Luminescence Properties of Lapis Lazuli Investigated by means of a Proton Micro-Beam

Lapis lazuli is a blue semi-precious stone that has been used since the Neolithic Era (VII millennium BC) for the manufacturing of precious carved artefacts. Our group is studying this rocks since 2008, mainly for a provenance study purpose [1]. In fact a systematic study of rock samples coming from different known sources could allow finding some markers discriminating various provenances. The possibility to identify the extraction sites of the rock in a non-invasive way, can give information on artefacts and consequently on ancient trade routes [2]. Lapis lazuli is an aggregate of several different minerals, in the form of crystals ranging from sub-micrometric to millimetric sizes. The possibility to analyse a single crystal is one of the main features to use a proton micro-beam to probe the samples. The main markers we found until now are related to trace elements present in some mineralogical phases, mainly in diopside and pyrite, and to luminescence properties of diopside and wollastonite [1-6]. Luminescence is a promising technique to discriminate lapis lazuli provenances also in a non-invasive way: using an in-air extracted microbeam, luminescence has been acquired both on precious artefacts [4,6] and archaeological samples [2]. For these reasons we decided to investigate the luminescent properties of other different minerals forming lapis lazuli, analyzing the semi-thin sections obtained from lapis lazuli samples from the Museo di Storia Naturale of the Università di Firenze.