Sedimentary Materials

1
Sedimentary Materials

• Sedimentary rocks cover 80 of the earths
  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 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

4
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?

5
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

6
Physical Weathering

• Joints and sheeting development in rocks
• Frost wedging, salt wedging, biologic wedging
• Thermal stress
• Abrasion 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 water
• What properties do you think make the ions in a
  mineral dissolve more easily?

Fe2
SiO2
olivine
Mg2
SiO2
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Chemical Weathering Vocabulary

• Hydrolysate dissolved material
• Resistate solid material left behind (didt
  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 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?
• Keqproducts / reactants
• KeqMg2Fe2SiO44- / olivineH2O

11
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
12
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
13
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!!

14
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

15
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

16
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

17

• Examples of graphical representations of mineral
  stability derived from thermodynamic calculations

18
Resistance to weathering

• Goldrich series ? empirical observation
  concerning what minerals weather before others

Remind you of anything??
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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.

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 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 halides (anion is Cl- or F-) and
  mineralization of silica

22
Clays

• Sheet Silicates aka Phyllosilicates

Si2O52- Sheets of tetrahedra
Phyllosilicates micas
talc clay minerals serpentine
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• Sheet Silicates aka Phyllosilicates

• Si2O52- 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

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Clays

• Term clay ALSO refers to a size (lt 1mm lt10-6 m)
• Sheet silicates, hydrous some contain up to 20
  H2O ? together with a layered structure and weak
  bonding between layers make them SLIPPERY WHEN
  WET
• Very complex (even argued) chemistry reflective
  of specific solution compositions

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Major Clay Minerals

• Kaolinite Al2Si2O5(OH)4
• Illite K1-1.5Al4(Si,Al)8O20(OH)4
• Smectites
• Montmorillonite (Ca, Na)0.2-0.4(Al,Mg,Fe)2(Si,Al
  )4O10(OH)2nH2O
• Vermicullite - (Ca, Mg)0.3-0.4(Al,Mg,Fe)3(Si,Al)4O
  10(OH)2nH2O
• 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)