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R. Schuster, G Habler, L. Tiede, O. Ageeva, R. Abart:
"Influence of the reaction interface orientation on the microstructure and texture of spinel formed by topotactic growth on corundum";
Talk: 3rd European Mineralogical Conference, Online; 2021-08-29 - 2021-09-02.

A natural about 150 μm thick spinel corona that formed around a centimeter-sized corundum single crystal through
chemical reaction between the corundum xenocrystal and the hosting basaltic melt was investigated by EBSD and μ-
CT analysis. The interface between corundum and spinel consists of straight segments of several 100 μm lateral
extension. A combination of EBSD and μ-CT was used to identify the full 3D orientation of the interface segments
with respect to the corundum lattice and to characterize the spinel microstructure and texture for different reaction
interface orientations. Our investigation revealed microstructural and textural features differing between segments
with different interface orientations, whereas other features are independent of the latter.
The common topotactic orientation relationship between corundum and spinel, where one of the ⟨111⟩ directions of
spinel is parallel to the c-axis of corundum and three ⟨110⟩ directions of spinel are parallel to the ⟨10-10⟩ directions
of corundum is roughly adhered to in all corona segments. This topotaxy results in a pronounced texture, where the
two spinel orientations compatible with the crystallographic orientation relationship dominate over other orientations,
regardless of the orientation of the corundum-spinel interface relative to the corundum lattice. However, the relative
prevalence of the two preferred texture components varies between corona segments and is controlled by the
orientation of the corundum-spinel interface relative to the crystallographic orientation of the two texture components
of spinel. Furthermore, the spinel grains exhibit small (up to about 7°) but systematic deviations from the ideal
topotactic orientations. These deviations result from rotations about the c- and a-axes of corundum, where the
activation of these rotations is controlled by the orientation of the corundum-spinel interface with respect to the lattice
of corundum. For rim segments associated with roughly prismatic corundum facets, the spinel rotation about the
corundum c-axis is always active, while rotation about a specific corundum a-axis only occurs for corona segments
corresponding to corundum facets that are oriented close to normal to that a-axis. The latter rotation shows a preferred
sense depending on the sign of direction of the a-axis with respect to the growth direction. Furthermore, the grain
shapes and grain boundary orientations in the spinel corona are controlled by both, the crystallographic orientation
of the corundum crystal and by the reaction interface orientation relative to the corundum lattice. Spinel twin
boundaries strictly follow the spinel {111} plane that is subparallel to the corundum basal plane, whereas the general
grain boundaries are preferentially oriented parallel to the spinel growth direction. Our findings have substantial
implications for the petrogenetic interpretation of reaction microstructures, as they show that particular reaction
interface orientations and the crystallographic orientations of the involved phases are critical for the development of
reaction microstructures