Order and disorder in sodium silicate glasses and melts at 10 GPa

We report experimental evidence, in conjunction with quantum chemical calculation, supporting chemical ordering among network polyhedra (e.g. Si-[5] and Si-[6]) in sodium silicate glasses quenched from melts at 10 GPa by using O-17 3QMAS (triple quantum magic angle spinning) NMR, which resolves new oxygen sites at high pressure, including Si-[5,6]-O-Si[4] and Na-O-Si[5,6]. The distribution of network polyhedra at 10 GPa, a controlling factor for transport and melting processes in the mantle, is not completely random but favors formation of oxygen linking dissimilar Si pairs such as Si-[5,6]-O-Si-[4]. The presence of non-bridging oxygens (NBOs) in highly coordinated units at 10 GPa, Na-O-Si[5,6], indicates that the formation of highly coordinated Si can also occur at initially depolymerized silicon tetrahedra such as Q(3) or Q(2). The methods and results given here yield improved prospects for understanding melt structures at high pressure and the atomistic origin of magmatic processes in the Earth's interior.