Presentation on theme: "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."— Presentation transcript:
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? Fe 2+ SiO 2 Mg 2+ olivine
Chemical Weathering Vocabulary Hydrolysate – dissolved material Resistate – solid material left behind (did’t dissolve) –More easily dissolved elements include alkali and alkaline earths (Na +, Ca 2+, K + ) Residual – product of hydrolysis reactions left behind (it can be physically weathered too…)
Mineral Dissolution Write a reaction: –Mg 0.5 Fe 0.5 SiO 4 + H 2 O 0.5 Mg Fe 2+ + SiO 4 4- 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? –K eq =[products] / [reactants] –K eq =[Mg 2+ ][Fe 2+ ][SiO 4 4- ] / [olivine][H 2 O]
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+K+ SiO 2 Na + feldspar 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 FeOOH s
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 –CaCO 3 + H + + H 2 O H 2 CO 3(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 O 2 is the oxidant Fe (II) 2 SiO 4 + ½ O 2 + H 2 O 2 Fe (III) OOH + SiO 2
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 O 2
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
Activity diagram showing the stability relationships among some minerals in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O 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… olivine amphibole pyroxene biotite K-feldspar quartz Ca-plagioclase Na-plagioclase 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.
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! –SiO 4 4-, CO 3 2-, SO 4 2- anionic groups, respectively Also consider halides (anion is Cl - or F - ) and mineralization of silica
Sheet Silicates – aka Phyllosilicates [Si 2 O 5 ] 2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine Clays talc pyrophyllite micas Display increasing order and lower variability of chemistry as T of formation increases
Clays Term clay ALSO refers to a size (< 1mm = <10 -6 m) Sheet silicates, hydrous – some contain up to 20% H 2 O together with a layered structure and weak bonding between layers make them SLIPPERY WHEN WET Very complex (even argued) chemistry reflective of specific solution compositions
Major Clay Minerals Kaolinite – Al 2 Si 2 O 5 (OH) 4 Illite – K Al 4 (Si,Al) 8 O 20 (OH) 4 Smectites: –Montmorillonite – (Ca, Na) (Al,Mg,Fe) 2 (Si,Al) 4 O 10 (OH) 2 *nH 2 O –Vermicullite - (Ca, Mg) (Al,Mg,Fe) 3 (Si,Al) 4 O 10 (OH) 2 *nH 2 O –Swelling 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)