Abstract
Oxidation of octahedral ferrous iron in biotite by saturated bromine water results in a loss of both octahedral and interlayer cations. The hydroxyl adjacent to vacant octahedral cation sites adopt an inclined orientation resulting in a more stable environment for interlayer cations. The only structural change accompanying these processes is a decrease in b-axis dimension which is linearly related to octahedral ferric iron content. These findings are in agreement with observations made on naturally weathered biotites.
Résumé
L’oxydation du fer ferreux octaédrique de la biotite par l’eau de brome saturée entraîne à la fois une perte de cations octaédriques et interfeuillets. L’hydroxyle adjacent au site cationique octaédrique vacant adopte une orientation inclinée entraînant un environnement plus stable pour les cations interfeuillets. Le seul changement de structure qui accompagne ces phénomènes est une diminution du paramètre b qui est relié linéairement à la teneur en fer ferrique octaédrique. Ces observations sont en accord avec celles que l’on peut faire sur les biotites altérées naturellement.
Kurzreferat
Die Oxydation von oktaedrischem Ferro-Eisen in Biotit durch gesättigtes Bromwasser ergibt einen Verlust an oktaedrischen sowie zwischenschichtigen Kationen. Die, freien oktaedrischen Kationenstellen benachbarten, Hydroxyle nehmen eine geneigte Richtung ein wodurch sich eine stabilere Umgebung für Zwischenschichtkationen ergibt. Der einzige, diese Vorgänge begleitende, Gefügewechsel ist eine Abnahme der b-Achsendimension, die in linearer Beziehung zu dem oktaedrischen Ferri-Eisengehalt steht. Diese Befunde stimmen überein mit Beobachtungen, die an natürlich verwitterten Biotiten gemacht wurden.
Резюме
Окисление октаэдрического железистого иона в биотите посредством насыщенной бромом водой, ведет к потере как октаэдрических так и межслоевых катионов. Гидроксил рядом с незаполненными октаэдрическими местонахождениями катионов принимают наклонную ориентацию в результате чего получается более стабильная среда для прослоенных катионов. Единственное структурное изменение сопровождающее эти процессы является уменьшение размера оси-b, линейно связанной с октаэдрическим содержанием железа. Эти данные совпадают с наблюдениями над естественно выветренными биотитами.
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Gilkes, R.J., Young, R.C. & Quirk, J.P. The Oxidation of Octahedral Iron in Biotite. Clays Clay Miner. 20, 303–315 (1972). https://doi.org/10.1346/CCMN.1972.0200507
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DOI: https://doi.org/10.1346/CCMN.1972.0200507