Sample in alkali granite
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Return to B.R.S.A mica mineral. This sample shows well the basal cleavage and the tendency toward forming 6-sided crystals when the crystal can grow freely. This specimen comes from a pegmatite, a coarse grained granitic rock.
Muscovite occurs occasionally in felsic igneous rocks. It is often recognized because light flashes sharply off its cleavages faces, almost like light off a mirror.