The mineral inclusions of two orange glass tesserae from paleo-Christian mosaics were investigated in order to derive the melting temperature reached during their production (sourced from Padua and Vicenza, Veneto region, Italy). In particular, clinopyroxene crystals were studied by single-crystal X-ray diffraction and electron microprobe WDS analysis. The crystals show C2/c symmetry, typical of disordered Ca/Na and Mg/Al distributions indicating high-temperature of formation (>700°C). The cation site populations were obtained by combining results from the two experimental techniques enabled us to derive the following stoichiometric formula: M2[Ca0.819Na0.172Mn0.006K0.003]M1[Mg0.765Fe3+0.210Cu0.015Ti0.006Zn0.006]T [Si1.933Al0.037Sn0.024]O6. This composition is very close to an ideal diopside (Di, CaMgSi2O6)–aegirine (Ae, NaFe3+Si2O6) solid solution (with composition Di80Ae20). The crystal-structure refinement shows that the Mn, K, Cu, Ti, Zn, Al and Sn “impurities” do not affect the bond lengths and atomic coordinates expected for a pure Di80Ae20. Previous studies report a temperature of formation of about 1250°C, which can be directly applied to the present case, thus inferring the same temperature for the production of the orange glass tesserae.
Pyroxene inclusions in paleo-Christian mosaic tesserae: a tool for constraining the glass manufacturing temperature
TONIETTO, SERENA;NESTOLA, FABRIZIO;SILVESTRI, ALBERTA;MOLIN, GIANMARIO;
2011
Abstract
The mineral inclusions of two orange glass tesserae from paleo-Christian mosaics were investigated in order to derive the melting temperature reached during their production (sourced from Padua and Vicenza, Veneto region, Italy). In particular, clinopyroxene crystals were studied by single-crystal X-ray diffraction and electron microprobe WDS analysis. The crystals show C2/c symmetry, typical of disordered Ca/Na and Mg/Al distributions indicating high-temperature of formation (>700°C). The cation site populations were obtained by combining results from the two experimental techniques enabled us to derive the following stoichiometric formula: M2[Ca0.819Na0.172Mn0.006K0.003]M1[Mg0.765Fe3+0.210Cu0.015Ti0.006Zn0.006]T [Si1.933Al0.037Sn0.024]O6. This composition is very close to an ideal diopside (Di, CaMgSi2O6)–aegirine (Ae, NaFe3+Si2O6) solid solution (with composition Di80Ae20). The crystal-structure refinement shows that the Mn, K, Cu, Ti, Zn, Al and Sn “impurities” do not affect the bond lengths and atomic coordinates expected for a pure Di80Ae20. Previous studies report a temperature of formation of about 1250°C, which can be directly applied to the present case, thus inferring the same temperature for the production of the orange glass tesserae.
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