Geoscience Reference
In-Depth Information
Table 7.16 Chemical Composition of Natural and Artificial Nepheline
Compound
Oxide Contents (wt%)
SiO 2 Al 2 O 3 Fe 2 O 3 FeO MgO CaO Na 2 OK 2 O
Nepheline natural (starting material) 43.63 32.37 0.41
0.17 0.76
0.36 15.94 6.0
Nepheline artificial (grown layer)
41.55 34.47 0.74
0.10
16.77 5.98
Nepheline single crystals can be grown in the field of its thermodynamic stabil-
ity as in the field of its instability at 300 C
400 C if the growth process is
initiated by nepheline seed. In the region of instability, single crystals of nepheline
grow on the seed but hydrocancrinate, Na 8 Al 6 Si 6 O 24 (OH) 2
,
T
,
nH 2 O, crystals grow on
450 C, nepheline and cancrinate grow simultaneously if
there are seeds of both nepheline and cancrinate.
Single crystals of (Na,K)AlSiO 4 were grown in the system Na 2 O
spontaneous nuclei. At T
.
a
K 2 O
a
Al 2 O 3 a
H 2 O using natural nepheline as a nutrient and as a material for
seeds. To minimize the mismatch between the seed and the grown layer, the potas-
sium was introduced into the starting NaOH solution. As a result, the composition
of the grown layer was close to the composition of seed plates ( Table 7.16 ).
The unit cell parameters are equal to a
SiO 2 a
5.16 ˚
12.64, c
(seed material),
5
5
5.12 ˚ (grown layer). At temperatures of 300
350 C in the growth
zone, the regeneration of seed plates takes place with the formation of the follow-
ing simple forms: hexagonal prisms (1010), monohedras (0001) and (0001) and
sometimes hexagonal pyramids (1011), (1012). The polar symmetry is observed
rarely, and the external habit usually corresponds to the point group 6/m or 6/
mmm. In this case, the growth morphology and growth rates of (0001) and (0001)
faces are the same, which may be caused also by twinning. A specific cellular
microstructure is observed in the transmission light; the cell size is 1.5
a
12.72, c
5
5
3 mm.
The mean values of the growth rates for different crystallographic directions are:
γ (1010): 0.04 mm/day
γ (0001): 0.12 mm/day
γ (1011): 0.4 mm/day
γ (1011) . γ (1210) . γ (0001) $ γ (0001) . γ (1010).
The faces (0001) and ð 1010 Þ grow on spiral mechanisms, the faces of pyramid
grow on normal mechanisms. Nepheline crystals exhibit interesting unidimensional
ionic conductivity. They show anamolously high dielectric constant parallels to the
c-axis [177] .
7.9.5 Zincosilicates
The earliest work on the hydrothermal synthesis of zincosilicates appeared during
1974 [178] . The study of zincosilicates carries a special significance with reference
to their crystal chemistry and phase stability, as they occur in nature under varying
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