Methods to analyze metastable and microparticulate hydrated and hydrous iron sulfate minerals

Abstract

We evaluate analytical methods for characterizing hydrated and hydrous iron sulfate minerals (HHIS) that are typically metastable in air or vacuum, commonly form micrometer-sized particles, and contain multi-valent and light elements (Fe²⁺, Fe³⁺, OH⁻, and H₂O) that may be challenging to quantify. We synthesized or obtained HHIS—szomolnokite, melanterite, rhomboclase, schwertmannite, ferricopiapite, paracoquimbite, and jarosite—as well as Fe-oxides. These nominally pure samples were characterized with X-ray diffraction (XRD), and then used to evaluate bulk analyses obtained from combined inductively coupled plasma, optical emission spectroscopy (ICP-OES), ion chromatography (IC), Mössbauer spectroscopy, and mass spectrometry. Integrated bulk analyses showed excellent agreement with the nominal formulas for the minerals. Because HHIS commonly form micro-sized particles—for example, HHIS found in acid mine drainage (AMD) environments and in martian meteorites—it is necessary to develop micro-analytical techniques. Microscopic mid-infrared spectroscopy allows the analyst to successfully discriminate among HHIS with minimal sample preparation on the small scale (~40 × 40 μm). For chemical analysis, electron probe microanalysis (EPMA) is preferred for samples that can be mounted, polished, coated, and that are stable under high vacuum; however, few HHIS meet those criteria. To characterize HHIS compositions, we show that multiple low-vacuum scanning electron microscopy (SEM) analyses of the same uncoated, unpolished mineral are required. Analysis of each mineral shows linear trends on ternary diagrams of 5×Fe-SO₄-O (where oxygen is in O, OH, and H₂O) that may be used to narrow down the HHIS mineralogy. Low-vacuum SEM also provides invaluable information about the geochemical and textural context of the samples. Our study provides protocols for microanalysis of these challenging, fine-grained, and metastable HHIS that may also be applied to other mineral groups.