AMPHIBOLES BETWEEN THE SHEETS: AN INTERESTING OCCURRENCE WITH EVEN MORE INTERESTING MORPHOLOGIES

During our on-going characterization of amphiboles associated with the former vermiculite mine near Libby, Montana, interesting amphibole morphologies were observed in the transmission electron microscope (TEM) in air samples collected in Libby by the Environmental Protection Agency (EPA). By relocating these amphiboles in the field emission scanning electron microscope (FESEM), high-resolution images were obtained that clearly show these features. Many of the amphiboles were perfect euhedral single crystals (several microns long and 200-500 nanometers wide) showing the classic (110) forms, while other single crystals appeared to be attached to sheet silicates and yet others appeared to have “wings” or “fins” attached to their edges, often less than one micron in width. Interestingly, the morphology of these amphiboles differs from those formed in the deposit by alterations of pyroxenes. Energy Dispersive X-Ray Analysis (EDX) and selected area electron diffraction from the TEM confirmed that these particles were amphiboles. The challenge was to determine the origin of these unique amphiboles.

The major component of the ore produced from the Libby mine was a mixture of sheet silicates, mainly intergrown vermiculite and hydrobiotite. These sheet silicates formed as an alteration of biotite; thus, it seemed possible that the amphiboles may have occurred associated with the sheet silicates. Sheet silicates were obtained from soil samples collected in Libby by the EPA. FESEM analysis revealed that these samples contained amphiboles with the same morphologies as observed in the air samples and were clearly intergrown into the sheet silicates. Several other unique features were observed, for example “band aids” and “patches” on the order of one to five microns wide that seemed to bind the amphiboles onto the surface of the sheet silicates. Some of the band aids had a smooth surface while others appeared “woven” with double strands of material at a 14 nanometer spacing. The band aids were so thin that they were electron transparent at 3 keV. Also, “thread-like” features ten's of microns long and only 5-10 nanometers in width were found growing on the surface. Thus these amphiboles exhibit a nonasbestiform habit growing not as bundles but as single crystals between the sheets of the vermiculite and hydrobiotite.