Stoichiometry of synthetic ulvoespinel single crystals

Spinel single crystals (up to 4 mm in size) of eight compositions along the FeFe (sub 2) O (sub 4) -Fe (sub 2) TiO (sub 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 (super 4+) and Fe (super 2+) progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the Fe (super 3+) content decreases from 0.49 to 0.13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2Fe (super 3+) <-->Fe (super 2+) +Ti (super 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.