Effects of non-stoichiometry on the spinel structure at high pressure: Implications for Earth’s mantle mineralogy

Abstract

A non-stoichiometric sample of spinel with composition T(Mg 0.4Al 0.6) M(Al 1.8□ 0.2)O 4 was investigated by single-crystal X-ray diffraction in situ up to about 8.7 GPa using a diamond anvil cell. The P( V) data were fitted using a third-order Birch–Murnaghan equation of state and the unit-cell volume V 0, the bulk modulus K T0 and its first pressure derivative K′ were refined simultaneously providing the following coefficients: V 0 = 510.34(6) Å 3, K T0 = 171(2) GPa, K′ = 7.3(6). This K T0 value represents the lowest ever found for spinel crystal structures. Comparing our data with a stoichiometric and natural MgAl 2O 4 (pure composition) we observe a decrease in K T0 by about 11.5% and a strong increase in K′ by about 33%. These results demonstrate how an excess of Al accompanied by the formation of significant cation vacancies at octahedral site strongly affects the thermodynamic properties of spinel structure. If we consider that the estimated mantle composition is characterized by 3–5% of Al 2O 3 this could imply an Mg/Al substitution with possible formation of cation vacancies. The results of our study indicate that geodynamic models should take into account the potential effect of Mg/Al substitution on the incompressibility of the main mantle-forming minerals (olivine, wadsleyite, ringwoodite, Mg-perovskite).