The use of operationally-defined sequential Fe extraction methods for mineralogical applications: A cautionary tale from M?ssbauer spectroscopy

摘要

Reactive iron minerals are crucial components of global nutrient cycles, directly controlling carbon transport and storage in marine sediments. Sequential selective extraction is frequently used for quantitatively characterising, and chemically isolating, individual Fe mineral phases. Reagent-specific mineral solubility is fundamental to the success of any sequential extraction, but is strongly affected by the varying physical and chemical morphology intrinsic to natural mineral samples. Natural sediment, rock, and soil samples often contain a mineral mixture, which further modifies solvent efficacy. ~(57)Fe M?ssbauer spectroscopy only probes the hyperfine interactions between next-nearest neighbouring atomic nuclei in the crystal lattice and is less affected by variation in mineral grain size and crystallinity than conventional, X-ray-based methods. In this study, we used M?ssbauer spectroscopy in a novel context to cross-calibrate and optimise a popular, but frequently misused, sequential Fe extraction protocol. Our results showed that incomplete and premature removal of the target Fe minerals could occur at nearly every stage of the extraction and, in many cases, the leachate Fe content did not represent the target phase at all. Crystalline, natural siderite and amorphous, synthetic goethite were detected in the M?ssbauer spectrum of the ammonium oxalate extraction for magnetite, after which all reactive Fe minerals should have been removed. Consistent with previous studies, and unlike many other clay minerals, nontronite was extracted as part of the highly reactive Fe pool, and in fact our data indicate that this mineral was extracted by the initial Na-acetate extraction that targets ‘carbonate-bound Fe’. Matrix effects appeared to cause variable yield efficiencies: synthetic goethite was successfully removed when present as an individual mineral yet persisted beyond its target extraction when present in an Fe mineral mixture. Although suitable for the quantif