Geoscience Reference
In-Depth Information
where the fluid phase is liquid alone or gas alone, the weight loss will be a measure
of solubility. However, special care must be exercised to avoid overshooting, to
ensure that the desired temperature is reached during warm up, to avoid losing
material before weighing, or weighing material that has precipitated during the
quenching. In many cases, the solubility obeys the van't Hoff equation and is line-
arly dependent on solution density
[91]
. The ratio of solubility to mineralizer con-
centration often gives clues to the species present
[92]
. Several additional
difficulties were encountered with the silicates, aluminosilicates, phosphates, and
so on. They were first noticed while studying the solubility of quartz. These diffi-
culties principally arise from phase separation of the solution under certain condi-
tions, and the formation of a “heavy” phase enriched with silicon
[93]
. For
example, it was shown by the authors of Ref.
94
, in connection with the solubility
of albite in H
2
O at 200
350
C, that under these conditions there was a partial
decomposition of the albite forming analcime. Hence, in this case, the true solubil-
ity of albite in water was not really being determined. Therefore, to obtain the true
solubility of any compound which forms the heavy phases or other
metastable phases, it has to be determined under conditions similar to its conditions
of synthesis, so as to determine its optimum growth conditions. Russian workers at
the Institute of Crystallography have designed an apparatus to suit the solubility
study of various compounds (Figure 3.22).
Let us consider the solubility studies of some selected compounds like zincite
and malachite. Solubility of a compound may be positive or negative. If the solubil-
ity of a compound increases with increasing temperature, it is positive. If the solu-
bility of the compound decreases with increasing temperature, it is negative. Some
compounds show positive solubility up to a certain temperature and then show neg-
ative solubility and vice versa. Quartz and berlinite form the best examples. The
majority of the earliest hydrothermal experiments, carried out in the nineteenth
century, had pure water as the mineralizer. Today we use a great variety of minera-
lizers, especially aqueous solutions of salts, acids, bases, etc., at very low concen-
trations. Likewise, there are a great numbers of nonaqueous solvents. Hannay and
Hogarth used an alcoholic solution as a mineralizer a century ago
[95]
. Usually
nonaqueous solvents are preferred when the required solubility cannot be reached
in an aqueous medium. However, the composition of the effective solvents used in
growing crystals is fairly standard. Pure water can be a good solvent at very high-
temperature and high-pressure conditions. The use of various acids, salts, and bases
considerably reduce the PT conditions. The commonly used nonaqueous solvents
are NH
3
, HF, HCl, HBr, Br
2
,S
2
Cl
2
,S
2
Br
2
, SeBr
2
,H
2
S
(C
2
H
5
)
3
,NH
4
Cl,
C
2
H
5
OH, CS
2
, CCl
4
,C
6
H
6
,CH
3
NH
2
, etc. In the past decade, the growth conditions
have been further reduced with the use of some acid solvents. For example, high
molar acids like HCl, H
2
SO
4
,H
3
PO
4
, HNO
3
, HCOOH are freely used in the growth
of various compounds and this has brought down the growth temperature to below
300
C. In recent years, mixed solvents have been used in the growth of some com-
pounds like AlPO
4
. The mixed solvents are found to be highly effective. The reduc-
tion in the growth temperature considerably reduces the working pressure, which in
turn helps in the use of simple apparatus. Even silica autoclaves can be used, which
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