An X-ray powder diffraction study of synthetic (Fe,Mn)2TiO4 spinel
Sedler, Ingo K.; Feenstra, Anne; Peters, Tjerk
European Journal of Mineralogy Volume 6 Number 6 (1994), p. 873 - 886
published: Nov 30, 1994
manuscript accepted: Jun 27, 1994
manuscript received: Apr 2, 1993
ArtNo. ESP147050606006, Price: 29.00 €
Abstract The (Fe,Mn)2TiO4 spinel solid solution was synthesized in steps of 10 mole % at 950°C, 1 bar total pressure and log fO2 of -16.30. Electron microprobe analysis indicates that all synthesized Fe-Mn titanate spinels are homogeneous but contain an additional small magnetite component in solid solution. The magnetite content is 6.8 mole % for ulvospinel and decreases with increasing manganese. As stoichiometric (Fe,Mn)2TiO4 oxide mixtures were used as starting material, additional Fe-Mn ilmenite occurs in the run products as a result of the Fe3O4 solution in spinel. Powder X-ray diffraction data, collected in the range 15-160° 2θ with CuKα radiation, indicate that the solid solution is continuous at 950°C with all members having the cubic crystal structure (space group Fd3m). Unit cell dimensions, oxygen positional factors, and isotropic displacement factors for Fe-Mn ulvospinel members were refined using the Rietveld method. The unit cell dimension increases linearly from 8.53374(8) Å for ulvospinel to 8.68063(7) Å for the Mn-endmember. The degree of inversion was estimated from X-ray powder data in three different ways, of which the method of optimizing the RB factor in the Rietveld refinement procedure is considered as the most accurate one. The results indicate that the whole Fe-Mn ulvospinel solid solution is 82-92 % inverse at 950°C. A change in degree of inversion with composition is not obvious from the data. The oxygen parameter u ranges from 0.2601(2) to 0.2615(2) within the solid solution. Calculated tetrahedral- oxygen bond lengths increase from 1.999(3) A for Fe2TiO4 to 2.046(3) A for Mn2TiO4, whereas the octahedral-oxygen bond lengths increase from 2.050(2) Å to 2.079(2) Å with XMn. Estimated effective ionic radii of Fe2+ and Mn2+ in spinel are slightly larger in IV-fold coordination and slightly smaller in Vl-fold coordination than values reported in previous studies.