Stoichiometry of synthetic ulvospinel single crystals

Spinel single crystals (up to 4 mm in size) of eight compositions along the FeFe(2)O(4)-Fe(2)TiO(4) solid-solution join, with more than 75 Mol% ulvospinel, were synthesized by use of a flux-growth method. The crystals were characterized by electron microprobe and Mossbauer spectroscopy. Results demonstrated that these ulvospinels are statistically stoichiometric. The atomic proportions of Ti(4+) and Fe(2+) progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the Fe(3+) content decreases from 0.49 to 0. 13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2Fe(3+) <-> Fe(2+) + Ti(4+). An average equilibrium temperature from coexisting spinel-ilmenite pairs of about 950 degrees C was estimated using the QUILF95 and Ghiorso-Sack models. In contrast to previous Studies based on non-stoichiometric samples sintered at higher temperatures, the present stoichiometric ulvospinel samples were grown from a melt under moderate temperatures. This fact indicates that the formation of vacancies is related to high-equilibration temperatures or growth mechanisms, whereas natural ulvospinel may very well be stoichiometric under normal magmatic conditions.