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For MgSiO₃ pPv only two studies report differential stress as a function of pressure (Merkel et al., 2007; Miyagi et al., 2010). A third high‐temperature study reported a differential stress of 0.25–1.5 GPa at a pressure range of 130–156 GPa and 2500–3000 K (Wu et al., 2017), however, in this study the MgSiO₃ pPv samples were sandwiched between layers of either NaCl or KCl and so it is likely that the measured stress levels reflect the flow strength of the NaCl or KCl pressure medium rather than that of MgSiO₃ pPv.

There is a large pressure gap between the highest‐pressure measurements on Brg at ~60 GPa and strength measurements on MgSiO₃ pPv at pressures > 130 GPa (Figure 2.3). The two measurements on MgSiO₃ pPv are similar though those of Merkel et al. (2007) (Figure 2.3, blue open squares) are higher than those of Miyagi et al. (2010) (Figure 2.3, green open triangles). For these measurements, Miyagi et al. (2010) used a moment pole stress model and as discussed above the small error bars (smaller than the symbols) only represent fitting errors. In contrast, the study of Merkel et al. (2007) has small errors (smaller than the symbols) but uses the method of Singh et al. (1998). The small error bars in this study are due to that fact that there are only small variations in stresses calculated on different lattice planes.

Differences in differential stress measurements between Miyagi et al. (2010) and Merkel et al. (2007) could be due to composition. A natural enstatite was used for synthesis in Merkel et al. (2007), whereas a synthetic MgSiO₃ glass was used as starting material in Miyagi et al. (2010). Another possibility is grain size. Visual comparison of raw diffraction images in these two studies shows spottier diffraction rings in Miyagi et al. (2010), which qualitatively indicates larger grain size, and may result in lower flow strength. Interestingly, if room temperature flow strength measurements on Brg are compared to pPv, the highest value measured in Brg is 11.9 GPa at a pressure of 52 GPa (Miyagi & Wenk, 2016). In pPv the highest differential stress value is 11.1 GPa at 177GPa (Miyagi et al., 2010). If one assumes that pressure increases the flow strength of Brg (as would be expected), then at room temperature pPv may be significantly weaker than Brg. A theoretical study by Ammann et al. (2010) suggests that high‐temperature strength of MgSiO₃ pPv may be much lower than Brg based on calculated diffusion coefficients. Likewise, experimental work on pPv analogs found that during high temperature deformation the pPv phase is ~5–10 time weaker than the perovskite (Pv) phase (Dobson et al., 2012; Hunt et al., 2009). If room temperature flow strength is lower and if diffusion is faster in MgSiO₃ pPv, there is a high likelihood that Brg is significantly stronger than MgSiO₃ pPv over a range of pressure and temperature conditions. If the strength contrast between Brg and pPv is as large as the analog studies indicate then pPv may be similar or even weaker than Fp.