Composition, Fe 3+ /∑Fe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, U.S.A.

Compositional data and Fe 3+ /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 Fe 3+ /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 Fe 3+ /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 (K 0.19 Na 0.32 ) A (Na 0.85 Ca 1.12 Mn 0.03 ) B (Mn 0.01 Mg 4.43 Fe 3+ 0.34 Fe 2+ 0.19 Ti 0.01 Al 0.02 )(Al 0.03 Si 7.97 O 22 )(OH 1.63 F 0.37 ). 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 Fe 3+ occupies M2, while Fe 2+ 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.