Ferroanthophyllite in Rockport grunerite: A transmission electron microscopy study

A transmission electron microscopy study reveals the microstructure of poly synthetically twinned, near end-member grunerite from Rockport, Massachuset ts. A small percentage of Pnma ferroanthophyllite of similar composition is present as thin slabs intergrown parallel to (100) of grunerite and as ind ividual mu -size crystals. HRTEM imaging shows that grunerite is free of ch ain-multiplicity faults. The observed microstructures are interpreted as a result of partial transformation of ferroanthophyllite to grunerite, a mech anism that is also supported by microstructures observed in the Pnmn protof erroanthophyllite from Cheyenne, Colorado, with similar composition. This s tudy supports the possibility that grunerite, ferroanthophyllite, and proto ferroanthophyllite may all possess true stability fields.