Abstract
Oriented hematite inclusions in prismatic sillimanite grains from a sapphirine-bearing granulite from Wilson Lake, Labrador, have been studied by optical microscopy and single crystal X-ray precession photography. The inclusions are lath-like in shape, 0.1 to 0.2 μm in smallest dimension, and elongated parallel to the sillimanite c-axis. In (001) section they are oriented parallel to sillimanite {110}. The crystallographic orientation of the hematite inclusions is [120] of hematite parallel to c-axis of sillimanite, c-axis of hematite inclined at 17.7° to the a-axis of sillimanite.
Although the hematite–sillimanite phase boundary orientation and hematite lattice rotation are approximately reproduced by two-dimensional lattice misfit (coincident lattice) calculations, significant discrepancies persist between the observed and refined interface parameters. However, further analysis reveals that the observed interface plane is a rational plane in both phases and is parallel to a potential common structural plane. Thus, the orientation of the hematite–sillimanite intergrowth appears to be controlled by topotaxy rather than lattice misfit. Petrographie details suggest that the hematite inclusions formed by a precipitation mechanism (probably exsolution). This is consistent with the complex and extensive retrograde history of the granulites.
