QAB 2023 Clastic Sedimentology & Petrography

AP Dr Abdul Hadi Abd RahmanRoom 16-03-29

Ext: 7039; e-mail: hadi_rahman@petronas.com.my

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)