Nature and properties of iron rich glaebules and mottles from some south-west Australian soils

GEODE~

ELSEVIER

Geoderma 71 (1996) 95-120

Nature and properties of iron rich glaebules and mottles from some south-west Australian soils Balwant Singh a b

a,*,

R.I. Gilkes

b

Department of Soil SLience, The University of Reading. Whztekmghts, P.O Box 233, Reading RG6 6DW, UK Department of SOli SCience and Plant Nutrition. The Unwersity of Western Australia, Nedlands, WA. 6907, Australia

Received I May 1995; accepted 30 November 1995

Abstract This paper compares the mineralogical and chemical composition, morphology and strength of co-existing matrix, mottles, weak glaebules and strong dense glaebules in some soils from south-western Australia. The mineralogy and morphology of mottles and weak glaebules are very similar to soil matrix, whereas most dense glaebules differ substantially from the soil matrix. These dense glaebules are very hard (more than 200 kg weight is required to break them) and consist mostly of hematite and maghemite with minor amounts of corundum, boehmite and quartz. In the comparatively more porous and weak glaebules, that are the more common forms occurring in lateritic soils, kaolinite, gibbsite and quartz are the major minerals with small amounts of goethite, hematite and anatase. Generally the strength of mottles ; soil matrix < glaebules < dense glaebules. Mottles contain ~ 3 times more dithionite-citrate-bicarbonate (DeB) Fe (median = 1.8%) and ~ 2.5 times more oxalate Fe (median = 614 j.Lg/g) than soil matrix, with smaller values ; 5% of the active Fe ratio [(oxalate Fe/DeB Fe) . 100]. Both types of glaebules contain much more Fe (5 to 10 times) than matrix with median values of Hel and DeB extractable Fe of 12.7 and 6.5%, respectively. The active Fe ratio for glaebules is also very high with a median value of 23%. The DeB Al content of mottles is about 2.5 times that of the soil matrix. whereas the oxalate Al content was about the same for both mottles and SOIl matrix. The amount of Al extracted from glaebules by DeB, oxalate and pyrophosphate reagents is significantly higher than for mottles and soil matrix samples. Oxalate extracted 1.5 times more Al compared to DeB reagent. Manganese was not concentrated in most mottles relative to co-existing matrix, and in glaebules It was only present in small amounts (median value of 104 j.Lg/g for Hel Mn).

, Corresponding author. Fax: (1734) 316660; E-mail: [email protected] 0016-7061/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0016-7061(95)00092-5

96

B. Singh, R.J. Gilkes / Geoderma 71 (J996) 95-120

AI-substitution in goethite in mottles was very similar to values for goethite in the soil matnx. The mineralogy of weak glaebules was similar to that of the soil matrix. Corundum and maghemite were only present in strong magnetic glaebules and were not present in the soil matrix.

1. Introduction Segregations in soils occur as a result of pedogenic processes (current or relict) that accumulate constituents by chemical and biological actions (McDonald et aI., 1990). These processes are common in many soils but vary in nature; for example segregations may be calcareous, manganiferous, ferruginous, aluminous or argillaceous. Ferruginous segregations are most common in soils of the tropical and sub-tropical regions and are the subject of this paper. Discrete ferruginous segregations have been called by a variety of names, for example nodules, concretions, septaria, glaebuIes, gravels, ferricrete and petroplinthite (Brewer and Sleeman, 1964; Pawluk and Dumanski, 1973; Eswaran and Raghu Mohan, 1973; Gallahar et aI., 1974a, 1974b; Shadfan et aI., 1985; King et aI., 1990). Mottles are essentially soil matrix with spots, blotches or streaks of subdominant colours that are di.fferent from the matrix colour and also different from the colour of ped surfaces (Northcote, 1979; McDonald et aI., 1990). There may be relationships between the presence of mottles in soil and the degree of wetness of the soil (Veneman et aI., 1976). Mottles may be good indicators of contemporary pedogenic processes and are commonly used in field descriptions of soil profiles (Northcote, 1979; Soil Survey Staff, 1975; McDonald et aI., 1990). Mottles are commonly continuous with soil matrix in respect of fabric and strength which contrasts with gravels or glaebules which are discrete bodies and show no continuity with the surrounding matrix. The mineralogical and chemical features of glaebules and mottles are not studied in details although a number of workers have investigated these materials (Gallahar et aI., 1974a, 1974b; Ojanuga and Lee, 1973; Wood and Perkins, 1976; Ibanga et aI., 1983). Published results are sometimes conflicting, for example Gallahar et aI. (l974b) found Fe oxides in glaebules from soils of Southern Coastal Plain of the U.S. were mostly amorphous while others were composed primarily of hematite. Glaebules with a low Fe content may consist of goethite, quartz and large amounts of kaolinite. The predominance of hematite in hardened materials has been reported by Gallahar et a1. (1974a, 1974b) and Wood and Perkins (1976). In petroplinthite goethite forms a strong, rigid framework contributing hardness to the petroplinthite in which kaolinite forms the matrix (Eswaran and Raghu Mohan, 1973). The degree of AI-substitution in iron oxides may be higher in softer materials and bigger crystals of goethite are present in hard materials (Shadfan et aI., 1985). A scanning electron microprobe study of concretionary lateritic soils from Brazil revealed structures ranging from a highly cemented type without apparent voids to a structure with voids and large degree of cementation (Carvalho, 1983). The development of maximum iron mottling in somewhat poorly drained environments was compared to well-drained and poorly drained environments by Richardson and Hole (1979) for a hydro sequence in northwestern Wisconsin. The Fe in well drained

