Peristerite unmixing in the plagioclases and metamorphic facies series

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

The peristerite plagioclases in the compositional range An2 to An17 are reviewed and it is concluded that they are unmixed into two phascs of composition about An0-2 and An26±2. No plagioclase in the bulk composition range An17 to An26 has so far been found to be unmixed.

Plagioclase crystals in metamorphic rocks probably nucleated in a disordered form (monoclinic or nearly so at the Ab-rich end). At the time of the last rccorded deformation, the albite in greenschist facies rocks was too ordered to dcvelop mechanical twins, whereas the plagioclase in almandine-amphibolite facies rocks was slightly but sufficicntly disordered to devclop mechanical twins. From this and crystallographic reasoning it is assumed that unmixing begins in partly disordered crystals and that ordering continues with the unmixing. Unmixed crystals should not develop mechanical twins. The top of the peristeritc solvus may well be at about 500°C , and it is probably raised by pressure to a slight extent.

In metamorphic rocks of the andalusite-sillimanite type (Miyashiro (1961)), the composition of the plagioclase increases regularly with increasing metamorphic grade, while in rocks of the kyanite-sillimanite type and of the low and high-pressure intermediate groups, it rna y show a jump in the range about. An7 to An20. It is considerd that this jump is due in some way to the peristerite solvus. This jump provides valid grounds for facies boundaries, but facies boundaries established on other grounds should not be modified too hastily, before work on the other minerals (especially the amphibolcs) has been done.