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Biotite Alteration in Deeply Weathered Granite. I. Morphological, Mineralogical, and Chemical Properties

Published online by Cambridge University Press:  01 July 2024

R. J. Gilkes
Affiliation:
Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia, Australia 6009
Anchalee Suddhiprakarn*
Affiliation:
Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia, Australia 6009
*
1Permanent address: Department of Soil Science, Kasetsart University, Bangkok, Thailand.
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Abstract

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Biotite in deeply weathered granitic rocks in southwestern Australia has altered to exfoliated grains composed of biotite, mixed-layer clay minerals, kaolinite, vermiculite, gibbsite, goethite, and hematite. Discrete vermiculite and vermiculite-dominant mixed-layer clay minerals are not major weathering products. Oxidation of octahedral iron in biotite is associated with ejection of octahedral cations, loss of interlayer K, and a contraction of the b-dimension of the biotite sheet. Si, Mg, Ca, Mn, K, and Na are lost from biotite during weathering, and Ti, Al, Ni, and Cr are retained. Fe and water have been added to the grains during weathering. Much Fe occurs as aggregates of microcystalline, aluminum-rich goethite particles on flake surfaces and within etchpits, with smaller amounts occurring as hexagonal arrangements of lath-shaped crystals of goethite on flake surfaces.

Резюме

Резюме

Биотит в глубоко выветренных гранитных породах в юго-западной Австралии был изменен в расслоенные зерна, состоящие из биотита, смешанно-слойных минералов, каолинита, вермикулита, гиббсита, гетита, и гематита. Дискретный вермикулит и смешанно-слойные глинистые минералы, в основном содержащие вермикулит, не являются основными продуктами выветривания. Окисление октаэдрического железа в биотите ассоциируется с выбросом октаедри-ческих катионов, потерей межслойного К, и сокращением Ь-измерения пластин биотита. Si, Мg, Са, Мп, К, и Nа высвобождается из биотита во время выветривания, а Тi, Аl, Ni, и Сг остаются. Во время выветривания в зернах появляется добавочная вода и Ре. Много Ре встречается в виде агрегатов микрокристаллических богатых алюминием гетитовых частиц на чешуйчатых поверхностях, меньшие количества встречаются в виде гексагонально расположенных пластинчатых кристаллов гетита на чешуйчатых поверхностях.

Resümee

Resümee

Der Biotit in den tiefgründig verwitterten, granitischen Gesteinen von Südwest-Australien ist in schuppige Körner umgewandelt, die aus Biotit, Wechsellagerungen, Kaolinit, Vermiculit, Gibbsit, Goethit, und Hämatit bestehen. Einzelne, diskrete Vermiculite und vorwiegend aus Vermiculit bestehende Wechsellagerungen kommen dagegen nicht oft als Verwitterungsprodukte vor. Die Oxidation des okta-edrischen Eisens im Biotit ist mit dem Austritt von oktaedrischen Kationen, dem Verlust von Kalium aus den Zwischenschichten und einer Verkleinerung der Biotitschicht in Richtung der b-Achse verbunden. Während der Verwitterung verarmt der Biotit an Si, Mg, Ca, Mn, K, und Na, während Ti, Al, Ni, und Cr zurückgehalten werden. Fe und Wasser werden den Körnern während der Verwitterung zugeführt. Ein großer Teil des Fe ist in Form von Aggregaten aus mikrokristallinen, aluminiumreichen Goethitpartikeln auf den Blättchenoberflächen und in Atzgruben vorhanden. Ein kleiner Teil kommt in Form von hexagon-alen Anordnungen aus leistenförmigen Goethitkristallen auf den Blättchenoberflächen vor.

Résumé

Résumé

La biotite dans des roches granitiques profondément altérées d'Australie du Sud-ouest a été altérée en grains exfoliés composés de biotite, de minéraux argileux à couches mélangées, de kaolinite, de vermiculite, de gibbsite, de goethite, et d'hématite. La vermiculite elle-même, et les minéraux argileux à couches mélangées à prédominance de vermiculite ne sont pas des produits d'altération majeurs. L'oxidation de fer octaédrique dans la biotite est associée avec l’éjection de cations octaédriques, avec la perte de l'intercouche K, et avec la contraction de la dimension-b de la feuille de biotite. Si, Mg, Ca, Mn, K, et Na sont perdus par la biotite pendant l'altération, et Ti, Al, Ni, and Cr sont retenus. Fe et de l'eau ont été ajoutés aux grains pendant l'altération. Beaucoup de Fe existe comme aggrégats de particules de goethite microcristallins et riches en aluminium sur des surfaces de lame et dans des crevasses gravées, avec des quantités moindres existant comme arrangements hexagonaux de cristaux de goethite en forme de latte sur les surfaces des lames.

Type
Research Article
Copyright
Copyright © 1979, The Clay Minerals Society

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