B. Singh, R.i. Gilkes / Geoderma 71 (J996) 95-120

97

soils was concentrated in the clay fraction and was more crystalline whereas in poorly drained soils iron was concentrated in the fine sand fraction and was dominantly amorphous or poorly crystalline. Similarly maximum Fe-Mn concretion formation was observed under moderately wet conditions and not in the wettest profile in a study of two hydrosequence on loess in Bavaria, Germany (Schwertmann and Fanning, 1976). Iron rich mottles and glaebules occur extensively in Western Australian (W.A.) soils and are distinctive features of the soils of lateritic origin in the region. Except some work on duricrust from bauxite mine areas (Loughnan and Sadleir, 1984; Anand and Gilkes, 1987b, 1987c), little published information is available on the properties of glaebules and mottles in W.A. soils and how these materials relate to the soil matrix. The present study was undertaken to determine micromorphological, chemical, mineralogical and physical properties of iron glaebules and mottles in some representative W.A. soils. In this study the term glaebule has been used as described by Brewer and Sleeman (1964), for hardened iron rich material of various sizes (2-77 mm diameter) and shapes, and which mayor may not a have a concentric coating.

2. Materials and methods Soil materials were collected from the south-west of Western Australia, comprising 31 mottles, 15 associated soil matrix samples and 70 glaebule samples some of which came from the 15 soils. Details of the sites and soils are available from the authors (Singh, 1991). For X-ray diffraction (XRD) study, the randomly oriented hand ground mottles and matrix samples were scanned from 4 to 65° 2 fJ using a scan speed of 0.02° 2fJ min-Ion a Philips 1050 goniometer (CuKa, 40 kV, 20 rnA). For glaebule samples 10% CaF2 by weight was added to the samples and the mixture was ground in a Tema Mill after adding ethanol as a lubricant to avoid damage to crystal structures. Corrections for peak shifts due to instrumental errors were made by reference to quartz for mottle and matrix samples, and CaFz for glaebules. Many XRD peaks of goethite were very broad and had low intensity making it impossible to measure their d-spacings accurately, consequently unit-cell dimensions could not be calculated and the method of calculating AI-substitution in natural goethite could not be used (Schwertmann and Carlson, 1994). Aluminium substitution in goethite was therefore determined from the c dimension of the unit cell obtained from 110 and 111 reflections of the mineral using the relationship obtained for synthetic goethites: mol% Al = 1730-572.0c and for 111 reflection using the relationship; mole% Al = 2086-850.7 dO 11) as described by Schulze (1984). Substitution of Al in hematite was determined from the a dimension of the hematite unit cell obtained from d( 11 0) using the relationship mole% Al = 3109-617.1 a (Schwertmann et aI., 1979). AI-substitution in maghemite was calculated from the relationship: mol%AI = where a in

a - 8.343

-0.00222

(Schwertmann and Fechter, 1984)

A was calculated from 220 reflection of maghemite.

98

B. Smgh, R.i. Gilkes / Geoderma 71 (J996) 95-120

Relative proportions of various minerals were calculated by comparing XRD peak intensity ratios and by reference to standard mixtures. Standard mixtures were used providing peak areas for the following reflections, 002 gibbsite, 110 goethite, 104 hematite, 220 maghemite, 002 muscovite, 001 kaolinite, 101 quartz, 101 anatase, 110 rutile and 220 for the internal standard calcium fluoride (CaF2 ) (Klug and Alexander, 1974). Dithionite-citrate-bicarbonate (DCB) and oxalate extractable Fe, Al and Mn were determined for matrix, mottles and ground glaebule samples using the procedures described by Mehra and Jackson (1960) and McKeague and Day (1966), respectively. The iron minerals in indurated materials are less readily dissolved by the DCB treatment (Taylor and Schwertmann, 1974) so for all the glaebule samples Fe, Al and Mn were also determined by boiling the samples for 30 min in 6M HCI. One extraction with DCB removed almost all the iron oxides from mottles and whole soil matrix samples. The strength of glaebules was measured by individually crushing 10 uniform size (8-10 mm) glaebules for each sample between parallel steel plates using a calibrated compression apparatus. The average value of the force required to break glaebules is recorded as the strength (kg). A hand held penetrometer was used to measure the strength (kg) of air-dried mottles and soil matrix. The density of glaebules was measured by immersing 10-15 pre-weighed glaebules in water in a volumetric cylinder and the increase in volume was measured. The surfaces of freshly broken fragments of mottles and glaebules were coated with carbon and examined using a Philips 505 SEM (scanning electron microscope) fitted with an EDAX 9900 energy dispersive system for quantitative analysis. After impregnating the samples with resin ultra-thin polished sections of the samples were prepared for optical microscopy, secondary electron SEM and energy dispersive analysis.

