Calculated Nominal Ranges of Refractive Indices for the Amphiboles Found in the Rainy Creek Igneous Complex
THE MICROSCOPE
2013, Volume 61:2, pp. 75–84
DOI
https://doi.org/10.59082/LFFD1588
AUTHORS
Matthew S. Sanchez and Mickey E. Gunter
ABSTRACT
The amphiboles occurring in the Rainy Creek Igneous Complex (RCC) near Libby, Montana, range in composition such that the following amphibole species have been identified: tremolite, winchite, richterite, magnesioriebeckite and magnesioarfvedsonite. The composition, optical and crystallographic values of only winchite and richterite have been determined. In an attempt to characterize the nominal (i.e. mean) refractive index for tremolite, magnesioriebeckite and magnesioarfvedsonite species, derivations of Gladstone-Dale constants specific to the RCC were attempted. It was not possible to obtain specific Gladstone-Dale constants using these data for the RCC amphiboles. Therefore, the Gladstone-Dale constants derived by Jaffe (1) for the alkali amphiboles were applied to the published electron probe microanalysis (EPMA) data from Meeker et al. (2) specific to tremolite and magnesioriebeckite, as well as the data from Sanchez et al. (3) for magnesioarfvedsonite. (Note: Dyar and Gunter [4], and references therein provide more information on amphibole nomenclature and correlations of refractive indices to mineral structures.) The range of nominal refractive indices for the tremolite, magnesioriebeckite and magnesioarfvedsonite are calculated as n=1.610 to 1.629, n=1.646 to 1.665 and n=1.641 to 1.652, respectively. The nominal range of refractive indices for the amphiboles occurring within the RCC is much smaller than the ranges for amphiboles under current International Mineralogical Association nomenclature for magnesioarfvedsonite and magnesioriebeckite and their solid solutions. Therefore, caution is required when using the optical properties.