Geoscience Reference
In-Depth Information
XY
Mg, Fe, Mn, Li, Al, and
so on. Tourmaline crystals exhibit pyroelectric properties and the importance of the
seeded growth of tourmaline was realized during the 1940s. The basic interest lies
in the colorless variety of tourmaline for pyroelectric applications. The earliest
attempts to obtain tourmaline single crystals were carried out through proper selec-
tion of nutrient and the solvent
[168,169]
. Robbins et al.
[170]
have carried out the
growth of tourmaline crystals using metastable glass of tourmaline composition
and the given pressure
Al
6
[Be
6
Si
6
O
27
(OH,F)
4
], where X
Na, Ca, K; Y
5
5
temperature diagrams of stability. Voskresenskaya
[171]
,
and Voskresenskaya and Barsukova
[172]
have carried out the synthesis of color-
less tourmaline on the seeds under hydrothermal conditions. The above authors
have carried out experiments in autoclaves with a volume of 200 cm
3
at tempera-
tures 550
600
C and pressure 2000 atm, using platinum liners. A mixture of oxi-
des or glass of tourmaline composition was used as the nutrient. Natural tourmaline
was used as seed crystals. The growth was carried out in the acidic media from
aqueous solutions H
3
BO
4
a
NaCl (NaF). The concentration of boric acid was
5
2.5 wt%. The seed orientation was along the (0001) face, polar
in character, and crystals usually developed simple forms (1011), (0221), and
(0112). However, in recent years, the hydrothermal growth of tourmaline is mainly
of academic interest and the focus is on the solid solutions of tourmaline. Besides,
there are some reports related to the gemological potential of the synthetic
tourmaline.
10 wt%, NaCl
B
7.9.4 Nepheline
Nepheline is an important mineral and has the orthosilicate formula (Na,K)AlSiO
4
,
with a framework of aluminosilicate having acentric space group P6
3
. Nepheline is
of interest as piezo- and pyroelectric material. The nepheline structure is related to
“tridymite intercalation stones,” which allows it to vary the Na/K ratio in a wide
interval. In nature, nepheline usually occurs as small glassy crystals or grains. The
natural nephelite always contains silica in excess and also small amounts of potas-
sium. Nepheline can be easily prepared artificially by fusing its constituents
together in the proper proportions. The earliest experiments on the artificial synthe-
sis began in the nineteenth century. Friedel
[173,174]
treated muscovite with
NaOH or KOH solution and water at 200
500
C and obtained nephelite. The end
members of this row are characterized by several polymorphs, and the nepheline
structural type is characteristic for Na/K ratios from 1 to 0.5.
Nepheline, NaAlSiO
4
, is stable in NaOH solutions with concentrations 3
15 wt%
at temperatures more than 450
C. Increase in temperature results in the expansion
of the stability field to 20
450
C, nepheline-
hydrate-I and nepheline-hydrate-II form from artificial cancrinate and sodalite.
However, under the same conditions, the nepheline phase is in equilibrium with
solution if synthetic nepheline is used as the starting material. The boundary nephe-
line hydrate, cancrinite, seems to be metastable. Nepheline (both synthetic and nat-
ural) is replaced by cancrinate at 200
22 wt% at 500
C. At 400
C
,
T
,
300
C and NaOH concentrations greater
than 3 wt%
[175,176]
.
