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Stoichiometry of synthetic ulvöspinel single crystals

Department of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden

Correspondence: ^* E-mail: ferdinando.bosi{at}uniroma1.it

Spinel single crystals (up to 4 mm in size) of eight compositions along the FeFe₂O₄-Fe₂TiO₄ solid-solution 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.

Key Words: Ulvöspinel • ilmenite • Mössbauer spectroscopy

This article has been cited by other articles:

				F. Bosi, U. Halenius, and H. Skogby Crystal chemistry of the magnetite-ulvospinel series American Mineralogist, January 1, 2009; 94(1): 181 - 189. [Abstract] [Full Text] [PDF]