pressure, water vapour will tend to condense to liquid water
(dew, mist, rain). The relative humidity expresses the actual
water vapour pressure in a given body of air as a percent-
age of the saturation vapour pressure at the temperature
The univariant curve dividing the liquid and vapour fields
ends abruptly at invariant point C, known as the critical
point of water. At this combination of P and T , the structural
distinction between liquid and gaseous states vanishes.
The two states merge into a single phase. At higher tem-
peratures and pressures, h 2 O exists as a homogeneous
single phase called a supercritical fluid, in which are com-
bined the properties of a highly compressed gas and a
superheated liquid. Some of the hydrothermal fluids respon-
sible for depositing ore bodies come into this category, and
the noun 'fluid' used alone often has this connotation in
geology. All liquid/gas systems become supercritical fluids
under sufficiently extreme conditions.
The atmospheric pressure ( P A ) isobar cuts the ice-water
phase boundary at exactly 0 °C ( T m ). Note that this phase
boundary has a negative slope , 3 a unique feature of the
ice-water system upon which every ice-skater uncon-
sciously depends. It expresses the fact that the melting
point of ice decreases as pressure is increased, so that ice
close to 0 °C can be melted simply by the application of
pressure, such as the skater's weight acting on the narrow
runner of the skate. This behaviour, like many other proper-
ties of water (Box 4.1), is unique among the common liq-
uids: melting points for most other materials rise with
increasing pressure, as the phase diagram for carbon diox-
ide (inset in Figure 2.2.1b) illustrates. See Exercise 2.2 at
end of this chapter.
only two polymorphs of carbon (i.e. ϕ < 3), there is no invari-
ant point (so that F > 0) in this phase diagram.
The H 2 O phase diagram
Figure 2.2.1b shows the phase equilibria between the famil-
iar forms of pure h 2 O as a function of pressure and tempera-
ture. Note that the axes have not been drawn to scale.
Vapour, liquid and solid coexist at only one point in the
diagram (0.06 × 10 5 Pa and 0.008 °C). This triple point 'T'
lies below atmospheric pressure, which is shown as
P A = 1 × 10 5 Pa (dashed line). The curve T-C shows the
vapour pressure at which liquid water and vapour are in
mutual equilibrium (the equilibrium or saturation vapour
pressure ) as a function of temperature. Along this curve, the
vapour is said to be saturated; at pressures below the
phase boundary T-C, however, the vapour is unsaturated
and no liquid water can form. The equilibrium vapour pres-
sure curve T-C rises with temperature to reach P A at 100 °C.
We can define the boiling point of pure water ( T b ) as the
temperature at which the equilibrium vapour pressure (the
univariant curve T-C) becomes equal to atmospheric pres-
sure. The vapour then exerts sufficient pressure to displace
its atmospheric surroundings and form bubbles in the liquid,
the everyday phenomenon of boiling. (Note that if the atmos-
phere pressure is lowered below P A , on a high mountain for
example, water can boil at temperatures lower than 100 °C.)
At room temperature (25 °C), the equilibrium vapour pres-
sure of water lies well below atmospheric pressure P A .
Atmospheric water vapour can be considered to exert a
partial vapour pressure in proportion to its concentration in
the air. If this partial pressure is below the saturation
vapour pressure, there will be a net evaporation of water
to vapour (clothes dry, puddles evaporate), whereas if the
partial pressure of water exceeds the equilibrium vapour
i.e. a negative gradient - see Figure A1b in Appendix A.
commonly shown inverted, with pressure increasing
downwards and with a depth scale added to the pres-
sure axis (cf. Figure 2.5.1). This orientation allows pres-
sure-dependent phase equilibria to be more easily
correlated with geophysical profiles of the crust and
upper mantle. Such diagrams are particularly helpful
for representing mantle melting processes as a func-
tion of depth (Box 2.5).
P v - T diagrams
Reactions in which all the reactants and products are
crystalline minerals such as those illustrated in
Figures 1.3a, 2.1 and 2.2 are known as 'solid-solid reac-
tions'. No vapour is involved, and its presence or
absence in the experiments is immaterial to the equi-
librium finally obtained (although it can accelerate
progress toward equilibrium). Figure 2.3 illustrates