Our time-resolved synchrotron X-ray diffraction of the crystallization of hematite (Fe2O3) from Fe chloride solutions heated between 150 and 200 oC has revealed that the precipitation of precursor akaganeite (β-FeOOH) is followed by the crystallization of hematite-like phases that are highly Fe deficient (Peterson et al., in press). Rietveld analyses of this incipient hematite yielded a refined occupancy for Fe of ~0.75, and the fraction of Fe gradually increased to unity with continued heating. We infer that Fe vacancies in the defective hematite nanocrystals were charge balanced by H+, indicating a composition of FeOOH for the first-formed hematite-like crystals. Are natural occurrences of hematite equally hydrous due to Fe defects? Breithaupt (1847) introduced the term “hydrohematite” for an Fe hydroxide from Bavaria that was physically similar to hematite but with a water content half that of goethite. Brush and Rodman (1867) described “hydrohematite” specimens from several localities in the eastern United States. However, “hydrohematite” was discredited by X-ray diffraction and thermogravimetric studies in the 1920s. We obtained pieces of Breithaupt’s “hydrohematite” from the TU Bergakademie Freiberg as well as specimens from localities identified by Brush and Rodman (1867) in the U.S. National Museum mineral collection, and we analyzed them by synchrotron X-ray diffraction and Fourier-Transform infrared (FTIR) spectroscopy. Our Rietveld refinements confirmed the high Fe vacancies in these samples, with refined Fe occupancies ranging from 0.84(1) to 0.89(1). Infrared absorbance spectra revealed distinct modes at ~800 and ~900 cm-1, which correspond closely to OH bending modes observed in goethite. We conclude that natural “hematite” can contain high concentrations of Fe vacancies due to the incorporation of water in amounts that approach those found in goethite and akaganeite.

On the join between goethite and hematite: X-ray diffraction and FT-IR studies of “hydrohematite”

249th National Meeting (2015)

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