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Fe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, U.S.A.
1 Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844–3022, U.S.A.
2 Department of Earth and Environment, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
3 University Research Office, University of Idaho, Moscow, Idaho 83844–3010, U.S.A.
4 Department of Geology, Washington State University, Pullman, Washington 99164–2812, U.S.A.
Correspondence: * E-mail: mgunter{at}uidaho.edu
Compositional data and Fe3+/
Fe ratios obtained by electron microprobe and Mössbauer analyses are given for a suite of three amphibole and amphibole-asbestos samples collected from the former vermiculite mine near Libby, Montana. A crystal structure analysis, compositional data, and Fe3+/Fe values for two samples from a previous study are also reported. The results confirm the conclusion drawn in the previous study that these amphiboles are dominantly compositions ranging from winchite to richterite. Mössbauer spectroscopy yielded Fe3+/Fe ratios from 58% to 72% for the five samples.
The crystal structure was determined for a single crystal selected from a bulk sample. Its formula (as determined by electron microprobe analysis and Mössbauer spectroscopy) is (K0.19Na0.32)A(Na0.85Ca1.12Mn0.03)B(Mn0.01Mg4.43Fe0.343+Fe0.192+Ti0.01Al0.02)(Al0.03Si7.97O22)(OH1.63F0.37). The refinement was carried out based on space group C2/m, with a = 9.879(2), b = 18.024(3), c = 5.288(1) Å, ß = 104.377(3)° and using data collected at room temperature. Mg is partitioned among the M1, M2, and M3 sites. All of the Fe3+ occupies M2, while Fe2+ is split between M2 and M3; Ca and Na fill the M4 site, while Na and K occupy the partially filled A site. The A-site occupancy is calculated as 0.51 based on chemical data, but only 0.48 based on X-ray diffraction results. Minerals with the former values would be classified as richterite and those with the latter as winchite.
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