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DIVISION S-9—SOIL MINERALOGY

Trace Elements in Manganese-Iron Nodules from a Chinese Alfisol

^a Department of Soil and Agrochemistry, Huazhong Agricultural University, Wuhan, China
^b Dipartimento di Scienze Animali, Vegetali e dell'Ambiente, Campobasso, Italy
^c Dipartimento di Scienze Chimico-Agrarie, Università di Napoli Federico II, Portici, Italy

* Corresponding author (colombo{at}unimol.it)

	ABSTRACT

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The morphological properties, the mineralogy, and the chemical composition of Mn-Fe nodules, collected from an Alfisol in Southern China (Wuhan), were studied by optical microscopy (OM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), total and sequential dissolution chemical analyses and differential x-ray diffraction (DXRD). This information is needed to gain a further understanding of nodule formation in soil environments and the influence of Mn and Fe oxides on the phyto-availability of both nutrients and toxic trace metals. Element quantification and mapping showed that Si, Al, Fe, Mn, Ca, K, and Ti were the main elements present in the nodules. Nodules were found to be highly variable in composition and in degree of banding of Fe and Mn. In spherical nodules, Fe and Mn were distributed mainly in concentric bands. With few exceptions, heavy metals were concentrated in the finest fraction (<2 µm). Copper and Ni were associated mainly with Mn, whereas Cr and V were associated with Fe. The majority (90%) of total Fe in the <2-µm fraction was extracted by ammonium oxalate and dithionite reagents. Most (61%) of total Mn was dissolved by hydroxylamine hydrochloride. It was ascertained by DXRD analysis that ferryhydrite and goethite were the poorly crystalline and crystalline Fe phases, and that lithiophorite and vernadite were the Mn oxides. Micromorphological, chemical, and mineralogical results suggest that the growth of nodules was initiated by flocculation of phyllosilicates in oxidative environments. The MnO₂ minerals may have inhibited recrystallization of the ferryhydrite.

Abbreviations: DCB, metals associated with crystalline Fe oxides • DXRD, differential x-ray diffraction • EDS, energy dispersive spectroscopy • HAHC, metals occluded in easily reducible Fe and Mn oxides • ICP-AES, inductively coupled plasma atomic emission spectroscopy • OM, optical microscopy • OXA, metals bound to amorphous materials • PZC, point of zero charge • RES, residual metals mainly in mineral lattice structure • SEM, scanning electron microscopy • TEM, transmission electron microscopy • XRD, x-ray diffraction • XRF, x-ray fluorescence spectrometry

	INTRODUCTION

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MANGANESE OXIDE AND hydroxide minerals are very important constituents in soils because Mn is an essential element for plant and animal nutrition. In addition, Mn minerals have a high sorption capacity for many heavy metals, and thereby, affect the phyto-availability of both nutrient and toxic trace elements (McKenzie, 1980; Huang, 1991). Metal cations are specifically adsorbed by Mn oxides when the surface becomes negatively charged with increasing soil pH. In general, the affinity of metal ions for the surface of Mn oxides is: Pb > Cu > Mn > Co > Zn > Ni (McKenzie, 1980). Thus trace elements adsorbed on the Mn minerals may be entrapped within nodules during nodule formation (McKenzie, 1989).

In soil, the reduction and oxidation of Fe and Mn, as a result of seasonal changes in soil moisture, contributes to the formation of nodules and concretions (Burns and Burns, 1975). Under reducing conditions, Fe and Mn oxides release Fe(II) and Mn(II) ions to the soil solution. When the soil dries out, Fe(II) and Mn(II) oxidize and precipitate, forming new metal oxides. Manganese is more mobile than Fe and requires a higher oxidation potential compared with Fe(II) (Lindsay, 1979). These differences contribute to the formation of alternate layers of Fe-rich and Mn-rich material observed in many nodules (White and Dixon, 1996). Oxidation of Fe(II) proceeds faster in the presence of Mn(IV) oxides (McKenzie, 1989). Birnessite may promote the crystallization of Fe hydrolytic products under certain E_H-pH conditions, forming lepidocrocite (-FeOOH), goethite (-FeOOH), akaganeite along with ferryhydrite (Krishnamurti and Huang, 1989a; Krishnamurti, 1997).

