Fig. 5: Schematic depiction of the orientation of the (0001) plane of the metastable high-pressure phase with respect to the quartz crystal with its a and c axes.

The orientation of this plane has been inferred from the indexing of diffraction peaks shown in the same colors in Fig. 1c (see also Supplementary Fig. 3). The exact orientation relationships between the rosiaite-structured high-pressure phase (Ros) and quartz (Qtz) are: (0001)Ros // (10\(\bar{1}\)0)Qtz and (10\(\bar{1}\)0)Ros // (0001)Qtz, (0001)Ros // (0001)Qtz and (10\(\bar{1}\)0)Ros // (10\(\bar{1}\)0)Qtz, (0001)Ros // (30\(\bar{3}\)2)Qtz and (11\(\bar{2}\)0)Ros // (11\(\bar{2}\)0)Qtz, and (0001)Ros // (30\(\bar{3}\)1)Qtz and (11\(\bar{2}\)0)Ros // (11\(\bar{2}\)0)Qtz.