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In this study, we use micro-X-ray absorption near-edge structures (μ-XANES) spectroscopy at the S  K -edge to investigate the oxidation state of S in natural magmatic-hydrothermal apatite (Durango, Mexico, and Mina Carmen, Chile) and experimental apatites crystallized from volatile-saturated lamproitic melts at 1000 °C and 300 MPa over a broad range of oxygen fugacities [(log( f  O 2  ) = FMQ, FMQ+1.2, FMQ+3; FMQ = fayalite-magnetite-quartz solid buffer]. The data are used to test the hypothesis that S oxidation states other than S 6+  may substitute into the apatite structure. Peak energies corresponding to sulfate S 6+  (~2482 eV), sulfite S 4+  (~2478 eV), and sulfide S 2−  (~2470 eV) were observed in apatite, and the integrated areas of the different sulfur peaks correspond to changes in  f  O 2   and bulk S content. Here, multiple tests confirmed that the S oxidation state in apatite remains constant when exposed to the synchrotron beam, at least for up to 1 h exposure (i.e., no irradiation damages). To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S 2− ), intermediate (S 4+ ), and oxidized (S 6+ ) S in variable proportions as a function of the prevailing  f  O 2   of the system.

Co-variability of S 6+ , S 4+ , and S 2− in apatite as a function of oxidation state: Implications for a new oxybarometer