The influence of crystallography and kinetics on phengite breakdown reactions in a low-pressure metamorphic aureole

Phengitic muscovites undergoing thermal decomposition due to contact with a metamorphic aureole at low-pressure were examined using transmission electron microscopy to study the various aspects of the breakdown process. Partial breakdown of muscovites yields a combination of cordierite, spinel, sillimanite, mullite, potassium-feldspar, biotite and phengite. Various regions within individual phengite grains undergoing partial decomposition show gradation in reaction and different end-products.