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Crystals of sodium tetrasilicate (Na 2 Si 4 O 9 ) have been grown at 6 GPa and 1000 °C to 9 GPa and 1500 °C using the MA6/8 superpress at Edmonton, and the X-ray structure was determined at room pressure (R = 5.0%). High-pressure sodium tetrasilicate is monoclinic, with a = 10.875(2), b = 9.326(1), c = 19.224(7) Å, β= 90.18(2)°, space group P2 1 /n, and D x  = 3.090 g/cm 3 . Si occurs in both tetrahedral and octahedral coordination, with  [6] Si:  [4] Si= 1:3; the structural formula is Na 6 Si 3 [Si 9 O 27 ], Z= 4. Nine-memberedrings of SiO 4  tetrahedra are collapsed around and interconnected by SiO 6  octahedra, at shared comers, giving a framework structure that is analogous to but distinct from those of K 2 Si[Si 3 O 9 J (wadeite-type) and K 2 Ge[Ge 3 O 9 J(A 2 Ge 4 O 9 -type). Although all nine independent SiO 4  tetrahedra have similar nearest-neighbor stereochemistries,  [4] Si-O- [4] Si bond angles vary markedly (130.6-172.1°). The Na cations are displaced to one side of framework cavities in irregular polyhedra, as in other framework structures, with six to eight bonds extending to 2.9 A. It appears that even transition-zone pressures do not dominate the stereochemical requirements of the large cations in determining the structures of the alkali and alkaline-earth aluminosilicates.

Sodium tetrasilicate: A complex high-pressure framework silicate (Na 6 Si 3 [Si 9 O 27 ])