Vanessa Léa

Moganite detection in silica rocks using Raman and infrared spectroscopy

The quantitative determination of moganite in flint and chert plays an important role in the characterisation of these silica rocks and in the study of their genesis and evolution. Both Raman and infrared (IR) spectroscopy promise to be rapid and cost-effective tools for such studies. However, the use of vibrational spectra of moganite in silica rocks is hampered by the proximity of specific moganite bands with IR and Raman vibrations bands of non-bridging Si-O in silanol (SiOH) groups of chalcedony, the main coexisting silica phase. This may result in spectral interferences that lead to an overestimation of the moganite concentration. In order to calibrate quantitative moganite detection using IR and Raman spectroscopy, the spectra of chalcedony/moganite mixtures were studied using spectral decomposition. Heat treatment of the samples prior to their analysis is found to reduce the contribution of chalcedony silanol-bands to the measurement of the moganite bands, facilitating in this way the interpretation of the spectra. A new calibration curve is proposed for quantitative moganite detection using Raman spectroscopy. Infrared spectroscopy is also found to be useful for moganite quantification: a molar absorption coefficient of 43 L/mol·cm for the specific moganite-band at 575 cm−1 is derived for the first time. The exact position of the specific IR and Raman moganite-bands is found to depend on whether the mineral occurs intermixed with chalcedony or in pure form. This study opens new prospects for quantitative moganite detection in silica rocks using vibrational spectroscopy.

The Urgonian chert from Provence (France): the intra-formation variability and its exploitation in petro-archeological investigations

Understanding details of stone tool procurement and transfers is for a major research avenue in improving our knowledge about prehistoric societies. The accuracy of the provisioning sources identifications is based on the establishment of large regional repositories. Recent studies show that specific investigations on the evolution of cherts were effective in distinguishing primary sources from the various secondary sources of a raw material. In this paper, we focus on another difficulty that is the distinction between different primary sources of the same geological layers.We consider the specific case of the Bedoulian cherts from southeastern France. This chert was exploited and circulated over large distances during the whole prehistoric record. It is particularly known to have been heat-treated during Late Chassey culture (Neolithic). We show in this paper that paleogeographical variability exists due to variations in the bioclastic and detrital components. With the support of foraminifera data, the granulometry of detrital quartz grain provides the possibility to distinguish between different primary sources. A first test in archeological contexts illustrates the efficiency of the method as well as indicates major changes in provisioning practices between upper Paleolithic and Neolithic groups.