The humid tropics: weathering and laterites
525
even within a single level. Thomas (1974) proposed a terminological simplification by
using three terms: bauxite for a duricrust rich in aluminium, silcrete where the silica
content is high, and the term laterite was restricted to materials of intermediate
composition.
Because laterite is generated from a residue resulting from the chemical weathering of a
rock, we can expect to find a direct relationship to the parent rock composition. At a
regional scale, some variations in the iron content have been recorded, while at a minor
scale no clear relations exist between the laterite chemical composition and the parent
rock. In this sense, the generation of bauxite is not always favoured by rocks rich in A1203
(McFarlane, 1983).
The main neoforming minerals in laterites are iron and aluminium oxides and
hydroxides, such as hematite (Fe203), goethite (FeO.OH), gibbsite [AI(OH)3], boehmite
(A10.OH), and diaspore (A10-OH), and clay minerals such as kaolinite and halloysite
[AlaSi401o(OH)18], both bearing the same composition. All the neoformed minerals tend
towards a higher stability according to the environmental circumstances and in humid
tropical zones they modify mainly by leaching. The kaolinite can proceed from feldspar
alteration or from a neoformed montmorillonite. The goethite directly derives from biotite
and hornblende, and the hematite from goethite by dehydration. Feldspar weathering can
directly originate gibbsite, although normally this mineral proceeds from the desilicification of kaolinite.
Laterites present different structures, frequently bearing nodules without inner
structure, and pisoliths (diameter > 2 mm) and ooliths (< 2 mm) with concentric layers.
These can appear as scattered or packed concretions, forming a cemented oolithic or
pisolithic laterite. The concentric structure forms by enrichment in the water table
fluctuating zone. The pedogenetic pisoliths are composed of loose material and present
irregular form and size, with a manganiferous external layer (McFarlane, 1976). Other
structural types in laterites are vermicular, vesicular, cellular, tubular, and other types,
whose differentiation is based on the morphology of hollows. This terminology is not
clearly defined and McFarlane (1976, 1983) only uses the term vermiform for characterizing laterites with inner tubes and cavities (Figure 20.21). The diameter of the conduits
is about 2 to 3 cm and they are commonly filled with kaolin. The origin of such structures
is complex. It is ascribed to termite activity (termite armouring of Erhardt, 1951) and
to plant root penetration. McFarlane (1976) considered that they result from precipitation
and they only appear in laterites formed by water table fluctuation. This author also
indicates that pisolithic laterites are immature laterites, while the vermiform ones are
mature laterites and derive from the former. Therefore, pisolithic structures can be found
alternating with vermiform ones.
3.8.2. Factors affecting generation
Laterites and bauxites appear in a wide variety of rock types (Mabbutt, 1961b; Goudie,
1973, among others), although rock mineralogy substantially influences the composition
of lateritic rocks (Thomas, 1994). Nevertheless, there exist zones where a given lithology
favours laterite development, whereas in other sites the same lithology inhibits its
generation. This seems to indicate the existence of other more determinant factors in its
development (McFarlane, 1976).
525
even within a single level. Thomas (1974) proposed a terminological simplification by
using three terms: bauxite for a duricrust rich in aluminium, silcrete where the silica
content is high, and the term laterite was restricted to materials of intermediate
composition.
Because laterite is generated from a residue resulting from the chemical weathering of a
rock, we can expect to find a direct relationship to the parent rock composition. At a
regional scale, some variations in the iron content have been recorded, while at a minor
scale no clear relations exist between the laterite chemical composition and the parent
rock. In this sense, the generation of bauxite is not always favoured by rocks rich in A1203
(McFarlane, 1983).
The main neoforming minerals in laterites are iron and aluminium oxides and
hydroxides, such as hematite (Fe203), goethite (FeO.OH), gibbsite [AI(OH)3], boehmite
(A10.OH), and diaspore (A10-OH), and clay minerals such as kaolinite and halloysite
[AlaSi401o(OH)18], both bearing the same composition. All the neoformed minerals tend
towards a higher stability according to the environmental circumstances and in humid
tropical zones they modify mainly by leaching. The kaolinite can proceed from feldspar
alteration or from a neoformed montmorillonite. The goethite directly derives from biotite
and hornblende, and the hematite from goethite by dehydration. Feldspar weathering can
directly originate gibbsite, although normally this mineral proceeds from the desilicification of kaolinite.
Laterites present different structures, frequently bearing nodules without inner
structure, and pisoliths (diameter > 2 mm) and ooliths (< 2 mm) with concentric layers.
These can appear as scattered or packed concretions, forming a cemented oolithic or
pisolithic laterite. The concentric structure forms by enrichment in the water table
fluctuating zone. The pedogenetic pisoliths are composed of loose material and present
irregular form and size, with a manganiferous external layer (McFarlane, 1976). Other
structural types in laterites are vermicular, vesicular, cellular, tubular, and other types,
whose differentiation is based on the morphology of hollows. This terminology is not
clearly defined and McFarlane (1976, 1983) only uses the term vermiform for characterizing laterites with inner tubes and cavities (Figure 20.21). The diameter of the conduits
is about 2 to 3 cm and they are commonly filled with kaolin. The origin of such structures
is complex. It is ascribed to termite activity (termite armouring of Erhardt, 1951) and
to plant root penetration. McFarlane (1976) considered that they result from precipitation
and they only appear in laterites formed by water table fluctuation. This author also
indicates that pisolithic laterites are immature laterites, while the vermiform ones are
mature laterites and derive from the former. Therefore, pisolithic structures can be found
alternating with vermiform ones.
3.8.2. Factors affecting generation
Laterites and bauxites appear in a wide variety of rock types (Mabbutt, 1961b; Goudie,
1973, among others), although rock mineralogy substantially influences the composition
of lateritic rocks (Thomas, 1994). Nevertheless, there exist zones where a given lithology
favours laterite development, whereas in other sites the same lithology inhibits its
generation. This seems to indicate the existence of other more determinant factors in its
development (McFarlane, 1976).