Submicroscopic Frosting on Eolian and Subaqueous Quartz Sand Grains

Abstract

Investigation of surface features on quartz sand grains of eolian origin from 20 desert and coastal dune environments from around the world reveals that chemical solution and redeposition of silica by the action of desert dew along with mechanical wind abrasion causes frosting and rounding of sand grains. Wind-induced grain impacts create small-scale fractures that are related to wind velocity and moisture as well as the roundness, degree of polish, and size of the impacting grains. This fracture pattern, which has been reproduced experimentally by simulating wind abrasion, is most pronounced on projections from the surface of the grains. Examination with a scanning electron microscope shows that this pattern consists of minute upturned plates of quartz that overlap each other and appear to be dipping in a direction possibly consistent with internal crystallographic planes. The close spacing of these fracture plates is responsible for the frosted appearance of many desert sands. These irregular fractured plates have a high chemical potential and will dissolve under favorable physicochemical conditions. This dissolved silica can be precipitated elsewhere as an amorphous layer on grain surfaces when the solution evaporates, resulting in a progressive rounding of sharp edges and subduing of irregular mechanical fracture patterns by transfer of minute amounts of silica. The occurrence, morphology, and distribution of diagnostic mechanical and chemical surface features allows differentiation between tropical desert sands, coastal dune sands, and periglacial sands. Closely spaced chemical etch pits formed on sand grains by “abrasion solution,” or during diagenesis, when grains are exposed to high p H solutions, also can result in a frosted appearance. Defrosting can be accomplished by addition of a thin amorphous layer of silica to the grain9s surface, or by polishing during turbulent subaqueous transport.