Figure 4. Molecular volume of various silica phases as a function of pressure.

They are calculated according to experimental equations of state given in refs ⁹, ²³, ⁵⁸ or implementing density-functional theory (DFT) with local-density approximation (LDA), starting from known structures⁵⁹,⁶⁰. Cristobalite X-I appears to be denser even than seifertite above about 100 GPa (experimental values). The theoretical structure of seifertite is about 3.5% denser than the experimental one, a characteristic limitation of the LDA functional. According to LDA, alpha-cristobalite would be the densest phase above ca. 50 GPa, but it has never been observed experimentally above 15 GPa. Generalized-Gradient approximation (GGA) calculations for seifertite and the two cristobalite X-I ordered phases are represented in the inset. See Supplementary Fig. 8 for E–V curves of other phases.