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Charges on the surfaces of two chlorites

Published online by Cambridge University Press:  09 July 2018

Angela A. Jones
Angela A. Jones*
Affiliation:
Department of Soil Science, University of Reading, Reading, Berks RG1 5AQ, UK
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Abstract

Two chlorites, sheridanite and clinochlore, have been examined to determine their surface charge characteristics. In order to increase their surface area and to produce a measurable surface charge, the chlorites were treated with 10−3m, 10−2m, and 10−1m HCl in 10−2m MgCl2 solutions. These treatments are shown not to alter the crystallinity of the chlorites and to produce a small pH-dependent negative charge which is not directly related to the total surface area. The clinochlore, (Si6.13Al1.84)(Al1.53FeIII0.53FeII0.18Mg9.52)O20(OH)16, is more readily attacked by the acids than the sheridanite, (Si5.43Al2.55)(Al2.90FeII0.05Mg8.86)O20(OH)16, and also produces material with greater surface area and pH-dependent, negative, surface charge. It is concluded that: (i) isomorphous substitutions in the lattice are not reflected in a permanent surface charge; (ii) the observed surface charge arises not only at the edges of the particles but also at points where the chlorite is predisposed to attack by acids; (iii) in the chlorite-acid system used, anions—probably mainly silicate—block positively charged sites.

Résumé

Résumé

Les caractéristiques des charges superficielles de deux chlorites, la shéridanite et le clinochlore, ont été déterminées. En vue d'accroître leurs surfaces spécifiques et de produire une charge superficielle mesurable, les chlorites ont été traitées par des solutions 10−2m en MgCl2 et 10−3m ou 10−2m ou 10−1m en HCl. On montre que ces traitements n'altèrent pas la cristallinité des chlorites et produisent une faible charge négative dépendant du pH qui n'est pas directement liée à la surface spécifique totale. Le clinochlore (Si6.13Al1.84)(Al1.53FeIII0.53FeII0.18Mg9.52)O20(OH)16, est plus rapidement attaqué par les acides que la shéridanite (Si5.43Al2.55)(Al2.90FeII0.05Mg8.86) O20(OH)16 et produit également un matériel à plus grande surface spécifique et une plus grande charge superficielle, négative, dépendant du pH. On en déduit: (i) la substitution isomorphique du réseau ne se reflète pas dans la charge superficielle permanente; (ii) la charge superficielle observée n'apparaît pas seulement au bord des particules, mais aussi à des points où la chlorite est prédisposée à êre attaquée par des acides; (iii) des anions—probablement des silicates—bloquent des sites chargés positivement dans les système chloriteacide utilisé.

Kurzreferat

Kurzreferat

Zwei Chlorite, Sheridanit und Klinochlor wurden zur Bestimmung ihrer Oberflächenladungsverhältnisse untersucht. Zur Oberflächenvergrößerung und um eine meßbare Oberflächenladung zu erhalten, wurden die Chlorite mit 10−3, 10−2 und 10−1m HCl in 10−2m MgCl2 Lösungen behandelt. Diese Behandlungen bewirkten keine Veränderung in der Kristallinität der Chlorite und erzeugten eine geringe pH-abhängige Ladung, welche in keinem direkten Bezug zur Gesamtoberfläche steht. Der Klinochlor (Si6.13Al1.84)(Al1.53FeIII0.53FeII0.18Mg9.52) O20(OH)16 wird durch die Säuren etwas leichter angegriffen als der Sheridanit (Si5.43Al2.55) (Al2.90FeII0.05Mg8.86)O20(OH)16 und liefert deshalb Material mit größerer Oberfläche und pH-abhängiger negativer Oberflächenladung. Es wird der Schluß gezogen, daß: (a) isomorpher Ersatz im Gitter nicht durch eine permanente Oberflächenladung wiedergegeben wird, (b) die beobachtete Oberflächenladung nicht nur an den Eckpositionen der Teilchen lokalisiert ist, sondern auch dan den gegenüber Säureangriffen empfindlichen Stellen des Chlorits, (c) in dem verwendeten Chlorit-Säure System Anionen—wahrscheinlich hauptsächlich Silicat—positiv geladene Positionen besetzt halten.

Resumen

Resumen

Dos cloritas, sheridonita y clinocloro, han sido examinadas para determinar sus caracteristicas de carga superficial. Al objecto de aumentat su superficie especifica y producir una carga superficial medible, las cloritas fueron tratadas con soluciones de Cl2Mg 10−2m en ClH 10−3m, 10−2m y 10−1m. Se ha demostrado que estos tratamientos no alteran la cristalinidad de las cloritas y producen una pequeña carga negativa dependiente del pH, que no está relacionada directamente con la superficie total. El clinocloro, (Si6.13Al1.84)(Al1.53FeIII0.53FeII0.18Mg9.52) O20(OH)16, es más fácilmente atacado por los ácidos que la sheridonita, (Si5.43Al2.55) (Al2.90FeII2.90Mg8.86)O20(OH)16, y dá lugar a un material con una mayor superficie especifica y una carga superficial negativa y dependiente del pH. Se concluye que: (i) las sustituciones isomórficas en la red se reflejan en una carga superficial permanente; (ii) la carga superficial observada proviene, no sólo de los bordes de las partículas sino también de los puntos donde la clorita está predispuesta al ataque con ácidos; (iii) en el sistema cloritaácido usado, los anionesprobablemente silicatos principalmentebloquean los puntos cargados positivamente.

Type
Research Article
Information
Clay Minerals , Volume 16 , Issue 4 , December 1981 , pp. 347 - 359
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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