Cation Distribution, Mössbauer Spectra, and Magnetic Properties of Ferripyrophyllite

Abstract

Abstract A sample of ferripyrophyllite, a mineral with ideal composition Fe 2 SiO 10 (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 δ = 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 δ = 0.43(1) mm/sec; Δ = 1.22(2) mm/sec and δ = 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+ 1.87 Mg 0.11 Al 0.09 ) VI (Si 3.80 Fe 3+ 0.16 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 B hf = 51.8 T. The relatively high Néel temperature compared with other dioctahedral ferric phyllosilicates provides further evidence that Fe 3+ cations tend to be ordered on M2 sites within the octahedral sheet.