Effect of pressure on the carbon speciation in silicate glasses and melts: Insights from multi-nuclear solid-state NMR

Abstract:

The pressure-induced structural changes in carbon-bearing silicate glasses and melts is essential to understand the changes in melt properties in the Earth interior and yield atomistic insights into the deep carbon cycle. Despite the extensive pioneering studies on carbon-bearing silicate glasses, spectroscopic and scattering studies at high pressure above ~4 GPa is limited due to the lack of suitable experimental probes. Here, we report the pressure-induced structural changes around C, Si and Al in albite and Na-trisilicate (Na₂O:SiO₂=1:3, NS3) glasses with varying pressure up to 8 GPa, using ²⁷Al, ²⁹Si and ¹³C solid-state high-resolution NMR. ²⁷Al 3QMAS NMR spectra for carbon-bearing albite glasses quenched from melts at high pressure up to 6 GPa show only ^[4]Al environments. The FWHM of ^[4]Al in albite glasses increases with increasing pressure, indicating that the overall densification of albite glasses at high pressure is accompanied by an increase in the topological disorder around Al. ²⁹Si MAS NMR spectra for NS3 glasses at high pressure up to 8 GPa show the presence of highly coordinated Si, ^[5,6]Si, which contributes to an increases in the total configurational disorder in the NS3 glasses with pressure. ¹³C MAS NMR spectra for carbon-bearing albite glasses show the presence of dominant fraction of CO₂, and minor amounts of CO₃^2-, and CO. At least three distinct carbonate species, such as ^[4]Si(CO₃)^[4]Si, ^[4]Si(CO₃)^[4]Al, and CO₃^2- were observed. Among those species, the increase in the fraction of ^[4]Si(CO₃)^[4]Al species is most prevalent. ¹³C MAS NMR spectra for NS3 glasses show the presence of carbonate species. The peaks position of the carbonate species shifts to lower frequency upon compression, suggesting the pressure-induced structural distortion of CO₃^2- in the glasses above 6 GPa. Spin-lattice (T₁) relaxation time for molecular CO₂ in carbon-bearing albite glasses increases with increasing pressure. T₁ relaxation time for CO₂ species at 6 GPa is 3.5 times longer than that at 1.5 GPa. The corrected solubility of carbon in albite glasses with T₁ relaxation increases linearly with increasing pressure. The current experimental results can shed light on experimental opportunity to provide the structural details of carbon in the Earth’s interior.