Analysis of Iron Oxidation in Garnets
Abstract The oxidation state of iron indicates the amount of oxygen present when a mineral is formed. If the environment was abundant in oxygen, many of the minerals in the assemblage will contain oxidized iron, Fe3+. If the environment is more reducing, there is likely to be more Fe2+. The amount of oxygen present influences the elements that interact in a magma, as well as which minerals form; this is known as oxygen fugacity (ƒo2). The fundamental ƒo2 directly describes the potential for multivalent cations to occur in one of its valence states and is therefore a direct measurement of the oxidation state. Iron is one of the most common multivalent rock-forming cations. Due to their differences in both size and charge, Fe2+ and Fe3+, can occupy different sites in a mineral structure. Given that oxidation state is important, it is then important to have ways of measuring the oxidation state. In the late 1960s, Mössbauer spectroscopy was first used to determine Fe2+ and Fe3+. Mössbauer, however, is a bulk technique that requires a large, homogenous sample. Accurate and precise Mössbauer measurements also require knowledge of the recoil-free fractions (ƒ) for Fe2+ and Fe3+ in specific sites for each mineral group, but these correction factors had not been determined for garnets. The first goal of this project was to measure ƒ for both Fe2+ and Fe3+ in garnet, in order to improve the accuracy of Mössbauer measurements of Fe3+/∑Fe in garnets. The resultant values for ƒ at 295K and 80K were 0.60 and 0.88 for Fe3+, 0.84 and 0.93 for Fe2+, respectively. It would be desirable to have a method of measuring samples at microanalytical scale, such as XANES spectroscopy. For the second part of my project, I compared corrected Mössbauer results to XANES measurements on a suite of 20garnet samples with varying amounts of Fe2+ and Fe3+. Many of the samples are from Gore Mountain, Barton Mine in New York, but due to the lack of significant Fe3+ content of the Gore Mountain garnets, a wider range of samples was chosen. The percentage of Fe3+ determined by Mössbauer was compared to the area, intensity, and energy of the pre-edge peaks in the XANES data to test how well the XANES technique can determine Fe2+ and Fe3+. Mössbauer spectroscopy and XANES results complement each other. Mössbauer and XANES data measure approximately the same percentage of Fe3+ content. The percentages of Fe3+ and Fe2+ according to Mössbauer and XANES, revealing that both techniques agree well within ± 8%, with the exception of 2 samples out of 20. This conclusion suggests that XANES studies of anisotropic minerals might be successful if, as in the case for this study, the optical orientation of the crystals is the same as the optical orientation of the standards. The combination of Mössbauer spectroscopy and XANES provides a promising outlook for microanalytical techniques in the near future.