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The behaviour of a synthetic 57Fe-doped kaolin: Mössbauer and electron paramagnetic resonance studies

Published online by Cambridge University Press:  09 July 2018

A. H. Cuttler
A. H. Cuttler*
Affiliation:
Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA
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Abstract

The thermal behaviour of a ferrous doped kaolin has been studied by Mössbauer spectroscopy and electron paramagnetic resonance spectroscopy. From the observations it is concluded that the iron substitutes trioctahedrally as Fe2+ in the ‘gibbsite-like’ sheet in place of dioctahedral aluminium. The g = 2 EPR signal is shown to be associated with these ferrous ‘cells’ which appear to occur in clusters. It is suggested that these ferrous cells are trapped within the normal dioctahedral aluminium structure. Dehydroxylation of the ferrous iron cells takes place between 623 and 673 K leading to the formation of an iron-rich pyroxene and, by 723 K, a ferric oxide. At temperatures > 723 K the pyroxene itself oxidizes to a second ferric oxide. The EPR signal changes at 623 K and disappears at 723 K. The signal is attributed to a trapped hole induced by X-irradiation, located near a silicon atom on the boundary between normal dioctahedral cells and trioctahedral Fe2+ cells. It is possible to extend the model to explain some puzzling features concerning the g = 2 EPR signals reported by other authors and to propose other effects which might result from the presence of these cells.

Résumé

Résumé

Le comportement thermique d'un kaolin dopé en Fe(II) a été étudié par spectroscopie Mössbauer et résonance paramagnétique électronique. On peut en déduire que le fer se substitue trioctaédriquement, sous forme de Fe2+, dans la couche du type gibbsite, à l'aluminium dioctaédrique. On montre que la signal de RPE (g = 2) est associé avec ces ‘mailles’ ferreuses qui semblent se présenter en amas. On suggère que ces mailles ferreuses sont piégées dans la structure dioctaédrique alumineuse normale. La deshydroxylation de ces mailles ferreuses se produit entre 623–673 K, conduisant à la formation d'un pyroxène riche en fer et vers 723 K, à un oxyde ferrique. A des températures supérieures à 723 K, le pyroxène lui-même est oxydé en un oxyde ferrique secondaire. Le signal RPE change à 623 K et disparaît à 723 K. Ce signal est attribué à une lacune piégée, induite par rayonnement X, localisée près d'un atome de silicium à la frontière entre les mailles dioctaédriques normales et les mailles trioctaédriques ferreuses. Il est possible d'étendre le modèle en vue d'expliquer quelques faits troublants concernant les signaux RPE (g = 2) rapportés par d'autres auteurs et de proposer d'autres effets qui semblent résulter de la présence de ces mailles.

Kurzreferat

Kurzreferat

Das thermische Verhalten von Fe(II)-dotiertem Kaolin wurde mittels Mössbauer Spektroskopie und Elektronenresonanz Spektroskopie untersucht. Aus den erhaltenen Daten wird geschlossen, daß in der ‘gibbsitähnlichen’ Schicht das dioktaedrische Aluminium durch trioktaedrisches Fe2+ substituiert wird. Es wird gezeigt, daß das g = 2 EPR Signal auf diese, anscheinend in Cluster auftretenden Fe (II)- ‘Zellen’ zurückzuführen ist. Weiterhin muß angenommen werden, daß die Fe(II)- ‘Zellen’ in einer normalen dioktaedrischen Aluminiumstruktur ‘gefangen’ sind.

Die Dehydroxylierung der Fe(II)- ‘Zellen’ findet zwischen 623–673 K statt und führt zur Bildung eines eisenreichen Pyroxens. Bei 723 K entsteht ein Eisen (II) oxid. Bei Temperaturen > 723 K wird der Pyroxen instabil und es bildet sich Eisen (III) oxid.

Das EPR-Signal ändert sich bei 623 K und verschwindet bei 723 K. Das Signal ist auf eine eingefangene Leerstelle, die durch Röntgenstrahlung entstanden ist, zurückzuführen, die neben einem Siliziumatom liegt und zwar auf der Grenze zwischen normalen dioktaedrischen Zellen und trioktaedrischen Fe2+-Zellen. Dieses Modell kann möglicherweise auch dazu verwendet werden solche schwierigen g = 2 EPR-Signale zu deuten, wie sie von anderen Autoren beschreiben werden. Ebenso können damit andere Erscheinungen abgeleitet werden, die sich aus der Existenz solcher ‘Zellen’ ergeben.

Resumen

Resumen

Se ha estudiado el comportamiento térmico de un caolín impurificado con material ferroso, mediante la espectroscopia de Mössbauer y la espectroscopia de resonancia paramagnética de los electrones. Se deduce de las observaciones que el hierro sustituye trioctaédricamente como Fe2+ al aluminio dioctaédrico en la lámina semejante a gibbsita. La señal g = 2 RPE se muestra que está vinculada a estas ‘células’ férreas que parecen producirse en grupos. Se sugiere que estas células férreas quedan atrapadas dentro de la estructura dioctaédrica normal del aluminio. La deshidroxilación de las células férreas tiene lugar a entre 623–673 K, conduciendo a la formación de un piroxeno rico en hierro y, a 723 K, un óxido férrico. A temperaturas > 723 K el propio piroxeno se oxida formando un segundo óxido férrico. La señal de resonancia paramagnética cambia a 623 K y desaparece a 723 K. La señal se atribuye a un hueco atrapado, inducido por la irradiación con rayos X, situado cerca de un átomo de silicio en el límite entre las células dioctaédric as normales y las trioctaédricas de Fe2+. Es posible ampliar el modelo para explicar algunas características confusas concernientes alas señales g = 2 RPE comunicadas por otros autores y para proponer otros efectos que pudieran resultar de la presencia de estas células.

Type
Research Article
Information
Clay Minerals , Volume 15 , Issue 4 , December 1980 , pp. 429 - 444
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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