gneiss - teaching hand specimen of a Proterozoic granitic gneiss from the San Gabriel Mountains, California
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This Proterozoic granite gneiss, an orthogneiss, was derived from granitic rocks that were deeply buried and metamorphosed. Pressure and heat caused the minerals to recrystallize and align in bands. Gneiss contains less mica than schist and does not split along mica layers. It shouldn't be a hard sell to convince students that this rock is metamorphic.
A gneiss can also be derived from sedimentary rocks and is often named for a mineral constituent of that gneiss, a biotite gneiss, for example.
George Barrow, who mapped rocks in the Scottish Highlands during the late 19th century, showed there was a metamorphic progression starting with shale as a parent rock. The series begins with slate, then phyllite, schist and gneiss. Students should be able to recognize examples.
The Barrovian metamorphic progression is often encountered where temperature and pressure increase gradually in areas of active mountain building, subduction and volcanic arcs.
This gneiss was collected from San Antonio Canyon in the San Gabriel Mountains, California. The pencil is 5" long, for scale.
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