phyllite - teaching hand specimen of schistose phyllite
Phyllites and schists are derived from clays, muds or shale in the sequence of increasing metamorphism through slate, phyllite, schist and gneiss. They can also be derived from fine grained igneous rocks. Phyllites are characterized by very small mineral grains and a satiny sheen. Schists are larger grained. This phyllite, one stage of metamorphism beyond a slate, has the satiny sheen of a phyllite but the "rice grain" mineral lumps show that it is on the way to becoming an even larger grained schist. A good example to show how one metamorphic rock type grades into another.
A student might see this as a phyllite. Another might see a schist. I would not argue with either, but I might ask why they called it what they did, accepting any reasonable answer.
Tilted back and forth, the surface of these specimens has a typical satiny sheen of a phyllite. Schists, with mica grains usually visible at arm's length, have at least 50% of the mineral grains in alignment. If less than 50%, the rock is a gneiss. This rock can be split along the layers of mica as can a schist, illustrating the origin of the name schist, from the Greek skhistos for split.
Have your students compare this schistose phyllite with the phyllite and schists we offer. This particular phyllite is from the Jurassic core of the El Paso Mountains in California, and though it is mapped as Mesquite Schist, that rock unit is variable from spotted slate to phyllite to schist.
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