Macroscopic characteristics of nodules and the distribution of Fe and Mn oxides within nodules have been the subject of several studies (Cescas et al., 1970; Phillippe et al., 1972; Gallaher et al., 1973; Schwertmann and Fanning, 1976; White and Dixon, 1996). Bands with different concentrations of Fe and Mn are often observed in nodules. High variability in the number and degree of expression of banding within or between nodules from a single site have been noted. Spherical nodules tend to be concentrically banded, whereas irregularly shaped nodules usually lack well-developed Fe and Mn banding. The variable distribution of Fe and Mn oxides within nodules is the result of the differences in redox potential required for the oxidation of the two elements and reflects variations in soil microenvironment surrounding the nodule.

A large number of Mn oxides have been found in soils and in special features such as nodules, segregations, concretions, and coatings. For example, lithiophorite, birnessite, vernadite, hollandite, pyrolusite, todorokite, cryptomelane, and husmannite have all been identified in soils (Taylor et al., 1964; Childs, 1975; Ross et al., 1976; Chukhrov and Gorschkov, 1981). In a complex matrix, such as soil, it is often difficult to identify Fe and Mn oxides by x-ray diffraction (XRD). Because of a low degree of crystallinity these minerals are characterized by broad and weak diagnostic XRD reflections which are often overlapped by more intense XRD peaks from other minerals. Various procedures have been proposed to extract Fe and Mn oxide minerals from nodules (McKenzie, 1989). Tokashiki et al. (1986) and Golden et al. (1993) combined selective dissolution procedures with XRD analysis. This approach was useful to identify and characterize the Mn minerals.

The aim of this research was to examine the macroscopic characteristics and the nature, mineralogy, and chemical composition of Fe and Mn oxides in nodules from an Alfisol, found in the Wuhan Region of Southern China. The total content of major and trace elements in the nodules was related to nodules characteristics.

	MATERIALS AND METHODS

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About 1000 samples (2 kg) of Mn-Fe nodules about 0.5 to 5.0 mm in diam. were collected at a 40-cm depth from the Bt horizon of a subacid Orthic Agrudalf developed on quaternary, siliceous, alluvial sediments, found in South Central China (Wuhan). The soil had a dark reddish brown (2.5 YR 3/4) clay loam surface horizon (0- to 15-cm depth) with wavy clear boundary and friable, granular structure. The Bt horizon (15- to 40-cm depth) was light yellowish brown (2.5 YR 5/4) with a clay loam to clay texture, wavy clear boundary and a moderately friable, blocky structure. It contained dark reddish brown (2.5 YR 5/4) nodules. Nodules were separated from the soil by wet sieving and stored at room temperature before grinding.

Particle-size Fractionation
About 500 nodules (1 kg of sample) of similar weight and size (1–10 mm) were ground in distilled water using an agate mortar. The material from the ground nodules was fractionated by wet sieving to separate the >125-, 125- to 50-, and <50-µm fractions. The <50-µm fraction was treated with sodium hexametaphosphate [(NaPO₃)₆] and further separated by sedimentation to separate the 50- to 10-, 10- to 5.0- and <5.0-µm fractions. Finally, the <5.0-µm fraction was centrifuged to separate the 5.0- to 2.0- and <2.0-µm fractions.

Elemental Analysis
Elemental composition of each particle-size fraction was determined by x-ray fluorescence (XRF) spectrometry (Si, Ca, Mg, K, and Na), using fused beads (flux consisting of lithium tetraborate, lithium carbonate, and lanthanum oxide) (Amonette and Sanders, 1994), and by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Fe, Al, Mn, Ba, Ti, Pb, Co, Zn, V, Zr, Ni, Cr, Cu, and Cd), after acid digestion of the samples (HF/HNO₃) (Sawhney and Stilwell, 1994).

Sequential Selective Dissolution
Four successive selective dissolutions were carried out on the <2.0-µm fraction according to the scheme reported in Table 1. The extractions were carried out at ambient temperature, in 100-ml polypropylene bottles, using 1 g of air-dried sample. After each chemical treatment, the extracts were separated by centrifuging at 1500 x g for 15 min and the residue was washed with deionized water. Manganese, Fe, Al, and Si in the extracts were determined by ICP-AES.

