Use of the Kubelka—Munk theory to study the influence of iron oxides on soil colour

The Kubelka–Munk theory, which is commonly applied to ‘pigment mixtures’, adequately predicted the colour of mixtures of synthetic haematite, synthetic goethite and deferrated soil powders. The theory illustrated the higher pigmenting effect of haematite compared with goethite. In mixtures containing haematite the calculated colour coordinates could be combined into simple ‘redness’ indices that were highly correlated with haematite content and were not appreciably influenced by goethite. The theory was also applied to the study of the chromatic characteristics of 98 soils, differing widely in their origin but having an organic matter content <2% in which haematite or goethite was the dominant Fe oxide. The theory showed that the average ‘soil’ haematite and ‘soil’ goethite had colours similar to those of their synthetic counter-parts, whereas the rest of the soil components could be considered as an essentially ‘grey’ matrix. It was also useful in predicting the haematite and goethite contents of soils either from several reflectance measurements of soil-white standard mixtures or from the indices of redness developed for the synthetic mixtures.     

高梯度磁场分离后土壤矿物组成的变化

The changes of clay mineral association after high-gradient magnetic separation(HGMS) treatment,and the effects of chemical and physical technologies on concentrating Fe oxides for mian soils in central and southern China were investigated by means of X-ray diffraction (XRD) and chemical analysis methods.Results indicated that the concentrating times of Fe oxides by HGMS treatment were the largest for 0.2-2μm size fraction in the examined soils .For the soils in which 2:1 phyllosilicates were dominant,concentrating times of iron oxides by HGMS treatment were larger than by 5 mol L^-1 NaOH treatment .Phyllosili-cates were decreased after HGMS treatment ;however,the decrease was less than that of kaolinite,The goethite/(goethite hematite) values in Fe oxides of the soils kept virtually constant after HGMS treatment.     

The Contrasting Effect of Goethite and Hematite on Phosphate Sorption and Desorption by Terre Rosse

Phosphate sorption and desorption in soils are markedly influenced by iron oxides, although little is known on how the common iron oxides differ in their behaviour towards added phosphate. In this study, we investigated phosphate sorption and desorption in the clay fractions of 12 Terre Rosse that ranged widely in Fe oxide content, had very low contents of oxalate-extractable Fe oxides and different hematite/goethite ratios. Phosphate sorption at an equilibrium concentration of 1 mg P 1^?1 was correlated with the goethite but not with the hematite content of the clay fractions. When phosphate was desorbed by electro-ultrafiltration, the difference in desorption half-time between untreated and deferrified clays was positively correlated with the goethite but not with the hematite content. These results suggest that goethite is more active than hematite in phosphate sorption and retention by soils.     

Transformation of haematite and Al-poor goethite to Al-rich goethite and associated yellowing in a ferralitic clay soil profile of the middle Amazon Basin (Manaus, Brazil)

The red and yellow colours of ferralitic soils in the tropics have for long intrigued pedologists. We have investigated the upward yellowing in a 10-m thick profile representative of the Ferralsols of the plateaux of the Manaus region of Brazil. We determined changes in the nature and crystal chemistry of their Fe oxides by optical and Mössbauer spectroscopy as well as Rietveld refinement of X-ray diffraction patterns. We attribute the upward yellowing of the soil to a progressive transformation of the Fe oxides at nearly invariant iron contents. Aluminium in contrast is strongly mobilized in the uppermost clay-depleted topsoil where there is preferential dissolution of kaolinite and crystallization of gibbsite. Haematite decreases from 35 to 10% of the Fe oxides from the bottom to the top of the profile and the particles become smaller (75–10 nm). Its Al for Fe-substitution remains almost unchanged (10–15 mol %). The average Al-substitution rate of goethite increases from 25 to 33 mol %, and its mean crystal diameter remains in the range 20–40 nm. The proportion of Al-rich goethite (33 mol %) increases at the expense of less Al-substituted Fe oxides (haematite and goethite). This conversion with restricted transfer of iron means that the amount of Al stored in Fe oxides gradually increases. Kaolinite, haematite and Al-poor goethite are thus witnesses of earlier stages of ferralitization of the soil. In contrast, Al-rich goethite and gibbsite initiate the alitization (or bauxitization) of the soil. They correspond to the last generation of soil minerals, which most likely reflects the present-day weathering conditions. The progressive replacement of kaolinite, haematite and Al-poor goethite by new generations of Al-rich goethite and gibbsite attests to greater activities of water and aluminium and smaller activity of aqueous silica in the topsoil than in the subsoil. We interpret this as a consequence of longer periods of wetting in the topsoil that could result from soil aging, more humid climate or both.     

