SYNCHROTRON XANES: AN ALTERNATIVE TECHNIQUE FOR MEASURING FE³⁺ CONTENT IN TOURMALINE

Tourmaline is an important gem mineral with a plethora of striking colors. The wide variety and intensity of these colors result, in part, from the presence of transition elements and their occupancy in the two types of octahedral sites in the tourmaline structure. One element that influences tourmaline’s color is the transition element Fe which can exist as Fe²⁺ and/or Fe³⁺. Currently, most tourmalines are chemically characterized by electron microprobe (EMP) analyses, however, the oxidation states of transition elements cannot be measured directly by EMP resulting in significant errors in stoichiometry when using EMP results alone. Direct measurement of Fe²⁺/Fe³⁺can be done by Mossbauer spectroscopy; however, this is not very commonly available. An alternative approach is to use Synchrotron-based X-ray Absorption Near Edge Spectroscopy (XANES). This technique allows the direct measurement of Fe²⁺ and Fe³⁺ of individual tourmaline crystals by comparing the spectral analysis to standards of known Fe²⁺ and Fe³⁺ contents.

The XANES analyses for this study were conducted on three selected samples of tourmaline with variable Mg and Fe²⁺/Fe³⁺contents: dravite, schorl, and povondraite. Fe K-edge spectra were recorded at the high energy absorption spectroscopy beamline at the LSU synchrotron. Bulk ratios of Fe³⁺/ΣFe and Fe²⁺/ΣFe measured for each sample were determined from the normalized spectral peaks via a linear combination fitting against spectra from standards of FeO and Fe₂O₃ within an uncertainty of +/-5% relative. This fitting technique results in iron contents of: 100% Fe²⁺ in dravite; 49% Fe³⁺/51 %Fe²⁺ in schorl; and 100% Fe³⁺ in povondraite. The direct percentages of Fe²⁺ and Fe³⁺ were used in combination with EMP data resulting in a more accurate structural formula. These data show that coupling EMP data with XANES data is a powerful approach to obtain more complete tourmaline chemistry when tourmaline contains transition elements. Such data sets can improve the interpretation of complex geochemistry and, thereby, enhance the understanding of the dynamic formation environments as well as color characteristics of tourmaline.