Lecture 24 sedimentary weathering

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 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 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 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 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 ã Fe(II)2SiO4 + ẵ O2 + H2O  Fe(III)OOH + SiO2 Chemical Weathering • Recap: How minerals dissolve? – Dissolution reactions • Ions dissolve in water, 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 O 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 • Examples of graphical representations of mineral stability derived from thermodynamic calculations 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