A molecular dynamics study of densification mechanisms in calcium silicate glasses CaSi2O5 and CaSiO3 at pressures of 5 and 10 GPa

Molecular dynamics is used to obtain models of (CaO)x (Si O2) 1-x glasses, with compositions Ca Si2 O5 (x=0.33) and CaSi O3 (x=0.50), at pressures of 5 and 10 GPa. At 5 GPa there are increases in Ca and Si coordinations for x=0.33, whereas for x=0.50 there is distortion of Ca ON polyhedra but no substantial change in coordination. At 10 GPa the Ca coordination increases by ?20% for x=0.33 and by ?10% for x=0.50. This increase is due to increased Ca bonds to bridging oxygens (Ob), since nonbridging oxygens (Onb) are already highly bonded to Ca, and the proportion of Onb is decreasing due to changes in the silica network. At 10 GPa there are ?20% of fivefold and a few percent of sixfold coordinated Si. Since the new Si-O bonds involve the conversion of Onb to Ob, there is a corresponding increase in the network connectivity. The x=0.50 glass is more resistant to deformation because there is less possibility to convert Onb to Ob due to lower Si content. The changes in Ca-O, Si-Ca, and Ca-Ca correlations are predicted to produce changes in the x-ray diffraction structure factor S (Q), including a shift of the first sharp diffraction peak to higher Q value