Determination of ferrous–ferric iron contents in tourmaline using synchrotron-based XANES

Abstract

The complex geochemistry of tourmaline makes it an important tool in determining its formational environment. Typically, tourmaline chemistry is analyzed through electron-probe microanalysis (EPMA), but this analytical tool cannot determine directly the oxidation states of transition elements such as Fe (Fe2+, Fe3+). Direct quantitative measurement of these cations is important in minerals to acquire a more complete chemical characterization and informative structural formula. Synchrotron-based X-ray Absorption Near Edge Spectroscopy (XANES) is a method to directly measure Fe2+ and Fe3+ in minerals, including tourmaline. This method utilizes advances in software and detector technology to significantly decrease data processing time and errors. Three tourmaline samples, dravite, povondraite, and oxy-schorl, analyzed by combining XANES and EPMA data, exhibit distinct ferrous–ferric contents using the pre-edge and absorption edge methods. These analyses reveal, respectively: 99.62–100 wt. % Fe2+ in dravite, 12.5–20.00 wt. % Fe2+ vs. 87.48–100 wt. % Fe3+ in povondraite, and 63.03wt. % Fe2+ vs. 36.98–36.41 wt. % Fe3+ in schorl. Information on the oxidation states of Fe results in enhanced charge-balanced constraints that allow improved estimation of the H contents in the tourmaline and a more accurate designation of the structural formula and classification of tourmaline species. Thus, XANES is a viable technique to obtain oxidation states of transition elements in tourmaline.