Iron oxides in lateritic soils from Western Australia

Lateritic soils developed from dolerite contained larger amounts of goethite and haematite than those developed from granite. The goethite/(goethite + haematite) ratio in granitic soils ranged from 0.55 to 1 and from 0.29 to 0.83 in doleritic soils. Maghemite ranged in abundance from 0 to 10% and it only occurred in duricrust. Mole % Al substitution ranged from 16 to 33% in goethite and from 2.5 to 10% in haematite and was similar for both granitic and doleritic soils. Al substitution in maghemite was <5%. A significant, positive correlation (P<0.01) occurred between Al substitution in goethite and the amount of gibbsite in the soil. The dehydroxylation temperature of goethite ranged from 292 to 334°C and was positively correlated with the mole % Al substitution. Goethite crystals varied in size from 16 to 26 nm and haematite crystals from 18 to 59 nm. Goethite and haematite crystals occurred as aggregates of subrounded platy crystals.
Iron oxides obtained by NaOH treatment contained much of the minor element contents of the soils; mean concentrations (μg g⁻¹) were: Zn 19.9, Cu 31, Mn 68, Ni 140, Co 32, Cr 394 and V 696. These minor elements were most abundant in iron oxides derived from dolerite, but were not preferentially associated with goethite or haematite.     

Types of Crystalline Iron Oxides and Phosphate Adsorption in Variable Charge Soils

The types,contents and morphologies of crystalline Fe oxides and their relations to phosphate adsorption on the clay fractions in soils with varable charge in southern China were investigated by means of XRD,TEM,EMA and chemical analysis methods.Results indicated that the types and contents of crystalline Fe oxides varied with the soils examined.The dominant crystaline Fe oxide was hematite in the latosols and goethites in the red soils.In yellow-brown soils,the only crystalline Fe oxide was goethite.The difference between Ald and Alo came mainly from the Al Substituting for Fe in the Fe oxides.The crystal morphology of goethite appeared mainly as subrounded flat or iso-dimensional rather than acicular particles,Hematities occurred in plates of various thickness,Their MCDa/MCDc ratios in the latosols and red soils were generally above 1.5 and below 1.5,respectively.The MCD values of goethites and hematites were 15-25nm and 20-35nm,and their specific surface areas were 80-120m^2/g and 35-75m^2/g,respectively.The goethite crystals were generally smaller,Variations of the total amounts of crystalline Fe oxides in clay fractions were not related to plhophats adsorption.The types,contents and morphologies of crystalline Fe oxides in the soils remarkably affected phosphate adsorption characteristics of the soils.The phosphate adsorption of goethite was much greater than that of hematite,The higher th MCDa/MCDc ratio of hematite,the lower the phosphate adsorption.     

氧化铁对土粒强胶结作用的矿物学证据

DCB脱铁处理高铁土壤前后用NaOH -超声波分散的四个粒级 (<2 μm、2～ 2 0 μm、2 0～ 2 5 0 μm和 2 5 0～2 0 0 0 μm)的矿物组成变化研究表明 :氧化铁对土壤颗粒有很强的胶结能力 ,它可与高岭石、蒙脱石等粘粒矿物胶结形成非常稳定的大颗粒团聚体 ,这些团聚体即使用NaOH -超声波也很难分散     

Iron oxide mineralogy of some soils of two river terrace sequences in Spain

The iron oxides of soils of two river terrace sequences in Spain which show an increasing degree of redness with age were studied. Clay fractions contained only small amounts of oxalate-extractable Fe. Goethite and hematite, the only crystalline Fe-oxides identified, were determined quantitatively by X-ray diffraction (XRD) after concentrating the Fe-oxides by boiling in $\text{5N}$ NaOH and subtracting the step-counted diffractogram of the deferrated clay from that of the non-deferrated clay, obtaining thus a “pure” Fe-oxide diffractogram. EDTA extracted hematite preferentially to goethite as is seen by loss of red colour and by XRD. A good correlation was found between the content of hematite in the fine earth and a redness rating based on Munsell notations.In the Guadalquivir River sequence, Fe_d and Fe_d/Fe_t increased with age. The amount of both goethite and hematite formed from silicate-Fe increased with soil age but hematite increased more than goethite, possibly due to the xeric soil environment. Also, goethite increased in crystallinity as indicated by a decrease in XRD line broadening and Fe_o/Fe_d ratios. No such trends were found in the Esla River sequence, possibly because the initial alluvium was already highly weathered as shown by high Fe_d/Fe_t values (0.8) irrespective of terrace level.Al substitution in goethite calculated from XRD increased with soil age, reflecting the increasing acidity of the soils. Al substitution in hematite was markedly lower.     

