Amphibole Asbestos Contamination of Soil In Libby, Montana from a Former Vermiculite Mine, or Not?

The former vermiculite mine near Libby, Montana has been in the national spotlight for almost a decade because of health-based issues associated with amphiboles occurring in the vermiculite ore. Amphibole asbestos occurring in the ore caused an increase in respiratory disease rates for the miners. Health concerns spread from the miners to their families and next to the residents of Libby as amphiboles were found in the air and soil in Libby.

There are two important geological points to consider in this issue: 1) What portion of the amphiboles from the deposit occur naturally in the soils? 2) Are there amphiboles in the soils that did not originate from the deposit? In response to the first question: the deposit is composed of easily erodible material, the area was glaciated during the Pleistocene, the deposit is located upstream from Libby which is built on Pleistocene lake sediments; thus, it would make geological sense that amphiboles from the deposit could occur in these sediments and predate mining. In regard to the second question: amphiboles are ubiquitous in the natural environment, especially in mountainous regions. Amphiboles from the deposit have been well characterized by several groups, because of this we can differentiate the amphiboles from the deposit from those forming in a different geological setting based on their composition.

We have collected soils in and around the Libby area and found they all contain amphiboles based on powder X-ray diffraction methods, and some contain over several weight percent. We have also determined that the majority of these amphiboles are not from the deposit, based on comparison of their compositions to those we have characterized from the deposit. However, amphiboles from the deposit have been found in soils that would predate mining.