Two samples of blueschist-facies metamorphic rocks from the Eastern Terrain of the Franciscan Complex were collected in Marin and Sonoma Counties, California. The specimens were thin-sectioned, polished and carbon coated for SEM analysis. Backscattered electron images were collected of the samples and chemical analyses performed on points of interest. The blueschist-facies indicator sodium-bearing amphibole, glaucophane, was selected for comparison between the samples. Sample RM-B4a from the Tiburon Peninsula showed consistency of glaucophane compositions throughout. Six crystals were analyzed and compositions were uniform within a standard deviation of about 10%. Sample SB7 from Shell Beach, Jenner, showed significant variation between the three glaucophane analyses, with element consituents varying to a standard deviation of almost 40%. The inconsistency of glaucophane composition in sample SB7 may be a result of original compositional variation in the unidentified protolith, or it may represent changes in pressure-temperature conditions during the course of metamorphic recrystallization. Further research is necessary to satisfy this question.

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