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Letter

Abstract | Letter | Acknowledgements | References | Supplementary Information
The entrainment or settling of silicate melts in the deep Earth is related to their physical properties, such as density and viscosity. These properties are linked to the atomic structure that controls their ascent or settling in the deep mantle. SiO₂ is the main component of silicate melts and is often used as a reference model to compare with the behaviour of other amorphous silicate compounds (Murakami and Bass, 2010

Murakami, M., Bass, J.D. (2010) Spectroscopic Evidence for Ultrahigh-Pressure Polymorphism in SiO2 Glass. Physical Review Letters 104, 025504.

) and melts at high pressure (Sanloup et al., 2013

Sanloup, C., Drewitt, J.W.E., Konopkova, Z., Dalladay-Simpson, P., Morton, D.M., Rai, N., van Westrenen, W., Morgenroth, W. (2013) Structural Change in Molten Basalt at Deep Mantle Conditions. Nature 503, 104–107.

) because of its network forming nature in silicate magmas. Most of the data on SiO₂ glass at high pressure have been obtained by X-ray diffraction (XRD) and show a change in the average coordination number (CN) from four- to six-fold starting at ~20 GPa and completing at pressures as low as 40 GPa (Benmore et al., 2010

Benmore, C.J., Soignard, E., Amin, S.A., Guthrie, M., Shastri, S.D., Lee, P.L., Jarger, J.L.  (2010) Structural and Topological Changes in Silica Glass at Pressure. Physical Review B 81, 054105.

; Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

). Between 40 and 130 GPa, it is unclear whether the CN increases above 6 (Prescher et al. 2017

Prescher, C., Prakapenka, V.B., Stefanski, J., Jahn, S., Skinner, L.B., Wang, Y. (2017) Beyond Sixfold Coordinated Si in SiO2 Glass at Ultrahigh Pressures. Proceedings of the National Academy of Sciences 114, 10041–10046.

) or plateaus around 6 (Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

) before increasing further above 140 GPa (Wu et al., 2012

Wu, M., Liang, Y., Jiang, J.-Z., Tse, J.S. (2012) Structure and Properties of Dense Silica Glass. Scientific Reports 2, 398.

). Brillouin spectroscopy measurements also suggest an increase in velocities, possibly related to a CN higher than six-fold, at pressures above ~140 GPa for SiO₂, 130 GPa for MgSiO₃, and 110 GPa for Al-rich silicates (Murakami and Bass, 2010

Murakami, M., Bass, J.D. (2010) Spectroscopic Evidence for Ultrahigh-Pressure Polymorphism in SiO2 Glass. Physical Review Letters 104, 025504.

, 2011

Murakami, M., Bass, J.D. (2011) Evidence of denser MgSiO3 glass above 133 gigapascal (GPa) and implications for remnants of ultradense silicate melt from a deep magma ocean. Proceedings of the National Academy of Sciences 108, 17286

; Ohira et al., 2016

Ohira, I., Murakami, M., Kohara, S., Ohara, K., Ohtani, E. (2016) Ultrahigh-Pressure Acoustic Wave Velocities of SiO2-Al2O3 Glasses up to 200 GPa.  Progress in Earth and Planetary Science 3, 18.

).

Many studies reported significant structural changes in silicate glasses at high pressure with potentially similar Si coordination changes in glasses and melts (Karki et al., 2007

Karki, B.B., Bhattarai, D., Stixrude, L. (2007) First-Principles Simulations of Liquid Silica: Structural and Dynamical Behavior at High Pressure. Physical Review B 76, 104205.

; Benmore et al., 2010

Benmore, C.J., Soignard, E., Amin, S.A., Guthrie, M., Shastri, S.D., Lee, P.L., Jarger, J.L.  (2010) Structural and Topological Changes in Silica Glass at Pressure. Physical Review B 81, 054105.

; Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

; Wu et al., 2012

Wu, M., Liang, Y., Jiang, J.-Z., Tse, J.S. (2012) Structure and Properties of Dense Silica Glass. Scientific Reports 2, 398.

; Sanloup et al., 2013

Sanloup, C., Drewitt, J.W.E., Konopkova, Z., Dalladay-Simpson, P., Morton, D.M., Rai, N., van Westrenen, W., Morgenroth, W. (2013) Structural Change in Molten Basalt at Deep Mantle Conditions. Nature 503, 104–107.

