Stoichiometry of synthetic ulvöspinel single crystals.

Spinel single crystals (up to 4 mm in size) of eight compositions along the FeFe2O4-Fe2TiO4 solidsolution join, with more than 75 mol% ulvöspinel, were synthesized by use of a flux-growth method. The crystals were characterized by electron microprobe and Mössbauer spectroscopy. Results demonstrated that these ulvöspinels are statistically stoichiometric. The atomic proportions of Ti4+ and Fe2+ progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the Fe3+ content decreases from 0.49 to 0.13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2Fe3+ ↔ Fe2+ + Ti4+. An average equilibrium temperature from coexisting spinel-ilmenite pairs of about 950 °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 ulvöspinel 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 ulvöspinel may very well be stoichiometric under normal magmatic conditions. [ABSTRACT FROM AUTHOR]