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Predicting Hydrogen Docking Sites in High-Pressure Silicates

This study explores the incorporation of hydrogen in mantle minerals and proposes a strategy to predict docking sites for hydrogen on minerals. The analysis focuses on the electron density distribution and topological bond properties to identify potential hydrogen positions in silicates such as wadsleyite and ringwoodite. Computational details, crystallographic orientation of O-H bonds, and critical points in minerals are examined to understand the role of hydrogen in their structures.

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Predicting Hydrogen Docking Sites in High-Pressure Silicates

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  1. Potential Docking Sites and Positions of Hydrogen in High-Pressure Silicates N.L. Ross, G.V. Gibbs Virginia Tech K.M. Rosso Pacific Northwest Laboratory

  2. Water in Minerals • Trace amounts of water can have profound effects on physical properties of minerals. • Nominally anhydrous mantle minerals can incorporate significant amounts of water (OH-) in their structure. • How is hydrogen incorporated into structures of mantle phases? • Why do wadsleyite and ringwoodite dissolve wt% H2O in structures whereas no detectable OH- found in MgSiO3 perovskite (Bolfan-Casanova et al. (2000)?

  3. Propose a Strategy to . . . • Predict docking sites for hydrogen on minerals • Crystallographic orientation of O-H Apply to high-pressure silicates

  4. Strategy • Calculate topological bond critical point properties of electron density distribution, including . . . • Laplacian, -2(r), and component curvatures of (r) ,1, 2, and 3 -2(r) =2(r)/x2 + 2(r)/y2 + 2(r)/z2 • Mapping of -2(r) identifies (3,-3) critical points that correspond to local concentration of and potenital proton docking positions Bader (1990)

  5. Different Views of H2O Lone Pairs • H 1s1 • O 1s22s22p4 (3,-3) Critical Points Electron density () and Laplacian (-2) of H2O: Bader (1990)

  6. Mapping of Valence Shell Charge Concentration (-2(r)>0) for H2O Gibbs et al. (2001)

  7. Hydrogen in Coesite(Gibbs et al. 2002, PCM) • H avoids O1, bonds to O2,O3,O4 and O5 • Very good agreement w/ Koch-Müller et al. (2001) IR study (see GV Gibbs, Session 5, Tues am)

  8. Computational Details • Electron density distributions for all phases calculated with CRYSTAL98(Pisani, 1996;Saunders et al., 1998; Pisani et al., 2000) • All-electron basis sets used: • The topological analysis of the electron density and of its Laplacian scalar fields were analyzed using TOPOND .

  9. Wadsleyite,-Mg2SiO4 & Ringwoodite,-Mg2SiO4 • Abundant minerals in transition zone • Can incorporate ~3 wt% H2O in structure(Smyth, 1987,1994; Gaspark, 1993;Inoue et al. 1995,1998;Kohlstedt et al., 1996;Kudoh et al., 1996,2000) IR spectra from Bolfan-Casanova et al. (2000)

  10. H in wadselyite: • Smyth (1987,1994) O1 • Downs (1989) O2 • Kudoh et al. (1996) O1..O1,O1…O3, O1..O4 • Kohn et al. (2002) ordered on 4 sites, most O1 (<0.4wt% H2O); disordered,14-17 sites (0.8-1.5 wt% H2O)

  11. H in ringwoodite • Kudoh et al. (2000): O-O pairs around MgO6 vacancies

  12. (3,-3) Critical Points in Wadsleyite, -Mg2SiO4 O1 O2 O3 O4

  13. Potential Docking Sites and Positions of H in Wadsleyite (001)

  14. WadsleyiteClusters around O1 and O2 O1-h1 [001] O2-h2 ~ [100]

  15. Potential H Positions associated with Mg Vacancies (100) slice of wadselyite

  16. (3,-3) Critical Points in Ringwoodite, -Mg2SiO4 O Compare with O3 and O4 in -Mg2SiO4 O3 O4

  17. Potential Docking Sites and Positions of H in Ringwoodite, -Mg2SiO4 (100) (110)

  18. Potential H Positions associated with Mg Vacancies in Ringwoodite

  19. Hydrogen in Stishovite • O-H  [001] IR spectrum from Pawley et al. 1993

  20. Potential Docking Sites and Positions of H in Stishovite • O-H  [001] (see GV Gibbs Session 5 – Tues am)

  21. MgSiO3 ilmenite and perovskite • No OH- detected in MgSiO3 perovskite [Meade et al. (1994) observed 2 pleochroic OH peaks] Bolfan-Casanova et al. (2000, EPSL)

  22. (3,-3) Critical Points in MgSiO3 Ilmenite • CP’s along edges and face of MgO6 octahedra: • “Avoid” SiO6 octahedra:

  23. Two potential H sites in MgSiO3 Ilmenite O-H  [001] w/in face of MgO6 H w/in MgO6 layers ~ along edges

  24. (3,-3) Critical Points in MgSiO3 Perovskite O1 O2 • No CP’s on O1 and only 1 CP on O2!

  25. Potential Docking Sites and Positions of H in MgSiO3 Perovskite • Mg vacancy and O-H  [110] [Similar to location of H in San Benito Perovskite proposed by Beran et al. (1996) Can. Min. ]

  26. CaSiO3 Perovskite • No (3,-3) Critical Points • May be due to Si-O-Si=180o • To incorporate H, need Ca vacancy • O-H not restricted to [100]c as MgSiO3 pv

  27. Summary

  28. Conclusions • Strategy based on mapping of -2 and location of (3,-3) critical points provides a powerful technique for location of potential H sites in minerals. • Future work includes introduction of trivalent cations, vacancies, etc. with H and see where H “docks”. Also let structure relax around proton sites.

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