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Soil Minerals in the Al2O3-SiO2-H2O System and a Theory of Their Formation

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Clays and Clay Minerals

Abstract

An attempt has been made to assemble the best thermodynamic information currently available for soil minerals in the Al2O3-SiO2-H2O system at 25°C and 1 atm. Montmorillonite is included by considering its aluminum silicate phase. Diagrams are presented so that the stability of the minerals can be visualized in relation to the ionic environment. Although the Al2O3-SiO2-H2O system is a very simple one compared to soils and sediments, the stability diagrams depict a mineral stability sequence and mineral pair associations that are in good agreement with natural relations.

According to the stability diagram, mineral pairs that can form in intimate association are gibbsite-kaolinite, kaolinite-montmorillonite, and montmorillonite-amorphous silica. Forbidden pairs are amorphous silica-kaolinite, amorphous silica-gibbsite, and montmorillonite-gibbsite. The formation of intimate mixtures of three or more of these minerals is also forbidden. The stability diagrams predict ion activity relationships that are in reasonable agreement with those obtained from soils and sediments.

Amorphous silica probably limits high silica levels, with montmorillonite also forming at high silica levels. Kaolinite forms at intermediate and gibbsite at low silica levels. These minerals in turn probably control the activity of aluminum ions at a level appropriate to the pH. The formation of gibbsite, kaolinite, montmorillonite and amorphous silica appears to be controlled by a combination of kinetics and equilibria. That is, the kinetic dissolution of unstable silicates appears to control the H4SiO4 level. The new mineral(s) most stable at that H4SiO4 level appear to precipitate in response to solution equilibria.

Résumé

On a tenté de rassembler toutes les informations thermodynamiques actuellement disponibles sur les minéraux terrestres dans le système Al2O3-SiO2-H2O a 25°C et 1 atm. La montmorillonite y est incluse compte tenu de sa phase de silicate d’aluminum. Des diagrammes sont présentés de façon à ce que la stabilité des minéraux puisse être représentée en relation avec l’environnement ionique. Bien que le système Al2O3-SiO2-H2O soit très simple par comparaison aux sols et sédiments, les diagrammes de stabilité décrivent une sequence de stabilité minérale et des associations minérales paires qui sont en accord avec les relations naturelles.

Selon le diagramme de stabilité, les paires minérales qui peuvent se former en association intime sont gibbsite-kaolinites, kaolinites-montmorillonites. et silice amorphe montmorillonite. Les paires interdites sont les silices amorphes kaolinites. silices amorphes gibbsites, kaolinites montmorillonites, et silices amorphes montmorillonites. La formation de mélanges intimes de 3 ou plus de ces minéraux est également interdite.

Le diagramme de stabilité prédit des rapports d’activité ionique en accord raisonable avec ceux obtenus à partir des sols et des sédiments. Les silices amorphes limitent probablement des niveaux de silice élevés, avec de la montmorillonite se formant également à ces memes niveaux de silice. Les kaolinites se forment à des niveaux intermédiaires et les gibbsites à des niveaux de silice très bas. A leur tour, ces minéraux contrôlent probablement les activités des ions d’aluminum à un niveau approprié au pH. La formation de gibbsite, de kaolinite, de montmorillonite et de silice amorphe paraît être contrôlée par une combinaison de cynétique et d’équilibre. C’est à dire que la dissolution cynétique de silices instables semble contrôler le niveau H4SiO4. Le nouveau minéral le plus stable à ce niveau H4SiO4, semble précipiter en réponse à la solution d’équilibre.

Kurzreferat

Es wurde versucht, die besten derzeit erhältlichen thermodynamischen Daten für Bodenminerale im Al2O3-SiO2-H2O System bei 25°C und 1 Atmosphäre zu vereinigen. Montmorillonit wurde unter Berücksichtigung seiner Aluminiumsilikatphase miteingeschlossen. Aus den beigefügten Kurvenbildern lässt sich die Beständigkeit des Minerals in Bezug auf die lonenumgebung beurteilen. Obwohl das Al2O3-SiO2-H2O System im Vergleich mit Böden und Ablagerungen ein sehr einfaches ist, zeigen die Beständigkeitskurven eine Mineralbeständigkeitsfolge und Mineral-paarungen, die gut mit natürlichen Verhältnissen übereinstimmen.

