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Mineralogy and Genetic Relationships of Tonstein, Bentonite, and Lignitic Strata in the Eocene Yegua Formation of East-Central Texas

Published online by Cambridge University Press:  02 April 2024

A. L. Senkayi ,
J. B. Dixon ,
L. R. Hossner ,
M. Abder-Ruhman  and
D. S. Fanning
A. L. Senkayi
Affiliation:
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
J. B. Dixon
Affiliation:
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
L. R. Hossner
Affiliation:
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
M. Abder-Ruhman
Affiliation:
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
D. S. Fanning*
Affiliation:
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
*
1Department of Agronomy, University of Maryland, College Park, Maryland 20742.
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Abstract

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A kaolinite-rich bed (tonstein) and an associated bentonite in the upper part of Yegua Formation at College Station, east-central Texas, were formed by in situ weathering processes in a late Eocene swamp. X-ray powder diffraction, infrared spectroscopy, petrographic studies, and scanning and transmission electron microscopy not only show that dioctahedral smectite and coarsely crystalline kaolinite are the dominant minerals in the bentonite and tonstein, respectively, but that cryptocrystalline halloysite and kaolinite are localized along the weathering front (transitional zone) between the tonstein and the bentonite. As weathering progressed, the cryptocrystalline minerals gradually recrystallized to yield the coarse books and vermicular growths of kaolinite characteristic of the tonstein.

Small amounts of cristobalite, sanidine, and euhedral zircon crystals with liquid or gaseous inclusions accord with the formation of the bentonite by alteration of volcanic ash. Clinoptilolite in the lignitic layer and sandstone below the bentonite probably formed from ions that were released during alteration of the volcanic materials to smectite, but clinoptilolite in the tonstein and overlying strata appear to have formed after kaolinization of the bentonite.

Резюме

Резюме

Пласт, обогащенный каолинитом (тонштейн) и ассоциированный бентонит в верхней части формации Егуа в Коледж Стейшин в восточно-центральном Тексасе, формировались путем виутри-пластовых процессов эрозии в болоте позднего эоцена. Рентгеновская порошковая дифракция, инфракрасная спектроскопия, петрографические исследования, а также сканнинговая и трансмисионная микроскопия указывали не только на то, что диоктаэдрический смектит и грубо кристаллический каолинит являются главными минералами в бентоните и тонштейне, соответственно, но что крип-токристаллический галлуазит и каолинит расположены вдоль фронта эрозии (переходная зона) между тонштейном и бентонитом. Когда эрозия прогрессировала, криптокристаллические минералы постепенно рекристаллизировались, чтобы производить грубые книгоподобные и червеобразные росты каолинита, характеристические для тонштейна. Небольшие количества кристобалита, санидина и идиоморфных кристаллов циркона с жидкими или газовыми включениями согласовывались с формированием бентонита путем изменения вулканического пепела. Клиноптилолит в слое бурого угля и песчаника ниже бентонита формировался, вероятно, из ионов, которые выделялись во премя видоизменения вулканических материалов в смектит, но клиноптилолит в тонштейне и перекрывающих породах, вероятно, образовывался после каолинизации бентонита. [E.G.]

Resümee

Resümee

Eine Kaolinit-reiche Lage (Tonstein) und ein damit zusammenhängender Bentonit im oberen Teil der Yegua Formation bei College Station, Ostzentraltexas, wurden durch in situ Verwitterungsprozesse in einem späteozänen Moor gebildet. Röntgenpulverdiffraktometrische, infrarotspektroskopische und petrographische sowie raster- und transmissionselektronenmikroskopische Untersuchungen zeigten nicht nur, daß dioktaedrischer Smektit und grobkristallisierter Kaolinit die vorherrschenden Minerale in dem Bentonit bzw. in dem Tonstein sind, sondern daß auch kryptokristalliner Halloysit und Kaolinit entlang der Verwitterungsfront (Übergangszone) swischen Tonstein und Bentonit auftreten. Wenn die Verwitterung fortgeschritten ist, rekristallisierten die kryptokristallinen Minerale allmählich und führten zu dem groben bücherförmigen und wurmähnlichen Wachstum von Kaolinit, das für den Tonstein charakteristisch ist.

Geringe Gehalte an Cristobalit, Sanidin, und idiomorphen Zirkonkristallen mit flüssigen oder gasförmigen Einschlüssen stimmen mit der Bildung des Bentonites durch Umwandlung von vulkanischer Asche überein. Klinoptilolit in den Lignitlagen und im Sandstein unter dem Bentonit wurde wahrscheinlich aus Ionen gebildet, die während der Umwandlung des vulkanischen Materials zu Smektit freigesetzt wurden. Der Klinoptilolit in dem Tonstein und den darüberliegenden Schichten scheint nach der Kaolinisierung des Bentonites gebildet worden zu Scin. [U.W.]

Résumé

Résumé

Un lit riche en kaolinite (tonstein) et une bentonite associée dans la partie supérieure de la Formation Yegua à College Station, centre-est du Texas, ont été formés par un procédé d'altération in situ dans un marais du Bas Eocène. La diffraction aux rayons-X, la spectroscopic infrarouge, des études petrographiques, et la microscopie électronique et à transmission d’électrons montrent non seulement que la smectite dioctaèdrale et la kaolinite grossièrement cristallisée sont les minéraux dominants dans la bentonite et la tonstein respectivement, mais que l'halloysite cryptocristalline et la kaolinite sont localisées le long du front d'altération (zone de transition) entre la tonstein et la bentonite. Au fur et à mesure de la progression de l'altération, les minéraux cryptocristallins se sont graduellement recristallisés pour donner les livres grossiers et les croissances vermiculaires caractéristiques de la tonstein.

De petites quantités de cristobalite, de sanidine, et de cristaux euhédraux de zircon avec des inclusions liquides ou gazeuses s'accordent avec la formation de la bentonite par altération de la cendre volcanique. La clinoptilite dans la couche lignitique et dans le grès en dessous de la bentonite s'est probablement formée à partir d'ions qui avaient été relâchés pendant l'altération de matériaux volcaniques en smectite, mais la clinoptilolite dans la tonstein et les strates susjacentes semble avoir été formée après la kaolinisation de la bentonite. [D.J.]

Keywords

BentoniteClinoptiloliteKaoliniteLigniteSmectiteTonsteinWeathering
Type
Research Article
Information
Clays and Clay Minerals , Volume 32 , Issue 4 , August 1984 , pp. 259 - 271
Copyright
Copyright © 1984, The Clay Minerals Society

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