Geoscience Reference
In-Depth Information
magnitude lower than the solubility at pH
.
pK
1
(H
2
S), this complex has to be
taken into account when such systems are modeled. The main reason for the
decreasing solubility of gold in high-temperature sulfide solutions is the change
in pH and/or total reduced sulfur concentration. The effect of temperature on the
solubility of gold in sulfide solutions is very strongly related to the total sulfur
concentration and the hydrothermal fugacity.
Crystallization of copper, silver, selenium, and tellurium in alkaline solutions
takes place in aqueous solutions of hydroxides of alkali metals at partial pressure
of hydrogen, 0.5
550
C.
For obtaining fairly large crystals, the experiments are carried out with temperature
gradients and in autoclaves with or without different liners. Glass carbon ampoules
are most effective in this respect. In experiments at T
40 atm, water pressure
600 atm, and temperature 350
.
300
C, Teflon liners are
used. As nutrient, respective metal oxide powders or metal pieces are used.
Popolitov
[16]
and Popolitov and Litvin
[17]
have studied the systems
,
Sb
H
2
O and obtained the crystals of Se and Te. They proposed a sim-
ple method of synthesis of Se and Te crystals in aqueous solutions of hydrochloric
acid in autoclaves provided with quartz ampoules.
The octahedral or more complex form of single crystals of copper have been
obtained in the cooler zone of the autoclave containing aqueous solutions of NaOH
KOH. In highly concentrated aqueous solutions of hydroxides of alkali metals and
at P
H
2
O
.
a
Te
a
I(Br)
a
often the platelike crystals form with (110) faces exhibiting
highly vitreous luster, and the crystal size varies from 1 to 3 mm
[18]
.
Silver crystallization conditions are very close to those of the crystallization of
copper. Crystals are usually platelike and show twinning. The crystal surfaces
exhibit metallic luster with a rise in temperature and silver dendrites form, resem-
bling its appearance in nature. The size varies from 0.1 to 0.5 mm
[19]
.
Selenium crystals have been obtained under hydrothermal conditions by Kolb
and Laudise
[20]
. The crystals are needlelike, prismatic, with metallic luster, col-
ored light gray. The size of the individual needles or crystals is up to 10
50 atm
;
12 mm
with a thickness of 0.2
0.3 mm.
Tellurium crystals have been obtained in aqueous NH
4
Cl solutions (8
15 wt%)
550
C, the partial pressure of hydrogen was
at temperatures of 450
80 atm, and
P
H
2
O
600 atm with a rise in the concentration of the solvent (up to 20 wt%). The
skeleton-like forms appear in Ref.
[21]
.
In recent years, the synthesis and growth of bulk crystals of native elements
under hydrothermal or solvothermal conditions does not carry a great significance,
and the present trend is the synthesis of nanoparticles of several elements for
specific applications.
Diamond is the most precious of the gemstone. The word “Diamond” originally
comes from the Greek word “Adams” which means “Unconquerable.” Diamonds
were formed at greater depths of 150
B
300 km below the surface of the earth's
crust under enormous pressure and very high temperature, millions and millions of
years ago. Diamond synthesis has become a very important activity. Several
research organizations and corporations have taken up diamond synthesis as a chal-
lenge in solid-state science. Man started using diamond several centuries
BC
.At
