Quantitative phase analyses of carbonate rocks containing Mg-rich calcite and non-stoichiometric dolomite by the Rietveld method yielded improved results when the substitutions are refined for either minerals. The refinement is constrained by the c-axis of the lattice for both minerals using the formula c = −1.8603 nMg + 17.061 for calcite, where nMg is the molar fraction of Mg replacing Ca, and c = 16.0032 + 0.8632Δn_Ca for dolomite, with Δn_Ca being the excess Ca in its B site. The one-step procedure was implemented into the Topas software and tested on twenty-two carbonate rock samples from diverse geological settings, considered analogues to petroleum system lithotypes of the pre-evaporite deposits of Southeastern Brazil. The case study spans over a wide range of calcite and dolomite compositions: up to 0.287 apfu Mg in magnesian calcite, and Ca in excess of up to 0.25 apfu in non-stoichiometric dolomite, which are maximum substitutions the formulas support. The method overcomes the limitations for the quantification of minerals by stoichiometry based on whole-rock chemical analysis for complex mineralogy and can be employed for multiple generations of either carbonate. It returns the mineral quantification with unprecedented detailing of the carbonates’ composition, which compares very well to spot analysis (both SEM-EDS and EMPA) if those cover the full range of compositions. The conciliation of the quantification results based on the XRD is also excellent against chemical analysis, thermogravimetry, and carbon elemental analysis.

Maghemite (γ-Fe₂O₃) is a mineral formed from magnetite oxidation at low temperatures, an intermediate metastable term of the magnetite to hematite oxidation and could be mixed with both. It has magnetic susceptibility similar to magnetite, crystal structure close to magnetite with which it forms a solid solution, while compositionally it equals hematite. Maghemite is thus easily misidentified as magnetite by Χ-ray diffraction and/or as hematite by spot chemical analysis in iron ore characterization routines. Nonstoichiometric magnetite could be quantified in samples of Brazilian soils and iron ores by the Rietveld method using a constrained refinement of the Χ-ray patterns. The results were confirmed by reflected light microscopy and Raman spectroscopy, thus qualitatively validating the method. Χ-ray diffraction with the refinement of the isomorphic substitution of Fe²⁺ by Fe³⁺ along the magnetite-maghemite solid solution could help to suitably characterize maghemite in iron ores, allowing for the evaluation of its ultimate influence on mineral processing, as its effect on surface and breakage properties.

On 5 November 2015, a large tailing deposit failed in Brazil, releasing an estimated 32.6 to 62 million m³ of iron mining tailings into the environment. Tailings from the Fundão Dam flowed down through the Gualaxo do Norte and Carmo riverbeds and floodplains and reached the Doce River. Since then, bottom sediments have become enriched in Fe(III) oxyhydroxides. Dissimilatory iron-reducing microorganisms (DIRMs) are anaerobes able to couple organic matter oxidation to Fe(III) reduction, producing CO₂ and Fe(II), which can precipitate as magnetite (FeO·Fe₂O₃) and other Fe(II) minerals. In this work, we investigated the presence of DIRMs in affected and non-affected bottom sediments of the Gualaxo do Norte and Doce Rivers. The increase in Fe(II) concentrations in culture media over time indicated the presence of Fe(III)-reducing microorganisms in all sediments tested, which could reduce Fe(III) from both tailings and amorphous ferric oxyhydroxide. Half of our enrichment cultures converted amorphous Fe(III) oxyhydroxide into magnetite, which was characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. The conversion of solid Fe(III) phases to soluble Fe(II) and/or magnetite is characteristic of DIRM cultures. The presence of DIRMs in the sediments of the Doce River and tributaries points to the possibility of reductive dissolution of goethite (α-FeOOH) and/or hematite (α-Fe₂O₃) from sediments, along with the consumption of organics, release of trace elements, and impairment of water quality.