Wider controls on soil and clay mineral formation

7 Wider controls on soil and clay mineral formation 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

A minority of unweathered rock-forming silicates, for example the micas, are already sheet silicates. It is not difficult to envisage that alteration could trans- 

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 

Group 1 cr 100 Soluble anions easily leached by water 

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