Gemäss den Beständigkeitskurven können folgende Mineralpaare in enger Association geformt werden: Gibbsit-Kaolinit, Kaolinit-Montmorillonit und Montmorillonit-amorphe Kieselsäure. Verboten sind Paare von amorpher Kieselsäure-Kaolinit, amorpher Kieselsäure-Gibbsit und Mont-morillonit-Gibbsit. Die Bildung enger Mischungen von drei oder mehr dieser Minerale ist ebenfalls ausgeschlossen. Die Beständigkeitskurven weisen auf Ionenaktivitätsbeziehungen hin, die recht gut mit den aus Boden- und Ablagerungsproben erhaltenen übereinstimmen.

Amorphe Kieselsäure schliesst wahrscheinlich hohe Kieselsäureniveaus aus während Montmorillonite auch bei hohen Kieselsäureniveaus geformt werden. Kaolinit bildet sich bei mittleren, und Gibbsit bei niedrigen Kieselsäureniveaus. Diese Minerale bestimmen wahrscheinlich die Aktivität der Aluminiumionen a einem dem pH entsprechenden Niveau. Die Bildung von Gibbsit, Kaolinit. Montmorillonit und amorpher Kieselsäure scheint durch eine Kombination von Kinetik und Gleichuf gewichten bestimmt zu werden, d.h. die kinetische Auflösung unbeständiger Silikate scheint das H4SiO4 Niveau zu bestimmen. Die neuen auf diesem H4SiO4 Niveau beständigsten Minerale scheinen durch die Lösungsgleichgewichte zur Ausfällung gebracht zu werden.

Резюме

Сделана попытка собрать наиболее достоверные термодинамические данные для минералов почв, образующихся в системе Аl2О3-SiO22О при 25°С и 1 атм. Монтмориллонит включен в связи с его алюмосиликатной составной частью. Диаграммы даны так, чтобы можно было составить представление о стабильности минералов в зависимости от их ионного окружения. Хотя система Аl2О3-SiO22О очень проста в сравнении с почвами и осадками, диаграммы эти изображают и последовательность стабильности и парные ассоциации минералов в хорошем согласии с природными соотношениями.

В соответствии с диаграммами стабильности, пары минералов, образующих тесные ассоциации, таковы: гиббсит—каолинит, каолинит—монтмориллонит, монтмориллонит—аморфный кремнезем. К запрещенным парам относятся: аморфный кремнезем—каолинит, аморфный кремнезем—гиббсит и монтмориллонит—гиббсит. Образование тонких смесей из трех (или более) минералов также запретно. Диаграммы стабильности позволяют предсказать соотношения активности ионов, удовлетворительно согласующиеся с найденными при изучении почв и осадков.

Аморфный кремнезем, вероятно, ограничивает верхний предел активности кремнезема, причем монтмориллонит образуется также при высокой активности кремнекислоты. Образование каолинита происходит при промежуточных, а образование гиббсита—при низких уровнях активности кремнезема. Эти минералы, вероятно, в свою очередь контролируют активность ионов алюминия в соответствии со значением рН. Образование гиббсита, каолинита, монтмориллонита и аморфного кремнезема, как кажется, контролируется совместным влиянием и кинетики и равновесия, т.е. кинетика растворения неустойчивых силикатов, по-видимому, контролирует уровень активности Н4SiO4; новый минерал или минералы, которые наиболее устойчивы при этом уровне активности Н4SiO4, повидимому, осаждаются в соответствии с равновесными отношениями в растоворе.

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This investigation was supported in part, by contract WP-01016 with the U.S. Public Health Service, and is published as Scientific Paper No. 3215 Washington Agr. Exp. Sta. Proj. 1885.

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Kittrick, J.A. Soil Minerals in the Al2O3-SiO2-H2O System and a Theory of Their Formation. Clays Clay Miner. 17, 157–167 (1969). https://doi.org/10.1346/CCMN.1969.0170304

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