Neutron diffraction on lapis lazuli: characterisation of rocks and artworks for a provenance study

Lapis lazuli is a semi-precious blue stone, being used for glyp- tic as early as 7000 years ago: jewels, amulets, seals and inlays are examples of objects produced using this material. Only few sources of lapis lazuli exist in the world due to the low probability of geological conditions in which it can be for- med [1], so that the possibility to associate the raw material to man-made objects is helpful to reconstruct trade routes. This is especially true for ancient contexts where there is an absence or scarceness of written evidences [2]. Although the Badakhshan mines in Afghanistan are widely considered as the only sources of the lapis lazuli in ancient times [2-4] other sources have been taken in consideration: Tajikistan (Pamir Mountains) [3,5], Pakistan (Chagai Hills) [5,6], Siberia (near Lake Baikal) [3], Iran [3] and Sinai [7] (these last two not geologically confirmed and their interpretations still debated [4]). Results obtained using physico-chemical analysis (AAS) on limited quantities of cut wastes from Shahr-i-Sokhta [5] are in agreement with a Pamir Mountains and Chagai Hills provenance, other than Badakhshan. Moreover, there is some evidences that also in Renaissance the Chagai Hills deposits was used as source of lapis lazuli to produce ultramarine blue pigment [6]. Nevertheless a systematic and exhaustive provenance study of the raw material used for artworks is still lacking and lapis lazuli provenance is then a still open question. Since art and archaeological objects produced using lapis lazuli are valuable, only non-invasive techniques can be used to identify the provenance of the raw materials. For this kind of investigation Prompt Gamma Activation Analysis (PGAA) was proposed and preliminary studies on rock samples were promising [8]. Our group started a provenance study of this material in 2007, using a protocol based on a multitechnique approach [9-11]. Due to the heterogeneity of lapis lazuli it is very dif- ficult to identify provenance markers analysing the mean elemental composition of the whole rocks or artworks, but the different phases composing lapis lazuli rocks have to be identified to give some clues about provenances. Thanks to the high penetration of neutrons, it has been possible to recognize the different phases composing several rocks, through the analysis of the diffraction patterns obtained at the time of flight (TOF) diffractometer INES (Italian Neutron Experimental Station) operating at ISIS facility (RAL, UK). In this contribution the results obtained on 13 rocks from 4 different provenances and on 3 artworks from the XIX cen- tury Savoy Collection (Museo Regionale di Scienze Naturali) [12] will be presented. The differences among the sources can be used to have some indications about the provenance of artworks and objects made in lapis lazuli whose origin is not known. These results can be extended to archaeological samples and can be a good starting point to answer still debated archaeological questions. Bibliography [1] Wyart J. et al, Gems & Gemmology 17 (1981) 184-190 [2] Tosi M., Gururajamanjarika, Studi in onore di G. Tucci, Ist. Univ. Orientale, Napoli (1974) 3-22 [3] Herrmann G., Iraq 30(1) (1968) 21-57 [4] Herrmann G. et al, Reallexikon der Assyriologie und Vorderasiatischen Archäologisch, ed. W. de Gruyter, Berlin (1983), 6: 489-492 [5] Casanova M., South Asian Archaeology, World Archaeology, Prehistory Press, 14 (1992) 49-56 [6] Ballirano P. et al, American Mineralogist, 91 (2006) 997-1005 [7] Nibbi A. “ Ancient Egypt and some Eastern Neighbours ”. Park Ridge, Noyes Publication (1981) 2: 33-55 [8] Zöldföldi J. et al, Proceeding of 34th Int. Symp. on Archaeometry (Zaragoza, Spain, 2004), (2006) 353-360 [9] Lo Giudice A. et al, Analytical and Bioanalytical Chemistry 395(7) (2009) 2211-2217 [10] Re A. et al, Nuclear Instruments and Methods in Physics Research B 269(20) (2011) 2373-2377 [11] Re A. et al, Applied Physics A 111(1) (2013) 69-74 [12] Gallo L.M., Cataloghi, XVI, Torino, (2004) 287pp