Quantification of allophane from Ecuador
Kaufhold, Stephan (BGR Bundesanstalt für Geowissenschaften und Rohstoffe)
Ufer, Kristian (Institute of Mineralogy. TU Bergakademie Freiberg)
Kaufhold, Annette (Martin-Luther-University Halle-Wittenberg. Institute for Soil Sciences and Plant Nutrition)
Stucki, Joseph W. (University of Illinois. Department of Natural Resources and Environmental Sciences)
Anastácio, Alexandre S. (University of Illinois. Department of Natural Resources and Environmental Sciences)
Jahn, Reinhold (Martin-Luther-University Halle-Wittenberg. Institute for Soil Sciences and Plant Nutrition)
Dohrmann, Reiner (LBEG Landesamt für Bergbau, Energie und Geologie)

Fecha:	2010
Resumen:	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 105°C produced a suitable reference state, and (2) water adsorption has no effect on quantification by XRD analysis.
Derechos:	Tots els drets reservats.
Lengua:	Anglès
Documento:	Article ; recerca ; Versió publicada
Materia:	Allophane ; Chemical Extraction Methods ; Differential Thermal Analysis ; Ecuador ; Mössbauer Spectroscopy ; Quantification ; X-ray Diffraction
Publicado en:	Clays and Clay Minerals, Vol. 58, Núm. 5 (2010) , p. 707-716, ISSN 1552-8367

DOI: 10.1346/CCMN.2010.0580509

10 p, 1.3 MB  Acceso restringido a la UAB

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