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.

Viti, C., & Ferrari, M. (2006). The nature of Ti-rich inclusions responsible for asterism in verneuil-grown corundums. EUROPEAN JOURNAL OF MINERALOGY, 18(6), 823-834 [10.1127/0935-1221/2006/0018-0823].

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

VITI, C.;
2006

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.
Viti, C., & Ferrari, M. (2006). The nature of Ti-rich inclusions responsible for asterism in verneuil-grown corundums. EUROPEAN JOURNAL OF MINERALOGY, 18(6), 823-834 [10.1127/0935-1221/2006/0018-0823].
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11365/18796