Presentation on theme: "Sedimentary Materials"— Presentation transcript:
1 Sedimentary 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!)
2 Once we weather the source material, the material is transported, deposited, compacted, and lithified, and maybe changed by reaction with groundwater (called diagenesis)
3 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 precipitatedPhysically weathered minerals/ rock fragments How are they transported?Water, wind, glaciers, gravityWhat processes are more selective to the size of the particle
4 Types of sedimentary rocks Detrital (a.k.a. clastic) form by compaction and lithification of clastic sediments or lithic fragmentsClasts are little grains or fragments of rocks (i.e. can be made of 1 or more minerals)Classification based on sizeChemical form by precipitation of minerals from water, or by alteration of pre-existing materialClassification based on chemical compositionBiogenic formed of previously living organic debrisHOWEVER Many sedimentary rocks are combinations of 2-3 of these types… WHY?
5 WeatheringLooking 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)Chemical
6 Physical Weathering Joints and sheeting development in rocks Frost wedging, salt wedging, biologic wedgingThermal stressAbrasion – through water, wind, glaciers, gravity, waves
7 Exfoliation or unloading Some rocks expand to to pressure release, uplift, heating/ cooling, etc. and break off in sheets
8 Chemical 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 waterWhat properties do you think make the ions in a mineral dissolve more easily?Fe2+SiO2olivineMg2+SiO2
9 Chemical Weathering Vocabulary Hydrolysate – dissolved materialResistate – 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…)
10 Mineral Dissolution Write a reaction: Mg0.5Fe0.5SiO4 + H2O 0.5 Mg Fe2+ + SiO44-Describe that reaction as an equilibrium expression which defines how much of the mineral can dissolve in a particular fluidWhat 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]
11 Aqueous SpeciesDissolved ions can then be transported and eventually precipitateMinerals which precipitate from solution are rarely the same minerals the ions dissolved out ofWhy would they be transported before precipitating?K+SiO2feldsparsmectiteNa+SiO2
12 Chemical Weathering II - hydrolysis Some minerals ‘weather’ directly to other mineralsMineral dissolves and immediately reprecipitates a new mineral at the surface of the originalFeldspars ClaysFe-bearing silicates to iron oxyhydroxidesolivineolivineFeOOHs
13 Acid/base reactionsMany minerals are affected by the pH of the solution they are insome form H+ or OH- when they dissolveSome dissolve much faster/ better in low or high pH solutionsCalcite weatheringCaCO3 + H+ + H2O H2CO3(g) + CaOH+Acid/ base chemistry important in mineral dissolution and precipitation!!
14 OxidationRecall that elements exist as different ions in a particular oxidation stateChanging that oxidation state can have a big effect on how well that element will dissolve and what minerals will form after it dissolvesOxidation (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 oxidantFe(II)2SiO4 + ½ O2 + H2O 2 Fe(III)OOH + SiO2
15 Chemical Weathering Recap: How do minerals dissolve? Dissolution reactionsIons dissolve in water, do not changeAcid-base reactionsIons dissolve in water through interaction with H+ or OH-Redox reactionsIons dissolve/ precipitate affected by interaction of ions in mineral or in water with O2
16 Chemical Weathering and Stability All minerals are described by a ‘stability’Thermodynamics defines this through an energy all energies are relativeEnergy 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 stableIn weathering environments,minerals that are weatheringare not stable, mineralsprecipitating ARE stable
17 Activity 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.This is what we will end up with after all the calculations and plotting are through. To calculate the positions of each of the boundaries shown above, we need to have thermodynamic data so we can calculate equilibrium constants for the reactions that occur at each of the boundaries. For example, along the boundary between the stability fields for the phases gibbsite and kaolinite, a reaction takes place involving these two phases. If we cross this boundary from the gibbsite field into the kaolinite field, the reaction is one in which gibbsite is converted to kaolinite. The thermodynamic data we require for this exercise are given in the following table.
18 Resistance to weathering Goldrich series empirical observation concerning what minerals weather before others…olivineamphibolepyroxenebiotiteK-feldsparquartzCa-plagioclaseNa-plagioclaseRemind you of anything??
19 What happens when granite is weathered?? First, unweathered granite contains these minerals:Na Plagioclase feldsparK feldsparQuartzLesser amounts of biotite, amphibole, or muscoviteWhat happens when granite is weathered?The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ionsThe Na+ and K+ ions will be removed through leachingThe biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
20 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 this?Quartz grains may be eroded, becoming sediment. The quartz in granite is sand- sized; it becomes quartz sand. The quartz sand will ultimately be transported to the sea (bed load), where it accumulates to form beaches.Clays will ultimately be eroded and washed out to sea. Clay is fine-grained and remains suspended in the water column (suspended load); it may be deposited in quiet water.Dissolved ions will be transported by rivers to the sea (dissolved load), and will become part of the salts in the sea.
21 Sedimentary MineralsWe will focus on some minerals which form from precipitation of dissolved ions other minerals in sedimentary rocks are derived from the source rocks!Clay, carbonate, and sulfate groups are key in sedimentary rocks – can ‘be’ the rock or cement fragments together!SiO44-, CO32-, SO42- anionic groups, respectivelyAlso consider halides (anion is Cl- or F-) and mineralization of silica
22 Sheet Silicates – aka Phyllosilicates [Si2O5] Sheets of tetrahedra Phyllosilicatesmicas talc clay minerals serpentineClays talc pyrophyllite micasDisplay increasing order and lower variability of chemistry as T of formation increases
23 Clays Term clay ALSO refers to a size (< 1mm = <10-6 m) Sheet silicates, hydrous – some contain up to 20% H2O together with a layered structure and weak bonding between layers make them SLIPPERY WHEN WETVery complex (even argued) chemistry reflective of specific solution compositions
24 Major Clay Minerals Kaolinite – Al2Si2O5(OH)4 Illite – K1-1.5Al4(Si,Al)8O20(OH)4Smectites:Montmorillonite – (Ca, Na) (Al,Mg,Fe)2(Si,Al)4O10(OH)2*nH2OVermicullite - (Ca, Mg) (Al,Mg,Fe)3(Si,Al)4O10(OH)2*nH2OSwelling clays – can take up extra water in their interlayers and are the major components of bentonite (NOT a mineral, but a mix of different clay minerals)