orthogneiss - mid to late Proterozoic, St. Jerome, Quebec - hand specimen

orthogneiss - Grenville Age mid to late Mesoproterozoic ~1 billion years old

St. Jerome, Quebec, Canada

This Proterozoic red gneiss is derived from granitic rocks that were deeply buried and metamorphosed. When derived from granitic rocks, a gneiss is called an orthogneiss. In this specimen, the pink is orthoclase feldspar, the white  mineral is quartz, and the black mineral is biotite.

Gneiss can also be derived from sedimentary rocks, and is often named for a mineral constituent of  that  gneiss, eg: garnet gneiss or  biotite gneiss.

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