ASBESTIFORM AMPHIBOLES OF THE FRANCISCAN COMPLEX, CENTRAL AND NORTHERN CALIFORNIA COAST RANGE

Concern over potential adverse health effects caused by natural occurrences of asbestos (NOA) in concentrations as low as 0.1 weight percent has led to increased scrutiny of amphibole containing rocks of all types. The Franciscan Complex of the North-Central Coast Ranges of California is known to have numerous occurrences of amphibole-containing mafic and ultramafic rocks including blueschist, eclogite, amphibolite, and talc-tremolite schist. Though much attention has been paid to occurrences of chrysotile present in California’s serpentinites, recent work as a result of the discovery of NOA amphibole at the Calaveras Dam site, Fremont, CA, during on-going construction activities to replace the existing dam has raised concerns about similar rocks found elsewhere in the Franciscan Complex. A suite of such rocks from the Coast Ranges ranging from San Benito Co. to Mendocino Co., CA, has been analyzed using a variety of optical and electron microscopy techniques including PLM, TEM w/ EDS and FESEM w/ EDS, in order to assess their potential asbestiform amphibole content. The range of asbestiform amphibole minerals identified is surprisingly numerous, though dominated by glaucophane, ferro-glaucophane, winchite, ferro-winchite, actinolite and tremolite which can form as much as ~80% by weight or more of the host rock. This work suggests considerable caution should be used when performing construction or mining related activities in Franciscan rocks of these types.