A HIGH-PRESSURE FORM OF AL2SIO5 AS A POSSIBLE HOST OF ALUMINUM IN THE LOWER MANTLE

ALTHOUGH iron, magnesium and calcium silicates are the principal components of the lower mantle, the significant amount of aluminium present must also be included in models of mantle geochemistry 1-4. The host mineral for aluminium and other trivalent ions is still open to debate. Candidates include a high-pressure form of MgAl2O4 (ref. 5), a form of (Ca,Mg)Al2Si2O8 with the hollandite structure 6 and Ca2AlSiO5.5 with a rhombohedral perovskite structure 7. Here we describe a new candidate formed by transforming CaAl2Si2O8 anorthite, Ca3Al2Si3O12 grossular and Mg3Al2Si3O12 pyrope in a diamond anvil cell at 2,500 K and 40-70 GPa. Analytical transmission electron microscopy reveals the formation of a new high-pressure phase of Al2SiO5, for which electron diffraction patterns show strong similarities to the V3O5 structure. We estimate that this phase could be present in the lower mantle in a volumetric proportion of about 5%.