Magnetic properties of sheet silicates; 2:1:1 layer minerals

Abstract

Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is