USING EBSD ANALYSIS TO INTERPRET GARNET MICROSTRUCTURES

Lattice orientations of garnet microstructures, obtained using electron-backscatter diffraction (EBSD) methods, were used to model garnet growth and deformation mechanisms. EBSD analysis uses the backscattered electrons (BSE) emitted from a specimen, in a scanning electron microscope (SEM), to form a diffraction pattern that is indexed. EBSD enables crystallographic orientation measurements of minerals as small as 1 micrometer, and the calculation of misorientation axes and angles (>1 degree) between any two measurements. EBSD data were used to interpret atoll-shaped garnets and elongated garnets.

Atoll garnets, from eclogite in the Ural Mountains, have rings of garnet that surround other phases, and that surround islands of garnet. EBSD analyses showed that the ring and islands of any individual atoll garnet have exactly the same lattice orientation, suggesting that they were originally one grain. We propose that the formation of these atoll garnets occurred in two stages: first, a poikiloblastic garnet grew; then, the core garnet was redistributed by diffusion to form the atoll shape, resulting in decreased surface to volume ratio, and lowered interfacial energy.

Elongated garnets, from amphibolite in Casco Bay, Maine, have a 4:1 length to width ratio. EBSD analyses revealed a dispersion of lattice orientations about a <100> axis, with almost no subgrain boundary development. We interpret the data to suggest the garnet underwent crystal plastic deformation at relatively low temperatures (c. 550 deg C).

These are two examples of how EBSD analysis was applied to interpret garnet microstructures. Because garnet is optically isotropic, it is difficult to observe and record microstructural details using light microscopy. The recent development of EBSD methods provides an effective means to quantitatively analyze variations in the lattice orientations of garnet and other minerals.