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

Dipartimento di Scienze della Terra, Università di Siena Via Laterina 8, I-53100 Siena, Italia

^* E-mail: vitic{at}unisi.it

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, = 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.

Key-words: Verneuil corundums, asterism, Ti-rich inclusions, HRTEM, twinning.