Fig. 2: SiO₂ reflectivity in the megabar regime.

The grey crosses show the error bars on our experimental data as one standard deviation of the distribution obtained via a Monte-Carlo method (see the Supplementary Notes 6 for details). Experimentally measured second-shock-front reflectivity of silica are shown at 1064 nm (coloured squares on the left of the grey crosses) and 532 nm (on the right) as a function of temperature and pressure. The reflectivity at 1064 nm and 532 nm (left and right circles) obtained from our calculations are also shown for comparison. Reflectivity data along the fused silica Hugoniot are shown at 532 nm¹⁶ (SESAME 7387 has been used for the shock-velocity-to-pressure relationship). The grey solid line is the α-quartz Hugoniot curve¹⁰. Colours of the curves along the α-quartz Hugoniot represent the reflectivity values at 532 nm¹⁰ (top) and 1064 nm³⁰ (bottom). Previous shock-velocity-to-pressure³¹ and shock-velocity-to-temperature¹⁰ relationships have been used. The stishovite Hugoniot curve has been calculated using previous data¹⁰, including reflectivity data at 532 nm. Different SiO₂ melting curve predictions^9,10,32 are also presented as solid-dashed purple, dash-dotted yellow and dash-double-dotted green lines.