Optical and Electron Microscopy
For OM observations, a large number of nodules (40) was impregnated with a polyester resin (Crystic). After hardening, thin sections were cut from the impregnated blocks (FitzPatrick, 1984) and observed with an optical Zeiss Axioplan 2 microscope (Carl Zeiss Ltd., Italy) using polarized and oblique incident light.

Scanning electron microscopic observations were made on selected areas of the thin sections with a Zeiss DSM 940 microscope (Carl Zeiss Ltd., Italy) equipped with a Link System INCA Energy Dispersive X-Ray Analyser (EDXRA). For SEM examination, the samples were mounted with colloidal C on aluminum stubs and coated with C. Quantitative x-ray analysis was performed at 20 kV using a working distance of 2.5 mm with a spot size of 5 µm and applying the ZAF standard matrix correction procedure to convert X-ray intensities into weight percents. Because of the low concentration of the elements of interest long exposures of up to 60 min were necessary for the elemental distribution maps, which resulted to be the result of the acquisition of about 20 images.

Transmission electron microscopic observations of the <2.0-µm fraction were made with a Philips CM12 electron microscope (Philips Industrie S.A., Wavre, Belgium) also equipped for energy dispersive x-ray microanalysis. For TEM examination suspended material was dried on formvar-coated Cu grids and subsequently coated with C to enhance conductivity. Electron diffraction patterns were obtained at 100 kV and calibrated with the diffraction pattern produced by a 10-nm thick preparation of Au using the same analytical conditions.

	RESULTS AND DISCUSSION

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Main Nodule and Soil Characteristics
As reported in Table 2, the nodules contained considerably higher amounts of Fe than Mn. Manganese was concentrated in the nodule by a factor of 48.5 compared with the bulk soil, Fe by a factor of 1.25. Generally, the pH of the ground nodule samples was higher than that of the soil. The nodules showed a low point of zero charge (PZC). The measured particle-size distribution indicates very low content of clay material in the nodule compared with the bulk soil. Incomplete dispersion of the original nodule ground material because of the strong cementing action by the Mn and Fe minerals could explain this observation.

View larger version (116K): [in this window] [in a new window]   Fig. 1. Optical micrographs of three representative nodules: (a) nodule embedded in resin block (oblique incident light); (b) thin section of nodule (plain polarized light); (c and d) thin section showing the soil matrix rim on a nodule (plain polarized light, crossed polarized light). Scale bar equals 0.5 mm.

View larger version (130K): [in this window] [in a new window]   Fig. 2. Scanning electron image showing (a) a spherical, banded nodule; (b) semiquantitative energy dispersion spectroscopy analysis of 21 points at equal distances across the nodule; x-ray mapping of (c) Fe and (d) Mn.

View larger version (172K): [in this window] [in a new window]   Fig. 3. Scanning electron image showing (a) an unbanded nodule; (b) semiquantitative energy dispersion spectroscopy analysis of 24 points at equal distances across the nodule; X-ray mapping of (c) Fe and (d) Mn.

View larger version (118K): [in this window] [in a new window]   Fig. 4. Scanning electron images (a, b, c, and d) and Fe and Mn distribution maps for selected nodules showing the variability in banding and chemical concentrations.

The proportion of Fe extracted by NH₄-oxalate and dithionite reagents were 35 and 55% of the total Fe content, respectively. The hydroxylamine hydrochloride treatment dissolved only 4% of the total Fe and 6% of the Fe was in the residual fraction. The dithionite reagent also extracted 22% of total Al. However, Al and Si were mainly concentrated in the residual fraction (70 and 93%, respectively).

The large amounts of Fe and Mn dissolved by OXA and HAHC, respectively, indicate that the nodules contain substantial amounts of poorly ordered Fe and Mn oxides. The very low percentages of the total Mn and Fe dissolved by OXA and HAHC, respectively, indicate that the Fe and Mn oxides exist as discrete phases in the nodules.

