Maghemite in Brazilian Iron Ores: Quantification of the Magnetite-Maghemite Isomorphic Series by Χ-ray Diffraction and the Rietveld Method, and Confirmation by Independent Methods

Renata Hiraga,Reiner Neumann,Otávio Gomes

doi:10.3390/min11040346

Renata Hiraga, Reiner Neumann + Show 1 more

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https://doi.org/10.3390/min11040346

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Abstract

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.

Highlights

The conversion of magnetite to hematite or goethite may pass through the metastable isostructural phase maghemite [3,4], which generally leads to the coexistence with a product called nonstoichiometric magnetite [5] consisting of a solid solution with varying atomic ratios of Fe2+ :Fe3+
At a first stage of this study, six samples that were supposed to bear maghemite were analyzed by X-ray diffraction (XRD) and the Rietveld method
Based on the results obtained for each sample, some were analyzed by optical microscopy aiming to find a sample where maghemite could be viewed and to confirm its identification by

Summary

Introduction

Iron ore concentrates and pellets are a major export product of Brazil. Its mineralogy is generally simple, the main iron-bearing minerals being hematite and magnetite in the Carajás (state of Pará, northern Brazil) ore, and hematite with goethite in very variable amounts and rare magnetite in the ones from the Quadrilátero Ferrífero (state of Minas Gerais, southeast of Brazil), as well as diverse gangue minerals [1,2]. The conversion of magnetite to hematite or goethite may pass through the metastable isostructural phase maghemite [3,4], which generally leads to the coexistence with a product called nonstoichiometric magnetite [5] consisting of a solid solution with varying atomic ratios of Fe2+ :Fe3+.

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