Chemical state of sulfur in natural and synthetic lazurite by S K-edge XANES and X-ray photoelectron spectroscopy

Lazurite near [Na (sub 0) Ca (sub 2) Al (sub 6) Si (sub 6) O (sub 24) (SO (sub 4) ,S) (sub 2) ] in composition was synthesized by using CaS + CaSO (sub 4) redox in hydrothermal-pressure experiments and reduction with Re foil in piston-cylinder experiments. A royal blue lazurite was formed by annealing at 1200 degrees 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 colourless sodalite phase of S-poor, Si excess composition. Experiments without a reducing agent gave hauyne. The chemical state of sulphur in the synthetic products and in lazurite fom Afghanistan and Baffin Island has been investigated by S K-edge X-ray absorption near-edge structure (XANES) and S 2p X-ray photoelectron spectroscopy (XPS). Sulphate is the dominant sulphur species in all samples of lazurite examined, and there are significant but subordinate amounts of sulphide bound only to Na (super +) cations. In addition, there is minor sulphite in some of the piston-cylinder products and minor native sulphur in the minerals. Natural lazurite differs markedly from its synthetic equivalent in chemical composition and S K-edge XANES and S 2p (sub 3/2) XPS spectra, but these differences appear to have little bearing on colour development. In hauyne, the cage sulphur species is exclusively sulphate.