Geoderma, 29 (1983) 27--39 27 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

G O E T H I T E A N D H E M A T I T E IN A C L I M O S E Q U E N C E IN S O U T H E R N B R A Z I L A N D T H E I R A P P L I C A T I O N IN C L A S S I F I C A T I O N OF K A O L I N I T I C S O I L S

N. KAMPF an0 U. SCHWERTMANN

Lehrstuhl f~r Bodenkunde, Technische Universit~t M~tnchen, 8050 Freising 12 (F.R.G.)

(Received October 31, 1981, accepted after revision June 23, 1982)

ABSTRACT

K~impf, N. and Schwertmann, U., 1983. Goethite and hematite in a climosequence in southern Brazil and their application in classification of kaolinitic soils. Geoderma, 29 : 27--39.

The goethite/hematite + goethite ratio of 11 profiles of Oxisols, Ultisols and Inceptisols along a 600 km E--W transect in S. Brazil varied systematically with climatic and soil factors. The ratio increased with decreasing mean annual air temperature, increas- ing excess moisture (rainfall minus evapotranspiration), increasing soil organic carbon and decreasing pH. This is explained in light of the current concept of goethite and hematite formation in soils: higher temperatures favor hematite, whereas higher excess moisture, higher organic carbon and lower pH favor goethite. A basis is suggested for using goetbite and hematite in the mineralogical classes at the family level of Soil Taxonomy.

INTRODUCTION

The wor ldwide d i s t r ibu t ion of soils shows the d o m i n a n c e of red and ye l l ow soils wi th high c h r o m a colors u n d e r warm h u m i d c l imates , such as the Medi t e r ranean area and m o s t t rop ica l and sub t rop ica l regions. In con t ras t , soils of t emperate h u m i d areas have d o m i n a n t l y ye l lowish colors wi th hues se ldom redder than 5YR. T h e ye l l ow and red colors are d e t e r m i n e d by goe th i te (Gt) and h e m a t i t e (Hm) , respec t ive ly ( S c h w e r t m a n n and Lentze , 1966) and the soil d i s t r ibu t ion m a y be l inked to t h a t o f the co r r e spond ing iron oxides.

The zonal and in t razona l occu r rence of goe th i t e and h e m a t i t e has been d o c u m e n t e d in m a n y places (Tay l o r and Graley , 1967; S c h w e r t m a n n , 1969; Fb l s te r e t al., 1971; L a m o r o u x , 1972 ; Fauck , 1974 ; Davey e t al., 1975; Resende , 1976) . In soils o f NW Tasmania , T a y l o r and Gra ley (1967) obse rved a decrease in the G t / H m ra t io which co r r e l a t ed wi th a decrease in a l t i tude f r o m 575 m to sea level, an increase in t e m p e r a t u r e f r o m 9 ° to 12°C, and a de-

*On leave from: Depto. Solos, Fac. Agronomia-UFRGS, Porto Alegre, Brazil.

0016-7061/83/0000--0000/$0.300 © 1983 Elsevier Scientific Publishing Company

28

crease in rainfall from 1650 to 1080 mm. The same tendency is described by Schwertmann et al. (1982) in an east--west soil sequence in southern Germany where the temperature increases from 7.5 ° to 10.5°C and the rainfall decreases from 1000 to 600 mm. Except in these two studies no quantitative relation- ships between climate (or pedoclimate) and Gt /Hm ratios are available.

This work at tempts to relate the goethite--hematite distribution in a soil climosequence from volcanic rocks in southern Brazil to the following envirom mental factors: (1) excess moisture (rainfall minus potential evapotranspira- tion calculated from annual values); (2) air temperature; (3) soil organic carbon (4) soil pH; and (5) Fe, Mn, and Cu contents of the soils.

SOILS

Along a 600 km east--west transect of the Planalto Sulriograndense in Rio Grande do Sul, Brazil (Fig. 1), elevation drops from 1200 to 90 m, annual rair~- fall decreases from 2450 to 1650 mm, and mean annual temperature increases from 14.1 ° to 20°C. Eleven soil profiles representing the major local soil units (Brasil, 1973) were sampled along the transect (Fig. 2). The soils are briefly de- scribed in Table I in terms of their major pedogenic features. The mineralogy is dominated by kaolinite, with goethite and hematite and minor amounts of Al-chlorite, quartz, cristobalite, halloysite, anatase, magnetite and ilmenite.

