Characterization of asbestiform glaucophane-winchite in the Franciscan Complex blueschist, northern Diablo Range, California

This paper presents a geologic and mineralogical investigation of asbestiform amphibole from blueschist in the Diablo Range, northern California. The analysis of fibers in air samples shows that the dominant blueschist amphibole composition ranges from glaucophane to subordinate winchite. In outcrop, blueschist amphibole exhibits a velvety luster, and its occurrence ranges from crosscutting veins to highly deformed foliated and lineated tectonites. TEM and SEM photographs reveal a highly fibrous habit typically associated with asbestiform amphiboles. Dimensional analysis reveals a mean fiber width of 0.27 mu m, and lengths and aspect ratios are shorter than reported for commercially exploitable asbestos, with a mean length of 2.8 mu m and mean aspect ratio of 11.5. The data are consistent with other research showing that the width population of fibers, and not length or aspect ratio, is the key indicator of the asbestiform habit. The blueschist data are compared to non-asbestiform hornblende amphibole in granitic rocks. The fiber concentration of the hornblende (458 MFG) is low compared to the blueschist amphibole (107,880 MFG) and chrysotile in serpentinite (196,066 MFG), indicating that under similar conditions, the exposure potential from cleaved particles could be several orders of magnitude lower than from asbestiform particles. The asbestiform habit of glaucophane is present at two other locations in the Franciscan Complex, suggesting that asbestiform blueschist amphibole may be characteristic within blueschist terranes, and rock formations containing asbestiform amphibole may be more common and widespread than previously assumed.