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Role of Iron Reduction in The Conversion of Smectite to Illite in Bentonites in the Disturbed Belt, Montana

Published online by Cambridge University Press:  01 July 2024

Eric Eslinger ,
Patrick Highsmith ,
Doyle Albers  and
Benjamin deMayo
Eric Eslinger
Affiliation:
Department of Geology, West Georgia College, Carrollton, Georgia 30118
Patrick Highsmith
Affiliation:
Department of Geophysical Sciences, Georgia Institute of Technology, Atlanta, Georgia 30303
Doyle Albers
Affiliation:
Department of Geology, University of Idaho, Moscow, Idaho 83843
Benjamin deMayo
Affiliation:
Department of Physics, West Georgia College, Carrollton, Georgia 30118
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Abstract

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Cretaceous bentonites were collected in outcrop from the Sweetgrass Arch and the Disturbed Belt in Montana. The mixed-layer illite-smectite (I/S) components of the bentonites from the Sweetgrass Arch have from 0 to 25% illite layers and no detectable structurai Fe2+, whereas the samples from the Disturbed Belt have from about 25 to 90% illite layers, and all contain Fe2+. A positive correlation (r = 0.89) exists between the percentage of structural iron that is Fe2+ and the amount of fixed interlayer K in the I/S.

The higher percentage of illite layers in the samples from the Disturbed Belt is attributed to reactions related to elevated temperatures caused by burial beneath thrust sheets. The increase in Fe2+/Fe3+ with increasing percentages of illite layers is tentatively attributed to a redox reaction involving the oxidation of organic matter. Although there is no statistical evidence for an increase in octahedral charge with an increase in illite layers when all the samples are considered together, iron reduction may have contributed as much as 10 to 30% of the increase in total structural charge that occurred in any given sample during metamorphism. The remaining structural charge increase can be attributed to the substitution of Al3+ for Si4+ in the tetrahedral sites.

Резюме

Резюме

Бентониты мелового периода были собраны в обнажениях Свитграс Арк и Дистербд Белт в Монтане. Смешанно-слойные компоненты бентонитов иллит-смектит (И/С) из Свитграс Арк имеют 0 до 25% слоев иллита и в них не было обнаружено структурного Fе2+, тогда как пробы из Дистербд Белт имеют примерно от 25 до 90% слоев иллита, и все содержат Fе2+. Положительная корреляция (г = 0,89) существует между процентным содержанием структурного жедеза (Fе2+) и количеством фиксированного межслоя К в И/С.

Большое процентное содержание слоев иллита в пробах из Дистербд Белт относится за счет реакций, присходящих при повышенных температурах, обусловленных залеганием бентонитов под надвиговыми пластами. Увеличение Fе2+/Fе3+ с повышением процентного содержания слоев иллита, на основе опыта, связывается с окислительно-восстановительной реакцией, включающей окисление органического материала. Хотя при рассмотрении всех образцов не существует статистических доказательств увеличения октаэдрического заряда с увеличением содержания слоев иллита, восстановление железа могло обусловить от 10 до 30% увеличения общего структурного заряда, который возник в каждом образце в течение метаморфизма. Остающееся увеличение структурного заряда может быть отнесено за счет замещения Аl3+ кремнием Si4+ в тетраэдрических слоях.

Resümee

Resümee

Kreidehaltige Bentonite wurden in Zutageliegen vom “Sweetgrass Arch” und “Disturbed Belt” in Montana gesammelt. Die Illit-Smektit (I/S) Anteile der Wechselschicht der Bentonite vom “Sweetgrass Arch” haben von 0 bis 25% Illitschichten und kein feststellbares strukturelles Eisen2”1”, wohingegen die Proben vom “Disturbed Belt” ungefähr von 25 bis 90% illitschichten haben und alle enthalten Eisen2+. Eine positive Abhängigkeit (r = 0,89) besteht zwischen dem Prozentsatz des strukturellen Eisen, welches als Eisen2”1” vorkommt und der Menge der gehärteten Zwischenschicht K im I/S.

Der höhere Prozentsatz von Illitschichten in den Proben vom “Disturbed Belt” wird Reaktionen zugeschrieben, die mit erhöhten Temperaturen verbunden sind, die durch Vergrabung unter Schubschichten erzeugt wurden. Die Zunahme in Eisen2+/Eisen3+ mit zunehmendem Prozentsatz von Illitschichten wird versuchsweise einer Redoxreaktion zugeschrieben, in der Oxidation von organischem Material beteiligt ist. Wenn alle Proben zusammen berücksichtigt werden, gibt es keine statistischen Beweise für die Zunahme in oktaedrischer Ladung mit Zunahme in Illitschichten. Eisenreduktion könnte jedoch soviel wie 10 bis 30% zu der Zunahme in gesamtstruktureller Ladung, welche in jeder gegebenen Probe vorkommt, beitragen. Der Rest der strukturellen Ladungszunahme kann der Substitution von Si4+ mit Al3+ in den tetraedrischen Plätzen zugeschrieben werden.

Résumé

Résumé

Des bentonites du Crétacé ont été collectionés d'un affleurement de Sweetgras Arch et du Disturbed Belt du Montana. Les constituants illite-smectite à couches mélangées (I/S) des bentonites de Sweet-gras Arch ont de 0 à 25% de couches d'illite et pas de Fe2+ détectible, alors que les échantillons du Disturbed Belt ont de 25 à 90% de couches d'illite, et tous contiennent Fe2+. Une corrélation positive (r = 0,89) existe entre le pourcentage de fer de composition qui est Fe2+ et la quantité de K fixe intercouche dans I/S.

Le pourcentage plus élevé des couches d'illite dans les échantillons du Disturbed Belt est attribué à des réactions liées à de hautes températures causées par l'enterrement sous des nappes de charriage. L'augmentation de Fe2+/Fe3+ allant de pair avec l'augmentation des pourcentages des couches d'illite est tentativement attribué à une réaction rédox impliquant l'oxidation de matière organique. Bien qu'il n'y ait aucune évidence statistique d'une augmentation de charge octaèdrique accompagnant une augmentation dans les couches d'illite lorsque tous les échantillons sont considérés ensemble, la réduction de fer peut avoir contribué de 10 à 30% à l'augmentation de la charge totale de composition qui s'est passée pendant le métamorphisme de tout échantillon. Le reste de l'augmentation de charge de composition peut être attribué à la substitution d'Al3+ à Si4+ dans les sites tétraèdriques.

Keywords

BentoniteIlliteIron reductionMixed layerMontanaSmectite
Type
Research Article
Information
Clays and Clay Minerals , Volume 27 , Issue 5 , October 1979 , pp. 327 - 338
Copyright
Copyright © 1979, The Clay Minerals Society

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