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| Titel |
IR-based Water Quantification in Nominally Anhydrous High-Pressure Minerals |
| VerfasserIn |
Monika Koch-Müller, Dieter Rhede |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032854
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| Zusammenfassung |
| Infrared spectroscopy is a powerful tool to determine traces of OH and H2O in minerals and
glasses. The application is based on the Beer Lambert law A = É*c*t, where A is the
absorbance, É the absorption coefficient, e.g. in L mol H2O-1 cm-2, c the concentration in
mol/L and t the thickness in cm. It has been shown in numerous experimental and theoretical
studies, i.e. Paterson (1982) and Libowitzky and Rossman (1997) that É generally increases
with decreasing wavenumbers. However, this general trend seems to be valid only for
hydrous minerals and glasses and should not be applied to water quantification
in nominally anhydrous minerals (NAMs) which incorporate traces of water in
their structures (e.g. Rossman 2006, Thomas et al. 2009). For example, Bell et al.
(2003) showed that if the general IR calibration of Paterson (1982) is adopted, the
water concentration of olivine is underestimated by about 25 %. A similar result
has been obtained by Deon et al. (2010) for Mg-wadsleyite. Thomas et al. (2009)
evidenced using a large variety of analytical methods that not using mineral-specific
IR-calibrations for the OH quantification in NAMs (e.g. SiO2 polymorphs and olivine) leads
to either underestimation as for olivine or overestimation of the water content as
for stishovite and coesite. Thus, to quantify the water content of NAMs mineral
specific absorption coefficients are needed but unfortunately only for a few minerals
available.
In this study we propose that within a polymorphic mineral series of the same
composition É positively correlates with the density and negatively with the molar volume of
the respective mineral phase. To prove this hypothesis we determined É-values for synthetic
hydrous ringwoodite samples ranging in composition from xMg = 0.0 to 0.6 by combining
results of FTIR-spectroscopy with those of Secondary Ion Mass Spectrometry. The É-values
plot well below the general calibration curves of Paterson (1982) and Libowitzky and
Rossman (1997) but follow the same trend, i.e. they increase with decreasing wavenumbers
of the OH bands from 59000 ± 6000 for the Fe-endmember to 85800 ± 10000
L mol H2O-1 cm-2 for the Mg-richest sample. From these data we can predict
an absorption coefficient for the iron-free Mg-endmember as 100000 ± 7000 L
mol H2O-1 cm-2. This value together with É-values for forsterite (Thomas et al.
2009) and wadsleyite (Deon et al. 2010) confirm the negative correlation with the
molar volume and positive correlation with the density within this polymophic
series. This allows us to predict absorption coefficients for some minerals, where
coefficients for one or better two of their polymorphs, either high- or low-pressure, are
available.
References
Bell, D. R., Rossman, G. R., Maldener, J., Endisch, D. & Rauch, F. (2003)
Hydroxide in olivine; a quantitative determination of the absolute amount and calibration
of the IR spectrum. Journal of Geophysical Research, B, Solid Earth and Planets
108.
Deon, F, Koch-Müller, M., Rhede, D., Gottschalk, M., Wirth, R. and Thomas, S.-M.
(2010) Location and quantification of hydroxyl in wadsleyite: new insights. Am Mineral (in
press).
Libowitzky, E. & Rossman, G. R. (1997) An IR absorption calibration for water in
minerals. Am Mineral 82, 1111-1115.
Paterson, M. S. (1982) The determination of hydroxyl by infrared absorption in quartz,
silicate glasses and similar materials. Bull Minéral 105, 20-29.
Rossman, G. R. (2006) Analytical methods for measuring water in nominally anhydrous
minerals. In: Keppler H, Smyth JR (eds) Water in nominally anhydrous minerals. Reviews in
Mineralogy and Geochemistry, vol 62. Mineralogical Society of America, Chantilly, pp
1-28.
Thomas, S.-M., Koch-Müller, M., Reichart, P., Rhede, D., Thomas, R., Wirth, R. and
Matsyuk, S. (2009) IR calibrations for water determination in olivine, r-GeO2 and SiO2
polymorphs. Physics and Chemistry of Minerals, 36, 489 - 509. |
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