MORPHOLOGICAL CHARACTERIZATION OF AMPHIBOLES USED IN IN VIVO STUDIES

Animal studies (i.e., in vivo studies) are often used to serve as a model for health effects of mineral dusts on humans. Critical to the success of these studies is characterization of the mineral's morphology, especially when dealing with amphiboles. This is true because it appears the health impact from inhalation of asbestiform amphiboles is greatly than their nonasbestiform counterparts. We undertook a study in collaboration with researchers from the University of Montana aiding them in characterizing amphibole morphology. The minerals they used were the so-called “Libby six mix” provided by the USGS (United State Geological Survey). The Libby six mix derives its name because it is a mixture of amphiboles from six of the thirty amphibole sampling locations collected by the USGS from the now closed vermiculite mine near Libby, Montana in 2000. We were provided with two sets of the Libby six mix deposited on duplicate filters for a total of four total filters; the mineral/solution formed an approximate 7 millimeter spot on each of the four filters.

Two separate microscopic methods were used to measure the particle's width and length and to determine, based on morphology, if the particles were fibers or fragments; the PLM (polarizing light microscope) was used for particles greater than one micron in width and the FESEM (field emission scanning electron microscope) was used for particles less than one micron in width. A total of 2400 particles – 1200 from the PLM and 1200 from the FESEM, and consisting of 300 particles from each of the four filters – were measured and then classified as fibers or fragments based on morphology or as a fiber if the particle's aspect ratio was greater than 3:1. Based on just the aspect ratio, 99% of the particles were fibers. Based on morphology, 34% were fibers, 46% fragments, and 20% could not be differentiated by the PLM; 34% were fibers, 55% fragments, and 11% could not be differentiated by the FESEM. More specifically, we found particles with low aspect ratios morphologically had a higher percentage of fragments, particles with high aspect ratios morphologically had a higher percentage of fibers, and particles with intermediate aspect ratios were difficult to classify as either fibers or fragments based on morphology.