Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts

E Bykova,M Hanfland,A Černok,N Dubrovinskaia,S I Simak,I A Abrikosov,C Prescher,J Tidholm,V B Prakapenka,M Bykov,H.-P Liermann,M P Belov,L Dubrovinsky,O Hellman

doi:10.1038/s41467-018-07265-z

Abstract

Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO2) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO6 octahedra, which, at odds with 3rd Pauling’s rule, are connected through common faces. Our results suggest that possible silicate liquids in Earth’s lower mantle may have complex structures making them more compressible than previously supposed.

Highlights

The local structure of melts, which is roughly characterized by the coordination number of silicon atoms and the way how the silicon polyhedra are interconnected at certain pressure–temperature conditions, can be modeled using molecular dynamics simulations[7,8], Bader’s atoms-in-molecules approach[9], or studied experimentally on silica or silicate glasses[10,11,12]
Recent experimental studies of silica glass suggest that the coordination number of silicon atoms drastically increases from 4 to 6 between 15 and 60 GPa and up to ~100 GPa, it is either constant[13] or increases[11] to 7
We show that high-pressure phases of coesite can be used as proxies of the local structure of high-pressure silica melts

Summary

Introduction

We report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. A very convincing method to obtain a structural model of noncrystalline silica material is to compare (or fit) experimental total scattering data with the pair-distribution function (PDF) of known crystalline phase(s)[14] In this manner, Keen and Dove[14] determined that the local structure of silica glass at ambient conditions has strong similarities with HPtridymite and β-cristobalite. The crystal structure of coesite’s high-pressure phases can give an insight into the mechanisms of silica glass densification

Methods

Results

Conclusion