Compositional dependence of elastic properties and density of glasses in the system anorthite-diopside-forsterite

The compressional (v_p) and shear (v_S) wave velocities of 20 glasses in the pseudoternary system anorthite (An)-diopside (Di)-forsterite (Fo) were measured by Brillouin spectroscopy as a means of constraining the effect of composition on the elasticity of glasses. The velocity data together with measured densities were used to calculate the elastic properties: Young's modulus (E), adiabatic bulk modulus (K_S), shear modulus (G), and Poisson's ratio (σ). The data show that different chemical constituents affect the density, velocities and elastic properties in a highly systematic way. All of the properties we examined are well described by ideal mixing of molar properties. The addition of MgO strongly increases the bulk, Young's and shear moduli. The results are compared to different models describing the elastic properties of glasses as a function of chemical composition. We show that the influence of magnesia is underestimated in previous models, in some cases due to a lack of appropriate experimental data. The relatively large values of elastic moduli for magnesia-rich glasses can be explained by the high bonding strength between magnesium and SiO₄ and AlO₄ tetrahedra. The strongest effect of magnesium is on the bulk modulus; the shear modulus is less affected. Therefore, Poisson's ratio shows a modest increase with increasing MgO content. The observed increase of Poisson's ratio with increasing MgO content may be explained by a reduced bond strength between tetrahedra due to the addition of MgO.