Iron Oxides in Selected Brazilian Oxisols: I. Mineralogy

DIVISION S-9-SOIL MINERALOGY

Iron Oxides in Selected Brazilian Oxisols: I. MineralogyM. P. F. Fontes* and S. B. Weed

ABSTRACTTwelve Oxisols from the Triangulo Mineiro region, Minas Gerais

state, Brazil, derived from four different parent materials were stud-ied to provide insight into their Fe-oxide mineralogy. The clay frac-tion (<2 MHI) of all soils consisted of kaolinite and Fe oxides(hematite and/or goethite); gibbsite and anatase were found in mostof the soils; maghemite was detected in several of them. Citrate-dithionite (CD) treatment of the soil clays showed hematite pref-erentially dissolved compared with goethite, and a higher dissolutionrate for poorly crystalline than for well-crystalline goethite. Thecalculated values for Al substitution in the Fe oxides, based on theCD extracts of the total clay corrected for Al soluble in acid am-monium oxalate, and of clay treated for gibbsite removal gave fairto good agreement with Al substitution determined by differential x-ray diffraction (DXRD) for those samples in which the Fe-oxidefraction was dominated by either goethite or hematite. Aluminum-substituted maghemite, detected by DXRD, was present only in soilsfrom mafic rocks, suggesting its formation through oxidation of themagnetite present in the parent material. Aluminum substitution,determined by DXRD, varied from 17 to 36 mol % for goethites, 6to 15 mol % for hematites, and 16 to 26 mol % for maghemites. Themean crystallite dimension (MCD,U) of some hematite samples de-termined from DXRD showed preferential crystal development inthe X-Y direction, suggesting a platy nature.

LAYER SILICATES have been extensively studied andtheir properties are quite well defined. Iron oxides

and oxyhydroxides (referred to collectively for sim-plicity as Fe oxides), though common secondary min-erals in highly weathered soils, are less wellunderstood. These minerals are commonly very finelydivided, may possibly occur as coatings on other min-erals, and are frequently present in small amounts.These factors often make them less readily detected byXRD. Therefore, techniques of selective dissolution(e.g., Mehra and Jackson, 1960; Coffin, 1963; Schwert-mann, 1964), DXRD (Schulze, 1981), and Mossbauerspectroscopy (Bowen and Weed, 1981) may be requiredto better evaluate the content, mineralogy, and foreign-ion substitution of the soil Fe oxides.

Iron oxides have high geochemical affinity for nu-merous other soil elements and Fe is often replacedby other metals in its octahedral position in the Fe-oxide structures. The most common substituting ionin soil Fe oxides is Al (Schwertmann and Taylor,M.P.F. Fontes, Dep. de Solos, Univ. Federal de Vicosa, Vicosa—MG 36570, Brazil; and S.B. Weed, Dep. of Soil Science, North Car-olina State Univ., Raleigh, NC 27695-7619. Paper no. 12455 of theJournal Series of the North Carolina Agricultural Research Service,Raleigh, NC 27695-7643. This paper was supported in part by theNational Science Foundation, Grant EAR-850656. Received 5 Feb.1990. "Corresponding author.

Published in Soil Sci. Soc. Am. J. 55:1143-1149 (1991).

1989). It is becoming increasingly apparent that thissubstitution is more the rule than exception in nature.

The objective of this study was to consider the con-tent, mineralogy, and Al substitution of the Fe oxidesin several Oxisols formed from different parent ma-terials in the state of Minas Gerais, Brazil.

MATERIALS AND METHODSArea of Study

Soil samples were obtained from the Triangulo Mineiroregion in Minas Gerais state in the central plateau of Brazil(Fig. 1). This region is bounded on the north and west bythe Paranaiba River and on the south by the Grande River,which intersect at the extreme southwest of the region.

