Geoscience Reference
In-Depth Information
140
. Si-atoms are found in sixfold coordination
in relatively small groups, basically in high-pressure silicates, and in fivefold coor-
dination in some of the organosilicate compounds
[1]
.
The structures of silicates and related compounds contain tetrahedral anions of
different configurations (single tetrahedron, pyrogroups, rings, chains, layers,
frameworks, and so on). Their comparative analysis considerably extends scientific
ideas on the structures of crystalline materials. The attention to silicates is con-
nected to their widespread abundance in the continental crust (
Table 7.2
), the
exceptional variability of tetrahedral anions in their structures, and a widespread
application of silicates in modern technology. Silicates are widely being used in
ceramic, optical, electronic, piezoelectric, optoelectronic, and other industries
[2]
.
The compounds that are nearer to silicates in the crystal chemical sense are
phosphates with over 350 minerals. According to Corbridge
[3]
, the term phosphate
may be used to include all phosphorus compounds which contain P
109
.47
0
;Si
O
a
Si
a
O
a
O
a
Si
5
5
O linkages.
Similarly, the author
[4]
calls all the simple and complex oxides obtained from
phosphoric acid solutions phosphates. For a free atom of phosphorus, the distribu-
tion of electrons in the valence cell is written as follows:
a
3s 3p
km mmk
ð
7
:
1
Þ
It means that the phosphorus atom can form the bonds due to 3p electrons and
another two owing to 3s electrons. Hence, the phosphorus atom participates in the
reaction with oxygen to exist in a trivalent state, forming phosphides and in a
pentavalent state forming phosphates. As a result of sp
3
hybridization (1s and 3p
orbitals of P), four similar energy orbits develop and during the interaction of phos-
phorus with oxygen, it leads to the formation of a stable tetrahedral group [PO
4
].
However,
the P
a
O bonds in such a tetrahedron will not be purely ordinary
(
σ
bonds), but they include a distinct fraction of short bonds (
π
bonds) as well. It is
well known that the P
O tetrahedra are susceptible to condensation reactions due
to the formation of bridging bonds
a
O (M). The possibility of
the development of such bonds make the p orbit of the bridging oxygen (which is
arranged perpendicular to the plane, POP) interact with the d orbits of the P atoms
to form
a
P
a
O
a
P
a
,orP
a
π
bonds. The bond lengths and bond angles in P tetrahedra are given as
Table 7.2 Bulk Content of Silicates in Continental Crust
[2]
Mineral
Percentage
Total
Feldspars
64.0
Pyroxenes
1
Amphiboles
9.0
Quartz
18.0
96.5%
Biotite
ivine
1.5
