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Dehydroxylation, Rehydroxylation, and Stability of Kaolinite

Published online by Cambridge University Press:  02 April 2024

Vernon J. Hurst
Affiliation:
Geology Department, University of Georgia, Athens, Georgia 30602
Albert C. Kunkle
Affiliation:
Research Department, J. M. Huber Corporation, Macon, Georgia 31298

Abstract

From hydrothermal experiments three pressure-temperature-time curves have been refined for the system Al2O3−SiO2−H2O and reversal temperatures established for two of the principal reactions involving kaolinite. The temperatures of three isobaric invariant points enable the Gibbs free energy of formation of diaspore and pyrophyllite to be refined and the stability field of kaolinite to be calculated. The maximal temperature of stable kaolinite decreases from 296°C at 2 kb water pressure to 284°C at water's liquid/vapor pressure, and decreases rapidly at lower pressures. On an isobaric plot of [H4SiO4] vs. °K-1, kaolinite has a wedge-shaped stability field which broadens toward lower temperature to include much of the [H4SiO4] range of near-surface environments. If [H4SiO4] is above kaolinite's stability field and the temperature is < 100°C, halloysite forms rather than pyrophyllite, an uncommon pedogenic mineral. Pyrophyllite forms readily instead of kaolinite above 150°C if [H4SiO4] is controlled by cristobalite or noncrystalline silica.

Kaolinite and a common precursor, halloysite, are characteristic products of weathering and hydro-thermal alteration. In sediments, relatively little halloysite has survived due to its low dehydration temperature and instability at low water pressure, but kaolinite commonly has survived since the Devonian Period. In buried sediments, the water pressure and [H4SiO4] requisite for stable kaolinite generally are maintained. In oxidized sediments and in pyritic reduced sediments, kaolinite commonly has survived, but where alkalies, alkaline earths, or aqueous iron has concentrated in the pore fluid, kaolinite has tended to transform to illite, zeolites, berthierine, or other minerals.

Резюме

Резюме

На основе гидротермальных экспериментов были усовершенствованы три кривые давление-температура-время для системы Al2O3-SiO2-H2O и были определены реверсные температуры для двух из числа основных реакций, включающих каолинит. Величины температуры трех изобарных инвариантных точек позволили усовершенствовать величину свободной энергии Гиббса образования диаспора и пирофиллита, а также рассчитать поле стабильности каолинитов. Максимальная температура стабильного каолинита уменьшается от 296°С при давлении воды 2 кбар до 284°С при давлении жидкость/пар (для воды) и уменьшается быстро при низших давлениях. На изобарной кривой зависимости [H4SiO4] от °K-1, каолинит имеет клинообразное поле стабильности, которое расширяется по направлению к низшим температурам, чтобы включить большую часть [H4SiO4] области близких к поверхности сред. Если [H4SiO4] больше, чем для поля стабильности каолинита и температура < 100°С, галлуазит образуется вместо пирофиллита, необычного педогенического материала. Пирофиллит легко образуется вместо каолинита при температуре свыше 150°С, если [H4SiO4] контролируется кристобалитом или некристаллическим кремнеземом.

Каолинит и обычний предшественник, галлуазит, являются характерными продуктами выветривания и гидротермальных изменений пород. В осадочных отложениях сохранилось сравнительно небольшое количество галлуазита вследствие его низкой температуры дегидратации и нестабильности при низких давлениях воды, тогда как каолинит обычно сохраняется со времени девонского периода. В захороненных осадочных отложениях необходимые для стабильного каолинита давление воды и количество [H4SiO4] в основном поддерживаются. В окисленных отложениях и в отложениях с уменьшенным количеством пирита каолинит обычно сохраняется, но каолинит стремится видоизмениться в иллит, цеолит, бертьерин или другие минералы там, где в жидкости пор сосредотачиваются щелочи, щелочные почвы или осадочное железо. [E.G.]

