Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review

$Page 1: Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review$
Minerals in the clay fraction of BrazilianLatosols (Oxisols): a review

C. E. G. R . SCHAEFER1 ,* , J . D . FABRIS2 AND J . C . KER3

1 Departamento de Solos, Universidade Federal de Vicosa, 36571-000 Vicosa, Minas Gerais, Brazil,2 Departamento de Quımica, UFMG, Campus - Pampulha, 31270-901 Belo Horizonte, Minas Gerais, Brazil, and

3 Departamento de Solos, Universidade Federal de Vicosa, 36571-000 Vicosa, Minas Gerais, Brazil

(Received 30 April 2007; revised 11 December 2007)

ABSTRACT: This review focuses on the clay mineralogy of the most important Brazilian soils: the

Latosols, which cover >60% of the country by area, and occur in association with other soils. They are

typically deep, highly-weathered soils, dominated by low-activity 1:1 clay minerals and Fe and Al

oxyhydroxides, with varying proportions of these minerals, depending on parent material and

weathering intensity. They are usually of low fertility, although eutric types also occur. Latosols are

generally correlated with Oxisols (American soil taxonomy) and Ferralsols (WRB system). Clay

mineralogy is typically monotonous: kaolinite, gibbsite, hematite, goethite, maghemite and Ti minerals

(mainly ilmenite and anatase) are the prominent mineral phases in the clay fraction. Some Latosols

developing on basalt from southern Brazil contain significant amounts of hydroxyl-interlayed

vermiculite. Among the pedogenic oxides the most frequent are goethite (a-FeOOH), indicated byyellowish colours (2.5Y�10YR; in the absence of hematite), and hematite (a-Fe2O3), which imbuesreddish colors (2.5YR�5R), even when present in very minor amounts. Maghemite (g-Fe2O3) is lessfrequent; it imparts a reddish-brown colour (5YR�2.5YR) and magnetic properties. Both goethite andhematite show Al-substitution, with a greater relative proportion in soil goethites. Hence, in similar

drainage conditions, goethite is less prone to dissolution than hematite. Most reddish Latosols also

contain maghemite, due to partial or complete oxidation of magnetite, which generally occurs naturally

or is fire-induced. Magnetite and/or maghemite are associated with trace elements which are important

in plant nutrition, such as Cu, Zn and Co. The contents of gibbsite in Latosols are extremely variable,

from a complete absence in brown Latosols, to 54% in red Latosols from mafic rocks. Relatively large

amounts of gibbsite are found in the clay fraction of these soils and this mineral is important in P

sorption in deeply weathered Latosols in association with goethite and hematite. Even though most

Latosols are dystrophic, some are eutrophic, revealing an unusually large base saturation in areas under

ustic regimes where the parent material is particularly rich in bases, such as basalts. This eutrophic

nature is attributed to the protecting role of micro-aggregates in ferric red Latosols, which retard base-

leaching from the inner aggregate. At the other extreme, some Brazilian Latosols are acric and

positively-charged in sub-surface horizons, as revealed by the relationship pH KCl > pH H2O. These

acric Latosols are the result of long-term weathering and intensive leaching, during which pH tends to

increase to values close to the zero point charge of Fe and Al oxides (between 6 and 7), greatly

increasing P adsorption, which is mainly attributed to gibbsite, goethite and hematite. Soil kaolinites in

Brazilian Latosols are mostly of low crystallinity, with Hughes and Brown indexes of between 6 and

15. In this review we have discussed the role of these clay-fraction minerals in soil genesis and fertility,

highlighting the marked role of inheritance from deeply-weathered parent material. Latosols typically

retain large amounts of Fe oxides, some of which are magnetic, with spontaneous magnetization

>1 J T�1 kg�1. In this regard, reddish Latosols developed from mafic rocks are the most representativemagnetic soils, and cover as much as 3.9% of Brazil. An overview of magnetic soils on four

* E-mail: [email protected]: 10.1180/claymin.2008.043.1.11

ClayMinerals, (2008) 43, 137–154

# 2008 The Mineralogical Society

representative examples of mafic lithologies is presented, together with some aspects of their Fe-oxide

mineralogy and related field and laboratory technqiues.

KEYWORDS: Latosols, Brazil, XRD, Oxisols, Fe oxide, kaolinite.

Deeply weathered soils, known as Latosols, are the

most common soils occurring in Brazil (Fig. 1).

Latosols, as identified in the Brazilian system,

correspond to Oxisols, Sols Ferralitiques or

Ferralsols of the American, French and FAO soil

classification systems, respectively. The term

‘Latosol’ is derived from ‘laterite’ and ‘solum’,

both of Latin origin, meaning brick or highly

weathered material and soil, respectively, and was

proposed by the American pedologist Charles E.

Kellog, in an American soil classification confer-

ence held in Washington in 1949 (Kellog, 1949;

Segalen, 1994). The introduction of this term as a

soil class was a way of placing highly-weathered

tropical soils in the same group. Until then they

were referred to as ‘laterite’ or ‘lateritic soils’

which had a general, but imprecise and ambiguous

definition, and placed soils with very distinct

characteristics in the same class (Cline, 1975;

Segalen, 1994).

Based on this definition, as well as on colour and

Fe content (sulphuric acid extraction) four types of

Latosol are now recognized in the Brazilian system

of soil classification (EMBRAPA, 2006), namely:

red Latosol, yellow Latosol, red-yellow Latosol and

brown Latosol. Until 1999, this class was separated

into seven types (ferriferous (LF), dusky-red (LR),

dark-red (LE), red-yellow (LV), yellow (LA) and

brown (LB) and Una variation (LU) (Oliveira et al.,

1992).

Latosols are considered polygenetic soils, since

they were subject to varying climatic conditions

throughout their development (Schaefer, 2001),

thereby homogenizing their chemical, morpholo-

gical and mineralogical characteristics. They are

considered soils with very simple, monotonous

FIG. 1. Distribution of Latosol mapping-units in Brazil (source: Camargo et al., 1988).

138 C. E. G. R. Schaefer et al.

mineralogy (Resende, 1976; Curi, 1983; Antonello,

1988; Oliveira et al., 1992; Bognola, 1995; Ker,

1995, 1997; Schaefer et al., 2004). In the coarse

fraction (silt and sand), quartz prevails, with trace

quantities of muscovite and some degraded

K-feldspars when derived from acid rocks.

Magnetite and ilmenite, with very small amounts

of quartz, prevail in the coarse fraction when

derived from basic rocks, such as basalts.

Magnetite can be an important source of trace

elements (Resende, 1976).

