Geoscience Reference
In-Depth Information
like isolated, infinite motifs of chains and layers. Here it is necessary to emphasize
the abundance of Ti octahedra sharing their edges in silicates, making it closer to
its neighbor diagonally in the periodic table, i.e., Al
3
1
, which can form tetrahedral
and octahedral coordinations with both Al octahedra and Ti octahedra sharing their
edges (epidote, kyanite, sillimanite, and andalusite). Such a formal substitution of
Al
3
1
in tetrahedra and Ti
4
1
octahedra does not change the charge balance in the
silicate structures. Considering the closeness of Al
Si frameworks, it is
necessary to show their principal difference of bonding with different valency and
coordination of silicon substitutes.
It is well known that all the variations in the framework silicates are connected
with the heterovalent substitution: Si
4
1
!
Si and Ti
Al
3
1
. If the framework is made up of
pure (SiO
2
), then in the isovalent substitution (but not isomorphic) of Si
4
1
!
Ti
4
1
,
the Ti
4
1
gets the additional oxygen atoms to form an octahedral coordination.
It is interesting to note that Zr
Hf forms an excellent solid solution because of
its unique chemical and crystal chemical closeness, which brings in the neighboring
elements, including the lanthanide series. The other elements which can form the
isomorphic substitution with Ti
4
1
are Nb
5
1
,Ta
5
1
,Zr
4
1
,Fe
3
1
, and Al
3
1
. All these
substitutions are obtained experimentally; however, in nature such a large substitu-
tion has not been found, e.g., Al
3
1
rarely substitutes for Ti
4
1
in minerals or vice
versa. Here, it is necessary to remember that both Al and Ti show chemically
amphoteric behavior which changes their coordination numbers significantly, so
also the ionic radii (rTi
4
1
5
0.68
˚
, rAl
3
1
5
0.51
˚
) plays an important role.
Therefore, a part of the Al
3
1
, octahedrally coordinated, can form the isomorphous
substitution with octahedrally coordinated Ti
4
1
.
One of the most common schemes observed in the heterovalent substitution of
Ti
4
1
!
Nb
5
1
shows 2Ti
4
1
!
Nb
5
1
1
Fe
3
1
. There are many such examples for
Ti
4
1
!
Nb
5
1
in the synthetic compounds like TiNbO
7
,Ba
6
Ti
2
Nb
8
O
30
,and,simi-
larly, YNbTiO
6
, a symmetric analog of the metamict mineral
[76
78]
.Onlyin
two mineral structures—epistolite, Na
2
(Nb, Ti)
2
Si
2
O
9
nH
2
O, and vuonnemite,
2Na
3
PO
4
—has such an isomorphism between Ti
4
1
and Nb
5
1
Na
4
TiNb
2
Si
4
O
17
been reported.
In spite of the isovalent behavior of these Ti
4
1
and Zr
4
1
atoms, their crystal
chemical relation is not as close as that of Ti
4
1
!
Nb
4
1
. The main reason is higher
ionic radii of Zr
4
1
(0.79
˚
) and also the different nature of bonding of Zr
O and
Ti
O, particularly the latter, which is appreciably different. Moreover, there is a
greater variation in the Ti
O octahedra, and also the
Ti
4
1
octahedra has a tendency to undergo condensation, and it is well known that
the coordination number of Ti
4
1
in quite a few structures goes down from 6 to 5
and even to 4. In contrast, the Zr
O bond distances in the Ti
O bond distances do not vary much in the Zr
octahedra, which show no tendency for undergoing condensation. Besides, in a
series of oxides (both simple and complex), the Zr
4
1
cations increase their coordi-
nation number from 6 to 7 and up to 8.
In recent years, a large number of experiments have been carried out on the
isomorphism of various titanium bearing compounds, which have technological
applications. BaTiO
3
, SrTiO
3
, alkali titanates, LiNbO
3
, ABO
4
(A
Sb, Bi; B
Nb,
5
5
