Abstract: Pyrophyllite is widespread in pelitic rocks of the Manning Canyon Shale in north central Utah, and the association of this mineral with other clay minerals, especially rectorite is related to the origin. The regular mixed-layer clay mineral rectorite seems to form as a result of the alteration of muscovite-paragonite during late stages of diagenesis and represents an intermediate metastable phase in the mineral paragenetic sequence. Pyrophyllite subsequently formed from the alteration of rectorite during advancing metamorphism and is the stable end member of the clay mineral assemblage.
Structural interpretations of rectorite found in the Manning Canyon Shale shows a regular, alternating sequence which consists of a fixed layer of 9·6 Å and an expandable layer, varying from 10 Å to 17 Å. With ethylene glycol saturation in the natural state a basal reflection of 26·60 Å is recorded.