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volatile constituent. The consequent existence of a vapour
in all the experiments means that the vapour pressure
was equal to the total pressure applied to the specimen.
The general symbol for vapour pressure is
P
V
; one can use
the more specific symbols
P
HO
2
4
or
P
CO
2
for the vapour
pressure of water and carbon dioxide respectively.
Figure 2.3 is therefore a vapour pressure-temperature
diagram, which can be analysed using the Phase Rule in
the same way as Figure 2.2. For example, if we choose the
components carefully in Figure 2.3 (Equation 2.4), we find
that only three are necessary to constitute all four phases:
3
Muscovite
2
Sanidine
+
Corundum
+
Vapour
KAlSiO AlOHO
38
F
23 2
1
since the composition of muscovite can be represented
as a combination of these three.
Point F
(4 phases = muscovite + sanidine +
corundum + water)
0
ϕ
= 4
500
1000
T
/
º
C
C
= 3
(3 components, KAlSi
3
O
8
+ Al
2
0
3
+ h
2
O)
4 +
F
= 3 + 2
Therefore
F
= 1
Figure 2.3
PT
HO
2
− diagram showing the 'dehydration
curve' of the mica muscovite (KAl
2
Si
3
AlO
10
(OH)
2
), showing
the
PT
HO
2
a
univariant
equilibrium.
− conditions at which it breaks down into the
assemblage sanidine (KAlSi
3
O
8
, an alkali feldspar) +
corundum (Al
2
O
3
) + vapour (H
2
O). Muscovite and sanidine
are aluminosilicates of potassium (K).
Le Chatelier's principle
Behind the empirical facts of mineral stability, as repre-
sented by the experimentally determined
P-T
diag-
rams in Figures 2.1, 2.2 and 2.3, there are some
important thermodynamic principles which will help
in the interpretation of phase diagrams.
The first of these concerns the distribution of phases in
a phase diagram. Why is kyanite stable at high pressures,
whereas andalusite can survive only at low pressure
(Figure 2.1)? What properties of the two minerals dictate
this behaviour? How is it that sillimanite is more stable
than either of them in the highest temperature range?
The answers to these questions lie in a simple princi-
ple enunciated by the French chemist Henri Louis Le
Chatelier in 1884:
when a system at equilibrium experi-
ences a change in physical conditions, the system will adapt
in a direction which tends to nullify the change
. In the pre-
sent context, the 'physical conditions' referred to are
pressure and temperature.
Consider a system comprising kyanite and andal-
usite in mutual equilibrium, for example under the
conditions represented by point D in Figure 2.1:
another important class of reactions, in which a volat-
ile constituent plays an essential role.
It shows a reaction involving water (the dehydration
of muscovite at high temperatures):
(
)
KAlSiO OH
KAlSiO
+
Al OHO
23 2
corundum vapour
+
3310
38
2
(2.4)
sanidine
afeldspar
musite
a mica
cov
(
)
(
)
and Figure 2.1.1a in Box 2.1 shows a similar reaction
involving carbon dioxide, which is important in the
metamorphism of siliceous limestones:
CaCO
+
SiO aSiO
+
CO
2
3
2
3
calcite
quartz
wollastonite
apyroxene like
m
vapour
(2.5)
(
-
ineral
)
Because molecules of H
2
O and CO
2
are involved in
these reactions, vapour pressure exerts a strong influ-
ence on the position of equilibrium.
The experiments from which these diagrams were pre-
pared were carried out in the presence of an excess of H
2
O
or CO
2
respectively (as a separate gas phase), so that at all
times the experimental charges were saturated with the
Al SiO lSiO
2
5
2
5
(2.6)
andalusite
kyanite
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