1. 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!)

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

9. 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…)

10. Mineral Dissolution Write a reaction:
Mg0.5Fe0.5SiO4 + H2O  0.5 Mg 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]

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
feldspar
smectite
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. 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.
This is what we will end up with after all the calculations and plotting are through. To calculate the positions of each of the boundaries shown above, we need to have thermodynamic data so we can calculate equilibrium constants for the reactions that occur at each of the boundaries. For example, along the boundary between the stability fields for the phases gibbsite and kaolinite, a reaction takes place involving these two phases. If we cross this boundary from the gibbsite field into the kaolinite field, the reaction is one in which gibbsite is converted to kaolinite. The thermodynamic data we require for this exercise are given in the following table.

18. 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??

19. 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. Sheet Silicates – aka Phyllosilicates
[Si2O5] 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

23. Clays Term clay ALSO refers to a size (< 1mm = <10-6 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

24. Major Clay Minerals Kaolinite – Al2Si2O5(OH)4
Illite – K1-1.5Al4(Si,Al)8O20(OH)4
Smectites:
Montmorillonite – (Ca, Na) (Al,Mg,Fe)2(Si,Al)4O10(OH)2*nH2O
Vermicullite - (Ca, Mg) (Al,Mg,Fe)3(Si,Al)4O10(OH)2*nH2O
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)