Micro-Raman spectroscopy for identification of calcite types in historical mortars: applications in archaeometry

Radiocarbon dating of mortars may present many issues in its application. The datable component is represented by anthropogenic calcite, namely lime binder of mortar, which is the carbon fraction to be isolated for dating [1]. Anthropogenic calcite is typically mixed with aggregates of several different compositions, even carbonaceous, and under-burnt rock fragments used for production of lime; may be present may alter the original carbon content. A complete characterization of the mortar before radiocarbon dating is mandatory to identifythe type of mortar and to develop an efficient analytical approach that allows to select the most suitable component of mortar for absolute dating. A further analysis on selected portions of sample is necessary, thus non-destructive techniques need to be found to identify the type of calcite in the sample [2]. This would allow to completely characterize the material truly used for dating, avoiding its contamination by other carbon sources. Given the widespread use of infrared spectroscopy to distinguish between calcites formed by different processes, from order/disorder in the calcite crystal structure [3-5], we extended this approach to Raman spectroscopy. A wide range of different carbonate rock (geogenic calcite) and lime binder of mortar (anthropogenic calcite), obtained, respectively, by different geological and archaeological/historical contexts were selected. A high-resolution micro-Raman spectrometer was used to explore the possibility to differentiate the calcite originated by anthropogenic or geological sources. Frequency, intensity (height) and peak area of the normal vibrations of carbonate groups were considered; Key Influence Factor analysis (KIF) and Principal component analysis (PCA) were used, respectively, for visual inspection and reducing dimensionality of dataset from different calcite types. Some parameters collected successfully classified calcite from different domains, opening new perspectives in Raman spectroscopy and heritage science