Volatiles under High Pressure - Physics and Chemistry of the Deep Earth - page 9

bands in the spectrum of olivine are only seen
at low Si activities that may favor the forma-
tion of Si 4 + vacancies, while other bands are
enhanced at low Mg 2 + activities and may there-
fore be associated with Mg vacancies (Matveev
et al ., 2001). The occurrence of Mg vacancies in
hydrous olivine was directly confirmed by sin-
gle crystal X-ray diffraction studies (Smyth et al .,
2006). As a general rule, when the protonation
of oxygen atoms is associated with cation vacan-
cies, the protons are not located on the vacant
cation site. Rather, their location is controlled
by hydrogen bonding interactions with neigh-
boring oxygen atoms. In the hydrogarnet (OH) 4
defect which corresponds to a fully protonated
SiO 4 tetrahedron with a vacant Si site, the pro-
tons are located on the outside of the tetrahedron
above the O-O edges (Lager et al ., 2005). Rauch
and Keppler (2002) demonstrated that certain OH
bands in the infrared spectra of orthopyroxene
only occur in the presence of Al and are therefore
assigned to coupled substitutions of H + and Al 3 + .
Similar studies have now also been carried out
for olivine and several bands have been identi-
fied that are related to various trivalent cations

and also to titanoclinohumite-like point defects
(Berry et al ., 2005; 2007). Table 1.2 compiles the
main substitution mechanisms for water in nom-
inally anhydrous mantle minerals.
Water solubility, i.e. the amount of water dis-
solved in a mineral in equilibrium with a fluid
phase consisting of pure water, can be described
by the equation (Keppler & Bolfan-Casanova,
2006):

A f H 2 O exp

,

H 1 bar V solid P

RT

+

c water = −

(1.2)
where A is a constant which essentially contains
the entropy of reaction and n is an exponent
related to the dissolution mechanism of OH:
n = 0.5 for isolated OH groups, n = 1 for OH

pairs (or molecular water), n 2 for the hydrog-

arnet defect. H 1 bar is the reaction enthalpy at
1 bar and V solid is the change in the volume
of the solid phases upon incorporation of water.
Experimentally derived parameters for Equation
(1.2) applied to the water solubility in a variety
of nominally anhydrous minerals are compiled in
Table 1.3.

=

Table 1.2 Hydrogen-bearing defects in nominally anhydrous minerals.
Substitution mechanism Mineral Reference

------------- Isolated protons -------------

H + + Al 3 + ↔ 2Mg 2 + orthopyroxene Mierdel et al . (2007)

H + + Al 3 + ↔ Si 4 + orthopyroxene Rauch & Keppler (2002)

(Fe 3 ,Cr 3 + ,Sc 3 + ,REE 3 + ) + H + ↔ 2Mg 2 + olivine Berry et al . (2007)

H + + B 3 + ↔ Al 3 + B-rich olivine Sykes et al . (1994)

H + + Li + ↔ Mg 2 + possibly in pyrope Lu & Keppler (1997)

--------------- Proton pairs ---------------

2H + ↔ Mg 2 + olivine Smyth et al . (2006)

enstatite Rauch & Keppler (2002)

wadsleyite Smyth (1987)

ringwoodite Smyth et al . (2003)

possibly MgSiO 3 perovskite Ross et al . (2003)

2H + + Mg 2 + ↔ Si 4 +

possibly ringwoodite Kudoh et al . (2000)

Ti 4 + + 2H + ↔ Mg 2 + + Si 4 + olivine Berry et al . (2005)

interstitial H 2 O feldspars Johnson & Rossman (2004)

--------- Cluster of four protons ----------

4H + ↔ Si 4 +

garnet Lager et al . (2005)

possibly in olivine Matveev et al . (2001)