Nucleation effects: hornfelsic texture.
In contrast to the porphyroblastic texture of the previous slide, the contact-metamorphic
hornfels, seen here at the same scale, shows abundant small grains of biotite.
Rapid heating has caused the biotite-forming reaction to be greatly overstepped,
allowing easy nucleation at very numerous sites.
Field of view is 2.5 mm across.
Cordierite-biotite hornfels, Comrie, Scotland, Sample 388.
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