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? Fe2+ SiO2 olivine 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? K+ SiO2 smectite feldspar Na+ SiO2
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 + ½ 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
Examples of graphical representations of mineral stability derived from thermodynamic calculations
olivine Ca-plagioclase pyroxene amphibole Na-plagioclase biotite K-feldspar quartz Resistance to weathering • Goldrich series empirical observation concerning what minerals weather before others… 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 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.