COMBINED SEM-EDS AND μ-XRF ANALYTICAL APPROACH FOR A PROVENANCE STUDY OF LAPIS LAZULI

摘要

Lapis lazuli is a semi-precious blue stone that is being used for different purposes since the ancient times. Archaeological objects like beads, gems, seals and small decorative artworks made of lapis lazuli are widely distributed in the Ancient East as early as the second half of the Ⅳth millennium BC, however, information about both its trade and the quarries exploited by different civilisations is still highly incomplete. The main sources of lapis lazuli in ancient times are widely considered the Badakhshan deposits in Afghanistan. However, other quarries (mainly the Pamir Mountains, Siberian area and Chagai Hills in Pakistan) could have possibly been exploited since antiquity. Despite several studies on lapis lazuli, have been carried out from the historical, geological, geochemical, and archaeometrical points of view, an exhaustive provenance study is still lacking. This study consists in a multi-analytical approach aimed at obtaining petrographic and minerochemical information in order to identify peculiar markers that reflects different supplying quarry districts for lapis lazuli. In addition to the Asian rocks (from Afghanistan, Pamir and Siberia), we included in this study rocks coming from the currently exploited reservoir of Ovalle (Chile). A petrographic and mineralogical characterisation of several samples was first performed to determine the texture and the mineralogical assemblage of the rock samples. Through this study it was recognized the characteristic lazurite-wollastonite association in the Chilean samples and the lazurite-diopside association for the Asian provenances, being lazurite the ubiquitous mineral responsible for the blue colour. In order to quantify both major and trace elements in mineral phases, the SEM-EDS measurements were integrated with measurements performed by means of a μ-XRF system (Eagle Ⅲ-XPL). Indeed, a prominent application of μ-XRF technique may be represented by the compositional analysis of trace elements in ornamental stones by a non-invasive approach. Hence, the trace element measurements were focussed on two mineral phases: diopside, usually associated with the lazurite in samples of Asian provenance, and pyrite, occurring in lapis lazuli from all provenances. The variation in the trace element contents in diopside (Ti, V, Mn, Cr, Sr and Zr) and pyrite (Ni and Cu), is significant to allow to ascertain the origin of the lapis lazuli rocks. The proposed trace elements approach has been finally applied to small polished slabs made in lapis lazuli used for artworks from the Museum of Natural History of Torino. They have been analyzed by means of μ-XRF only, being a non-invasive technique and, therefore, suitable for cultural heritage studies.