The formation of silt material for terrestrial sediments: Particularly loess and dust

There are certain controls which operate to demarcate the nature and size range of quartz particles in terrestrial detrital sediments. The particles fall into discrete and definable populations, which are separated by 'Tanner gaps'. Quartz sand nature is determined largely by a eutectic-like reaction, which takes place in the original igneous rock. This reaction delivers a fine mix of quartz and feldspar units, which on weathering deliver quartz sand. The formation of quartz silt is more contentious; it appears that silt production is essentially controlled by the presence in larger quartz particles of 'Moss defects', crystalline defects introduced into the quartz structure by earlier events, e.g. the high-low displacive transformation in cooling quartz. There appear to be two modes in airborne quartz particles (which have been called large dust and small dust). The larger mode is the loess mode (around 20-60 μm) and the smaller mode falls into the classification with high-level, long-travel dust (mode around 2-6 μm). The quartz in small dust is around the same size as the clay mineral agglomerate particles (CMA) which form a considerable proportion of high level dust clouds. The CMA particles form from lake deposits; the size is controlled by the openness of the packing in the lake sediments. This can be demonstrated by a simple Monte Carlo model.