Order-disorder phenomena in naturally occurring metal oxides.

Abstract

In this investigation order-disorder phenomena and related structure-property relationships have been investigated by means of $\rm\sp{57}Fe$ Mossbauer spectroscopy on four natural metal oxides: galaxite, $\rm MnAl\sb2O\sb4;$ franklinite, $\rm ZnFe\sb2O\sb4;$ iwakiite, $\rm Mn\sb{1.1}Fe\sb{1.9}O\sb4,$ and ilvaite, $\rm CaFe\sb3Si\sb2O\sb8O(OH).$ These Mossbauer spectroscopic studies have been complemented by the following characterization and property measurement techniques: electron microprobe analysis, magnetic susceptibility and x-ray diffractometry. Galaxite from Bald Knob, North Carolina, has been found to contain $\rm Fe\sp{3+}$ ordered on the octahedral sites based on Mossbauer spectroscopic data, contrary to earlier studies reporting the presence of $\rm Fe\sp{2+}$ in galaxite. In the course of this investigation, it has also been discovered that a widely-advertised galaxite specimen from Bo Phloi, Thailand is, indeed, not galaxite but rather a lherzolite spinel. An unusual franklinite from Langban, Sweden, one of the first authenticated occurrences outside of Franklin, New Jersey, has been found to be very similar to stoichiometric zinc ferrite, having a high zinc content with exclusive ordering of $\rm Fe\sp{3+}$ on the octahedral site with no indication of $\rm Fe\sp{2+}.$ In addition, a magnetite phase, which is believed to be an exsolution product, also present in this franklinite specimen, was found to be similar to natural and synthetic magnetite on the basis of its Mossbauer parameters. There are indications of some foreign ions in solid solution but the purity level of the magnetite phase approximates that of high quality single phase magnetite single crystals. Clustering of manganese and iron in iwakiite has been confirmed by Mossbauer spectroscopic analysis of iwakiite samples heated at 1000 K far 17 and 77 hours. These heat treatments resulted in the formation of an iron rich phase $(\alpha$-hematite) and a disordered manganese rich phase. Investigations at 298 K of the electronic properties of ilvaite using Mossbauer spectroscopy have revealed a trend in the spectroscopic profiles with Mn content. Magnetic susceptibility measurements of ilvaite have indicated a negative correlation between manganese content and the magnetic ordering temperature of the $\rm Fe\sp{2+}$ M2 sites. The structure-property relationships in ilvaite have been found to be complex and no simple correlations have emerged from this investigation.