Silt:2-62 μm, 9-4φ

The particles in finer-grained detrital sediments are usually composed of quartz. They fall into two size grades: sand (2 mm-62 μm) and silt (62 μm-2 μm). These size gradings conceal important geological processes since there are geological controls on both the quartz sand and silt populations. Sand nature is largely controlled by geochemical reactions, for example in cooling granite from which it is eventually released by weathering action. The quartz is among the last part of the rock system to solidify and the eutectic-like reaction which occurs ensures that the quartz exists as small crystalline units, each suffering considerable cooling and recrystallisation stresses. Silt is broken quartz, and has traditionally been considered to lack a specific geological control. The situation is confused by the large range of materials that fall into the size category, but there do seem to be distinguishable modes and possibly comminution limits within the 60 μm span. The controls operating in this size range are probably the critical concentration of 'Moss' defects in the quartz particles. Moss postulated the formation of specific crystal defects in the quartz formed in granites. These affect sand formation and it is possible that they also control the mode size of silt particles. Within the silt range there may be several usefully definable populations, as Moss proposed for sand. The Quaternary appears to be a silt-rich period due to tectonic and glacial activity, but silt production is apparent throughout the sedimentary record. Very long-term silt producing processes are required. To produce silt in nature, on a large scale, very energetic processes are required. Many processes that are believed to generate silt particles have been listed. However, large-scale production is essentially due to glacial grinding, or to intense weathering processes in high, cold, tectonically active mountain regions. The region of High Asia (of crustal overlap) is a major generator of silt particles. These form the productive alluvial soils of north India, most of the nation of Bangladesh, the loess deposits along the Syr-darya and Amu-darya rivers in Central Asia, and the great loess deposits of North China. Some claims have been made for silt production in hot deserts. Large amounts of very fine aerosolic dust are produced, but in terms of loess-sized particles this is a small-scale process and leads to modest, disputed deposits. The loess deposits around the Sahara usually have smaller mode sizes (e.g., Nigeria) or larger mode sizes (e.g., Libya, Tunisia) than the true Chinese mode at 25 μm. This paper aims to introduce the distinguishing properties of sand and silt, discuss in detail the possible processes of silt formation in relation to the evidence of what is produced, describe the major silt populations, and conclude on the most likely formation process(es). In particular three silt ranges are discussed in relation to the R-size diagram: C, 2-5 μm, D1, 20-30 μm, and D2, ~60 μm. These can represent Canadian quickclay (QQ), Chinese loess and the ideal Loughborough Loess (LL) and North African loess (NAL). It is intended that this paper will put into context the diverse work in the area, and in so doing shed light on what is an often overlooked subject.