Electron micrography of mineral surfaces subject to wind-blast erosion

Abstract

The erosional results of two different kinds of wind-blast experiments on isometric minerals, ranging from 2 to 6 in hardness on the Mohs scale, are compared and illustrated, chiefly by electron micrography. One prolonged experiment employed only normal laboratory air. In the other experiment, silt-sized projectiles were added to the air that was blown over one of two similar fluorite specimens for a period of 50 hr during a five-month wind-blast test of the two specimens. In all cases, wind erosion occurred. However, in the second experiment, it was greatly accelerated on the test specimen which was wind-blasted by the air with the added projectiles. Also, these experiments demonstrated in detail where the aerodynamic and vorticity erosion processes concentrate to effect erosion on the shapes that were used. Although wind moves as a fluid, the interfacial flow lines along which erosion occurs form a complex reticulum. Each flow line is attracted to a center of low pressure. Such centers may be within traveling vortices along a flow line but often are at stationary points such as at vortex centers in irregularities along sharp margins where air escapes. Such vortices oriented the flow lines variously so that they merged or even transected one another, despite the use of a single wind direction. Likewise, traveling vortices along flow lines developed pit chains which show all stages of development from isolated scores and pits to fine U-shaped fluting. Thus vorticity plays a major erosive role, and wind can erode in the total absence of sand. Furthermore, aerodynamic erosion can affect every surface of an object, even the basal surface, and may even be greater on the lee face than on the windward face, and the suspended tools produce more definitive erosion than does sand grain impact. In addition, shape and size of the target, its orientation to the wind, inclination of faces, surficial texture and irregularities, and internal constitution are all important determinants of the character, locations, and rates of erosion.