Epitaxial nucleation of garnet on biotite in the polymetamorphic metapelites surrounding the Vedrette di Ries intrusion (Italian Eastern Alps)

Garnet in a metapelite from the aureole of the Vedrette di Ries tonalite, Eastern Alps, is found in two distinctive microstructural settings relative to a large single crystal of biotite: it forms a narrow polycrystalline layer in a reaction zone between plagioclase and biotite, and nearby it occurs in clusters of crystals interspersed with the biotite. In the reaction zone, morphological and microchemical evidence document multiple nucleation events along the interface between biotite and plagioclase, with ilmenite inclusions marking the original plagioclase–biotite boundary. Electron backscatter diffraction analysis shows that for all garnet in both microstructural settings, (110)grt || (001)bt with [100]grt || [100]bt. This single, recurring orientation for all garnet crystals adjacent to the biotite grain implies that nucleation was dominantly epitaxial during crystallization of the garnet. The occurrence is remarkable for the clear genetic relationships revealed by the microstructures and for its preservation of the mica substrate on which the garnet nucleated, which allows unambiguous determination of not only the coincident lattice planes but also the coincident lattice directions involved in the epitaxy.