Dolomite and Mg Calcite as Mineral Thermometers in Mortar Binders: A High Resolution Raman Spectroscopic Study

This paper suggests the use of high-resolution Raman scattering bands of Mg–Ca carbonates as posteriori thermometer minerals in archaeometric studies. Therefore, the thermal behavior of two dolomite samples and the hydration and carbonation reaction in air of the decomposition products were investigated by Raman microspectroscopy. The increase in the calcination temperature resulted in the formation of – Raman silent MgO and – inert Mg calcite at 700 °C–750 °C. In contrast, the decarbonation, hydration, and recarbonation of sample material exposed to 750 °C–900 °C in a muffle furnace led to the appearance of Mg-free calcite. High spectral resolution Raman spectroscopy enabled a spectral distinction between these two groups due to differences in the band parameters (peak position, bandwidth) of the vibrational (v1, v4, L) modes of calcite. In combination with Raman microspectroscopic mapping, this spectral information represents a new approach for the estimation of burning temperatures of medieval high-fired gypsum mortars via natural dolomite impurities. Thus, the results of this work highlight the importance and potential of Raman microspectroscopy as a thermometric tool for elucidating the thermal history of anthropogenic fired materials, with potential applications for archaeometry and art technology, as well as for quality controls in the frame of the production of mineral mortar binders and ceramics or bricks, respectively.