The formation of silt material for terrestrial sediments: Particularly loess and dust I.J. Smalley * , R. Kumar, K. O’Hara Dhand, I.F. Jefferson * , R.D. Evans Midlands Loess Group, Civil Engineering: School of Engineering, University of Birmingham, Birmingham B15 2TT, United Kingdom Received 26 July 2004; received in revised form 17 June 2005; accepted 23 June 2005 Abstract 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 dTanner gapsT. 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 dMoss defectsT, 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 Am) and the smaller mode falls into the classification with high-level, long-travel dust (mode around 2–6 Am). 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. D 2005 Elsevier B.V. All rights reserved. Keywords: Sand; Silt; Clay; Sedimentary particles; Clay material agglomerates; Monte Carlo lacustrine structures; Aeolian dust; Loess 1. Introduction Certain particles dominate the clastic particulate detrital sedimentary environment. We are all familiar with sand, silt and clay but their very familiarity has meant that they have been, in many ways, neglected. Quartz particles tend to dominate here and the size grading systems give us a continuous range from 2 mm to less than 2 Am. The grading systems, because of the continuous ranges they offer, have the effect of obscuring geologically controlled modes which may exist in the sand, silt and clay ranges. We argue, in this discussion, that there are various (many yet unappre- ciated) geological factors controlling the production of sand silt and clay particles for particulate sediments. This argument is set against the background of a wider 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.06.011 * Corresponding authors. I.J. Smalley can be contacted at Tel.: +44 0116 224 8409. I.F. Jefferson can be contacted at Tel.: +44 121 414 7972. E-mail addresses: Smalley@loessletter.com (I.J. Smalley), I.Jefferson@Bham.ac.uk (I.F. Jefferson). Sedimentary Geology 179 (2005) 321 – 328 www.elsevier.com/locate/sedgeo