Synthesis and characterization of pyroaurite

Ten samples of pyroaurite, [Mg8-x)Fe(III)x(OH)16]x+[(CO3)x/2·yH2O]x− with x varying between 1.45 and 3.07, have been synthesized by oxidation of Fe(II) with air in magnesium nitrate solutions at different pH and using different rates of oxidation. The morphology and crystal size of the precipitates vary as a function of x and hydrothermal treatment following precipitation. The unit cell parameter a0 varies insignificantly with x, whereas c0 decreases linearly with increasing x. From infrared spectroscopy the only interlayer anion present is carbonate. The carbonate symmetry is distorted in the samples with disordered layer stacking and also following heating of the sample in KBr tablets. The absorption bands due to stretching vibrations of structural OH (3MgOH, 2MgFe(III)OH, Mg2Fe(III)OH) varies continuously in position and intensity as a function of x. The parameters of the Mossbauer spectra at 298 K reveal a constant isomer shift (0.35 mm s−1) and increasing quadrupole splitting with increasing values of x. Considering the chemical analysis and known speciation of anions and cations the ideal formula reveal a charge imbalance between +0.77 and −1.09 eq per formula unit and depends linearly on x. This may be explained by amphoteric properties of the hydroxyl groups. A decreasing hydrogen bond length in the interlayer with increasing x values may explain the major decrease in c0. For constant ac this implies an increasing distortion of the Fe-octahedra contributing to an increase of the mean quadrupole splitting of the Mossbauer spectra.