THE EARLIEST EVENTS IN THE PALAEODUST CYCLE

Before the dust cycle can begin the dust has to be made. There are various types of airborne dust and the dust type affects aspects of the dust cycle. Many materials are found in dust but the commonest dust unit is probably a small quartz particle. We propose that there are basically two types of quartz dust particle: large dust (say coarse and very coarse silt size 60-16um) and small dust (of fine and very fine silt sizes 8-2um); between the two size ranges is a 'Tanner gap', a region where there is an observed shortage of particles. A natural control exists which affects the formation of these particle populations- the bimodal nature of quartz dust is controlled at the particle-making stage. The key events are at the [not dust] to [dust] stage, rather than at the {dust on ground} to {dust in air} stage. The control is the presence of 'Moss defects' in the proto-quartz particles. The quartz formation in source granitic rocks is affected by a eutectic-type reaction which controls sand particle size and by the high-low displacive quartz transformation which introduces defects (usually dislocation clusters) into the quartz structure. The type and distribution of these defects affects silt formation- and dust production. Large dust does short range suspension travel and forms loess deposits. Small dust does long range suspension travel and crosses oceans. We have carried out a series of grinding experiments on Leighton Buzzard sand which appear to indicate that the internal defects in the quartz do have an effect on the size distribution in the comminuted product. Energy levels can be identified which control quartz dust production.