ENERGY IN GEOCHEMICAL PROCESSES - Chemical Fundamentals of Geology and Environmental Geoscience - page 12

(a) (b)

Aragonite

B

G

Y

G aragonite

G calcite

p

A

X

Calcite X p Y

Pressure

Surface conditions

O

Temperature

Figure 1.3 Stability of CaCO 3 polymorphs. (a) Pressure-temperature phase diagram showing the stability fields of calcite
and aragonite. The line A-B is the phase boundary, indicating the P-T conditions under which calcite and aragonite are able
to coexist in stable equilibrium. (b) Variation of free energy ( G ) of calcite and aragonite over a range of pressures along the
isothermal line X-Y in (a).

of the wooden block in Figure 1.2a. At a lower pressure
(X) nearer to the surface, however, the position is
reversed: calcite has the lower free energy and is there-
fore the stable mineral. The lines representing the free
energy of calcite and aragonite as a function of pres-
sure cross over in Figure 1.3b at a point marked p. Here
the two minerals have equal molar free energies, and
are therefore in chemical equilibrium with each other.
Point p therefore marks the position in Figure 1.3b of
the phase boundary appearing in Figure 1.3a.
Imagine transporting a sample of aragonite from the
conditions represented by point Y to a new location (at a
shallower depth in the Earth's crust) having the pressure
and temperature coordinates of point X. Under the new
conditions aragonite will no longer be the stable mineral
and it will tend to achieve a state of lower free energy by
recrystallizing to calcite. This transformation may not
occur immediately, however, because the status of arag-
onite is similar to the wooden block in position B. The
same three points made above in relation to Figure 1.2
can be reiterated for the calcite-aragonite system:

(b) Many other forms of energy are associated with
calcite and aragonite under such conditions, but in
discussing thermodynamic stability we are con-
cerned only with free-energy differences between
alternative states. This has the important conse-
quence that free energy needs only to be expressed
in relative terms, referred to a convenient but
arbitrary common point, a sort of thermodynamic
'sea-level'. All important applications of thermo-
dynamics involve the calculation of free-energy
differences between the various states of the sys-
tem being considered, and the notion of an absol-
ute scale of free-energy values, analogous to the
absolute temperature scale, is unnecessary and
inappropriate.
(c) In spite of not being stable under near-surface con-
ditions (Figure 1.3), aragonite is quite a common
mineral, and may survive for long periods of geol-
ogical time on the surface of the Earth. Like con-
figuration B in Figure 1.2a, aragonite may give the
appearance of being in a stable state in such cir-
cumstances, even though its free energy clearly
exceeds that of calcite. (Under certain circum-
stances, aragonite may actually crystallize under
near-surface conditions: for example, the shells of

(a) Calcite, having the lower free energy, will be the
stable form of calcium carbonate under the lower
pressure conditions defined by X.