In situ diffraction studies of magnesium silicate liquids

Primitive MgO–SiO2 liquids dominate the early history of the Earth and Terrestrial planets. The structures of these liquids and structure-dependent properties, such as viscosity and diffusion, are considered important in the evolution of these planets, however, MgO–SiO2 liquids are refractory and do not form glasses easily and it is difficult to measure the structure of these liquids. Container-less synthesis techniques have been used to produce glasses that range in composition from 50 to 33% SiO2, corresponding to the compositions of two important mantle minerals: enstatite and forsterite. The structure of these glasses has been determined using combined neutron and high-energy diffraction and show changes in the short-range order as a function of composition. These changes include a jump in Mg–O coordination number at the limit to the formation of the silicate network in forsterite composition glass. These results imply a similar change in the structure of the liquid. Accordingly, the structures of forsterite and enstatite liquids have been determined using high-energy X-rays and a specialized sample environment, a containerless levitator. The main qualitative structural differences between MgSiO3 and Mg2SiO4 glasses are also observed in the melt. Liquid MgSiO3 is interpreted as forming a relatively ‘strong’ network of SiO4 tetrahedra, whereas the Mg2SiO4 liquid is ‘‘fragile’’ and dominated MgOn (n = 4, 5, 6) polyhedra and highly mobile oxygen ions. The results differ significantly from previously reported X-ray diffraction data for liquid MgSiO3.