Geoscience Reference
In-Depth Information
According to Ringwood
[28]
, high-pressure transformations of silicates can be
studied on the basis of their germanate analogs. This idea can be understood if one
takes into account that the ratio r
cation
/r
oxygen
should be increased under high pres-
sure. At atmospheric pressure, rGe
4
1
is greater than rSi
4
1
for 20%. Therefore, the
structures of germanates at normal pressure reflect many specific features of high-
pressure deformations in silicates. As an example, the authors compare the high-
pressure structural changes in quartz
[29]
with chemical deformations in its struc-
ture, where Si-atoms were partially substituted by Ge.
These crystals bear composition (Si
0.86
Ge
0.14
)O
2
which is considered to be
unique because, hitherto, the GeO
2
concentration in the synthetic quartz crystals
never exceeded 0.13 molar%. The common tendencies in quartz under high pres-
sure and with the substitution of Si for Ge are connected with: (i) A decrease in
angle T
O;
(iii) a similar change in tilt angles between tetrahedra (
Figure 7.3a
); and (iv) an
increase in tetrahedral distortion (
Table 7.4
)
[30]
.
The structural comparison in silicates and germanates allows explanation of the
different crystal chemical phenomena. One of them is connected with the question
of why GeO
2
does not crystallize with coesite structure while stishovite—a form of
GeO
2
—mineral argutite is known even in nature.
The structural changes of coesite, under high pressure are, connected with signif-
icant dispersion in Si
a
O
a
T(
Figure 7.2
); (ii) a decrease in intertetrahedral distances O
a
Si angles. It is noteworthy that the smallest angle sharply
decreases. Moreover, under high pressure, the temperature factor of O
1
, which par-
ticipates in bond Si
a
O
a
Si with an angle equal to 180
, increases up to about
30%, while the temperature factors of other atoms decrease. Substitution of Ge for
Si would reinforce the tendencies and would lead to structural distortions, in other
words, to the displacement of O
1
and the distortion in angle Si
a
O
1
a
180
.
The tetrahedral complexes are being extensively studied and characterized,
owing to their applications in technologies such as laser, superionic, ferroelectric,
ion-exchange, piezoelectric, optoelectronics, and a host of others. We initially
a
O
1
a
Si
5
Conc.
Ge, at%
P
(Kbar)
80
61.4
55.8
60
48.6
37.6
40
20.5
20
135
135
140
T - O - T
Figure 7.2 Decrease in T
a
O
a
T with the substitution of Si for Ge
[7]
.