Iron oxide mineralogy of Terre Rosse and Rendzinas in relation to their moisture and temperature regimes

Terre Rosse and Rendzinas occur on limestones close together under the same climatic conditions. Their differences in iron oxide mineralogy are believed to be due to differences in the pedoclimate. We monitored the soil moisture tension and the soil temperature over nearly 3 years in Terra Rossa-Rendzina pairs on hard limestone and soft chalk in Israel. The Terre Rosse dried out more rapidly than the Rendzinas mainly because of their smaller water-holding capacity. In summer, average soil temperatures in the Rendzinas were significantly lower than in the Terre Rosse. Haematite dominates the red Terre Rosse and goethite the yellower Rendzinas. This difference appears to be related to the soil climate, specifically the moisture regime of the two soils, and supports the hypothesis that release of Fe and formation of ferrihydrite in Terre Rosse during the wet winter is followed by transformation to haematite during marked desiccation in the dry summer. By contrast, wetter soil conditions in the Rendzinas direct the formation of Fe oxides more towards goethite. either directly or by transformation of ferrihydrite via solution.     

Iron oxide and clay minerals and their relation to colours of red and yellow podzolic soils near Sydney,Australia

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.     

Spectroscopic control of iron oxide dissolution in two ferralitic soils

Second derivative diffuse reflectance spectroscopy (DRS) in the visible range has been used to characterize changes in colour and identify the nature of Fe oxides which withstand reduction during experimental yellowing of reddish materials. It is accepted that haematite dissolves preferentially and faster than goethite, and Al-substitution controls the dissolution kinetics of Fe oxides. However, DRS has shown that haematite is more resistant than predicted and that some Fe-oxides, probably trapped within kaolinite particles, are inaccessible to solvents. DRS allows the nature of dissolved phases at each deferration step to be determined and changes in Al-content of residual phases throughout deferration to be followed. It also demonstrated that Helmholtz coordinates correlate very well with changes in Fe-oxide mineralogy and are preferable to redness ratings when monitoring differential dissolution of Fe oxides through colour measurements. DRS is a powerful and sensitive technique for monitoring the dissolution of Fe oxides in soils.     

Trace magnetic minerals to detect redox boundaries and drainage effects in a marshland soil in western France

The marshlands of western France provide the opportunity to relate the magnetic properties of a recent sedimentary clay soil to pedogenesis, including the impact of agriculture and drainage on magnetic mineralogy. We studied a plot of drained land that had been ploughed up to 1998 and under grass since. A new thermomagnetic method was used to identify and to quantify roughly the magnetic minerals, which represent less than 1 g kg⁻¹. The most abundant Fe oxides are haematite (45%) and goethite (45%). However, trace amounts of the ferrimagnetic minerals maghemite (4%), magnetite (3%) and, to a lesser extent, iron sulphides (1%) provide most of the magnetic signal. This signal allowed us to identify magnetic horizons that relate to the soil horizons. The topsoil is characterized by a strong magnetic enhancement (4-fold), and the thickness of this layer increases close to the drains. Relative contents of ferrimagnetic phase also increase laterally with decreasing distance to the drains. Magnetic enhancement coincides with the most aerated and developed layer, in which water circulation, root colonization and hence soil fertility are greater. Ferromagnetic minerals sensu lato, especially maghemite and magnetite, can be considered as mineralogical tracers partly and indirectly reflecting soil fertility of these clay-rich marshland soils.     

