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Data on the magnetic susceptibility, magnetization, Mössbauer spectra, magnetic neutron scattering, and computer simulation of the magnetic ground state of representative Fe-rich trioctahedral phyllosilicates are reviewed. The principal exchange interaction, between Fe2+ in edge-sharing octahedra in the octahedral sheets, is ferromagnetic with J = 1–2 K; intersheet coupling is about 20 times weaker. Anisotropy due to the Fe2+ may be represented by a trigonal crystal field which stabilizes the 5Alg orbital singlet as the ferrous ground state, the trigonal, hard axis being normal to the sheets.

Ferrous end members of the trioctahedral series order as planar antiferromagnets at 20–30 K. The planar antiferromagnetic ground state can tolerate the small amounts of Fe3+ found in greenalite or minnesotaite; however, it is destabilized in biotite and thuringite, in which octahedral sheets contain more than a small percentage of ferric pairs, in favor of a spin glass state having only short-range ferromagnetic correlations (~ 10 Å) in the planes. Except for a reduction by a factor of 4 in the magnetic ordering temperature, the bulk magnetic properties of the spin glass and planar antiferromagnetic states are remarkably similar.

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