Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2009 Weathering When you talk about sedimentary rocks, you have to talk about weathering Weathering- Clastic rocks - conglomerates, sandstones, mudrocks are composed of fragments and solid weathering products of pre-existing rocks - Even carbonate rocks and cements, whose constituents are precipitated from seawater by biological organisms, are made from ions that come from the weathering process Two aspects to Weathering- Mechanical Weathering - physical breakup of rocks - Chemical Weathering - chemical breakdown of minerals in the presence of water - Mechanical Weathering is trivial compared to chemical weathering, due to the extraordinary dissolving power of H2O Mechanical Weathering- Abrasion by wind- or water-carried fragments - Frost-wedging is most important agent of mechanical weathering - water freezes to ice 9% volume increase - most important where water is liquid in daytime and freezes every night - most important mechanism for increasing surface area of rocks Chemical Weathering-H2O is polar; therefore, good at dissolving ions -H2O dissociates into H+ and OH- -H+ reacts readily with minerals -Weathering is probably aided by organic acids and microorganisms -Different minerals weather at different rates - Weathering of minerals depends on their chemical stability in the weathering environment: low T, low P, high H2O, oxidizing - What factors probably control the weathering rates of minerals? - Consider olivine, biotite, quartz and plagioclaseGoldich’s Weathering Series – see handoutNote similarity to Bowen’s Reaction Series, Why?Summarizes susceptibility of minerals to weathering Weathering Reactions of OrthoclaseStep 1: 3 KAlSi3O8 + 2 H+ + 12 H2O --> KAl3Si3O10(OH)2 + 6 H4SiO4 + 2 K+orthoclase illite(~muscovite) soluble silicaStep 2: 2 KAl3Si3O10(OH)2 + 2 H+ + 3 H2O --> 3 Al2Si2O5(OH)4 + 2 K+illite kaolinite All feldspars weather similarly-React with H2O and H+ -Release silica in solution and cations -Produce clay minerals (sheet silicates) -Albite + H2O + H+ = Sodium montmorillonite + H4SiO4 + Na+ -Anorthite + H2O + H+ = Calcium montmorillonite + H4SiO4 + Ca2+ Montmorillonite- Montmorillonite formula: (Na,Ca)(Al,Mg)2(Si4O10)(OH)2-nH2O - Montmorillonites are EXPANDING clays (unlike illite and kaolinite)- Al is essential in all clay minerals (i.e., Al in weathered silicates goes into clay minerals) - Mg silicates also weather to form montmorillonite Iron in minerals weather differently-Fe in most ferromagnesian minerals is reduced (Fe2+), because they’re formed in reducing conditions (low oxygen) -Surface waters are very oxygen-rich, i.e., oxidizing -Fe2+ released during weathering immediately oxidizes to Fe3+ -Fe3+ precipitates rapidly as EXTREMELY INSOLUBLE Fe(OH)3 and other hydroxides Weathering of pyroxene, for exampleCaFeSi2O6 (Fe part of augite) + H2O + H+ = Calcium montmorillonite + H4SiO4 + Ca2+ + Fe(OH)3 The most common products of weathering are:- Quartz - Clay minerals-kaolinite, illite, montmorillonite - Cations in solution - Ferric hydroxides and oxides (insoluble) from the weathering of mafic minerals Which will weather more rapidly, basalt or granite? Why? Weathering of Basalt- What will be the most common sand-sized fragments? - What will be the most common mineral weathering products? - Weathered ash deposits form bentonite - a mixture of clay minerals, mostly montmorillonite - expand when wet, very slippery!Weathering of Granite- Where does alteration first occur? - Disaggregation of grains: forms "grus" - Surface (esp. of feldspars) gets soft and punky, why? - What are the mineral products of granite weathering? Grus: disaggregated grains of quartz and feldspar from
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