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. Schists 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 Sierra Pelona, Los Angeles County, California. They have a textbook surface sheen and a student will easily recognize this as a schist.
Sierra Pelona is on the Pacific Plate, just south of the San Andreas Fault. It sheds Pelona Schist northwards across the fault onto the North American Plate. You can walk on the north side of the fault and pick up pieces of Pelona Schist a good distance to the east of Sierra Pelona, which is being carried off to the northwest on the Pacific Plate at about 6 cm per year.
Mica schists are the most common, with the mica making a schist easy to recognize. In these specimens the mica grains are small, but tilted back and forth, the surface has a silvery sheen. Schists have at least 50% of the mineral grains in alignment. If less than 50%, the rock is a gneiss. This rock can be split along the layers of mica, illustrating the origin of its name, from the Greek skhistos for can be split.
Schists are often described by their dominant mineral such as garnet schist or biotite schist. Chlorite schist is largely composed of chlorite - Greek chloros = green.
Stilpnomelane was described in 1827 by Ernst Friedrich Glocke from an occurrence in Moravia in the Czech Republic. The name is from the Greek stilpnos for shiny, and melanos for black, though the color varies from yellowish to greenish, brown and black. It is often found in radiating groups of flaky crystals.
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