The present landscape (Resende, 1976) is best visualizedas a broad, three-level terrain (Fig. 2) that is lowest andslightly undulating in the west, where the Paranaiba andGrande Rivers meet, and higher and flatter in two stepstoward the east. The highest level (Pedons 1, 2, and 3: Fig.Ib; Fig. 2) is a table land (Chapada) of clayey-textured soilsformed in Cenozoic sediments. Sandstone is the dominantparent material for the soils of the Triangulo Mineiro region(Pedons P4, P5, P6, and P7). Through exposure by erosion,mafic (mainly diabase) rocks became the main parent ma-terial for upland soils along the major rivers (Pedons P8, P9,P10, and Pll). Other important parent materials for somesoils towards the northwest are the rocks of the Araxa Group(Pedon PI2). These are pre-Cambrian rocks, mainly gneissand schists, which constitute the outcrop of the crystallinebasement.

The climate of the region of interest (EMBRAPA, 1982),according to the Koppen classification, is predominantly Aw,minor-scale Cwa, where Aw is tropical savanna climate, drywinters, rainy summers, and temperatures of the coldestmonth above 18 °C, and Cwa has dry winters and rainysummers but temperatures of the coldest month are below18 °C. The mean annual temperature increases from about20 °C in the east to 24 °C in the west. The soil temperatureregime is isohyperthermic. The soil moisture regime for allthe region is generally considered to be ustic, although VanWambeke (1981) classified it as udic. Total annual precip-itation is between 1300 and 1700 mm, with no consistentpattern across the region.Sample Selection

Oxisols representing the main soil-geomorphic units weresampled for the study (Table 1). Selected soil properties areshown in Table 2.

The main objective in sample selection was to permit com-parison of Fe-oxide formation in different environments.Soils were selected mainly on the basis of parent material.Therefore, whenever suitable, they are grouped and referredto as follows:Abbreviations: CD, citrate-dithionite; XRD, x-ray diffraction;DXRD, differential x-ray diffraction; Fe0, ammonium oxalate ex-tractable poorly crystalline Fe; Fed, CD-extractable total free Fe ox-ides; TGA, thermogravimetry analysis; HIM, hydroxy-Al-interlayered mineral; MCD4W, mean crystallite diameter.

1143

1144 SOIL SCI. SOC. AM. J., VOL. 55, JULY-AUGUST 1991

150km

Fig. 1. Map showing the Triangulo Mineiro region and its association with the state of Minas Gerais, Brazil: (a) map of Brazil; shaded areais the Triangulo Mineiro region; shaded plus netted area is Minas Gerais; (b) road map of the Triangulo Mineiro region showing samplingsites.

1. Soils from clayey sediments: soils derived from Cen-ozoic clayey sediments (PI, P2, P3).

2. Soils from sandstone-derived materials: Bauru sand-stone (P4, P5, P6, P7).

3. Soils from mafic rock-derived material: basalt (P8, P9,PIO, PI 1).

4. Soil from schist: soil developed in situ from schist(P12).

The soils developed in the clayey sediments and sandstoneare generally low in terms of ionic concentration of theirenvironment; those developed in mafic rocks and schist arecharacterized by a richer ionic environment, i.e., weatheringof primary minerals in these rocks maintains a higher so-lution concentration of ions. Within each soil-geomorphicunit, different conditions are represented:

1. In the clayey Chapada soils, PI and P2 are in the samewell-drained environment; P3 has a variable water ta-ble, i.e., a moderately well-drained environment (EM-BRAPA, 1982).

2. In the sandstone-derived soils, P4 developed in a wetterenvironment due to its topographic position, P6 hassome influence of basalt, and P7 is influenced by acalcareous cement from the original sandstone. Soil P6is under grass vegetation and P7 is under forest, whichputs both in a higher C-production system, comparedwith P4 and P5, which are under cerrado vegetation(savanna).

3. In the soils from mafic rocks, P9 and PI 1 are underforest vegetation, while P8 and PIO are currently undercultivation; cerrado was the initial vegetation.

