Synthesis and crystal structure of LiNbO3-type Mg3Al2Si3O12: A possible indicator of shock conditions of meteorites

Abstract

LiNbO 3 -type Mg 2.98(2) Al 1.99(2) Si 3.02(2) O 12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) A, c = 12.6885(8) A, V = 255.23(3) A 3 , with Z = 6 (hexagonal, R 3 c ). The average A -O and B -O distances of the A O 6 and B O 6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A - and B -site cations and oxygen (2.073 and 1.833 A, respectively). The present compound has the B -site cations at the octahedral site largely shifted along the c axis compared with other LiNbO 3 -type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B -site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of B O 6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A -site cation and oxygen during decompression. This also explains why there is no back-transition from the Pv-structure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K.