EBSD ANALYSIS OF HIGH ANGLE, LOW ENERGY GRAIN BOUNDARIES IN GARNET

Garnets that appear to be single crystals based on their morphology as seen in outcrop or thin section may in fact contain multiple grains that possess different lattice orientations separated by high angle boundaries. Garnets with 1, 2, or 3 grains occur in schists from Townshend Dam, Vermont (USA). The grains are juxtaposed along high angle boundaries with misorientation angles of 47-61°; the boundaries cut across the garnets from rim to rim and have no relationship to chemical zoning or distribution of mineral inclusions. The boundaries possess misorientations that correspond to coincidence site lattices (CSL) for cubic crystals (e.g., Σ11, 17b). CSL boundaries are low energy configurations and therefore more stable than general boundaries. Their boundary diffusivity is typically smaller than that of general grain boundaries, but presumably still much higher than the bulk diffusivity in the garnet. For all our measured CSL boundaries, the neighboring grains share a {110} plane, which is also the dodecahedral face of garnet. There is no evidence for intracrystalline deformation in the garnets, so these composite garnets likely evolved from crystals that coalesced during growth. Coalescence occurred early – before garnet zoning developed – and therefore the garnet ‘nuclei' cannot be detected with compositional maps. The presence of CSL misorientations with matching {110} planes – for which several variants exist due to the high crystallographic symmetry of cubic garnets – may indicate that these boundaries represent low energy configurations that are preserved because they are stable during garnet coalescence and growth. For comparison, we analyzed a cluster of 3 morphologically distinct garnets (a central garnet touching 2 garnets that are not in contact with each other). Misorientation angles and axes between the garnets all correspond to a CSL: 41° about [100] (Σ29a); 54.8° about [110] (Σ11 or Σ41c); and 43° about [100] (Σ29a). We propose that early interface-controlled growth leads to euhedral garnet crystals that coalesce with increased likelihood in special misorientations; the euhedral morphology may be partially eliminated during subsequent diffusion-limited growth, but the garnets preserve evidence for their earlier history in their crystallographic orientation.