FONTES & WEED: IRON OXIDES IN SELECTED BRAZILIAN OXISOLS: I. 1145

Table 1. Soil classification, clay mineralogy t, and citrate-dithionite-extractble (Fed) and oxalate-extractable (Fe,,) Fe of the clay fraction.Soilpedon

PIP2P3P3P4P5P6P7P8P9P10PllP12

Brazilian classification J

Red-Yellow LatosolDark-Red LatosolRed-Yellow LatosolRed-Yellow LatosolDark-Red LatosolDark-Red LatosolDark-Red LatosolDark-Red LatosolDusky-Red LatosolDusky-Red LatosolDusky-Red LatosolDusky-Red LatosolDark-Red Latosol

U.S. taxonomyt

Typic AcrustoxTypic AcrustoxAquic AcrustoxAquic AcrustoxQuartzipsammentic HaplustoxQuartzipsammentic HaplustoxTypic AcrustoxTypic AcrustoxTypic AcrustoxTypic AcrustoxHumic EutrustoxRhodic AcrustoxTypic Acrustox

Samplingdepth§

BBABBBBBBBBBB

Kaolinite

600660323340790825930930490627556699340

Gibbsite———— gkg-'

3433006436654472NDHND490340412231280

Fea

41.839.034.756.4

111.5127.3124.0132.4186.1161.0143.4161.892.4

Fe0

0.950.991.671.863.123.995.093.564.383.904.994.242.28

Fe^Fe,

0.0230.0250.0480.0330.0290.0310.0410.0270.0240.0240.0350.0260.025

t Kaolinite and gibbsite determined by thermogravimetry.$ Approximate correlation with U.S. soil taxonomy.§ Sampling depths were 0-20 cm for A and 100-120 cm for B. Except for soil P3, results for all depths in a given soil were similar.H Not detected.

H a

IH d

8,9,10,114,5,6,7 1,2,3

12

Fig. 2. Idealized geology of the Triangulo Mineiro region showingsample locations: (a) clayey chapada (Cenozoic sediments); (b)sandstone (Bauru formation); (c) mafic rocks (Serra Geral for-mation); (d) pre-Cambrian gneiss and schist (Araxa group). Num-bers refer to pedons sampled.

4. The Oxisol formed from schist represents a relativelyrich ionic environment for the formation of the Fe ox-ides, because it still has primary minerals in the coarsefraction.

The soils were sampled at depths of 0 to 20, 100 to 120,and 200 to 220 cm. Emphasis is placed on the 100-cm ma-terials, referred to as B materials, although materials takenat the three depths were very similar (except for Soil P3,which had very distinct morphological A and B horizons).

Chemical and Physical AnalysesSubsamples were treated for organic-matter destruction

with NaCIO (Anderson, 1961), dispersed ultrasonically, andthe clay fraction (<2 /im) separated by gravity settling ofthe coarser components (Jackson, 1979). The clays were Nasaturated and freeze dried.

The following analyses were performed on the total-clayfraction (<2 jim): ammonium oxalate extraction for poorlycrystalline Fe oxides (Fe0) (Schwertmann, 1964); CD ex-traction for total free Fe oxides (Fed) (Coffin, 1963); gibbsiteremoval by heating the sample at 75 °C in 1.25 M NaOHfor 1 h (Mckenzie and Robertson, 1961, as in Kampf andSchwertmann, 1982); TGA in a N2 atmosphere using aDuPont1 Series 99 Thermal Analyzer (DuPont Co., Wil-mington, DE) with a thermobalance module; XRD using a

1 The use of trade names in this publication does not imply en-dorsement by the North Carolina Research Service of the productnamed, or criticism of similar ones not mentioned.

Diano XRD-700 diffractometer with Cu-Ka radiation anda diffracted beam monochromator; differential x-ray diffrac-tion (DXRD) using a Diano XRD-700 diffractometer (DianoCorp., Woburn, MA) modified for step-scanning and em-ploying Co-Ka radiation. The d,0, line of powdered quartz,or the dul line of powdered silicon was used as a standard,which was either added to the clay as an internal standardor run separately under the same instrument settings. Resultswere comparable by either method. Thetaplus X-ray Dif-fraction Software (Scientific Microprograms, Raleigh, NC)was used to obtain the difference patterns.

