Cation Distribution, Mössbauer Spectra, and Magnetic Properties of Ferripyrophyllite

J. M. D. Coey, F. V. Chukhrov and B. B. Zvyagin
Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland
Institute of Ore Geology, Petrography, Mineralogy and Geochemistry, Academy of Sciences of the U.S.S.R., Moscow 109017 U.S.S.R.

Abstract: A sample of ferripyrophyllite, a mineral with ideal composition Fe2Si4O10(OH)2, was analyzed by Mössbauer spectroscopy, magnetization and magnetic susceptibility measurements. The iron is entirely ferric, and the main feature of the Mössbauer spectrum at room temperature is a single absorption line with isomer shift σ = 0.36(1) mm/sec due to iron in M2 sites. The line is broadened by a slight quadrupole splitting Δ = 0.18(1) mm/sec, the smallest yet reported for a sheet silicate. Weaker features corresponding to quadrupole doublets with relative intensities of 7% and 8% having σ = 0.43(1) mm/sec; Δ = 1.22(2) mm/sec and σ = 0.14(1) mm/sec; Δ = 0.59(2) mm/sec are assigned to ferric iron in nonequivalent octahedral and tetrahedral sites, respectively, giving the following formula, based on published chemical analysis: Ca 0.05 (Fe 3+ Mg 0.11 Al 0.09 ) VI ( Si 3.80 Fe 3+ Al 0.04 ) IV O 10 (OH) 2 , where Ca is in interlayer sites. Ferripyrophyllite orders antiferromagnetically at 18(2)°K. The Mössbauer spectrum at 4.2°K consists of a single, resolved magnetic pattern with hyperfine field Bhf = 51.8 T. The relatively high Néel temperature compared with other dioctahedral ferric phyllosilicates provides further evidence that Fe3+ cations tend to be ordered on M2 sites within the octahedral sheet.

Key Words: Cations • Ferripyrophyllite • Iron • Magnetic properties • Mössbauer spectra • Pyrophyllite

Clays and Clay Minerals; June 1984 v. 32; no. 3; p. 198-204; DOI: 10.1346/CCMN.1984.0320307
© 1984, The Clay Minerals Society
Clay Minerals Society (www.clays.org)