ALBERT

Description: 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.

Description: The combination of high-resolution X-ray computed tomography (HRXCT) with electron backscattered diffraction (EBSD) analysis has the potential to reveal evidence for epitaxial nucleation of garnet porphyroblasts containing an included fabric. HRXCT measures the fabric orientation near the inferred nucleation site at the crystal centre, whereas EBSD establishes the crystallographic orientation of the porphyroblast relative to the attitude of the included fabric. Using the orientation of the central fabric as a proxy for the attitudes of the basal planes of micas on which the garnet may have nucleated, one can determine if low-index planes in garnet—(100), (110), or (111)—are parallel to (001) of mica, which would correspond to known epitaxial relationships between the two minerals. We test the feasibility of this approach with a set of exploratory measurements for six garnet porphyroblasts from Passo del Sole (Switzerland) and five from the Nevado Filabride Complex (Spain); we also report unsuccessful attempts at such measurements in rocks from Harpswell Neck (USA). The results do not deviate appreciably from those expected of a set of measurements drawn from a parent population with a random distribution of orientations. However, considering the size of the present measurement uncertainties in relation to the precision required to recognize an epitaxial relationship unambiguously, it is not possible in this data set to exclude epitaxy as a nucleation mechanism. The principal limitation to this approach is the need for high precision in determining the attitude of the included fabric at the site of nucleation, which depends largely upon the choice of suitable samples. Crystals ideal for such studies would contain a non-crenulated, planar (not helicitic) foliation, defined by numerous small, well-aligned inclusions. With careful sample selection, this approach could be a viable technique for evaluating the prevalence in nature of epitaxial nucleation of garnet porphyroblasts.