Hexagonal Ba-ferrite: a good model for the crystal structure of a new high-pressure phase CaAl4Si2O11?

A new calcium aluminosilicate phase of composition CaAl4Si2O11 has been encountered amongst the transformation products of CaAl2Si2O8 (anorthite composition) at 14 GPa (Gautron et al., 1996). X-ray diffraction (XRD) confirms that its crystal structure is essentially the same as that of a new complex Ca-Al-silicate (abbreviated CAS phase) first reported by Irifune et al. (1994). The crystal structure of the CAS phase has been investigated by transmission electron microscopy (TEM). It has a hexagonal unit cell with lattice parameters a = 5.4 Angstrom and c = 12.7 Angstrom, and its space group is either P6(3)mc, <P(6)over bar 2c> or P6(3)/mmc. It is proposed that this CAS phase has a six-layer, close-packed structure so that Z = 2 and density is 3.94 g cm(-3), reasonable for a phase stable at transition-zone pressures. The most plausible model for the structure of this phase arises from published refinements of hexagonal Ba-ferrites. This postulated P6(3)/mmc structure consists of octahedral layers, 3/4 occupied, separated by 12-coordinate Ca atoms, and by Al and Si in face-shared octahedra and in complex trigonal bipyramidal polyhedra, i.e. some Si would be five-fold coordinated. Observed TEM and XRD data are compared with calculated reflection intensities for this CAS model. (C) 1997 Elsevier Science B.V.