X-ray Diffraction and Differential X-ray Diffraction Analysis
X-ray diffraction analysis of the nodules revealed, in both 5.0- to 2.0- and <2.0-µm size fractions, the presence of illite (1.01- and 0.502-nm XRD peaks), kaolinite (0.719- and 0.257-nm XRD peaks) and quartz (0.426- and 0.335-nm XRD peaks) (data not shown). The more intense reflections of these well crystallized phases made it difficult to detect the Mn and Fe oxides. To more clearly identify the Mn and Fe oxides, the DXRD technique (Schulze, 1981) was applied to the clay fraction after sequential selective dissolution (Fig. 5 and 6) . Furthermore, with the intent to improve detection of Mn oxides both Cu-K and Co-K radiations were used.

View larger version (27K): [in this window] [in a new window]   Fig. 5. Differential x-ray diffraction diagrams (Cu-K radiation) obtained by subtracting x-ray diffraction (XRD) patterns for the (a) HAHC- and (b) OXA-treated clay fraction (<2 µm) from the XRD pattern for the untreated sample. Fr represents ferryhydrite, Gt represents goethite, Lt represents lithiophorite, V represents vernadite.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Differential x-ray diffraction diagrams (Co-K radiation) obtained by subtracting the x-ray diffraction (XRD) patterns for the (a) HAHC + OXA- and (b) HAHC + OXA + DCB-treated clay fraction (<2 µm) from the XRD pattern for the untreated sample. Fr represents ferryhydrite, Gt represents goethite, Lt represents lithiophorite.

The HAHC treatment preferentially removed Mn oxides. However, the presence of broad peaks at 0.254 and 0.222 nm also indicated some ferryhydrite dissolution. The effective removal of ferryhydrite was obtained when the HAHC treated clay fraction was extracted with acid oxalate reagent (Fig. 5b and 6a). The last selective treatment (HAHC + OXA + DCB) dissolved preferentially goethite (Fig. 6b), which was also partially dissolved after HAHC + OXA treatment, as evidenced by 0.419-, 0.245-, and 0.150-nm peaks in the DXRD patterns (Fig. 6a).

The poorly crystalline Mn oxide (birnessite-like mineral), which is negatively charged at neutral pH (PZC 2.0), has been reported to adsorb large amounts of positively charged Fe hydroxy-polymers, perturbing the crystallization processes of Fe minerals (Krishnamurti and Huang, 1989b). This behavior could account for the apparent coexistence of ferryhydrite with poorly crystalline Mn phases both dissolved by the HAHC treatment. The formation of goethite increases at low concentration of Mn, more alkaline pH, and E_H values below that needed for the formation of hematite (Cornell and Giovanoli, 1987; Krishnamurti and Huang, 1989a; Schwertmann and Taylor, 1989). The formation of goethite and hematite are by competing mechanisms: hematites form by dehydration and internal rearrangement of ferryhydrite (pH close to neutral, oxidizing conditions), whereas goethite forms at pHs that promote dissolution of ferryhydrite and nucleation of goethite (pH < 6.0, reducing conditions) (Schwertmann, 1985; Cornell et al., 1989).

Differential x-ray diffraction observations suggest that nodule growth starts with a deposit of ferryhydrite as a nucleating agent, followed by formation of goethite when the activity of the Mn²⁺ in the solution decreases and the (E_H-pH) oxidation potential rapidly increases. The intergrowth of Fe and Mn phases depending on the E_H-pH variation would explain the coexistence of poorly ordered Fe and Mn minerals in intimate association in some bands and the different distribution of these elements in other bands.

Transmission Electron Microscopy and Electron Diffraction Observations
Transmission electron microscopic observations of the <2.0-µm fraction revealed small (10–30 nm in diam.) subrounded particles of lithiophorite, yielding an electron diffraction pattern displaying characteristic spacings at 0.945, 0.471, 0.314, 0.237, and 0.188 nm (Fig. 7) . Agglomerate masses of particles with a fine-grained, indeterminate morphology were also detected. The electron microdiffraction pattern of these aggregates showed continuous fine diffuse rings at 0.267, 0.247, 0.224, 0.172, 0.152, and 0.132 nm (Fig. 8) which are consistent with poorly ordered or very fine particle-size goethite (Cornell and Schwertmann, 1996).

View larger version (89K): [in this window] [in a new window]   Fig. 7. (a) Transmission electron micrograph and (b) electron diffraction pattern showing lithiophorite in the nodule <2.0-µm fraction.

