Mineralogy and grain morphology of the aeolian dune sand of Toshka area, southeastern Western Desert, Egypt

Abstract

Abstract This work investigates the mineralogical properties and the morphology of aeolian quartz sand in the Toshka dune area, southeastern Western Desert of Egypt. The dunes are composed mostly of subangular to subrounded fine to medium quartz sands together with minor proportions of feldspars and lithic fragments. Significant roundness variations are observed both between different dune types and within the dune sites. Barchan sands are the most rounded, whilst sand shadows are angular and fine. Barchans are also relatively enriched in polycrystalline quartz, where most of these grains are more rounded and more durable than monocrystalline quartz. Roundness characteristics of the Toshka sands are considerably managed by their grain size as medium quartz grains are more rounded than the finer ones. Angularity is a characteristic property of most monocrystalline quartz grains that exhibit undulose extinction. This maintenance of undulose extinction in quartz perhaps as a result of less stability and minimal transport from the source in the vicinity of the dune fields, as shown by the increase of angularity of quartz and short-distance transfer of these grains. These Toshka sand grains also show signs of a fluvial transportation signature. The analysis of grain surface features by scanning electron microscope (SEM) performed on dune sand grains from the Toshka area showed that quartz grains demonstrate a variety of surface textures indicating mechanical and chemical processes developed during transportation of these grains in subaqueous and aeolian environments, respectively, reflecting the mixed mode of formation of these dunes.