Abstract
Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe2+ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe2+/Fe total ratio on An content demonstrates that Fe2+ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe3+ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe3+ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe3+, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.
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Hofmeister, A.M., Rossman, G.R. Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy. Phys Chem Minerals 11, 213–224 (1984). https://doi.org/10.1007/BF00308136
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DOI: https://doi.org/10.1007/BF00308136