Quartz sand grain size and surface morphology in some tropical soils

This study was conducted on catenae at Nam Phong and Khon Buri, Thailand to identify thespatial variation in quartz sand grain morphology using SEM and particle size distribution data.Profiles were sampled at six positions; on the summit (SU), shoulder (SH), upper midslope (UM),lower midslope (LM), footslope (FS) and toeslope (TS) of the Nam Phong catena which hasdeveloped on sandstone in the upper part and shale in the lower part. Four profiles developedfrom basalt and colluvium on crest (CT), backslope (BS), footslope (FS) and valley floor (VF)were sampled on the Khon Buri catena.Dissolution and precipitation features on quartz grain surfaces including pits, silicaprecipitation, crystal growth and adhering forms were present in soils at all positions. Evidence ofmechanical damage including conchoidal fracture, angular edges, rounded edges and cracks wasalso present on quartz grains from all positions on both catenae. The particle size distributionsindicate that changes in size sorting due in part to colluvial transportation and addition haveoccurred in both catenae. For the Nam Phong catena, sand content increases and clay contentdecreases from summit to footslope. There is a large increase in the clay content in the toeslopesoil due to a combination of sedimentation, crystallization of clay minerals from solution and apossible contribution of finer material from underlying shale. In the Nam Phong catena theparticle size distribution also indicates a discontinuity of parent material for shoulder, midslopeand toeslope soils, possibly due to colluvium over residuum. Summit to footslope soils arecomposed mostly of transported sandstone material over weathered sandstone and the toeslope soilhas developed on residual shale beneath surficial transported sandstone material, and this profilealso contains authigenic clay minerals. The abundant quartz in Khon Buri Soils is inconsistentwith a purely basaltic origin so colluvial or aeolian materials have been introduced into theprofiles. Particle size analysis and quartz grain surface analysis did not provide unambiguousinformation on the origins of soil materials or their weathering history for either catena. This ispossible due to the catenae being in a tropical region, where intense chemical weathering ispervasive and to the diverse origins of the soil materials.