form these to clay minerals. Alteration is most likely in the interlayer areas, espe-
cially at damaged crystal edges. The clay mineral formed will depend on the com-
position of both the original mineral and the ions substituted during alteration.
For example, the replacement of K+in muscovite by Mg^2 +would lead to the
formation of magnesium smectite.
Topography influences most of the soil-forming factors (Section 4.6). Where
all the soils in an area of varied topography have formed on the same parent mat-
erial, the soils differ principally due to changes in relief and drainage. These soils
are described as part of a catena (‘chain’) and are intrinsically linked to the land-
scape of a region or area. Two key processes control the development of a soil
catena: (i) erosion and subsequent transport and deposition of eroded material;
and (ii) leaching and subsequent transport and deposition of dissolved materials.
The latter process is dependent on soil chemistry and in the tropics where rain-
fall is high, a chemical catena develops.
Wet tropical environments promote dissolution and transport of a number of
soil constituents. Chemical species have been categorized into four groups by their
relative mobility during rock weathering (Table 4.8). At high altitude in the
tropics, high rainfall causes mobilization of Group I soluble ions and oxyanions
from the soil. These easily dissolved species (see Section 5.2) are carried in solu-
tion down slope to lower altitude where they accumulate. Thus, the tropical catena
is characterized by distinct soil end-members (Fig. 4.16). The high-altitude
end-member is iron- and silica-rich Groups III and IV and base cation-deficient
(Box 4.11), and called an oxisol or ferralsol depending on the classification system
used (Plate 4.1, facing p. 138). By contrast, the low altitude end-member is base
cation-rich and also clay-rich and known as a vertisol (swelling clay soil).
The high-altitude oxisols (ferralsols) are of low fertility on account of their
low cation exchange capacity (CEC) (Section 4.8). Furthermore, the high iron
concentrations can prove toxic to some plants and animals while the formation
The Chemistry of Continental Solids 105Table 4.8Mobility of different chemical species in relation to rock weathering. Modified
from Polynov (1937).
Species Mobility* Comments†Group I Cl- 100 Soluble anions easily leached by water
SO 42 - 57
Group II Ca^2 + 3.00 Relatively soluble cations, easily leached by water
Na+ 2.40
Mg^2 + 1.30
K+ 1.25
Group III SiO 2 0.20 Relatively insoluble element, typically present as
quartz grains
Group IV Fe 2 O 3 0.04 Highly insoluble elements present as Fe and Al
Al 2 O 3 0.02 oxides
- Most mobile=100.
†The reasons for differing solubility are discussed in Section 5.2 and depicted in Fig. 5.2.