Some reddish Latosols are magnetic due to the

presence of up to ~10 wt.% of magnetite (ideal

formula, Fe3O4) or maghemite (Fe8/3&1/3O4, where

& = vacancy). Both oxides have the spinel

structure, with cations distributed among tetrahedral

and octahedral oxygen coordination sites, and both

are ferrimagnetic. The geographical distribution of

the major magnetic soils in Brazil, their importance

to agriculture, and the main mechanisms in the

genesis of magnetic Fe oxides in selected pedo-

domains reported to date, were recently reviewed

by Fabris & Coey (2002). Soil Fe oxides are widely

variable in their composition, crystal structure,

grain size and morphology. These characteristics

impose some difficulties on their study by current

chemical and physical techniques, including

Mossbauer spectroscopy. The assignment of the

elemental chemical composition of individual

mineral phases is normally limited by the complex

mineralogical association of Fe oxides in soils.

Where sophisticated methods such as energy

dispersive X-ray (EDX) spectroscopy or extended

X-ray absorption fine structure (EXAFS) are

available for point-by-point analysis, or where

sub-samples containing the sole phase of interest

can be separated for conventional analysis, the

chemical formulae of the minerals can be allocated,

provided some details of their chemical structure

can be inferred from other techniques, e.g. the

mineral type, using X-ray diffraction (XRD).

In this paper we present a review on the clay

mineralogy of Latosols, which represent the

dominant soils in Brazil, covering >60% of the

country’s surface area (Fig. 1). As highly weathered

soils, they are dominated by low-activity 1:1 clay

minerals and Fe and Al oxyhydroxides, with

varying proportions of these minerals depending

on parent material and weathering intensity. They

are usually of low fertility status, although eutric

types also occur. Emphasis was placed on recent

results from studies of magnetic soils derived from

mafic lithologies in Brazil, including: (1) data on

their occurrence and parent-rock lithology; (2) their

detection in the field; and (3) characterization of

magnetite and maghemite in selected mafic litho-

domains. Also, details of some techniques involved

in their mineralogical study are discussed briefly, as

part of a methodical approach to characterizing

minerals of pedogenic origin, particularly kaolinite

and Fe oxides,

CLAY MINERALOGY � GENERALASPECTS

Latosols comprise soils at advanced weathering

stages, with consequent concentration of 1:1 clay

minerals and oxides (including oxyhydroxides and

hydroxides). Goethite (a-FeOOH) and hematite

(a-Fe2O3) are amongst the most abundant pedo-genic Fe oxides, and are identified by a yellowish

colour (2.5Y�10YR) in the absence of hematite

and reddish colour (even when hematite is present

in very minor amounts; 2.5YR�5R), respectively.Less frequent is maghemite (g-Fe2O3), which has areddish-brown colour (5YR�2.5YR) and magneticproperties (Resende, 1976; Curi, 1983; Santana,

1984; Kampf & Schwertmann, 1983; Dick, 1986;

Kampf et al., 1988a,b; Fontes & Weed, 1991;

Bognola, 1995; Ker, 1995; Fernandes, 2000). The

degree of weathering, expressed most reliably by

the Ki (Si:Al ratio) values obtained in the sulphuric

extract, is variable, ranging from very small

amounts such as 0.32 in gibbsitic-oxidic Latosols

to 2.1 in brown Latosols rich in hydroxy-

interlayered vermiculite (Table 1).

The amounts of kaolinite, gibbsite, hematite and

goethite vary according to several factors including

parent material, weathering intensity and drainage

conditions. Smaller amounts of hydroxy-inter-

layered vermiculite, illite, anatase, rutile, maghe-

mite and even halloysite are frequently observed in

Latosols. Generally, the clay fraction of Latosols is

dominated by kaolinite and Fe and Al oxides, with

smaller amounts of other components. In the next

section we discuss the most important minerals

found in the clay fraction of these soils and further

aspects related to their occurrence.

Fe oxides

Iron oxides, a generic term which includes Fe

oxides, hydroxides and hydrous oxides, are amongst

the major components of the clay fraction of

Clay fraction minerals of Brazilian Latosols: a review 139

TABLE1.ChemicaldataforselectedLatosolsfromBrazil:concentrationofelementsextractedfromtheclayfractionbysulphuricacid:molarratiosKi{;Kr{and

FeextractedbyCDB(Fe d)andammoniumoxalate(Fe o).DatabasedonRodrigues-Netto(1996).

Sample

Latosol

SiO2

Al 2O3

Fe 2O3

TiO2

P2O5

K2O

MgO

Total

Ki

Kr

Fe d*

Fe o*

Fe d/Fe o

no.

—————————————wt.%—————————————

RN1

RLItabirite

15.2

20.7

42.9

0.6

0.27

0.24

0.08

79.9

1.24

0.53

51.0

1.21

42

RN2

RLBasalt

10.7

27.5

25.9

4.5

1.53

0.00

0.05

70.0

0.66

0.41

29.0

1.78

16

RN4

BLVolcanics

34.1

27.5

17.9

1.9

0.18

0.10

0.08

81.7

2.10

1.48

18.8

0.61

31

RN6

RLLimestone

8.9

42.0

15.6

1.2

0.13

0.09

0.02

67.8

0.36

0.29

17.2

0.53

32

RN7

RLTuffite

15.2

39.2

16.1

0.8

0.11

0.85

0.09

72.3

0.66

0.52

17.1

0.38

45

RN9

RYLFe-richtuffite

8.1

42.9

14.1

2.1

0.16

0.00

0.01

67.3

0.32

0.27

14.8

0.34

43

RN10

RYLFe-richgneiss

30.8

31.8

15.1

1.8

0.09

0.00

0.01

79.6

1.65

1.26

16.2

0.45

36

RN11

RYLGneiss

28.7

35.3

14.8

0.7

0.11

0.00

0.01

79.6

1.38

1.09

15.5

0.18

86

RN13

RYLClayeysediments

10.9

42.0

13.5

3.5

0.17

0.00

0.01

70.0

0.44

0.36

12.8

0.27

47

RN14

YLTertiarysediments

40.5

34.1

5.7

1.8

0.04

0.00

0.01

82.2

2.02

1.82

4.0

0.15

26

RL:redLatosol;RYL:red-yellowLatosol;BL:brownLatosol;YL:yellowLatosol.

*Feamountexpressedas%Fe 2O3

{ :Ki=SiO2/Al 2O3

{ :Kr=SiO2/(Al 2O3+Fe 2O3)


Latosols (Kampf et al., 1988), and of their

taxonomic equivalents, Oxisols. They are usually

dispersed in the soil mass as fine particles with

varying crystallinity and may coat clay minerals or

be associated with organic complexes (Oades, 1963;

Fontes et al., 1992).

