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

Abstract

Compositional data and Fe 3 + /ZFe ratios obtained by electron microprobe and Mossbauer 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 + /ZFe 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. Mossbauer spectroscopy yielded Fe 3 + /ZFe 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 Mossbauer spectroscopy) is (K 0 . 1 9 Na 0 . 3 2 ) A (Na 0 . 8 5 Ca 1 . 1 2 Mn 0 . 0 3 )B(Mn 0 . 0 1 Mg 4 . 4 3 Fe 3 + 0.34Fe 2 + 0.19Ti 0 . 0 1 Al 0 . 0 2 )(Al 0 . 0 3 Si 7 . 9 7 O 2 2 )(OH 1 . 6 3 F 0 . 3 7 ). The refinement was carried out based on space group C2/m, with a = 9.879(2), b = 18.024(3), c = 5.288(1) A, β = 104.377(3)° and using data collected at room temperature. Mg is partitioned among the Ml, 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.