T118 Petrologic Mineralogy—The Study of Minerals in Context (Posters): In Honor of Charles V. Guidotti

CHARACTERIZATION OF HISTORICAL AMPHIBOLE SAMPLES FROM THE FORMER VERMICULITE MINE NEAR LIBBY, MONTANA U.S.A

Paper number: T118-6
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

A suite of amphiboles collected from the Libby vermiculite deposit at different time periods were assembled: the 1920's - obtained from the Harvard and Smithsonian museums; 1960's - obtained from the Museum at Montana Tech, and 1999 - collected by M.E. Gunter. Goals of the project were to: 1) characterize these amphiboles and compare them with past work; 2) to determine if the chemical variation of the amphiboles had changed through time; and 3) to ascertain if correlations exist between amphibole morphology and chemistry. Amphibole morphology ranges from perfect single crystals with distinct (110) and (100) faces on the order of 50 to 200 microns, to fine grained, massive amphiboles on the order of a few microns in size. Chemical compositions were determined by electron microprobe using wavelength dispersive spectrometry on polished grain mounts and thin sections; the ratio of ferrous to ferric iron was determined using Mössbauer spectroscopy. Correlations of chemistry to morphology (i.e., asbestiform/non-asbestiform) were investigated using multivariant statistical techniques.

Compositions ranging from sodic, calcic-sodic, and calcic group members all were found to be aluminum poor with magnesium # greater than 0.92. Using current amphibole nomenclature, the majority of species identified by chemistry were richterite and winchite, with minor magnesio-arfvedsonite and tremolite. These species determinations and distributions agree with earlier works. Each of the three time periods of samples have a somewhat unique elemental distribution, and individual samples show less variation than that of the entire suite. Based on major and minor elemental compositions, there is no discernible correlation between morphology and composition; however, the variation in chemistry of the fine-grained, massive amphiboles appeared greater compared to the single crystals. Current work is under way using laser ablation mass spectrometry, to obtain trace element compositions, and Mössbauer spectroscopy on range of morphologies to attempt to further correlate the trace elemental composition and the oxidation state of iron to morphology.