Thermal behaviour of cinnabar, α-HgS, and the kinetics of the β-HgS (metacinnabar) → α-HgS conversion at room temperature

Ballirano, P., Botticelli, M. and Maras, A. (2014) Thermal behaviour of cinnabar, α-HgS, and the kinetics of the β-HgS (metacinnabar) → α-HgS conversion at room temperature. European Journal of Mineralogy, 25(6), pp. 957-965. (doi: 10.1127/0935-1221/2013/0025-2341)

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Cinnabar use on painting is historically documented from ancient times, together with its tendency to darken. The most accepted mechanism for this kind of deterioration considers cinnabar (α-HgS) transformation into its polymorph metacinnabar, β-HgS, but recent studies have cast doubt on this hypothesis. On this background, an in situ high-temperature X-ray powder diffraction study of the thermal behaviour of cinnabar has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the α-HgS (trigonal) to β-HgS (cubic) phase transition occurs at 673 K and is completed at 698 K due to kinetics reasons. The thermal expansion of cinnabar is fairly isotropic, the a-parameter being slightly softer against heating. A fast cooling of metacinnabar at room temperature (T) produced a mixture of both polymorphs. The analysis of the kinetics of the α-HgS- β-HgS transformation at room-T shows a low rate of conversion, but not low enough to be consistent with a persistence of metacinnabar for a time span of 2000 years as hypothesized in reference data for Pompeian frescoes.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Botticelli, Dr Michela
Authors: Ballirano, P., Botticelli, M., and Maras, A.
College/School:College of Arts > School of Culture and Creative Arts > History of Art
Journal Name:European Journal of Mineralogy
Publisher:Schweizerbart Science Publishers
ISSN (Online):1617-4011

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