). Such changes seem also independent of the measurement time as illustrated by the first sharp diffraction peak position measured within hours (Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

), minutes (Benmore et al., 2010

Benmore, C.J., Soignard, E., Amin, S.A., Guthrie, M., Shastri, S.D., Lee, P.L., Jarger, J.L.  (2010) Structural and Topological Changes in Silica Glass at Pressure. Physical Review B 81, 054105.

; Prescher et al. 2017

Prescher, C., Prakapenka, V.B., Stefanski, J., Jahn, S., Skinner, L.B., Wang, Y. (2017) Beyond Sixfold Coordinated Si in SiO2 Glass at Ultrahigh Pressures. Proceedings of the National Academy of Sciences 114, 10041–10046.

), or even a few seconds (Sanloup et al., 2013

Sanloup, C., Drewitt, J.W.E., Konopkova, Z., Dalladay-Simpson, P., Morton, D.M., Rai, N., van Westrenen, W., Morgenroth, W. (2013) Structural Change in Molten Basalt at Deep Mantle Conditions. Nature 503, 104–107.

). Similarly, density measurements of SiO₂ glasses are in very good agreement with each other, regardless of the starting glass material (Meade and Jeanloz, 1987

Meade, C., Jeanloz, R. (1987) Frequency-Dependent Equation of State of Fused Silica to 10 GPa. Physical Review B 35, 236–244.

; Petitgirard et al., 2017

Petitgirard, S., Malfait, W.J., Journaux, B., Collings, I.E., Jennings, E.S., Blanchard, I., Kantor, I., Kurnusov, A., Cotte, M., Dane, T., Burghammer, M., Rubie, D.C. (2017) SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters 119, 215701.

), and are also in agreement with the quenched liquid from molecular dynamic (MD) simulations (Wu et al., 2012

Wu, M., Liang, Y., Jiang, J.-Z., Tse, J.S. (2012) Structure and Properties of Dense Silica Glass. Scientific Reports 2, 398.

). Still, the mechanisms associated with such high densification and compressibility (Petitgirard et al., 2017

Petitgirard, S., Malfait, W.J., Journaux, B., Collings, I.E., Jennings, E.S., Blanchard, I., Kantor, I., Kurnusov, A., Cotte, M., Dane, T., Burghammer, M., Rubie, D.C. (2017) SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters 119, 215701.

) and their link to local structural changes, remains debated (Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

; Prescher et al. 2017

Prescher, C., Prakapenka, V.B., Stefanski, J., Jahn, S., Skinner, L.B., Wang, Y. (2017) Beyond Sixfold Coordinated Si in SiO2 Glass at Ultrahigh Pressures. Proceedings of the National Academy of Sciences 114, 10041–10046.

)  and requires more precise investigations on the changes in local structure with pressure.

The discrepancy in CN evolution arises because of the use of non-elemental specific probes and due to the fact that XRD and Brillouin spectroscopy measurements only give access to the bulk structure and cannot probe the electrons directly involved in the chemical bonds that reflect structural rearrangements. Determining the pair distribution function (PDF) from X-ray total scattering (XRD) requires other parameters such as the density and careful background subtraction in order to obtain the average CN, and thus is not a direct method. Furthermore, natural systems with several cations (Si, Mg, Ca, Al, Fe) would be very complicated to analyse with XRD due to the overlap of cation-oxide contributions in the PDF. For instance, the CN of Si and Mg as well as the individual Si-O and Mg-O bond distances could not be solved using PDF in a recent report on MgSiO₃ glass (Kono et al., 2018

Kono, Y., Shibazaki, Y., Kenney-Benson, C., Wang, Y., Shen, G. (2018) Pressure-Induced Structural Change in MgSiO3 Glass at Pressures near the Earth’s Core–mantle Boundary. Proceedings of the National Academy of Sciences of the United States of America 115, 1742–1747.

), and requires complementary analyses as well as further improvement in high pressure PDF studies.

X-ray Raman scattering (XRS) (Sternemann and Wilke, 2016

Sternemann, C., Wilke, M. (2016) Spectroscopy of Low and Intermediate Z Elements at Extreme Conditions: In Situ Studies of Earth Materials at Pressure and Temperature via x-Ray Raman Scattering. High Pressure Research 36, 275–292.

and reference therein) spectroscopy allows for measurements of X-ray absorption edges of light elements relevant for Earth compositions (Si, Mg, Al, Ca, S, O, Fe) using X-ray energies of 9.7 keV with a resolution of 0.7 eV. Only a few experiments have reported data using XRS on silica at high pressure on the O K-edge (Lin et al., 2007

Lin, J.-F., Fukui, H., Prendergast, D., Okuchi, T., Cai, Y.Q., Hiraoka, N., Yoo, C.-S., Trave, A., Eng, P., Hu, M.Y., Chow, P. (2007) Electronic Bonding Transition in Compressed SiO2 Glass. Physical Review B 75, 012201.

