57Fe Mössbauer measurements and electronic structure calculations on natural lawsonites

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.