Geoscience Reference
In-Depth Information
t
2
g
weaker repulsion
Six oxygen atoms
form an
octahedral site
e
g
stronger repulsion
Figure 1.7
Crystal field effect on a transition element, such as Fe, Mn, Cr, in octahedral coordination. Six
oxygen atoms form the apexes of the octahedron. The
t
2
g
orbitals of the transition element lie
between the oxygen atoms, while the
e
g
orbitals point toward them. The filling of an
e
g
orbital
by electrons therefore has to overcome excess repulsion energy compared with a
t
2
g
orbital.
e
g
+
3
Δ
/5
−
2
Δ
/5
Spherical
field
t
2
g
Octahedral
field
No field
Figure 1.8
Splitting of the d energy levels in the octahedral site of (
Fig. 1.7
)
. If the ion is inserted into a
spherical site, the repulsion to overcome is symmetric for the electrons on all the d orbitals. In an
octahedral site, which is particularly common in silicate rocks, the two
e
g
orbitals are subjected to
stronger repulsion than the three
t
2
g
orbitals. For the same overall interaction energy, and a
difference
in bonding energy between
t
2
g
and
e
g
(the crystal field stabilization energy), the
energy shift of the
t
2
g
orbitals is
−
2
/
5, while the shift for the
e
g
orbitals is
+
3
/
5. In a
tetrahedral site, the situation would be reversed.
2. Ferrous iron Fe
2
+
has electronic formula [Ar]3d
6
4s
0
. Once the lowest three
t
2
g
orbitals
are occupied by one electron each, there are two options: (i) if
is less than the repul-
sive energy of electron-electron pairing, the next electron will fit an upper
e
g
orbital or,
(ii) if
is large, it will pair with an electron on the
t
2
g
orbital. Two Fe
2
+
configurations
are therefore possible in each case (
Fig. 1.9
) which receive their denomination from
the way electronic spins add up. The most abundant iron in the mantle corresponds to
the small
case and is referred to as “high-spin” Fe. The energy gain due to crystal
