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Published byKelley Moore Modified over 8 years ago
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TRACE METAL TRANSFER FROM ROCKS TO THE PEDO- AND HYDRO- SPHERES
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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
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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
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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
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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
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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)
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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)
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Weathering Control on Surface Water Major Element Chemistry Mechanisms controlling world water chemistry (see Handout) Weathering Precipitation Evaporation/Crystallization
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A WEATHERING EXAMPLE Composition of a granitic-type rock and of its weathering products Determines chemical and biological properties of water
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Weathering and Control on water Chemistry Weathering – Precipitation- Evaporation/Crystallization
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