ES2 Metamorphic Rocks - Metamorphic Microstructures #17
Pseudomorph of biotite after garnet.
The brown patch at the centre of the field of view has the regular outline of a garnet. It consists of a more-or-less random aggregate of biotite flakes. Elsewhere in the thin section relics of garnet remain in these aggregates, which are said to pseudomorph the original mineral. This is the result of polymetamorphism - a thermal overprint on a regionally-metamorphosed rock.

Field of view is 2.5 mm across.
Polymetamorphic mica-schist, Eastern Alps, Sample DW-0222.

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DJW - August 1997