2.3 WEATHERING
33
2.3.3.1 Laterite
The term laterite, from the Latin later, a brick, was originally proposed by Buchanan
(1807) to describe the red soils of parts of India, notably those developed on the plateau basalts of the Deccan (Patel, 1987). Subsequently, the precise definition of the term
has become somewhat diffuse (Sivarajasingham et al., 1962). As generally understood,
laterite is the product of intensely weathered material. It is rich in hydrated iron and
aluminium oxides, and low in humus, silica, lime, silicate clays, and most other minerals (Meyer, 1997). Analyses of Ugandan laterites given by McFarlane (1971) show between 40 and 50% iron oxide, and between 20 and 25 % of both alumina and silica. In
physical appearance laterite is a red-brown earthy material which, though often friable,
can harden rapidly on exposure to the atmosphere, a property useful in brick-making.
Laterites often show pisolitic and vermiform structures (Plate 5A). The pisolites are
rounded, concentrically zoned concretions up to a centimeter or more in diameter.
Vermicular laterite consists of numerous subvertical pipes of hard laterite in a friable
matrix.
Lateritic soils are widely distributed throughout the humid tropics, occurring
throughout India and Africa, as already mentioned, and also in South America. It has
been argued already that residual deposits such as laterite require prolonged intensive
chemical weathering for them to reach maturity. This will occur fastest in areas of low
relief, where rates of erosion are low. High initial iron content in the parent rock is also
a further significant factor. Thus laterites are often best developed on plateau basalts
and basic intrusive rocks. Thin red laterite bands are commonly found separating fossil basalt lavas.
2.3.3.2 China Clay~Kaolin
China clay, or kaolin, is the name given to the rock aggregate of the mineral kaolinite. This is the hydrated aluminosilicate clay mineral (A1203"2SiO2"2H20). China clay
occurs in three geological situations, only one of which is, strictly speaking, a residual
weathering product. First, it is formed by the hydrothermal alteration of feldspars in
granite. The granites of southwest England are an example (Bristow, 1968). Secondly,
china clay is formed by intensive weathering of diverse rock types, but particularly
those which are enriched in aluminosilicates, such as shales, and acid igneous and metamorphic rocks. Thirdly, china clay can be transported short distances from hydrothermal and residual deposits to be resedimented in lacustrine environments. Here the kaolin beds occur associated with sands and coals or lignites. The "ball clays" of Bovey
Tracey were deposited in an Oligocene lake, which received kaolin from rivers draining
the Dartmoor granites (Edwards, 1976). The residual kaolins are very variable in composition. With increasing iron content they grade into the laterites, and with desilicification they grade into the bauxites. Where kaolin forms as a residue on granitoid rocks,
large quantities of quartz may be present that must be removed before the kaolin can
be put to use. China clay is economically important in the ceramic and paper-making
industry.
33
2.3.3.1 Laterite
The term laterite, from the Latin later, a brick, was originally proposed by Buchanan
(1807) to describe the red soils of parts of India, notably those developed on the plateau basalts of the Deccan (Patel, 1987). Subsequently, the precise definition of the term
has become somewhat diffuse (Sivarajasingham et al., 1962). As generally understood,
laterite is the product of intensely weathered material. It is rich in hydrated iron and
aluminium oxides, and low in humus, silica, lime, silicate clays, and most other minerals (Meyer, 1997). Analyses of Ugandan laterites given by McFarlane (1971) show between 40 and 50% iron oxide, and between 20 and 25 % of both alumina and silica. In
physical appearance laterite is a red-brown earthy material which, though often friable,
can harden rapidly on exposure to the atmosphere, a property useful in brick-making.
Laterites often show pisolitic and vermiform structures (Plate 5A). The pisolites are
rounded, concentrically zoned concretions up to a centimeter or more in diameter.
Vermicular laterite consists of numerous subvertical pipes of hard laterite in a friable
matrix.
Lateritic soils are widely distributed throughout the humid tropics, occurring
throughout India and Africa, as already mentioned, and also in South America. It has
been argued already that residual deposits such as laterite require prolonged intensive
chemical weathering for them to reach maturity. This will occur fastest in areas of low
relief, where rates of erosion are low. High initial iron content in the parent rock is also
a further significant factor. Thus laterites are often best developed on plateau basalts
and basic intrusive rocks. Thin red laterite bands are commonly found separating fossil basalt lavas.
2.3.3.2 China Clay~Kaolin
China clay, or kaolin, is the name given to the rock aggregate of the mineral kaolinite. This is the hydrated aluminosilicate clay mineral (A1203"2SiO2"2H20). China clay
occurs in three geological situations, only one of which is, strictly speaking, a residual
weathering product. First, it is formed by the hydrothermal alteration of feldspars in
granite. The granites of southwest England are an example (Bristow, 1968). Secondly,
china clay is formed by intensive weathering of diverse rock types, but particularly
those which are enriched in aluminosilicates, such as shales, and acid igneous and metamorphic rocks. Thirdly, china clay can be transported short distances from hydrothermal and residual deposits to be resedimented in lacustrine environments. Here the kaolin beds occur associated with sands and coals or lignites. The "ball clays" of Bovey
Tracey were deposited in an Oligocene lake, which received kaolin from rivers draining
the Dartmoor granites (Edwards, 1976). The residual kaolins are very variable in composition. With increasing iron content they grade into the laterites, and with desilicification they grade into the bauxites. Where kaolin forms as a residue on granitoid rocks,
large quantities of quartz may be present that must be removed before the kaolin can
be put to use. China clay is economically important in the ceramic and paper-making
industry.