Lazurite with a composition close to [Na6Ca2 Al6Si6O24(SO4,S)2] has been synthesized by employing CaS + CaSO4 redox in hydrothermal-pressure experiments and reduction with Re foil in piston-cylinder experiments. A royal blue lazurite formed by annealing at 1200°C and 0.5 GPa; it has an Al-rich framework (Al/Si between 1.1 and 1.2) and coexists with minor amounts of a colorless sodalite phase of S-poor, Si-excess composition. The sodalite phase of experiments without a reducing agent is haüyne. The chemical state of sulfur in the synthetic products and in lazurite from Afghanistan and Baffin Island, Nunavut, has been investigated by S K-edge X-ray absorption near-edge structure (XANES) spectroscopy and S 2p X-ray photoelectron spectroscopy (XPS). Sulfate is the dominant species of sulfur in all samples of lazurite studied, and there are significant but subordinate amounts of sulfide bound only to Na+ cations. In addition, there is minor sulfite in some piston-cylinder products and minor native sulfur in the minerals. Polysulfides are not present above minimum levels of detection. Natural lazurite differs markedly from its synthetic equivalent in chemical composition and S K-edge XANES and S 2p3/2 XPS spectra, but these differences appear to have little bearing on color development. In synthetic haüyne, the cage sulfur species is exclusively sulfate.
- X-ray absorption near-edge structure (XANES)
- X-ray photoelectron spectroscopy (XPS)