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 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 particle Types 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 Chemical Physical Weathering Joints and sheeting development in rocks Frost wedging salt wedging biologic wedging Thermal stress Abrasion through water wind glaciers gravity waves Exfoliation or unloading Some rocks expand to to pressure release uplift heating cooling etc and break off in sheets 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 water What properties do you think make the ions in a mineral dissolve more easily SiO2 olivine Fe2 Mg2 SiO2 Chemical 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 SiO2 K feldspar Na SiO2 smectite Chemical 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 oxyhydroxides olivine olivine FeOOHs Acid 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 O2 H2O 2 Fe III OOH SiO2 Chemical 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 O2 Chemical 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 stable 6 Muscovite Quartz 7 Amorphous silica 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 log aK aH 5 K feldspar 4 3 Gibbsite Kaolinite 2 1 Pyrophyllite 0 6 5 4 3 log aH SiO 0 4 4 2 1 Resistance to weathering Goldrich series empirical observation concerning what minerals weather before others olivine Ca plagioclase pyroxene amphibole Na plagioclase biotite K feldspar quartz Remind 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 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 finegrained 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 Sedimentary Minerals We 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 respectively Also consider