Coordination of Fe,Ga and Ge in high pressure glasses by Mössbauer,Raman and X-ray absorption spectroscopy,and geological implications

Mössbauer spectra of glasses of NaFeSi₃O₈ and 3NaAlSi₂O₆ · NaFeSiO₄ starting compositions consist of a dominant Fe³⁺ and subordinate Fe²⁺ quadrupole-split doublet, in agreement with previous work. Fe³⁺ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe³⁺ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO₂, NaGaSi₃O₈ and NaGaSiO₄ glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge⁴⁺ in GeO₂ from 1 bar to 14 kbar, of Ga³⁺ in NaGaSi₃O₈ from 1 bar to 28 kbar and of Ga³⁺ in NaGaSiO₄ from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses.XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO₄ glasses and crystalline NaGaSiO₄ were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga³⁺ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga³⁺ is four-coordinated with oxygen in all three samples with a Ga³⁺-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO₄ (nepheline-type structure) and vitreous NaGaSiO₄ beyond the first coordination shell of Ga³⁺. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga³⁺ in NaGaSiO₄ glasses with pressures up to 25 kbar.Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al³⁺ and Si⁴⁺. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.