) and on the Si L-edge (Fukui et al., 2008

Fukui, H., Kanzaki, M., Hiraoka, N., Qai, Y.C. (2008) Coordination Environment of Silicon in Silica Glass up to 74 GPa: An X-Ray Raman Scattering Study at the Silicon L Edge. Physical Review B 78, 012203.

). However, the last report does not show any evidence for changes in Si CN, because of low signal and energy resolution, and concluded that Si remains predominantly 4-fold coordinated up to 74 GPa.

Here we report XRS measurements on both Si L_2,3- and O K-edges in SiO₂ up to 110 GPa with twice the pressure range and data measured in one run, with a three-fold improvement in energy resolution at 0.7 eV. Such quality and resolution are now possible thanks to: i) the new spectrometer at the ID20 beamline at the ESRF, ii) development of diamond anvils of 500 μm thickness, allowing a five-fold gain in transmission, and iii) a new data extraction scheme. We complemented our measurements on the the quenched SiO₂ melt at high pressure with first principles spectral calculations by solving the Bethe-Salpeter Equation (BSE) using the OCEAN code (Gilmore et al., 2015

Gilmore, K., Vinson, J., Shirley, E.L., Prendergast, D., Pemmaraju, C.D., Kas, J.J., Vila, F.D., Rehr, J.J. (2015) Efficient Implementation of Core-Excitation Bethe-Salpeter Equation Calculations. Computer Physics Communications 197, 109.

) based on trajectories from ab initio molecular dynamic (AIMD) simulations by Wu et al. (2012)

Wu, M., Liang, Y., Jiang, J.-Z., Tse, J.S. (2012) Structure and Properties of Dense Silica Glass. Scientific Reports 2, 398.

. (See Supplementary Information for experimental and calculation details).

We found a striking match between our experimental spectra of cold compressed glass and the calculated spectra of the quenched high pressure melt with: (i) two major discontinuities at ~20 GPa and ~60 GPa, and (ii) similar structures for the glass and the quenched melt from MD simulation. In addition, we measured the six-fold coordinated reference stishovite up to 60 GPa. It shows that Si in the glass does not reach complete 6-fold coordination at high pressure, but rather only approaches a CN of 6 at 110 GPa as corroborated by the MD simulations. At the same time, O fulfills 3-fold coordination at 40 GPa. With this unique data set we can establish the electronic and coordination changes in SiO₂ to core-mantle boundary (CMB) pressures, influencing the macroscopic properties of SiO₂.

Our experimental data show significant changes in shape for both the Si L_2,3-edge (Fig. 1a) and O K-edge (Fig. 1c) at high momentum transfer (|q|). These changes are observed for both edges at similar pressures and are well reproduced by our calculated spectra (Fig. 1b-d; spectra were shifted in energy loss to the experimental edge onsets). We report precise edge onsets as a function of pressure for both elements from our experimental data for the glass up to 110 GPa and the reference stishovite up to 60 GPa. All the data for Si low |q|, stishovite spectra and edge onset values can be found in the Supplementary Information (Figs. S-2, S-3, S-6 and Table S-1).