The low Feo /Fe d ratio of the soils {<0.05) indicates that the iron oxides are largely crystalline. Advanced weathering of the Oxisols and Ultisols is in- dicated by high Fed /Fe t ratios of 0.75 to 0.99, whereas the Inceptisols tend to have somewhat lower ratios in the range 0.46--0.85.

METHODS

Fine soil fraction (~2 rnrn )

The pH(H20) was measured with a soil--solution ratio of 1:2.5 after 1 h equilibration and organic carbon by dry combust ion with a Strbhlein- Carmograph 8. Oxalate extractable iron (Feo) {Schwertmann, 1964), di- thionite extractable iron (Fed) (Mehra and Jackson, 1960), and total iron (Fet), total manganese (Mnt), and total copper (Cu t ) (HF--HC104--H~SO4 dissolu- tion} were determined. Fe, Mn and Cu were measured in the extracts by AAS (Perkin-Elmer 420).

Clay fraction (< 2 pm)

The clay fraction was obtained by sedimentation after dispersing 10 g soil in 500 ml water brought to pH 8.5 with NaOH, followed by a 3-min ultrason- ic t reatment and 1 h of shaking. The clay suspension was kept as such or dried at 50°C. The iron oxides were concentrated by treating the clay fraction with boiling 5M NaOH for 60 rain to remove 1:1 layer silicates and gibbsite

29

1600

1500

1900

RIO F~RANOE DO SUL STATE

MEAN ANNUAL RAINFALL (ram) AND

LO[ATtON OF SOIL PROFILES •

?00 ~1600

1200

r' \

1 3 0 0 ~ ' ~ ' ~ - ' - ~

~30(

ScQte

~200

Fig. 1. Mean annual rainfall (ram) and locations of soil profiles (.) in the state of Rio Grande do Sul, southern Brazil.

(Norrish and Taylor, 1961; K~impf and Schwertmann, 1982a) prior to quanti- tative estimation by X-ray diffraction. Self-supporting powder mounts were made in a Perspex holder by backfilling and then gently pressing the sample against filter paper to minimize preferred orientation. The diffractograms were obtained with a Philips PW 1050 diffractometer equipped with a graphite monochromator using CoKa radiation (25mA, 35 KV) and 1/2 ° 20/min scan speed (2 sec TC, 1 ° divergence slit, 0.2 mm receiving slit, 1 ° scattering slit,

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31

400 or 1000 cps). The samples were X-rayed three times after re-pressing the sample into the holder each time. Goethite and hematite were quantified by comparing the integrated peak areas of the goe th i t e ( l l 0 ) and hematite(102) lines with those of goethite and hematite standards extracted from soils. The accuracy of this method was checked by comparing the amount of Fe as cal- culated from the amount of Gt+Hm determined by XRD (FeXRD) with the amount of Fe determined chemically in the DCB-extract (Fe d ). The relation- ship between FeXR D and Fed was FeXR D = 0.97. Fe d -1 .07 (n = 45, r = 0.96) and was therefore sufficiently accurate (for details see K~mpf and Schwertmann, 1982 b).

RESULTS

From the mechanisms of hematite and goethite formation as deduced from synthesis experiments it may be expected that the following soil environ- mental factors could influence the Gt/Hm ratio in soils: soil temperature, soil moisture (replaced by mean annual temperature and excess moisture due to lack of soil data), organic matter content, pH, and total Fe and other elements (Schwertmann and Taylor, 1977). The relationship of each to the Gt/Gt+Hm ratio was tested.

32

Air temperature

In both Ultisols and Oxisols an S-shaped relationship between the Hm/Gt+ Hm ratio (the Hm/Gt+Hm ratio rather than the Gt/Gt+Hm ratio was used in this case because the former shows more clearly the direct influence of temperature on the formation of hematite) and the mean annual air tempera- ture was obvious (Fig. 3). At ~< 15°C goethite was the only Fe oxide mineral found, between 15.5 and 17.5°C the Hm/Gt+Hm ratio increased rapidly, reach ing a maximum of about 0.8 but never 1.0. In additional samples from central Brazil goethite occurred in a Typic Haptorthox even under an isothermic (21.5°C) and ustic (1311 mm) climate and in a Typic Acrorthox under an iso- hyperthermic (22.9°C) and ustic (1564 mm) climate. Also, so far we have never found a soil high in hematite which did not contain at least some goeth- ite. It appears therefore that conditions in soils rarely occur where only hematite can form without the formation of at least some goethite.

