gems, lapis lazuli, Ultramarine pigment, Geo-sourcing, Sodalite structure, CO₂ clathrate

The source of the previously unassigned weak band at 2340 cm⁻¹ that is occasionally observed in the infrared spectrum of natural lapis lazuli pigment is here attributed conclusively to CO₂ trapped in the β-cage of lazurite. In addition, the geo-sourcing potential of this infrared feature for lapis lazuli is tested and found to be more ambiguous than previously suggested. The CO₂ absorption band is in fact observed in lapis lazuli samples from numerous disparate geographic locales including Afghanistan, Canada, Myanmar, Siberia, Tajikistan, and the Ural Mountains, as well as mineral hauyne samples from Germany and Italy. The spectral feature was absent from lapis lazuli samples from California, Chile, Colorado, and New York. Furthermore, poor quality lapis lazuli samples, regardless of locale, highly processed natural pigment samples, and all samples of synthetic ultramarine blue, green, and violet pigments were devoid of encapsulated CO₂.