Abstract

Sequences of four edge-shared FeO6 octahedra in the babingtonite structure are interrupted by the substitution of Mn2+ ions for Fe2+ and by Mn-induced stacking faults. These compositional and structural features influence Mössbauer spectral parameters of babingtonite, particularly at low temperatures where magnetic ordering of Fe occurs. In babingtonite from Massachusetts containing small amounts of Mn, the temperature of magnetic ordering, TM, is as high as 18.9 K. However, TM decreases with increasing Mn content and is lowest in Norwegian babingtonite (TM = 14.5 K) known to contain high densities of chain periodicity faults (CPFs). The magnetic hyperfine splitting parameter, H, determined from 4.2-K Mössbauer spectra is highest for Massachusetts babingtonite, but decreases in other specimens containing progressively higher Mn concentrations. Ratios of Fe3+/(Fe2+ + Fe3+) determined from computed peak areas of 4.2-K and 295-K spectra are highest in Mn-rich babingtonite. Correlations of isomer shift parameters for Fe2+ and Fe3+ in babingtonite with other mixed valence Fe2+-Fe3+ minerals indicate that the low velocity Fe2+ and Fe3+ peaks are almost exactly superimposed in 295-K Mössbauer spectra of babingtonite.

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