Geoscience Reference
In-Depth Information
Figure 7.89 Photograph of Li
3
B
5
O
8
(OH)
2
crystal
(magnification
3
25)
[364]
.
calculating the pressure. The experiments were carried out by spontaneous crystalli-
zation. The starting materials such as lithium carbonate and boric acid were taken in
the following molar ratio:
Li
2
O:B
2
O
3
5
ð
0
:
4
0
:
7
Þ
:
ð
2
:
5
4
:
3
Þ
ð
7
:
29
Þ
2
2
2). Since
the crystallization took place through spontaneous nucleation, the temperature of
the furnace was slowly increased to control the rate of nucleation. In the majority
of the experiments, the percent filling varied from 50 to 65. The experimental con-
ditions under which Li
4
B
7
O
12
Cl crystals obtained are given in
Table 7.30 [369]
.
These experiments yielded crystals up to 2 mm in size. In the majority of the
experiments, Li
4
B
7
O
12
Cl crystals obtained were twinned and the size varied up to
3 mm. The twinning was also controlled and the transparency of the crystals was
also enhanced by means of increasing the growth temperature, quenching of the
autoclaves, and accurate monitoring of the experimental conditions. The rate of
heating and the rate of cooling of the autoclaves and the establishment of equilib-
rium conditions in the system control the defect formation in these crystals.
LiH
2
B
5
O
9
. The synthesis of LiH
2
B
5
O
9
crystals has been carried out for the first
time by Byrappa et al.
[370]
. LiH
2
B
5
O
9
crystallizes within a narrow region in the
Li
2
O
The mineralizer solution HCl was taken with a definite molarity (1.5
H
2
O system.
This new compound belonging to the monoclinic system space group (P2
1
/a)
has been obtained under hydrothermal conditions at 250
C and 100 bar pressure.
The starting materials such as LiBO
2
,B
2
O
3
, and solvent 1.5 M HCOOH were taken
in Teflon liners. The Li
2
O:B
2
O
3
:H
2
O molar ratio was 1
a
B
2
O
3
a
3.5. The
experiments were carried out using Morey type of autoclaves. The % fill was var-
ied from 50 to 60. Several mineralizers were tried and the HCOOH was found to
be a better mineralizer with its molarity varying from 1.5 to 2 M.
The formation of LiH
2
B
5
O
9
must have taken place according to the following
reaction:
1.5:.5.5
6:3
2LiBO
2
1
2HCOOH
!
2HCOOLi
1
B
2
O
3
1
H
2
O
ð
7
:
30
Þ
5H
3
BO
3
!
ð
:
Þ
HCOOLi
1
LiH
2
B
5
O
9
1
CO
1
7H
2
O
7
31
