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Feldspar Weathering in Lateritic Saprolite

Published online by Cambridge University Press:  02 April 2024

R. R. Anand ,
R. J. Gilkes ,
T. M. Armitage  and
J. W. Hillyer
R. R. Anand
Affiliation:
Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia 6009, Australia
R. J. Gilkes
Affiliation:
Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia 6009, Australia
T. M. Armitage
Affiliation:
Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia 6009, Australia
J. W. Hillyer
Affiliation:
Electron Microscopy Center, University of Western Australia, Nedlands, Western Australia 6009, Australia
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Abstract

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Feldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.

Резюме

Резюме

Фельдшпаты в гранитовом сапролите в юго-западной Австралии изменялись в галлуазит, каолинит и гиббсит, без доказательства присутствия некристаллических материалов. Вторичные минералы находятся обычно как внутренные смеси в зернах фельдшпата, но дискретные зоны гиббсита или галлуазита-каолинита также присутствуют. Перемены в химическом микроокружении внутри микроскопических зон в зернах, кажется, контролируют тип и расположение вторичных минералов. Видоизменение обоих плагиоклаза и щелочных фельдшпатов включало соответственно растворение в почвенном растворе и кристаллизацию галлуазита, каолинита, и гиббеита из этого раствора. Значительно видоизмененные зерна фельдшпата состоят из изъеденных фрагментов фельдшпата, окруженных сильно пороватой сетью субгедральных до евгедральных плоских кристаллов каолинита и гиббеита, либо сферических и войлочных аггрегатов галлуазита. [E.G.]

Resümee

Resümee

Die Feldspäte in einem granitischen Rückstandsgestein von SW-Australien haben sich in Halloysit, Kaolinit, und Gibbsit umgewandelt, wobei es keinen Hinweis für nichtkristallisierte Substanzen gibt. Die sekundären Minerale treten meist eng vermischt in umgewandelten Feldspatkörnern auf. Es gibt jedoch auch begrenzte Zonen, in denen Gibbsit oder Halloysit-Kaolinit getrennt auftreten. Variationen im chemischen Mikrobereich innerhalb von Mikrometer-großen Zonen in den Körnern kontrollieren offensichtlich die Art und Verteilung der Sekundärminerale. Die Umwandlung von Plagioklas und Alkalifeldspat umfaßt konkruente Auflösung durch Bodenlösung und die Kristallisation von Halloysit, Kaolinit und Gibbsit aus dieser Lösung. Stark umgewandelte Feldspatkörner bestehen aus angelösten Feldspatfragmenten, die in einem stark porösen Gerüst aus hypidiomorphen bis idiomorphen tafeligen Kristallen von Kaolinit und Gibbsit liegen oder in einem Gerüst aus kugeligen und dichten Aggregaten von Halloysit eingebettet sind. [U.W.]

Résumé

Résumé

Des feldspars dans de la saprolite granitique en Australie du sud-ouest se sont altérés en halloysite, kaolinite et gibbsite, sans évidence de matériel non-cristallin. Les minéraux secondaires sont communément présents en tant que mélanges intimes au sein de grains de feldspar altéres, mais des zones discrètes de gibbsite ou d'halloysite-kaolinite sont également présentes. Des variations dans le microen-vironement chimique au sein de zones de taille micrométrique dans les grains ont apparemment contrôlé le type et la distribution de minéraux secondaires. L'altération à la fois de plagioclase et de feldspars alkalins a impliqué la dissolution congruente de solution de sol et la cristallisation d'halloysite, de kaolinite et de gibbsite à partir de cette solution. Les grains de feldspar fort altérés consistent en des fragments gravés de feldspar enfoncés au sein d'une matrice très poreuse de cristaux sousédraux à euédraux de kaolinite et de gibbsite, ou d'aggrégats sphériques et feutrés d'halloysite. [D.J.]

Keywords

FeldsparGibbsiteHalloysiteKaoliniteNoncrystalline materialWeathering
Type
Research Article
Information
Clays and Clay Minerals , Volume 33 , Issue 1 , February 1985 , pp. 31 - 43
Copyright
Copyright © 1985, The Clay Minerals Society

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