The coordination environment is a central issue in condensed matter physics and planetary science, as it governs structural diversity and physical properties under extreme conditions. In particular, hypercoordinate states of silicon—one of the most abundant elements in the universe—are key to understanding both the mineralogy of planetary interiors and the limits of chemical bonding theory beyond the octet rule. Here, we extend the concept of “chemical pressure,” originally developed in materials science, to planetary environments by investigating the interaction between $\mathrm{C}{\mathrm{O}}_2$ and $\mathrm{Si}{\mathrm{O}}_2$. Our first-principles calculations reveal that $\mathrm{C}{\mathrm{O}}_2$ can chemically compress $\mathrm{Si}{\mathrm{O}}_2$ and stabilize a novel eightfold-coordinated silicon carbonate, $\mathrm{Si}\mathrm{C}{\mathrm{O}}_4$, in the pressure range of 560–780 GPa. This stabilization occurs nearly 100 GPa earlier than in pure $\mathrm{Si}{\mathrm{O}}_2$ (∼650 GPa), representing a recombination of $\mathrm{Si}{\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ at megabar conditions after their decomposition above 26 GPa. As major constituents of planetary interiors, such Si-C-O compounds may influence carbon sequestration, migration, and release within planets. Moreover, $\mathrm{Si}\mathrm{C}{\mathrm{O}}_4$ exhibits lower density and higher thermal conductivity than $\mathrm{Si}{\mathrm{O}}_2$, suggesting enhanced heat transport and accelerated cooling processes in planetary interiors. These findings enrich the high-pressure phase diagram of the Si-C-O system and provide insights into planetary evolution and the fundamental limits of bonding under extreme compression.

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• High-pressure studies
• Crystal structures
• Minerals
• Density functional theory