RESULTS AND DISCUSSIONClay Mineralogy

The general mineralogy of the clay fractions of mostof the soils consists of kaolinite, goethite, and hema-tite; gibbsite occurs in some samples and muscovitewas seen in the sample of the soil derived from schist.The predominance of crystalline forms of Fe oxides isreflected in the low Fe0/Fed values (Table 1).

The TGA results (Table 1) show the amount of ka-olinite to be twice that of gibbsite in Soils PI and P2;the opposite occurs in Soil P3, although these soilsformed from the same parent material. Soils PI andP2 are from a well-drained environment and P3 froma moderately well-drained environment, suggestingthat the seasonal alternation between reducing and ox-idizing conditions in P3 may result in silicate destruc-tion (Buol et al., 1989), in this case, kaolinite. In thesesoils, if the microbial populations are adapted to tem-porary saturation of the soil with water, much of thefree Fe oxide may be reduced to Fe2* within 1 to 2 moof waterlogging (van Breemen, 1987). At the onset ofreducing conditions, Fe2+ replaces the exchangeableA13+ and, when the soil becomes reoxidized, A13+ reap-pears in the exchangeable form (Gate and Sukhay,1964). At least part of the A13+ may come from de-struction of kaolinite caused by the exchangeable H+

produced by the oxidation of Fe2+ (Brinkman, 1979;Coleman, 1962). What is observed is residual increasein gibbsite and consequent decrease in kaolinite asseen in Soil P3. Soil P3 also shows a 1.4-nm peak inthe XRD pattern, which corresponds to hydroxy-Al-interlayered material (HIM). The presence of HIM in


Table 2. Selected properties of soil materials (EMBRAPA, 1982).

Samplef

P1AP1BPICP2AP2BP2CP3AP3BP3CP4AP4BP4CP5AP5BP5CP6AP6BP6CP7AP7BP7CP8AP8BP8CP9AP9BP9CPICAP10BP10CP11APUBPI 1CP12AP12BP12C

Sand

150130120190170170360280210830770760830800760470350370800690720180110140190140130230200190809080

410350380

Particle sizeSilt

—— gleg- ——905050807080

10011080607080405040

130130140505060

210180170250180180120110120300140160180170210

pHClay

760820830730760750540610710110160160130150200400520490150260220610710690560680690650690690620770760410480410

H2O

4.95.35.44.95.66.44.85.15.55.74.95.15.25.05.45.05.25.36.45.86.04.45.15.55.85.85.84.85.45.56.76.57.05.25.65.7

KC1

4.04.75.34.45.05.94.05.05.74.83.94.04.14.14.34.14.34.25.84.84.84.04.65.65.05.05.53.95.35.86.15.96.14.34.75.4

Cation-exciangecapacity

cmol,. kir1

7.73.82.78.64.01.07.12.91.56.02.92.23.02.41.98.33.42.98.41.71.39.44.62.7

14.94.42.76.52.51.64.13.30.08.22.71.4

Organic C

gkg-18.29.06.1

17.08.04.3

15.35.72.18.52.21.04.73.21.2

17.84.53.4

14.41.91.0

17.58.84.4

29.78.64.7

12.87.04.6

39.79.64.4

16.54.41.5

t A = 0-20 cm; B = 100-120 cm; C = 200-220 cm depth.

Soil P3 agrees with the model proposed by Brinkman(1979). In this model, alteration and decompositionof clay minerals accompany the translocation and seg-regation of Fe that occur as a consequence of alter-nating reduction and oxidation of Fe compoundsunder periodically water-saturated conditions. Thisleads to the formation of hydroxy-Al interlayers in 2:1clay minerals in seasonal wet acid soils.