View larger version (89K): [in this window] [in a new window]   Fig. 8. (a) Transmission electron micrograph and (b) electron diffraction pattern showing goethite in the nodule <2.0-µm fraction.

	CONCLUSIONS

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Considerable amounts of metals were associated with the Fe and Mn minerals. Heavy metals tended to concentrate in the finest fraction (<2 µm) separated from the nodules. Concentrations of Cu and Ni were significantly correlated with Mn, whereas Cr and V were correlated with Fe. Metal ions may be specifically adsorbed on Mn and Fe oxides through interactions with deprotonated surface hydroxyl groups to form mono and binuclear inner sphere complexes. Ferryhydrite, characterized by very small particle size and thus exhibiting a large specific surface area, might have had a key role in the nodule sorption capacity for heavy metals.

The study of the distribution pattern of Fe-Mn oxides in the nodules provides interesting information about the redox history of the pedoenvironment and the fate of some heavy metals in soil.

	ACKNOWLEDGMENTS

 
The authors are grateful to three anonymous referees and to the SSSAJ Associate Editor Dr. David A. Laird for their comments, which improved the presentation of the results, and for the helpful review of the manuscript. Thanks are due to Dr. Lucia Maiuro (CSIM of Molise University, Campobasso), for her valuable assistance with the scanning electron microscope and energy dispersive spectroscopic analyses. This paper was supported by Project No. 49771049 of the National Natural Science Foundation of China. Paper represents Journal Series N. 200 from the DISCA.

