Sedimentary Minerals and Particles - Clastic Minerals and Particles

8/13/2019 Sedimentary Minerals and Particles - Clastic Minerals and Particles

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1/5/2014 Geology 3153 Part 3B. 1

Part 3. Sedimentary Minerals and Particles.

3A. Textural Attributes.1. Basic Textural Attributes.

2. Graphical and Quantitative Treatment of Grain Size Data.

3B. Clastic Minerals and Particles.1. Review of Macroscopic Properties of Common Clastic Minerals.2. Quartz.3. Feldspars.

4. Rock Fragments.5. Accessory Minerals.

3C. Carbonate Minerals and Particles.1. Carbonate Minerals.2. Basic Carbonate Chemistry.3. Carbonate Particles.

3D. Clay Minerals.1. Common Clay Minerals and Their Crystal Structure.2. Stability and Properties of Clays.3. Clay Mineral Sources.


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3B. Clastic Minerals and Particles.3B.1. Review of Macroscopic Properties of Common Clastic Minerals.

See notes; review from Physical Geology Lab.


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3B. Clastic Minerals and Particles.3B.2. Quartz.

Recall from our discussion of weathering, quartz is the most commonframework grain of the average sandstone.

a. Types of Quartz.

1) monocrystalline quartz = single crystal unit

2) polycrystalline quartz with straight boundaries = multiple crystalunits with straight contacts

3) polycrystalline quartz with sutured boundaries = multiple crystal

units with sutured contacts

4) quartz framework with abraded quartz overgrowth


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a. Types of Quartz.

1) monocrystalline quartz = single crystal unit


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a. Types of Quartz.

2) polycrystalline quartz with straight boundaries = multiple crystalunits with straight contacts


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a. Types of Quartz.

3) polycrystalline quartz with sutured boundaries = multiple crystalunits with sutured contacts


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a. Types of Quartz.

4) quartz framework with abraded quartz overgrowth

first cycle; abradedoriginal framework

burial; quartzovergrowth cement

recycled; abradedquartz cement


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b. Sources of Quartz Types.

monocrystallineacidic to intermediate plutonic or volcanic igneous

polycrystalline straightacidic to intermediate plutonic igneous

polycrystalline sutured folitated metamorphic

quartz with abraded quartz overgrowth cementsedimentary

Detrital quartz grains from igneous rocks tend to be equant toslightly blade shaped, and those from metamorphic tend to prolateshaped.


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3B. Clastic Minerals and Particles.3B.3. Feldspars.

Feldspars are less common overall in clastic sedimentary rocks as aframework grain.

K-feldspars are more common than Ca- or Na-feldspars. Why?

Conditions conducive to producing feldspar framework grains:

dry climate;high relief;combinations of dry climate and high relief.

Sources of feldspar clastic debris:

all igneous rocks, in particular acidic to intermediate plutonic igneous;feldspar-bearing metamorphic (e.g. gneiss);volcanic (look for compositional zoning within grain).


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3B. Clastic Minerals and Particles.3B.4. Rock Fragments.

Rock fragments, also known as lithic fragments, are very usefulindicators of the pre-existing rock contributing sediment.

Although any type of rock fragment is possible, there is a bias towardfiner textured rock fragments:

a fundamental size limit for recognition of rock fragments isbased on the texture of the original rock: rock fragments can beno smaller than the texture of the pre-existing rock;

otherwise mineral fragments result.

Consider clastic grain size diminution and recognition of graniticrock fragments.

Type of rock fragments directly reflect the source rock type(s).


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3B. Clastic Minerals and Particles.3B.5. Accessory Minerals.

Heavy Minerals:

A number of minerals have a density higher than quartz (= 2.65 g/cc).These are given the general name heavy minerals. This fact is utilizedto separate the heavy from the light minerals by grain settling

through a column of heavy liquid.

Most heavy minerals are mechanically stable. However, many are notchemically stable, so they do not fair well under chemical weatheringand diagenesis.


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3B. Clastic Minerals and Particles.3B.5. Accessory Minerals.

Heavy minerals, stability and their source rocks.