Citrate-Dithionite ExtractionSuccessive extractions using CD (Coffin, 1963) were

performed on the total-clay fraction (<2 ^m). MoreFed was obtained from the clays from soils formed inmaterial derived from mafic rocks than from the othersoils, and 90 to 98% of the Fed in these soils was re-moved in the first extraction (Table 3). The soil fromschist released about 80% of its Fed in the first treat-ment. The soils from clayey sediments exhibited a var-iable behavior within the group. The sample from P3released about 96% of the Fed in the first extraction,whereas samples from the well-drained PI and P2 gaveabout 69 and 61%, respectively.

The hematitic soils (see Table 4) generally showeda high amount of Fed extracted in the first treatment(Table 3), with very little Fed extracted in subsequenttreatments. In the gpethitic samples, with the excep-tion of P3A, Fed continued to be released through fourtreatments. Dissolution kinetics of hematite and goe-thite in strong acids (Schwertmann, 1984a,b, 1988b;

Schwertmann and Latham, 1986) show preferentialdissolution of hematite compared with goethite, ahigher dissolution rate for poorly crystalline comparedwith well-crystalline goethite, and a decreased disso-lution rate for highly Al-substituted goethite. Similarresults have been reported for dissolution by CD (Tor-rent et al., 1987). Thus, the kind of oxide, its crystal-Unity, and its Al substitution are important factors inhematite and goethite dissolution. This appears to ex-plain the observation that the hematitic samples hadalmost all of the Fed extracted in the first treatment,while the goethitic samples had much Fed remaining.The explanation for Sample P3A is that its Fe oxide,although primarily goethite:, is of very low crystallinityand has a relatively high surface area («110 m2/g)(Fontes and Weed, 1991). If Al substitution and Fe-oxide crystallinity are competitive effects with refer-ence to dissolution rate, the low crystallinity in thiscase exerted greater influer ce—an almost complete re-moval of the goethite in the first extraction.

Estimates of Aluminum SubstitutionAluminum substitution in Fe oxides based on CD

extractions of all samples, both total clay and clay fromwhich gibbsite was removed, were calculated and arepresented in Table 4. The rationale to calculate Alsubstitution from the CD extraction is based on thepremise that this methodology is specific enough toremove only the Fe oxides and that the Al measured


Table 3. Iron removed by consecutive citrate-dithionite extractions(Fed) of total clay samples.t

Treatment no.

Samples Total Fe4 10 to-'

2 3<Mt nf tntal Fn .

4

Soils from clayey sedimentsPIP2P3AP3B

P4PSP6P7

P8P9P10Pll

P12

42.0(0.2)39.1(0.2)34.756.4

111.6(1.6)126.9(1.4)124.4(3.9)136.3(6.5)

182.8(5.5)160.1(5.0)143.9(1.7)161.0(1.3)

68.8(0.8)60.6(1.7)95.796.4

14.9(0.4)21.3(0.6)3.33.0

9.7(0.4)11.3(1.3)0.70.4

6.6(0.1)6.8(0.0)0.30.2

Soils from sandstones84.4(0.8) 10.6(0.7)92.8(1.3) 6.0(0.6)95.7(3.0) 3.8(0.2)96.8(4.0) 2.9(0.3)

Soils from mafic rocks

3.8(0.4) 1.2(0.1)0.9(0.0) 0.3(0.0)0.4(0.0) 0.1(0.0)0.3(0.0) 0.1(0.0)

96.4(2.8)97.1(3.1)95.9(1.2)97.2(0.6)

3.2(0.5)2.6(0.1)3.5(0.0)2.5(0.3)

0.3(0.0)0.2(0.0)0.5(0.0)0.2(0.0)

0.1(0.0)0.1(0.0)0.1(0.0)0.1(0.0)

Soil from schist89.6(5.9) 79.1(4.6) 14.0(1.4) 4.7(0.4) 2.2(0.1)

f Means of samples from 0-20(A), 100(B), and 200(C) cm profile depths,except P3. Values in parentheses are standard deviations.| Sum of Fe,, from four successive extractions.

