Glasses of stoichiometric cordierite composition
Mg2(Al1−xFex)4Si5O18,
containing very low iron contents (x=0.015 and 0.005) have been investigated
by 57Fe Mössbauer spectroscopy. At higher Fe concentrations
spinel exsolution has been observed in X-ray powder patterns. To allow
Mössbauer 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 In
water saturated O2 stream. Glasses were crystallized to
cordierite by heating In air at 1100 to 1400°C. 57Fe-Mössbauer
spectra 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 to
metallic iron). X-ray powder patterns exhibit no additional phases. But, the
295 K and 5 K Mössbauer spectra of treated and untreated glasses and of
cordierltes exhibit broad lines, which have been fitted by applying internal
magnetic hyperfine fields H of ca. 500 kG. These lines are
attributed to heterogeneously nucleated, superparamagnetic (MgFe)
(AlFe)2O4 spinels of complex compositions. From the
calculated subspec-tra one may conclude that 35 to 65% of the total iron,
depending on preparation conditions, is incorporated in the spinels.