By contrast, smectite clays develop in poorly drained sites. On the basaltic
island of Hawaii, soil clay mineral type changes in the sequence smectite–kaoli-
nite–gibbsite as rainfall amount increases (Fig. 4.18). A similar, generalized zona-
tion has been proposed for clay mineral distribution with depth in soils, again
based on the degree of leaching (Fig. 4.19).
Intense leaching favours kaolinite formation since cations and H 4 SiO 4 are
removed, lowering the silicon : aluminium ratio and favouring the 1 : 1 structural
arrangement. Less intense leaching favours a higher silicon : aluminium ratio,
allowing the formation of various 2 : 1 clay minerals, depending on the supply of
cations. For example, the weathering of basalt provides abundant magnesium for
the formation of magnesium smectite. In the most intense tropical weathering
environments, all of the silica is removed, favouring the formation of gibbsite,
which can be thought of as a 0 : 1 arrangement (i.e. only octahedral sheet present;
Fig. 4.19).
The Chemistry of Continental Solids 109300Precipitation/evaporation (mm)2700
2100900
5Relative depth of weathering profile increasing25
10(^2015)
Temperature (
°C)
1500
Large amounts of soil organic
matter promote aggressive
weathering conditions
Savannah
Tropical
Steppes Savannah forest
Semidesert
and desert
Tundra
Taiga-spodosol
zone
Rock debris – little
alteration chemically
Moderate amounts of soil
organic matter promote
weathering reactions
Illite – smectite clay minerals
formed by acid hydrolysis
KK
Kaolinite clay minerals
formed by hydrolysis
plus leaching
Gibbsite formed by
excess leaching
Laterites and bauxites formed
as residua in heavily leached
conditions
++Fresh rock
Precipitation evaporationPotential Temperature
Fig. 4.17Schematic relationship between weathering zones and latitude–vegetation–climatic
zones. The influence of relief (mountainous areas), where soils are typically thin, is not
included. After Strakhov (1967). With kind permission of Kluwer Academic Publishers.