Electronic bonding transition in compressed<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>SiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>glass

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near $K$-edge spectra of ${\mathrm{SiO}}_{2}$ glass to $51\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ are consistent with those of quartz and coesite, whereas the spectra above $\ensuremath{\sim}22\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in ${\mathrm{SiO}}_{2}$ glass between $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $22\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed ${\mathrm{SiO}}_{2}$ glass.