To determine if the composition or morphology of amphiboles occurring in the Rainy Creek vermiculite deposit near Libby, Montana, has changed as the deposit has been mined over the last 70 years, we assembled amphiboles collected in the ~1920s, ~1960s, and 1999. Amphibole morphologies range from ~20–50 μm prismatic single crystals to massive aggregates of fine-grained, asbestiform amphiboles only a few micrometers in size. Compositional data obtained by electron-microprobe analysis (EPMA) using wavelength-dispersive spectrometry and Fe3+/∑Fe ratio determined by Mössbauer spectroscopy showed that calcic, sodic–calcic, and sodic group amphiboles are present in each suite. The samples all have Mg numbers > 0.90. According to the current nomenclature for amphibole species, the majority of amphiboles analyzed are winchite or richterite. There are a few exceptions: one of the 1960 samples was tremolite, while one of the ~1920s samples was fluoro-magnesio-arfvedsonite; the latter was characterized optically. Compositions generally concur with previous characterizations of the amphiboles originating from the Rainy Creek Complex, but composition variations occur both within each temporally-related sample suite and among suites. Therefore, suggestions that the bulk composition of amphiboles in the deposit has changed over the 70 years of mining are unsupported. Overall, there is no correlation between major and minor element composition and morphology. However, fibrous and prismatic crystal trace element concentrations from a sample from the 1920s suite were analyzed using laser ablation mass spectrometry. Based on the trace elements the separate morphologies could be distinguished by cluster analysis, but only within this single sample.

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