Grain‐size separation of haematite in the <5μm range for rockmagnetic investigation

Abstract

Grain‐size separation of the <5 μm fraction of a natural haematite into six fractions has been accomplished by means of centrifuging. Separation was repeated up to 10 times under the same centrifuging conditions to obtain grain‐size fractions with a minimal quantity of undesired grains smaller than the lower limit for each fraction (the so‐called tailing). to calculate the grain‐size distribution of each fraction the total grain‐size range is divided into 0.1 μm increments; the travelling distance for each increment is calculated using Stokes' Law starting with a homogeneous suspension for each separation run. the travelling distance is recalculated to a virtual suspension volume, which in turn is related to the actually extracted suspension volume. In this way a normalized distribution is obtained for 0.1 μm increments for each separation run. to arrive at the grain‐size distribution for each fraction the calculation is repeated for the appropriate number of runs under the same centrifuging conditions and the normalized distribution is fitted with an actually determined grain‐size distribution for the total grain‐size range. Electron microscope observation of the separated fractions yielded good agreement between calculated and observed grain‐size ranges. With these well‐determined grain‐size fractions rockmagnetic parameters can be determined for the geologically important <5 μm grain‐size range for weakly magnetic material, like haematite or goethite. Copyright © 1991, Wiley Blackwell. All rights reserved