Sanchez, M.S., R.J. Lee and D.R. Van Orden, “Extinction Characteristics of Six Tremolites with Differing Morphologies”, Microscope, Vol. 56(1), pp. 13-27, 2008.
There has been considerable discussion in the literature related to the use of standard optical properties of commercial asbestos minerals for the classification of amphibole minerals found in raw materials as either asbestiform or as non-asbestos (1-4). The goal of this study was to ascertain if there is a relationship between particle morphology and extinction characteristics in monoclinic tremolite amphiboles. Six tremolitic amphiboles were chosen for this study: three are fibrous(five from natural sites (i.e., mining locales)) and one is the NIST SRM 1867a tremolite standard. The morphology of these tremolites ranged from blocky to asbestiform. A particle-by-particle analysis was performed to determine extinction characteristics and the number of EPA-defined asbestos characteristics. In general, zero or near-zero extinction angles correlate to the number of asbestiform characteristics. Exceptions to this occur when a non-fibrous tremolite has (100) parting as a result of twinning or when fibrous tremolites are elongated in the g_-crystallographic direction, therefore preferentially lying on (010). However, when using extinction characteristics in conjunction with morphology, the differences between habit is discemable. The three fibrous tremolites have different fiber widths and, as such, exhibit different extinction characteristics. The smaller the diameter of the fibers, the greater number of particles exhibiting parallel extinction. The sample with the smallest widths (< 0.5 µm) displayed parallel extinction for all particles. When fiber diameters are small (<1 µm) optically visible fibers are in fact bundles of fibrils and are not single crystals; they therefore show parallel extinction.