3. Results and discussion

3.1. Properties of mottles and matrix The mottles ranged in colour from light yellow (2.5Y 7/4), orange (7.5YR 6/8) to reddish brown (2.5YR 4/8) compared to light grey (7.5Y 8/2) to yellow orange (lOYR 7/8) colours for soil matrix. The size of mottles mostly varied from 5-20 mm in diameter and they had faint to distinct outlines and had a sharp to clear boundary. Fractured surfaces and polished thin sections of a representative mottles are shown in Figs. lA, Band 2. Thin sections and secondary electron (SE) images (figures not shown here) of fractured surfaces of samples show the fabric of the mottles where quartz grains are within a porous matrix consisting mainly of Fe and Al oxides and kaolinite, and minor amounts of feldspar. The kaolinite and iron rich matrix surrounding the quartz grains gives a honey comb structure in sample Y80.2 (2) as shown in Fig. lB. The mottles in both the samples were made up of the same minerals and fabric as in the soil matrix except for some enrichment of iron oxides in the mottles (Fig. 2). The most frequently occurring minerals in matrix and mottles as estimated from XRD patterns in decreasing order of abundance are quartz, kaolinite, goethite and anatase.

B. Singh, R.J. Gilkes / Geoderma 71 (J996) 95-120

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Fig. 1. Secondary electron images of fractured surfaces of mottle samples. (A) #126.2 (I), (B) #Y80.2(2), and glaebules samples, (C) #P40.2(I), (D) #CI18.1(I) and (E) #N60.2(1). Magmfied view oflenticular goethite aggregates in a vOId as indicated by an arrow in (E) is shown in (F).

Illite, hematite and gibbsite were present in small amounts in only a few samples. Rutile was present in small amounts (:0;; 5%) in both mottles and matrix samples from some alluvial soils of the Swan Coastal Plain area. The most significant difference in mineralogy between mottles and their associated soil matrix was the presence of higher amounts of goethite in most mottles and of hematite in a few of them. These iron oxides give rise to the distinct yellowish orange to reddish brown colours of mottles.

B. Singh, R.J. Gilkes / Geoderma 71 (J996) 95-120

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As iron oxides were not present at high concentration in mottles X-ray diffraction lines for any contained hematite could not be measured accurately. Consequently Al substitution values for hematite in this occurrence have not been reported. Similarly Al substitution in goethite for most soil matrix samples could not be measured without prior concentration of iron oxides. Substitution of Al in goethite for mottles ranged from zero to 36 mol% with a mean value of 21 mol% AI. This value is quite similar to the mean value of 20 mol% Al substitution for the iron oxide concentrates from the same soil samples (Singh and Gilkes, 1992). For nine soils the level of Al substitution in goethite in mottles was about the same as for goethite in associated soil matrix (Singh and Gilkes, 1992) (Fig. 3). Despite of the small number of samples the relationship between the two values is highly significant with a near zero intercept and a slope value of 0.88 which is not significantly different from one. The force required to break mottles has a median value of 2.6 kg which is similar to the strength of the soil matrix which has a median strength value of 2.4 kg (Fig. 4A, B). This low strength is probably due to the low concentration of Fe in mottles being insufficient to cement the soil matrix. The distributions of DCB and oxalate Fe and Al values for 31 mottles and 15 associated soil matrix samples are shown in Figs. 4 and 5. The DCB and oxalate Fe values although being small (median values 1.8%, 0.61 mg/g) were mostly higher for mottles compared to soil matrix samples (0.62%, 0.25 mg/g). The percentage of DCB Fe dissolved by oxalate for soil matrix (5.7%) was significantly higher than for mottle samples (3.5%) (Fig. 4g, h).

Fig. 2. (A) An optical micrograph of a polished thin section of a yellow brown mottle containing iron rich yellow brown to bright red areas. There are fine to coarse, fractured, sub-angular quartz grains (#Y80.2(2». (B) Magnified view of the yellow brown matrix under cross-polanzers. (C) EDAX spectrum of the red area of the mottle Illdicated by an arrow (c) III (A) showing that Fe is a major constituent. (D) EDAX spectrum of yellow brown part of the mottle indicated by an arrow (d) in (A) showing that Fe IS a subordinate to Al and much Si due to quartz is present.

102

B. Singh, R.l. Gilkes / Geoderma 71 (]996) 95-120

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B. Singh, R.I. Gilkes / Geoderma 71 (]996) 95-120

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DeB extractable Al was generally higher in mottles (median = 2912 fLgj g) compared to the soil matrix (median = 1113 fLgj g) (Fig. Sa, b). The median values of oxalate Al for mottles (I035 fLgj g) and soil matrix (963 fLgj g) did not differ significantly. For soil matrix samples most of the DeB Al was also extracted by oxalate

reagent with a median value of 95.9% which is much higher than the value of 35.9% for mottle samples. The ratio of DeB Al in mottles relative to soil matrix (enrichment factor) is significantly higher (median value = 1.6) than the corresponding value for oxalate Al

104

B. Singh. R.i. Gilkes / Geoderma 71 (1996) 95-120

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B. Singh. R.J. Gilkes / Geoderma 71 (J 996) 95-120

