Geology Reference
In-Depth Information
chemical reaction, as in Equation 1.1. For the water/
vapour equilibrium:
which direction the reaction will proceed under the
conditions being considered. A negative value of Δ G
indicates that the products are more stable - have a
lower free energy - than the reactants, so that the
reaction can be expected to proceed in the forward
direction. If Δ G is positive, on the other hand, the
'reactants' will be more stable than the 'products',
and the reverse reaction will predominate. In either
case, reaction will lead eventually to a condition
where Δ G = 0, signifying that equilibrium has been
Now let us see how these principles apply to miner-
als and rocks.
The equilibrium symbol (⇋) represents a balance
between two competing opposed 'reactions' taking
place at the same time:
'Forward' reaction:
liquid → vapour
reactant product
'Reverse' reaction:
liquid ← vapour
product reactant
By convention, the free-energy change for the forward
reaction (Δ G ) is written:
Stable, unstable and metastable minerals
The terms 'stable' and 'unstable' have a more precise
connotation in thermodynamics than in everyday
usage. In order to grasp their meaning in the context of
minerals and rocks, it will be helpful to begin by con-
sidering a simple physical analogue. Figure 1.2a shows
a rectangular block of wood in a series of different
positions relative to some reference surface, such as a
table top upon which the block stands. These config-
urations differ in their potential energy, represented by
the vertical height of the centre of gravity of the block -
shown as a dot - above the table top. Several general
principles can be drawn from this physical system
which will later help to illuminate some essentials of
mineral equilibrium:
Each G can be expressed in terms of molar enthalpy
and entropy values obtained from published tables
(Equations 1.8 and 1.9). Thus
In this equation Δ H is the heat input per mole required
to generate vapour from liquid (the latent heat of
evaporation). In the context of a true chemical reac-
tion, it would represent the heat of reaction (strictly the
enthalpy of reaction). If Δ H for the forward reactions
is negative, heat must be given out by the reaction,
which is then said to be exothermic ('giving out
heat'). A positive value implies that the reaction will
proceed only if heat is drawn in from the surround-
ings. Reactions that absorb heat in this way are said
to be endothermic ('taking in heat'). Δ S represents
the corresponding entropy change between liquid
and vapour states.
The values of H vapour , H liquid , S vapour and S liquid can be
looked up as molar quantities for the temperature of
interest (e.g. room temperature ≃ 298 K) in published
tables. In this case, Δ H and Δ S can be calculated by
simple difference, leading to a value for Δ G (taking
care to enter the value of T in kelvins, not °C). From
the sign obtained for Δ G , it is possible to predict in
(a) Within this frame of reference, configuration D has
the lowest potential energy possible, and one calls
this the stable position. At the other extreme, con-
figurations A and C are evidently unstable , because
in these positions the block will immediately fall
over, ending up in a position like D. Both clearly
have higher potential energy than D.
(b) In discussing stable and unstable configurations,
one need not consider all forms of energy pos-
sessed by the wooden block, some of which
(for  example, the total electronic energy) would
be difficult to quantify. Mechanical stability
depends solely upon the relative potential energies
of - or energy differences between - the several
configurations, and not on their absolute energy
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