Magnetic order in silicate minerals (invited)

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Abstract
Silicates are the most common Earth materials. Many iron‐rich solid solution series order magnetically but ordering temperatures rarely exceed 100 K, even for the iron end‐members. Minerals with sheet and chain structures are discussed, with emphasis on the magnetic properties of greenalite, cronstedtite, biotite, and crocidolite determined by magnetization, susceptibility, and Mössbauer measurements and by magnetic neutron diffraction. The principal magnetic interactions in the ferrous minerals are ferromagnetic, due to near 90° Fe–O–Fe bond angles for edge‐sharing octahedral sites, yet all of them order antiferromagnetically as the ferromagnetic sheets or chains are coupled by relatively weak antiferromagnetic interactions. A spin flop transition occurring in fields of 1–15 kOe at 4.2 K is analyzed in terms of effective anisotropy and exchange fields. Magnetocrystalline anisotropy arises from the Fe2+ ion which is in an orbital singlet 5A1g, lz=0 ground state in an effectively trigonal crystal field. The hard trigonal axis in sheet silicates lies close to the crystllographic c direction and approximately normal to the plane of the layers. Almost all the iron‐rich silicates show some degree of mixed valence in that Fe2+ and Fe3+ occupy crystallographically equivalent sites, but thermally‐activated electron hopping is rare. Among the sheet silicates it has been found only in cronstedtite [SiFe3+]2{Fe2+, Fe3+}3O5(OH)4.

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