Composition, Fe3+/ΣFe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, U.S.A.

Mickey E. Gunter 1 , M. Darby Dyar 2 , Brendan Twamley 3 , Franklin F. Foit Jr. 4  and Scott Cornelius 4
  • 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.

Abstract

Compositional data and Fe3+/ΣFe 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+/ΣFe 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+/ΣFe 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 (K0.19 Na0.32)A(Na0.85 Ca1.12Mn0.03)B(Mn0.01Mg4.43Fe3+ 0.34Fe2+0.19Ti0.01Al0.02)(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, b = 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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