ELASTICITY AND STRUCTURE OF NA257FESI3O8 GLASS TO 14 GPA BY BRILLOUIN AND RAMAN SPECTROSCOPIES

Structure and compressibility of silicate melts are important in understanding the thermodynamic and fluid dynamic properties of magmatic systems in the Earth’s interior. Elastic and structural properties of silicate glasses (analogs of melts) and their pressure dependences are therefore important for understanding the structure and compressibility of silicate melts. Although iron and sodium oxides are major magma components, only a few studies have been conducted on Fe-bearing sodium silicate glasses. Fe³⁺ is the most abundant redox state of iron in the majority of oxide glasses. Its presence in melts affects crystalline nucleation, as well as their rheological and thermodynamic properties.

We have carried out Brillouin and Raman spectroscopic studies on Na₂⁵⁷FeSi₃O₈ glass (analog of aegirine crystal) with 95% enrichment of iron in the ⁵⁷Fe isotope up to 12 and 14 GPa, respectively. In contrast to the compressive wave velocity (V_P), the shear wave velocity (V_S) anomalously decreases with pressure, reaching a minima at 2.3 GPa, and then increases. Density and the bulk, shear, and Young’s moduli and Poisson’s ratio for this glass as a function of pressure up to 12 GPa are presented. The best polynomial fit of the density versus pressure data yields: r = 2.71642 + 0.06202´P -0.00153´P². The measured bulk modulus K and its pressure derivative are obtained as K = 41.5 ± 0.6 GPa, K_o¢ = 5.9 ± 0.2. Shear and Young’s moduli, and Poisson’s ratio show a marked modification around 2.3 GPa. These results are discussed in light of the structural changes inferred from the Raman data.