L. Gautron et al., HEXAGONAL BA-FERRITE - A GOOD MODEL FOR THE CRYSTAL-STRUCTURE OF A NEW HIGH-PRESSURE PHASE CAAL4SI2O11, Physics of the earth and planetary interiors, 102(3-4), 1997, pp. 223-229
A new calcium aluminosilicate phase of composition CaAl4Si2O11 has bee
n encountered amongst the transformation products of CaAl2Si2O8 (anort
hite composition) at 14 GPa (Gautron et al., 1996). X-ray diffraction
(XRD) confirms that its crystal structure is essentially the same as t
hat of a new complex Ca-Al-silicate (abbreviated CAS phase) first repo
rted by Irifune et al. (1994). The crystal structure of the CAS phase
has been investigated by transmission electron microscopy (TEM). It ha
s 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), re
asonable for a phase stable at transition-zone pressures. The most pla
usible model for the structure of this phase arises from published ref
inements of hexagonal Ba-ferrites. This postulated P6(3)/mmc structure
consists of octahedral layers, 3/4 occupied, separated by 12-coordina
te 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 coordi
nated. Observed TEM and XRD data are compared with calculated reflecti
on intensities for this CAS model. (C) 1997 Elsevier Science B.V.