Clay Minerals, (2008) 43, 137–154
Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review C. E. G. R. SCHAEFER1,*, J. D. FABRIS2
AND
J. C. KER3
1
2
Departamento de Solos, Universidade Federal de Vic¸osa, 36571-000 Vic¸osa, Minas Gerais, Brazil, Departamento de Quı´mica, UFMG, Campus - Pampulha, 31270-901 Belo Horizonte, Minas Gerais, Brazil, and 3 Departamento de Solos, Universidade Federal de Vic¸osa, 36571-000 Vic¸osa, Minas Gerais, Brazil
(Received 30 April 2007; revised 11 December 2007)
AB ST R ACT : 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 by yellowish colours (2.5Y 10YR; in the absence of hematite), and hematite (a-Fe2O3), which imbues reddish colors (2.5YR 5R), even when present in very minor amounts. Maghemite (g-Fe2O3) is less frequent; it imparts a reddish-brown colour (5YR 2.5YR) and magnetic properties. Both goethite and hematite 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 baseleaching 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 representative magnetic soils, and cover as much as 3.9% of Brazil. An overview of magnetic soils on four
* E-mail:
[email protected] DOI: 10.1180/claymin.2008.043.1.11
# 2008 The Mineralogical Society
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C. E. G. R. Schaefer et al. 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 conference held in Washington in 1949 (Kellog, 1949; Se´galen, 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; Se´galen, 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, morphological 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).
Clay fraction minerals of Brazilian Latosols: a review
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 pedodomains 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 Mo¨ssbauer 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
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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 lithodomains. 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 ASPECTS
GENERAL
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 pedogenic 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 a reddish-brown colour (5YR 2.5YR) and magnetic properties (Resende, 1976; Curi, 1983; Santana, 1984; Ka¨mpf & Schwertmann, 1983; Dick, 1986; Ka¨mpf 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 hydroxyinterlayered 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-interlayered vermiculite, illite, anatase, rutile, maghemite 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
RL Itabirite RL Basalt BL Volcanics RL Limestone RL Tuffite RYL Fe-rich tuffite RYL Fe-rich gneiss RYL Gneiss RYL Clayey sediments YL Tertiary sediments
RN1 RN2 RN4 RN6 RN7 RN9 RN10 RN11 RN13 RN14
15.2 10.7 34.1 8.9 15.2 8.1 30.8 28.7 10.9 40.5
20.7 27.5 27.5 42.0 39.2 42.9 31.8 35.3 42.0 34.1
42.9 25.9 17.9 15.6 16.1 14.1 15.1 14.8 13.5 5.7
0.6 4.5 1.9 1.2 0.8 2.1 1.8 0.7 3.5 1.8
0.27 1.53 0.18 0.13 0.11 0.16 0.09 0.11 0.17 0.04
0.24 0.00 0.10 0.09 0.85 0.00 0.00 0.00 0.00 0.00
0.08 0.05 0.08 0.02 0.09 0.01 0.01 0.01 0.01 0.01
79.9 70.0 81.7 67.8 72.3 67.3 79.6 79.6 70.0 82.2
SiO2 Al2O3 Fe2O3 TiO2 P2 O 5 K2 O MgO Total ————————————— wt.% —————————————
RL: red Latosol; RYL: red-yellow Latosol; BL: brown Latosol; YL: yellow Latosol. * Fe amount expressed as % Fe2O3 { : Ki = SiO2/Al2O3 { : Kr = SiO2/(Al2O3 + Fe2O3)
Latosol
Sample no. 1.24 0.66 2.10 0.36 0.66 0.32 1.65 1.38 0.44 2.02
Ki
0.53 0.41 1.48 0.29 0.52 0.27 1.26 1.09 0.36 1.82
Kr
51.0 29.0 18.8 17.2 17.1 14.8 16.2 15.5 12.8 4.0
Fed*
1.21 1.78 0.61 0.53 0.38 0.34 0.45 0.18 0.27 0.15
Feo*
42 16 31 32 45 43 36 86 47 26
Fed/Feo
TABLE 1. Chemical data for selected Latosols from Brazil: concentration of elements extracted from the clay fraction by sulphuric acid: molar ratios Ki{; Kr{ and Fe extracted by CDB (Fed) and ammonium oxalate (Feo). Data based on Rodrigues-Netto (1996).
140 C. E. G. R. Schaefer et al.
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Clay fraction minerals of Brazilian Latosols: a review
Latosols (Ka¨mpf 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 yellow or 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; Ka¨mpf 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 redpigmenting 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; Ka¨mpf 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 Ka¨mpf 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 (Ka¨mpf 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 complexation, 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, concentrated 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 Alsubstitution 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 ( 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 increases P adsorption, which reaches up to 3.5 mg P/g of soil (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 nonacric 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 environmental conditions. Overall, wetter and warmer
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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; Varaja˜o 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).
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C. E. G. R. Schaefer et al.
Fernandes (2000) Fernandes (2000) Fernandes (2000) Fernandes (2000) Fernandes (2000) Fernandes (2000) Ker (1995) Ker (1995) Ker (1995) Ker (1995) 11.0 (A horizon); 12.6 (B horizon) 15.0 (A horizon); 13.5 (B horizon) 13.2 (A horizon); 13.8 (B horizon) 7.7 (A and B horizons) 5.8 (A horizon) 9.7 (A horizon); 9.4 (B horizon) 14 (B horizon) 15 (B horizon) 9 (B horizon) 8 (B horizon) Tertiary clay sediments - Barreiras Saprolite from gneiss Saprolite from gneiss Basalt with sandstone layers Limestone Basalt Tertiary clay sediments Gneiss Basalt Basalt Yellow Latosol (R25-26) Red-yellow Latosol (R 19-20) Red-yellow Latosol (R21-22) Red Latosol (R1-2) Red Latosol (R13-14) Red Latosol (R11-12) Yellow Latosol (K20) Red-yellow Latosol (K11) Red Latosol (K2) Red Latosol (K16)
Reference HB CI Parent material Soil and sample no.