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which showed no systematic enrichment (::::: O. For Fe the enrichment factor was much greater than 1 and was slightly higher for oxalate Fe (median = 3.0 than for DeB Fe (median = 2.7). The enrichment factors in mottles relative to soil matrix for median values of DeB and oxalate Fe are 1.7 and 3.1 times the corresponding enrichment values for AI. Detectable DeB and oxalate extractable Mn were present in only 9 samples of mottles and their associated soil matrix samples. DeB Mn values ranged from 4 to 666 jLg/g. No crystalline Mn mineral was identified by XRD but this would not be expected in view of the small amounts of Mn present and the poor crystallinity of soil Mn minerals. In some samples the values of DeB and oxalate Mn were significantly higher in soil matrix compared to mottles showing that the pedogenic processes that are responsible for formation of iron accumulation may not lead to a concomitant enrichment in Mn. These results show that there is a concentration of Fe and to a lesser extent Al in mottles relative to soil matrix values. The crystalline Al minerals kaolinite and gibbsite were present in about the same amounts in both mottle and matrix samples. The significantly higher values of DeB Al for mottles is consistent with the high extent of Al substitution in goethite and the higher goethite content of mottles than matrix. The high values of Al substitution in goethite for the mottles indicate that Al was relatively abundant in soil solution in the pedoenvironment during crystallisation of this goethite. Goethite formed under strongly acid weathering conditions of free drainage is highly Al substituted and goethite that has crystallised in poorly drained environments has a lower extent of Al substitution (Fitzpatrick, 1988). In these profiles present day drainage is mostly moderate to good and it appears that Fe has been concentrated in mottles in an environment where Al activity is relatively high. Lepidocrocite which is widely associated with reductomorphic conditions was not present in these samples supporting the field observation that soil drainage was moderate to good where goethite had crystallised. The dominance of goethite over lepidocrocite in mottles is consistent with the model that states that an increasing rate of oxidation and the presence of Al in the soil solution system favour goethite formation (Taylor and Schwertmann, 1978; Taylor, 1987; Schwertmann and Taylor, 1989). Goethite is also favoured over hematite in the presence of Al 3+ , as Al in soil solution can bring ferrihydrite back into solution that allows polymerisation and nucleation of aluminous goethite (Taylor, 1988). The dominance of goethite compared to hematite in most of the mottles may be favoured by low pH, the presence of organic compounds in soil solution and high H 2 0 activity (Schwertmann and Taylor, 1989).

Fig. 6. (A) An optical micrograph of a polished thin section of a glaebule from a water-logged soil (#P40.2(J)). (B) Magmfied view of a part of the glaebule exhibiting sub-rounded to rounded quartz grams characteristic of coastal dunes. Sand grains are surrounded by an iron oxide rich matrix as indicated by an arrow. (C) EDAX spectrum of a mm 2 area in (B), composed of much quartz and lesser iron OXides. (D) EDAX spectrum of the small area of the matrix surrounding quartz grains indicated by an arrow in (B), the matrix contains mostly iron OXides with the contents of Si, Al and Ti being due to minor amounts of very fine quartz, kaolinite and rutile.

106

B. Singh, R.J. Gilkes / Geoderma 71 (J 996) 95-120

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B. Singh. R.I. Gilkes / Geoderma 71 (J996) 95-120

3.2. Properties of glaebules

Three dominant types of glaebules were recognised during this investigation (i.e. those for seasonally water-logged soils, pisolitic and magnetic) and will be discussed separately. In most instances the glaebules were not associated with mottles so that comparison of mottles, matrix and glaebules from single soil samples could not be made. 3.2.1. Micromorphology of glaebules Glaebules from water-logged profiles: Glaebules from the Swan Coastal Plain area are abundant in soils at the junction of coastal dunes to the west and alluvial soils to the east. These soils are water-logged during winter. The secondary electron image of fractured glaebules and optical micrograph of polished thin sections shows sub-rounded to round quartz grains (Figs. Ic, 6a, b). The quartz grains are strongly cemented by an iron oxide rich matrix The sub-rounded to rounded shape of the quartz grains is consistent with their origin in coastal sand dunes (Fig. 6). The matrix contains much Fe with small amount of Si and Al which is consistent with the high goethite content (Fig. 6d). The small amount of Al in the matrix surrounding the quartz grains in the absence of any crystalline mineral of Al is consistent with the 11 mol% Al substitution in goethite as determined by XRD. The glaebules probably form by mobilisation of iron under waterlogged conditions in winter which precipitates at the oxidation front and as the soil dries in summer. This seasonal induration by iron oxides has resulted in the formation of discrete hard glaebules in a sandy matrix. Pisolitic glaebules: These glaebules commonly occur as free or cemented constituents of lateritic profiles on the Darling Plateau of Western Australia and perhaps are the most common form of the glaebules occurring in these soils. Micrographs of the fracture surface show the porous structure of the glaebules and some of the voids are filled with platy goethite aggregates (Fig. IE, F). A similar morphology for goethite crystals was observed by Eswaran and Raghu Mohan (1973) and Shadfan et al. (1985) in iron-rich petroferric materials. There is a concentric coating around the inner core as shown in Fig. 7B. The external matrix material around the concentric coating has hardened. The mineralogy of the bulk pi solites consists mainly of kaolinite, gibbsite and quartz with minor amounts of goethite, hematite and anatase. EDAX spectra indicating the chemical composition of inner core matrix, concentric coating and external matrix are shown in Fig. 7. The external matrix is dominated by Si and Al with small amounts of Fe and Ti (Fig. 7D). The major minerals in this part of the glaebule are quartz and gibbsite with small amounts of goethite and ilmenite. The concentric coating consist pre-dominantly of Al minerals with small amounts of Si and minor amounts of Fe and Ti (Fig. 7E). The concentration of Fe is high in the dark red inner core matrix compared to concentrations in concentric coatings and outer matrix. Other dominant elements in the inner core are Al and Si; some Ti is also present (Fig. 7F). Several hypotheses have been proposed to explain the development of a concentric fabric in glaebules. For example the thin, distinct laminae as observed in this glaebule sample may indicate formation in waves during alternating wet and dry periods with

