Cation distributions and related order-disorder phenomena in the naturally occurring Mn-spinels: iwakiite, galaxite and franklinite, have been investigated by means of 57Fe Mrssbauer spectroscopy. Previous measurements on an iwakiite sample indicated the existence of Mn-rich and Mn-poor regions. This interpretation has been confirmed in this investigation by further measurements on annealed iwakiite samples. The 57Fe Mrssbauer spectrum of galaxite from Bald Knob, NC, is a well-resolved quadrupole doublet and indicates the presence of a single crystallographic Fe 3+ species. A "galaxite" sample from Thailand exhibited a complex spectrum of Fe 2+ and Fe 3+ quadrupole doublets: This sample has been misidentified and should be regarded as a member of the (Mg, Fe)(AI, Fe)204 series. The 57Fe hyperfine parameters of a rare franklinite from L~gban, Sweden, are very close to those for synthetic zinc ferrite, confirming electron microprobe results of an unusually high zinc content.

An unusually pure Mnx Fe3−x O4 sample (x≂1.3) from a regionally metamorphosed ore body is expected to exhibit an approach to thermodynamic equilibrium unattainable in synthetic materials. This expectation is confirmed by the observation of (1) a small tetragonal distortion and (2) a complete ordering of Fe on the B sublattice. 57Fe Mössbauer spectra, obtained at 298, 175, 125, and 92 K and in an external field of 30 kG at 92 K, exhibit magnetic hyperfine splitting requiring the fitting of two subspectra, I and II. At 298 K, Heff =418 kOe, α=0.36 mm s−1 , and ε≊0 for the more intense subspectrum I, whereas for II Heff =403 kOe, δ=0.34 mm s−1 , and ε≂−0.15 mm s−1 . The two subspectra are not split by a 30 kG external field. Subspectra I and II correspond to Fe3+ ions located on a single magnetic sublattice. Subspectrum II is assigned to Fe3+ ions in regions with locally, large tetragonal distortions as a consequence of Mn3+clustering, which has often been invoked to explain the crystal chemistry and physical properties of Mnx Fe3−x O4 .

Naturally occurring samples of iwakiite, Mrtl.* Fe,,vOI, have been annealed for 17 and 77 h at 1000 K in an investigation of order-disorder phenomena in Mn-Fe oxide spinels. TWO distinct magnetic hyperfine patterns, which correspond to Fe-and Mn-rich phases, were observed in the 57Fe Mijssbauer spectra for the two annealed samples. While the spectrum of the Fe-rich phase corresponds to a magnetically and crystallographically highly ordered cY-Fe,O,-like phase, the Mn-rich phase exhibits considerable disorder, as indicated by very broad linewidths and a low value for Neff. Annealing for longer periods of times leads to a considerable increase in the ordering of the Mn-rich spine1 phase, as evidenced by the considerable narrowing of the lines, from re2.40 mms-' to I'= 1.17 mm s-', and an increase. in HErf from 318 to 417 kOe at 29s K. The Fe-rich, cw-Fe20,-like phase is not affected by further annealing, as expected. At 85 K, the "'Fe Miissbauer spectrum of the Mn-rich. spine1 phase continues to exhibit rather broad lines, indicating the presence of significant cation disorder.

Ilvaite, a mixed-valence iron silicate, rivals magnetite in the complexity of the phenomena exhibited in connection with the simultaneous occurrence of Fe2+ and Fe3+ on equivalent sites. Inasmuch as the effects of impurities on the several phase transitions and electron delocalization have been explored little, if at all, an investigation of the effect of manganese impurities by means of measurements on a large suite of samples has been undertaken. Significant variations in magnetic properties are observed even for low impurity levels. For a Mn2+ content of 0.5 wt %, the low temperature magnetic phase transition is at ∼40 K, for a Mn2+ content of 1.2%, the transition temperature is less than 20 K and for a Mn2+ content of 4.5%, the transition appears to be well below 10 K. In addition to the quantitative variations, the 1/χ versus T curves are qualitatively different for samples with different concentrations of manganese. The 57Fe Mössbauer spectra are also qualitatively different at 298 K with pronounced differences in the resolution of the two Fe2+ patterns and in their relative intensities. In agreement with earlier reports, no evidence for magnetic ordering at 120 K has been observed in the susceptibility curves; the ordering is, however, exhibited in the 57Fe Mössbauer spectra obtained at 82 K. The exploration of the effect of Mn2+ impurities has permitted rare insights to be gained on the relationship between the bulk and local, site-specific properties of ilvaite. Further progress is expected to result from electrical conductivity and x-ray diffraction measurements.