Characterization of the lapis lazuli from the Egyptian treasure of Tôd and its alteration using external µ-PIXE and µ-IBIL

Lapis lazuli is among the earliest and most prized ornamental stone worked to produce carvings, beads and inlays. The first lapis lazuli workshop excavated in Pakistan was dated from the 4 millennium BC and the use of this gem is attested since the 3 millennium BC in Mesopotamia and Ancient Egypt. Lapis lazuli is an heterogeneous metamorphic rock composed of minerals of the sodalite group like lazurite to which it owes its blue color intermixed with other minerals like calcite, diopside and pyrite. The only proven source of Lapis lazuli in Antiquity is Badakshan in Afghanistan while there are sources in Tajikistan and Russia. This work focuses on the lapis-lazuli of the Egyptian treasure of Tôd, dating from Amenemhat II (1911-1876 BC.), third king of the 12 dynasty, that was excavated in 1936 from the basement of a temple at Thebes and now conserved in the Louvre museum, Paris. The treasure contains thousands of blocks of raw lapis lazuli, minute fragments, beads and carvings stylistically dated to various periods. After with the corpus of lapis lazuli objects found in the royal tombs at Ur, it constitutes the largest set of ancient lapis left. The treasure of Tôd first raises the question of the use of lapis lazuli in ancient Egypt and of the relations with Mesopotamia and the producing countries, because there is no source of lapis in Egypt. In addition, most of Tôd artefacts show today a dull, whitish and stained appearance. It is important to understand the alteration process that has affected those elements during their burial in their copper chests, in order to help their preservation and possibly restore their initial appearance. A series of samples from the treasure were selected to be analysed using the 30 µm-size new external microbeam line of the AGLAE facility at the C2RMF. The system relies upon five fast counting SDD X-ray detectors coupled with an original scanning system combining vertical magnetic beam steering and horizontal target translation that allows to acquire PIXE maps (e.g. 200 x 200 pixels) on the entire artefacts within a short time (<30 min) [1]. The GUPIX-based processing of the raw maps yielded quantitative element maps that permitted to identify the mineral phases (lazurite, sodalite, pyrite, diopside, calcite, etc.) and their corresponding trace elements (Ba, Ti, As, Ni, etc). The ionoluminescence spectrum recorded for each pixel using a 200-900 nm fiber optic spectrometer provided an additional fingerprint of the luminescent mineral phases. The first objective was to identify the alteration products observed on the Egyptian artefacts, many deriving from the decomposition of the pyrite FeS whose nature was confirmed,using µ-XRD and Raman spectroscopy. The second goal was to compare the chemical fingerprints for artefacts from different periods to see if they match the historical deposit of Badakshan, Afghanistan, despite the alteration products. Indeed, the previous studies conducted on the reference lapis lazuli samples of certified provenance have demonstrated that it is possible to distinguish the lapis sources (Chile, Russia, Tajikistan, Afghanistan) using PIXE.