The humid tropics: weathering and laterites
513
1983), and on the chemistry and mineralogy of the weathering profiles (Grant, 1969;
Sueoka, 1988; Geological Society, 1990). As an example, Grant (1969) correlated
abrasion pH to the relation between the most mobile elements (Na, K, Ca, and Mg) and
the residual clay minerals. The lower the index, the more intense the weathering. Sueoka
(1988) proposed the chemical weathering index, which consists of dividing the sum of the
molecular weight percentages of A1, Fe, and Ti oxides plus H20, by the rest of elements
(alkali, alkaline earth cations, and silica). Thomas (1994) included other indexes related to
the weathering degree.
As a consequence of the destruction of crystalline structures by weathering processes,
several cations are released. Those with high solubility are leached by water, while others
remaining in the residue are restructured, giving rise to neoformed minerals (Bland and
Rolls, 1998). The most common ones are clay minerals, quartz, and oxihydroxides of iron
and aluminium, which form lateritic hardpans.
Clay minerals are hydrated silicates of aluminium, iron, and magnesium ordered into
sheets, the reason for which they are named "phyllosilicates." The tetrahedral sheets are
formed by Si surrounded by four O ions, while the octahedral layers consist of six O or OH
ions located around A1, Mg, or Fe cations that are placed in a central position. These two
basic sheets can be grouped in three different ways. First, a tetrahedral sheet joints an
octahedral one to form structures of the type 1:1. Clay minerals of this type are those of
the kaolinite-halloysite group. Secondly, an octahedral sheet can be sandwiched between
two tetrahedral sheets. This type of arrangement gives rise to the 2:1 phyllosilicates, which
are represented by the smectite group (montmorillonite, beidellite, and nontronite) and the
illites. The third ordering case results from the joining of an octahedral sheet between
adjacent 2:1 sheets, and this type is known as 2:1:1 clay minerals, among which chlorite is
the most common.
The genesis of the clay minerals is complex due to the wide variety of weathering
environments. They can also originate as a result of weathering of non-phyllosilicate
minerals. Cations, silicon and alumina are released and they reorganise to from clay
minerals. When the altered mineral is a phyllosilicate, weathering is mainly produced in a
solid state (Birkeland, 1984).
Studies by Sherman (1952) on basalt weathering in Hawaii indicated an increase in
the clay percentage with precipitation, as well as changes in the mineralogy of the rock.
Figure 20.9 shows how montmorillonite is a typical mineral of low precipitation regimes
that transforms into kaolinite under conditions of higher leaching. When precipitation is
high the soil desilicifies and iron and aluminium oxihydroxides are formed. The figure also
indicates that if a dry period occurs, the alteration residue enriches in iron and titanium
oxides. When precipitation takes place throughout the whole year the main constituents
are aluminium oxihydroxides.
Taking into account the dominant weathering products, several weathering zones can
be distinguished in the humid tropics (Strakhov, 1967) (Figure 1.8). In this sense, Pedro
(1966, 1968) differentiated:
1. Bisiallitization zone with the formation of clay minerals of the 2:1 type
(montmorillonite, 4SiO2.A1203.H20), in areas with a precipitation lower than 500 mm.
2. Monosiallitization zone with kaolinite (2SiO2.A1203.2H20) as the predominant
mineral in the regolith, in zones with a precipitation of 500 to 1200/1500 mm.
513
1983), and on the chemistry and mineralogy of the weathering profiles (Grant, 1969;
Sueoka, 1988; Geological Society, 1990). As an example, Grant (1969) correlated
abrasion pH to the relation between the most mobile elements (Na, K, Ca, and Mg) and
the residual clay minerals. The lower the index, the more intense the weathering. Sueoka
(1988) proposed the chemical weathering index, which consists of dividing the sum of the
molecular weight percentages of A1, Fe, and Ti oxides plus H20, by the rest of elements
(alkali, alkaline earth cations, and silica). Thomas (1994) included other indexes related to
the weathering degree.
As a consequence of the destruction of crystalline structures by weathering processes,
several cations are released. Those with high solubility are leached by water, while others
remaining in the residue are restructured, giving rise to neoformed minerals (Bland and
Rolls, 1998). The most common ones are clay minerals, quartz, and oxihydroxides of iron
and aluminium, which form lateritic hardpans.
Clay minerals are hydrated silicates of aluminium, iron, and magnesium ordered into
sheets, the reason for which they are named "phyllosilicates." The tetrahedral sheets are
formed by Si surrounded by four O ions, while the octahedral layers consist of six O or OH
ions located around A1, Mg, or Fe cations that are placed in a central position. These two
basic sheets can be grouped in three different ways. First, a tetrahedral sheet joints an
octahedral one to form structures of the type 1:1. Clay minerals of this type are those of
the kaolinite-halloysite group. Secondly, an octahedral sheet can be sandwiched between
two tetrahedral sheets. This type of arrangement gives rise to the 2:1 phyllosilicates, which
are represented by the smectite group (montmorillonite, beidellite, and nontronite) and the
illites. The third ordering case results from the joining of an octahedral sheet between
adjacent 2:1 sheets, and this type is known as 2:1:1 clay minerals, among which chlorite is
the most common.
The genesis of the clay minerals is complex due to the wide variety of weathering
environments. They can also originate as a result of weathering of non-phyllosilicate
minerals. Cations, silicon and alumina are released and they reorganise to from clay
minerals. When the altered mineral is a phyllosilicate, weathering is mainly produced in a
solid state (Birkeland, 1984).
Studies by Sherman (1952) on basalt weathering in Hawaii indicated an increase in
the clay percentage with precipitation, as well as changes in the mineralogy of the rock.
Figure 20.9 shows how montmorillonite is a typical mineral of low precipitation regimes
that transforms into kaolinite under conditions of higher leaching. When precipitation is
high the soil desilicifies and iron and aluminium oxihydroxides are formed. The figure also
indicates that if a dry period occurs, the alteration residue enriches in iron and titanium
oxides. When precipitation takes place throughout the whole year the main constituents
are aluminium oxihydroxides.
Taking into account the dominant weathering products, several weathering zones can
be distinguished in the humid tropics (Strakhov, 1967) (Figure 1.8). In this sense, Pedro
(1966, 1968) differentiated:
1. Bisiallitization zone with the formation of clay minerals of the 2:1 type
(montmorillonite, 4SiO2.A1203.H20), in areas with a precipitation lower than 500 mm.
2. Monosiallitization zone with kaolinite (2SiO2.A1203.2H20) as the predominant
mineral in the regolith, in zones with a precipitation of 500 to 1200/1500 mm.