Coordination change of Ge⁴⁺ and Ga³⁺ in silicate melt with pressure

The coordination environments of ∼2000 ppm Ge⁴⁺ and Ga³⁺ in silicate glasses of three compositions based on the CaO-MgO-Al₂O₃-SiO₂ (CMAS) system were determined as a function of pressure by X-ray absorption near edge structure (XANES) spectroscopy. Glasses of 11 additional compositions were examined to check for any effect of composition on coordination at low pressure (1 atm to 1 GPa). Glasses were synthesised at pressures from 1 atm to 10 GPa using either a melt-quench method or glass-annealing method (annealing near the glass transition temperature, ∼780 °C). The glasses also contained H₂O and CO₂, which were quantified by attenuated total reflectance infrared spectroscopy, using an estimated density-normalised calibration factor for CO₂. While changes in average coordination number are sensitively monitored by the changes in the XANES spectra, quantification of these changes requires assuming that the XANES spectra of crystalline substances of known structure provide analogues for the glasses. Linear combination fits of the spectra using the crystalline analogues indicate that the average coordination numbers of Ge⁴⁺ and Ga³⁺ increase from 4 at ambient pressure to approximately 5 at 10 GPa. Ge⁴⁺ and Ga³⁺ have higher average coordination numbers in annealed glasses compared to those from quenched melts prepared at the same pressure up to 4 GPa, but at 5–6 GPa quenched melts and annealed glasses record similar coordination numbers. Coordination changes occur at lower pressures in more depolymerised melt compositions, and are therefore relevant to natural melts of the mantle. The partition coefficients of Ge and Ga between melts and coexisting minerals are likely to be affected by such coordination changes.