Resümee

Resümee

Aus hydrothermalen Experimenten wurden drei Druck-Temperatur-Zeit-Kurven für das System Al2O3-SiO2-H2O bestimmt, und die Temperaturen für zwei der wichtigsten Kaolinitreaktionen gewonnen. Die Temperaturen von drei isobar invarianten Punkten ermöglichen die Bestimmung der Gibbs'schen Freien Energie für die Bildung von Diaspor und Pyrophyllit und die Berechnung des Stabilitätsfeldes von Kaolinit. Die maximale Temperatur für stabilen Kaolinit nimmt von 296°C bei 2 kBar Wasserdampfdruck auf 284°C bei gesättigtem Wasserdampfdruck ab und verringert sich sehr schnell bei niedrigeren Drucken. Auf einem isobaren Diagramm, in dem [H4SiO4] gegen °K-1 aufgetragen ist, hat Kaolinit ein keilförmiges Stabilitätsfeld, das sich gegen niedrigere Temperaturen hin verbreitert, um viel von [H4SiO4]-Bereich der Oberflächenzone mit einzuschließen. Wenn [H4SiO4] über dem Kaolinitstabilitätsfeld liegt, und die Temperatur unter 100°C ist, dann bildet sich eher Halloysit als Pyrophyllit, ein unübliches Bodenmineral. Pyrophyllit bildet sich sehr leicht anstelle von Kaolinit bei Temperaturen über 150°C,wenn [H4SiO4] durch Cristobalit oder nichtkristallisiertes SiO2 kontrolliert wird.

Kaolinit und eine häufige Übergangsphase, Halloysit, sind typische Produkte der Verwitterung und hydrothermalen Umwandlung. In Sedimenten ist relativ wenig Halloysit aufgrund seiner niedrigen Dehydratationstemperatur und seiner Instabilität bei niedrigem H2O-Druck zu finden, während Kaolinit im allgemeinen seit dem Devon überlebt hat. In Versenkungssedimenten bleiben der für stabilen Kaolinit geforderte H2O-Druck und die notwendige [H4SiO4]-Aktivität im allgemeinen erhalten. In oxidierten Sedimenten und in pyritisch reduzierten Sedimenten bleibt Kaolinit gewöhnlich erhalten. Wenn jedoch Alkalien, Erdalkalien oder hydratisiertes Eisen in den Porenlösungen konzentriert sind, dann wandelt sich Kaolinit leicht in Illit, Zeolithe, Berthierit und anderen Minerale um. [U.W.]

Résumé

Résumé

A partir d'expériences hydrothermiques, 3 courbes pression-température-temps ont été rafinées pour le système Al2O3-SiO2-H2O et des températures de revers ont été établies pour deux des réactions principales impliquant la kaolinite. Les températures de trois points invariants isobariques permet le rafinement de l’énergie libre de Gibbs de formation de la diaspore et de la pyrophyllite et le calcul du champ de stabilité de la kaolinite. La température maximale de kaolinite stable décroit de 296°C a 2 kb de pression d'eau a 284°C à la pression liquide/vapeur d'eau, et décroit rapidement à des pressions plus basses. Sur un diagramme isobarique de [H4SiO4] vs. °K-1, la kaolinite a un champ de stabilité éffilé à trois coins qui s’élargit vers la température plus basse pour inclure une grande partie de la gamme [H4SiO4] d'environements proches de la surface. Si [H4SiO4] est au delà du champ de stabilité de la kaolinite et la température est < 100°C, l'halloysite est formée plutôt que la pyrophyllite, un minéral pédogénique peu commun. La pyrophyllite est formée promptement à la place de la kaolinite au delá de 150°C si [H4SiO4] est contrôlée par la cristobalite ou par la silice non cristalline.

La kaolinite et un précurseur commun, l'halloysite, sont des produits caractéristiques de l'altération à l'air et hydrothermique. Dans des sédiments, relativement peu d'halloysite a survécu à cause de sa température de déshydratation basse et de son instabilité à de basses pressions d'eau, mais la kaolinite a communément survécu depuis la période dévonienne. Dans des sédiments ensevelis, la pression d'eau et l’[H4SiO4] nécéssaires pour la kaolinite stable sont généralement maintenues. Dans des sédiments oxidés et dans des sédiments pyritiques réduits, la kaolinite a communément survécu, mais là où des alkalins, des terres alkalines, ou du fer aqueux a été concentré dans les fluides de pores, la kaolinite a eu tendance à se transformer en illite, zéolite, berthierine ou en d'autres minéraux. [D.J.]

Type
Research Article
Copyright
Copyright © 1985, The Clay Minerals Society

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