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Mössbauer, Thermomagnetic, and X-Ray Study of Cation Ordering and High-Temperature Decomposition in Biotite

Published online by Cambridge University Press:  01 July 2024

V. I. Bagin
Affiliation:
Institute of Geophysics, Academy of Sciences, Moscow, U.S.S.R.
T. S. Gendler
Affiliation:
Institute of Geophysics, Academy of Sciences, Moscow, U.S.S.R.
L. G. Dainyak
Affiliation:
Geological Institute, Academy of Sciences, Moscow, U.S.S.R.
R. N. Kuz'min
Affiliation:
Moscow State University, Moscow, U.S.S.R.
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Abstract

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To explain the large values of the quadrupole splittings and line widths in the Mössbauer spectrum of oxybiotite, a formalized analysis of the distribution of effective charges in the anion network was conducted using an idealized model of the biotite octahedral layer. Based on this analysis, a qualitative physical model has been proposed, according to which the Mössbauer spectrum consists of a superposition of six quadrupole doublets. The proposed model explains the appearance of residual magnetization well below the biotite decomposition temperature. Annealing the biotite at 1070°K leads to the formation of nuclei of the magnetic phase in chains consisting of R3+ and vacancies. This phase is the decomposition product of biotite at 1370°K, i.e., a ferrispinel having Fe2−xMgAlxO4 (x = 0.5–0.6) composition.

Резюме

Резюме

Для объяснения больших значений квадрупольного расщепления и полуширины линии в Мессбауэровском спектре оксибиотита, в данной работе проводится анализ распределения формальных эффективных зарядов в узлах анионной сетки для идеализированной модели октаэдрического слоя биотита. На основе этого анализа предлагается физическая модель, согласно которой мессбауэровский спектр оксибиотита представляет собой суперпозицию шести дублетов квадрупольного расщепления. Предложенная модель позволяет объяснить возникновение остаточной намагниченности задолго до температуры распада биотита. Отжиг биотита при 1070°К приводит к образованию зародышей магнитной фазы в цепочках, состоящих из R3+ и вакансий, являющейся продуктом распада биотита при 1370°К.

Resümee

Resümee

Um die großen Werte der Quadrupol-Aufspaltung und der Linienbreiten im Mössbauerspektrum von Oxybiotit zu erklären, wurde unter Verwendung eines idealisierten Modells der Biotit-Oktaederschicht eine formalisierte Analyse der Verteilung der effektiven Ladungen im Anionen-Netzwerk durchgeführt. Aufgrund dieser Analyse wurde ein qualitatives physikalisches Modell vorgeschlagen, nach dem das Mössbauerspektrum aus einer Überlagerung von 6 Quadrupol Dubletten besteht. Das vorgeschlagene Modell erklärt das Auftreten von Restmagnetisierung weit unter der Zersetzungstemperatur des Biotits. Glüht man den Biotit bei 1070°K, so führt das zur Bildung von Keimen der magnetischen Phase in Ketten, die aus R3+ und Leerstellen bestehen. Diese Phase ist das Zersetzungsprodukt von Biotit bei 1370°K, d.h. einem Fer- rispinell mit der Zusammensetzung Fe2-xMgAlxO4 (x = 0,5–0,6). [U.W.]

Résumé

Résumé

Pour expliquer les valeurs élevées des scissions quadrupolaires et des largeurs des droites dans le spectre de Mössbauer de l'oxybiotite, une analyse formalisée de la distribution de charges éffectives dans le circuit des anions a été faite, utilisant un modèle idéalisé de la couche octaédrique de la biotite. Sur la base de cette analyse, un modèle physique qualitatif a été proposé, selon lequel le spectre de Mössbauer consiste en une superposition de six doublets quadrupolaires. Le modèle proposé explique l'apparition de magnétisation résiduelle bien en dessous de la température de décomposition de la biotite. Si on tempère la biotite à 1070°K, on obtient la formation de noyaux de la phase magnétique en chaînes consistant de R3+ et de vides. Cette phase est le produit de décomposition de la biotite à 1370°K, c.à.d., un ferrispinel de composition Fe2-xMgAlxO4 (x = 0.5–0.6). [D.J.]

Type
Research Article
Copyright
Copyright © Clay Minerals Society 1980

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