Summary
Chemical analytical and pyrolytical methods have been used to study the Fe+2/Fe+3 ratios and dehydroxylation reactions in synthetic biotites. It has been found for the biotites with Fe/(Fe + Mg) of 20 to 70 mole % that the oxidation degree decreases from 26 to 16% with increasing iron. Based on the measured amounts of water and hydrogen released during pyrolysis it is inferred that the deprotonization is a dominant reaction at low temperatures (T ⩽ 600°C), accompanied by dehydration as the temperature increases. Depending on the composition, a complete dehydroxylation takes place at T ⩾ 900 °C, and the measured amount of water corresponds to the iron oxidation degree in the starting samples. The results of this study have important implications with respect to determination of the formation conditions of biotite-bearing rocks, and also for improvement of the techniques for determination of different valence of iron and water.
Résumé
Des méthodes chimiques et pyrolitiques ont été utilisées pour l'étude des rapports Fe+2/Fe+3 et de la réaction de la déhydroxilation en biotites synthétiques. On a trouvé pour les biotites avec Fe/(Fe + Mg) de 20-70 mole % que le degré d'oxidation décroît à partir de 26 jusqu'à 16% pendent que le contenu du fer s'accroît. Sur la base de la quantité d'eau et hydrogène liberée pendant la pyrolyse, on infère que la déprotonisation est une réaction dominante à températures basses (T = 600°C), mais quand la température s'accroît, la déprotonisation est accopagnée de la déhydratation. Dépendant de la composition il y a une déhydroxilation complète à T = 900°C, et la quantité de l'eau mesurée correspond au degré d'oxidation du fer dans les specimens initials. Les résultats de cette étude infuencent la détermination des conditions de formation des roches contenantes biotite et aussi l'amélioration des techniques de la détermination du fer de valences différentes et de l'eau.
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Tikhomirova, V.I., Konilov, A.N. & Koshemchuk, S.K. The degree of oxidation of iron in synthetic iron-magnesian biotites. Mineralogy and Petrology 41, 41–52 (1989). https://doi.org/10.1007/BF01164809
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DOI: https://doi.org/10.1007/BF01164809