under normal soil pH ranges (Table 4.8) has been used to formulate a chemical
index of alteration (CIA); using molecular proportions:
eqn. 4.17
where CaO* is CaO in silicate minerals (i.e. excludes Ca-bearing carbonates and
phosphates). Thus, CIA values approaching 100 are typical of materials formed
in heavily leached conditions where soluble calcium, sodium and potassium have
been transported away from the weathering site. Kaolinite clays attain such values
(Table 4.9), whereas illites and smectites have CIA values around 75–85 (Table
4.9). In comparison, unleached feldspars have CIA values around 50.
The CIA predicts that kaolinite will form under heavily leached conditions,
and this is confirmed by observations in tropical weathering regimes. On stable
well-drained land surfaces where weathering and leaching have been prolonged,
the oxisols (ferralsols) develop kaolinitic and, in extreme cases, gibbsitic clay min-
eralogies (Fig. 4.17). Such sites are mantled by iron-rich (laterite) and aluminous
(bauxite) surface deposits (Plate 4.1). These surface deposits can become thick
enough to prevent further interaction between surface waters and bedrock, low-
ering the rate of subsequent bedrock weathering.CIA=+++()Al O Al O 23 23 CaO* Na O 23 K O 2 ¥ 1000108 Chapter Four
Table 4.9Chemical index of alteration (CIA) values for various crustal materials. Data from
Nesbitt and Young (1982), Maynard et al. (1991) and Taylor and McLennan (1985).
Material CIA
Clay minerals
Kaolinite 100
Chlorite 100
Illite 75–85
Smectite 75–85
Other silicate minerals
Plagioclase feldspar 50
Potassium feldspar 50
Muscovite mica 75
Sediments
River Garonne (southern France) suspended load 75*
Barents Sea (silt) 65*
Mississippi delta average sediment 64*
Amazon delta muds 70–75
Amazon weathered residual soil clay 85–100
Rocks
Average continental crust (granodiorite) 50
Average shales 70–75
Basalt 30–40
Granite 45–50
* Value calculated using total CaO rather than CaO* (see text).