Magnetic and optical properties of kaolinite particles in aqueous dispersion

A magneto-optical method is described for measuring the magnetic properties of micron and submicron sized particles in suspension. The results show that platelet-like kaolinite particles with iron impurities have a permanent magnetic moment and an anisotropy of magnetic susceptibility. The directions of the permanent moment and the axis of smallest magnetic susceptibility both lie in the platelet plane. A theory of magneto-optic birefringence is used to derive values of the permanent magnetic moment and the magnetic susceptibility anisotropy for a range of particle sizes, the permanent moment being approximately proportional to particle area and the susceptibility anisotropy to the particle volume. The directions and magnitudes of the moments and the anisotropies are consistent with the presence of fine particles of α–Fe2O3 on the kaolinite particles, having magnetic properties different considerably from macroscopic α–Fe2O3 samples.