Geoscience Reference
In-Depth Information
The ratio of SiO
2
/CaO (degree of silification)
regularly increases 0.5
!
0.75
O] radical leads to the transition from discrete ortho, diortho group
to infinite chains and then infinite ribbon [Si
6
O
17
]
N
. In most of the experiments,
quartz also crystallizes together with xonotlite.
In the second case, the degree of silification increases insignificantly (0.33
!
1.0. [Si
a
!
0.4
!
0.62
!
0.66) and morphotropic transitions in [Si
a
O] radical takes place as
follows: [SiO
4
]
[SiO
4
][Si
3
O
10
].
Nd
3
1
-doped Ca
2
Al
2
SiO
7
(gehlenite) is an efficient new laser material
[191]
,
since its structure exhibits some disorder and can accept active rare earth ions. The
unit cell is tetragonal (melilite type) with special group P42
lm
.
Figure 7.38
shows
the Na
3
1
emission spectrum corresponding to the
4
F
3/2
!
!
[SiO
4
]
2
[Si
3
O
10
]
!
4
I
11/2
transition (a) in geh-
lenite and (b) in YAG.
In the system Na
2
O
H
2
O under hydrothermal conditions below
350
C, two phases crystallize: BaSiO
3
a
BaO
a
SiO
2
a
nH
2
O
[192]
and BaSi
2
O
5
3H
2
O
[193]
,in
the BaCl
2
1
Na
2
H
2
SiO
4
solution, the low temperature of growth and the nature of
the solution do not form silicates when BaO/SiO
2
.
1:1. At temperatures 450
550
C, pressures 400
1200 atm, and BaO/SiO
2
from 7.5:1 to 1:10, the following
phases form: BaSiO
3
,Ba
2
Si
3
O
8
, BaSi
2
O
5
(sanbornite) and Ba
5
Si
4
O
12
(OH)
2
,
NaBa
3
Si
2
O
7
(OH)
[194]
.
Figure 7.39
shows NC diagram of crystallization in the
system Na
2
O
H
2
O, and
Figure 7.40
shows the schematic diagram of
the crystal morphology of the barium silicates. From the NC diagram, the bound-
aries of crystallization of BaSi
2
O
5
and BaSi
3
O
8
are controlled only by NaOH con-
centration. Further,
a
BaO
a
SiO
2
a
the boundaries between monocationic barium silicate and
Figure 7.38 Na
3
1
emission
spectrum corresponding to the
4
F
3/2
4
I
11/2
transition in (a)
gehlenite and in (b) YAG
[191]
.
(a)
(b)
1030
1050
1070
1090
1100
λ
(nm)