Goethite (a-FeOOH), which accounts for yellowor brownish soil colours (2.5Y�5YR), and hematite(a-Fe2O3), for the red colors (5R to 5YR), are the

main Fe forms present in Brazilian Latosols

(Resende, 1976; Curi, 1983; Dick, 1986; Kampf et

al., 1988; Fontes, 1991). Goethite is considered the

most stable form, found in many different

environments, and appears to be the dominant

form present in Latosols (Resende, 1976). In red

Latosols derived from mafic rocks, the yellowish

colours of goethite are masked by the high red-

pigmenting power of hematite (Resende, 1976).

Hematite, a less stable mineral, is generally

negligible or absent in yellow soils, regardless of

the total Fe content (Resende, 1976; Curi, 1983;

Dick, 1986; Kampf et al., 1988; Macedo & Bryant,

1987).

The information available on amounts, types,

isomorphous substitution and crystallinity of Fe

oxides in Brazilian Latosols is based mainly on the

excellent review of Kampf et al. (1988), which is

difficult to access for most scientists. The amount

of Fe in Latosols varies from 0.7% to 44% Fe, with

80�100% of it pedogenic, mostly goethite, hematite

and maghemite, as indicated by the Fe(CBD)/

Fe(H2SO4) ratio (Kampf et al., 1988). Hematite,

as a proportion of the dominant oxide to the sum of

hematite and goethite (Hm/Hm+Gt ratio), ranges

from 0 to 0.97. Maghemite is found in abundance in

reddish, hematitic Latosols developed from mafic

and itabiritic rocks. The varying colours (red,

brown and yellow) of Latosols are due to the

varying proportions of hematite and goethite. In

southern Brazil, where udic to perudic systems

combined with thermic to mesic regimes prevail,

Hm/Hm+Gt is more clearly related to the present

climate, whereas in isohyperthermic regimes in

central Brazil, the large variations in goethite and

hematite contents are related to parent material,

bioclimatic conditions or drainage.

If soil-environment conditions are favourable, i.e.

low silica activity in solution and small amounts of

organic matter, which result in less Fe complexa-

tion, ferrihydrite, a less crystalline mineral phase,

alters to hematite through internal rearrangement

and dehydration. These conditions are typical of

free-drainage systems characterized by high

temperature and enough water to cause greater

weathering rates and silica leaching. In contrast, if

the environmental conditions are not adequate,

ferrihydrite may dissolve, allowing goethite to

form in its place. In poorly drained soils,

however, hematite may also be present, concen-

trated in mottles forming soft plinthites. In this

case, it is postulated that in periods of good

drainage (low watertable), a localized accumulation

of Fe3+ occurs.

Substitution of Fe by Al occurs in goethite and

hematite. Aluminium substitution in goethite and

hematite in Brazilian Latosols is common, ranging

between 7 and 40 mole% Al in goethite (Table 2)

and 4�17 mole% Al in hematite. Rodrigues-Netto

(1996) provided a comprehensive study of Al-

substitution in Hm and Gt in Latosols from Brazil,

illustrating that the large variability depends on soil

class and parent material, as illustrated in Table 3.

The small Fe-oxalate/Fe-DCB values (<0.03)

indicate the dominance of the crystalline forms of

TABLE 2. Isomorphic Al substitution in soil goethite from Brazilian Latosols.

Soils Locations Samples Al subst. (mole%) Reference

Latosols and Cambisols Southern Brazil 47 11�25 1Latosols, Cambisols,brown earths

Southern Brazil 10 13�22 2

Latosols Western Minas Gerais 6 ~30 3Latosols Central Plateau 12 24�36 4All Latosol classes Various regions 17 9�40 5, 7Latosols North Brazil 14 7�33 6

1 � Schwertmann & Kampf (1985); 2 � Palmieri (1986); 3 � Resende (1976); 4 � Curi (1983); 5 � Ker (1995);6 � Moller (1991); 7 � Rodrigues-Netto (1996).


Fe oxides. However, their crystallinity is low

(structural-perfection level and small crystal size)

compared to Fe oxides from ores. Al-rich Fe oxides

commonly have larger specific surface areas (SSA)

and reactivity values. In Latosols, however, this

relationship is not always observed (Gualberto et

al., 1987; Ker, 1995).

It is unclear whether there is a trend of gibbsite-

rich Latosols which are richer in Al-substituted

goethites, as data on the subject are conflicting (e.g.

in Table 3, RN 12, kaolinitic, and RN 13, gibbsitic,

both contain large amounts of Al-substituted

goethite). Rezende (1980) observed an inverse

relationship between the Al-substitution in goethite

and gibbsite content for the Latosols of Minas

Gerais State, with substitution rates varying

throughout the profile. What seems clear is that

highly aluminized goethites tend to be less

susceptible to reduction (Macedo & Bryant, 1987)

in Brazilian Latosols. The presence of organic

compounds, biological activity and high water

tables, all favour Fe-oxide dissolution in Latosols.

Maghemite is another common Fe oxide in

Brazilian soils, especially in red Latosols derived

from itabirite and mafic rocks (Curi, 1983; Kampf

et al., 1988; Ker, 1995). Its formation seems to be

related to total or partial oxidation of magnetite

(Resende et al., 1988) or to the fire-induced

transformation of pedogenetic Fe oxides (Viana et

al., 2004). There is good correlation of maghemite

amounts with trace-element contents, particularly

Cu, Zn, Ni and Mn (Ker, 1995), in the case of soils

developed from mafic rocks, as shown in Table 4.

Gibbsite

During weathering of silicate minerals, we can

observe the release of Fe, Al, Si, Ca, Mg, Na, K,

etc. The alkali elements tend to leach out of the

system due to their greater solubility. Silicon is less

soluble than these ions but more soluble than Fe

and Al, and, as a result, can also leach depending

on the drainage conditions. For gibbsite, stability is

reached at pH 5.2 (Lindsay, 1979). Hence, all Al

released during weathering precipitates as gibbsite

if the pH is greater than the silica potential in the

soil environment, which must be low initially.

Although gibbsite is considered a commonly-

occurring mineral in various soils under diverse

climatic conditions, relatively large contents of this

mineral tend to occur in Brazilian Latosols, where

weathering and leaching processes are more intense.

Two basic mechanisms have been proposed to

explain gibbsite’s origin in soils: (1) rapid

dissolution in the initial phases of Al-silicate

weathering (primarily feldspars), where gibbsite

TABLE 3. Values of R (=Hm/(Hm+Gt)) and isomorphic substitution (IS) of Al in hematite (Hm) and goethite (Gt)

(modified from Rodrigues-Netto, 1996). These soils are referred to in Table 1.