in the extract is structural Al in those oxides (Bighamet al., 1978). The results do not verify this assumption.In all samples, the calculated amount of Al substitu-tion increased from the first to the third extraction(data not shown), suggesting either that Fe oxides be-came increasingly difficult to reduce as Al content in-creased (Torrent et al., 1987), or that Al came fromother sources, possibly including (i) silicate clays at-tacked by CD, (ii) poorly crystalline (amorphous) Aloxides, or (iii) gibbsite partially dissolved during theCD treatment (Curi, 1983). The dissolution of silicateclays, mainly poorly crystalline kaolinite and/or hal-loysite may have had some influence, judging fromthe high amounts of Si found in the extract. When theAl that was extracted by oxalate (Table 3) was sub-tracted from the Al extracted by CD, and the Al sub-stitution for the total-clay samples was recalculated,the decrease in apparent Al substitution was evident(Table 4).

Comparison of the data from the total-clay samples,total minus oxalate Al, and samples with gibbsite re-moved (Table 4) indicates that there is a general de-crease in the calculated Al substitution in that sameorder. If the treatment to remove gibbsite dissolved amajor part of the Al sources other than Fe oxides, theCD treatment of this sample is more likely to give anaccurate estimate of the average Al substitution in thecrystalline Fe oxides. The remaining question is thepossible transformation of goethite to hematite underthis treatment, which would usually lower the calcu-lated value for Al substitution, since hematite nor-mally contains much less Al in its structure than doesgoethite (Schwertmann, 1988a).

Differential X-ray Diffraction AnalysesDifferential XRD patterns (patterns of selected total

clays minus patterns of deferrated clays) showed peaksfor hematite in patterns for all but two of the samples

Table 4. Aluminum substitution of Fe oxides calculated from thefirst citrate-dithionite extraction of total clay samples, of samplestreated for gibbsite removal, and from differential x-ray diffraction(DXRD) of total clay.

Sample

P1BP2BP3AP3BP4BP5BP6BP7BP8BP9BP10BPUBP12B

Totalclay

42314126201276

1212141325

Correctedf

37233522181056

1011121123

Gibbsiteremoved

29253018168559898

20

Goethite}:mole%

262820_17__—___—25

DXRDHematite))

_11—99

1086

1515131411

Maghemitel

__—_—__—20261716-

t Aluminum extracted by CD minus that extracted by oxalate.i From Schulze (1982): mol % Al = 1730 - 572c, where c is the unit cell

parameter.§ From Schwertmann et al. (1979): mol % Al = 647(5.0376 - a), where a is

the unit cell parameter.H From Schwertmann and Fechter (1984): mol % Al = 450(8.343 - a), where

a is the unit cell parameter.

and peaks for goethite and maghemite in some sam-ples (Table 4).

The presence of maghemite, based on the 0.295-nmpeak, was noted in surface samples as well as samplesobtained at 1- and 2-m depths of all soils from maficrocks. Schwertmann and Fechter (1984) provided di-rect and indirect evidence for Al substitution in mag-hemite and reasoned that the presence of Al in thestructure of soil maghemites would make it less likelythat they could have been formed through the oxi-dative weathering of magnetite inherited from parentrock. Also, greater proportions of maghemite in thesurface soil layers, as is often found, emphasize therole that fire could play in maghemite formation.Schwertmann and Fechter (1984) concluded that Al-substituted maghemites that occur in the upper hori-zons of tropical and subtropical soils might have beenformed by the heating of Al-substituted goethites orhematites in the presence of organic matter. The re-sults from our study suggest that fire was not the agentof formation of maghemite for these Brazilian Oxisols,although they are Al substituted. Support for this con-clusion includes:

1. There is a very good relationship between thepresence of maghemite and the type of parentmaterial, i.e., all of the soils derived from maficrocks contained maghemite, but none of theother soils from clayey sediments, sandstone, orschist showed evidence for maghemite. It is un-likely that, of 12 different soils in the same region,only the ones from a given parent material wouldbe exposed to fire.

