Ilmenite-type solid solutions between MgSiO 3 and Al 2O 3 and some structural systematics among ilmenite compounds

Abstract

Complete solid solutions with the ilmenite structure from pure MgSiO 3 to 75% MgSiO 3·25% Al 2O 3 have been synthesized in the pressure region between 240 and 300 kbar at 1000–1400°C in a diamond-anvil cell coupled with laser heating. The results suggest that complete solid solutions with the ilmenite structure might be formed between MgSiO 3 and Al 2O 3 under high pressure-high temperature conditions. The lattice parameters for the ilmenite solid solutions between MgSiO 3 and Al 2O 3 deviate from ideality in the same manner as those found by Berry and Combs along the FeVO 3–Fe 2O 3 join. For the ordered A 2+ B 4+O 3 ilmenite-type compounds, c o is determined primarily by the size of the relatively large A 2+ cation, whereas a o depends strongly on the radii of both A 2+ and B 4+ cations. Such systematics might account for the fact that c o and a o for the ilmenite-type MgSiO 3 are respectively smaller and larger than those for Al 2O 3. The lattice parameters a o and molar volumes for the A 2 3+O 3 corundum-type compounds and the disordered A 2+B 4+O 3 ilmenites follow a different trend and are, therefore, readily distinguished from the ordered A 2+B 4+O 3 ilmenites.