3 results
Quantification of Allophane from Ecuador
- Stephan Kaufhold, Kristian Ufer, Annette Kaufhold, Joseph W. Stucki, Alexandre S. Anastácio, Reinhold Jahn, Reiner Dohrmann
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- Journal:
- Clays and Clay Minerals / Volume 58 / Issue 5 / October 2010
- Published online by Cambridge University Press:
- 01 January 2024, pp. 707-716
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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.
Layer Charge Density of Smectites — Closing the Gap Between the Structural Formula Method and the Alkyl Ammonium Method
- Stephan Kaufhold, Reiner Dohrmann, Joseph W. Stucki, Alexandre S. Anastácio
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- Journal:
- Clays and Clay Minerals / Volume 59 / Issue 2 / April 2011
- Published online by Cambridge University Press:
- 01 January 2024, pp. 200-211
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The layer charge density (LCD) of montmorillonite represents the permanent negative charge, its most important property. The LCD can be determined by two different methods, the structural formula method(SF M) and the alkylammonium method (AAM). Other methods of determining the LCD are calibrated against one or the other of these. The results of the two methods differ systematically: SFM values are larger than AAM values and the difference increases with increasing layer charge density. In the present study, the critical parameters of both methods were considered quantitatively in order to identify the most likely reason for the systematic difference. One particularly important argument against the validity of the SFM is that typical SFM values correspond to unrealistically large CEC values that have never been reported. In addition, SFM does not consider the variable charge which causes cations to be adsorbed to the outer surface (at pH >4). In contrast to minor constituents, which can of course also affect SFM values, the variable charge can explain only part of the systematic difference. The exchange of pure smectite samples with both Cu-trien andalkyla mmonium revealedthe presence of non-exchangeable, nonstructural cations (Na, K, Ca). These cations, together with 10% (or more) variable charge, may explain the differences in LCD values. The non-exchangeable, non-structural cations could stem from undetected traces of feldspar or volcanic glass. The present samples indicated that the systematic difference in LCD values between the two methods is related to the amount of non-exchangeable, non-structural cations only, indicating that the two LCD methods probe different features of smectites. Using the SFM on pure smectite provides a value for the total number of charges (permanent with andwithout fixed (= non-exchangeable, non-structural) cations plus variable charge). The AAM, on the other hand, provides the charge density of the exchangeable cations (without variable charge).
A Mössbauer Spectroscopic Study of Aluminum- and Iron-Pillared Clay Minerals
- Amina Aouad, Alexandre S. Anastácio, Faïza Bergaya, Joseph W. Stucki
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- Journal:
- Clays and Clay Minerals / Volume 58 / Issue 2 / April 2010
- Published online by Cambridge University Press:
- 01 January 2024, pp. 164-173
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The placement of metal oxide pillars between clay mineral layers modifies their physical-chemical properties, including surface area, acidity, and catalytic activity. Aluminum is the most commonly used pillar cation, but the use of Fe offers a distinct opportunity to expand the range of catalytic behavior. The purpose of this study was to prepare Fe-pillared Laponite and montmorillonite and to characterize the resulting Fe phase(s). Laponite or montmorillonite suspension was mixed with different pillaring solutions containing Al oligomer and/or Fe oligomer with Fe:(Al+Fe) percent ratios ranging from 0 to 100%. The Al oligomer was obtained by hydrolysis of A1C13·6H2O with NaOH at pH 4.4 and the Fe oligomer was prepared by FeCl3 hydrolysis with Na2CO3 at pH 2.2. The pillared clay was obtained by adding the oligomer to the clay suspension, then heating to 300°C for 3 h. The Fe oligomer and the pillared clay minerals were characterized by variable-temperature Mössbauer spectroscopy, X-ray powder diffraction, and chemical analysis. The unheated Fe oligomer was akaganeite, an Fe oxyhydroxide phase. Heating the Fe oligomer to 300°C transformed the akaganeite to hematite, but heating it in the presence of the clay protected it, at least partially, from this transformation, creating instead a phase which resembled a more poorly ordered akaganeite or a mixture of akaganeite and poorly ordered hematite. Mixing of Al and Fe oligomers in the pillaring solution had no effect on the magnetic hyperfine field of the Fe pillars, indicating that Al forms separate pillars rather than substituting for Fe in the pillar. A small fraction (4%) of the Fe pillar resisted reductive dissolution by citrate-bicarbonate-dithionite.