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TRACE METAL TRANSFER FROM ROCKS TO THE PEDO- AND HYDRO- SPHERES.

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Presentation on theme: "TRACE METAL TRANSFER FROM ROCKS TO THE PEDO- AND HYDRO- SPHERES."— Presentation transcript:

1 TRACE METAL TRANSFER FROM ROCKS TO THE PEDO- AND HYDRO- SPHERES

2 ROCK WEATHERING 1 Physical weathering 2 Biological weathering 3 Chemical weathering: Acid dissolution of minerals Congruent and incongruent dissolution Primary and secondary minerals Balancing weathering chemical reactions DEFINITIONS

3 Rain Water as Weathering Agent  Solubility of CO 2 in water  pH of natural rain and pristine waters  Example determination of pH of pristine rain water  Determine the pH of rain water if the following is given:  T =25 0 C, P CO2 =10 -3.5 bar  The chemical equilibria applicable to the problem are:  CO 2 + H 2 O  H 2 CO 3 k 1 = 10 -1.47 (1)  H 2 CO 3  H + + HCO 3 - k 2 = 10 -6.35 (2) Answer: pH ~ 5.7

4 Balancing Weathering Rxns  Select the mineral that undergoes weathering and on the left side of the weathering equation place  Selected mineral + CO 2 + H 2 O  Example: CaAl 2 Si 2 O 8 (anorthite) + CO 2 + H 2 O  Select/identify the secondary mineral formed as result of chemical weathering and place on the right side of the equation  CaAl 2 Si 2 O 8 + CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 (kaolinite)  Balance aluminum atoms (Al)  CaAl 2 Si 2 O 8 + CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 In this case, Al already balanced (2 on each side)  Release and balance cations  CaAl 2 Si 2 O 8 + CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 + Ca 2+  Set bicarbonate (HCO 3 - ) to balance electrical charges  CaAl 2 Si 2 O 8 + CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 + Ca 2+ + 2HCO 3 -  Balance Si with H 4 SiO 4  CaAl 2 Si 2 O 8 + CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 + Ca 2+ + 2HCO 3 - (Si is already balanced in this case)  Set the number of CO 2 molecules to balance carbon atoms in HCO 3 -  CaAl 2 Si 2 O 8 + 2CO 2 + H 2 O  Al 2 Si 2 O 5 (OH) 4 + Ca 2+ + 2HCO 3 -  Use water molecule(s) on the left to balance hydrogen (H)  CaAl 2 Si 2 O 8 + 2CO 2 + 3H 2 O  Al 2 Si 2 O 5 (OH) 4 + Ca 2+ + 2HCO 3 -  Count oxygen atoms (O) on both sides of the equation to check balance  [O] left = 15and [O] right = 15

5 Example Weathering Rxns  Write and balance the weathering equations of the following conversions:  Ca 2 Mg 4 AlSi 7 AlO 22 (OH) 2 to Al 2 Si 2 O 5 (OH) 4  CaMg(CO 3 ) 2 to Al 2 Si 2 O 5 (OH) 4

6 4 Tracer: –Element or compound that is or can be assumed nearly inert during weathering –Examples include: Al 2 O 3, ZrO 2, and TiO 2 5 Enrichment factor (EF) –The EF of an element “X” in the environmental compartment receiving the product of weathering can be determined as follows: DEFINITIONS (Cont’d)

7 Illustration of Atmospheric EF vs. Physicochemical Parameters 1.Predictive approaches for metal enrichment in the atmosphere (see handout) 2.Trace metal atmospheric EF vs. boiling point of oxides (see handout)

8 Weathering Control on Surface Water Major Element Chemistry  Mechanisms controlling world water chemistry (see Handout)  Weathering  Precipitation  Evaporation/Crystallization

9 A WEATHERING EXAMPLE Composition of a granitic-type rock and of its weathering products  Determines chemical and biological properties of water

10 Weathering and Control on water Chemistry Weathering – Precipitation- Evaporation/Crystallization


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