B. Singh. R 1. Gilkes / Geoderma 71 (1996) 95-120

109

desiccation following formation of each lamina (Brewer, 1976). The concentric coating around an inner red core with an undifferentiated fabric may indicate two phases of genesis. The inner red core might have originated by accumulation of sesquioxides in a void from solution. The undifferentiated fabric of the outer matrix surrounding the concentric coating indicates some changes in pedological environment between conditions responsible for the formation of the concentric coating and outer matrix. Magnetic glaebules: Magnetic glaebules are common in upland sites in lateritic areas of the Darling Range of Western Australia. These glaebules are opaque in thin section so that reflected light and electron images were used in their investigation. The secondary electron image of glaebule shows a dense, uniform fabric (Fig. ID). The optical micrograph of a polished thin section shows a uniform opaque matrix containing a few quartz grains (Fig. 8A). The matrix is mainly composed of Fe and Al minerals with little Ti and Si (Fig. 8). This is consistent with the presence of much AI-substituted hematite (mol% Al = 14.8%) and maghemite as identified by XRD in the bulk sample using CaF2 as an internal standard (Fig. 8D(j)). Corundum and boehmite are major constituents of the residue after iron oxide dissolution in HCI (Fig. 8d(ii)). The chemical composition of a bulk sample of this material was 47.1% Al z0 3 , 5.8% Si0 2, 44.2% Fe 2 0 3 and 0.9% Ti0 2 . The very high content of Al 20 3 appears to be inconsistent with the contents of crystalline Al minerals as indicated by XRD and indicate that some amorphous form of Al is also present in addition to corundum and boehmite. 3.2.2. Mineralogy and chemical composition of glaebules The distribution of values of abundance for major minerals in all glaebules is shown in Fig. 9 and illustrates the considerable variation in composition that exists. Kaolinite and quartz with median values of 30 and 25%, respectively were the two most abundant minerals present in most iron glaebules. Goethite and hematite were the most common iron minerals and were present in most samples. Goethite content of glaebules ranged from 0 to 18% with a median value of 3%, whereas hematite ranged from 0 to 50% and had a median value of 8%. Maghemite was present in 21 out of a total of 70 samples with contents ranging from 0 to 14%. Maghemite was a common mineral in black and red dense compact glaebules where hematite was the most abundant mineral, and in these glaebules kaolinite was either absent or present in very small amounts (Fig. 8). Similar observations have also been made by Taylor and Schwertmann (1974) and Coventry et al. (1983) who explained this association on the basis of the inhibiting effects of aluminium in solution on the formation of the y-phase iron oxides maghemite and lepidocrocite (Taylor and Schwertmann, 1978). However these glaebules contain 47.1 % Al 20 3 which indicates that maghemite may not have formed via solution but by some other mechanism like by heating in bush fires. Gibbsite was present in 42 glaebules samples with values of abundance ranging from o to 20% but in most samples it was a minor constituent « 5%) (Fig. 9f). Small amounts of mica, anatase and rutile were also present in a few glaebule samples. Minerals constituting glaebules are the same as those present in the corresponding soil matrix but generally in glaebules the quartz content is smaller, the content of kaolinite is slightly greater and iron oxides are much more abundant. Mobilisation and concentration of iron in iron glaebules can be through (j) movement into glaebules of

B. Singh, R.J. Gilkes / Geoderma 71 (]996) 95-120

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B. Singh, R.i. Gilkes / Geoderma 71 (J996) 95-120

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suspensions containing colloidal material, (ij) reduction of Fe H phases to soluble and mobile Fe2+ in the soil and migration of Fe 2 + into the glaebule where it precipitates as Fe H oxides and (iii) formation of soluble Fe H complexes that migrate into the glaebule (Coventry et aI., 1983). The precipitation of Fe and AI phases on silica from soluble AI-Fe (III) hydroxy species would also tend to increase the Fe and AI over Si in glaebules (Taylor et aI., 1990). The increase in kaolinite and iron relative to quartz suggest that process (i) may have been involved to some extent. The greater concentration of Fe minerals and presence of maghemite (y-Fez03) which contains minor amounts of Fe z+ in some samples (Taylor and Schwertmann, 1974) indicates that some iron may have been mobilised as dissolved Fe2+. In glaebules hematite is more abundant than goethite and hematite content is significantly positively correlated with HCI and DCB extractable Fe (Table 1). This is consistent with some past observations that high rate of Fe release favours hematite formation over goethite (Schwertmann and Taylor, 1989). The median value of the goethitej(goethite + hematite) ratio for glaebules was 0.40 compared to 0.53 to 0.94 for iron oxide concentrates from the soil matrix of W.A. soils (Singh and Gilkes, 1992). Anand and Gilkes (l987a) reported mean values of goethitej(goethite + hematite) of 0.75 and 0.57 for bauxitic soils on acidic and mafic parent materials from south-western Australia. In a study of 33 ferruginous soil concretions from various parts of Australia, Taylor and Schwertmann (1974) reported a similar mineralogy to that described here and with higher hematite than goethite contents. The presence of high amounts of hematite in glaebules has been observed in many other studies (Taylor and Schwertmann, 1974; Coventry et aI., 1983; Anand and Gilkes, 1987b). Hematite in glaebules might have formed under an elevated temperature, low moisture content and relatively high concentration of AI in solution; conditions which are consistent with past climatic fluctuations in the region (Anand and Gilkes, 1987b). Ferrihydrite which may be a necessary precursor for hematite formation is favoured by a high rate of Fe release to soil solution, warm climate and a low soil organic matter content (Schwertmann, 1988). Aluminium substitution in goethite was measured for the 21 samples where this was possible without prior concentration and values ranged from 11 to 31 mol% with a median value of 24 mol% AI substitution. For hematite AI substitution ranged from 2 to 26 mol% with a median value of 13 mol% AI substitution. AI substitution in hematite for iron glaebules may extend to higher values than normally reported for soils (Schwertmann and Kampf, 1985; Anand and Gilkes, 1987a). Iron oxide concentrates from soils of this area had values up to 23 mol% AI substitution in hematite (Singh and Gilkes, 1992) and a similar range of values for AI substitution in hematite has been

