Transformations of clay minerals in black earth and red loams of basaltic origin.

Abstract

The development in the Toowoomba area, Queensland, of red, black, and transitional soil types from basalt is typical of similar processes in many parts of eastern Australia. Systematic samples from this area have been investigated mineralogically by differential thermal analysis, X-ray diffraction, and other methods. Montmorillonite appears to be the clay mineral first formed under present weathering conditions. The further transition to red soils in certain situations is due to partial decomposition of montmorillonite to minerals of the kaolin group by loss of alkalis and iron, the latter being stabilized in the upper horizons as hydrated oxides. Further decomposition of kaolin minerals produces gibbsite. Changes in the nature and amount of clay minerals are thought to correspond with stages in the evolution of basalt soil types.