HYDRATED AND HYDROUS IRON SULFATE SYNTHESIS AND ANALYSIS ON THE BULK AND MICRO-SCALES

Hydrated and hydrous iron sulfates (HHIS) are common in acid rock drainage settings and they are sensitive indicators of the environment (e.g., pH, relative humidity, temperature and PO₂). However, HHIS are metastable minerals that are commonly found in mixtures of numerous sulfates; thus it is necessary, but difficult, to analyze them on the micro-scale.

We have synthesized or obtained HHIS – szomolnokite, melanterite, rhomboclase, schwertmannite, ferricopiapite, paracoquimbite and jarosite – and characterized them by x-ray diffraction (XRD) analysis. New methods were developed for synthesis of ferricopiapite and paracoquimbite. Bulk analytical and micro-analytical techniques were evaluated to characterize these nominally pure samples, as well as iron oxide and iron carbonate samples. Inductively coupled plasma optical emission spectroscopy (ICP-OES), ion chromatography (IC) and mass spectrometry methods were used to achieve an integrated bulk analysis of each sulfate, including the light elements. Mössbauer spectra were obtained to determine Fe³⁺/Fe²⁺. Overall, the bulk results agree well with ideal compositions for the minerals.

Electron probe microanalysis (EPMA) and low vacuum (~30 Pa) scanning electron microscope (SEM) techniques were used to characterize the samples at the micro-scale. For micro-analysis, EPMA is preferred for samples that may be mounted, polished and coated, and are stable under high vacuum. To successfully characterize the compositions of HHIS that are metastable in vacuum or difficult to mount and coat, it is necessary to use multiple SEM analyses of the same uncoated, unpolished mineral. Our analyses of each mineral show linear trends on ternary diagrams of 5*Fe-SO₄-O (where O is in O, OH and H₂O) and these trends allow for the determination of compositions of candidate minerals, but do not allow direct determination of mineralogy. Mineralogy may be determined by other small-scale spectroscopic or micro-XRD methods and we found that micro-infrared (IR) spectroscopy (mid-IR to near-IR) may be used to discriminate between some HHIS. However, low vacuum SEM provides invaluable information about the geochemical and textural context of the samples, while narrowing down the mineralogy.