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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) Å,  c  = 12.6885(8) Å,  V  = 255.23(3) Å 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 Å, 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.

Synthesis and crystal structure of LiNbO 3 -type Mg 3 Al 2 Si 3 O 12 : A possible indicator of shock conditions of meteorites