The atomic scale structure and chemistry of (111) twins in natural MgAl2O4 spinel crystals were analyzed by using complementary methods of transmission electron microscopy. To obtain a 3-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections [¯110] and [11¯2]. Using conventional electron diffraction and high-resolution transmission electron microscopic analysis, it was shown that (111) twins in spinel could be crystallographically described by a 180° rotation of the O sub-lattice normal to the twin composition plane. This operation generated a local hexagonal close-packed stacking into an otherwise ccp lattice and maintained a regular sequence of kagome and mixed layers. In addition to rotation, no other translations were present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by means of energy-dispersive X-ray spectroscopy using a variable beam diameter technique, which was best suited for analyzing the chemical compositions of twin boundaries at the sub-nm scale. The variable beam diameter and energy-dispersive X-ray spectroscopic measurements indicated that the (111) twin boundary in spinel was Mg-deficient. Quantitative analyses of high-resolution transmission electron microscopic (phase contrast) and HAADF-STEM (Z-contrast) images of the (111) twin boundary confirmed that Mg2+ ions were replaced by Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure was closely related to the BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (taaffeite) group of intermediate polysomatic minerals. On the basis of these results, it was concluded that the formation of (111) twins in spinel was a preparatory stage in polytype/polysome formation (taaffeite) and was a result of the thermodynamically favorable formation of hexagonal close-packed stacking due to Be incorporation on the {111} planes of the spinel structure at the nucleation stage of crystal growth. The twin structure grew, provided that surrounding geochemical conditions permitted its formation. The incorporation of Be introduced a 2-dimensional anisotropy, and exaggerated growth of the crystal along the (111) twin boundary.

Structure and Chemistry of (111) Twin Boundaries in MgAl2O4 Spinel Crystals from Mogok. N.Daneu, A.Rečnik, T.Yamazaki, T.Dolenec: Physics and Chemistry of Minerals, 2007, 34[4], 233-47