Packing and angle of repose of naturally sedimented fine silica solids immersed in natural aqueous electrolytes

Six grades of near uniformly sized silica solids (ranging in median size from 15 μm to 160 μm) were sedimented in various natural aqueous electrolytes. The influence of surface forces on these silica-electrolyte systems was then inferred from measurements of the packing coefficient, C, of each sediment. It was found that the packings were uninfluenced by surface chemical forces when the grade size was greater than about 100 μm. For smaller grades immersed in naturally concentrated aqueous electrolytes, C was found to decrease from an initial value of about 0·50, and this was explained by the onset of surface chemical attractive forces. For smaller grades immersed in pure water, C was found to increase to a maximum of 0·58. Three explanations for this phenomenon, namely a difference in shape with grade size, a decrease in intensity of deposition, and the onset of surface chemical repelling forces were examined and the latter explanation seemed the most probable. Measurements of the angle of repose for the sediments also showed the influence of surface chemical repelling as well as attractive forces in fine silica solids. The results may be used to aid interpretation of the fabric and stability of pre-lithified sedimentary units.