Published online by Cambridge University Press: 14 March 2018
Size graded fractions of ground quartz and other silica minerals are prepared by gravity settling for particles greater than 2 microns Stokes' diameter and by centrifugal methods for smaller sizes. The size distributions of the prepared fractions are determined by counting and sizing with an optical microscope or with an electron microscope. The volume-surface diameter ΣND3/ΣND2 is used both for calculating the specific surface of the fractions and for comparing the projected area diameters used in counting, with the Stokes' diameters.