Differences in Fe-redox for asbestiform and nonasbestiform amphiboles from the former vermiculite mine, near Libby, Montana, U.S.A.

Abstract

We obtained oriented Fe-XANES spectra on three amphibole bundles of fiber (i.e., asbestiform morphology), two single-crystal fragments (i.e., nonasbestiform morphology), and one aggregate of thousands of smaller particles (termed herein a "puffball") by use of a spindle stage mounted on beamline X26A, National Synchrotron Light Source, Brookhaven National Lab. These particles of differing morphology were obtained from a single rock sample collected at the former vermiculite mine near Libby, Montana, U.S.A. The XANES spectra vary as a function of orientation for the single-crystal fragments and bundles of fibers and were the same regardless of orientation for the puffball. Differences in the spectra for the single-crystal fragments and bundles of fibers indicate variations in Fe oxidation states between the two differing morphologies. Comparison to a calibration line based on amphibole standards allowed determination of Fe3+/ςFe values of 0.69 for the bundles of fibers and 0.60 for the single-crystal fragments, while the puffball had a Fe3+/ςFe value of 0.66. These values agree well with our earlier Mössbauer data on a bulk sample of this same material, with Fe 3+/ςFe of 0.65. To our knowledge, these are the first data that show compositional differences between asbestiform and nonasbestiform amphiboles from the same sample. Because health effects appear to vary with amphibole morphology, these results suggest that iron oxidation, mineral morphology, and potential health effects may all be interrelated.