Browsing by Author "Whitworth, AJ"

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    Iron isotope exchange and fractionation between jarosite and aqueous Fe(II)
    (Elsevier, 2020-11-05) Whitworth, AJ; Brand, HEA; Frierdich, AJ
    Jarosite 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.
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    Iron isotope geochemistry and mineralogy of jarosite in sulfur-rich sediments
    (Elsevier, 2020-02-01) Whitworth, AJ; Brand, HEA; Wilson, SA; Frierdich, AJ
    Jarosite is a common mineral in acidic, sulfate-rich environments where it is critical in regulating the acidity of aquatic systems and the mobility of trace elements and potential contaminants. This research aims to understand jarosite formation and recrystallization in these environments by examining the stable iron isotope geochemistry of jarosite at two coastal sites in Victoria, Australia: Fossil Beach and Southside Beach. Jarosite occurs at high abundance as beds, veins, surface coatings and nodules within oxidized zones of sulfidic sediment outcrops and as pebbles, cobbles, and boulders at the base of the outcrops within the intertidal zone, making these two beaches ideal natural laboratories. Synchrotron powder X-ray diffraction (XRD) and ICP-MS results indicate that samples are comprised predominantly of natrojarosite, often with substantial K substitution. Rietveld refinement of XRD patterns shows that most jarosite samples are a solid-solution of Na-K jarosite, differing from previous observations that (near-)end-member mixing predominantly occurs in nature. The iron isotope composition of the jarosite samples have δ56Fe values between −1.91 and +1.24‰ (relative to IRMM-014), an exceptionally large range that partially overlaps with the δ56Fe values of the sulfidic sediment precursor (−0.54 to +1.30‰). There is a negative relationship between the alkali ratio [Na/(Na + K)] and iron isotope composition, with the heavier iron isotopes preferentially partitioned into K-rich jarosite. The large range in δ56Fe values of jarosite likely results from a combination of the variable δ56Fe values of the precursor sulfides, thermodynamic differences between Na- versus K-bearing jarosite, and an open-system Rayleigh distillation during jarosite formation. © 2020 Elsevier Ltd.
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    Meteorites on the Nullarbor Plain, insights from synchrotron powder diffraction
    (Universities Space Research Association, 2019-03-19) Brand, HEA; Langendam, AD; Whitworth, AJ; Alkemade, SL; Mitchell, JT; Davis, A; Stephen, NR; Tomkins, AG
    Introduction: The Australian deserts are an excellent place to search for meteorites, the dry warm climate limits changes on the surface allowing meteorites to remain in place for hundreds, if not thousands of years. Additionally, the Nullarbor plain – one of the largest limestone karst systems in the world provides an additional benefit in colour, the light limestone contrasting the black meteorites well. Over the past decade a group from Monash have been searching for these meteorites and with moderate success have collected over 200 new meteorites. This represents approximately 1/5 of Australia’s meteorite collection. Although the Nullarbor provides a fairly stable environment, there are still variations in the weathering of these meteorites and it is important to establish if this is just a result of time on the surface or if there is also a location and local environment factors. While these meteorites have been studied using optical and SEM techniques, synchrotron XRD (SXRD), represents a fast way to gain detailed bulk mineralogy of these samples to complement and add to the existing data. It can also be combined with geo-spatial data associated with the samples to model and determine weathering patterns for the meteorites on the Nullarbor. To this end we plan to study a wide selection of Australian Meteorites of various classes using SXRD to determine the phases present, with particular sensitivity to minor phases, both original and weathered mineral phases. Fig. 1. Context image to show meteorite collection sites. The meteorites described here are a mixture of officially described meteorites and new, as yet, unclassified meteorites from the Nullarbor as well as having a range of compositions. The samples were chosen as being a large enough sample, or multiple fragments, so that a representative sample (~0.5g) could be crushed while leaving enough for other analyses. Figure 1 shows the collection area for these meteorites. The meteorites which are not officially classified have been named for the date they were found (DDMMYY) and then alphabetically for the order they were found that day.
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    Nickel exchange between aqueous Ni(II) and deep-sea ferromanganese nodules and crusts
    (Elsevier, 2019-12-05) Hens, T; Brugger, J; Etschmann, BE; Paterson, DJ; Brand, HEA; Whitworth, AJ; Frierdich, AJ
    Deep-sea ferromanganese (Fe-Mn) nodules and crusts are rich in traditional and non-traditional metals with both current and emerging economic value. Mn(III,IV) oxides (e.g., phyllomanganates) are important host phases for these metals (e.g., Ni), which are structurally incorporated during nodule and Fe-Mn crust formation. Recrystallization of phyllomanganates can be catalyzed by aqueous Mn(II) (Mn(II)aq) during (bio)geochemical Mn redox cycling. The fate of structurally incorporated metals during such recrystallization of Mn(III,IV) oxides remains, however, poorly constrained. Here, we use a 62Ni isotope tracer to determine the exchangeability of dissolved Ni with structurally incorporated Ni in two deep-sea Fe-Mn nodules and one Fe-Mn crust. Ni exchange between solid and solution was investigated during reactions in 1 mM Mn(II)aq and in Mn(II)-free solutions under variable pH conditions (pH 5.5 and 7.5) over time. Sample characterization shows that all samples are of hydrogenetic or mixed hydrogenetic-diagenetic origin and Ni is preferentially associated with the phyllomanganates. Our Ni exchange experiments reveal that in some samples up to 25% of incorporated Ni is exchangeable with the fluid after 14 days. The prevalent reaction pathways exhibit pH-dependent behavior during phyllomanganate recrystallization and differ between sample types, with Mn(II)aq enhancing Ni exchange in the Fe-Mn crust-fluid system and Ni exchange being independent of Mn(II)aq concentrations in the Fe-Mn nodule-fluid systems. The exchangeability of structurally-incorporated Ni in Fe-Mn nodules and crusts indicates a labile behavior that potentially makes it available for biogeochemical processes in the marine environment. © 2019 Elsevier B.V