Scientists in the mid-1800s identified the natural occurrence of exceptionally Fe-deficient and hydrous varieties of hematite that they named “hydrohematite” or “turgite”. However, these mineral names were discredited in the 1920s based on an interpretation that they are mixtures of hematite and goethite. We have investigated five natural specimens that were labeled as “turgite” in the historic Genth Collection at Penn State University. To determine their water contents and Fe concentrations, we have analyzed five natural hydrohematite samples using electron probe microanalysis (EPMA), synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis mass spectrometry (TGA-MS). Hydrohematite is differentiated from hematite by the substitution of 3H+ for Fe3+, and it can be represented by a general chemical formula of Fe1-x/6Hx/2O1.5, with x=0 in stoichiometric hematite. Our EPMA study revealed that the iron occupancy (1-x/6) in the five pure hydrohematite specimens ranged from 0.80-0.85. Rietveld analyses of our synchrotron XRD data further confirmed Fe concentrations of 80-86% relative to hematite for all measured samples. Moreover, the unit-cell volumes for natural hydrohematite were 0.4-1.0% smaller than that of stoichiometric hematite (Blake et al. 1966). The presence of hydroxyl groups in hydrohematite was supported by broad peaks at 3200 and 900 cm-1 in FTIR spectroscopy. TGA-MS analysis of hydrohematite from Salisbury, CT revealed 5.2 wt% total H2O, consistent with a composition halfway between FeOOH and Fe2O3. TEM of an Arkansas hydrohematite specimen showed that crystallite sizes range from 5-10 nm, and they aggregate to form fibers elongate along [101]*. The fiber dimensions were up to 1 mm in length and up to 0.2 mm in width. We therefore suggest that hydrohematite can tolerate high Fe deficiencies and exist metastably for millions of years.

Characterization of a highly Fe-deficient and hydrous natural hydrohematite

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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