Mineral weathering processes in podzolic soils on granitic materials and their implications for surface water acidification

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Abstract
Many areas where concern has arisen over the recent acidification of their surface waters contain podzolic soils developed upon granitic materials. The mineral weathering processes in these soils are reviewed and the ways in which such processes could be altered by acid deposition are considered. The most weatherable primary minerals are plagioclase feldspar, biotite and fine-grained dioctahedral mica and their decomposition, which is mediated by organic acids, leads to the formation of proto-imogolite allophane, vermiculite and smectite. Other clay minerals occurring in podzols such as kaolinite and halloysite, as well as gibbsite, are probably inherited from previous weathering cycles, although a present-day pedogenic origin should not be ruled out entirely. The possibility is considered that the partial replacement of soil organic acids by mineral acids introduced through precipitation could bring about significant differences in mineral weathering, leading to more acid soils with more dissolved ionic aluminium in soil and stream waters. The weathering products involved in the mobilization of aluminium from podzols are likely to be proto-imogolite allophane and hydroxy Al-interlayered vermiculite or smectite. Another possibility is that sand and silt-sized plagioclase feldspars are more reactive than previously realized.