Transmission electron microscopic study of coexisting pyrophyllite and muscovite: direct evidence for the metastability of illite

Transmission electron microscopy has been used to characterize coexisting pyrophyllite and muscovite in low-grade metamorphosed pelites from Witwatersrand and northeastern Pennsylvania. The Witwatersrand sample consisted largely of porphyroblasts of interlayered muscovite and pyrophyllite in a fine-grained matrix of the same phases. The textures of both samples were consistent with an equilibrium relationship between pyrophyllite and muscovite. The Pennsylvania sample also contained NH₄-rich illite, kaolinite, and an illite-like phase having intermediate Na/K, which collectively imply non-equlibrated low-grade conditions. -from Authors