Received for publication January 2, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Adriano, D.C. 1986. Trace elements in the terrestrial environment. Springer Verlag, New York.
• Amonette, J.E., and R.W. Sanders. 1994. Nondestructive techniques for bulk elemental analysis. p. 1–48. In J.E. Amonette and L.W. Zelazny (ed.) Quantitative Methods in Soil Mineralogy. Misc. Publ., SSSA, Madison, WI.
• Burns, R.G., and V.M. Burns. 1975. Mechanisms for nucleation and growth of manganese nodules. Nature 255:130–131.
• Cescas, M.P., E.H. Tyner, and R.S. Harmer III. 1970. Ferromanganiferous soil concretions: A scanning electron microscope study of their micropore structures. Soil Sci. Soc. Am. Proc. 34:641–644.
• Chao, T.T. 1972. Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Soil Sci. Soc. Am. Proc. 36:764–768.
• Childs, C.W. 1975. Composition of iron-manganese concretions from some New Zealand soils. Geoderma 13:141–152.
• Chukhrov, F.V., and A.I. Gorschkov. 1981. Iron and manganese oxide minerals in soils. Trans R. Soc. Edinburgh, Earth Sci. 72:195-200.
• Cornell, R.M., and R. Giovanoli. 1987. Effect of manganese on the transformation of ferryhydrite into goethite and jacobsite in alkaline media. Clays Clay Miner. 35:11–20.[Abstract]
• Cornell, R.M., R. Giovanoli, and P.W. Schneider. 1989. Review of the hydrolysis of iron (III) and the crystallization of amorphous iron (III) hydroxide hydrate. J. Chem. Technol. Biotechnol. 46:115–134.
• Cornell, R.M., and U. Schwertmann. 1996. The iron oxides. Wiley-VCH, New York.
• FitzPatrick, E.A. 1984. Micromorphology of soils. Chapman and Hall, London.
• Gallaher, R.N., H.F. Perkins, and D. Radcliffe. 1973. Soil concretions: I. X-ray spectrographic and electron microprobe analyses. Soil Sci. Soc. Am. Proc. 37:465-469.
• Giovanoli R. 1980. Vernadite is random stacked birnessite. Mineral. p. 251–253. In Deposita (Berlin 15).
• Golden, D.C., J.B. Dixon, and Y. Kanehiro. 1993. The manganese oxide mineral, lithiophorite, in an Oxisol from Hawaii. Aust. J. Soil Res. 31:51–66.
• Huang, P.M. 1991. Kinetics of redox reactions on surfaces of manganese oxides and its impact on environment quality. p. 191–230. In D.L. Sparks and D.L. Suarez (ed.) Rates of soil chemical processes in soils. SSSA Spec. Publ. 27. SSSA, Madison, WI.
• Jackson, M.L. 1979. Soil chemical analysis—Advance course. M. L. Jackson, Madison, WI.
• Krishnamurti, G.S.R. 1997. Soils components with variable charge with special reference to iron oxides. Dipartimento di Scienze Chimico-Agrarie. Università di Napoli Federico II.
• Krishnamurti, G.S.R., and P.M. Huang. 1989a. Influence of Mn²⁺ and pH on the formation of iron oxides from ferrous chloride, and ferrous sulphate solutions. Clays Clay Miner. 37:451–458.[Abstract]
• Krishnamurti, G.S.R., and P.M. Huang. 1989b. The catalytic role of birnessite in the transformation of iron. Can. J. Soil Sci. 67:533–543.
• Lindsay, W.L. 1979. Chemical equilibria in soil. John Wiley & Sons, New York.
• McKenzie, R.M. 1989. Manganese oxides and hydroxides. p. 439–466. In J.B. Dixon and S.B. Weed (ed.) Minerals in soil environment. 2nd ed. SSSA Book Ser. 1, SSSA, Madison, WI.
• McKenzie, R.M. 1980. The adsorption of lead and other heavy metals on oxides of manganese and iron. Aust. J. Soil Res. 18:61–73.
• Murray, J.W. 1974. The surface chemistry of hydrous manganese dioxide. J. Colloid Interface Sci. 46:357–371.
• Phillippe, W.R., R.L. Blevins, R.I. Barnhisel, and H.H. Bailey. 1972. Distribution of concrections from selected soils of the Inner Bluegrass Region of Kentucky. Soil Sci. Soc. Am. Proc. 36:171–173.
• Ross, S.J., D.P. Franzmeier, and C.B. Roth. 1976. Mineralogy and chemistry of manganese oxides in some Indiana soils. Soil Sci. Soc. Am. J. 40:137–143.
• Sawhney, B.L., and D.E. Stilwell. 1994. Dissolution and elemental analysis of minerals, soils and environmental samples. p. 49–82. In J.E. Amonette and L.W. Zelazny (ed.) Quantitative Methods in Soil Mineralogy. Misc. Publ., SSSA, Madison, WI.
• Schulze, D.G. 1981. Identification of some iron oxide minerals by differential X-ray diffraction. Soil Sci. Soc. Am. J. 45:437–440.[Abstract/Free Full Text]
• Schwertmann, U. 1964. Differenzierung der Eisenoxyde des Bodens durch Extraktion mit Ammoniumoxalat-Lösung. Z. Pflanzenern., Dung, Bodenkunde 105:195–202.
• Schwertmann, U. 1985. The effect of pedogenic environments on iron oxide minerals. Adv. Soil Sci. 1:71–200.
• Schwertmann, U., and D.S. Fanning. 1976. Iron-manganese concretions in hydrosequences of soils in loess in Bavaria. Soil Sci. Soc. Am. J. 40:731–738.[Abstract/Free Full Text]
• Schwertmann, U., and R.M. Taylor. 1989. Iron oxides. p. 379–438. In G.B. Dixon and S.B. Weed (ed.) Minerals in soil environments. 2nd ed. SSSA Book Ser. No. 1, SSSA, Madison, WI.
• Soil Survey Staff. 1996. Soil survey laboratory methods manual. Soil Survey Investigations Rep. No. 42. Version 3.0. USDA-NRCS, National Soil Survey Center, Lincoln, NE.
• Taylor, R.M., R.M. McKenzie, and K. Norrish. 1964. The mineralogy and chemistry of manganese in some Australian soils. Aust. J. Soil Res. 2:235–248.
• Tokashiki, Y., J.B. Dixon, and D.C. Golden. 1986. Manganese oxides analysis in soils by combined X-ray diffractions and selective dissolution methods. 50:1079–1084.
• Uzochukwu, G.A., and J.B. Dixon. 1985. Manganese oxide minerals in nodules of two soils of Texas and Alabama. Soil Sci. Soc. Am. J. 50:1358–1363.
• White, G.N., and J.B. Dixon. 1996. Iron and manganese distribution in nodules form a young Texas vertisols. Soil Sci. Soc. Am. J. 60:1254–1262.[Abstract/Free Full Text]

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