Clay mineralogy and chemistry of seven pedons formed in volcanic ash, Tanzania

Abstract

Clay mineralogy and chemistry of seven pedons developed in Quaternary volcanic ashes under different moisture regimes in northern Tanzania have been studied. Total carbon, nitrogen and acid-oxalate soluble aluminum contents of soils increased and pH (H 2O, M-KCl), exchangeable basic cations and available phosphorus contents decreased with increasing mean annual rainfall. High contents of clays in soil samples (410–860 g/kg) are believed to indicate advanced stages of weathering. An aluminum-humus complex, allophane and gibbsite predominated in soils under a perudic moisture regime, kaolinite and halloysite (0.7 and 1.0 nm) in those under udic to ustic moisture regimes. Resilication resulting from the seasonal drying of moisture regimes is considered to inhibit the release of “active” aluminum in the weathering of volcanic materials but to favour the authigenic formation of kaolinite and halloysite. Substantial amounts of vermiculite-chlorite intergrades were observed associated with fine-grained quartz in a perudic soil and mica in an aridic soil. Oxygen isotopic composition of fine aerosol-sized quartz suggests an eolian origin. Origin of clay mica is also discussed in the light of the oxygen isotope composition.