Incorporation of H+ in nominally anhydrous minerals
The presence of water in the earth's crust and mantle has enormous effects on geodynamic processes, because the incorporated hydrogen changes the physical properties of the minerals such as the melting point, rheological behavior (hydrolytical weakening) and the transformation kinetics. Besides in nominally hydrous minerals such as amphibole, serpentine and talc, hydrogen may be stored in nominally anhydrous minerals (NAMs) such as coesite, pyroxene, garnet and olivine. The hydrogen in these minerals is structurally bound as point defects. An appropriate method for detecting traces of hydrogen in minerals is IR spectroscopy. However, Transmission electron microscope analyses of olivine (Khisina et al., 2001) and clinopyroxene (Koch-Müller et al., 2004) showed that the presence of nm-sized hydrous phase inclusions might influence the IR-spectra. Consequently, the amount of hydrogen solved in the crystal structure will be overestimated. Therefore, for correct interpretation of IR-spectra knowledge of the microstructure of the investigated grains is required. We also apply confocal Raman-spectroscopy in combination with the comparator technique (Thomas, 2000) to quantify traces of H in NAMs (Thomas et al. 2008, 2009).
© Prof. Monika Koch-Müller, GFZ
- Image of an omphacite crystal and some IR spectra taken on the spots indicated by the lines with synchrotron radiation. The beam size is illustrated by the red square. In combination with TEM investigations we could show that the spectra contain contributions from nm-sized inclusions of sheet silicates.
Currently we investigate the incorporation mechanism and the amount of hydrogen in nominally anhydrous minerals such as:
We apply transmission electron microscopy to check for nm-sized inclusions and use Secondary Ion Mass Spectrometry to quantify the H and trace element concentrations. In combination with IR spectroscopy we determine mineral-specific absorption coefficients for NAMs (e.g. Koch-Müller and Rhede, 2010). To get insight into the location of hydrogen in the structure we apply linear polarized conventional as well as synchrotron IR radiation (Electron storage ring BESSY at Helmholtz Centre Berlin for Materials and Energy) at ambient condition and in-situ as a function of pressure and/or temperature.
M. Koch-Müller, R. Wirth, D. Rhede, B. Wunder, M. Gottschalk
Ulrich Schade, Helmholtz Centre Berlin for Materials and Energy
N. Khisina, Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
S. Matsyuk and M. Taran, Institute of Geochemistry, Mineralogy and Ore Formation, The National Academy of Science s of Ukraine, Kyiv, Ukraine
Sylvia-Monique Thomas, Department of Geoscience, University of Nevada, Las Vegas, USA