Surface morphology of quartz grains from tropical soils and its significance for assessing soil weathering

The surface morphology of quartz grains can indicate the degree of weatheri ng of soil material. We have compared two methods of assessing the relative weathering of soils on the basis of differences in the surface morphology of quartz sand grains in a catena of soils in Rwanda. One method is based o n the presence or absence of surface features indicative of weathering or f reshness, while the other uses the frequency and size of dissolution etch p its. A ranking of relative weathering could be obtained using the first tec hnique for the slightly and somewhat weathered soils but not for the weathe red soils. On the other hand, weathering trends and differences between the horizons studied were detected in weathered soils using the second method. The introduction of more specific definitions of the weathering classes us ed in the latter method leads to clear improvement of the inter-observer re producibility of the weathering classification. The surface features on the quartz grains suggest that the soil at the summ it is less weathered than the other soils of the sequence. Quartz grains fr om the well-drained soils on the slopes, which are subjected to more intens e leaching and thus to stronger chemical weathering, have more triangular e tch pits and chatter marks. In the imperfectly drained soils in the valley bottom quartz grains are less etched because dissolution is inhibited by th e oversaturation in silica of the drainage waters.