Geoscience Reference
In-Depth Information
on a series of powder diffraction spectra collected during the course of a chemical
reaction to determine the amount of each crystalline component as a function of
time
[55]
. This has several advantages when a number of processes are occurring
simultaneously as the powder method can determine the amount of each phase and
the rate at which it takes place, in a mixture of phases. The hydrothermal cell
consists of a stainless steel container with a Teflon lining. The top of the cell is con-
nected to a control manifold, which contains a pressure bleed valve, a bursting disk
assembly, and a pressure transducer which is used to measure the pressure inside the
cell. The cell operates up to 40 bar at a maximum temperature of 230
C and is pro-
tected by a thin metal foil-bursting disk, which bursts at pressure beyond 65 bar. The
sample holder is a cylinder with a volume of 30 cm
3
. The cell wall thickness is
3 mm and is reduced to 1 mm at the beam entry and exit points.
Figure 3.20
shows
the schematic diagram of a hydrothermal reaction cell.
PVT Apparatus
Several designs have been made for PVT measurements in highly corrosive
systems. These devices usually consist of standard pressure manometers. Kolb and
Laudise
[56]
have designed a PVT apparatus (
Figure 3.21
) for highly corrosive
fluids
(e.g., berlinite systems). Also, this has been used to measure the pressure in
the H
2
O
NaOH system. The pressure takeoff is lined with platinum and
connected to a water capillary of small capacity which goes to the pressure gauge.
Several other designs have been proposed by the Special Construction Bureau,
Institute of Crystallography, Moscow.
a
SiO
2
a
Apparatus for Solubility Determination
The solubility of substances under hydrothermal conditions is fundamental informa-
tion concerning their crystal growth. Solubility measurements are usually carried
out by the weight loss method. This approach is effective only for compounds
having positive temperature coefficients of solubility. For compounds with negative
solubility coefficients, the possibility of further weight loss of the specimen due
to increased solubility during the quench, or even at room temperature, must be con-
sidered
[57]
. In such cases, solubility measurements are carried out in specially
designed autoclaves. However, the solubility and phase-equilibria studies are usu-
ally carried out in Tuttle cold-cone-sealed autoclaves or small autoclaves without
a pressure gauge. Sometimes, other available autoclaves are also used if the quench-
ing can be carried out effectively. The quenching ensures the maintenance
of conditions in the autoclave corresponding to equilibrium conditions at test tem-
perature. The Special Construction Bureau, Institute of Crystallography, Russian
Academy of Sciences, Moscow, Russia, has designed several solubility measure-
ment apparatuses consisting of single autoclave or multiple autoclaves (by creating
a liquid thermostat).
Figure 3.22
shows an assembly for studying solubility simulta-
neously in two autoclaves
[58]
.
