Libby vermiculite, contaminated with asbestos

Vermiculite from the mine that operated near Libby, Montana (from the early 1920s until 1990) was contaminated with asbestos and other fibrous amphibole minerals, crystalline silica, and talc.

The U.S. Geological Survey has characterized the respirable fraction of asbestiform amphiboles contaminating the Libby vermiculite as approximately 84% winchite, 11% richterite, and 6% tremolite (Meeker et al. 2003).

The raw Libby ore was estimated to be 21–26% asbestos by weight; the mill feed was 3.5–6.4% asbestos; airborne dust in the dry mill was 40% asbestos; and the vermiculite concentrate shipped to numerous processing plants in the United States and other countries was 0.3–7.0% asbestos before expansion.