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Alkali Cation Selectivity and Fixation by Clay Minerals

Published online by Cambridge University Press:  01 July 2024

Dennis D. Eberl
Dennis D. Eberl*
Affiliation:
Department of Geology, University of Illinois, 61801, Urbana, Illinois, USA
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Abstract

Two variables must be considered when calculating exchange free energies (ΔG°ex) for 2:1 clays: (1) anionic field strength, as expressed by equivalent anionic radius (ra), and (2) interlayer water content, as expressed by interlayer molality. For smectites that are in a state of high hydration, interlayer molality is determined by the cations undergoing exchange. Thus ΔG°ex for an exchanging cation pair can be calculated solely from measurements of ra. ra is related to layer charge per half unit cell (C) and ab unit cell area (A) by: ra = (-A/8πC)1/2. The layer charge necessary for cation fixation can be predicted by calculating the ra at which cation exchange with an illite structure expresses a AG°ex equal to that of exchange with a smectite structure. The theory can also be applied qualitatively to understand the high selectivity of illite for Cs+, the fixation of K+ rather than Na+ in shales during diagenesis, the stability of illite over muscovite in the weathering environment, and cation segregation in smectite.

Резюме

Резюме

При вычислении свободных энергий обмена (ΔC°ех) для глин 2:1 необходимо рассматривать две переменненные (1) силу анионного поля, выраженную в виде эквивалентного анионного радиуса (ra), и (2) содержание межслойной воды, вырвженное в виде межслойной мольности. Для смективов, которые находятся в состоянии высокой гидротации, межслойная мольность обуслав- левается катионами, повергающимися обмену. Таким образом, ΔC°eх для пары обмениваемых катионов может быть вычислена исключительно по данным измерений га. Величина га связана с зарядом слоя на половину элементарной ячейки, (С), и с площадью (А) элементарной ячейки ab соотношением га = (-А/8π-С)12. Заряд слоя, необходимого для фиксации катиона, может быть предсказан вычислением га при котором катионный обмен с иллитовой структурой выражает ДСех равный ΔG°ex обмена со смектитовой структурой. Эту теорию можно также применить качественно, чтобы понять высокую селективность иллита для Cs+, фиксацию K+ вместо Na+ в сланцах во время диагенеза, устойчивость иллита по сравнению с мусковитом в среде выветривания и сегрегацию катионов в смектите. [N.R.]

Resümee

Resümee

Zwei Variable müssen bei der Berechnung der freien Austauschenergie (ΔG°ex) bei 2:1 Tonen berücksichtigt werden: (1) die Anionenfeldstärke, wie sie durch den äquivalenten Anionenradius (ra) ausgedrückt wird und (2) der Zwischenschichtwassergehalt, wie er durch die Zwischenschichtmolalität gegeben ist. Bei Smektiten, die sich in einem Stadium großer Hydratation befinden, wird die Zwischenschichtmolalität durch die austauschbaren Kationen bestimmt. Daher kann ΔG°ex für ein Austauschkationenpaar nur durch die Bestimmung von ra berechnet werden. Der Wert von ra steht in Beziehung zur Schichtladung pro halber Einheitszelle (C) und ab Einheitszellenbereich (A) durch: ra = (-A/8πC)1/2. Die Ladungsdichte, die für die Kationenfixierung notwendig ist, kann vorausgesagt werden, indem der ra berechnet wird, bei dem der Kationenaustausch mit einer Illitstruktur ein ΔG°ex gleich dem eines Kationenaustausches mit einer Smektitstruktur ausdrückt. Diese Theorie kann auch qualitativ verwendet werden, um die große Selektivität des Mit für Cs+ zu verstehen, die Fixierung von K+ vor Na+ in Schiefertonen während der Diagenese, die Stabilität des Illits gegenüber Muskovit unter Verwitterungsbedingungen und die Kationenentmischung in Smektit. [U.W.]

Résumé

Résumé

Deux variables doivent être considérées lorsqu'on calcule l’échange d’énergies libres AG°ex des argiles 2:1: (1) la force de champ anionique, exprimée par le rayon équivalent ra, et (2) la teneur en eau interfeuillet, exprimée par la molalité interfeuillet. Pour les smectites qui sont dans un état d'hydratation élevée, la molalité interfeuillet est déterminée par les cations subissant l’échange. De cette manière, ΔG°ex pour une paire de cations échangeants ne peut être calculé qu’à partir de la mesure de ra. La valeur ra est apparentée à la charge de feuillet par Vi maille (C) et par surface de maille ab (A) par ra = (-A/8πC)1/2. La charge de feuillet nécessaire pour la fixation de cation peut être prédite en calculant la valeur ra à laquelle l’échange de cation avec une structure illite exprime une valeur AG°ex égale à celle de l’échange avec une structure smectite. La théorie peut aussi être appliquée qualitativement pour comprendre la haute sélectivité de Milite pour Cs+, la fixation de K+ plutôt que Na+ dans les roches argileuses pendant la diagénèse, la plus grande stabilité de Milite que de la muscovite dans un environement d'altération, et la ségrégation de cations dans la smectite. [D.J.]

Keywords

Cation exchangeCation fixationCation selectivityEquivalent anionic radiusFree energy of exchangeIlliteSmectite
Type
Research Article
Information
Clays and Clay Minerals , Volume 28 , Issue 3 , June 1980 , pp. 161 - 172
Copyright
Copyright © Clay Minerals Society 1980

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