Transcript Potential Sites for H in Forsterite

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.
• 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 OHfound in MgSiO3 perovskite (Bolfan-Casanova et
al. (2000)?
Propose a Strategy to . . .
• Predict docking sites for hydrogen on
minerals
• Crystallographic orientation of O-H
Apply to high-pressure silicates
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)
Different Views of H2O
Lone Pairs
• H 1s1
• O 1s22s22p4
(3,-3) Critical Points
Electron density
() and Laplacian
(-2) of H2O:
Bader (1990)
Mapping of Valence Shell Charge
Concentration (-2(r)>0) for H2O
Gibbs et al. (2001)
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)
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:
Ion
Basis set
Reference
Si
Mg2+
65-111G
8-511G
Ca2+
O2-
86-511d3G
8-411G
Darco et al. 1993
McCarthy & Harrison,
1994
Mackrodt et al., 1993
McCarthy & Harrison,
1994
4+
• The topological analysis of the electron density and
of its Laplacian scalar fields were analyzed using
TOPOND .
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)
H in wadselyite:
• Smyth (1987,1994)
• Downs (1989)
• Kudoh et al. (1996)
O1
O2
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)
H in ringwoodite
• Kudoh et al. (2000):
O-O pairs around MgO6 vacancies
(3,-3) Critical Points in Wadsleyite, -Mg2SiO4
O1
O2
O3
O4
Potential Docking Sites and Positions
of H in Wadsleyite
(001)
Wadsleyite
Clusters around O1 and O2
O1-h1 [001]
O2-h2 ~ [100]
Potential H Positions associated
with Mg Vacancies
(100) slice of wadselyite
(3,-3) Critical Points in
Ringwoodite, -Mg2SiO4
O
Compare with O3 and O4 in -Mg2SiO4
O3
O4
Potential Docking Sites and Positions
of H in Ringwoodite, -Mg2SiO4
(100)
(110)
Potential H Positions associated with
Mg Vacancies in Ringwoodite
Hydrogen in Stishovite
• O-H  [001]
IR spectrum from
Pawley et al. 1993
Potential Docking Sites and Positions
of H in Stishovite
• O-H  [001]
(see GV Gibbs Session 5 – Tues am)
MgSiO3 ilmenite and perovskite
Bolfan-Casanova et al. (2000, EPSL)
• No OH- detected in MgSiO3 perovskite
[Meade et al. (1994) observed 2 pleochroic OH peaks]
(3,-3) Critical Points in MgSiO3 Ilmenite
 CP’s along
edges and face
of MgO6
octahedra:
• “Avoid” SiO6
octahedra:
Two potential H sites in MgSiO3 Ilmenite
O-H  [001]
w/in face of MgO6
H w/in MgO6 layers
~ along edges
(3,-3) Critical Points in MgSiO3 Perovskite
O1
O2
• No CP’s on O1 and only 1 CP on O2!
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. ]
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
Summary
Phase
-Mg2SiO4
- Mg2SiO4
Stishovite
MgSiO3 ilm
MgSiO3 pv
Potential docking sites for H
Docking sites for H possible on all O:
O1-H ( [001])
02-H (~ [100])
O3 and O4 (MgO6 vacancy)
Each O has 3 potential H sites
assoc. w/ MgO6 vacancy, disorder likely
O-H  [001]
2 unique docking sites for H:
O-H  [001] (face of MgO6)
O-H assoc. O…O edges of MgO6
1 unique docking site for H:
No H on O1, only on O2 (assoc. w/ MgO12
vacancy)
O-H  [110]
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.