Effect of mica on particle-size analyses using the laser diffraction technique

New sizing technologies commonly make particle-size analyses more routine, simpler, and less time consuming. The proliferation of new technologies led Syvitski et al, (1991) to recommend that "new particle size instruments should no longer have their results compared with those of the classical methods of sieving and pipetting." However, care is required to ensure that results are broadly comparable to those obtained using established methodologies, especially where sediment classification and environmental processes are being interpreted using traditional schemes. The progressive development of new classification schemes based on modern particle-size analysis techniques will eventually mitigate this problem. Laser particle size analyses of Upper Neogene micaceous terrigenous deposits from New Zealand have yielded some significantly different grain-size distributions compared to traditional sieving methods. These differences typically escalated with increasing sand content, h series of test samples spiked with increasing amounts of mica demonstrate that very small amounts of mica ( 2 wt%) have the potential to significantly alter the grain size characteristics obtained using a laser particle sizer compared to sieve analyses. This is probably due to the mica particles having (a) a higher light-scattering property, and (b) large numbers of particles per unit volume, resulting in mica being overestimated.