Characterization of granites by 57Fe Mössbauer spectroscopy
Two granite complexes in Egypt, a sodic type and an aluminous type are characterized by Mössbauer spectroscopy. Mössbauer spectra (MS) of the sodic granite show a major doublet of ferric (Fe3+) iron that is attributable to octahedral coordination (M1) sites plus/minus a tetrahedron Fe3+ doublet plus/minus a doublet of ferrous (Fe2+) iron on the M1 sites plus/ minus another Fe2+ (M1) doublet and a sextet of Fe3+. The sextet is attributed to α-Fe2O3 (hematite) and the other Fe components are due to NaCaFeSi2O6 (aegirine-augite) plus/minus minor contributions from (Ca2(Mg, Fe)5(Si, Al)8O22(OH)2 (magnesium-hornblende). Changes in the quadrupole splitting and width line of Fe2+ ions are likely composition-related. The MS of the aluminous-type granite, on the other hand, shows evidence only of single doublets containing Fe2+ or Fe3+ in the octahedral M1 sites, with parameters that remain almost constant. This consistency implies that the existing minerals - K(Mg, Fe2+)3 (Al, Fe3+)Si3O10(OH, F)2 (biotite), (Mg, Fe)6(Si, Al)4O10(OH)8 (clinochlore), (Na, K)Ca2(Fe, Mg)5(Al, Si)8O22(OH)2 (ferrohornblende and magnesiohornblende) - have similar iron positions. The intensity of iron oxidized (Fe3+/ΣFe) for the sodic granite is 79.1 to 100% and for the aluminous granite, 28.4 to 38.2%. The observed Fe3+/ΣFe differences between the two granites are source-related and consistent with distributions of other redox-sensitive elements.
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