Sorption and desorption of cobalt by soils and soil components

The sorption of Co by individual soil components was studied at solution Co concentrations that were within the range found in natural soil solutions. Soil-derived oxide materials sorbed by far the greatest amounts of Co although substantial amounts were also sorbed by organic materials (humic and fulvic acids). Clay minerals and non-pedogenic iron and manganese oxides sorbed relatively little Co. It is considered that clay minerals are unlikely to have a significant influence on the sorption of Co by whole soils. Cobalt sorbed by soil oxide material was not readily desorbed back into solution and, in addition, rapidly became non-isotopically exchangeable with solution Co. In contrast, Co was relatively easily desorbed from humic acid and a large proportion of the Co sorbed by humic acid remained isotopically exchangeable. Cobalt sorbed by montmorillonite was more easily desorbed than that sorbed by soil oxide but less easily than that sorbed by humic acid. Cobalt sorption isotherms for whole soils at low site coverage were essentially linear and the gradients of isotherms increased with pH. A comparison of isotherm gradients for whole soils and individual soil components supported the suggestion that Co sorption in whole soils is largely controlled by soil oxide materials.     

铁氧化物与土壤表面电荷性质的关系

The relationship between iron oxides and surface charge characteristics in variable charge soils (latosol and red earth) was studied in following three ways.(1)Remove free iron oxides (Fed) and amorphous iron oxides (Feo) from the soils with sodium dithionite and acid ammonium oxalate solution respectively.(2) Add 2% glucose (on the basis of air-dry soil weight) to soils and incubate under submerged condition to activate iron oxides,and then the mixtures are dehydrated and air-dried to age iron oxides.(3) Precipitate various crystalline forms of iron oxides onto kaolinite.The results showed that free iron oxides (Fed) were the chief carrier of variable positive charges.Of which crystalline iron oxides (Fed-Feo) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges,and did little influence on negative charges.Whereas the amorphous iron oxides (Feo),which presented mainly fas a coating with a large specific surface area,not only had positive charges,but also blocked the negative charge sites in soils.Submerged incubation activated iron oxides in the soils,and increased the amount of amorphous iron oxides and the degree of activation of iron oxide,which resulted in the increase of positive and negative charges of soils.Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide,and also led to the decrease of positive and negative charges.Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges.Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges.Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.     

Stability behavior of soil colloidal suspensions in relation to sequential reduction of soils

Flocculation and dispersion of colloidal particles of nine inorganic paddy soils were studied mainly based on turbidity measurements of the suspensions of soils which were previously incubated at 28°C under in vitro waterlogged conditions. After 1-week of incubation, the turbidity of the soils except for 1) two soils containing larger amounts of sodium salts and 2) one soil containing larger amounts of Fe and Al oxides, significantly decreased, and colloidal particles flocculated with 1) a decrease in soil Eh and 2) an increase in electric conductivity (EC). During the 3- to 4-week period of waterlogging, the turbidity of the three soils significantly increased with the 1) decrease in EC and 2) increase in pH of the soils although the Eh remained low. Infrared (IR) absorption analysis showed that the suspended colloidal particles consisted of layer silicates from respective soil clays. Oxidation of suspensions of waterlogged soils by air-bubbling led to an increase in turbidity with the 1) increase in Eh, and 2) decrease in pH, EC, and water-soluble Fe₂₊ concentration. It was suggested that the stability of the soil colloidal suspensions was affected by soil reduction with alterations in ionic species and their concentrations at clay surfaces and in soil solutions.     

The use of boron isotopes to evaluate boron uptake by rape grown in acid soil treated with boron containing goethite

Boron (B), taken up by plants, comes mainly from boron adsorbed by soil constituents, in particular by metal hydrous oxides, organic matter, and edges of clay minerals. The extent and availability of B adsorbed or occluded by soil minerals is unknown due to the lack of a methodology for probing activity of this type of boron. In this study, ¹⁰B labeled boron‐containing goethites, i.e., goethite with adsorbed B (ad‐B‐goethite) and occluded B (oc‐B‐goethite), were added individually to an Ultisol for pot experiments to probe soil B bioavailability. The fraction of soil B extracted from B‐containing goethite showed a sigmoidal extraction pattern similar to that of B adsorbed on soil minerals. The rape seedling uptake of B from ad‐B‐goethite treatment of soil was close to that from soil background (50%), while that from oc‐B‐goethite treatment of soil was about 66%. The B absorbed from both B‐containing goethite and soil was mainly accumulated in the shoot; less than a tenth of the B was accumulated in the root. In summary, the behavior of B in B‐containing goethite was generally similar to that of soil B, indicating that B containing goethite can be used to probe migration of B from soil to plant.     

