schist - teaching student specimens of shimmering silvery quartz mica schist with wavy parting from the Black Hills of South Dakota - Unit of 5 specimens
The silver quartz-mica schists of the Precambrian Mayo Formation are exposed along Highway 16 west of Custer, South Dakota. Biotite is the predominant mica. Tilted back and forth, the quartz imparts a shimmering silvery flash. Derived from sedimentary rocks, it was metamorphosed from graywacke and silty shale. Graywackes are essentially dirty sandstones.
Schist is part of the metamorphic series that begins with a shale parent, proceeding with increasing regional metamorphism through slate, phyllite, schist and gneiss. If the gneiss is partly melted, migmatite forms. Complete melting produces an igneous rock in the family of granite.
Students should be able to identify a schist when it is tilted back and forth from the satiny sheen usually given by mica flakes that have been squeezed into alignment. A schist usually breaks along the plane of the mica. Metamorphism has given this schist a wavy parting surface. The silvery shimmer and wavy parting are spectacular.
Collected along North Pole Road in the Berne Quadrangle. USGS Professional Paper 297F covers the area. Here's an interesting old discussion on slate and schist from the Journal of Geology in 1897. The page enlarges so you can read it. Shales are fissile, meaning they weather into chips. So are some slates.
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