Micro-sectoriality in hydrothermally grown ruby crystals: the internal structure of the boundaries of the growth sectors

This study explores the fine structural details of the boundaries between the growth micro-sectors in a ruby crystal grown hydrothermally on a nonsingularly oriented (01) seed. The samples were examined using IR-spectroscopy and HRTEM-analysis, demonstrating that the interfaces of the micro-sectors serve as ‘traps’ for OH-groups often observed in grown crystals. Counter to what has previously been reported, a significant proportion of these OH-groups is incorporated into ruby crystals in an orderly manner, forming diaspore-like layers growing epitaxially on the corundum (012) lattice planes. The tensions on the boundaries between the micro-sectors result in local increase of internal pressure, making the diaspore-like phase stable. The assumption is made that the mechanism discussed in the article can explain the occurrence of OH-groups in the structure of nominally anhydrous minerals, such as MgSiO3 (akimotoite) which is structurally similar to corundum.