Heterogeneous Nucleation of Superparamagnetic Phases in“Fe-Bearing Cordierite-Glasses

Abstract

Glasses of stoichiometric cordierite composition Mg₂(Al_1−xFe_x)₄Si₅O₁₈,containing very low iron contents (x=0.015 and 0.005) have been investigatedby ⁵⁷Fe Mössbauer spectroscopy. At higher Fe concentrationsspinel exsolution has been observed in X-ray powder patterns. To allowMössbauer spectroscopy at very low Fe-concentrations (<0.8 weight* Fe₂O), starting materials were doped with 100%⁵⁷Fe. Glasses prepared in air at 1560°C were also oxidized Inwater saturated O₂ stream. Glasses were crystallized tocordierite by heating In air at 1100 to 1400°C. ⁵⁷Fe-Mössbauerspectra of all samples are governed by a doublet typical for Fe²⁺in octahedral coordination (IS: 1.09–1.15, QS: 2.00–2.31 in MM/s relative tometallic iron). X-ray powder patterns exhibit no additional phases. But, the295 K and 5 K Mössbauer spectra of treated and untreated glasses and ofcordierltes exhibit broad lines, which have been fitted by applying internalmagnetic hyperfine fields H of ca. 500 kG. These lines areattributed to heterogeneously nucleated, superparamagnetic (MgFe) (AlFe)₂O₄ spinels of complex compositions. From thecalculated subspec-tra one may conclude that 35 to 65% of the total iron,depending on preparation conditions, is incorporated in the spinels.