2. There is no clear trend of increasing maghemitein the upper horizons, and similar amounts ofmaghemite, based on XRD peak intensities, werefound at all depths, including 2 m. Fire wouldnot have had so deep an effect.

Aluminum-substitution data from DXRD areshown in Table 4. The Al substitution in goethite was


determined from the c dimension of the goethite unitcell obtained from the dn0 and dm spacings {c = [I/(dn,)2 - l/(d100)2]-1/2} and the relationship: mol % Al= 1730 - 572c (Schulze, 1982). Aluminum substi-tution of hematite was estimated from the a dimensionof the hematite unit cell, obtained from the dno spac-ing and using the relationship: mol % Al = 647(5.0376— a), from Schwertmann et al. (1979), derived forhematites synthesized at pH 7 and 70 °C. Aluminumsubstitution of maghemite was determined from thea dimension of the maghemite unit cell, obtained fromthe d22o spacing and using the relationship: mol % Al= 450(8.343 - a) (Schwertmann and Fechter, 1984),which was derived from 11 highly weathered soils, anatural maghemite, and several Al-substituted mag-hemites obtained by heating Al-substituted goethitesin the presence of sucrose. The data show that thegoethites contain the highest Al substitution, in agree-ment with other results for Brazilian Oxisols (Bigham,1977; Curi, 1983; Fabris et al., 1985; Santana, 1984;Schwertmann and Kampf, 1985). If the degree of Alsubstitution for goethites reflects the environment inwhich they formed, the data indicate that Soils PI andP2 are strongly weathered, nonhydromorphic, andformed in an acid environment where the activity ofAl was high; it was readily incorporated into the goe-thite structure (Fitzpatrick and Schwertmann, 1982).The goethite from the moderately well-drained P3 haslower Al substitution than that from Soils PI and P2,though all are from the same parent material. How-ever, it is still more highly substituted (;»20 mol%)than goethites of hydromorphic environments, whichare normally 2 to 10% substituted (Fitzpatrick andSchwertmann, 1982). Reducing conditions from thecombined effects of organic matter (a source of energyfor the microbial population) and the short-term watersaturation may preferentially reduce hematite overgoethite (Macedo and Bryant, 1989). The fluctuatingwater table is such that reduced conditions are notmaintained long enough to reduce all of the Fe oxidesbut only hematite, and the Fe2+ is leached from thesystem. This preferential removal of hematite agreeswith the Mossbauer data (Fontes et al., 1991), whichshow only goethite in Sample P3A. Thus the goethiteis probably not newly formed, but rather a remnant

Table 5. Mean crystallite dimension (MCD ,̂) of goethite and he-matite as determined from differential x-ray diffraction of total-clay samples.

SampleGoethite Hematite

110 nm 111 nm 012 nm 104 nm 110 nm 113 nmP1BP2BP3AP3BP4BP5BP6BP7BP8BP9BP10BPUBP12B

34.127.421.319.622.0—_—_—_—24.3

22.224.420.1____—___—18.9

-

__32.0_30.727.620.631.5_28.9-

-

_21.226.227.828.428.222.928.725.923.7-

-

_40.342.637.894.685.233.635.035.035.837.6

-

_—34.033.333.233.824.630.730.727.2-

of the original goethite with a higher Al substitutionthan goethite formed in a hydromorphic environment.It is subjected to the perioiiic reduced conditions andall related reactions, which may result in some loss ofstructural Al. This goethite ends up with a little lessAl than the other goethites of the soils from clayeysediments.

The Al substitution of the hematites ranges from 6to 15 mol %, well below the levels in the goethites.According to Schwertmann and Kampf (1985), a ratioof Al substitution of about 0.5:1 for hematite/goethitein the same clay sample can be expected.

