23 - Lecture notes for Clay Mineralogy

Chemical weathering and clay mineral formation

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Suggested reading:

Berner E. K. and Berner R. A. (1987) The global water cycle: Geochemistry and Environment: Prentice-Hall, Inc., Englewood Cliffs, NJ, Pages 155-171.

Lasaga A. C. (1981) Rate laws of chemical reactions. In Kinetics of geochemical processes vol. 8, (ed. A. C. Lasaga and J. Kirkpatrick), Mineralogical Society of America, Blacksburg, VA, 1-67.

Nagy, K. L.,Blum, A. E., & Lasaga, A. C. (1991). Dissolution and precipitation kinetics of kaolinite at 80°C and pH 3: The dependence on solution saturation state. American Journal of Science, 291, 649-686.

Nagy, K. L., & Lasaga, A. C. (1992). Dissolution and precipitation kinetics of gibbsite at 80°C and pH 3: The dependence on solution saturation state. Geochimica et Cosmochimica Acta.

Nagy, K. L.,Steefel, C. I., Blum, A. E., & Lasaga, A. C. (1990). Dissolution and Precipitation of kaolinite: Initial results a 80° C with application to porosity evolution in a sandstone. In I. D. Meshi & P. J. Ortoleva (Eds.), Prediction of Reservoir quality through chemical modeling (pp. 85-102). Tulsa: A.A.P.G.

Recall that the nature of clay formed during the weathering process depends upon three factors:

• The mineralogical and textural composition of the parent rock.
• The composition of the aqueous solution.
• The nature of the fluid flow (i.e., rate of water flow and pore network)
  

Water composition and flow rate (i.e., Rates of reaction)

The formation of clays in the weathering environment is primarily the result of reaction of protons (H⁺ found in soil waters) with primary silicate minerals.

Although the primary source of protons is a result of the production of organic acids (not the absorption of atmospheric CO2 by rain water) the production of acids can be viewed as the production of carbonic acid.

4H2C2O4 + 2O2 --> 8CO2 + 4H2O

and recalling that carbonic acid forms by the reaction,

H2O + CO2--> H2CO3

Immediately,

H2CO3 --> HCO3^- + H⁺

Example 1.

A typical weathering reaction can therefore be represented by,

7 H2O + H⁺ + NaAlSi3O8 ---> Al(OH)3 + Na⁺ + 3 H4SiO4

The stability fields for gibbsite, kaolinite, idealized smectite (Na-beidellite) and end-member albite are considered in the diagram below. The brackets indicate concentration in moles per liter.

1. If the water does not leave the system (i.e., very slow hydrologic flow rates) then the concentration of orthosilisic acid and the ratio of sodium to acid increase with time.

2. With time, the solution composition "evolves" and moves toward the NE part of the diagram.

3. When the gibbsite - kaolinite boundary is encountered, then the gibbsite and excess H4SiO4combine to form kaolinite.

2Al(OH)3 + 2H4SiO4 ---> Al2Si2O5(OH)4 + 5H2O

4. Silica released by dissolution of albite is used to make kaolinite. Therefore, H4SiO4 concentration does not change. The composition of the fluid evolves vertically, towards the north end of the diagram.

5. Once all the gibbsite is consumed, the fluid composition then evolves towards the NE again.

6. At the smectite - kaolinite boundary, both H4SiO4 and Na are used to form smectite. The fluid composition tracks along the kaolinite - smectite boundary until all the kaolinite is gone.

7. Again, the composition tracks to the NE until the albite field is reached and dissolution/ppt reach equilibrium.

Example 2.

The above scenario is for a closed system. Soils are open systems.

The faster the flow rate, the shorter the contact time of solution with the primary minerals.
Soils developed in Hawaii diplay the effect of different weathering products from a parent (basalt).

The diagram below shows the effect of rain fall versus the percentage of clays formed in soils.

Example 3

Daniel Springs metagabbro weathering.

