A NEW HIGH-PRESSURE FORM OF MGAL2O4

MAGNESIUM aluminium spinel (MgAl2O4) is a common constituent of low-pressure peridotite xenoliths, and is an important host mineral for aluminium and other trivalent cations in the shallow upper mantle. Shock-wave compression data for MgAl2O4 demonstrated 1 that spinel would transform to denser phases at pressures greater than 40 GPa, although there was much uncertainty in the pressure estimation. Later, spinel was found to transform under static compression to the denser oxide mixture (MgO periclase + Al2O3 corundum 2,3) at pressures above 15 GPa (ref. 4). Detailed analyses of the shock-compression data, however, suggested the presence of a still denser phase of MgAl2O4 spinel at much higher pressures 5,6. Here we report the transformation of MgAl2O4 spinel to a new high-pressure form at pressures above 25 GPa in a multi-anvil high-pressure apparatus. The new MgAl2O4 phase has a structure similar to that of CaFe2O4 (calcium ferrite) and its zero-pressure density is 3.937(3) g cm-3, which is approximately 2% denser than the lower-pressure assemblage of periclase + corundum. We suggest that this high-pressure form of MgAl2O4 may be an important host of aluminium in the Earth's lower mantle.