Hematite in the soils from clayey sediments andfrom sandstone showed the lowest Al substitution,probably because (i) the goethite present would takeup most of the available Al, and (ii) the amount ofavailable Al would be lower. In fact, Soils P6 and P7from sandstone, which showed the lowest Al substi-tution of any hematites, dp not contain any gibbsite.Schwertmann and Kampf (1985) reported that, forsoils of central Brazil, highly Al-substituted goethitesare usually associated with gibbsite in the same sam-ple; however, they point out that it probably does notmean that gibbsite acts as a source of Al, but morelikely indicates that both ijibbsite and highly Al-sub-stituted goethite characterize a pedogenic environ-ment of strong desilication. The soils with hematiteshaving higher Al substitution are the soils from maficrocks. Gibbsite is present in each of these samples,showing the same relationship described above.

Aluminum substitution determined by DXRDshows fair agreement with the values calculated fromthe CD extract of the samples treated to remove gibb-site (Table 5), for samples determined by XRD to con-tain only hematite. Agreement for samples containingonly goethite (P1B and P3A) is poor. Sample P3A,treated for gibbsite removal, showed Al substitutionin the goethite of 30 mol %:, whereas DXRD gave «20mol %. Part of the Al found by CD treatment probablycame from a source other than the Fe oxides. Somegibbsite remained in this particular sample after thetreatment for its removal. On the other hand, theDXRD-derived value may have been influenced bythe small size of this goetbite, although correction forline shifting due to small crystallite size (Schulze,1982) was made. Consideration was also given to theeffects of small particle size on the line positions ofhematite (Helge Stanjek, 1990, personal communica-tion). The shifts were negligible for the hematites ob-served in our study.

Aluminum-substitution values for maghemites inSoils P8, P10, and PI 1 are in the range of maximumsubstitution reported by Schwertmann and Fechter(1984); one value (Soil P9) is well above this maximum.As Al-substitution values in maghemites from Oxisolsformed on mafic rocks have not been reported and nostudy has shown the maximum Al-substitution for mag-hemites, the validity of this value is uncertain.

Mean crystallite dimensions (MCDhkj) of goethitesand hematites were calculated using the Scherrer for-mula (Klug and Alexander, 1954) and are shown inTable 5. The values for MCDUO and MCD1U for goe-thite lie between 18 and 34 nm and show a relationshipclose to 1:1, indicating similar crystal development


along all three crystallographic axes. This is commonfor soil goethites (Schwertmann, 1988a); synthetic goe-thites characteristically exhibit greater MCDm thanMCDUO, indicating an acicular morphology and pref-erential growth in the z direction.

The MCDAW values for hematites show largerMCD,10 for all the samples, indicating greater crystaldevelopment in the x-y, compared with the z directionand a platy nature of hematite crystals. According toSchwertmann et al. (1979) and Barren and Torrent(1984), hematites synthesized in the presence of Alshow pronounced differential growth, i.e., reducedgrowth in the z direction, whereas in Al-free systemshomogeneous growth is observed. Although in ourstudy there is no Al-free hematite, the data show clear-ly that the greater the Al substitution in the hematite,the more homogeneous the growth. Samples P6B andP7B have the least amount of Al substitution and theyexhibit the least homogeneous growth as shown bycomparison of MCD110 vs. MCDm2, MCD104, andMCD113. The apparent discrepancy between these re-sults and those of the other authors indicates that thereare factors, in addition to those imposed in well-de-fined laboratory syntheses, that affect the morphologyof crystals.

The MCDhkl data indicate a trend for the soil he-matites to have a slightly better crystallinity comparedwith the goethites. This trend is most evident if thehematites of the soils from sandstone are comparedwith the goethites in general. With the hematites fromdifferent sources, the trend seems to be for the platynature and good crystallinity to decrease from the soilsfrom sandstone and clayey sediments to soils frommafic rocks.

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Iron Oxides in Selected Brazilian Oxisols: I. Mineralogy