Crystal structure of CaMg2Al6O12, a new Al-rich high pressure form

The crystal structure of CaMg2Al6O12, a new high-pressure phase synthesized at 21.8 GPa and 1200 degreesC using a multi-anvil apparatus, was solved by a newly developed structure program and refined by Rietveld analysis of th e powder X-ray diffraction profile. The structure is hexagonal with cell co nstants a = 8.7616(2) and c = 2.7850(1) Angstrom, and space group P6(3)/m. The structure of this phase contains double chains of edge shared AlO6 octa hedra running along the c axis. Three double chains share corners to form s ixfold positions in which octahedrally coordinated Mg atoms reside. The lar ge Ca atoms are randomly distributed at ninefold sites with half-occupancy in the hexagonal tunnel. Previously reported Al-rich silicate phases could possibly have the same structure. This structure could thus qualify as one of the possible major host phases for aluminum in the lower mantle.