Abstract 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.

中文翻译：

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黄钾铁矾和含水 Fe(II) 之间的铁同位素交换和分馏

摘要 黄钾铁矾是调节酸性硫酸盐环境中酸度和污染物的关键矿物之一，其 Fe 同位素组成可能揭示其随时间的形成、转变和重结晶。对其铁同位素组成的解释需要了解黄钾铁矾与其他含铁矿物和水相物质之间的平衡铁同位素分馏因子。在这里，我们使用三同位素法 (54Fe-56Fe-57Fe) 在酸性条件下探索了黄钾铁矾和 Fe(II)aq 之间的 Fe 同位素交换和分馏。通过使合成黄钾铁矾和天然钠钾铁矾与两种富含 57Fe 的 Fe(II)aq 溶液反应，达到平衡的逆转方法，这些溶液的初始 56Fe/54Fe 比率高于和低于预测的平衡值。没有观察到溶解的 Fe(II) 浓度随时间发生变化，但 Fe(II)aq 的 57Fe/56Fe 比朝着系统质量平衡下降，表明尽管没有净 Fe( II) 吸附（误差范围内）。pH 值和 Fe 同位素交换之间存在负相关关系，Fe 同位素交换随着 pH 值的降低而进行。这可以通过水合氢钾铁矾的溶解和钠钾铁矾的再沉淀、耦合的 H3O+ - Na+ 交换或黄钾铁矾分解来解释，尽管从 XRD 中没有发现 Fe-羟基氧化物相。与 Fe(II)aq 交换的黄钾铁矾中 Fe 原子量的计算表明黄钾铁矾再结晶仅限于几个百分比。当 Fe(II)aq 的初始 δ56Fe 值大于假定的平衡值时，其同位素值随时间显着下降，而 Fe(II)aq 的 δ56Fe 值在其初始值低于疑似平衡成分时随时间增加。在每种情况下，Fe(II)aq 的同位素组成接近相似的值，提供了达到平衡的高度置信度。Fe(II)aq-黄钾铁矾和 Fe(II)aq-钠黄钾铁矾平衡分馏因子在 22 °C 的计算分别为 -2.26‰ (±0.27‰, 2σ) 和 -2.19‰ (±0.18‰, 2σ)。这表明在 Fe(II) 存在下黄钾铁矾重结晶期间，随着较轻的同位素被分馏成 Fe(II)，黄钾铁矾预计会变得同位素更重。这些值不同于从 NRIXS 光谱和分子建模得出的估计分馏因子。实验和理论之间的差异可能反映了表面交换，这在我们的研究中很可能与预测的矿物整体热力学性质相比。