The nature of Ti-rich inclusions responsible for asterism in verneuil-grown corundums

Verneuil-grown star corundums, with variable zoning and colour (ruby and sapphire), host three sets of acicular inclusions at 120°. Inclusions are up to 20-30 μm long and 0.1-0.4 μm wide. Their cross-sections match the visible-light wavelength and explain the optical diffraction responsible for sharp six-arms stars in cabochon gemstones. Inclusions are elongated parallel to corundum {110} faces, and they are {110} polysynthetically twinned. Electron diffraction and high-resolution transmission electron microscopy show that inclusions are almost isostructural with the corundum matrix, even if slightly distorted to a monoclinic lattice with a = 5.00 Å, b = 4.63 Å, c = 13.08 Å, γ = 118°. Nanochemical data point to a TiO2 stoichiometry, thus indicating the occurrence of Ti4+ cations, coupled with vacancies, within the distorted corundum-like structure (having just 1/2 of the octahedral sites occupied). Close to the apical terminations of TiO2 needles, local stoichiometry is consistent with the presence of Ti2+ cations, formed by redox disproportionation during post-growth thermal annealing.