Microstructures of Metamorphic Rocks
Most size distributions show a peak at intermediate grain size, with fewer grains in small size classes. Various feedback mechanisms potentially affect the nucleation rate in the later stages of reaction and account for the left-hand tail of the size distribution:
- Site saturation: suitable sites for easy nucleation get used up.
- Chemical potential gradients around growing grains suppress further nucleation in their vicinity (significant if the transport step is slow).
- Endothermic reaction removes heat from the rock, and overstepping decreases (significant if overall reaction rate is fast).
- Multivariant reaction: when reaction takes place over a temperature interval, overstepping decreases if reaction rate outstrips heat supply.
We believe that multivariant reaction is the most important factor.
Here are two further examples of measured size distributions with their possible interpretations.
Very small left tail suggests andalusite formed by discontinuous reaction, small b parameter (compared to other andalusites) indicates slow heating.
Long left tail suggests cordierite formed by multivariant reaction, larger b parameter (compared to andalusite in same rock) suggests easier nucleation or slower reaction.