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We did not observe changes in the spectra in the first 14-17 GPa and confirm that four-fold coordinated ^[4]Si (Figs. 2, 3) remains predominant. At 17 GPa, the peak ratio of the Si L_2,3-edge spectra changes with a reduction of the second peak at 108.5 eV and a shift of the edge onset from 104.9 eV (at ambient) to 105.3 eV (at 17 GPa). Between 19 and 27 GPa we observed that the two peaks merge into a broader single peak (Fig. 1a) with a clear edge onset shift to higher energy of 106.2 eV (Fig. 2a). For the O K-edge, rapid changes between 17 and 19 GPa were also observed, with a second peak appearing at ~544.7 eV (Fig. 1c) and the edge onset shifting from 536.3 eV (ambient) to 536.7 eV (at ~19 GPa) and up to 537.4 eV at ~27 GPa (Fig. 2b). The latter illustrates an evolution of coordination also for the oxygen atoms. These changes in spectral shapes are associated with a slight drop in pressure of 2 to 5 GPa during the measurements (Figs. 1, 2). Above 27 GPa, the edge onset of both edges increases linearly with pressure: the Si L-edge reaches 107.0 eV and the O K-edge 538.6 eV at 56 GPa. Our data on stishovite compressed to 63 GPa show a linear trend of the edge onset for this canonical 6-fold reference phase, for both edges. All the Si L-edge onset values for the glass fall below that of stishovite, but tend to approach similar values around 60 GPa (Fig. 2a). For oxygen, the edge onset of the glass becomes similar to that of stishovite at around 30 GPa. This indicates that the oxygen coordination (O-Si) in the glass approaches quite rapidly a CN of 3, like that in stishovite at such pressure (Fig. 2b).  This observation is consistent with a mixture of 5- and 6-fold coordinated Si at these pressures and likely pinpoints the disappearance of ^[4]Si. Further, it may even indicate that Si is arranged in a mixture of edge-sharing octahedra and trigonal bipyramids, as both polyhedra require O with CN of 3. Such fine features in the electronic topology of O and Si can only be addressed using XRS, which measures the electrons directly involved in the bonding.

Full size image | Download in Powerpoint

Full size image | Download in Powerpoint

The calculated spectra of the quenched melt from MD show similar changes, although the peak ratio changes at lower pressure (14 GPa, Fig. 2b). These changes are completed for both edges at 26 GPa, similar to the experiment. Above this transition, at 27 GPa, spectra for both edges resemble the ambient stishovite reference with a single peak for the Si L_2,3-edge and a doublet for the O K-edge (Fig. 1a-d). These spectral shapes could indicate that the 6-fold CN has been completed for a pressure as low as 30 GPa. However, our MD simulations indicate that between 20 to 27 GPa, a rapid decrease of ^[4]Si occurs, which is replaced by a mixture of an intermediate five-fold coordinated ^[5]Si species with ^[6]Si up to 60 GPa (Fig. 3b). The calculated spectra for both ^[5]Si and ^[6]Si are nearly identical with only minor differences, which explains the aspect of the Si L-edge of glass being similar to the 6-fold spectra. The lack of a ^[5]Si reference for SiO₂ combined with the similarity of ^[5]Si and ^[6]Si spectra makes it difficult to observe a distinct signal of ^[5]Si in experimental and calculated spectra (Fig. S-4).

At 60 GPa, our data show a further transition, with a broadening of the silicon peak (Fig. 1a) and an edge onset shift for both edges during the measurements (Fig. 2; detailed in Fig. S-5). Such changes are due to a re-arrangement of the glass structure. We also noticed a slight drop of pressure, which is likely related to a considerable structural change such as observed near the 20 GPa transition. Further, above 70 GPa, the Si L-edge onset in the glass still remains lower than the 6-fold CaCl₂ reference coordinated structure (Fig. 2). In this pressure range, between 60-70 GPa, the MD simulations indicate a clear drop in ^[5]Si species and an increase of ^[6]Si (Fig. 3b). Thus, we interpret this transition as a near completion of six-fold coordination and disappearance of ^[5]Si. It is also possible that the structural re-arrangement at 60 GPa mimics the stishovite to CaCl₂ transition in the solid as observed for the MgSiO₃ system (Kono et al., 2018

Kono, Y., Shibazaki, Y., Kenney-Benson, C., Wang, Y., Shen, G. (2018) Pressure-Induced Structural Change in MgSiO3 Glass at Pressures near the Earth’s Core–mantle Boundary. Proceedings of the National Academy of Sciences of the United States of America 115, 1742–1747.

). However, the signatures of the transition are consistent with the one at ~20 GPa which marked the disappearance of ^[4]Si for a mixture of ^[5]-[6]Si. We can also observe that the shift of edge onset with pressure for both Si and O (Fig. 2a,b) follows exactly the trend given by the CN evolution as a function of pressure from MD simulations (Fig. 3a). The changes in slope for the different domains of Si species from MD (Fig. 3a,b) are in perfect agreement with our measured edge onset shift with pressure. Thus, we can interpret the breaks in the edge onset slopes as a function of pressure as a good marker for the Si coordination change.

Above 60-70 GPa, measured and calculated spectra agree very well up to 110 GPa (Figs. 1, 3). The spectral shape remained similar with a shift of both edge onsets to higher energies (Fig. 2a,b). The MD calculations show that ^[7]Si-coordination only starts to appear at ~110 GPa and becomes significant at ~150 GPa (Fig. 3b), corroborating Brillouin spectroscopy measurements with an observed increase of sound velocities at such pressure in SiO₂ (Murakami and Bass, 2010

Murakami, M., Bass, J.D. (2010) Spectroscopic Evidence for Ultrahigh-Pressure Polymorphism in SiO2 Glass. Physical Review Letters 104, 025504.

