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Potential Docking Sites and Positions of Hydrogen in High-Pressure Silicates

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. Water in Minerals. Trace amounts of water can have profound effects on physical properties of minerals.

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Potential Docking Sites and Positions of Hydrogen 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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