Abstract
Three iron-rich 1:1 clay minerals, greenalite [Si2]{Fe 2+3 }O5(OH)4, berthiérine [Si, Al]2{Fe2, Mg, Fe3+, Al}3 O5(OH)4 and cronstedtite [Si, Fe3+]2{Fe2+, Fe3+}3O5(OH)4 have been studied by Mössbauer spectroscopy, magnetization measurements and neutron diffraction to determine their magneticproperties. The predominant magnetic coupling is ferromagnetic for pairs of ferrous ions in the octahedral sheet, but antiferromagnetic for ferric pairs. The crystal field at Fe2+ sites in greenalite and berthiérine is effectively trigonal with an orbital singlet l z=0 as ground state. These mainly ferrous minerals order magnetically at 17K and 9K respectively. The magnetic structure of greenalite consists of ferromagnetic octahedral sheets, with the moments lying in the plane, coupled antiferromagnetically by much weaker interplane interactions. The ratio of intraplane to interplane coupling is of order 50, so the silicate has a two-dimensional aspect, both structurally and magnetically. Although the overall magnetic order is established as antiferromagnetic by neutron diffraction, the magnetization curves resemble those of a ferromagnet because of the very weak interplane coupling. Cronstedtite orders antiferromagnetically around 10K. Moments within the planes are antiferromagnetically coupled. The magnetism has no particular two-dimensional character because exchange paths between the layers are provided by the ferric cations present in the tetrahedral sheets.
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Coey, J.M.D., Ballet, O., Moukarika, A. et al. Magnetic properties of sheet silicates; 1:1 layer minerals. Phys Chem Minerals 7, 141–148 (1981). https://doi.org/10.1007/BF00308232
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DOI: https://doi.org/10.1007/BF00308232