THINGS ONCE KNOWN NOW FORGOTTEN? THE USE OF OPTICAL MINERALOGY IN REGULATORY METHODS: THE LIBBY MONTANA EXAMPLE

In June 2008 the United States Environmental Agency published the third version of a site-specific analysis protocol using Polarized Light Microscopy dealing with the amphibole asbestos in the soils around Libby, Montana. While the adaptations of regulatory methods specific to superfund-sites is commendable, this protocol in its current form contains deviations from standard nomenclature of the optical properties of biaxial minerals, amphiboles and amphibole asbestos, and does not take into full account the research done on the amphiboles that occur in the Rainy Creek Igneous Complex, the source of the vermiculite deposit. Using the published research done on the compositional, physical, and optical properties of these amphiboles the results show: 1) using current amphibole nomenclature a large range of refractive indices are possible for minerals with the same name as those found in the complex, the result is the permissible reported range of refractive indices is too large 2) using the compositional and crystallographic data from multiple sources the mean refractive indices were calculated using Gladstone-Dale constants, this also demonstrates that the mean predicted refractive index for all known amphiboles occurring at the mine are narrower in range than the listed values, 3) measurement of optical properties as described are not consistent with the principles of optical mineralogy and how it is applied, and 4) the method fails to discriminate the regulated asbestiform growth habit of amphibole from the non-regulated forms both of which are documented to occur in the deposit. Furthermore a recent publication demonstrated the occurrence of amphiboles in the Libby basin not associated with the deposit (see Gunter and Sanchez 2009 http://ammin.geoscienceworld.org/cgi/reprint/94/5-6/837), and the optical properties of these so-called non-deposit amphiboles would overlap with those permissible in the method.