A new high-pressure form of MgAl₂O₄

Abstract

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 demonstrated¹ 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 + A1₂O₃ 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 ∼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.