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