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0 15 16 17 18 19 20 6()0 860 10'00 1200

Mean annual air temperature (°[} Ramfatt rn+nus [vapotransptrubon imm)

Fig. 3. Re l a t i onsh ip b e t w e e n H m / G t + H m ra t io of A and B ho r i zons and m e a n annua l air t empe ra tu r e .

Fig. 4. Re l a t i onsh ip b e t w e e n G t / G t + Hm ra t io o f A and B ho r i zons and excess mo i s tu re (rainfall m i n u s po t en t i a l evapo t r ansp i r a t i on ) .

Excess moisture

The excess of rainfall over evapotranspiration was taken as a measure of the amount of moisture available for pedogenic processes. The Gt/Gt+Hm ratios increased exponentially from 0.20 to 0.95 as excess moisture increased from 550 to 1000 mm (Fig. 4). Above ca. 1000 mm goethite seems to be the only Fe oxide formed. An additional Inceptisol from an area with 1850 mm of ex- cess moisture (not included in Fig. 4) also contained only goethite.

Organic carbon

An S-shaped correlation seemed also to exist between the Gt/Gt+Hm ratio

33

and the organic C percentage in the A1 horizon (Fig. 5). Between 2 and 3% organic C the Gt /Gt+Hm ratio increased steeply, whereas above 3% organic C hemati te is no longer formed to any great extent. The influence of organic components on goethite and hemati te formation can also be deduced from the individual profiles. As the conten t of organic C decreases with increasing depth, a tendency toward a lower Gt/Gt+Hm ratio is apparent in Inceptisols (r = 0.94) but less evident in most of the Oxisols and Ultisols (Fig. 6).

0.8" !

Off E =.

0.~"

L~

0,2- /

o i Orgoni: C~rbon(%)

Fig. 5. Relationship between Gt/Gt + Hm ratio and organic carbon of A1 horizons of soils.

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Fig. 6. Relationship between Gt/Gt + Hm ratio and organic carbon of all soil horizons.

pH(HaO)

A recent in-vitro experiment in which ferrihydrite was stored at 24°C showed that goethite increased at the expense of hemati te when the pH de- creased from 8 to 4 (Schwertmann, 1981). Although there is considerable scat~

34

ter in the data, an analogous negative relationship exists between Gt/Gt+Hm and soil pH(H20) within the range of 4.2--5.8 if the soils are grouped climat- ically into those below and those above 16°C mean annual temperature (Fig. 7). This again shows that temperature is an important factor. The somewhat al- though not significantly higher r-value of 0.86 for the lower-temperature group of soils as compared to the one for the higher-temperature group seems to indicate that pH is a more important factor in the lower-temperature range, whereas temperature is more important in the higher-temperature range in a,> cordance with the results plotted in Fig. 3. Naturally, if the temperature is t oc~ low for the formation of hematite, pH would no longer have any influence.

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Fig. 7. Rela t ionship be tween G t / G t + Hm ratio and pH(H20) of A and B horizons for two different ranges of mean annual air t e m p e r a t u r e

Content of Fe, Mn and Cu

There was no correlation between the Gt/Gt+Hm ratio and either Fe t or Fe d. Soils from rhyolite-dacite or sandstone/basalt with <8% Fe t covered the same range of Gt/Gt+Hm ratios as those from basalt with > 8% Fe t. Thus, the, total amounts of Fe in these soils does not seem to be a factor influencing the ratios of the two Fe oxides•

In his in-vitro experiments Nalovic (1974) found that heavy metals (Cu, Cr, Mn) coprecipitated with ferrihydrite retarded its transformation to goethite and hematite in favor of hematite alone. In the softs we studied, however, no relation was found between the Gt /Gt+Hm ratio and Mnt+Cu t or Mnt+Cut/Fet Also, no relation existed between Mn t or Cut and Fe o/Fe d.

