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 Mesozoic deep-water sand, silt and calcareous and siliceous mud locally interlayered with basalt flows. 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.
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 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.
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