FTIR Imaging of lapis lazuli from Sar-E-Sang (Badakhshan, Afghanistan)

FTIR focal-plane-arrays (FPAs) of detectors enable different imaging modalities with a spatial resolution comparable to micro-Raman but without any damage of the sample. This technique has been applied with great success to the study of organic or biological materials (Petitbois et al., 2008), however very few applications have been so far reported in Earth Sciences. One example is provided by the study of the distribution of water in volcanic glasses (Wysoczanski and Tani, 2006). In this work we characterize a sample of lapis-lazuli from the type locality at Sar-e-Sang (Badachshan, Afghanistan) by combining SEM, EMP, X-ray fluorescence imaging, X-ray diffraction and FTIR spectroscopy. Lapis-lazuli is an ornamental stone that has been widely used in antiquity; art works consisting of this deep-blue material have been found in archeological excavations of the middle-east to north Africa since the neolithic age (Barthelemy de Saizieu, B., 1988). Of the (few) worldwide sources of lapis lazuli, the Sar-e-Sang deposit located in the Badakhshan province of Afghanistan is the most famous for the finest quality of its material, and is often cited as the source for most of the lapis lazuli used for decorative purposes up to Reinassance. Lapis lazuli is a complex rock rather than a pure mineral. At Sar-e-Sang, it origin is related to regional methamorphism on dolomitic limestones, and typically consist of a fine-grained assemblage of feldspathoid-type minerals, pyrite, calcite, wollastonite and dolomite (Wyart et al., 1981). Carbon dioxide has been recently reported in lapis lazuli from numerous geographic localities (Smith and Klinshaw II, 2008) and tested as a possible tool for geo-sourcing natural ultramarine pigments in archeological or art conservation studies. These works have been systematically done on powdered materials, therefore the carrier of CO2 has not been identified with certainty. Recently, Bellatreccia et al. (2009) have shown that carbon dioxide is a widespread constituent in sodalite-group minerals, which are among the main constituents of lapis lazuli. In the present work we use high-resolution FTIR imaging to examine in detail the distribution of CO2 across an apparently single crystal of lazurite from the Sar-e-Sang deposit. SEM and optical observations show that the sample is composed of a very fine-grained mixture of different minerals; combining XRD and EMP analyses these could be identified as albite + sodalite + lazurite + dolomite. FTIR imaging shows that extremely high CO2 contents are exclusively associated with lazurite. References Barthelemy de Saizieu, B. (1988) Les sépultures néolithiques de Mehrgarh. Réfexions sur les cultes funéraires anciens. PhD thesis, Université de Paris X, 2 vv. Bellatreccia, F., Della Ventura, G., Piccinini, M., Brilli, M. (2009) Mineralogical Magazine, in press. Petibois C., Piccinini M., Cestelli-Guidi M.A., Déléris G. and Marcelli A. (2009) Nature Phot. 3, 177 Smith, G.D., Klinshaw II, R.J. (2008) Wyart, J., Bariand, P., Filippi, J. (1981) Gems and Gemmology, 184-190. Wysoczanski, R., Tani, K. (2006) J. Volcan. Geoth. Res., 156, 302-314.