SPECTROSCOPIC GEO-SOURCING OF LAPIS LAZULI USED IN ARTISTS' PIGMENT PRODUCTION

Of the few worldwide sources of lapis lazuli, the Sar-e-Sang deposit located in Afghanistan has garnered the most attention for its high quality and purported use as a source of artists' pigment in antiquity. Previously, scientists at the Getty Conservation Institute (GCI) reported a sharp infrared (IR) spectral feature at 2340 cm-1 said only to be found in the Afghanistan mineral. This newly discovered IR band was proposed as a means of identifying historical artists' pigments which were obtained through trade routes extending to Afghanistan. Although unverified in the original case study, the feature was tentatively attributed to häuyne. These hypothesis were tested by analyzing microsamples of mineral specimens from lapis deposits around the world, as well as minerals typically associated with or mistaken for lapis lazuli.

More than 50 natural and synthetic mineral specimens and pigments have so far been collected for the study. These samples include examples of synthetic and natural ultramarine pigments, blue mineral specimens such as lazurite, lazulite, häuyne, sodalite, scorzalite, and afghanite, as well as specimens of lapis lazuli rocks from Afghanistan, Chile, Lake Baikal, the Ural Mountains, Tajikistan, California, Colorado, Canada, and Burma.

Of the samples analyzed to date, the IR absorption band has in fact been found in all of the lapis lazuli specimens except for those from California and Chile. When present, the absorbing moiety is not always seen in every dark blue microsample, hinting at its heterogeneous composition within the mineral. The feature has not been observed in pure specimens of other blue minerals known to associate with lazurite in lapis lazuli, including the earlier suspected häuyne, nor in any synthetic ultramarine. Manipulations of lapis lazuli microsamples using heat and acid – as well as spectral comparisons with sulfide compounds - suggest that the source of the 2340 cm-1 band might be one of the polysulfide enclathrates previously identified as the source of color in lapis lazuli. While the presence of this spectral feature does not appear to facilitate the geo-sourcing of historic pigments, these experiments have enriched our understanding of the comparative mineralogy of lapis lazuli worldwide.