Sample Soil class/parent material R1 R2 R3 IS-Gt IS-Hm————— % ————— —— mole% ——

RN1 RL Itabirite 74 74 74 9 0RN2 RL Basalt 69 76 73 20 13RN4 BL Volcanics 42 46 44 20 4RN6 RL Limestone 45 49 47 37 8RN7 RL Tuffite 40 36 38 27 7RN9 RYL Fe-rich tuffite H.A. H.A. 0 32 H.A.RN10 RYL Fe-rich gneiss 9 1 5 33 *RN11 RYL Gneiss 6 0 3 27 *RN13 RYL Clayey sediments H.A. H.A. 0 30 H.A.RN14 YL Tertiary sediments H.A. H.A. 0 32 H.A.

1 Calculated using the formulae: 1 � [0.55 + 0.57.log(AGt111/AHm110)] (AGt111 = area of the goethite 111reflection; AHm110 = area of the hematite 110 reflection)2 Calculated using the formulae: 1 {[ AGt110/(0.708.AHm104)] + 1} (AGt110 = area of the goethite 110reflection; AHm104 = area of the hematite 104 reflection)3 Mean of R1 and R2

H.A.: hematite is absent* Hematite in small amounts: IS not determined.


may be one of the first products of neo-formation;

and (2) intense, long-term weathering, including the

progressive dissolution of kaolinite (desilicifica-

tion). In both pathways, free drainage, low silica

activity and small base concentrations in solution

are required (Gomes, 1976; Macıas Vasquez, 1981).

Although gibbsite may originate from a wide

variety of parent materials, in Brazil greater

amounts of this mineral are found in soil developed

from Fe-rich rocks (Moniz, 1967; Resende, 1976;

Curi, 1983; Santana, 1984). This tendency,

according to Resende (1976), seems to be related

to one or both of the following facts: (1) Fe-rich

rocks are originally silica-poor, which favours

gibbsite formation instead of kaolinite; and

(2) free Fe oxides absorb silica, reducing its

ability to complex Al and form kaolinite.

Where leaching is not very intense, the Al

released may penetrate the interlayer space of clay

minerals, especially vermiculite (Ker, 1997). This

process was referred to as the ‘‘anti-gibbsitic effect’’by Jackson (1964). The brown Latosols of south

Brazil appear to follow Jackson’s model (Ker &

Resende, 1990).

The amounts of gibbsite in Brazilian Latosols are

presented in Table 5. These values are extremely

variable, ranging from a complete absence in brown

Latosols (RN4) to as much as 54% in red Latosols

from volcanic tuffs (RN9). Using scanning electron

microscope energy dispersive X-ray spectroscopy

(SEM-EDS) studies, Schaefer et al. (2004) demon-

strated the importance of gibbsite at the micro-

aggregate scale in P sorption of deeply weathered

Latosols, which is consistent with previous studies

of P adsorption (Leal, 1971; Bahia Filho et al.,

1983; Dick, 1986; Ker, 1995).

EUTR IC LATOSOLS AND

GIBBS IT IC LATOSOLS : TWOEXTREMES

Even though the majority of Latosols are

dystrophic, some are eutrophic, with a large base

saturation. These unusual Latosols are from regions

of ustic climate and where the parent material is

particularly rich in bases, such as basalt or

limestone. Moura Filho & Buol (1976) have

attributed this eutrophy to an effective protecting

role of micro-aggregates in ferric red Latosols,

which retard or impede the leaching of K, Ca and

Mg from the inner aggregate. When crushed, these

aggregates reveal a considerable increase in the rate

TABLE4.ConcentrationsofCo,Ni,Cu,ZnandMndeterminedbyHFtotal-digestionofair-driedsoilB-horizons(BrazilianLatosols).

Soilclass

Location

Parent

Co

Ni

Cu

Zn

Mn

(sampleno.)

material

——————————(mg/kg)——————————

RedLatosol(K1)

Cravinhos-SP

Basalt

95

69

174

102

791

RedLatosol(K2)

RibeiraoPreto-SP

Basalt

n.d.

56

178

122

1100

RedLatosol(K17)

Silvania-GO

Amphibolites

n.d.

50

52

72

338

RedLatosol(K23)

Catalao-GO

Pedidotite

n.d.

73

96

110

1712

RedLatosol(K26)

Dourados-MS

Basalt

91

66

365

120

929

RedLatosol(K29)

Patos-MG

Tuffite

n.d.

327

242

140

2917

RedLatosol(K30)

NovaLima-MG

Itabirite

n.d.

021

33

293

RedLatosol(K16)

Piracicaba-SP

Claysediments

513

43

40

164

Red-yellowLatosol(K8)

Silvania-GO

Claysediments

n.d.

15

36

40

193


AbreCampo-MG

Gneiss

22

36

45

42

209

YellowLatosol(K20)

Acara-PA

TertiaryBarreirasclaysediments

19

11

16

43

96

Source:Ker(1995)

n.d.:notdetermined


TABLE5.ThegibbsitecontentinBrazilianOxisolsbasedonchemicalanalysisandDTA-TGdata,Feoxidesandkaolinitecontents,pHH2O/pHKClandpointof

zerocharge(pzc).

Sampleno.

Class

Gibb.*

Hem.*

Goeth.*

Kaol.*

pHH2O

pHKCl

DpH

PZC

Fe d

SumOx

——————wt.%——————

—wt.%—

RN1

RLItabirite

11.2

40.5

14.2

33.6

5.4

5.4

0.0

5.6

51.0

65.9

RN2

RLBasalt

27.7

28.9

10.7

27.3

6.1

5.5

�0.6

5.0

29.0

67.3

RN4

BLVolcanics

0.0

10.4

13.2

70.5

5.0

4.4

�0.6

4.5

18.8

23.5

RN6

RLLimestone

53.0

11.5

12.9

21.2

5.4

5.1

�0.3

4.8

17.2

77.4

RN7

RLTuffite

41.0

8.8

14.4

23.9

5.5

5.3

�0.2

5.1

17.1

64.3

RN9

RYLFe-richtuffite

54.0

0.0

24.2

19.5

5.3

5.0

�0.3

4.8

14.8

78.2

RN10

RYLFe-richgneiss

1.5

1.3

24.0

71.3

4.5

4.5

0.0

4.7

16.2

26.8

RN11

RYLGneiss

12.0

0.6

21.7

64.9

5.2

5.2

0.0

5.1

15.5

34.3

RN13

RYLClayeysediments

49.5

0.0

20.2

26.5

5.2

4.9

�0.3

4.7

12.8

69.7

RN14

YLClayeysediments

0.0

0.0

6.1

90.1

4.8

4.3

�0.5

4.1

4.0

6.1

RL:redLatosol;RYL:red-yellowLatosol;BL:brownLatosol;YL:yellowLatosol.

