Geoscience Reference
In-Depth Information
[155]
. The characteristic inclusions are the particles of gold, grayish white “bread-
crumb” inclusions, phenakite crystals, and rare fragments of the seed
[156]
.
Hydrothermal synthesis of emerald in USSR began in 1965
[157]
. At present, it
has been improved to obtain commercially faceted emeralds
[158]
. The Russian
method of hydrothermal growth of emerald has been reported in detail by
Schmetzer
[159]
. To cite the growth conditions of the Russian synthetic emerald
from his paper:
Growth temperature: 590
620
C
Temperature gradient: 45
C, 20
100
C, 70
130
C
Pressure: 816
1530 kg/cm
2
Solvent: acidic solution or flour-bearing solutions of complex components
Nutrient: natural beryl or oxides of Be, Al, and Si
Vessel: stainless steel (200
800 ml)
Seed: plate cut parallel to (5 5 10 6)
Under these growth conditions, a seed plate of 6 cm
2
in size increases its weight
by 10 gm in about 20 days, and an emerald layer is 5
7 mm in this period of time.
0.3 mm/day. Cr
3
1
,Fe
2
1
,Fe
3
1
,Ni
3
1
,
and Cu
2
1
have been detected through chemical analysis as color-associated transi-
tion elements. Fe
2
1
and Cu
2
1
ions were assigned to the tetrahedral sites, and Ni
3
1
,
Cr
3
1
, and Fe
3
1
ions to the octahedral sites
[159]
. Evolution of ions from the inner
wall of the autoclave is possible. It is, therefore, unknown what ions were added
during the process of growth. The length of the crystal is 6.1 cm, width 1 cm, thick-
ness 0.7 cm. The Russian synthetic emerald crystals grown for commercial
aims commented on by Klyakhin are 10
This corresponds to a growth rate of 0.2
2
2.5 cm in width
and up to 2 cm in thickness of grown layers. Their total thickness varies within
0.8
12 cm in length, up to 2
1.4 cm.
The optical properties and physical features of commercial synthetic emeralds
are presented by Sinkankas
[160]
.
Table 7.15
lists the properties of various syn-
thetic emeralds including the Russian hydrothermally grown emerald.
Kodaira et al.
[161]
obtained hexagonal prism crystals having a maximum size
of 0.3 mm in length from 0.1 N NaOH solution at 750
C. Using 0.5 N or 1 N
NaOH solution as solvents, beryl crystals were precipitated with phenacite at
500
600
C. Temperatures higher than 650
C resulted in phenacite crystals with a
small amount of beryl.
Figure 7.32
shows the hexagonal-prism-shaped crystals
grown from 0.1 N NaOH solution.
What is most significant in the hydrothermal growth of beryl is that Al and Be
hydroxides are highly reactive; the growth process is usually stopped when phena-
kite crystals start developing at the bottom of the vessel. The growth rate and mor-
phology depend upon temperature, pressure,
T, size of the crushed quartz
dopants, and so on. High growth rates produce too many inclusions.
As the sources of natural colored gemstones continue to shrink and the popula-
tion growth of the world continues to increase, the size of the market for gem mate-
rials such as synthetic emerald is expected to increase (tempered, however, by
occasional reverses in the world economy).
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