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| Titel |
Comprehensive structural and chemical (CO2, Fe/Fe Mg, H2O) investigations of Mg-Fe cordierite with micro Raman spectroscopy |
| VerfasserIn |
U. Haefeker, R. Kaindl, P. Tropper |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067571
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| Zusammenfassung |
| The Mg-Fe silicate cordierite with the idealized formula (Fe, Mg)2Al4Si5O18 occurs
as a hexagonal and an orthorhombic polymorph with disordered/ordered Al-Si
distribution on the tetrahedral sites. Most of the natural cordierites are fully ordered.
Six-membered rings of (Si,Al)O4 are piled in the direction of the crystallographic c-axis and
form channels, laterally and vertically linked by additional (Al, Si) tetrahedrons.
Mg and Fe in varying fractions occupy the octahedrally coordinated M-sites. CO2
and H2O (and other volatiles) can be incorporated into the structural channels,
thus cordierite can be used for paleofluid reconstruction. The vibration energies of
incorporated volatiles, their interaction with the lattice and variations of certain
lattice-vibration energies caused by the Mg-Fe exchange can be determined with Raman
spectroscopy, allowing chemical quantifications and structural investigations. A method
for the semi-quantitative determination of CO2-contents of natural cordierites by
Kaindl et al. (2006) was optimized and enhanced by Haefeker et al. (2007). CO2
contents can be measured in single crystals and thin sections with an error of ± 0.05 -
0.09 wt.-%. Based on the Mg-Fe exchange with garnet, cordierite can be used as a
geothermobarometer. Recent investigations of synthetic Mg-Fe cordierites with XFe = 0 – 1
have shown a linear downshift of six selected lattice peaks between 100 and 1250
cm-1 with the Mg-Fe contents. Correlation diagrams allow an estimation of the
Mg-Fe contents in synthetic and natural samples. The experimental data are in
good agreement with the results of quantum-mechanical calculations of the Raman
spectra of Mg- and Fe cordierite (Kaindl et al., 2011) allowing the assignment of
the peaks to specific vibrations of tetrahedral and octahedral sites. Natural Mg-Fe
cordierites are mainly orthorhombic with a fully ordered Al/Si distribution on the
tetrahedral sites. However, the disordered hexagonal polymorph is observed in
many experiments. Raman spectroscopy allows easy distinguishing between the two
polymorphs by the splitting of a characteristic peak at ~569 cm-1. Crystallographic and
Raman spectroscopic data of the Fe endmember polymorphs are rare in literature,
therefore, Raman and single-crystal x-ray data of synthetic samples were collected and
compared with the well-known Mg and Mg-Fe cordierites. First compositional
Raman maps show a relation between the degree of ordering of Fe cordierite and the
amount of water incorporated into the channels. The effects of water incorporation
on the Raman spectra of Mg cordierites is currently being evaluated. Preliminary
investigations indicate a downshift of the peak at ~1186 cm-1with increasing water
contents.
Literature:
Kaindl, R., Tropper P., Deibl, I. (2006) A semi-quantitative technique for determination
of CO2in cordierite by Raman spectroscopy in thin sections. Eur. J. Mineral, 18,
331-335
Haefeker, U. (2007) Verbesserte semiquantitative Analyse von CO2 in natürlichem
Cordierit mit Hilfe der Mikro-Raman-Spektroskopie. Unpublished master thesis. University
of Innsbruck, 86p
Kaindl, R., Többens, D. M., Haefeker, U. (2011) Quantum-mechanical calculations of the
Raman spectra of Mg- and Fe-cordierite. American Mineralogist, 96, 1568-1574 |
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