High pressure rhombohedral perovskite phase Ca2AlSiO5.5 |
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| Authors: | J D Fitz Gerald A E Ringwood |
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| Institution: | 1. Research School Earth Sciences, Australian National University, G.P.O. Box 4, Canberra, Australia
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| Abstract: | A new high pressure phase with the composition Ca2AlSiO5.5 has been synthesized using an MA-8 apparatus operating at 1700° C and 16 GPa. The phase possesses a structure analagous to CaSiO3 perovskite but with half the Si atoms replaced by Al, and charge balance provided by vacancies in the oxygen sub-lattice. The unit cell possesses a lattice parameter, ao = 3.706 ± 0.003 Å (room P and T value), based on a simple cubic perovskite structure. However, electron diffraction shows that a superstructure has developed parallel to one of the {111} cubic planes with a wavelength of 10.70 Å (equals 5 × ¦d111¦), so that the Ca2AlSiO5.5 cell must be described formally as rhombohedral with a = 11.12 Å and  = 27.27 degrees. This rhombohedral cell is metrically cubic, since the distortion of the cubic cell is not determinable from X-ray diffraction patterns obtained so far. The calculated density of this high pressure phase Ca2AlSiO5.5 is 3.64 gm·cm-3. This low density is related in part to the large proportion of oxygen vacancies present in the structure. Because of the low density, this phase is unlikely to be a significant mineralogical constituent of the lower mantle, unless the phase is characterized by extreme compressibility. However, the identification of the phase may be of significance in showing how A12O3 can be accommodated in silicate perovskite via replacement of SiVI in octahedral sites accompanied by production of one oxygen defect for every 2 Al atoms substituted. The possibility that this mode of substitution might be relevant to the incorporation of Al2O3 in MgSiO3 perovskite warrants further study. |
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