Soft X-ray Absorption Spectroscopy and Imaging of Sulfur in Lapis Lazuli

Since antiquity, lapis lazuli has been highly valued across many cultures for its bright blue color. Due to the material’s significance, there has long been interest in understanding its color variations and determining its geographic origin, whether used as the processed pigment ultramarine in painted works of art (e.g. paintings and manuscripts) or the raw lapis lazuli stone in cultural heritage objects (e.g. jewelry and inlaid decorations). While the most well-known source of lapis lazuli is Afghanistan, there are several other sources, including sites in Tajikistan, Iran, Russia, Canada, and Chile. Naturally occurring lapis lazuli contains the blue mineral lazurite (Na6Ca2(Al6Si6O24)(SO4,S3,S2,Cl,OH)2) with a variety of accessory minerals that are common to many of the known geological deposits—e.g. hauyne (Na3Ca(Si3Al3)O12(SO4)), pyrite (FeS2), calcium carbonate (CaCO3), and diopside (MgCaSi2O6). Much of the current research on the provenance of lapis lazuli has focused on the spectroscopic characteristics, the composition, and the overall distribution of the accessory minerals within the whole rock [1-4]. For example, work in our laboratory has shown that diopside inclusions in lapis lazuli sometimes have a fluorescent response to infrared wavelengths which may be characteristic of the geological deposit [5]. One challenge with basing provenance on the composition and distribution of accessory minerals is that the accessory minerals are mostly removed during the production of the ultramarine pigment, a process that requires crushing, sorting, and soaking the rock in an alkaline solution. Thus, a technique that focuses on in-situ analysis of the lazurite mineral alone, which remains chemically unaltered during this intensive processing, is desirable. Expanding on our previous work, this study therefore focuses on the lazurite component of lapis lazuli.