蒸腾潜能的模型研究及估算中国土壤湿度状况

The morphologies of goethites in latosol,red soils,yellow-brown soil and the paddy soils developed from red soils were studied in comparison with the morphology of synthetic goethite by means of the X-ray diffration,transmission electron microscopy and energy dispersive X-ray analysis.The synthetic goethite displayed acicular particles elongated in the c-direction.The goethites in the latosol,red soils and yellowbrown soil were platy particles stretched in two directions or isodimensional particles,and those in the paddy soils from red soils were acicular,short columnar,palty or isodimensional particles,Various morphologies of the goethites probably suggested their different dominant crystal faces,surface charge distribution and surface adsorption characteristics.     

Forms and pedogenetic distribution of extractable iron and aluminum in selected soils of Nigeria

The content of various forms of Fe and Al in six well-drained soil profiles sampled from different parts of Nigeria was determined by selective extraction methods. Dithionite-Fe (total free Fe oxides) content increases with the increase of depth. The oxalate-extractable Fe (amorphous Fe oxides) constitutes less than 10% of the total free Fe oxides throughout the profiles. The active Fe ratio decreases with the increase of profile depth, suggesting that larger proportions of Fe oxides are present as crystalline forms in the lower horizons of these well-drained profiles. Little or no relationships were found in the case of Al.The constant clay/dithionite-Fe ratio within the four profiles from the wetter southern part of Nigeria indicates the co-migration of clay and Fe oxides from the A horizon into the B horizon (lessivage). However, this relationship was not observed in the two soil profiles sampled from the drier northern part of the country.The need for expansion or alteration of the present U.S.D.A. system of soil classification is emphasized.     

Lateritic and redoximorphic features in a faulted landscape near Manaus, Brazil

The soils and sediments of the uplands in the Manaus region are described and analysed along a representative cross‐section. There are two broad types of features, lateritic and redoximorphic. Their formation is linked to two main processes acting under contrasted hydrological regimes. The first process, acting under well‐drained conditions, is lateritization. It has transformed strongly weathered sediment into soil and led to depletion of silica (mainly quartz) as well as to relative accumulation of both kaolinite and iron oxides (haematite and goethite). Crystallographic changes observed in the latter have resulted from alternating dissolution and crystallization cycles without significant transfer of iron and alumina. However, in the uppermost soil, dissolution of kaolinite has prevailed over crystallization, leading to depletion of clay and the formation of tiny crystals of gibbsite disseminated throughout the groundmass. The second process results from the development of reducing conditions in groundwater giving redoximorphic features in lateritic soils and sediments. In the sediments, iron has been depleted by regional aquifers to form a pallid zone. In the soil, large amounts of iron and minor amounts of alumina, mainly from aluminous goethite, have been mobilized at first in small patches, which with further mobilization and vertical transfer of these elements have increased in size and have led to the formation of bleached horizons over thin iron pans. Iron has crystallized predominantly as haematite in the iron pans and alumina as large crystals of gibbsite in soil voids. Formation of impervious iron pans holds up fluctuating perched groundwater in the overlying horizons depending on rainfall events. Neotectonic events (formation of uplifted blocks and small grabens) have markedly altered the hydrological regimes. In the uplifted blocks, the soil has been deeply truncated and iron loss has been checked in the uppermost sediment. By contrast, mobilization of iron has been initiated at various places in the soil of the small grabens. In this way tectonic events have checked mobilization of iron in sediments but activated it in soils, leaving spectacular fingerprints on the landscape.     

Highly weathered soils in the east coast hinterland of Southern Africa with thick, humus-rich A1 horizons

Strongly weathered red and yellow soils with thick (490–900 mm) humic Al horizons (Haplohumox and Palchumults) derived from sandstones and basic igneous rocks, and occurring near the east coast of Southern Africa, are described and discussed in terms of their distribution, morphology, texture, mineralogy, chemistry, genesis and classification. The high organic matter content (2–5%C) of the Al contributes significantly to a high pH-dependent negative charge, probably to poorer crystallinity of goethite and kaolinite and to the transformation of haematite to goethite. Varying proportions of kaolinite and gibbsite reflect different soil ages within these old landscapes. The yellow colour of aluminous goethite, the main pedogenic iron oxide, is masked in the Al by organic matter and in red B2 horizons by haematite. Temperature may have influenced the broad pattern of occurrence of red and yellow B2 horizons. These soils need not, as previously suggested, have developed from the weathering products of an ancient laterite. Neither Soil Taxonomy nor the South African soil classification system accommodates the soils entirely satisfactorily and possible improvements to the latter are discussed.