STRUCTURE OF GLASSES IN THE SYSTEMS MG2SIO4-FE2SIO4, MN2SIO4-FE2SIO4, MG2SIO4-CAMGSIO4, AND MN2SIO4-CAMNSIO4

We have measured polarized micro-Raman spectra for twelve orthosilicate glasses containing varying proportions of Ca2+, Fe2+, Mg2+, and Mn2+, and one FeOSiO2 composition slightly enriched in silica relative to fayalite. We have also obtained selected area electron diffraction data for glasses having the compositions of forsterite (Mg2SiO4), fayalite (Fe2SiO4), and tephroite (Mn2SiO4). Our Raman data indicate that: (1) CaMg olivine glasses all have similar structures consisting of isolated SiO4 tetrahedra with subordinate short-chain polymers; (2) addition of Ca2+ causes reduction in the effective SiO vibrational force constants; (3) in the absence of iron, a proportion of manganese occurs in tetrahedral MnO4 complexes which are not network formers; (4) silicon-oxygen bonds are shorter in glasses than in the isocompositional crystals; (5) significant polymerization occurs in fayalite-bearing glasses. The diffraction results for forsterite glass show evidence for cryptocrystalline periclase regions no larger than 40 Å, probably resulting from a melt free oxygen fraction which cannot be quenched to glass. Fayalite and tephroite glass diffraction patterns show no such evidence for a free oxygen fraction. Therefore, it is proposed that the high degree of polymerization of fayalite-bearing glass results from network-forming Fe2+ ions. © 1990.