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  • 1
    Language: English
    In: Clays and Clay Minerals, 2010, Vol.58(5), pp.707-716
    Description: 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: Allophane ; Chemical Extraction Methods ; Differential Thermal Analysis ; Ecuador ; Mössbauer Spectroscopy ; Quantification ; X-ray Diffraction
    ISSN: 0009-8604
    E-ISSN: 1552-8367
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  • 2
    Language: English
    In: Catena, 2004, Vol.56(1), pp.67-83
    Description: Soils conforming to Andosols in the World Reference Base (WRB) of Soil Resources have not yet been recognised in Germany. The aims of this study were to (i) show whether some German soils developed in volcanic parent materials are Andosols, (ii) investigate their pedogenetic development and (iii) determine their local distribution. We selected a soil on Quaternary basalt in the Eifel region (the "Windsborn" profile), and a soil from the summit of the Tertiary basaltic stratovolcano Vogelsberg (the "Kohlerwald" profile) northwest of Frankfurt am Main. The Windsborn soil is classified as a Silic Andosol. Allophane contents are near 10%, and allophane globules were identified by transmission electron microscopy (TEM). By progressively constraining field parameters in GIS-generated maps, we found that Andosols in the Mosenberg chain are confined to a few isolated occurrences. The Kohlerwald profile in the Tertiary Vogelsberg is an acid, base-poor and strongly weathered Aluandic Luvisol. Metal-humus complexes are the dominant colloidal phase. In the topsoil, both Fe (sub p) and Al (sub p) are in excess of Fe (sub o) and Al (sub o) values. Our data suggest the presence of phases like hydroxy-Al (e.g. from Al-interlayers or poorly ordered gibbsite) and opaline silica that are easily attacked by the alkaline (pH 9.5-10.5) pyrophosphate extractant, but may be resistant to the acid (pH 3) oxalate procedure. We conclude that there are limited occurrences of silandic Andosols in the Eifel area, Germany. The Windsborn soil is in an early pedogenetic development stage, but pedogenesis in the Kohlerwald soil has advanced beyond the Andosol phase to the stage of clay translocation. Therefore the existence of aluandic Andosols in Germany seems possible, but remains to be proven.
    Keywords: Volcanic Soils ; Pedogenesis ; Soil Classification ; Allophane ; Andosols ; Sciences (General) ; Geography ; Geology
    ISSN: 0341-8162
    E-ISSN: 1872-6887
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  • 3
    Language: English
    In: Geoderma, 2007, Vol.138(1), pp.1-11
    Description: Halloysite and gibbsite, although known to require quite different conditions for their formation, commonly occur together in the same horizon in oxisols derived from andesitic parent materials in tropical Costa Rica. We selected two soils of similar parent material, but of different ages and soil moisture regimes to identify possible clues to the coexistence of these two minerals. We employed selective dissolution procedures, X-ray fluorescence analysis and X-ray diffraction on field moist and air dry bulk soil samples to investigate how mineralogy changes with depth. We further separated the size fraction 〈 2 μm by means of sedimentation after organic matter and iron oxide removal to obtain more specific information on the phyllosilicate mineralogy of the clay size fraction. We found both soils to be depleted of primary minerals and pedogenesis to have progressed to advanced weathering stages particularly in the subsoils. Gibbsite XRD signal intensities were linearly and significantly related to weathering indices, corroborating the residual nature of gibbsite as an endproduct of weathering processes. The Si-bearing quartz and kaolinite-group minerals were enriched in the topsoils, indicating (i) their independence from a primary mineral Si source and (ii) the existence of a mechanism capable of protecting them against the continuous tropical weathering pressure. As we found no indications for retrospective additions of soil material through mass movement or aeolian additions, we believe a vegetation dependent, biological pumping mechanism to be the most plausible explanation for the presence of silica bearing minerals in the La Selva topsoils. The vertical distribution of 1.0 nm halloysite and its accumulation in the lower reaches of the wetter alluvial soil suggest that this metastable mineral forms as a result of Si enrichment where the residence time of the pore water is long enough to allow for Si concentrations to exceed the halloysite precipitation threshold. Taken together, our evidence indicates gibbsite in the La Selva soils to be the endproduct of intense tropical weathering, while the presence of hydrated halloysite seems to have mainly kinetic reasons and is most probably coupled to the contemporary soil moisture regime.
    Keywords: Halloysite ; Gibbsite ; Allophane ; X-Ray Diffraction ; Silicon Cycling ; Tropical Soils ; Rain Forest ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 4
    Language: English
    In: Clays and Clay Minerals, 2009, Vol.57(1), pp.72-81
    Description: In Ecuador, DINAGE (known today as the Servicio Geológico Nacional) and the German Federal Institute for Geosciences and Natural Resources have discovered a huge allophane deposit covering an area of 〉4000 km 2 . This study presents the results from an investigation of a 16-m thick vertical sequence from this deposit, supposedly the weathering product of two different volcanic ash deposits. In particular, the distribution of alkali metals within the uppermost layer indicates that the weathering process is still ongoing. According to the mineralogical composition, an allophane-rich layer (allophane facies) could be distinguished from the underlying halloysite-rich layer (halloysite facies). A 2-m thick transition zone is characterized by the presence of gibbsite and intermediate specific surface area values. Only a few imogolite fibers could be identified (by scanning electron microscopy), indicating the dominance of allophane over imogolite in the allophane facies. Single allophane particles were investigated by atomic force microscopy, though this method was less accurate than transmission electron microscopy with respect to the determination of the primary particle diameter. Carbon isotope analysis ( 14 C) suggested an age of ∼20,000 y for the allophane layer. Within the allophane facies, a 4-m thick layer occurs containing 70–80 wt.% allophane with an N 2 -BET specific surface area of 〉300 m 2 /g. Based on infrared and energy-dispersive X-ray diffraction measurements, an Al/Si ratio of 1.3–1.4 was established for this allophane, which is between Al-rich and Si-rich allophane. The allophane layer may be of economic value due to the large allophane content, the small amount of organic matter, and the significant thickness of the deposit.
    Keywords: AFM ; Allophane ; Ecuador ; Nanoparticle ; Special Clay
    ISSN: 0009-8604
    E-ISSN: 1552-8367
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