An Experimental Investigation of the Relative Strength of the Silica Polymorphs Quartz, Coesite, and Stishovite

In this study, quartz, coesite, and stishovite were deformed concurrently with an olivine reference sample at high pressure and 850 ± 50 °C. Olivine deformed with an effective stress exponent (n) of 6.9 +3/-1, which we interpret to indicate that the Peierls creep deformation mechanism was active in the olivine. Quartz and coesite had very similar strengths and deformed by a mechanism with n = 2:8 +1.2/−0.9 and 2:9 +1.3/-0.9, respectively, which are consistent with previous measurements of power law creep in these phases. Stishovite deformed with n = 8.1 +3.7/-2.7 and was stronger than both olivine and the other silica polymorphs. The high stress exponent of stishovite is greater than that typically observed for power law creep, indicating it is probably (but not certainly) deforming by Peierls creep. The rheology of SiO2 minerals appears therefore to be strongly affected by the change in silicon coordination and density from fourfold in quartz and coesite to sixfold in stishovite. If the effect of Si coordination can be generalized, the increase in Si coordination (and density) associated with bridgmanite formation may explain the tenfold to 100‐fold viscosity increase around 660 km depth in the Earth.