DISCOVERING THE TRUE MORPHOLOGY OF AMPHIBOLE MINERALS: COMPLIMENTARY TEM AND FESEM CHARACTERIZATION OF AMPHIBOLE PARTICLES IN MIXED MINERAL DUST

This study involved the development and application of an analytical protocol using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), and field emission scanning electron microscopy (FESEM) techniques for the particle-by-particle characterization of mixed mineral dust. This protocol was developed as part of a continuing effort to characterize amphibole particles that occur as accessory minerals around the former vermiculite mine located near Libby, Montana. Media attention has been focused on Libby since the fall of 1999 due to health concerns over potential amphibole asbestos exposure occurring in and around the mine. There is still ongoing debate on the various types of asbestos and non-asbestos amphibole particles present in the mixed mineral dust and their relative quantities and potential health risks. Therefore, the distinction among asbestos and non-asbestos amphiboles is important from both a scientific and regulatory standpoint.

In this study, the Yamate Level III TEM method for asbestos analysis of air samples was supplemented by FESEM imaging. The FESEM procedure involved relocation of amphibole particles analyzed by TEM/EDS/SAED and the collection of secondary electron (SE) images of the full structure, both structure ends, and the structure surface. Three-dimensional FESEM stereo pair images were also acquired for the full structure and the particle surface to provide additional morphological and depth perception information. Features key to distinguishing between asbestos and non-asbestos amphibole particles, such as overall particle shape and surface roughness, are readily apparent in the FESEM SE and stereo pair images.

To facilitate data review, all images, spectra, and diffraction patterns were imported into a digital database. Mineral particles were classified by various categories, such as particle shape, end and side geometries, and surface texture. The organization and viewing capacity of the database were useful for reviewing and interpreting data, sharing technical results with other experts, and training new analysts. This study demonstrated that FESEM imaging and digital database utilization are essential elements for accurate characterization of complex mixed mineral dusts.