Geoscience Reference
In-Depth Information
analogous compounds, such as borates, vanadates, arsenates, and sulfates, have not
been studied in detail and they do not have any systematic structural classification.
This is probably connected with their lesser abundance in nature and also lesser
degree of structural diversities.
One of the main crystal chemical problems is connected with predictions of
structural transformations of elevated temperatures and pressures. The hydrothermal
technique is especially very handy in this context.
The diversity of (Si,O)-complexes described in an earlier section has been attrib-
uted to silicate stability at relatively lower depths. The lower coordination number
of oxygen
[2,3]
in these structures makes them unstable in the mantle. High-
pressure structural studies
[17]
show the decrease of Si
O distance until the “criti-
cal” value—1.59
˚
—leads to a change of Si-tetrahedra by Si-octahedra. There is a
certain advantage in such transformations, because Si-octahedra with longer
distances Si
a
O can be linked not only by corners, but also by edges and even by
faces. Thus, the coordination number of oxygen becomes greater, and at the same
time the distribution of Si-atoms becomes more compact
[18]
. The “critical”
value 1.59
˚
is not accidental and is confirmed by the correlation between
interatomic distances and angles in Si-tetrahedra
[19]
. Lg2/d(Si
a
a
O)avg/
5
Lgd
3.06
˚
a
+
a
a
a
(Si
Si)
2
Lg
(Si
O
Si)/2 with
fixed
Si
Si
distances
5
and
1.59
˚
. This value was obtained
+
(Si
a
O
a
Si)
max
5
180
, one gets d(Si
a
O)
min
5
in the structure of forsterite Mg
2
Si0
4
at P
14.9 GPa
[20]
.
The stability of Si-tetrahedra depends on the properties of nontetrahedral
M-cations. The presence of M-cations with a high value of electronegativity leads
to a decrease in the coordination of oxygen atoms. Consequently, under high
pressures,
5
these structures transform into structures with Si-octahedra more
easily
[21]
.
McDonald and Cruickshank
[22]
have shown that in structures with electronega-
tive M-cations, the distances Si
O terminal become more
equal. According to Brown
[23]
, this effect is followed by a decrease of (Si
a
O bridging and Si
a
O)
avg
distances in Si-tetrahedra. In other words, chemical deformations connected with
the presence of M-cations with high electronegativity are similar to the high-
pressure structural deformations.
The concept of emphasizing the dominating role of cations in the structure refor-
mation is used for the interpretation of structural transformations under high pres-
sure. The compressibility of cationic polyhedra (
a
) is inversely proportional to the
polyhedral charge density z/d
[24]
, where d is the distance between central cation
and O-atoms. The behavior of Si-tetrahedra mainly depends on the compressibility
of the M-polyhedra, which can be considered as initial
[25]
. For example, Mg
2
SiO
4
under high pressure successively transforms from
β
α
- and
β
-modifications with
decreasing ratios (Mg
). The signifi-
cant compressibility of the Mg-octahedra during the transformation from the
a
O)/(Si
a
O): 1.291 (
α
), 1.860 (
β
), and 1.251 (
γ
α
-to
γ
a
the
-form initiates the unexpected expansion of the Si-tetrahedra; d(Si
O) in
1.655
˚
[26]
. It is noteworthy that
the structural alterations in these compounds as in some other silicates result in
oxygen atom packing ordering
[27]
.
1.636
˚
while in
α
-Mg
2
SiO
4
5
γ
-Mg
2
SiO
4
5
