Weber, S. U., Grodzicki, M., Geiger, C. A., Lottermoser, W., Tippelt, G., Redhammer, G. J., Bernroider, M. and Amthauer, G. (2007) Fe-57 Mossbauer measurements and electronic structure calculations on natural lawsonites. Physics and Chemistry of Minerals, 34 (1). pp. 1-9. DOI 10.1007/s00269-006-0121-y.

Abstract

Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by Fe-57 Mossbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. Fe-57 Mossbauer spectra are consistent with the assumption that high-spin Fe3+ substitutes for Al in the octahedrally coordinated site. The Mossbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with Fe-57 hyperfine parameters typical for octahedrally coordinated high-spin Fe2+. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe3+ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe3+ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe2+. However, it cannot be excluded that Fe2+ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H2O molecule.

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