Evidence for anomalously large degree of polymerization in Mg2SiO4 glass and melt

Ab-initio molecular dynamics simulation of forsterite (Mg2SiO4) melt at 2273 K shows the presence of nearly 40% of the Si atoms as (Si2O7)(6-) dimers. This result is directly corroborated by the Si-29 nuclear magnetic resonance spectrum of bulk Mg2SiO4 glass, prepared by container-less levitation techniques. The presence of a large excess of bridging O atoms associated with the (Si2O7)(6-) dimers in forsterite glass and melt is in sharp contrast with their complete absence in crystalline forsterite. Such structural differences between the crystal and the melt can have important implications in understanding the dynamics of crystallization and segregation in a primordial magma ocean and the continuing chemical differentiation of the Earth.