gneiss - large hand/display specimen of a Proterozoic granite gneiss from California

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.

An orthogneiss is deprived from igneous rocks. When derived from sedimentary rocks, the gneiss is a paragneiss. A gneiss is often named for a mineral constituent of that gneiss, a biotite gneiss, for example.

This gneiss was collected from San Antonio Canyon in the San Gabriel Mountains, California. 

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.

The pencil is 51/2" long for scale.  These are cut flat on one side but not polished. Both sides are shown.

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

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