phyllite - teaching hand specimen of a silvery gray green Paleozoic phyllite from the Shoo Fly Complex
Phyllite is just beyond slate in the metamorphic sequence from slate to gneiss. Further metamorphism would convert clays to mica and result in a schist. Phyllites have a definite sheen because tiny grains of mica and other minerals not visible in a slate have been aligned by increased heating and pressure and have grown larger.
This phyllite is from a broad band of metamorphic rocks on the west side of the Sierra Nevada in California, which originated as sediments deposited in a series of subduction zones that existed during the Paleozoic along the western edge of North America. During the Late Jurassic Nevadan orogeny, they were subjected to a period of intense metamorphism and were added to the continent.
The Shoo Fly Complex rocks are the oldest of the metamorphic belt. The original sediments were derived from the continent, deposited on the ocean floor to the west, and in the mid-Devonian, were subducted. Subsequent subduction shoved slabs of younger sediments under the Shoo Fly, like a stack of pancakes, though with the oldest on top, a reversal of the normal sedimentary sequence of older at the bottom, youngest on top.
Phyllite is from the Latin for leaf. The sheen is from tiny grains of mica, chlorite or graphite. While slate has a flat cleavage, phyllite tends break in wavy sheets. This phyllite has a textbook silvery sheen that a student can't miss.
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