22 - Lecture notes for Clay Mineralogy

Chemical weathering and clay mineral formation

---

Suggested reading:

Garrels, R.M. and Christ, C.M. (1965) Solutions, minerals and equilibria, Freeman and Cooper. 450 pages.

Nordstrom D. K. and Munoz J. L. (1985) Geochemical Thermodynamics: Benjamin/Cummings Publishing Co., Inc., Menlo Park, CA, Pages 270-281.

Construction of stability diagrams.

Graphical representation of mineral phases and the effect of aqueous fluid composition and temperature on their stability's.

Recall Gibb's phase rule.

f = c - p + 2

where:

• f = number of degrees of freedom
• c = number of components in the system
• p = number of phases in the system

variables include; T, P, µi , µj ...

Example: K2O - Na2O - Al2O3 - SiO2 - H2O system

In a five component system at constant T and P, description of a single phase requires 4 degrees of freedom

f = 5 - 1 + 2 = 6 - 2 (T,P) = 4

If the 4 variables are taken to be activities and 2 are held constant then we can make a 2 dimensional coordinate system where any reaction between any two pair of phases will produce univariant curve. It follows then that any reaction between three phases will be invariant.

The specific choice of variables is both dictated and represented by the formulas of the reacting species.

In the above system we might choose the aqueous species:

aK⁺, aNa⁺, aH⁺, aH4SiO4^o

Procedure for constructing activity diagrams

This example is simplified for the purpose of demonstration. The Athens Gneiss can be represented by mineral assemblage: Quartz, Muscovite, Albite, Microcline, Kaolinite and Gibbsite.

Simplifying assumptions:

• Aquesous solution is always present
• Al is always in a solid phase
• Si concentrations is fixed by quartz saturation
• P and T are constant

I. Write reactions for mineral pairs using ions you wish to plot.

Kao + 5 H2O = 2 Gib + 2 H4SiO4^o

Qtz + 2 H2O = H4SiO4^o

2 Mic + 9 H2O + 2H⁺ = 2K⁺ + H4SiO4^o + Kao

2 Alb + 9 H2O + 2H⁺ = 2Na⁺ + H4SiO4^o + Kao

Mus + 9 H2O + H⁺ = 3 Gib + 3 H4SiO4^o + K⁺

2 Mus + 3 H2O + 2 H⁺ = 3 Kao + 2 K⁺

Mic + 7 H2O + H⁺ = Gib + 3 H4SiO4^o + K⁺

3 Mic + 12 H2O + 2 H⁺ = Mus + 2 K⁺ + 6 H4SiO4^o

Alb + 7 H2O + H⁺ = Gib + 3 H4SiO4^o + Na

3Alb + K⁺ + 3H⁺ + 12 H2O = Mus + 3Na +H + 6H4SiO4^o

Alb + K⁺ + H⁺ = Mic + Na + H⁺

Recall: If DG = 0 , then the reaction will not proceed in either direction (at equilibrium state). In this case,

DG^o = - RT ln K

To calculate the DGreaction the DG^oformation must be obtain from a a data table such as:

Robie R. A., Hemingway B. S., and Fisher J. R. (1984) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (10+e5 pascals) pressure and higher temperature: Bulletin 1452, U.S. Geological Survey, Washington DC.

For the reaction: Kaolinite + 5 H2O = 2 Gibbsite + 2H4SiO4^o

• DG^of Gibbsite = -1155 kJ/mol
• DG^of Kaolinite = -3799 kJ/mol
• DG^of H2O = -237 kJ/mol
• DG^of H4SiO4^o = -1308 kJ/mol

DGreaction = the products - reactants

DGreaction = (2 x -1155) + (2 x-1308) - (-3799) - (5 x -237) = -59 kJ/mol

therefore solve for K,

K = 4.2 x 10^-11

K = (aH4SiO4^o)²

or

log K = -10.4

log K = 2 log aH4SiO4^o = -10.4

The coordinate system is therefore define by the ratios:

aK⁺ / aH⁺

aNa⁺ /aH⁺

Because kaolinite and gibbsite and quartz do not contain sodium or potassium, their stability's are govern by the silica activity and temperature.

If quartz saturation is always maintained at an activity of aH4SiO4^o = 10^-3.95 the direction of reactions can be assessed.

For example in step II above, it was shown that for the reaction of kaolinite ---> gibbsite

2 log aH4SiO4^o = -10.4

Therefore, under these conditions, the equilibrium activity of silica is 10^-5.4.

The reaction will be driven to the left and kaolinite is more stable than gibbsite under these conditions.

Form of equations is y = mx +b

where

y = log (aNa+ / aH+)

x = log (aK+ / aH+)

m is the log coefficient

b = zero intercept