MÖSSBAUER SPECTROSCOPY OF AMPHIBOLE-ASBESTOS FROM LIBBY, MONTANA: IMPLICATIONS FOR ASBESTOS CLASSIFICATION

Over the past two years amphibole-asbestos contained in vermiculite ore mined in Libby, Montana has gained national attention because of the high mortality rate of those who worked in the mine. Previously, the regulated mineral species tremolite-asbestos has been reported as the asbestos mineral there. The correct species name is important for regulatory reasons because amphibole-asbestos has been regulated based on species name and not, as would make more mineralogical sense, based on group name (i.e., ALL amphibole-asbestos should be regulated). Also, some health-effects workers have proposed the Fe content of minerals have implication on mineral reactions in the lung.

The Fe³⁺ content of Libby amphiboles is important to their characterization and chemical classification as pointed out by Wylie and Verkouteren (2000). Mössbauer spectroscopy at 12-300 K is used to measure the Fe³⁺/Fe²⁺ in the amphibole. Ferric iron contents ranged from 56%-76% of the total Fe in the five samples studied. Thus, it is apparent that previous wet chemical (bulk) analyses of these samples, such as that by Larsen (1942), probably underestimated the Fe³⁺ contents of these samples because impurities were not recognized. When considered along with uranium extraction data for hydrogen contents and EMPA results of Gunter et al. (2001) showing significant Na and K contents, these data allow the proper formulas for various samples of Libby amphibole to be determined. The resultant formulas support the suggestion of Wylie and Verkouteren (2000) that these samples are winchite. Winchite is not currently considered "asbestos" by any regulatory agency, but it should be.