Metamorphic aragonite in the glaucophane schists of Cazadero, California

Abstract

Metamorphic aragonite is the dominant polymorph of CaCO ₃ within concordant and discordant carbonate-rich lenses and veins in the glaucophane schist belt exposed along Ward Creek, Cazadero, California. This schist sequence is part of a larger metamorphic belt within the Franciscan Formation of Jurassic and Cretaceous age. Petrofabric study shows that the fabric element of the aragonite marbles is compatible with the lineation of the enclosing schists. A careful chemical, optical, and X-ray study of 4 purified aragonite specimens shows that this metamorphic aragonite conforms in every respect with the physical properties recorded from aragonite. The mineral is biaxial negative, 2V approximately 18 degrees , alpha = 1.530 + or - .001, beta = 1.680 + or - .002, and gamma = 1.685 + or - .001. A maximum of 1.26 molecular percent SrCO ₃ was found by chemical analysis. The presence of aragonite as a metamorphic mineral within these glaucophane schists suggests that the pressure and temperature conditions for this facies may be unique. Experimental work on the inversion of calcite to aragonite has shown that aragonite is the stable high pressure polymorph of CaCO ₃ ; the formation of aragonite in the glaucophane schists indicates that pressures greater than 4000 bars prevailed during metamorphism. Tectonic stresses developed during the formation of the glaucophane schists may have increased the pressure considerably above the lithostatic load pressure and promoted the stable formation of aragonite marbles.