Crystal chemistry of the magnetite-ulvospinel series

Spinet single crystals of 19 compositions along the magnetite-ulvospinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations. the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mossbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate. and tetrahedral bond length increase with increasing ulvospinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation. the Akimoto model (T)(Fe(1-X)(3+)Fe(X)(2+))(M)(Fe(2+)Fe(1-X)(3+)Ti(X))O(4) describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction (T)Fe(2+) + (M)Fe(3+) = (T)Fe(3+) + (M)Fe(2+), which causes (M)Fe(2+) not equal 1 and (T)Fe(2+)/Ti not equal 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti approximate to 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the (T)Fe(3+) + (M)Fe(3+) = (T)Fe(2+) + (M)Ti(4+) chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvospinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.