New markers to identify the provenance of lapis lazuli: trace elements in pyrite by means of micro-PIXE

Lapis lazuli is one of the oldest semi-precious stone, being used for glyptic as early as 7000 years ago to produce jewels, amulets, seals, inlays, etc. Only few sources of lapis lazuli exist in the world due to the low probability of geological conditions in which it can form, so that the possibility to associate the raw material to man-made objects helps to reconstruct trade routes. Since art objects produced using lapis lazuli are valuable, only non-destructive investigations can be carried out to identify the provenance of the raw material. Ion Beam Analyses (IBA) are a good candidate for this task, especially if performed with a microbeam, because lapis lazuli is a rock composed by crystals of different minerals dimension from some micrometers to hundreds of micrometers. Some markers to distinguish among lapis lazuli from different provenances using IBA were presented in previous papers [1,2]: they are based on the presence or absence of a mineral phase in samples from a certain provenances, trace elements inside a phase or luminescence spectrum peculiar for a certain provenance. In this work a systematic study performed on pyrite crystals present in 24 lapis lazuli samples from four quarries is presented. It provided a multi-technique approach: optical microscopy and SEM-EDX to find out the crystals of interest and micro-PIXE measurements (both in vacuum at INFN Legnaro laboratory and with external beam at INFN LABEC laboratory in Florence) to evaluate the trace elements content inside the same crystals. Results are promising and the experimental differences among different provenances are significant. [1] A. Lo Giudice et al., Analytical and Bioanalytical Chemistry, 395 (2009) 2211-2217 [2] A. Re et al., Nucl. Instrum. & Meth. B, 269 (2011): 2373-2377