34
2 WEATHERING AND THE SEDIMENTARY CYCLE
2.3.3.3 Bauxite
Bauxite is a residual weathering product composed of varying amounts of the aluminium hydroxides boehmite, chliachite, diaspore, and gibbsite (Valeton, 1972). It
takes its name from Les Baux near Aries, in France (Plate 5A). The bauxite minerals
are formed by the hydrolysis of clay minerals, principally kaolinite:
H20 + A1203"2SiO2"2H20 = A1203nH20 + 2SIO2.2H20
water +
kaolinite
aluminium
+
silicic acid
hydroxide
Bauxite thus needs the presence of pure leached clay minerals as its precursor. This occurs in two geological settings. Bauxite is often found overlying limestone formations
as, for example, in the type area of southern France, and in Jamaica. In these situations
the calcium carbonate of the limestones has been completely leached away to leave an
insoluble residue of clays. These have been desilicified to form the bauxite suite minerals (Bardossy, 1987).
In the second situation, as seen in Surinam in South America (Valeton, 1973), bauxite occurs as a weathering product of kaolinitic sediments that forms a thin veneer on
Pre-Cambrian metamorphic basement. In both these geological settings it is apparent
that the formation of bauxite is the terminal phase of a sequence of weathering that included kaolinization as an intermediate step. Bauxite is of great economic importance
because it is the only source of aluminium.
2.3.4 Economic Significance and Conclusion
From the preceding description of weathering it should be apparent that it is necessary
to understand the processes of weathering to gain insight into sediment formation. To
conclude this chapter it is appropriate to summarize some of the major points and to
draw attention to the economic significance of weathering. It is important to remember that weathering separates rock into an insoluble residue and soluble chemicals. The
residue, composed largely of quartz grains and clay particles, ultimately contributes to
the formation of the terrigenous sands and shales. The soluble products of weathering
are ultimately reprecipitated as the chemical rocks, such as limestones, dolomites, and
evaporites. Most terrigenous sediments are the product of weathered granitoid rocks
and preexisting sediment. Basic igneous and volcanic rocks and limestones are very susceptible to chemical weathering and contribute little material to the insoluble residue.
Figure 2.10 shows how, in a terrain of diverse rock types, only a few lithologies contribute to the adjacent sediment apron. Aside from these generalities, weathering is important because of the way it can concentrate minerals into deposits that are worth exploiting economically. The bauxites and china clays have already been mentioned.
There are four main ways in which weathering is economically important in a geological context. Weathered profiles are often buried beneath unconformities. These are
commonly zones of enhanced porosity, both in sands (see Section 8.5.3.4) and especially in carbonates (see Section 9.2.7). The product of limestone weathering consists
of nothing but calcium and carbonate ions that are carried away in solution. Limestone
2 WEATHERING AND THE SEDIMENTARY CYCLE
2.3.3.3 Bauxite
Bauxite is a residual weathering product composed of varying amounts of the aluminium hydroxides boehmite, chliachite, diaspore, and gibbsite (Valeton, 1972). It
takes its name from Les Baux near Aries, in France (Plate 5A). The bauxite minerals
are formed by the hydrolysis of clay minerals, principally kaolinite:
H20 + A1203"2SiO2"2H20 = A1203nH20 + 2SIO2.2H20
water +
kaolinite
aluminium
+
silicic acid
hydroxide
Bauxite thus needs the presence of pure leached clay minerals as its precursor. This occurs in two geological settings. Bauxite is often found overlying limestone formations
as, for example, in the type area of southern France, and in Jamaica. In these situations
the calcium carbonate of the limestones has been completely leached away to leave an
insoluble residue of clays. These have been desilicified to form the bauxite suite minerals (Bardossy, 1987).
In the second situation, as seen in Surinam in South America (Valeton, 1973), bauxite occurs as a weathering product of kaolinitic sediments that forms a thin veneer on
Pre-Cambrian metamorphic basement. In both these geological settings it is apparent
that the formation of bauxite is the terminal phase of a sequence of weathering that included kaolinization as an intermediate step. Bauxite is of great economic importance
because it is the only source of aluminium.
2.3.4 Economic Significance and Conclusion
From the preceding description of weathering it should be apparent that it is necessary
to understand the processes of weathering to gain insight into sediment formation. To
conclude this chapter it is appropriate to summarize some of the major points and to
draw attention to the economic significance of weathering. It is important to remember that weathering separates rock into an insoluble residue and soluble chemicals. The
residue, composed largely of quartz grains and clay particles, ultimately contributes to
the formation of the terrigenous sands and shales. The soluble products of weathering
are ultimately reprecipitated as the chemical rocks, such as limestones, dolomites, and
evaporites. Most terrigenous sediments are the product of weathered granitoid rocks
and preexisting sediment. Basic igneous and volcanic rocks and limestones are very susceptible to chemical weathering and contribute little material to the insoluble residue.
Figure 2.10 shows how, in a terrain of diverse rock types, only a few lithologies contribute to the adjacent sediment apron. Aside from these generalities, weathering is important because of the way it can concentrate minerals into deposits that are worth exploiting economically. The bauxites and china clays have already been mentioned.
There are four main ways in which weathering is economically important in a geological context. Weathered profiles are often buried beneath unconformities. These are
commonly zones of enhanced porosity, both in sands (see Section 8.5.3.4) and especially in carbonates (see Section 9.2.7). The product of limestone weathering consists
of nothing but calcium and carbonate ions that are carried away in solution. Limestone