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Specimens of muscovite from Siluro-Devonian Appalachian granites of the Gander zone in New Brunswick were studied by 57Fe Mössbauer spectroscopy, microprobe analysis and X-ray powder diffractometry. Chemical compositions, corresponding structural formulae and powder patterns indicate that they are dioctahedral true micas of 2M1 polytype. Mössbauer spectroscopy shows that these muscovites fall into two groups having distinct spectra, despite an absence of systematic differences in their chemical compositions, X-ray patterns, unit-cell parameters, and Fe3+/Fetotal ratios. In the first group, two distinct and well-resolved viFe2+ spectral contributions occur whereas, in the second group, a single but broader viFe2+ contribution occurs. All spectra from both groups have viFe3+ contributions. These observations are confirmed by quadrupole splitting distribution (QSD) analyses of the spectra. Spectra from the first group clearly show a bimodal distribution of quadrupole splittings for Fe2+, with a dominant contribution at ~3.0 mm/s and a minor one at ~2.1 mm/s. In the second group, the spectra show a broad unimodal distribution of QSDs for Fe2+. We attribute the 3.0 and 2.1 mm/s QSD components to Fe2+ in cis and trans octahedral sites, respectively. Muscovites from our second group may have Fe2+ in both cis and trans sites but these cannot be resolved, as is usually the case, for example, with trioctahedral micas. In group one, cis/trans populations provide measures of the degree of cation order and of the density of vacancies on the cis sites. Simple models based on average unit cell site dimensions are found not to hold. Local effects seem to dominate, with Fe2+ showing no systematic preference for cis or trans sites. 相似文献
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Constantin Crâciun 《光谱学快报》2013,46(10):579-590
Abstract The influence of octahedral replacement of Al3+ by Fe3+ on the infrared spectra of montmorillonites in the domain of OH vibrations is presented. The obtained results suggest that in the domain of OH-bending vibration, the absorption band from 870 cm?1 (Al3+ -OH-Fe3+) can not be detected in the spectra of montmorillonites when the octahedral Fe3+ content is below 0.10-0.15 (per half unit cell) In the domain of OH-stretchtng, the increasing Fe3+ -for-Al3+ substitution in the dioctahedral series of smectites causes a continous shift of the absorption band ascribed to M3+ - OH - M3+ vibrations to lower frequencies. 相似文献
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