Mineral Stability Diagrams

 

I. Introduction

 

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

     environment.

 

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    

conditions.

 

          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

     mineral gibbsite.

 

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+

          and H4SiO4.

 

          c. Such systems are best handled on a differ-

          ent type of diagram called an activity  or

Korjinski diagram.

 

 

 

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

          to form?

 

          b. What minerals are stable in a given

          geochemical environment?

 

          c. What ions or molecules are consumed

          or produced when an unstable mineral

          reacts in a given geochemical environ-

          ment?

 

          d. How does the water evolve chemically

          when reactions occur in a closed system

          with a small water/rock ratio?