Accurate determination of ferric iron in garnets

Abstract

Numerous techniques are available to determine the amount of Fe 2+ and Fe 3 + in minerals. Calculating Fe 2+ and Fe 3 + by charge-balance using electron probe microanalysis (EPMA) data is the most common method, but several studies question the usefulness and accuracy of this approach (Canil and O’Neill 1996; Dyar et al. 1993, 2012; Lalonde et al. 1998; Li et al. 2005; McGuire et al. 1989; Schingaro et al. 2016; Schmid et al. 2003; Sobolev et al. 2011). We compile and compare data for natural garnets that have been analyzed by both EPMA and Mossbauer spectroscopy. Comparison of Fe 3 + / ∑Fe determined by charge-balance vs. Mossbauer spectroscopy shows an approximate 1:1 correlation. The EPMA data set of Dyar et al. (2012) is reexamined and it is shown that disagreement between EPMA and Mossbauer for their data is not nearly as bad as reported. Data for charge-balance vs. Mossbauer spectroscopy are compared and show that the EPMA/charge-balance approach provides a suitable alternative when other methods are not practical.