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Quantification of Allophane from Ecuador

Published online by Cambridge University Press:  01 January 2024

Stephan Kaufhold ,
Kristian Ufer ,
Annette Kaufhold ,
Joseph W. Stucki ,
Alexandre S. Anastácio ,
Reinhold Jahn  and
Reiner Dohrmann
Stephan Kaufhold*
Affiliation:
BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany
Kristian Ufer
Affiliation:
TU Bergakademie Freiberg, Institute of Mineralogy, 09596 Freiberg, Germany
Annette Kaufhold
Affiliation:
Martin-Luther-University Halle-Wittenberg, Institute for Soil Sciences and Plant Nutrition, Weidenplan 14, D-06108 Halle, Germany
Joseph W. Stucki
Affiliation:
Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois, USA
Alexandre S. Anastácio
Affiliation:
Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois, USA
Reinhold Jahn
Affiliation:
Martin-Luther-University Halle-Wittenberg, Institute for Soil Sciences and Plant Nutrition, Weidenplan 14, D-06108 Halle, Germany
Reiner Dohrmann
Affiliation:
BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany LBEG, Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, D-30655 Hannover, Germany
*
* E-mail address of corresponding author: [email protected]
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Abstract

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Allophane is a very fine-grained clay mineral which is especially common in Andosols. Its importance in soils derives from its large reactive surface area. Owing to its short-range order, allophane cannot be quantified by powder X-ray diffraction (XRD) directly. It is commonly dissolved from the soil by applying extraction methods. In the present study the standard extraction method (oxalate) was judged to be unsuitable for the quantification of allophane in a soil/clay deposit from Ecuador, probably because of the large allophane content (>60 wt.%). This standard extraction method systematically underestimated the allophane content but the weakness was less pronounced in samples with small allophane contents. In the case of allophane-rich materials, the Rietveld XRD technique, using an internal standard to determine the sum of X-ray amorphous phases, is recommended if appropriate structural models are available for the other phases present in the sample. The allophane (+imogolite) content is measured by subtracting the amount of oxalate-soluble phases (e.g. ferrihydrite). No correction would be required if oxalate-soluble Fe were incorporated in the allophane structure. The present study, however, provides no evidence for this hypothesis. Mössbauer and scanning electron microscopy investigations indicate that goethite and poorly ordered hematite are the dominant Fe minerals and occur as very fine grains (or coatings) being dispersed in the cloud-like allophane aggregates.

Allophane is known to adsorb appreciable amounts of water, depending on ambient conditions. The mass fraction of the sample attributed to this mineral thus changes accordingly; the choice of a reference hydration state is, therefore, a fundamental factor in the quantification of allophane in a sample. Results from the present study revealed that (1) drying at 105ºC produced a suitable reference state, and (2) water adsorption has no effect on quantification by XRD analysis.

Keywords

AllophaneChemical Extraction MethodsDifferential Thermal AnalysisEcuadorMössbauer SpectroscopyQuantificationX-ray Diffraction
Type
Article
Information
Clays and Clay Minerals , Volume 58 , Issue 5 , October 2010 , pp. 707 - 716
Copyright
Copyright © Clay Minerals Society 2010

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