Magnetic Susceptibility of Mica

Abstract

THE magnetic susceptibilities of various natural micas have been measured by Wilson¹ and Nilakan-tan² ; the latter also measuring the anisotropy. It was found that for micas which are substantially free from inclusions of magnetite, and in which the iron is therefore present in the crystal lattice, the paramagnetic susceptibility is proportional to the iron content. It was therefore surprising that Eitel and Dietzel³ should report that synthetic mica containing only 0·05 per cent of iron is still paramagnetic, since a small cleavage flake orientated itself parallel to the magnetic field. A rough calculation shows that a mineral corresponding to their synthetic fluor-phlogopite (KMg₃AlSi₃O₁₀F₂) should have a specific diamagnetic susceptibility of – 0·35× 10^-6 C.G.S. units, and using the usual Bohr magneton value for iron of P_B=5 and calculating the paramagnetic susceptibility of iron from the relation p^b=2·84√Ψ Ï where is the susceptibility per gram-ion, one finds that 0·2 per cent of iron corresponds to a paramagnetic susceptibility of just + 0·35 × 10^-6 C.G.S. units. A mica with a greater iron content should therefore be paramagnetic, but with less iron should be diamagnetic. Eitel and Dietzel suggested that the anomalous paramagnetism found by them was connected in some way with the highly anisotropic crystal structure of mica.