Fig. S. (A) A polished thin section of a magnetic glaebule (#CllS.1(J)) showing an almost unifonn opaque matrix with a few mlilled cracks and enclosed quartz grains (transmitted light). (B) Magnified view of a section of the glaebule under reflected light showing randomly oriented cracks and VOIds which are filled with aluminous material. Variations in reflectivity in the matrix are due to segregations of maghemite (light grey) and hematite (dark grey). (C) EDAX spectrum of a matrix area similar to that shown in (B). (D) X-ray diffraction patterns of the glaebule. (i) Original sample. Arrows indicate the positions of the strongest reflectIOns of corundum. (ii) Residue after HCl treatment, corundum reflectIons are shaded. (Q = quartz, H = hematite, M = maghemite. C = corundum, B = boehmite and CaF1 = calcium fluonde).

•• ••

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Property

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Table I Lmear correlatton coeffiCients for relattonshlps between some propertIes of Iron glaebules

10 077 089 -016 -006 031 •• 038 •• 026 • 014

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B. Smgh, R.J. Gilkes / Geoderma 71 (J996) 95-120

113

(1))

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Fig. 9. Frequency distribution for values of density, strength (force) and contents of various minerals in 70 non glaebules.

114

B. Smgh, R 1. Gllkes / Geoderma 71 (J 996) 95-120

reported for a wide range of petroferric materials by Ibanga et al. (1983). The high degree of AI-substitution in hematite may suggest their origin from aluminous goethite which rejects it Al only at high temperatures ( - 800°C). The presence of elements such as Ti in hematite may also cause a shift in the peaks used for the measurement of Al substitution of hematite so that values of Al substitution derived by XRD must be treated with some caution. For maghemite the 220 spacing which is free from interferences from other minerals varied from 2.938 to 2.951 A which is similar to the range of values reported for glaebules from Australia (2.938 to 2.954 A - Taylor and Schwertmann, 1974) and South Africa (2.937 to 2.948 A - Fitzpatrick, 1988). The values of Al substitution in maghemite for these samples calculated from XRD data ranged from no substitution (i.e. - 2%) to 15% mole% Al substitution with a mean value of 5%. Anand and Gilkes (1987c) reported Al substitution values ranging from 0-4 mol% for maghemites from lateritic bauxite from the Darling Range, W.A. and considered these values to be not significantly different from zero, whereas Schwertmann and Fechter (1984) found 0-14 mol% Al substitution in soil maghemites from Australia and New Caledonia. Since there is no other evidence like chemical analysis to prove the AI-substitution in maghemite, these results should be treated with some caution. Many hypotheses have been advanced for the formation of maghemite in soil (e.g. oxidation of magnetite, oxidation of green-rust at pH 7-8, transformation of other pedogenic oxides by heating at approximately 300-500°C in the presence of organic matter (Schwertmann, 1988). The mechanism of maghemite formation proposed by Coventry et al. (1983) is that a fluctuating water table concentrates Fe 2 + in soil pores and subsequent oxidation leads to the formation of hematite and/or maghemite. Rapid oxidation of all Fe2+ in such situations could lead to the precipitation of ferrihydrite (precursor of hematite) whereas slower oxidation, which allows an interaction between Fe3+ hydroxy species formed on oxidation and remaining Fe 2 + hydroxy species in solution would yield maghemite. Although reducing conditions probably do not commonly occur in the near surface horizons of lateritic profiles in the region at the present time this might have been possible in the past. Formation of maghemite due to heating of soil by the bush-fires which occur frequently in uncleared forests may also be a mechanism and this origin is supported by the association of corundum and maghemite in laterites in the Darling Range, W.A. (Anand and Gilkes, 1987c). Very high concentrations of Fe as hematite and maghemite together with minor amounts of quartz are present in magnetic black glaebules (Fig. 8) compared to a mineralogy dominated by kaolinite and quartz with small amounts of goethite, hematite and gibbsite in orange and more porous glaebules (Fig. 7). These are supposed precursors of magnetic glaebules but this very different explanation does not support the proposal that orange glaebules are altered to magnetic black glaebules simply by heating in bush-fires. The distributions of various forms of extractable Fe, Al and Mn in glaebules are given in Fig. 10. HCI extracted most Fe and Mn followed by the amounts soluble in DCB and smallest amounts of Fe and Mn were extracted by oxalate. HCI extracted about twice the amount of Fe (median 12.72%) extracted by DCB (median = 6.49%) and about four times the amount extracted by oxalate (median = 3.20%). The one extraction in DCB used for these samples evidently does not dissolve all the crystalline

115

B. Singh. R.l. Gilkes / Geoderma 71 (J996) 95-120 (a)

Median = 12.72%

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0

o

10

1II

Sl ..,

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80

l100 «XI IlOO 8XI 1000 1200

OldMn
Fig. 10. Frequency distribution for HCI, DCB and oxalate soluble Fe, Al and Mn in 70 glaebule samples.

