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Identification and Structure of Two Types of Allophane from Volcanic Ash Soils and Tephra

Published online by Cambridge University Press:  01 July 2024

R. L. Parfitt ,
R. J. Furkert  and
Teruo Henmi
R. L. Parfitt
Affiliation:
Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, New Zealand
R. J. Furkert
Affiliation:
Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, New Zealand
Teruo Henmi
Affiliation:
Ehime University, Matsuyama, Japan
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Abstract

Samples containing allophane with molar Al/Si ratios from 1.0 to 2.0 have been examined by infrared spectroscopy, X-ray fluorescence, and phosphate adsorption methods. The infrared spectra of allophane with Al/Si ratios close to 2.0 showed that the wall of the allophane spherules is made up of imogolite structural units similar to “proto-imogolite.” X-ray fluorescence gave no clear evidence of Al in tetrahedral sites (AlIV), while pyridine adsorption results suggested that a small number of Bronsted acid sites (AlIVOH) are present in silica-rich allophanes. Lewis acid sites (AlH2O) are present in both silica-rich and alumina-rich allophanes. The results suggest that the framework for the allophane structure is an Al octahedral sheet.

Allophanes with Al/Si ratios close to 1.0 contain condensed silicate units either on the outside surface of the Al octahedral sheet, giving rise to a halloysite-like structure, or bonded on the inside surface of an imogolite-like structure. Allophanes with Al/Si ratios between 1.0 and 2.0 appear to be mixtures of the “proto-imogolite” structure and the allophane (Al/Si = 1.0) structure.

Резюме

Резюме

Образцы, содержащие аллофан с молярными отношениями Al/Si от 1,0 до 2,0, были исследованы методами инфракрасной спектроскопии, рентгеновской флюоресценции, и фосфатной адсорбции. Инфракрасные спектры аллофана с отношениями Al/Si близкими к 2,0 показали, что оболочка аллофановых сфер состоит из имоголитовых структурных единиц похожих на «прото-имоголит». Рентгеновская флуоресценция не дала ясного доказательства присутствия Аl в тетраэдрических структурах (AlIV), тогда как результаты пиридиновой адсорбции указывают на то, что небольшое количество кислотных структур Бронстеда (AlIVOH) присутствует в аллофанах богатых кремнеземом. Кислотные структуры Льюиса (АlН2O) присутствуют как в аллофанах богатых кремнеземом, так и в аллофанах богатых алюминием. Результаты показывают, что основой для аллофановой структуры является октаэдрический лист.

Аллофаны с пропорциями Al/Si близкими к 1,0 содержат уплотненные силикатные единицы. Возможные расположения этих единиц находятся на наружной поверхности октаэдрического листа А1 для галлуазитообразной структуры или связаны с внутренней поверхностью имоголитообразной структуры. Аллофаны с отношениями Al/Si между 1,0 и 2,0 представляют смесь «протоимоголитовой» структуры и аллофан (Al/Si = 1,0) структуры. [N.R.]

Resümee

Resümee

Proben, die Allophan mit molaren Al/Si-Verhältnissen von 1,0 bis 2,0 enthielten, wurden mittels Infrarotspektroskopie, Röntgenfluoreszenz, und Phosphatadsorptionsmethoden untersucht. Die Infrarotspektren der Allophane mit Al/Si-Verhältnissen nahe 2,0 zeigten, daß die Wand der Allophankügelchen aus Einheiten mit Imogolitstruktur aufgebaut sind, die dem “Proto-Imogolit” ähnlich sind. Die Röntgen-fluoreszenzuntersuchung brachte keinen eindeutigen Hinweis für AI auf Tetraederplätzen (AlIV). Die Ergebnisse der Pyridinadsorption deuten darauf hin, daß in den Silizium-reichen Allophanen eine kleine Anzahl von Bronstedsäureplätzen (AlIVOH) vorhanden sind. Lewissäureplätze (A1H2O) sind sowohl in Silizium-reichen als auch in Aluminium-reichen Allophanen vorhanden. Dieses Ergebnis deutet darauf hin, daß das Gerüst der Allophanstruktur eine Aluminiumoktaederschicht ist.

Allophane mit Al/Si-Verhältnissen nahe 1,0 enthalten kondensierte Silikateinheiten. Die möglichen Lagen für diese Einheiten sind auf der äußeren Oberfläche der Aluminiumoktaederschicht, wodurch sie eine Halloysit-ähnliche Struktur ergeben, oder sie sind an die innere Oberfläche einer Imogolit-ähnlichen Struktur gebunden. Allophane mit Al/Si-Verhältnissen zwischen 1,0 und 2,0 scheinen Mischungen aus der “Proto-Imogolit”-Struktur und der Allophan (Al/Si = 1,0) struktur zu sein. [U.W.]

Résumé

Résumé

Des échantillons contenant des proportions molaires Al/Si de 1,0 à 2,0 ont été examinés par des méthodes de spectroscopie infrarouge, de fluorescence aux rayons X, et d'adsorption de phosphate. Le spectre infrarouge de l'allophane avec des proportions Al/Si près de 2,0 montre que la paroi des sphérules d'allophane est composée d'unites structurales d'imogolite semblables à la “proto-imogolite.” La fluorescence aux rayons X n'a pas démontré clairement la presence d'Al dans les sites tetraèdres (AlIV), tandis que les résultats d'adsorption de pyridine suggèrent qu'un petit nombre de sites acides Bronsted (AlIVOH) sont présentes dans les allophanes riches en silice. Des sites acides Lewis (AlH2O) sont présents à la fois dans les allophanes riches en silice et dans ceux riches en aluminium. Les résultats suggèrent que la charpente pour la structure allophane est un feuillet Al octaèdre.

Des allophanes avec des proportions Al/Si près de 1.0 contiennent des unités de silice condensées. Les positions possibles pour ces unités sont sur la surface extérieure du feuillet octaèdre Al pour donner une structure semblable à l'halloysite, ou liées à la surface intérieure d'une structure semblable à l'imogolite. Les allophanes ayant des proportions d'entre 1,0 et 2,0 semblent être des mélanges de la structure “protoimogolite” et de la structure semblable à l'allophane (Al/Si = 1,0). [D.J.]

Keywords

AllophaneDeuterium exchangeHalloysiteImogoliteInfrared spectroscopy
Type
Research Article
Information
Clays and Clay Minerals , Volume 28 , Issue 5 , October 1980 , pp. 328 - 334
Copyright
Copyright © Clay Minerals Society 1980

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