schist - teaching hand specimen of fine-grained gray mica schist

This mica schist most commonly has evenly distributed very small flakes of the biotite look-alike stilpnomelane, giving it the shiny surface typical of all schists. However, the mica in these examples is muscovite. Schists, as this one does,  vary greatly in composition and appearance. They are derived from clays, muds and shales and fit into a metamorphic sequence of slate, phyllite, schist and gneiss in order of increasing metamorphosis. They can also be derived from fine grained igneous rocks. This schist, the Pelona Schist, was derived from arkosic sediments being shed off the western edge of the North American continent. About 80 million years ago, the subducting oceanic plate flattened out below the continent, taking the overlying arkosic sediments under the North American crust. When the East Pacific Rise hit the continent, the continental crust was pulled apart, allowing exhumation of the Pelona and the related Rand and Orocopia schists. These specimens were collected on Sierr