJarosite is one of the critical minerals that regulates acidity and contaminants in acid-sulfate environments and its Fe isotope composition may shed light on its formation, transformation and recrystallization over time. Interpretation of its Fe isotope composition requires understanding the equilibrium Fe isotope fractionation factor between jarosite and other Fe-bearing minerals and aqueous species. Here we explore Fe isotope exchange and fractionation between jarosite and Fe(II)aq under acidic conditions using the three-isotope method (54Fe-56Fe-57Fe). A reversal-approach to equilibrium was applied by reacting synthetic jarosite and natural natrojarosite with two 57Fe-enriched Fe(II)aq solutions that had initial 56Fe/54Fe ratios above and below the predicted equilibrium value. No change in dissolved Fe(II) concentrations were observed with time but the 57Fe/56Fe ratio of Fe(II)aq decreased towards the system mass balance, suggesting a high degree of equilibration of the fluid with the solid phase despite no net Fe(II) sorption (within error). There is a negative relationship between pH and Fe isotope exchange, with Fe isotope exchange proceeding as pH decreases. This may be explained by dissolution of hydronium jarosite and reprecipitation of natrojarosite, coupled H3O+ - Na+ exchange, or jarosite decomposition, although no Fe-oxyhydroxide phases were identified from XRD. Calculation of the amount of Fe atoms in jarosite that exchanged with Fe(II)aq indicates that jarosite recrystallization was limited to a few percent. When the initial δ56Fe value of Fe(II)aq was greater than the presumed equilibrium value its isotopic value substantially decreased with time whereas the δ56Fe values of Fe(II)aq increased with time when it had an initial value below the suspected equilibrium composition. In each case, the isotopic composition of Fe(II)aq approached similar values, providing a high degree of confidence of an attainment of equilibrium. Calculation of the Fe(II)aq–jarosite and Fe(II)aq-natrojarosite equilibrium fractionation factors at 22 °C were −2.26‰ (±0.27‰, 2σ) and −2.19‰ (±0.18‰, 2σ), respectively. This indicates that during jarosite recrystallization in the presence of Fe(II), jarosite is expected to become isotopically heavier as lighter isotopes are fractionated into Fe(II). These values differ from the estimated fractionation factors derived from NRIXS spectroscopy and molecular modeling. The differences between experiments and theory may reflect surface exchange, which was likely in our study, versus predicted bulk thermodynamic properties of the mineral. © 2020 Elsevier B.V.en_AU
dc.identifier.articlenumber119802en_AU
dc.identifier.citationWhitworth, A. J., Brand, H. E. A., & Frierdich, A. J. (2020). Iron isotope exchange and fractionation between jarosite and aqueous Fe (II). Chemical Geology, 554, 119802. doi:10.1016/j.chemgeo.2020.119802en_AU
dc.identifier.issn0009-2541en_AU
dc.identifier.journaltitleChemical Geologyen_AU
dc.identifier.urihttps://doi.org/10.1016/j.chemgeo.2020.119802en_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/12173en_AU
dc.identifier.volume554en_AU
dc.language.isoenen_AU
dc.publisherElsevieren_AU
dc.subjectIron isotopesen_AU
dc.subjectRecrystallizationen_AU
dc.subjectFractionationen_AU
dc.subjectIon exchangeen_AU
dc.subjectMetallographyen_AU
dc.subjectMetallurgyen_AU
dc.subjectSulfate mineralsen_AU
dc.subjectThermodynamic propertiesen_AU
dc.titleIron isotope exchange and fractionation between jarosite and aqueous Fe(II)en_AU
dc.typeJournal Articleen_AU
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