*Amountofgivenmineralintheclayfraction.

Fe d=CDB-extractableFeaswt.%Fe 2O3

SumOx=sumofoxidecontents;Gibb.+Hem.+Goeth.+Kaol.


of P, Ca, Mg and K release, confirming that

physical protection does occur.

Another feature commonly found in Brazilian

Latosols is the occurrence of acric and positively

charged subsurface horizons. This is illustrated by

pH KCl > pH H2O. These acric Latosols are

typical of the highland planation surfaces, where

long-term weathering has resulted in intensive

leaching of the Latosol mantle (Rolim-Neto et al.,

2004). During weathering, pH tends to increase to

values close to the point of zero charge (pzc) of

the Fe and Al oxides (6�7). This greatly increasesP adsorption, which reaches up to 3.5 mg P/g ofsoil (Ker, 1995; Rolim Neto et al., 2004). Besides

their positively charged nature, subsurface B

horizons of the acric soils are generally very

fluffy and porous, allowing chemical leaching to

reach greater depths compared with other non-

acric Latosols.

Kaolinite

Kaolinite is probably the most abundant mineral

in the majority of Brazilian Latosols, except for the

most weathered and gibbsitic types (e.g. Latosols

RN 2, 5, 6, 7 and 9 � see Table 5). It originates

from the alteration of a variety of primary minerals,

especially feldspars and micas, or secondary

minerals (2:1 clay dissolution), in different environ-

mental conditions. Overall, wetter and warmer

climates and free-draining conditions (but not

excessive silica leaching), and low pH favour

kaolinite genesis (Jackson & Sherman, 1953;

Keller, 1957). These conditions are common in

the tropics and account for the mineral’s great

abundance in Latosol clay fractions.

Soil kaolinite is usually of lower crystallinity

than kaolinite from geological deposits (Hughes &

Brown, 1979; Varajao et al., 2001). Many empirical

methods have been applied to establish kaolinite

crystallinity indexes. Most of them are based on

XRD results, where there are relationships between

the intensity of some peaks and base-line and

kaolinite crystallinity.

The HB crystallinity index (Hughes & Brown,

1979) is the most well used and is based on the

relationship between h1 and h2, where h1 refers to

the peak intensity at ~22 and 17º2y, or 24 and

20º2y for Cu-Ka and Co-Ka radiation, respectively,with h2 representing the depression observed near

44º or 37.6º2y for these radiations (Fig. 2).Using the HB index, Ker (1995) found values

ranging from 6 to 15 for kaolinites in the Fe-free

clay fraction of Brazilian Latosols with various Fe

contents (Table 6). These values are consistent with

those reported by Hughes & Brown (1979) for

African soils and well below those found for highly

crystalline kaolinites from elsewhere (Table 7).

Fernandes (2000), using the same procedure,

showed HB index values ranging from 8 to 15

FIG. 2. Kaolinite peaks from two regions of the XRD patterns used to calculate the crystallinity index (based on

Hughes & Brown, 1979).


(Table 6), with no clear relationship between

kaolinite crystallinity and the total Fe content of

kaolinite, as suggested by Moniz (1967) and

Mestdagh et al. (1980).

Hydroxy-interlayered vermiculite (HIV) and

other 2:1 minerals in Latosols

In many Latosols, minor quantities of hydroxy-

interlayered vermiculite (HIV) have been detected

in the clay fraction. However, in most brown

Latosols from southern Brazil, large amounts of

HIV are observed (Potter & Kampf, 1981; Ker &

Resende, 1990; Bognola, 1995), and appear to be

related to a marked trend of soil-cracking upon

desiccation (Fig. 3). In these soils, the Al-inter-

layering in the vermiculite crystal can block

exchange sites, greatly decreasing the CEC of

these soils and promoting a so-called ‘‘anti-gibbsite’’ effect (Jackson, 1964). According to Ker& Resende (1990), the large expansion-contraction

revealed in these brown Latosols at field scale is

due to the large SSA, although chemically they

behave as a low-CEC clay.

Rodrigues-Netto (1996) used XRD to identify

traces of 2:1 clays in Latosols from Brazil

(Table 8). Clay samples after Fe-removal by DCB

were analysed following treatments at 25, 135, 300

and 500ºC, and also after Mg2+ and glycerol

treatment, which allowed identification of 2:1 clays.

Traces of 2:1 clays have been detected even in

deeply-weathered gibbsitic red Latosols (such as

RN5, RN6 and RN7) with very small Ki values.

Also, important K reserves are found in these soils,

as illustrated by the large amounts of K2O in the

clay fraction (RN5 and RN7). This element is

associated with illite and HIV in Brazilian Latosols,

either as a discrete mineral or enclosed within

kaolinite laths, as shown by Melo et al. (2002) and

Varajao et al. (2001). The abundance of non-

exchangeable K in Brazilian Latosols is directly

related to the presence of illite within the kaolinite

laths, as well as to the presence of minor quantities

of primary minerals in the sand fraction (Melo et

al., 2003).

Clay mineralogy of Latosols quantified using

sulphuric-acid extraction and DCB analyses

In Brazilian Latosols, the amounts of elements

extracted by sulphuric acid and of Fe extracted by

DCB in the clay fraction can be allocated to

minerals identified by XRD, allowing mineralogical

quantification. Table 9 shows the amount of each

mineral phase resulting from this allocation

procedure (Rodrigues-Netto, 1996) adjusted to

100% (a calculated mean of 92% recovery, with

3% standard deviation, was considered satisfactory).TABLE6.HBkaolinitecrystallinityindexvalues(CI)forvariousLatosolB-horizonsfromBrazilandageologicaldeposit.Datacompiled

from

Fernandes(2000),Ker(1995)andHughes&Brown(1980).

Soilandsampleno.

