L08_Sedimentary_Rocks_Chemical_Weathering

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Unformatted text preview: l and chemical weathering and are found in the distal reaches of long river systems): Common Some<mes Rare quartz feldspars olivines clay minerals micas pyroxenes amphiboles limestone note: beach sand is primarily quartz, except when there is a local source of an alterna<ve material (Hawaii—olivine sands; Caribbean—crushed coral (limestone) sands). • Quartz (SiO2): hard, tough, very low chemical solubility. Zero CO2 sequestra<on poten<al. • clays: very son, low toughness, but are ‘chemical end- points’ (as products of chemical weathering themselves, clays are hard to weather any farther). Low to no CO2 sequestra<on poten<al. • feldspars: intermediate hardness and lower toughness. Intermediate chemical solubility. Variable CO2 sequestra<on poten<al (Ca- feldspars = high, Na,K feldspars = low to no) • olivines, pyroxenes: hard, rela<vely tough, but high chemical solubility, generally high CO2 sequestra<on poten<al • calcites, dolomites (including limestones, marbles): rela<vely son and weak, high chemical solubility. No CO2 seq. marble tombstone showing effects of chemical erosion by acidic rainfall Chemical Sedimentary Rocks Clas<c Sedimentary Rocks Limestones are (bio)chemical sedimentary rocks. The full cycle: Chemical weathering of crustal rocks produces Ca, Mg ions and dissolved bicarbonate. These are carried by rivers to the seas. First Step of Urey Reac<on: H2O + CO2 + CaSiO3 à༎ SiO2 + Ca++ + CO3- - + H2O Some later event leads to satura&on of water with respect to carbonate mineral, leading to precipita&on of the carbonate mineral. For example: Ca++ + CO3- - à༎ CaCO3 This process is usually facilitated (or driven!) by biological organisms coral cocolithophore benthic (bodom- dwelling) foraminifera tests (shells) planktonic (surface water) foraminifer Coccolithophore blooms near southern England and Alaska Limestone Cliffs, Southeast Asia White Cliffs of Dover, SE England. Wikipedia These kind...
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This document was uploaded on 03/26/2014 for the course GEC AS.270.103 at Johns Hopkins.

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