IRON REDOX IN FIBERS AND FRAGMENTS OF AMPHIBOLES FROM THE FORMER VERMICULITE MINE NEAR LIBBY, MONTANA

Exact mechanisms of asbestos disease causations are still being studied by medical researchers. There is ongoing debate about the differing health effects of asbestiform vs. non-asbestiform amphiboles, which can occur together with no apparent compositional differences. To help address these unresolved issues, we are studying the oxidation state of Fe in well-characterized amphiboles of both habits from the former vermiculite mine near Libby, Montana. The bulk sample of this amphibole is 65% Fe³⁺.

X-ray absorption near-edge spectroscopy was used to measure Fe³⁺ contents in one fragment, one puff-ball (randomly-oriented cryptocrystals), and two fibers. Each sample was mounted on a glass fiber inserted into an individual goniometer head. A polarizing light microscope was used to identify the X, Y, and Z optical orientations of each crystal, and three mutually-perpendicular directions for the puff-ball. The goniometer heads were taken to beamline X26A at the National Synchrotron Light Source, and mounted so that each crystallographic direction could be made to coincide with the incident polarized X-ray beam of the synchrotron. Using mutually-orthogonal Kirkpatrick-Baez silica mirrors to produce a focused beam of monochromatic x-rays <10 µm in diameter, the structure of the Fe K absorption edge was scanned from 7080-8100 eV using a high resolution Si(311) monochromator. Pre-edge peaks were extracted and fit using Lorentzian lineshapes to determine their component peaks.

All spectra of all samples in all orientations were dominated by a prominent peak with a centroid at 7114.33(3) eV; this likely represents Fe³⁺ in all samples. In spectra of the puff-ball and the two fibers, the second-largest peak, was at ~7113.07(10) eV. In the fragment, the latter peak is present at lower energies (7112.30 in X and 7112.62 eV in Y); this shift suggests that the fragment is slightly more reduced than the fibers. This second peak is likely a combination of superimposed Fe²⁺ and Fe³⁺ contributions In two of the fiber samples, we can resolve structure in this composite peak, showing peaks at ~7113.03 eV and 7111.86 eV that might represent contributions from Fe²⁺ and Fe³⁺, respectively. Overall, if the fibers represent the bulk Fe³⁺ at 65% Fe³⁺, then these data suggest that the fragments might be ~50-55% Fe³⁺.