Fe minerals present in glaebules and DeB has been reported to be quite ineffective in removing crystalline Fe minerals from dense magnetic concretions (Taylor and Schwertmann, 1974). There is an about 7 to 20 times increase in the Fe content of glaebules relative to associated soils. The amount of Fe extracted by one DeB treatment was about half of the amount extracted by the 6M Hel treatment (Fig. 11 a). The proportion of oxalate Fe (poorly crystalline Fe oxides) to free iron oxides has been described as the active Fe ratio (Juo et aI., 1974) but may be misleading where DeB extracts only a minor proportion of the total Fe. For this reason the proportion of oxalate Fe relative to Hel Fe will be considered as the active Fe ratio for glaebules in this work. The median value of 23% for the active Fe ratio for glaebules indicates the presence of high amounts of poorly crystalline or amorphous Fe compounds (Fig. lIb). The median value of this ratio for 97 soil sample was 13.6% (Singh and Gilkes, 1991) and the corresponding values for soil matrix and mottle samples were 5.7% and 3.5%, respectively. In addition

B. Singh, R.i. Gilkes / Geoderma 71 (]996) 95-120

116

Median = 53.8%

=

(b)

Median 23.0%

(c)

Median. 48.2%

1II

(d)

Median =38%

z

(e)

Median '" 66.2%

Median = 165.7%

1II

o

10 1II 31 ., III III 'JI III 111100 (DCB AlIHCI Al)"100

0 1II ., III III 100 120 140 160 (Oxl AlIHCI Al)'100

Median '" 25.6% (h)15

Median '" 22.7%

Median =88.6%

1II

Olll.,1II111100

(DCB MnlHCI Mn)'100

OlOlll31.,lIlal'JIal (Oxl MnlHCI Mn)'100

III aI 'JI aI aI 100 110120 130

(Oxl MnlDCB Mn)'100

Fig. II. Frequency distribution for values of the proportion (percentage) of DeB to Hel, oxalate to Hel and oxalate to DeB extractable Fe, Al and Mn for 70 glaebule samples.

to the presence of high amounts of poorly crystalline Fe compounds in glaebules, energetic grinding of glaebule samples in a Tema Mill and the presence of microcrystalline iron oxides which are partly soluble in oxalate (Campbell and Schwertmann, 1984) may also be responsible for the high values of active Fe. Generally active iron values of < 10% have been reported for highly weathered soil materials (Iuo et aI., 1974; Ibanga et aI., 1983). Much higher values in the range of 30 to 60% occur for soils from temperate regions (McKeague et aI., 1971). It is worth pointing here that non-oxalate soluble goethite in soils are commonly very small in size (10-20 nm) and have high surface area (~ 100 m 2 g - I) and thus by no means are chemically inactive (Schwertmann and Kampf, 1985; Singh and Gilkes, 1992). In the case of Al the sequence for the amounts extracted by various reagents was HCI > oxalate > DCB (Fig. 10). HCI extractable Al (median = 3.77%) probably includes

B. Singh. R.J Gilkes / Geoderma 71 (] 996) 95-120

117

amorphous or poorly crystalline Al (oxalate AI) compounds, some Al present in the structure of iron oxides and some Al present in poorly crystalline kaolinite and other silicates. The median value for the percentage of HCI Al extracted by DCB is about 38% which is approximately the amount of Al substituting in Fe oxides. Small crystals of Al substituted iron oxides may also be dissolved by oxalate reagents (Campbell and Schwertmann, 1984) and the oxalate reagent extracts more Al than does DCB from iron glaebule samples (Fig. lOe, 0. Oxalate and DCB extracted about equal amounts of Mn with median values of 22 and 25.5 j-tg/ g, respectively compared to 104 j-tg/ g extracted by Hel (Fig. 109, h, i). With the probable absence of discrete Mn minerals, the Mn may be present in an amorphous form, within other mineral or as Mn sorbed onto iron oxide minerals. Very high values (median = 88.6%) of oxalate Mn as a percentage of DeB Mn also indicate that the Mn is present in an amorphous form (Fig. IIi). Specific sorption of Mn onto the surfaces of iron oxides (Schwertmann and Taylor, 1989) may be the main reason for the presence of high amounts of Mn in some glaebules as indicated by very strong positive relationships existing for HCI and DCB soluble Fe and Mn, (Table 1). although Mn can also substitute for Fe in the structure of goethite (Stiers and Schwertmann, 1985).

