Hunt, S; Whitaker, ML; Bailey, E; Mariani, E; Stan, CV; Dobson, D; (2019) An experimental investigation of the relative strength of the silica polymorphs quartz, coesite and stishovite. Geochemistry, Geophysics, Geosystems , 20 (4) pp. 1975-1989. 10.1029/2018GC007842.

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Abstract

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 6.0_{+3.1}^{-2.2}, 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 4‐fold in quartz and coesite to 6‐fold in stishovite. If the effect of Si‐coordination can be generalised, the increase in Si‐coordination (and density) associated with bridgmanite formation may explain the 10‐100 fold viscosity increase around 660km depth in the Earth.

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