Determination of Oxidation Slates, CoordinationNumbers and ratios of Iron Cations in Trioctahedral Micas by Mossbauer Spectroscopy

Category Petrology
Group GSI.IR
Location 20th symposium on geosciences
Holding Date 16 April 2008

     Mossbauer speetra of 56 biotite speeimens from granitic rocks of the Canadian Appalachians are examined to evaluate the oxidation state and site occupancics of fe cations. Mineral analyses were obtained by wavelength – dispersive X-ray spectrometry using the JEOL 8900 Superprobe.
All room temperature spectra of biotite exhibit remarkable similarity and have three main absorption peaks centered at about -0.1,+1,0 and +2,3 mm/s, which respectively correspond to 1) the low-energy line of both octahedral Fe2+ and octahedral Fe3+ quadrupole doublets, 2) the high-energy lines of octahedral Fe3+ quadrupole doublets. None of these spectra shows a shoulder at about +0.4 mm/s corresponding to high energy-lines of quadrupole doublets of Fe3+ in tetrahedral sites.
In their study of biotite compositions from granitic rocks of the South Mountain batholith of the Canadian Appalachinas, MacKenzic and Clarke (1974) determined the Fe3+/Feratio of biotite by conventional wet-chemical methods and obtained values ranging from 0.06 to 0.37 for the 18 specimens. The spread of Fe3+/Fe values obtained by Mossbauer spectroscopy, from 0.05 to 0.23, including several specimens of the same batholith is considerably smaller. This reflects the poor precision of conventional wet-chemical determinations as performed in most laboratories. Mossbauer spectroscopy is far more precise than most commonly-performed oxidimetric wet-chemical methods. In other words, the wet-chemical methods appear to overestimate ferric. Iron contents. Probably because of oxidation of Fe2+ during the acid decomposition.

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