Depth functions of the Gt/Gt÷Hm ratio

Two typical depth functions for the Gt/Gt+Hm ratio are compared with the degree of weathering as measured by the Fed/Fe t ratio (Fig. 8}. In both pro-

35

0 A~

B1

~2 ~

B2~

B3

i0,2 0,.4 0~6 018

Gt;Gt*Hm FedlF

\ /\

8Sn

)din

0 AI

A3

BI

B2

22] 3B

0~2 0~$ 0~6 Oti

5t/G \

)din

10

20

.~0

Fig. 8. Depth functions of Gt/Gt+Hm and Fed/Fe t for profiles 8Sa and 3D.

files a higher Gt/Gt+Hm ratio in the C horizon coincided with a Fed/Fe t ratio < 0.6. In two other profiles (not shown) the Fed/Fe t ratio remained > 0.6 at the greatest depth sampled and here the Gt/Gt+Hm ratio was not higher in the C than in the B horizon. In profile 3D, which contains 30 kg/m 2 of organic C, the Gt/Gt+Hm ratio increased again in the upper B and the A horizon, where- as in profile 8Sa with only 17 kg/m 2 of organic C, no such increase in the Gt/Gt+Hm ratio was noticeable.

DISCUSSION

The results will be discussed in light of our concept of goethite and hemat- ite formation (Schwertmann and Taylor, 1977). Iron from silicates and other primary sources released into solution may lead to a direct crystallization of goethite if the solubility product of goethite, but not the much higher one of ferrihydrite, is exceeded. If the solubility product of ferrihydrite is exceeded, however, ferrihydrite will be formed and may then transform either to hemat- ite via a dehydration--recrystallization process or, alternatively, to goethite via dissolution of ferrihydrite. The ferrihydrite then acts as an Fe source for crystal growth of goethite like any other Fe source. Factors favoring hematite over goethite formation are therefore those which favor ferrihydrite formation, such as a high release rate of Fe and a low concentration of organic com- pounds to complex the Fe, thereby allowing a somewhat higher concentration of inorganic FeIII ions. In addition, once ferrihydrite is formed, hemati te is favored over goethite with increasing temperature because dehydration is in- volved in the ferrihydrite -~ hematite transformation.

The actual causal relationship between the various environmental factors and the hematite--goethite relationship cannot be elucidated from our study,

36

however, because of the strong intercorrelations among the factors considered, such as temperature and excess moisture (r = -0 .86) and even soil organic mat- ter and pH (r = 0.86). Even so, the type of relations found may be suitable to test the general concept described above.

Both higher air temperature and lower excess moisture may induce a higher soil temperature which, in turn, favors hematite over goethite because it favors the dehydrat ion of ferrihydrite to hematite and accelerates the turn-over of organic matter. A limiting temperature above which hematite can form can not be given because other factors play a role as well. Thus, although hematite, occurred at ca. 8°C mean annual temperature (Schwertmann et al., 1982) in Bt horizons of Alfisols on calcareous Wi]rmian gravel in the northern foreland ~f the Alps, hematite was not detected below 15°C in the Brazilian soils. In a study on NW Tasmanian soils (Taylor and Graley, 1967) hematite was de- tected at a mean annual temperature of 9°C. Independent of temperature, low excess moisture may also favor dehydration of ferrihydrite to hematite by lowering the activity of water in the soil. This is supported by in-vitro experi- ments of Torrent et al. (1982) who noticed an increasing rate of hemat- ite formation from ferrihydrite as the relative humidi ty of the system dropped from 99% to ca. 70%.

The effect of excess moisture on the distribution of yellow (hematite free) and red (hematite containing) soils can also be seen within the same climatic region. An example of this kind is described by Williams and Coventry (1979) in N. Queensland where, within a toposequence, Yellow and Grey Earths are associated with a high perched water table due to the underlying rock at shal- low depths, whereas Red Earths prevail in well-drained situations with water tables and parent rocks at greater depths.

The positive correlation between the concentrat ion of organic compounds and the Gt /Gt+Hm ratio observed in this study indicates that organic com- pounds favor goethite formation. This can be explained by their Fe-complex- ing ability which prevents formation of ferrihydrite and thereby that of hematite.

The influence of pH has been explained elsewhere (Schwertmann, 1981). The formation of goethite from ferrihydrite appears to be positively related to the concentrat ion of [Fe(OH)2 ]+ monomers which has its maximum around pH 4 and decreases with increasing pH, thereby favoring hematite over goeth- ite. The findings of this s tudy are consistent with the earlier ones.

Finally, the depth function of Gt /Gt+Hm has to be explained. The dom- inance of goethite in the A horizon, if rich in organic matter, can be explained as above. With increasing depth the influence of organic matter weakens and this favors the formation of hematite. In the lower B and particularly in the C horizon the Gt /Gt+Hm ratio again increases if the proport ion of unweathered non-oxidic Fe (e.g., from silicates) also increases (lower Fed/Fet) . It is as- sumed that at this earlier stage of weathering the rate of Fe release is some- what lower, leading to less ferrihydrite and, in turn, to less hematite. In addi- tion, the maximum daily temperature is lower and excess moisture is higher in

37

these parts of the profile. In extreme cases, such as deep saprolites, goethite may be the only oxide formed.

