Fig. 3.

Conditions for observations of different hydrous silica phases. (A) Pressure–temperature conditions for the observations of different phases. The error bars are the $1\sigma$ estimated uncertainties. The vertical lines show the pressures where new polymorphs begin to appear. The phase boundaries in the anhydrous ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ system (thin dashed lines) are shown for comparison (14, 15). (B) A provisional phase diagram of ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$–${\mathrm{H}}_2$O at 1400 to 1500 K (SI Appendix, section 6 has details). Although temperature is different, we also considered a data point with pure Ct from LHDAC (orange circle, $\sim$1700 K). The data point provides insights on temperature-dependent changes of the phase boundaries. d, anhydrous; h, hydrous.