Parentmaterial

HBCI

Reference

YellowLatosol(R25-26)

Tertiaryclaysediments-Barreiras

11.0(A

horizon);12.6(Bhorizon)

Fernandes(2000)

Red-yellowLatosol(R19-20)

Saprolitefromgneiss

15.0(A


Fernandes(2000)

Red-yellowLatosol(R21-22)

Saprolitefromgneiss

13.2(A


Fernandes(2000)

RedLatosol(R1-2)

Basaltwithsandstonelayers

7.7(A

andBhorizons)

Fernandes(2000)

RedLatosol(R13-14)

Limestone

5.8(A

horizon)

Fernandes(2000)

RedLatosol(R11-12)

Basalt

9.7(A


Fernandes(2000)

YellowLatosol(K20)

Tertiaryclaysediments

14(Bhorizon)

Ker(1995)


Gneiss

15(Bhorizon)

Ker(1995)

RedLatosol(K2)

Basalt

9(Bhorizon)

Ker(1995)

RedLatosol(K16)

Basalt

8(Bhorizon)

Ker(1995)


TABLE 7. Specific surface area (SSA) and mean crystal dimension (MCD) estimated for the 001

reflection of kaolinites from clay samples of Brazilian Latosol B-horizons (based on Ker, 1995).

Sample Soil classification ——— Kaolinite ———MCD 001 SSA(nm) (m2/g)

Kaolin reference1 Kaolin � Georgia 107 15K2 Red Latosol from basalt 18 50K18 Red Latosol 34 30K26 Red Latosol from basalt 18 50K16 Red Latosol 24 39K28 Red-yellow Latosol (high Fe content) 49 23K11 Red-yellow Latosol 25 38K20 Yellow Latosol (kaolinitic) 34 30K25 Brown Latosol 17 52K14 Brown Latosol 28 35K4 Yellow Latosol (gibbsitic) 49 23

1Georgia kaolinite (n.312) as reference

FIG. 3. XRD patterns of the clay fraction of selected brown Latosols from southern Brazil, showing the

abundance of HIV (Ker & Resende, 1990): (a) Mg-saturated, deferrified clay treated with ethylene glycol;

(b) K-saturated at 25ºC; and (c,d,e,f) K-saturated and heated to 100, 200, 300 and 550ºC, respectively.


The <100% total of the minerals may be attributed

to the organic matter content and water adsorbed by

clay minerals.

All Latosols have goethite and kaolinite in the

clay fraction. Hematite is absent in soils with colour

>7.5 YR and gibbsite is absent in less weathered

soils, with Ki values >2.0. In some Latosols, the

amount of gibbsite is greater than kaolinite. Anatase

values can reach 5.3% in Latosols developed from

basalt.

Magnetic properties of Latosols and related

materials

Soil surveyors in Brazil often use a hand magnet

as a field test for judging the nature of the parent

material, especially for separating basalts from non-

mafic lithologies, where magnetism is usually low

(Resende et al., 1986). In this respect, spontaneous

magnetization has been shown to be a better

mineralogical parameter than magnetic suscept-

ibility (MS), but a good linear relationship has

been established between MS obtained at field and

relatively high field (0.5 Tesla) using a modified

analytical balance. Magnetization values for some

reference Fe minerals present in Latosols show a

wide variation, which can be explained inter alia by

varying amounts of isomorphous substitution of Ti

and Al for Fe (Resende et al., 1988).

Magnetization data for Brazilian Latosols are

related to both Fe content and colour (Resende et

al., 1988). Magnetization of the clay fraction of

Latosols is greater in redder soils and in those

richer in Fe; it shows a marked decrease in soils

with increasing yellow colour and less Fe. Magnetic

Latosols are closely associated with Fe-rich parent

TABLE 9. Clay mineral contents adjusted to 100% in selected Latosols (compiled from Rodrigues-Netto, 1996)

Soil Class Hem. Goet. Gibb. Kaol. Anatase SiO2 2:1 clayminerals

————————————— wt.% —————————————

RN1 RL Itabirite 40.5 14.2 11.2 33.6 0.6 0.0 0.0RN2 RL Basalt 28.9 10.7 27.7 27.3 5.3 0.0 0.0RN4 BL Volcanics 10.4 13.2 0.0 70.5 2.0 4.0 0.0RN6 RL Limestone 11.5 12.9 53.0 21.2 1.3 0.0 0.0RN7 RL Tuffite 8.8 14.4 41.0 23.9 0.8 0.0 11.0RN9 RYL Fe-rich tuffite 0.0 24.2 54.0 19.5 2.3 0.0 0.0RN10 RYL Fe-rich gneiss 1.3 24.0 1.5 71.3 1.9 0.0 0.0RN11 RYL Gneiss 0.6 21.7 12.0 64.9 0.8 0.0 0.0RN13 RYL Clayey sediments 0.0 20.2 49.5 26.5 3.9 0.0 0.0RN14 YL Clayey sediments 0.0 6.1 0.0 90.1 2.0 1.8 0.0

Abbreviations as in Table 1.

TABLE 8. Types of 2:1 clay minerals and some mineralogical properties of the studied Latosols.

Soil Class Ki value Mineral K2O1 MgO1 2:12

concentrationK2O

3 amountin illite

———————— wt.% ————————

RN3 Red Latosol 1.92 HIV 0.00 0.06 tr �RN4 Brown Latosol 2.10 HIV 0.10 0.08 tr �RN5 Red Latosol 0.58 Illite + HIV 1.04 0.03 11.6 8.9RN6 Red Latosol 0.36 HIV 0.09 0.02 tr �RN7 Red Latosol 0.66 Illite + HIV 0.85 0.09 9.8 8.7

1 Determined in the sulphuric extract of the clay fraction.2 Based on 100% clay.3 Calculated by: % K2O6100/% 2:1 mineral.tr = traces


TABLE10.Magneticsusceptibilityatthefine-earth,clay,siltandsand-sizefractionsofLatosolB-horizonsfromsouthernandsoutheasternBrazil(basedon

Resende

etal.,1988).

Magneticsusceptibility

Latosol

Alti-

tude

Lithology

WetMunsell

colour

Clay

Fe 2O3

Fine-

earth

Fine-sand

(0.005�0.02mm)

Silt

(0.002�0.05mm)

Clay

(<0.002mm)>0.149

mm

—(wt.%)—

——————————(m

3kg�1610�8)——————————

RL

1200

Itabiritesandhematiticphyllites

10R3/6

58

55.8

7353

12970

4078

2535

3884

RYL

850

Gneissandacidmigmatites

7.5YR5/8

57

8.1

10

27

125

13

12

RYL

950

Mesocraticgneiss

7.5YR4/5

69

14.7

48

114

220

23

34

YL

480

Tertiarysediments

10R5/6

49

5.2

11

18

253

23

19

RL

�Claysediments

2.5YR3/7

59

11.1

565

573

448

642

504

RYL

645

Basalts

4YR4/4

53

30.0

5886

14465

2546

1046

2467

RL

760

Basalts

1.5YR3/4

75

34.2

7296

9932

12797

3885

7239

RL

�Sandstone

1.5YR3.5/6

24

3.6

67

71

643

118

111

RL

640

Basalts

1YR3/4

82

29.6

8391

19146

16652

5317

8398

RL

760

Basalts

1YR3/5

88

22.9

2258

8292

3680

1883

2105

BL

920

Basalts

3.5YR3.5/5

85

23.1

803

4525

2222

458

730

BL

1100

Basalts

5YR3.5/5

79

24.0

653

3971

2430

77

696

BL

910

Acidmetamorphicrocks

1.5YR4/8

63

7.7

107

62

204

77

80

RYL

860

Migmatites

10YR5/8

50

10.4

218

948

96

999

AbbreviationsasinTable1.