For two reasons we believe that the higher amount of hemati te in the B than in the C horizon is due to hematite formation from the remaining Fe re- serves as the rate of weathering increases rather than from dehydrat ion of goethite already present. First, at surface temperatures the goethite -~ hematite transformation is energetically unfavorable. Secondly, hematite forms readily in vitro at ambient temperatures from ferrihydrite (Schwertmann, 1964, 1981).

So far, the distribution of hematite and goethite has been interpreted with regard to the present pedogenic conditions. Probably, however, most of the soils are rather old and therefore polygenetic, although no direct evidence for this comes from profile morphology. It is possible, for example, that the goeth- ite in the A horizon may be the result of a more recent hematite -~ goethite transformation caused by a moister and/or cooler, i.e. C-preserving, climate (Schwertmann, 1971). The fact that significant correlations exist between (pedo)climatic factors and the present distribution of goethite and hematite seems to indicate that similar differences in these factors must have existed during former stages of soil development.

Goethitic and hematitic mineralogy classes

According to Soil Taxonomy (Soil Survey Staff, 1975) all the soils studied belong to the kaolinitic class at the family level. Within this mineralogy class no further differentiation is possible on the ground of a crystallinity index for kaolin (Hughes and Brown, 1979), as all the values fall into the narrow range of 3 .7-5 .8 , wi thout any trend along the transect or within the profiles. Neither was such a trend visible in the minor amounts of Al-chlorite occurring mainly in the upper parts of the profiles.

As shown above, the Gt /Gt+Hm ratio appears a useful criterion for further mineralogical differentiation. So far Fe oxides are only considered in the ferritic and oxidic classes which are based on the total amount of oxides. But these are, in fact, not mineralogical criteria. The reason for this might be the general belief that the common soil iron oxides are difficult to quantify or even to identify because they occur in low concentration or are amorphous to X-rays. Methods for their identification and quantification have, however, been recently improved (Schulze and Dixon, 1979; K~impf and Schwertmann, 1981; Schulze, 1981). Therefore, it is suggested that goethitic and hematitic mineralogy classes be included at the family level. Besides their genetic im- portance, such classes may also be helpful for management purposes of trop- ical and subtropical soils because soils high in goethite may fix more phos- phate than those high in hematite (Bigham et al., 1978).

A possible way to include goethite and hematite in the mineralogical classes is to divide the kaolinitic class into a kaolini t ic-goethi t ic and a kaolinitic-- hematitic class. The limit for the classes may be tentatively set at a Gt/Gt+Hm

38

ratio of 0.60. This limit would be different for other mineralogical classes such as the illitic and montmorillonitic (should be smectitic) classes because of lower tendencies in the respective softs for hematite to be formed.

If the quantification of hematite can be done by simple color measurements (Munsell notation, remission spectroscopy) (Torrent et al., 1980; Schwertmann et al., 1981; K~mpf, 1981) the use of these mineralogical classes may be faciL itated. In the softs used for this study the borderline value of Gt/Gt+Hm = 0.60 coincides with a dry Munsell hue of 6.25YR and a moist hue of approximately 3.75YR.

ACKNOWLEDGEMENTS

The authors are grateful to Mr. D.G. Schulze from this institute for critical- ly reading the manuscript. One of us (N. Kiirnpf) acknowledges the financial support by MEC-CAPES, Brazil (Grant 4617/76).

REFERENCES

Bigham, J,M., Golden, D.C., Buol, S.W., Weed, S.B. and Bowen, L.H., 1978b. Iron oxide mineralogy of well-drained Ultisols and Oxisols. II. Influence on color, surface area, and phosphate retention. Soil Sci. Soc. Am J., 42: 825--830.

Brasil, 1973. Levantamento de reconhecimento dos solos do Estado do Rio Grande do Sul. Min. Agrie., DNPA, DPP Bol. T~cn. 30, Recife, 431 pp.

Davey, B.G., Russel, J.D. and Wilson, M.J., 1975. Iron oxide and clay minerals and their relation to colors of red and yellow podzolic soils near Sydney. Australia. Geoderma, 14 : 125--138.

