Mineral weathering in two soils, contrasting in colour and clay distribution, formed from granodiorite, was studied by X-ray diffraction, chemical and optical techniques. The feldspars, hornblende, and quartz become comminuted and appear to be weathering entirely by solution. The main source of clay is from the weathering of biotite which gives hydrobiotite, vermiculite, smectite and kaolin. Clay ... [Show full abstract] coatings lining cracks and voids are restricted mostly to the weathered rock and lower part of the soil, and their main source is from decomposing biotite nearby. This weathering study permits details of the clay distribution to be postulated. The clay-rich horizons seem to be formed by settling of residual material as other constituents are lost by solution. The resulting reduced permeability would be expected to increase surface run-off and lateral flow through the upper part of the soil, giving rise to the clay-depleted upper horizons. Clay illuviation seems to be insignificant in forming the horizon of maximum clay content.