TABLE 6. HB kaolinite crystallinity index values (CI) for various Latosol B-horizons from Brazil and a geological deposit. Data compiled from Fernandes (2000), Ker (1995) and Hughes & Brown (1980).
(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 hydroxyinterlayered vermiculite (HIV) have been detected in the clay fraction. However, in most brown Latosols from southern Brazil, large amounts of HIV are observed (Po¨tter & 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-interlayering in the vermiculite crystal can block exchange sites, greatly decreasing the CEC of these soils and promoting a so-called ‘‘antigibbsite’’ 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 Varaja˜o et al. (2001). The abundance of nonexchangeable 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).
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Clay fraction minerals of Brazilian Latosols: a review 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
Kaolin reference1 K2 K18 K26 K16 K28 K11 K20 K25 K14 K4
Soil classification
Kaolin Georgia Red Latosol from basalt Red Latosol Red Latosol from basalt Red Latosol Red-yellow Latosol (high Fe content) Red-yellow Latosol Yellow Latosol (kaolinitic) Brown Latosol Brown Latosol Yellow Latosol (gibbsitic)
——— Kaolinite ——— MCD 001 SSA (nm) (m2/g) 107 18 34 18 24 49 25 34 17 28 49
15 50 30 50 39 23 38 30 52 35 23
1
Georgia 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.
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C. E. G. R. Schaefer et al. TABLE 8. Types of 2:1 clay minerals and some mineralogical properties of the studied Latosols.
Soil
Class
Ki value
RN3 RN4 RN5 RN6 RN7
Red Latosol Brown Latosol Red Latosol Red Latosol Red Latosol
1.92 2.10 0.58 0.36 0.66
Mineral
K2O1
MgO1
2:12 K2O3 amount in illite concentration ———————— wt.% ————————
HIV HIV Illite + HIV HIV Illite + HIV
0.00 0.10 1.04 0.09 0.85
0.06 0.08 0.03 0.02 0.09
tr tr 11.6 tr 9.8
8.9 8.7
1
Determined in the sulphuric extract of the clay fraction. Based on 100% clay. 3 Calculated by: % K2O6100/% 2:1 mineral. tr = traces 2
The 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 nonmafic 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 susceptibility (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
RN1 RN2 RN4 RN6 RN7 RN9 RN10 RN11 RN13 RN14
Class
Hem.
Goet.
Gibb.
Kaol.
Anatase
SiO2
RL Itabirite RL Basalt BL Volcanics RL Limestone RL Tuffite RYL Fe-rich tuffite RYL Fe-rich gneiss RYL Gneiss RYL Clayey sediments YL Clayey sediments
40.5 28.9 10.4 11.5 8.8 0.0 1.3 0.6 0.0 0.0
14.2 10.7 13.2 12.9 14.4 24.2 24.0 21.7 20.2 6.1
11.2 27.7 0.0 53.0 41.0 54.0 1.5 12.0 49.5 0.0
33.6 27.3 70.5 21.2 23.9 19.5 71.3 64.9 26.5 90.1
0.6 5.3 2.0 1.3 0.8 2.3 1.9 0.8 3.9 2.0
0.0 0.0 4.0 0.0 0.0 0.0 0.0 0.0 0.0 1.8
Abbreviations as in Table 1.
2:1 clay minerals ————————————— wt.% ————————————— 0.0 0.0 0.0 0.0 11.0 0.0 0.0 0.0 0.0 0.0
1200 850 950 480
RL RYL RYL YL RL RYL RL RL RL RL BL BL BL RYL
Lithology
Itabirites and hematitic phyllites Gneiss and acid migmatites Mesocratic gneiss Tertiary sediments Clay sediments Basalts Basalts Sandstone Basalts Basalts Basalts Basalts Acid metamorphic rocks Migmatites
Abbreviations as in Table 1.
640 760 920 1100 910 860
645 760
Altitude
Latosol
10 R 3/6 7.5 YR 5/8 7.5 YR 4/5 10 R 5/6 2.5 YR 3/7 4 YR 4/4 1.5 YR 3/4 1.5 YR 3.5/6 1 YR 3/4 1 YR 3/5 3.5 YR 3.5/5 5 YR 3.5/5 1.5 YR 4/8 10 YR 5/8
Wet Munsell colour
Fe2O3
58 57 69 49 59 53 75 24 82 88 85 79 63 50
55.8 8.1 14.7 5.2 11.1 30.0 34.2 3.6 29.6 22.9 23.1 24.0 7.7 10.4
— (wt.%) —
Clay
7353 10 48 11 565 5886 7296 67 8391 2258 803 653 107 218
12970 27 114 18 573 14465 9932 71 19146 8292 4525 3971 62 948
4078 125 220 253 448 2546 12797 643 16652 3680 2222 2430 204 96
2535 13 23 23 642 1046 3885 118 5317 1883 458 77 77 9
3884 12 34 19 504 2467 7239 111 8398 2105 730 696 80 99
Magnetic susceptibility FineFine-sand Silt Clay >0.149 earth (0.005 0.02 mm) (0.002 0.05 mm) (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, magnetite, 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 Mo¨ssbauer 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 subsurface horizons, as revealed by the pH KCl > pH H2O. These acric Latosols are the result of longterm 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 greatly increases 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. 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 deep-weathered parent material. REFERENCES
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