Quantification of Sulfur Speciation in Apatites: Comparison of the XAS , XPS and EPMA Techniques

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Abstract
Quantifying sulfur (S) speciation in apatite‐group minerals (apatites) is critically important for understanding magmatic and magmatic‐hydrothermal processes but has generally been restricted to individual analytical techniques. In this study, we have quantified sulfur speciation in a suite of apatites (including fluorapatite, hydroxylapatite and chlorapatite) using three complementary analytical techniques: S K ‐edge X‐ray absorption near‐edge structure (XANES), S 2 p X‐ray photoelectron spectroscopy (XPS), and S K α wavelengths with electron probe micro analysis (EPMA). Our results show that all three techniques identify S 6+ as the dominant sulfur species, with minor contributions from S 4+ , in the apatite samples investigated in this study. Sulfur K ‐edge XANES offers the most precise quantification of S 6+ /ΣS values (where ΣS = S 6+ + S 4+ + S 2‐ ) from 0.93 to 1.0, with the standard deviation from ±0.01 to ±0.06, although it requires synchrotron access and careful calibration. Sulfur 2 p XPS analyses yield a similar trend in S speciation but lower S 6+ /ΣS values from 0.69 to 0.78, attributable to partial radiation‐induced reduction of S 6+ to S 4+ related to prolonged X‐ray exposure. Similarly, the EPMA‐based (S K α wavelength shift) technique is capable of distinguishing between S 6+ and S 4+ , and yields S 6+ /ΣS values ranging from 0.56 to 0.88, again indicating significant radiation‐induced reduction. Therefore, the XPS and EPMA‐based techniques are also capable of quantifying sulfur speciation in apatites but require accurate corrections for radiation‐induced sulfur reduction. This study highlights the strengths and limitations of these three analytical techniques, with XANES offering the highest sensitivity. At the same time, XPS and EPMA, although less sensitive, are surface‐specific and widely accessible, respectively. It also guides their applications for quantifying sulfur speciation in apatites.
Funding Information
  • Natural Sciences and Engineering Research Council of Canada (RGPIN 03255‐2023)
  • National Natural Science Foundation of China (41921003)

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