A REVIEW: MINERALOGY AND DIMENSIONAL CHARACTERISTICS OF AMPHIBOLES FROM THE VERMICULITE DEPOSIT, RAINY CREEK COMPLEX, LIBBY, MONTANA

Inhalation of amphibole associated with vermiculite mined at Libby Montana has resulted in excess pulmonary fibrosis, lung/respiratory cancer and mesothelioma in exposed populations. Exposures to workers have been reported in excess of 100 f/cc (before 1965); dust suppression processes dropped exposure to < 1 f/cc by mid 1980. Amphibole is highly variable in both composition and morphology. Its abundance is reported at greater than 20% in the ore, 2-7% in the concentrate and < 1% in the expanded product. Amphibole compositions consistent with winchite, richterite, tremolite, magnesio-riebeckite, edenite, and magnesio-arfvedsonite have been reported, although winchite makes up the majority of the material (84%), with richterite (11%) and tremolite (6%) compose almost all the remaining. Particle measurements from samples of mine products and exfoliated material demonstrate that amphibole in the ore and exfoliated products are essentially identical and they are characterized by lower mean aspect ratios, smaller mean widths, shorter lengths, and larger variances in width than populations of commercial asbestos. Width distributions from bulk and airborne particles are tri-modal, with modes abundances established by TEM that center around 0.2, 0.4, and 0.8µm, supporting optical studies that describe a highly variable morphology including populations of cleavage fragments and asbestos fibers. No data are available on compositional variability with dimensions, although data from other locations suggest such correlations might be expected. Comparison of Libby populations to airborne amphibole particles from vermiculite operations at Enoree, SC, where excess asbestos-related diseases have not been reported, and to commercial asbestos may be helpful in delineating those properties most likely to be correlated to human disease. Libby amphibole particles are longer, narrower and have a higher aspect ratio than amphiboles found at Enoree, SC, but are wider with lower aspect ratios than grunerite-asbestos (amosite) and riebeckite-asbestos (crocidolite). Frequencies of length, width and aspect ratio and regression of log width on log length with associated standard error of the estimate illustrate the population characteristics and facilitate comparison among these samples.