Abstract
Pelitic schists in the contact aureole of the late-Alpine Vedrette di Ries granodiorite show an epitaxial replacement of kyanite by Fe-rich staurolite. The reaction is observed within nodules of fine-grained staurolite crystals (100–200 μm) containing oriented lamellae of kyanite.
Microprobe, backscattered electron microscopy (BSEM), and high-resolution transmission electron microscopy (HRTEM) investigations demonstrate the presence of submicroscopic intergrowths of staurolite and kyanite. The orientation of both phases, aSt||bKy and CSt||cKy, is in agreement with previous observations. Individual staurolite and kyanite lamellae are between one unit cell and some tens of unit cells wide. Detailed observation of the termination of staurolite lamellae within kyanite suggests that the replacement occurs by a solid-state reaction, in which the primary cubic close-packed O framework of kyanite is preserved. The epitaxial replacement implies a reorganization of the site occupancy accompanied by addition of Fe + Mg + H and removal of Si + Al from the reaction site.
The epitaxial replacement of metastable kyanite occurred during contact metamorphism within the andalusite stability field, under temperature and pressure conditions of 500–550 °C and >2.9 kbar, respectively.