). It seems unlikely that a CN > 6 is formed at pressures above 60 GPa, because there is no evidence of a further densification of SiO₂ glass compared to crystalline phases (Petitgirard et al., 2017

Petitgirard, S., Malfait, W.J., Journaux, B., Collings, I.E., Jennings, E.S., Blanchard, I., Kantor, I., Kurnusov, A., Cotte, M., Dane, T., Burghammer, M., Rubie, D.C. (2017) SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters 119, 215701.

), unlike in GeO₂ for which a CN > 6 has been recorded at 60 GPa (Kono et al., 2016

Kono, Y., Kenney-Benson, C., Ikuta, D., Shibazaki, Y., Wang, Y., Shen, G. (2016) Ultrahigh-Pressure Polyamorphism in GeO2 Glass with Coordination Number > 6. Proceedings of the National Academy of Sciences of the United States of America 113, 3436–3441.

) where the glass density may equal or even cross that of the crystal (Hong et al., 2007

Hong, X., Shen, G., Prakapenka, V.B., Rivers, M.L., Sutton, S.R. (2007) Density Measurements of Noncrystalline Materials at High Pressure with Diamond Anvil Cell. Review of Scientific Instruments 78, 103905.

). At 60 GPa, a change in compressibility was measured for SiO₂ (Petitgirard et al., 2017

Petitgirard, S., Malfait, W.J., Journaux, B., Collings, I.E., Jennings, E.S., Blanchard, I., Kantor, I., Kurnusov, A., Cotte, M., Dane, T., Burghammer, M., Rubie, D.C. (2017) SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters 119, 215701.

) with a saturation of the density increase for higher pressure. A density crossover with solids has not been reported for SiO₂, precluding that the glass reaches a CN higher than the solid.

These results are quite different from those obtained by a recent XRD analysis (Prescher et al. 2017

Prescher, C., Prakapenka, V.B., Stefanski, J., Jahn, S., Skinner, L.B., Wang, Y. (2017) Beyond Sixfold Coordinated Si in SiO2 Glass at Ultrahigh Pressures. Proceedings of the National Academy of Sciences 114, 10041–10046.

), which suggests an average CN of 6 or higher for pressures of 40 GPa and above, but more consistent with a previous report that show a plateau of CN = 6 at around 40 GPa (Sato and Funamori, 2010

Sato, T., Funamori, N. (2010) High-Pressure Structural Transformation of SiO2 Glass up to 100 GPa. Physical Review B 82, 209604.

). Our data show a more detailed analysis of the CN with a mixture of five- and six-fold species in this pressure range. The average CN calculated from XRD should be viewed carefully because of the large uncertainties (at least 10 %) associated with the method. Our XRS measurements, confronted to the canonical 6-fold reference systems using the same technique, give a direct and precise coordination for Si using the edge onset shift. XRS measurements also yield information on the oxygen coordination, evolving from ^[2]O to ^[3]O with a completion at lower pressure than for Si. Thus, XRS brings important evidence that the electronic shell around the two atoms can be compressed and re-arranged in a different way than the simple hard sphere model would explain (Du and Tse, 2017

Du, X.P., Tse, J.S. (2017) Oxygen Packing Fraction and the Structure of Silicon and Germanium Oxide Glasses. Journal of Physical Chemistry B 121, 10726–10732.

),  which is often used to model the oxygen packing fraction. The same compression mechanism takes place in GeO₂ glass as measured with X-ray emission spectroscopy (Spiekermann et al., 2019

Spiekermann, G., Harder, M., Gilmore, K., Zalden, P., Sahle, C.J., Petitgirard, S., Wilke, M., Biedermann, N., Weis, C., Morgenroth, W., Tse, J.S., Kulik, E., Nishiyama, N., Yavaş, H., Sternemann, C. (2019) Persistent Octahedral Coordination in Amorphous GeO2 Up to 100 GPa by Kβ'' X-Ray Emission Spectroscopy. Physical Review X 9, 011025.

