Download - Sedimentary Materials
QAB 2023 Clastic Sedimentology & Petrography
AP Dr Abdul Hadi Abd RahmanRoom 16-03-29
Ext: 7039; e-mail: [email protected]
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)
TransportAll weathered products can be transportedDissolved 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 rocksDetrital (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?
WeatheringLooking at the rock cycle, key
to forming sedimentaryrocks is weathering (or
erosion) of pre-existing rocks (or organisms…)
Types of weathering:Physical (a.k.a. mechanical)Chemical
Physical WeatheringJoints and sheeting development in rocksFrost wedging, salt wedging, biologic wedgingThermal stressAbrasion – through water, wind, glaciers,
gravity, waves
Exfoliation or unloadingSome rocks expand to to pressure release,
uplift, heating/ cooling, etc. and break off in sheets
Chemical WeatheringHow 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
SiO2
Mg2+
olivine
Chemical Weathering Vocabulary
Hydrolysate – dissolved materialResistate – 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 DissolutionWrite a reaction:
Mg0.5Fe0.5SiO4 + H2O 0.5 Mg2+ + 0.5 Fe2+ + SiO4
4-
Describe that reaction as an equilibrium expression which defines how much of the mineral can dissolve in a particular fluidWhat 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 SpeciesDissolved ions can then be transported and
eventually precipitateMinerals which precipitate from solution are
rarely the same minerals the ions dissolved out of
Why would they be transported before precipitating?
K+
SiO2
SiO2
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 originalFeldspars Clays Fe-bearing silicates to iron oxyhydroxides
olivine
olivine
FeOOHs
Acid/base reactionsMany minerals are affected by the pH of the
solution they are insome form H+ or OH- when they dissolveSome dissolve much faster/ better in low or
high pH solutionsCalcite 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 WeatheringRecap: 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 relativeEnergy 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
log aH4SiO40
-6 -5 -4 -3 -2 -1
log
(a
K+/a
H+)
0
1
2
3
4
5
6
7
KaoliniteGibbsite
Muscovite
K-feldspar
Pyrophyllite
Qua
rtz
Am
orph
ous
silic
a
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.
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 MineralsWe 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!SiO4
4-, CO32-, SO4
2- anionic groups, respectively
Also consider halides (anion is Cl- or F-) and mineralization of silica
Sheet Silicates – aka Phyllosilicates
[Si2O5]2- Sheets of tetrahedra Phyllosilicates
micas talc clay minerals serpentine
Sheet Silicates – aka Phyllosilicates
[Si2O5]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%
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
Major Clay MineralsKaolinite – 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)2*nH2O
Vermicullite - (Ca, Mg)0.3-
0.4(Al,Mg,Fe)3(Si,Al)4O10(OH)2*nH2OSwelling 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)