Stability, metastability, and elastic properties of a dense silica polymorph, seifertite

Abstract

Dense silica polymorphs with sixfold coordinated Si have been found in SNC and lunar meteorites and may be important minerals for silica‐rich components in the lower mantle. However, the stable crystal structure in the lower mantle and properties of dense silica remain controversial. Under stable heating and quasi‐hydrostatic stress conditions, we found that the CaCl2 type undergoes a phase transition to the α‐PbO2type (seifertite) at 130–140 GPa and 2500 K. Our data suggest that this phase transition occurs at a greater depth than the perovskite → postperovskite transition in the lowermost mantle. The molar volume measured at 1 bar is the smallest among the reported silica polymorphs, therefore having the highest calculated density and in excellent agreement with recent first‐principles calculations. The greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field. Our data combined with the Hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field. We found very little change in bulk sound speed across the CaCl2‐type → seifertite transition. If shear wave velocity decreases at the transition to seifertite as suggested by some computational studies, this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the D″discontinuity.