Mineral Stability Diagrams
A. When minerals are involved in chemical reactions they are either consumed or produced depending on which way the reaction goes. So, the reactant minerals are unstable, but the product minetrals are stable while the reaction progresses. If the reaction reaches equilibrium, both reactant and product minerals are stable. We, therefore, take two different approaches to investigating the stability of minerals:
1. We can determine the environmental conditions
necessary for minerals to coexist at equilibrium.
2. We can determine which minerals are stable and
which are unstable in a particular geochemical
B. The theoretical basis for such studies are:
1. The Law of Mass Action
2. The relationship between the standard free
energy change of a chemical reaction and its
equilibrium constant at 25oC.
G = - RT lnK
3. Conversions among minerals that form only
at high temperatures and presures are not shown
on diagrams drawn at temperature and pressure
of the Earth's surface.
C. Geochemists use various types of mineral stability
diagrams to display the results of these investigations.
II. Types of stability diagrams
A. Partial pressure or fugacity diagrams
1. We've already worked with this type of diagram
2. This transparency shows a partial pressure
diagram for O2 and S2 and this is what it tells us
3. Although partial pressures of gases are con-
venient for comparisons of mineral relations,
except for CO2, they are so low as to have
little physical significance under Earth surface
a. The calculated values are not verifiable
by direct experiment and they are not easy
to translate into the environmental conditions
observed in the field.
4. Therefore, we need other types of diagrams
B. Eh/pH diagrams
1. We'll work on these this afternoon and I'll
talk about their usefulness then.
2. Basically tell us the redox and pH conditions
under which various minerals are stable.
C. Solubility diagrams
1. We've already worked on one of these for the
Al(OH)3 (s) + 3 H+(aq) = Al3+ (aq) + 3 H2O (l)
This diagram is complicated by the fact that
Al forms different species in solution
depending on the pH.
REVIEW WHAT THE LINES ON THE TRANSPARENCY MEAN
Point out that axes are activities of species and that these diagrams are therefore a form of activity diagram
2. Tell us about the chemistry of a solution as
a mineral dissolves and the conditions under
which a mineral will dissolve.
3. All minerals initially dissolve congruently when placed in pure water. That is, they dissolve to give a phase with the same composition as the starting mineral.
a. For example,
NaCl (s) = Na+ (aq) + Cl- (aq)
Mg2SiO4 (s) + 4 H+(aq) = 2 Mg2+(aq) + H4SiO4 (aq)
b. This dissolution is CONGRUENT because the proportions of Mg and Si in the resultant solution are the same as in the mineral.
4. At some point the dissolution of some minerals
becomes incongruent, that is they begin to form
solids of a different composition and a liquid
that contains some of the components as dissolved
ions. The reaction then becomes IRREVERSIBLE.
a. K-feldspar dissolution proceeds congruently
at first yielding a solution with the same
proportions of K, Al and Si until the
activities of Al3+ ions in solution increase
sufficiently to stabilize kaolinite.
b. After that point, K-feldspar dissolves
incongruently to form kaolinite plus K+
c. Such systems are best handled on a differ-
ent type of diagram called an activity or
D. Activity diagrams
1. We'll start on these in a couple of weeks
but here is an example.
2. Provide useful information about:
a. What environmental conditions are
required to allow a particular mineral
b. What minerals are stable in a given
c. What ions or molecules are consumed
or produced when an unstable mineral
reacts in a given geochemical environ-
d. How does the water evolve chemically
when reactions occur in a closed system
with a small water/rock ratio?