Indices of weathering for soils and palaeosols formed on silicate rocks

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Abstract
Most chemical weathering indices assume aluminium is an immobile element. However, eluvial processes in many soils and palaeosols are responsible for the movement of aluminium, in the form of layer lattice silicates of submicron size, from the surface to horizons lower in the profile. Application of commonly used indices to such soils and palaeosols leads to the inaccurate conclusion that A horizons are less weathered than B horizons. The problem is illustrated with reference to two soils, one formed in feldspathic sandstone, the other in a granite, and a chronosequence of five soils formed in Quaternary alluvium. A new index based on resistant heavy minerals in the 20–90 μm fraction is proposed: WR = [(CaO + MgO + Na2O)/ZrO2] (molecular proportions) where ZrO2 is a measure of zircon. Several other minerals may be substituted for zircon depending on abundance and degree of alteration as detected by scanning electron microscopy. Their concentrations can be determined by elemental measurements, for example boron for tourmaline, yttrium for xenotime, titanium for rutile.