Geoscience Reference
In-Depth Information
In general, the alkali borates exhibit essentially framework structures made up
of [BO
4
] tetrahedra and [BO
3
] triangles. Among alkali borates, only one compound
shows a chain arrangement, some four compounds show islands, and two com-
pounds show a layered arrangement. The ring arrangement in general is absent. On
the other hand, the phosphates, silicates, and sulfates show extensive chain and ring
arrangements and the crystals essentially belong to the lower symmetry. The alkali
borates, because of their basic structural units made up of [BO
3
] triangles and
[BO
4
] tetrahedra, crystallize both in higher and in lower symmetry systems and
show wide morphological variations. Similarly, the growth experiments clearly
show that any slight change in any of the growth parameters would drastically
change the resultant products. Hence, the study of morphology of alkali borates is
very interesting and a wide range of morphological variations has been observed
which exhibit some definite relationship with reference to the type of the structure,
compound, and growth conditions.
Borates were almost ignored by hydrothermal crystal growers. It was only
during the last decade that we become acquainted with an in-depth knowledge
about the behavior of boron in the hydrothermal solutions and its interaction with
various metal cations. Although not much has been done on the alkali borate sys-
tems, several other borate systems like alkali transitional metal borate systems have
been studied in detail. Today we have data available on several borate systems like:
Na
2
O
a
CdO
a
B
2
O
3
a
H
2
O, Li
2
O
a
CdO
a
B
2
O
3
a
H
2
O
[349]
,Na
2
O
a
ZnO
a
B
2
O
3
,
Li
2
O
a
CdO
a
B
2
O
3
a
H
2
O
K
2
O
a
ZnO
a
B
2
O
3
a
H
2
O,
ZnO
a
B
2
O
3
a
H
2
O
[350]
,
MgO
H
2
O
[352]
, under hydrother-
mal conditions. More than 25 different borate components have been obtained by
hydrothermal technique and many of them show interesting luminescence, nonlin-
ear, superionic, properties. For example, the Li
2
O
a
CdO
a
B
2
O
3
a
H
2
O
[351]
,Li
2
O
a
ZnO
a
B
2
O
3
a
H
2
O system has
been studied under hydrothermal conditions within the temperature range
250
a
ZnO
a
B
2
O
3
a
450
C (see
Figure 7.81a
c
)
[352]
. A series of lithium borates like LiBO
2
,
Li
2
B
4
O
7
,Li
3
B
5
O
8
(OH)
2
and mixed borates like Li
4
Zn
3
B
4
O
11
,Li
8
Zn
4
B
6
O
17
have
been obtained in this system. Further, the authors
[352]
have added MnO into the
nutrient in a very small quantity and the experiments have been carried out using
copper ampoules. It is well known that all the zinc compounds activated with
Mn
2
1
show strong luminescence in green or orange regions
[353]
. Hence, the
authors have added a small quantity (0.5 mol %) of Mn
2
1
into the system
M
2
O
Li, Na, K)
[352]
.
Further, it is known that Mn and Cu are the activators of Li
2
B
4
O
7
for thermally
stimulated luminescence dosimeters
[354,355]
. Here it is appropriate to quote the
earlier work of Garret et al.
[346]
who while growing Li
2
B
4
O
7
observed that only
small crystals of Li
2
B
4
O
7
could be obtained because of the highly corrosive nature
of Li
2
O
a
ZnO
a
B
2
O
3
a
H
2
O(M
5
B
2
O
3
that melts the platinum crucibles. Therefore, the use of copper
ampoules for the borate system is doubtful and copper might contribute to the crys-
tallization of various other phases just like the addition of MnO in the system
Li
2
O
a
H
2
O as reported earlier
[352]
. The authors have studied this
Li
2
O
a
B
2
O
3
a
H
2
O system under
a
ZnO
a
B
2
O
3
a
(T
5
200
300
C,
lower
PT
conditions
P
5
50
200 bar) and discovered several new phases like Li
4
H
2
B
2
O
6
and LiH
2
B
5
O
9
.