The class project on the Danial Springs meta-gabbro is simialr to the work of Mike Velbel. He examined water chemistry and petrology of weathered terrain in the Blue Ridge Province (metamorphic plagioclase, garnet, hornblende assemblage).

Soil, saprolite and bed rock were examined (saprolite is a rock that has weathered in place with texture retained).

Rates of reaction

How is the rate of this reaction assessed?

There are three basic types of rate laws that are used to describe the kinetics of a reaction.

Zeroth Order:

This is the case where the rate of change in concentration (C) of a component with time (t) is related to some constant (k).

dC / dt = -k

or in integral form,

C = Co - k t.

This means that the rate of reaction is independent of the initial amount of material present. The plot below depicts the change in concentration with time assuming Co = 100 and arbitratry k values.

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First Order:

dC / dt = -k C

or in integral form,

C = Co e^{-k t}

The rate of change is proportional to the amount of material present (e.g., radioactive decay).

Rates can also be related to the difference between the actual concentration (C) of a solution and the equilibrium (i.e., steady-state) concentration (Cs).

dC / dt = k(Cs - C)

or in integral form,

(Cs - C) / (Cs - Co) = e^{-k t}

The plot below depicts the change in concentration with time assuming Co = 100 and arbitratry k values.

Second (nth) Order:

dC / dt = kCⁿ

or in integral form,

(1 / C^n-1) - (1 / Co^n-1) = (n-1) kt

Rates can also be related to the difference between the actual concentration (C) of a solution and the equilibrium (i.e., steady-state) concentration.

for C < Cs

dC / dt = k (Cs - C) ⁿ

for C > Cs

dC / dt = k (C - Cs) ⁿ

The plot below depicts the chabge in concentration with time assuming Co = 100 and arbitratry k values.

Determination of rates of reaction

Which law(s) applies to chemical weathering environments and how are rates determined?

One effective approach to determining rate of reaction is via flow through reactions cells. This allow conditions to exist both far and close to equilibrium. They also simulate open system conditions (as opposed to closed-system bomb type experiments).

Let's consider the dissolution and precipitation reaction of kaolinite where,

Al2Si2O5(OH)4 + 6H⁺ = 2Al³⁺ + 2H4SiO4 + H2O

At steady-state the number of moles of silicon or aluminum in solution leaving the reaction cell will not change with time;

dN / dt = 0

In a flow through system the, rate of change of fluid composition due to the dissolution or precipitation of kaolinite can be given as:

dN / dt = qv DM + 2AkaoRkao

where:

• qv = volume flow rate of fluid (L sec^-1)
• DM = molarity of component in the output solution minus the input.
• A = total surface area = mass (g) x surface area (m² g^-1)
• R = reaction rate (mol m^-2 sec^-1)
• 2 = stochiometric coefficient for 2 Al or Si per mole of kaolinite.
  

Rearranging (at steady state dN/dt = 0):

Rkao = (qv DM ) / 2Akao

Equilibrium is represented by Keq = (a²Al³⁺ a²H4SiO4^o) / (a⁶H+)

Recall that

W = IAP / Keq

If the there is no change in the input and output composition of the flow through reactor (i.e., steady state) then activities can be measured directly (if the distribution of aqueous species is determined, i.e., speciation calculations must be performed using known thermodynamic data) and the IAP determined.

Keq is determined by the bracketing of dissolution and precipitation (loss or gain of species).

Once K is known, then the saturation state is known.

Recent work by Nagy et al have demonstrated that a general non-linear rate law for dissolution and precipitation at a constant P can be written as

Rate = -k (1 - exp(DGr / RT ))ⁿ

where:

• Rate = net rate of reaction (mol m^-2 sec^-1)
• k = rate coefficient (mol m^-2 sec^-1)
• n = order of reaction
• DGr = Gibb's free energy of the overall reaction
  

DGr= RTln(W) = 0 at equilbrium

The figure below has been adapted from figure 7 Nagy et al. (1991) and figure 6 from Nagy et al. (1992).

Note: As DGr --> 0 then the rate law becomes linear.