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Sedimentary MaterialsSlide 2TransportTypes of sedimentary rocksWeatheringPhysical WeatheringExfoliation or unloadingChemical WeatheringChemical Weathering VocabularyMineral DissolutionAqueous SpeciesChemical Weathering II - hydrolysisAcid/base reactionsOxidationSlide 15Chemical Weathering and StabilitySlide 17Resistance to weatheringWhat happens when granite is weathered??Granite weathering, continuedSedimentary MineralsSlide 22ClaysMajor Clay MineralsSedimentary Materials•Sedimentary rocks cover 80% of the earth’s surface but only comprise ~1% of the volume of the crust (they are generally NOT dense either!)•Once we weather the source material, the material is transported, deposited, compacted, and lithified, and maybe changed by reaction with groundwater (called diagenesis)Transport•All weathered products can be transported•Dissolved ions are transported until they get to a final destination (such as the ocean) and/ or are precipitated•Physically weathered minerals/ rock fragments  How are they transported?–Water, wind, glaciers, gravity•What processes are more selective to the size of the particleTypes of sedimentary rocks•Detrital (a.k.a. clastic)  form by compaction and lithification of clastic sediments or lithic fragments–Clasts are little grains or fragments of rocks (i.e. can be made of 1 or more minerals)–Classification based on size•Chemical  form by precipitation of minerals from water, or by alteration of pre-existing material–Classification based on chemical composition•Biogenic  formed of previously living organic debris•HOWEVER  Many sedimentary rocks are combinations of 2-3 of these types… WHY?Weathering•Looking at the rock cycle, key to forming sedimentary rocks is weathering (or erosion) of pre-existing rocks (or organisms…)•Types of weathering:–Physical (a.k.a. mechanical)–ChemicalPhysical Weathering•Joints and sheeting development in rocks•Frost wedging, salt wedging, biologic wedging•Thermal stress•Abrasion – through water, wind, glaciers, gravity, wavesExfoliation or unloading•Some rocks expand to to pressure release, uplift, heating/ cooling, etc. and break off in sheetsChemical Weathering•How do we dissolve stuff?–Ions dissolve into water based on properties of that ion and how easily the mineral ‘releases’ it into the water–What properties do you think make the ions in a mineral dissolve more easily?Fe2+SiO2SiO2Mg2+olivineChemical Weathering Vocabulary•Hydrolysate – dissolved material•Resistate – solid material left behind (did’t dissolve)–More easily dissolved elements include alkali and alkaline earths (Na+, Ca2+, K+)•Residual – product of hydrolysis reactions left behind (it can be physically weathered too…)Mineral Dissolution•Write a reaction:–Mg0.5Fe0.5SiO4 + H2O  0.5 Mg2+ + 0.5 Fe2+ + SiO44- •Describe that reaction as an equilibrium expression which defines how much of the mineral can dissolve in a particular fluid–What aspects of fluid composition do you think might affect how much of a mineral can dissolve?–Keq=[products] / [reactants]–Keq=[Mg2+][Fe2+][SiO44-] / [olivine][H2O]Aqueous Species•Dissolved ions can then be transported and eventually precipitate•Minerals which precipitate from solution are rarely the same minerals the ions dissolved out of•Why would they be transported before precipitating?K+SiO2SiO2Na+feldsparsmectiteChemical Weathering II - hydrolysis•Some minerals ‘weather’ directly to other minerals•Mineral dissolves and immediately reprecipitates a new mineral at the surface of the original–Feldspars  Clays –Fe-bearing silicates to iron oxyhydroxidesolivineolivineFeOOHsAcid/base reactions•Many minerals are affected by the pH of the solution they are in–some form H+ or OH- when they dissolve–Some dissolve much faster/ better in low or high pH solutions•Calcite weathering–CaCO3 + H+ + H2O  H2CO3(g) + CaOH+• Acid/ base chemistry important in mineral dissolution and precipitation!!Oxidation•Recall that elements exist as different ions in a particular oxidation state•Changing that oxidation state can have a big effect on how well that element will dissolve and what minerals will form after it dissolves•Oxidation (where a reduced ion loses an electron to an oxidant) is important in the weathering of many minerals at the surface of the earth where O2 is the oxidant•Fe(II)2SiO4 + ½ O2 + H2O  2 Fe(III)OOH + SiO2Chemical Weathering•Recap: How do minerals dissolve?–Dissolution reactions•Ions dissolve in water, do not change–Acid-base reactions•Ions dissolve in water through interaction with H+ or OH-–Redox reactions•Ions dissolve/ precipitate affected by interaction of ions in mineral or in water with O2Chemical Weathering and Stability•All minerals are described by a ‘stability’•Thermodynamics defines this through an energy  all energies are relative•Energy changes depending on the conditions  i.e. some minerals are more stable than others at high P and T; change the P and T conditions and different minerals are more stable•In weathering environments, minerals that are weathering are not stable, minerals precipitating ARE stablelog aH4SiO40-6 -5 -4 -3 -2 -1log (aK+/aH+)01234567KaoliniteGibbsiteMuscoviteK-feldsparPyrophylliteQuartzAmorphoussilicaActivity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica.Resistance to weathering•Goldrich series  empirical observation concerning what minerals weather before others…olivineamphibolepyroxenebiotiteK-feldsparquartzCa-plagioclaseNa-plagioclaseRemind you of anything??What happens when granite is weathered??•First, unweathered granite contains these minerals: –Na Plagioclase feldspar –K feldspar –Quartz –Lesser amounts of biotite, amphibole, or muscovite •What happens when granite is weathered?•The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions •The Na+ and K+ ions will be removed through leaching •The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.Granite weathering, continued•The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering. •Weathered rock is called saprolite. •What happens after


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UVM GEOL 110 - Sedimentary Materials

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