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

Abstract

Mössbauer spectra of glasses of NaFeSi 3O 8 and 3NaAlSi 2O 6 · NaFeSiO 4 starting compositions consist of a dominant Fe 3+ and subordinate Fe 2+ quadrupole-split doublet, in agreement with previous work. Fe 3+ 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 Fe 3+ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO 2, NaGaSi 3O 8 and NaGaSiO 4 glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge 4+ in GeO 2 from 1 bar to 14 kbar, of Ga 3+ in NaGaSi 3O 8 from 1 bar to 28 kbar and of Ga 3+ in NaGaSiO 4 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 NaGaSiO 4 glasses and crystalline NaGaSiO 4 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 Ga 3+ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga 3+ is four-coordinated with oxygen in all three samples with a Ga 3+-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 NaGaSiO 4 (nepheline-type structure) and vitreous NaGaSiO 4 beyond the first coordination shell of Ga 3+. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga 3+ in NaGaSiO 4 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 Al 3+ and Si 4+. 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.