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Goethite in an alfisol and an ultisol of Southern Taiwan
Abstract
One Alfisol and one Ultisol were selected from Kenting National Park in southern Taiwan to study their iron oxide minerals and Al substitution for Fe in these oxides by the differential x-ray diffraction (DXRD). The differences of physical and chemical properties between these two soils are mostly in soil pH and base saturation percentage. Both soils contain appreciable percentages of iron oxide (54 to 63 g/kg Fe d) that is predominantly goethite. Goethite was identified in the clay fraction of A and Bt horizons of both soils by the DXRD method. High crystalline index values (0.7 in Fe dTFe 1 or 0.6-0.7 in (Fe d-Fe 0)/Fe 1) and very low iron activity ratio (0.04-0.15 in Fe 0/Fe d) indicated little evidence for the presence of ferrihydrite in these soils formed in tropical environment. Calculation of Al substitution for Fe was based on x-ray diffraction (XRD) measurement of d-spacings on the d 111 and d 111-110 which indicate about 10 to 15% in two studied soils. The Al substitution for Fe is about the same amount in both soils yet there is evidence for slightly higher substitution in the goethite of the Alfisol than in the Ultisol. The results also suggested that there is evidence of a modest increase in Al for Fe substitution in the goethite of subsoils versus the top soils.
In the last 50 years, soil survey in Taiwan has made great contribution to the soil database and to good interpretation of soil management, fertilizer recommendation, and environmental quality, especially the detailed soil survey of Taiwan rural soils (1962–1976, 1974–1979), Taiwan hill land (1980–1988), contaminated soils (1984–1990), productivity grade of Taiwan upland (1986–1990), Taiwan forest soils in mountains (1993–2002), and Taiwan rural soils (1998–2005). Now, we face the time to develop the strategies of soil survey in Taiwan for the future. The major elements discussed include the soil survey techniques developed for contaminated soils, the National Soil Information System, regional soil museum, and to decide in which direction we can go.
The morphologically distinct materials in Darling Range lateritic duricrust (i.e. loose and cemented pisoliths, concretions, matrix, pisolith coatings and void coatings) exhibit goethite/(goethite + hematite) ratios ranging from 0.15 for individual pisoliths to 1.0 for void coatings. Mole % Al substitution ranged from 20 to 34% in goethite and from 2 to 15% in hematite. Goethite and hematite in pisoliths and concretions were mostly highly Al substituted. Al substitution in goethite was positively related (P < 0.01) to Al substitution in hematite. Al substitution in maghemite was less than 5%. Goethite crystals ranged in size from 130 to 260 A. Hematite crystals ranged from 140 to 520 A, and were systematically smaller in pisoliths. Crystal size of goethite and hematite decreased with increasing Al substitution. Hematite crystals were usually about 50% larger than goethite crystals in the same sample, and crystal sizes of goethite and hematite were positively correlated (P < 0.01). Goethite and hematite occurred as aggregates of subrounded platy crystals. Differences in the properties of goethite and hematite between morphologically distinct materials in single hand specimens are indicative of the complex history of these duricrusts.
Ferruginous concretions from various Australian and German soils and samples of bog iron ore from various European localities were examined by X-ray diffraction and electron microprobe techniques. The association between phosphorus and iron in these samples was looked at from the point of view of differences in iron mineralogy and genesis. When surface area is considered it appears that hematite, goethite and possibly maghemite show phosphorus adsorption levels in the same order of magnitude as those obtained on synthetic preparations of these minerals. In most cases a high phosphorus content is associated with a high iron, but the converse is not true, so that although the iron minerals are definite sinks for the phosphorus in the environment a linear relation cannot be expected due to the variation in time and spatial distribution of these two elements. In the younger European soils the phosphorus/iron ratios are generally higher (average 0.015), reaching values as high as 0.05 in some bog iron ores formed in gleys, whereas in the older Australian soils the average ratio is 0.002.
The iron oxide and clay minerals in some typical red and yellow podzolic soils from New South Wales have been investigated by X-ray diffraction and infra-red spectroscopy. The dominant iron oxide mineral is goethite containing about 13–14 mol % AlOOH, this being the mineral which gives the yellow soils their characteristic colour. The red soils also contain finely divided hematite which masks the colour of the goethite. Lepidocrocite was not detected in any of the soils examined. The dominant clay minerals are kaolinite and dioctahedral interstratified illite-smectite, the latter being more concentrated in the finer clay fractions, especially in soils developed on calcareous greywacke. In the red podzolic soil developed on Ashfield shale, illite-smectite is strongly interlayered with well-ordered aluminous material. Dickite occurs in this soil.
