Phengite Schist
“High-pressure mica from subduction zones”
Phengite schist is a metamorphic rock rich in phengite — a silicon-rich variety of white mica that forms under high-pressure conditions in subduction zones. Its presence signals that the rock was once dragged to great depths along a subducting plate before being exhumed back to the surface.
phengite, quartz, garnet, chloritoid, lawsonite (occasional), rutile
Dora-Maira Massif (Italy), Sesia Zone (Italy), Cycladic Islands (Greece), New Caledonia, Tianshan (China)
4-6
Phengite's chemical composition acts as a geological barometer — the amount of silicon substituted into its crystal structure directly correlates with the pressure at which it formed.
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