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% H 2 O in structures whereas no detectable OH - found in MgSiO 3 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)/  x 2 +  2  (r)/  y 2 +  2  (r)/  z 2 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 H 2 O H 1s 1 O 1s 2 2s 2 2p 4 Electron density (  ) and Laplacian (-  2  ) of H 2 O: Bader (1990) (3,-3) Critical Points Lone Pairs

6. Mapping of Valence Shell Charge Concentration ( -  2  (r)>0) for H 2 O 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,  -Mg 2 SiO 4 & Ringwoodite,  -Mg 2 SiO 4 Abundant minerals in transition zone Can incorporate ~3 wt% H 2 O 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% H 2 O); disordered,14-17 sites (0.8-1.5 wt% H 2 O)

11. H in ringwoodite Kudoh et al. (2000): O-O pairs around MgO 6 vacancies

12. (3,-3) Critical Points in Wadsleyite,  -Mg 2 SiO 4 O3O4 O1 O2

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

14. Wadsleyite Clusters 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,  -Mg 2 SiO 4 O Compare with O3 and O4 in  -Mg 2 SiO 4 O3O4

17. Potential Docking Sites and Positions of H in Ringwoodite,  -Mg 2 SiO 4 (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. MgSiO 3 ilmenite and perovskite No OH - detected in MgSiO 3 perovskite [Meade et al. (1994) observed 2 pleochroic OH peaks] Bolfan-Casanova et al. (2000, EPSL)

22. (3,-3) Critical Points in MgSiO 3 Ilmenite  CP’s along edges and face of MgO 6 octahedra: “Avoid” SiO 6 octahedra:

23. Two potential H sites in MgSiO 3 Ilmenite O-H  [001] w/in face of MgO 6 H w/in MgO 6 layers ~ along edges

24. (3,-3) Critical Points in MgSiO 3 Perovskite No CP’s on O1 and only 1 CP on O2! O2O1

25. Potential Docking Sites and Positions of H in MgSiO 3 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. CaSiO 3 Perovskite No (3,-3) Critical Points May be due to Si-O-Si=180 o To incorporate H, need Ca vacancy O-H not restricted to [100] c as MgSiO 3 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.