The nature of freshly-precipitated and aged hydrated ferric oxides prepared by the addition of ferric chloride to KOH was investigated by the use of scanning and transmission electron microscopy, X-ray diffraction, i.r. absorption, and pH 3·0 ammonium oxalate extraction. The results show the fresh material to be essentially non-crystalline hydrated ferric oxide, which when aged at 60 °C and high pH rapidly crystallizes as goethite, without any indication of coexisting hematite. The various methods were evaluated as indices of crystallinity for aging materials. The acid ammonium oxalate method was shown to extract selectively only the non-crystalline portion of such mixtures. The use of X-ray diffraction analysis for estimating aging stage requires elimination of the preferred orientation of the goethite crystals. While both the oxalate and X-ray methods can detect as little as 2 per cent crystallinity, the oxalate method is probably superior for quantitative determinations as it depends directly on an inherent difference in the solubility of the crystalline and non-crystalline materials, rather than on a technique dependent intensity measurement. The use of the intensity of the OH bending vibrations of the infrared absorption spectra can also potentially detect as little as 2 per cent crystallinity, but the procedure is probably less useful for quantitative determinations than the oxalate or X-ray methods because of the problem of evaluating the area under the peaks.
Mineralogical and chemical properties of Madison soil derived from quartz mica schist reflected the degree of weathering. The Madison soil had sandy surface horizons and clayey 13 horizons. The clay in surface horizons contained more mica than clay in the subsoils; a maximum of about 40 per cent mica in the coarse clay of surface horizons. Intergradient chlorite-vermiculite was present in appreciable quantities in clays of sur- face horizons. The presence in the whole soil of B2 horizons of about 15 per cent gibbsite and 12 per cent free iron oxides indicated a high degree of weathering. Kaolinite and gibbsite increased with depth and were the major clay minerals in all clay fractions in the lower portion of the profiles. Kaolinite and gibbsite also were important accessory minerals of the coarse and medium silt fractions and increased in percentage with depth. The B3 horizons had much of the morphology of the schist. Results indicated a high weathering intensity in the subsoil and leaching of weathering products.
Highly weathered soils of the humid tropics may exhibit loss of aggregate stability and dispersion of hydroxyl-bearing soil constituents as a result of management-induced changes in soil solution chemistry. The effects of pH and 1:1 "indifferent' electrolyte concentration on the surface charge and flocculation of colloids extracted from four representative kaolinitic tropical soils were examined by microelectrophoresis and light scattering methods. The pH and ionic strength dependence of colloidal stability observed for the soils is consistent with an electrostatic stabilization mechanism among their clay-sized particles. -Authors
Samples of 9 red and brown closely associated soils developed from highly calcareous materials in central Tuscany, Italy, were analyzed to determine: the mineralogical properties of the iron oxides, the relationship between redness and hematite content, and the relationship between parent material and iron oxide mineralogy.Goethite and hematite were the only crystalline iron oxides detected in the A, E and B horizons studied. X-ray data indicated that they were either poorly crystalline or of small particle size. A high proportion (67 to 95%) of the total iron was extracted by dithionite, and oxalate-extractable Fe was less than 10% of Fed in most of the samples. The extent of Al substitution in goethite was positively correlated (R2=0.68) to crystal dimension along the 110 direction. This finding is consistent with those for aluminous goethites synthesized at 25°C but at odds with some published data for natural goethites.Redness rating of dry crushed samples, as determined with Munsell colour charts, was positively correlated (R2=0.79) with hematite content. Redness rating, however, would provide only a crude estimate of hematite content.The soils with the lowest content of total iron had by far the lowest hematite content; nearly all of the crystalline iron oxide was goethite. This lends support to the hypothesis that parent material influences the development of iron oxides in soils.
The relative importance of iron oxides to the aggregation status of soil has been evaluated for thirteen samples collected from B horizons of Inceptisols and Alfisols. The particle size distributions before and after selective dissolution treatments were performed to assess the capacity of various iron oxides to aggregate soil particles. Iron extractable with NH4-oxalate was more effective in aggregation than goethite and hematite. The capacity of iron oxides to aggregate clay-sized particles was not significantly different in samples containing goethite and hematite from those having goethite only. Goethite and hematite in clay and in sand fractions had similar Al substitution and mean crystallite dimensions but different reductive dissolution rates in dithionite.