). A CN above 6 for Si may only form for pressures above 140 GPa with a significant increase of ^[7]Si species as suggested by the MD results (Fig. 3b). The increase in sound velocity in SiO₂ at 140 GPa measured by Brillouin spectroscopy (Murakami and Bass, 2010

Murakami, M., Bass, J.D. (2010) Spectroscopic Evidence for Ultrahigh-Pressure Polymorphism in SiO2 Glass. Physical Review Letters 104, 025504.

) could then be due to a densification of the glass linked to an evolution of the CN of Si above 6 (Wu et al., 2012

Wu, M., Liang, Y., Jiang, J.-Z., Tse, J.S. (2012) Structure and Properties of Dense Silica Glass. Scientific Reports 2, 398.

).

In summary, we find a striking match between the spectra obtained from experimental data and the ones calculated from the quenched melt (Figs. 1, S-2, S-4). We observe two structural changes at ~20 GPa and ~60 GPa related to changes of the electronic environment of Si and O. This may also occur in silicate melts (Sanloup et al., 2013

Sanloup, C., Drewitt, J.W.E., Konopkova, Z., Dalladay-Simpson, P., Morton, D.M., Rai, N., van Westrenen, W., Morgenroth, W. (2013) Structural Change in Molten Basalt at Deep Mantle Conditions. Nature 503, 104–107.

), with large influences on the chemical and physical properties of melts at such pressures, such as change in melt viscosity (Meade and Jeanloz, 1988

Meade, C., Jeanloz, R. (1988) Effect of Corrdination Change on the Strength of Amorphous SiO2. Science 241, 1072–1074.

) or partitioning of elements (Sanloup et al., 2013

Sanloup, C., Drewitt, J.W.E., Konopkova, Z., Dalladay-Simpson, P., Morton, D.M., Rai, N., van Westrenen, W., Morgenroth, W. (2013) Structural Change in Molten Basalt at Deep Mantle Conditions. Nature 503, 104–107.

). In the context of a magma ocean, it may affect the chemical segregation, thermal evolution of the CMB, and mobility of melts during magma crystallisation. The transition at 60-70 GPa is quite marked in the glass and corresponds to the near-disappearance of the ^[5]Si in favour of the ^[6]Si species in the quenched melt (Fig. 2b). This provides a good explanation for the change of compressibility at such pressures in the density data (Petitgirard et al., 2017

Petitgirard, S., Malfait, W.J., Journaux, B., Collings, I.E., Jennings, E.S., Blanchard, I., Kantor, I., Kurnusov, A., Cotte, M., Dane, T., Burghammer, M., Rubie, D.C. (2017) SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters 119, 215701.

), but also for the variation of strength of the glass (Meade and Jeanloz, 1988

Meade, C., Jeanloz, R. (1988) Effect of Corrdination Change on the Strength of Amorphous SiO2. Science 241, 1072–1074.

). Our MD results indicate that silica does not exceed six-fold coordination at pressures of the deep mantle (Fig. 3b). Depolymerised melt compositions, containing network modifying cations (e.g., Mg, Ca, Al) have shown stronger densification at lower pressures, closer to the CMB as illustrated by Brillouin measurements on MgSiO₃ and Al-rich SiO₂ (Ohira et al., 2016

Ohira, I., Murakami, M., Kohara, S., Ohara, K., Ohtani, E. (2016) Ultrahigh-Pressure Acoustic Wave Velocities of SiO2-Al2O3 Glasses up to 200 GPa.  Progress in Earth and Planetary Science 3, 18.

) glasses. The depolymerised nature of such compositions would facilitate the increase of CN > 6 for Si at pressures of the lower mantle producing negatively buoyant silicate at the CMB during the early Earth’s formation or today in the modern mantle.

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Acknowledgements

We acknowledge the ESRF for provision of beamtime under the proposal ES-431. We acknowledge N. Dubrovinskaia for providing us the stishovite standard material. We wish to thank Dr. Yunfeng Liang for providing the ab initio MD trajectories used in this study. We acknowledge the support of ID20 beamline staff, M. Moretti Sala and C. Henriquet. SP is financed by a DFG grant (PE 2334/1-1). The Scios Focus Ion Beam at BGI was financed by a DFG grant No. INST 91/315-1 FUGG. CS and CW would like to thank M. Tolan for discussion and general support of these activities and acknowledge funding by the BMBF (05K13PE2 and 05K16PE1).

Editor: Simon Redfern

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References

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Supplementary Information

The Supplementary Information includes:

• Experimental Details
• BSE Spectral Calculations
• Data Analysis Details
• Figures S-1 to S-6
• Table S-1
• Supplementary Information References

Download the Supplementary Information (PDF).