Mobilities of alkalis, alkaline earths and halogens during weathering

Concentrations of alkali metals, alkaline earths and halogens were monitored in an Australian weathering profile and in Brazilian soils and bauxite. The materials analysed are considered to represent a range of chemical weathering histories, which are distinguished with respect to their degree of leaching by the dominant aluminosilicate assemblages. During initial weathering concentrations of alkalis and alkaline earths are similar or enriched relative to average crustal abundances. As weathering proceeds depletion is attenuated by the capacity of a particular element to participate in adsorption and ion-exchange processes. Strong depletion of alkalis and alkaline earths is common as weathering advances, and clay minerals are replaced by Al-oxide phases (e.g., gibbsite), with much lower capacities for exchanging and incorporating ions. Halogen concentration trends are more variable. F retention during weathering is likely due to substitution in mineral phases for hydroxyl groups. Cl and I are commonly observed to accumulate as weathering advances and some of these accumulations may be due to atmospheric additions.