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.


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.

Purchase article
Get instant unlimited access to the article.
Log in
Already have access? Please log in.

Log in with your institution

Journal + Issues