AbstractThe size distributions of carefully screened √2 size fractions of ground materials were determined using a Microtrac laser diffractometer. The size distributions were fitted to the empirical functionP(x) = 1/[1 + (x50/x)λ] whereP(x) is cumulative mass fraction less than Microtrac size,x. The standard deviation of λ for a 50 second test time was approximately 0.5, so that seventeen test times give a mean with an estimated error within ± 0.25 (95% confidence level). Values of λ were 5.44, 4.82, 4.40 for a coal ground under different conditions, indicating statistically different shape distributions; the average value for mica was 2.91 owing to the high aspect ratio of the particles. The ratio ofx50to the geometric mean sieve size was 1.2 for the coal and 0.70 for