materials (Table 10), as magnetization in the clay

fraction is only high in soils derived from mafic,

tuffitic and itabiritic rocks. Coarse fractions of these

soils have strong magnetic susceptibility due to

magnetite or maghemite (Table 10). Since magne-

tite is usually rich in certain trace elements, the

magnetic behaviour can indicate chemically-rich

pedosystems (Resende et al., 1988). The separation

of magnetic Latosols seems to be justified by the

greater agricultural productivity of these soils.

Hence, Latosols derived from mafic rock have a

greater potential for agricultural development than

Latosols derived from other rocks in Brazil.

Parent-rocks of fresh and altered amphibolite (or

dolerite), basalt, diabase and tuffite were sampled at

four sites with a tropical climate in southern Brazil

(locations specified in Table 11; Fabris et al.,

1999).

The laboratory procedures used to prepare the

samples have been described in detail elsewhere

(Fabris & Coey, 2002). Chemical analyses of the

fresh and altered rock samples were performed by

dissolving the sample in HF + HCl + HNO3. The

Fe2+ was determined by dissolving separate samples

in concentrated HCl in a CO2 atmosphere.

Chemical compositions were obtained via point

analyses on thin sections using SEM-EDS. Powder

XRD patterns were obtained using a diffractometer

equipped with a graphite diffracted-beam mono-

chromator using Cu-Ka radiation. Further informa-tion on the methods used can be found in Fabris et

al. (1999) and Fabris & Coey (2002). Mossbauer

spectra were recorded in a conventional constant

acceleration transmission spectrometer with a57Co/Rh source. The various chemical treatments

used are reviewed in Fabris & Coey (2002).

The main magnetic minerals in some mafic litho-

domains of Brazil are summarized in Table 12.

Magnetic Fe oxides vary widely in chemical

composition. For amphibolite (or dolerite), tholeiitic

basalt and diabase, Ti or Al are the main

isomorphic substituents of the Fe oxides.

Magnetite and maghemite from volcanic ash

(tuffite) are richer in Ti and Mg. Their saturation

magnetization values range from 18 to 54 J T�1

kg�1. Lattice parameters of the cubic structure are

strongly affected by the oxidation state of Fe and

the presence of isomorphously substituted ions. The

Ti4+ tends to increase unit-cell dimension a in

magnetite but has little or no effect on maghemite;

Mg2+ tends to decrease a in magnetite and increase

it in maghemite. The combined effect in more

complex compositions gives rise to a wide range of

values. Greater a values are found in the (Ti,Mg)-

rich magnetite of tuffite (Table 12), although values

are quite variable for both magnetite and maghe-

mite from this rock (Fabris et al., 1999).

The earliest mineralogical studies assigned

magnetite to the coarser fractions of magnetic soils

developed on mafic lithologies (Resende et al.,

1988). However, several recent Mossbauer studies

have shown that only maghemite is actually found in

all fractions of those soils and in their altered parent-

rock, at least from tropical regions (Pinto et al.,

1998; Goulart et al., 1998). These results suggest

that the stability of magnetite is related to weath-

ering conditions during pedogenesis.

Ti-rich mineral phases are generally associated

with magnetic Fe oxide. Except for tuffite, where

anatase is the main Ti-mineral, ilmenite occurs as

lamellae in the Fe-oxide spinels of magnetic pedons

(Doriguetto et al., 1998).

Magnetic soils forming on mafic rocks are

widespread in central, southeastern and southern

Brazil (Resende et al., 1988; Fabris et al., 1998).

They can be detected in the field with a hand

magnet and their magnetization measured with a

portable soil magnetometer. Maghemite, which

varies widely in its composition and in some of

its physical characteristics, is the Fe oxide mostly

TABLE 11. Locations of the sampled mafic rocks beneath the red magnetic

Latosols (Oxisols), in the wet and dry tropical regions of Brazil.

Lithology ————— Location —————City - state Geographical coordinates

Amphibolite Vicosa - MG 20o 27’ 00’’ S; 42o 31’ 48’’ WDiabase Sao Carlos - SP 22o 00’ 36’’ S; 47o 32’ 24’’ WBasalt Tupaciguara - MG 21o 33’ 36’’ S; 50o 18’ 00’’ WTuffite Patos de Minas - MG 18o 21’ 00’’ S; 46o 19’ 12’’ W


TABLE12.Dominantlithology,Feoxide,origin,chemicalformula,unit-celldimensionandmagnetizationofthemainmagneticFeoxidesinsomemaficlitho-

domainsofBrazil.

Lithology

Feoxide1

Origin1

Proposedchemicalformula3

Unit-celldimension

a(nm)

Magnetization,s/J

(T�1kg�1)

Amphibolite(ordolerite)

Mh

FR

Fe 1.91Ti 0.53Al 0.03Mn0.02Zn0.01&0.50O4

0.8359(1)

N.A.

Mh

AR

[Fe 0.77Ti 0.22Zn0.01]{Fe 1.19Ti 0.26Mn0.02Al 0.04&0.49}O4

0.8348(3)

33

Amphibolite(ordolerite)

Mt

FR

N.A.4

0.8392(2)

545

Mh

AR

N.A.

0.8313(1)

477

TholeiiticbasaltMt

FR

N.A.

0.839(1)

455

Mh

AR

N.A.

0.8340(1)

N.A.

Mh

Soil

[Fe 0.92Al 0.08]{Fe 1.43Ti 0.18&0.39}O4

0.8319(5)

49

Diabase

Mt

FRl

N.A.

0.8409

N.A.

Mh

AR

N.A.

0.836(5)

33

Mh

Soil

[Fe 1.06(5)Si 0.04(3)]{Fe 1.30(6)Al 0.21(6)Cr 0.01(1)Ca 0.01(1)Mn0.03(6)Ti 0.02(2)&

0.34(1)}O4

0.8347(9)

33

Tuffite

Mh

AR

[Fe 0.88Si 0.01Mg0.11]{Fe 0.96Mg0.30Ti 0.32Al 0.07Cr 0.03Mn0.02&0.30}O4

0.8380(2)

18�31

Mh

Soil

Fe 2.11Ti 0.31Mg0.21&0.37O4

0.8360

29

Mt

7Fe 1.54

3+Fe 0.47

2+Mg0.39

2+Ti 0.36

4+Al 0.05

3+Si 0.01

4+Mn0.01

4+&0.17O4

0.8412(5)

N.A.

