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In general… High-temperature minerals + water = weathering products + dissolved ions

In general… High-temperature minerals + water = weathering products + dissolved ions. Hydration/hydrolysis reactions. Depend on pH – acid vs. alkaline conditions. 2 NaAlSi 3 O 8 + 2H + + H 2 O  Al 2 Si 2 O 5 (OH) 4 + 2 Na + + 4 SiO 2

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In general… High-temperature minerals + water = weathering products + dissolved ions

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  1. In general… High-temperature minerals + water = weathering products + dissolved ions • Hydration/hydrolysis reactions. Depend on pH – acid vs. alkaline conditions 2 NaAlSi3O8 + 2H+ + H2O  Al2Si2O5(OH)4 + 2 Na+ + 4 SiO2 Na-feldspar (albite) kaolinite (clay) • “Carbonic acid” reactions. Involve dissolved atmospheric CO2 or CO2 respired by plants CO2 + H2O  H2CO3 H+ + HCO3- “carbonic acid” “bicarbonate” 2 KAlSi3O8 + CO2 + 3H2O  Al2Si2O5(OH)4 + 2 K+ + 2HCO3-+ 4 SiO2 K-feldspar kaolinite (clay) (consumes CO2)

  2. In general… High-temperature minerals + water = weathering products + dissolved ions • Oxydation reactions. “Oxidation” is removal of an electron from an ion • (e.g., Mn2+  Mn3+ + e-) 2 Fe2+ + ½ O2+ 2H2O  Fe2O3 + 4 H+ Fe2+in silicate dissolved O2 hematite (Fe3+) acid

  3. About weathering… • Weathering and biology: • Biological activity (modification of chemical micro-environments near root fibers, colonization of mineral surfaces by micro-organisms, accumulation of organic decay products) modifies soil chemical environment, enhancing weathering. - Atmospheric CO2 is ~380 ppm, but respiration by plant roots can result in concentrations up to 100,000 ppm (10%) in soil gases! Soil waters acquire high concentrations of HCO3- (bicarbonate) and H2CO3 (carbonic acid) and are effective in weathering.

  4. About weathering… • Weathering and climate • Chemical weathering of silicate minerals consumes atmospheric CO2, which is ultimately deposited in the oceans in Ca and Mg carbonates (marine limestones). Silicate weathering is the most important long-term regulator of atmospheric CO2 level (multi-million yr timescales) • Weathering requires flux of water for reactions, and weathering rates are fastest under warm and humid (tropical) climate. • Where on Earth does weathering rate approach zero? • What happens if you uplift a mountain range and weather it intensely?

  5. About weathering… • Weathering and soils • Weathering reactions produce hydrated and/or oxidized minerals • (clays, hydroxides, oxides). • Soluble species K+, Na+, Ca2+, Mg2+… are leached away • Insoluble species Al3+, Fe3+, Ti4+ are concentrated in the residue • Where do you expect Ca2+ to be concentrated? Ocean or surface of an old continent? • How might chemical weathering and ore deposits be related?

  6. Summary: • Rates of soil production controlled by • Flux of water • Contact time • Reactivity of mineral assemblage • pH (acidity vs. alkalinity) • temperature (tropical vs. glacial) • biota

  7. Physical weathering processes: Changes in rock volume (expansion from chemical changes like hydration; relaxation of confining stress; thermal expansion)

  8. Physical weathering processes: joints

  9. Physical weathering processes: Changes in rock volume (expansion from chemical changes like hydration; relaxation of confining stress; thermal expansion)

  10. Physical weathering processes: Changes in shape or volume of voids (frost wedging, salt weathering)

  11. Physical weathering processes: Changes in shape or volume of voids (tree roots, animal burrows)

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