Micro‑ to nano‑scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

International audienceThe pseudomorphic transformation of magnetite into hematite (martitization) is widespread in geological environments, but the process and mechanism of this transformation is still not fully understood. Micro- and nanoscale techniques--scanning electron microscopy, focused ion bean transmission electron microscopy, and Raman spectroscopy--were used in combination with X-ray diffraction, Curie balance and magnetic hysteresis analyses, as well as Mössbauer spectroscopy on martite samples from a banded iron formation (2.9 Ga, Dharwar Craton, India), and from lateritic soils, which have developed on siliciclastic and volcanic rocks previously affected by metamorphic fluids (Minas Gerais, Brazil). Octahedral crystals from both samples are composed of hematite with minor patches of magnetite, but show different structures. The Indian crystals show trellis of subhedral magnetite hosting maghemite in sharp contact with interstitial hematite crystals, which suggests exsolution along parting planes