Mh

7Fe 1.74

3+Mg0.42

2+Ti 0.42

4+Al 0.08

3+Si 0.01

4+Mn0.01

4+&0.33O4

0.8382(5)

N.A.

1Mt=magnetite;Mh=maghemite.

2FR=freshrock;AR=alteredrock.

3[]and{}denotetetrahedralandoctahedralsites,respectively.&=cationvacancies.

4N.A.=Notavailable

5Calculatedfromreportedmagnetizationvalueofthemagneticextractofthefreshrock,assumingthattheFeallocationtomagnetite,asgivenbytherelativearea

oftheMossbauerspectrum,reflectsthemineralproportioninthesample.

6Calculatedfromreportedmagnetizationvalueofthemagneticextractofthealteredrock,byassumingthattheFeallocationtomaghemite,asgivenbythe

relativeareaoftheMossbauerspectrum,reflectsthemineralproportioninthesample.

7Magneticcrystalsfromthesamplerock


responsible for the large magnetization of these

tropical soils, although residual lithogenic magnetite

may be found in some pedons, such as those

developing on tuffite (Fabris et al., 1999). The

mafic-rock magnetite is relatively unstable under

tropical conditions.

The fundamental mechanisms associated with the

relatively fast oxidation of magnetite to maghemite

during pedogenesis are not well understood and

further research is needed to establish a reliable

model of these reactions in the soil environment.

Many laboratory techniques provide physical and

chemical information for the characterization of

magnetic soil minerals. Among them, Mossbauer

spectroscopy plays a central role in the study of the

Fe-bearing phases, particularly Fe oxides. Magnetic

measurements are also of fundamental importance

in the advancement of magnetic soil studies. A

complete picture of the characteristic physical and

chemical features of the magnetic Fe oxides and

related phases in tropical soils is still to be

achieved.

CONCLUS IONS AND F INALREMARKS

Brazilian Latosols, which cover >60% of the

country’s surface, are typically highly weathered

soils, dominated by low-activity 1:1 clay minerals

and Fe and Al oxyhydroxides, with varying

proportions of these minerals, depending on parent

material and soil-drainage conditions. They have

low fertility and a monotonous clay-mineral

assemblage of kaolinite, hematite, goethite, magne-

tite, maghemite and Ti minerals (mainly ilmenite

and anatase) as prominent mineral phases. Among

the pedogenic oxides, the most frequent are goethite

(a-FeOOH), indicated by yellowish colours

(2.5Y�10YR) in the absence of hematite, and

hematite (a-Fe2O3) which gives reddish colours

(2.5YR�5R), even when present in very minor

amounts. Less frequent is maghemite (g-Fe2O3),which gives a reddish-brown colour (5YR�2.5YR)and has magnetic properties.

Both goethite and hematite show Al substitution,

with a greater relative proportion in the former.

Hence, goethite is less prone to dissolution than

hematite when in similar drainage conditions. Most

reddish Latosols also contain maghemite, due to

partial or complete oxidation of magnetite, which

generally occurs naturally or is fire-induced.

Magnetite and/or maghemite in Latosols are

associated with trace elements important in plant

nutrition, such as Cu, Zn and Co and have been

studied intensively by Mossbauer spectroscopy.

Latosols typically retain large amounts of Fe

oxides, some of which are magnetic, with

spontaneous magnetization >1 J T�1 kg�1. In this

regard, reddish Latosols developed from mafic

rocks are the most representative magnetic soils

and cover as much as 3.9% of Brazil. Magnetic Fe

oxides vary widely in chemical composition. For

amphibolite (or dolerite), tholeiitic basalt and

diabase, Ti or Al are the main isomorphic

substituents in the Fe oxides. Magnetite and

maghemite from volcanic ash (tuffite) are richer

in Ti and Mg. Their saturation magnetization values

range from 18 to 54 J T�1 kg�1. Lattice parameters

of the cubic structure are strongly affected by the

oxidation state of Fe and the presence of

isomorphously substituted ions. The Ti4+ tends to

increase unit-cell dimension a in magnetite but has

little or no effect on maghemite; Mg2+ tends to

decrease a in magnetite and increase it in

maghemite.

The gibbsite contents in Latosols are extremely

variable, from a complete absence in brown

Latosols up to 54% in red Latosols from volcanic

tuffs. Gibbsite is an important mineral in terms of

sorption of P in deeply-weathered Latosols, in

association with goethite and hematite. Gibbsitic

Latosols are acric and positively charged in sub-

surface horizons, as revealed by the pH KCl > pH

H2O. These acric Latosols are the result of long-

term weathering and complete leaching, during

which pH tends to increase to values close to the

pzc of the Fe and Al oxides (6�7). This greatlyincreases P adsorption, which is attributed mostly to

gibbsite.

The majority of soil kaolinites in Brazilian

Latosols are of low crystallinity, with Hughes &

Brown (1979) crystallinity index values of 6�15. Inthis review we have discussed the role of these

clay-fraction minerals in soil genesis and fertility,

highlighting the marked role of inheritance from

deep-weathered parent material.

REFERENCES

Antonello L.L. (1988) Mineralogy deferrified clay

fractions in B horizon of pedons of VIIIth

International Soil Classification Workshop. Pp.

109�138 in: International Soil Classification

Workshop: Classification, Characterization and


Utilization of Oxisols, 8. EMBRAPA, SMSS, AID,

UPR, Rio de Janeiro, Brazil.

Bahia Filho A.F.C., Braga J.M., Resende M. & Ribeiro

A.C. (1983) Relacao entre adsorcao de fosforo e

componentes mineralogicos da fracao argila de

latossolos do Planalto Central. Revista Brasileira

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Camargo M.N., Jacomine P.K.T., Carvalho A.P.J. &

Olmos I.L. (1988) The Brazilian classification of

Latosols. Pp. 190�202 in: International Soil

Class i f ica t ion Workshop: Class i f ica t ion,

Characterization and Utilization of Oxisols, 8.

EMBRAPA, SMSS, AID, UPR, Rio de Janeiro,

Brazil.

Cline M. (1975) Origin of the term Latosol. Soil Science

Society of America Proceedings, 39, 162�163.Curi N. (1983) Lithosequence and Toposequence of

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Documents

Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review