Micromechanics of quartz sand breakage in a fractal context

• Sand and silt are not final resistant products of glacial abrasion.
• Crystallographically defected sand and silt are inherently breakable.
• Quartz breakdown lasts until the fractal dimension tends to an intrinsic maximum.
• Internal defects in quartz are independent from quartz size and energy input.

From a Quaternary science perspective, sand-sized quartz as well as silt-sized quartz is often acknowledged as final products of glacial abrasion through different evolution mechanisms. This view challenges the existence of any universal comminution process, which may relate the formation of detrital quartz sand and silt. The contribution of grain size, energy input, and crystalline integrity in the scale of quartz crushability has long been the matter of much debate. The present empirical work examines the micromechanics of sand-to-silt size reduction in the quartz material. A series of grinding experiments was performed on Leighton Buzzard Lower Greensand using a high-energy agate disc mill. Analogous conditions to glacial abrasion are provided due to the combined abrasion between grains' asperity tips, and also between grains and rotating smooth tungsten carbide pestle. Simulation of discontinuous grain breakage allowed the examination of grains' crystalline defects. To enable an objective assessment of micromechanics of size reduction, measurements of particle and mode size distribution, fractal indexes and micro-morphological signatures were made. The crushing approach was probed through varied grinding times at a constant energy input, as well as varied energy inputs at constant grinding time. Breakage pathway was inspected via laser diffraction spectroscopy and transmission light microscopy. Results suggested that the grain breakdown is not necessarily an energy-dependent process. Non-crystallographically pure (amorphous) quartz sand and silt are inherently breakable materials through a fractal breakdown process. Results also revealed that the internal defects in quartz are independent from size and energy input.