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Understanding Chemical Weathering and its Impact on Minerals

Learn about high-temperature minerals reacting with water to form weathering products through hydration/hydrolysis and oxidation reactions. Explore how weathering is influenced by pH, climate, and biological activity, impacting soil composition and mineral concentration. Discover the factors affecting weathering rates, such as water flux, mineral reactivity, and environmental conditions.

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Understanding Chemical Weathering and its Impact on Minerals

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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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