Fauck, R., 1974. Les facteurs et les m6eanismes de la pedogenese dans les sols rouges et jaunes ferrallitiques sur sables et gr6s en Afrique. Cab. ORSTOM, S6r. P6do]., XII: 69--72.

Fblster, H., Noshrefi, N. and Ojenuga, A.G., 1971. Ferrallitic pedogenesis on metamorphic rocks, SW-Nigeria. P6dologie, XX(1 ): 95--124.

Hughes, J,C, and Brown, G., 1979. A crystallinity index for soft kaolins and its relation to parent rock, climate and soft maturity. J. Soil Sci., 30: 557--563.

K~mpf, N., 1981. Die Eisenoxidmineralogie einer Klimasequenz yon Bbden aus Eruptiva in Rio Grande do Sul, Brasilien. Dissertation Techn. Univ. Miinchen, Weihenstephan.

K~/mpf, N. and Schwertmann, U., 1982a. The 5M NaOH concentration method for iron oxides in soils. Clays Clay Miner. In press.

K~/mpf, N. and Sehwertmann, U., 1982b. Quantitative determination of goethite and hematite in kaolinitic soils by X-ray diffraction. Clay Miner. In press.

Lamoroux, M., 1972. Etat et comportement du fer dans les sols form,s sur roches carbo- nat~es au Liban. Sci. Sol, 1: 85--101.

Mehra, O.P. and Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. 7th. Nat. Conf. Clays Clay Miner,, pp. 317--327.

Nalovic, L., 1974. Recherches g6ochimiques sur les $16ments de transition dans les sols; Etude exp6rimentale de l ' influence des ~16ments traces sur le comportmente du fer et l '6volution des compos~s ferrif6res au cours de la p6dog6n6se. Th~se Doctorat, ORSTOM, Paris.

Norrish, K. and Taylor, R.M., 1961. The isomorphous replacement of iron by aluminium in soil goethites. J. Soil Sci., 12: 294--306.

39

Resende, M., 1976. Mineralogy, chemistry, morphology and geomorphology of some soils of the Central Plateau of Brazil. Ph.D. Thesis Purdue Univ., Lafayette.

Schulze, D.G., 1981. Identification of soil iron oxide minerals by differential X-ray dif- fraction. Soil Sci. Soc. Am. J., 45: 437--440.

Schulze, D.G. and Dixon, J.B., 1979. High gradient magnetic separation of iron oxides and other magnetic minerals from soil clays. Soil Sci. Soc. Am. J., 43: 793--799.

Schwertmann U., 1964. Differenzierung der Eisenoxyde des Bodens dutch Extraktation mit Ammoniumoxalat-LSsung. Z. Pflanzenern~ihr. Bodenkd., 105: 195--202.

Schwertmann U., 1969. Die Bildung yon Eisenoxidmineralen. Fortschr. Miner., 46: 274--285.

Schwertmann U., 1971. Transformation of hematite to goethite in soils. Nature (London), 232: 624--625.

Schwertmann U. and Lentze, W., 1966. Bodenfarbe und Eisenoxidform. Z. Pflanzenern~ihr. Bodenkd., 115: 209--214.

Schwertmann U., 1981. The influence of pH on goethite and hematite formation from ferrihydrite at room temperature. Int. Clay Conf., 7th, Bologna, Abstract No. 268.

Schwertmann U. and Taylor, R.M., 1977. Iron oxides. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. Soil Sci. Soc. Am., Madison, Wisc., pp. 145--180.

Schwertmann, U., Murad, E. and Schulze, D.G., 1982. Is there Holocene reddening (hematite formation) in soils of axeric temperate areas? Geoderma, 27 : 209--224.

Soil Survey Staff, 1975. Soil Taxonomy: A Basic System for Soil Classification for Making and Interpreting Soil Surveys. U.S. Dept. Agric. Handb., No. 436,134 pp.

Taylor, R.M. and Graley, A.M., 1967. The influence of ionic environments on the nature of iron oxides in soils. J. Soil Sci., 18: 341--348.

Torrent, J., Schwertmann, U. and Schulze, D.G., 1980. Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma, 23: 191--208.

Torrent, J., Guzman, R. and Parra, M.A., 1982. Influence of relative humidity on the crys- tallization of Fe(III) oxides from ferrihydrite. Clays Clay Miner., 30: 337--340.

Williams, J. and Coventry, R.J., 1979. The contrasting soil hydrology of red and yellow earths in a landscape of low relief. The hydrology of areas of low precipitation. Proc. Canberra Symp., 128: 385--395.