slate - black Jurassic slate from the Mariposa Formation, Placer County, California - hand specimen
This slate is from the Jurassic Mariposa Formation, in the Mother Lode region of the Sierra Nevada of California. Slate is metamorphosed from a shale or fine grained sedimentary rock. Further metamorphism results in a phyllite, which has a satiny surface sheen from microscopic flakes of mica aligned in the plane of the bedding. Even more metamorphism results in a schist, where the mica flakes are large enough to be seen.
To distinguish a slate from a shale, lick it. A slate will smell only very faintly muddy or not muddy at all. A shale will generally have a muddy smell when wet. This slate originated as muddy marine sediments and is commonly auriferous. It was collected near Yankee Jim's in Placer County, California.
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