Conditions for Formation of Allophane in Soils

Abstract The Al/Si atomic ratio of allophane was obtained from analyses of soil horizons for a range of New Zealand soils and for a wide range of volcanic soils from around the world. Imogolite‐like allophane (Al/Si = 2) occurs very frequently in soil horizons with >1 or 2% allophane, suggesting that it is more stable, or is more readily formed, in soils than other allophanes. Generally, allophane forms from glass (at pH(H 2 O) 5 to 7), and feldspar and/or biotite (at pH about 5) in soils with udic moisture regimes and with good drainage. Under these conditions silicic acid and hydroxy‐aluminum cations react to give allophane. A pH >4.7 is required for allophane to precipitate. The Si‐rich allophanes are present in environments where Si in soil solution is likely to be high, and Al‐rich allophanes are present in acid soils where the Si in soil solution is likely to be low. Examples are given of some different types of soils that contain allophane.