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

Authors: Viti, Cecilia; Ferrari, Marco

Source: European Journal of Mineralogy, Volume 18, Number 6, November 2006 , pp. 823-834(12)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

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.

Keywords: VERNEUIL CORUNDUMS; ASTERISM; TI-RICH INCLUSIONS; HRTEM; TWINNING

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2006/0018-0823

Publication date: 2006-11-01

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