ES2 Metamorphic Rocks - Metamorphic Microstructures #11
Nucleation effects: Porphyroblasts.
This is a regionally-metamorphosed rock from the garnet zone of the Scottish Dalradian. The slow rate of temperature increase through the equilibrium condition for the biotite-producing reaction has allowed time for biotite to grow on the first few successful nuclei in the rock, before the temperature has risen sufficiently to allow more indiscriminate nucleation. Each thin section contains only about ten large biotite crystals or porphyroblasts.
Field of view is 2.5 mm across.
Garnet-biotite schist, Dalradian garnet zone, Sample 7153.
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DJW - August 1997