Determination of cis and trans Fe2+ populations in 2M1 muscovite by Mössbauer spectroscopy

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⁺/Fe_total 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.