3.2.3. Density and strength of glaebules The density of iron glaebules ranged from 1.90 to 3.78 g/cm 3 with a median value of 2.42 g/ cm 3 (Fig. 9a). These values are a consequence of glaebules being mostly mixtures of kaolinite and quartz (- 2.6 g/cm 3 ) with iron oxides (- 5 g/cm 3 ). There were mostly significant positive relationships between the density of glaebules and HCI Fe, DeB Fe, hematite and maghemite contents (Table 1). The high densities of hematite (5.25 g/cm 3 ) and maghemite (4.88 g/cm 3 ) and the role of these minerals in cementing glaebules is probably responsible for these relationships. There was consequently a systematic decrease in the density of glaebules with increasing kaolinite and quartz contents and also with goethite/(goethite + hematite) ratio. Thus indurated glaebules rich in hematite and maghemite were generally denser than porous glaebules rich in kaolinite and quartz. The strength of glaebules varied greatly with the force required to break glaebules ranging from 3.9 to 265 kg with a median value of 23 kg (Fig. 9b). Highly significant positive relationships exist between strength and Hel Fe, DeB AI, hematite and maghemite contents (Table 1). The strength of glaebules was also very closely related to density (r = 0.81, Table 1). Increasing contents of kaolinite, quartz and increased goethite /(goethite + hematite) are associated with a decrease in the strength of glaebuIes. A positive relationship between DCB Fe and the strength of glaebules was reported by Shadfan et al. (1985) for a wide range of iron-rich glaebules. The association of kaolinite with the softness of lateritic materials was demonstrated for a wide range of samples from Africa by Alexander and Cady (1962), and they indicated that hardening of the material was nearly always accompanied by a (relative) loss of kaolinite. The disappearance of kaolinite, leaving gibbsite, goethite and hematite as major constituents of glaebules in laterite in Venezuela was also observed by Schorin (1980). Glaebules have diverse chemical and mineralogical compositions, and micromorphol-

118

B. Singh, R 1. Gilkes / Geoderma 71 (1996) 95-120

ogy probably indicating different pedoenvironment conditions of formation. Some properties of glaebules indicate that their genesis occurred in a pedoenvironment quite different to the existing now.

Acknowledgements

The authors are grateful to Mr. Terry Armitage for his assistance. Balwant Singh acknowledges the AIDAB for financial assistance and H.A.U. Hisar (India) for grant of study leave during the study. The authors are grateful to Professor Udo Schwertmann and an anonymous referee for their valuable review and comments on the manuscript.

References Alexander, L.T. and Cady, 1.G., 1962. Genesis and hardening of latente in soils USDA Tech. Bull. 1282.90 pp. Anand, R.R and Gilkes, R.1., 1987a. Iron oxides in lateritic soils from Western Australia. 1. Soil SCI., 38: 607-622. Anand, R.R and Gilkes, R 1 , 1987b. Variations m the properties of Iron oxides within individual specimens of laterilic duncrust. Aust. 1. Soil Res., 25: 287-302. Anand, R R and Gilkes, R.l., 1987c. The association of maghemite and corundum in Darling Range latentes, Western Australia. Aust. 1. Soil Res., 25: 303-311. Brewer, R, 1976 Fabric and Mineral Analysis of Soils. R.E. Krieger Publ. Co., New York Brewer, R and Sleeman, 1.R., 1964. Glaebules: their definitIOn, classification and interpretatIOn. 1. Soil Sci., 15 66-80. Campbell, A.S. and Schwertmann, U., 1984. Iron oxide mineralogy of placic honzons. 1. SOlI Sci., 35: 569-582. Carvalho, 1.B, 1983. Study of the microstructure of lateritic concretIOnary soils using scanning electron microscope. In: A.1. Melfi and A. Carvalho (Editors), Internalional Seminar on LateritisatIOn Processes, Sao Paulo, Brazil, 1982, pp. 569-576. Coventry, R.1 , Taylor, RM. and Fitzpatrick, R.W., 1983. Pedological slgmficance of the gravels in some red and grey earths of central north Queensland. Aust. 1. Soil Res., 21: 219-240. Eswaran, H. and Raghu Mohan, N.G., 1973. The microfabnc of petroplinthite. Soil Sci. Soc. Am. Proc., 37: 79-82. Fitzpatrick, RW., 1988. Iron compounds as indicators of pedogemc processes. Examples from the Southern Hemisphere In: 1 W. Stucki, B.A. Goodman and U. Schwertmann (Editors), Iron in Soils and Clay Minerals. Reidel, Dordrecht, pp. 351-396. Gallahar, RN., Perkins, H.F. and Tan, K.H., I 974a. Classification, composition and mineralogy of iron glaebules in a southern coastal plam soil. Soil Sci., 117: 155-164. Gallahar, R.N., Perkins, H.F. and Tan, K.H., 1974b. ChemICal and mineralogical changes in glaebules and enclosmg soil with depth m a plinthic soil. Soil SCI., 117: 336-342. Ibanga, 1.1., Buol, S.W., Weed, S.B. and Bowen, L.H., 1983. Iron oxides in petrofemc materials. Soil Sci Soc. Am. 1., 47: 1240-1246. luo, A.S.R, Moonnann, F.R. and Maduakor, H.O., 1974. Fonns and pedogenetic distribution of extractable iron and aluminum in selected soils of Nigeria. Geoderma, II: 167-179. King, H.B., Torrance. 1 K., Bowen, L.H. and Wang, c., 1990. Iron concretions m a Typic Dystrochrept in Truwan. SOlI Sci. Soc. Am. 1., 54: 462-468. Klug, H.P. and Alexander, L.E., 1974. X-ray Diffraction Procedures for Polycrystalhne and Amorphous Materials. 2nd ed. Wiley, New York. Loughnan, F.C. and Sadleir, S.B., 1984. Geology of established bauxite producing areas m Australia. In: L.

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