A non-stoichiometric sample of spinel with composition T(Mg0.4Al0.6)M(Al1.8h0.2)O4 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 V0, the bulk modulus KT0 and its first pressure derivative K0
were refined simultaneously providing the following coefficients: V0 = 510.34(6) A ̊ 3, KT0 = 171(2) GPa, K0 = 7.3(6). This
KT0 value represents the lowest ever found for spinel crystal structures. Comparing our data with a stoichiometric and natural
MgAl2O4 (pure composition) we observe a decrease in KT0 by about 11.5% and a strong increase in K0 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 Al2O3 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).
