Cordierite is a unique case of a microporous mineral stable under geological conditions spanning from the amphibolite facies to UHT metamorphism to crustal anatexis (Harley et al., 2002; Bertoldi et al., 2004). Because of its structural channels, it is able to trap H2O and CO2 (Armbruster and Bloss, 1980), and for this reason the analysis of the volatile constituents of cordierite may help constraining the geological conditions and the composition of coexisting fluids during its formation (Vry et al., 1990; Harley et al., 2002). It follows that the quantitative evaluation of the channel constituents is crucial in petrologic studies. We address here this point by studying, using a multidisciplinary approach, a set of cordierite samples from different occurrence and with different H2O/CO2 content. The aim of the study was a calibration of reliable absorption coefficients to be used for the quantitative microanalysis of H2O and CO2 in cordierite based on single-crystal polarized-light FTIR spectroscopy. The specimens were fully characterized by a combination of techniques including optical microscopy, single-crystal X-ray diffraction, EMP (electron micro probe), SIMS (secondary ion mass spectrometry), and FTIR spectroscopy. All cordierites are orthorhombic Ccmm; the 2Vα optic axis angle is linearly related to the CO2 content. According to the EMP data, the Si:Al ratio is always close to 5:4; XMg ranges from 76.31 to 96.63 and additional octahedral constituents occur in very weak amounts. Extraframework K and Ca are negligible, while Na may reach values as high as 0.84 apfu. SIM spectrometry shows H2O up to 1.52 and CO2 up to 1.11 wt%. Optically transparent single-crystals were oriented using a spindle-stage and examined under polarized light. On the basis of the literature data and the polarizing behaviour, the observed bands were assigned to water molecules in two different orientations and to CO2 molecules in the structural channels. The spectra also show the presence of 13C and 18O, and weak amounts of CO in the samples. FTIR imaging done using the novel FPA detector system clearly shows that at a m-scale the distribution of H and C in this mineral may be significantly inhomogeneous, and this must be taken into account when collecting analytical data for petrological purposes. This feature is probably less important for carbon molecules than it is for H2O, whose distribution within the crystal can be strongly affected by the geological history of the mineral after its formation. Armbruster, T. e Bloss, F. D. (1982) Am. Min., 67, 284-91. Bertoldi, C., Proyer, A., Garbe-Schönberg, D., Behrens, H., Dachs, E. (2004) Lithos 78, 389-409. Harley, S.L., Thompson, P., Hensen, B.J., Buick, I.S. (2002) J Metam Geol 20, 71-86. Vry, J.K., Brown, P.E., Valley, J.V. (1990) Am Min 75, 71-88.

Radica, F., Bellatreccia, F., DELLA VENTURA, G., Cavallo, A., Capitelli, F., Harley, S. (2011). The quantitative analysis of H2O and CO2 in cordierite using single-crystal polarized-light FTIR microspectroscopy, 4, 208.

The quantitative analysis of H2O and CO2 in cordierite using single-crystal polarized-light FTIR microspectroscopy

BELLATRECCIA, FABIO;DELLA VENTURA, Giancarlo;
2011-01-01

Abstract

Cordierite is a unique case of a microporous mineral stable under geological conditions spanning from the amphibolite facies to UHT metamorphism to crustal anatexis (Harley et al., 2002; Bertoldi et al., 2004). Because of its structural channels, it is able to trap H2O and CO2 (Armbruster and Bloss, 1980), and for this reason the analysis of the volatile constituents of cordierite may help constraining the geological conditions and the composition of coexisting fluids during its formation (Vry et al., 1990; Harley et al., 2002). It follows that the quantitative evaluation of the channel constituents is crucial in petrologic studies. We address here this point by studying, using a multidisciplinary approach, a set of cordierite samples from different occurrence and with different H2O/CO2 content. The aim of the study was a calibration of reliable absorption coefficients to be used for the quantitative microanalysis of H2O and CO2 in cordierite based on single-crystal polarized-light FTIR spectroscopy. The specimens were fully characterized by a combination of techniques including optical microscopy, single-crystal X-ray diffraction, EMP (electron micro probe), SIMS (secondary ion mass spectrometry), and FTIR spectroscopy. All cordierites are orthorhombic Ccmm; the 2Vα optic axis angle is linearly related to the CO2 content. According to the EMP data, the Si:Al ratio is always close to 5:4; XMg ranges from 76.31 to 96.63 and additional octahedral constituents occur in very weak amounts. Extraframework K and Ca are negligible, while Na may reach values as high as 0.84 apfu. SIM spectrometry shows H2O up to 1.52 and CO2 up to 1.11 wt%. Optically transparent single-crystals were oriented using a spindle-stage and examined under polarized light. On the basis of the literature data and the polarizing behaviour, the observed bands were assigned to water molecules in two different orientations and to CO2 molecules in the structural channels. The spectra also show the presence of 13C and 18O, and weak amounts of CO in the samples. FTIR imaging done using the novel FPA detector system clearly shows that at a m-scale the distribution of H and C in this mineral may be significantly inhomogeneous, and this must be taken into account when collecting analytical data for petrological purposes. This feature is probably less important for carbon molecules than it is for H2O, whose distribution within the crystal can be strongly affected by the geological history of the mineral after its formation. Armbruster, T. e Bloss, F. D. (1982) Am. Min., 67, 284-91. Bertoldi, C., Proyer, A., Garbe-Schönberg, D., Behrens, H., Dachs, E. (2004) Lithos 78, 389-409. Harley, S.L., Thompson, P., Hensen, B.J., Buick, I.S. (2002) J Metam Geol 20, 71-86. Vry, J.K., Brown, P.E., Valley, J.V. (1990) Am Min 75, 71-88.
2011
Radica, F., Bellatreccia, F., DELLA VENTURA, G., Cavallo, A., Capitelli, F., Harley, S. (2011). The quantitative analysis of H2O and CO2 in cordierite using single-crystal polarized-light FTIR microspectroscopy, 4, 208.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11590/174887
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