ISSN:
1572-9540
Keywords:
muscovite
;
Mössbauer spectroscopy
;
cis
;
trans
;
New Brunswick
;
site populations
;
vacancies
;
dioctahedral
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1012659830325
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