A deuterium-hydrogen fractionation mechanism in magnesium oxide |
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Authors: | Friedemann Freund Heinz Wengeler Reinhard Martens |
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Institution: | Mineralogisches Institut, Universität zu Köln, Zülpicher Str. 49, D-5000 Köln 1, Fed. Rep. Germany |
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Abstract: | Water dissolved in a nominally anhydrous oxide like MgO forms not only OH? ions, as is usually assumed, but also molecular H2 and peroxy anions O22?. The specific lattice site where this reaction occurs has been identified for the model-type MgO structure: it is the fully OH? compensated cation vacancy. In partially deuterated synthetic MgO, molecular D2 forms preferentially to H2, by a factor of the order of 4. Three methods have been used to study this very large D/H fractionation experimentally: mass spectrometry, electron spin resonance and infrared spectroscopy. All give consistent results and confirm that internal D/H fractionation occurs and is a function of the temperature. Theoretically the D/H fractionation is explained by the lower zeropoint energy of D2 with respect to H2, amplified by secondary processes through which the molecular hydrogen species become mobilized and eventually lost via diffusion. The possible consequences of such internal D/H fractionation processes are indicated for the interpretation of water and hydrogen release patterns, with varying D/H ratios, from terrestrial and extraterrestrial samples. |
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