17 - Lecture notes for Clay Mineralogy

The Geochemistry of clay minerals
Suggested reading:

Background reading on principles of physical chemistry.

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

Berner R. A. (1980) Early Diagenesis: A Theoretical Approach: Princeton University Press, Princeton, NJ

Denbigh K. (1981) The principles of chemical equilibrium - 4th edition: Cambridge University Press, Cambridge, U.K.

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.

Wood B. J., and Fraser D. G. (1984) Elementary Thermodynamics for Geologist: Oxford University Press, Oxford

Sposito G. (1994) Chemical equilbria and kinetics in soils: Oxford University Press, New York, 268.


Review of physical chemistry - The purpose is to provide a brief outline of physical chemical principles important to the study of clay mineralogy. Emphasis will be placed on the understanding of:

* chemical equilibria
* thermodynamics of electrolyte solutions
* crystal nucleation, growth and dissolution
* colloid chemistry

It will be assumed that you have some background in geochemical principles (i.e., concepts taught in first year college chemistry, mineralogy and introductory geochemistry or aqueous chemistry).

Equations useful to clay mineralogy can be derived from a few basic chemical thermodynamic equations. These include:

1. The combined first and second laws of thermodynamics:

dU = TdS - dW+ µ idn i+ µj dn j ....

(1)

where:

2. Some expressions for work

where:


3. Definitions of enthalpy H (heat content) and Gibbs free energy G.

where:

H = U + PV (4)

G = H - TS (5)

4. Definition of chemical activity.


µ
i = µio + RTln a i (6)

where:

µ io = chemical potential in a standard state* of component i.
a
i = activity of component i.
R = gas constant.

* caution there are several different choices of standard states.

Items 1-4 can be considered as definitions. These definitions are combined with the criterion that the free energy for a chemical reaction at equilibrium is equal to zero. From mathematical derivation, equations are developed to study the chemical reactivity of clay minerals in sedimentary, weathering and diagenetic environments.

An important derivation from equations 4 and 5 includes:

dG = U + PdV + VdP - TdS - SdT (7)

and substituting of equations 1 and 2 into equation 7 yields,

dG = VdP - SdT + µ i dn i + µj dn j + ... (8)


From equation 8 through partial differentiation,


From equation 8 through double differentiation,

where,

Equation 8 at constant P and T can be integrated at constant µ to give;

G = µi ni + µj nj +.... (17)


Equilibrium Processes


The change in concentration of any particular clay mineral phase occurs in response to the chemical state of the system. The primary processes that control the chemical state of a system include:

1. advection
2. diffusion
3. chemical reaction

The chemical reaction term is the driving force to compositional change and can be written in the general form:

aA + bB --> gC + dD,

 

where; A, B are the reactants and C,D are the products and a, b, g and d are the relative number of moles of each.


If the rate of chemical reaction is very rapid (in spite of advective and diffusive processes)and chemical equilibrium is maintained, then a thermodynamic approach to chemical reaction is possible.

In reality, NO net reaction can occur at equilibrium. However, the assumption is that there is so little kinetic impedance to the reaction that reactions occur at very small departures from equilibrium.

How does one determine equilibrium concentrations?

This is expressed by way of the thermodynamic equilibrium constant which is related to the Gibbs free energies (G) of the reactants and products.

Let:

K = thermodynamic equilibrium constant

then one can write:

Which can be stated as the activities (a) of the product over the reactants, each raised to the power of the relative molar abundance (at this point activities are assumed to be equal to molar concentrations, althought this not the case; see below).

The difference in free energy (
DG) is a useful parameter to assess the tendency or likelihood for a given chemical reaction to proceed.



For a chemical reaction to proceed spontaneously in the direction of the arrow,
D G must be less than zero (there must be a decrease in free energy).

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

DGo = - RT ln K (20)



Many calculations are done at 25° C (this is the form in which data are tabulated See Robie reference).

At 25° C (298° Kelvin)


DGo = - 1.364 log K (log = base 10) (21)



To adjust K for chemical reactions at different temperature and pressures, the correction can be made with the following equations:




Activity - Once the equilibrium constant K is obtained from the DG of reaction, the activities must be converted to concentrations (which is typically the measure that is made).

Activity Coefficients

The activity for dissolved species in solution can be expressed as

a = gm (24)

where:

g= conventional molal activity coefficient (approaches one as solutions become more dilute).
m = molality (moles per kg of H
2O).

In terms of concentration of per unit volume of pore solution (C):


where
r*w = mass of water per unit volume of interstitial solution.


In near surface conditions
r*w = 1 therefore,

a = gm

 

The solid solutions can be expressed as

a = lX

where:

l = rational molal activity coefficient (for major components as X approaches one so does l).
X = mole fraction of an end-member in a solid solution.

In most sedimentary systems we deal with end-members therefore, X = 1 and
l= 1.

Hence: a = 1 for solids.

Also most pore waters are sufficiently dilute, that the activity of H
2O can be considered to be one.

a H2O = 1


For surface species adsorbed onto the surface of a clay, there is a special convention adopted where:

a = y C

where:

y= surface activity coefficient
C = mass absrobed per unit mass of total sedimentary solids.