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. Tile proliferation of new technologies le d Syvitski et al, (1991) to recommend that "new particle size instruments s hould no longer have their results compared with those of the classical met hods of sieving and pipetting." However, care is required to ensure that re sults are broadly comparable to those obtained using established methodolog ies, especially where sediment classification and environmental processes a re being interpreted using traditional schemes. The progressive development of new classification schemes based on modern particle-size analysis techn iques will eventually mitigate this problem. Laser particle size analyses of Upper Neogene micaceous terrigenous deposit s from New Zealand have yielded some significantly different grain-size dis tributions compared to traditional sieving methods. These differences typic ally escalated with increasing sand content, h series of test samples spike d with increasing amounts of mica demonstrate that very small amounts of mi ca (< 2 wt%) have the potential to significantly alter the grain size chara cteristics obtained using a laser particle sizer compared to sieve analyses , This is probably due to the mica particles having (a) a higher light-scat tering property, and (b) large numbers of particles per unit volume, result ing in mica being overestimated.