524
Climatic Geomorphology
into blocks with an iron instrument, although it hardened when dried. This is the reason
why it is used for making bricks, from which the Latin term laterite is derived. In the
transition to the 20th century laterite begun to be defined as a function of its chemical
composition, by using the content of iron, aluminium and the relationship between
SiO2, A1203, and Fe203. Although numerous laterite definitions have been proposed
(for example, Alexandre and Cady, 1962; Sivarajasingham et al., 1962; Norton, 1973;
Schellmann, 1981), little difference exists among them. A laterite is considered to be
formed in environments with intense chemical weathering, lacking alkali and alkaline
earth elements, enriched in iron and aluminium, and with important quantities of quartz
and kaolinite. It hardens by wetting and drying.
A controversy exists between the pedological and the geomorphological concepts of
laterite. Pedologists understand a lateritic soil as a residual one, iron-enriched in its upper
part under conditions of wetting and drying, whereas for geomorphologists laterites
generate under special geomorphic conditions and the precipitates are produced in the
water table fluctuation zone. For this reason, pedologists consider this type of laterite as a
podsolic tropical soil. All this has led to a differentiation between pedogenetic laterites
and the ones generated by a fluctuating water table (McFarlane, 1976). This controversy
transforms into a difficult agreement when trying to explain soil thickness of 60 m or more
(Hays, 1967), in which case it is hard to argue for a pedogenetic origin. Following Goudie
(1973), this last type requires a thin horizon, although in most cases this does not occur.
3.8.1. Constitution and structure
Chemical analyses of A1203, Fe203, SIO2, and clay minerals from laterites can be
represented in triangular diagrams, which permit the making of compositional
differentiations. Some authors use Fe203, A1203, and kaolinite percentages (Bardossy
and Aleva, 1990), while Dury (1969) and Schellmann (1981) substituted SiO2 in the
diagram for kaolinite (Figure 20.20). These studies show a wide compositional variety,
02
SiO 2
Figure 20.20. Ternary diagram with different types of duricrusts as a function of their chemical
composition (after Dury (1969), in Thomas (1994), Figure 4.2).
Climatic Geomorphology
into blocks with an iron instrument, although it hardened when dried. This is the reason
why it is used for making bricks, from which the Latin term laterite is derived. In the
transition to the 20th century laterite begun to be defined as a function of its chemical
composition, by using the content of iron, aluminium and the relationship between
SiO2, A1203, and Fe203. Although numerous laterite definitions have been proposed
(for example, Alexandre and Cady, 1962; Sivarajasingham et al., 1962; Norton, 1973;
Schellmann, 1981), little difference exists among them. A laterite is considered to be
formed in environments with intense chemical weathering, lacking alkali and alkaline
earth elements, enriched in iron and aluminium, and with important quantities of quartz
and kaolinite. It hardens by wetting and drying.
A controversy exists between the pedological and the geomorphological concepts of
laterite. Pedologists understand a lateritic soil as a residual one, iron-enriched in its upper
part under conditions of wetting and drying, whereas for geomorphologists laterites
generate under special geomorphic conditions and the precipitates are produced in the
water table fluctuation zone. For this reason, pedologists consider this type of laterite as a
podsolic tropical soil. All this has led to a differentiation between pedogenetic laterites
and the ones generated by a fluctuating water table (McFarlane, 1976). This controversy
transforms into a difficult agreement when trying to explain soil thickness of 60 m or more
(Hays, 1967), in which case it is hard to argue for a pedogenetic origin. Following Goudie
(1973), this last type requires a thin horizon, although in most cases this does not occur.
3.8.1. Constitution and structure
Chemical analyses of A1203, Fe203, SIO2, and clay minerals from laterites can be
represented in triangular diagrams, which permit the making of compositional
differentiations. Some authors use Fe203, A1203, and kaolinite percentages (Bardossy
and Aleva, 1990), while Dury (1969) and Schellmann (1981) substituted SiO2 in the
diagram for kaolinite (Figure 20.20). These studies show a wide compositional variety,
02
SiO 2
Figure 20.20. Ternary diagram with different types of duricrusts as a function of their chemical
composition (after Dury (1969), in Thomas (1994), Figure 4.2).