Short-range-ordered Fe(III) minerals such as ferrihydrite (Fh) are ubiquitous in the environment, are key players in biogeochemical cycling, and sorb trace elements and nutrients. As such, it is important to be able to identify the presence of such minerals in natural samples. Fh is commonly observed to be X-ray amorphous and cannot be easily analyzed using X-ray diffraction, meaning that spectroscopic methods such as X-ray absorption or 57Fe Mössbauer spectroscopy (MBS) are necessary for accurate identification and quantification. Despite decades of research into Fh using MBS, there is a discrepancy in the literature about the exact parameters applicable to the mineral when measured at liquid helium temperature. Fh is frequently fitted with either one, two, or three hyperfine sextets with little interpretation applied to the meaning of each, which is problematic as a one sextet model does not account for the asymmetric lineshape frequently observed for Fh. Here, we address inconsistencies in the fitting of Fh and provide a more standardized approach to its identification by MBS. We present a systematic comparison of different fitting methods, notably based on Lorentzian and Voigt functions. We suggest that the most suitable approach to fitting pure Fh at liquid helium temperature is with two sextets (A and B) fitted using an extended Voigt-based function with the ability to apply probability distributions to each hyperfine parameter. 2-line Fh: A (δ = 0.49 mm/s; ε = 0.00 mm/s; Bhf = 50.1 T) and B (δ = 0.42 mm/s; ε = –0.01 mm/s; Bhf = 46.8 T) 6-line Fh: A (δ = 0.50 mm/s; ε = –0.03 mm/s; Bhf = 50.2 T) and B (δ = 0.40 mm/s; ε = –0.05 mm/s; Bhf = 47.1 T). We interpret the two sextets to be due to either differences in the coordination environment of iron, i.e., in tetrahedral or octahedral sites, the presence of a disordered surface phase, or a combination of both. We hope that provoking a discussion on the use of MBS for Fh will help develop a greater understanding of this mineral, and other short-range ordered iron minerals, which are so important in environmental processes.

中文翻译：

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使用穆斯堡尔光谱法对液氦温度下水铁矿的修正分析

水铁矿 (Fh) 等短程有序的 Fe(III) 矿物在环境中无处不在，是生物地球化学循环的关键参与者，可吸附微量元素和营养物质。因此，能够识别天然样品中此类矿物质的存在非常重要。通常观察到 Fh 是 X 射线无定形的，无法使用 X 射线衍射轻松分析，这意味着光谱方法如 X 射线吸收或 57Fe 穆斯堡尔光谱 (MBS) 对于准确识别和量化是必要的。尽管使用 MBS 对 Fh 进行了数十年的研究，但文献中关于在液氦温度下测量时适用于矿物的确切参数存在差异。Fh 经常与一个、两个或三个超精细六重奏相匹配，对每个的含义几乎没有解释，这是有问题的，因为一个六重奏模型不能解释 Fh 经常观察到的不对称线形。在这里，我们解决了 Fh 拟合中的不一致问题，并提供了一种更标准化的方法来通过 MBS 对其进行识别。我们对不同的拟合方法进行了系统比较，特别是基于 Lorentzian 和 Voigt 函数。我们建议在液氦温度下拟合纯 Fh 的最合适方法是使用扩展的基于 Voigt 的函数拟合两个六重奏（A 和 B），该函数能够将概​​率分布应用于每个超精细参数。2 线 Fh：A（δ = 0.49 mm/s；ε = 0.00 mm/s；Bhf = 50.1 T）和 B（δ = 0.42 mm/s；ε = –0.01 mm/s；Bhf = 46.8 T）6 - 线 Fh：A（δ = 0.50 mm/s；ε = –0.03 mm/s；Bhf = 50.2 T）和 B（δ = 0.40 mm/s；ε = –0.05 mm/s；Bhf = 47.1 T）。我们解释这两个六重体是由于铁的配位环境的差异，即在四面体或八面体位点中的差异，无序表面相的存在，或两者的组合。我们希望引发关于将 MBS 用于 Fh 的讨论将有助于更好地了解这种矿物以及其他在环境过程中非常重要的短程有序铁矿物。