Water solubility in nominally anhydrous minerals measured by FTIR and 1H MAS NMR: the effect of sample preparation |
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| Authors: | H Keppler M Rauch |
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| Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, 95440 Bayreuth, Germany e-mail: Hans.Keppler@uni-bayreuth.de, DE
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| Abstract: | Samples of enstatite and forsterite were crystallized in the presence of a hydrous fluid at 15 kbar and 1100 °C. Water contents
in quenched samples were measured by 1H MAS NMR and by FTIR. If the samples were prepared in the same way, similar water concentrations were obtained by both methods.
There is no evidence that one or the other method would severely over or underestimate water contents in nominally anhydrous
minerals. However, measured water contents vary by orders of magnitude depending on sample preparation. The lowest water contents
are measured by polarized FTIR spectroscopy on clear, inclusion-free single crystals. These water contents probably reflect
the real point defect solubility in the crystals. Polycrystalline material shows much higher total water concentrations, presumably
due to hydrous species on grain boundaries, growth defects, and in submicroscopic fluid inclusions. Grinding the sample in
air further increases water concentration. This effect is even more pronounced if the sample is ground in water and subsequently
dried at 150 °C. Polarized FTIR measurements on clear single crystals of enstatite saturated at 15 kbar and 1100 °C give 199 ± 25 ppm
by weight of water. The spectra show sharp and strongly polarized bands. These bands are also present in spectra measured
through turbid, polycrystalline aggregates of enstatite. However, in these spectra, they are superimposed on much broader,
nearly isotropic bands resulting from hydrous species in grain boundaries, growth defects, and submicroscopic fluid or melt
inclusions. Total water contents for these polycrystalline aggregates are between 2000 and 4000 ppm. Water contents measured
by FTIR on enstatite powders are 5300 ppm after grinding in air and 12 600 ppm after grinding under water und subsequent drying
at 150 °C.
Received: 25 June 1999 / Revised, accepted: 4 October 1999 |
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| Keywords: | Enstatite Forsterite 1H NMR FTIR Water Nominally anhydrous minerals |
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