Heavy Stability Source Rock Comments

Mineral chemical mechanical Metamorphic Igneous Sedimentary

Garnet M H X

Rutile H H X X

Zircon H H X X X source from microanalysis

Corundom H H X rare

Kyanite M M X

Amphibole M M X X type can determine source

Olivine L L X

Pyroxene L L X X type can determine source


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3B. Clastic Minerals and Particles.

Closing Remark: The framework composition of clasticsediments gives clues to the preexistingrock that was weathered in theuplifted source area (see Rock Cycle) !!


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3C. Carbonate Minerals and Particles.1. Carbonate Minerals.2. Basic Carbonate Chemistry.

3. Carbonate Particles.



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3C. Carbonate Minerals and Particles.

Much of what we have focused on is mainly applicable to clasticsediments and sedimentary rocks.

Here our attention turns to the minerals and particles that are theprincipal framework and matrix constituents of biochemical sedimentsand sedimentary rocks known as carbonates.

minerals involve the carbonate ion (C03)

as we shall see later, carbonate minerals are sensitive to post-depositional changes (i.e. diagenesis).


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3C. Carbonate Minerals and Particles.3C.1. Carbonate Minerals.

3 most important minerals:

Calcite = CaCO3higher-order crystal system than aragoniterelatively more stable than agragonite

Aragonite = CaCO3lower-order crystal system than calcite (psuedomorph of calcite)relatively unstable compared to calcitewith time and higher T&P readily converts to calcite or dolomite

Dolomite = CaMg(CO3)2higher-order crystal system (same as calcite)difficult to precipitate from sea waterrequires elevated concentrations, particularly Mg


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3C. Carbonate Minerals and Particles.3C.2. Basic Carbonate Chemistry.

From these basic chemical relationships, it is clear that natural

processes driving the reactions to a reduction in carbonic acidpromotes carbonate sediment production, for example:

higher water T drives off CO2higher water salinity increases Ca and/or Mg concentrations

WaterSediment

Ca+2+ 2HCO3-CaCO3

Calcite or Aragonite

H2CO3

Carbonic Acid

+

H20 + CO2

Atmosphere


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3C. Carbonate Minerals and Particles.3C.3. Carbonate Particles.

Carbonate sedimentary rocks are composed of all four components ofthe generic sedimentary rock. However, carbonates differ fromclastics in that carbonate:

framework grains are derived at or near site of deposition;

matrix is abundant in carbonate sedimentary environments

cement occurs early after carbonate sediment deposition;

pore systems in carbonates are more complex


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3C.3.a. Carbonate Grains.

Carbonate grains come in wide variety of sizes, shapes and bulkdensities (= density of combined solid and fluid phases containedwithin solid). Thus, it is no surprise to find a wide variety of grain

sizes deposited together (an another example of hydraulicequivalence).

We will consider just some of the more commonly occurring grains.

Intraclasts:

fragments of reworked carbonate sediment

intra = from within


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Skeletal:

pieces of the exoskeletons of invertebrate organisms;

aragonite or calcite original composition

note: scale in figure under-represents typical size


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Peloid:

silt- to very fine sand-sized grains of microcrystalline calcite(= micrite);

usually well rounded blades; roundness not by abrasion

origins:-- endolithic algal micritization of grain-- feeding pellets

-- fecal pellets

note: scale over-representstypical peloid


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Lumps:

composite grain made of peloids covered by thin film of micrite

formed by early lithification and/or cohesive attraction ofpeloids and micrite matrix


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Oncolith:

also a coated grain (nucleus + coating);

several cms diameter;

irregular concentric coating;

formed by algal (sticky mat) accretion on upper grain surface

ooids and oncoliths form in high-energy environments were grainsare nearly constantly agitated (ooids) or occasionally toppled(oncoliths)


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Pisolith:

several mm diameter coated grain;

irregular concentric coating;

formed by direct or biologically mediated calcite precipitation;

found in caverns and soils (e.g. calcrete)


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3C.3.b. Carbonate Matrix.

Carbonate matrix is made up of microcrystalline aragonite, whichconverts to calcite with burial, or calcite. It is commonly referred toas micrite or carbonate mud.

in modern sediment, typically


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3C.3.b. Carbonate Matrix.

Origins of micrite:

1) physical abrasion;2) biological abrasion (think of parrot fish eating coral);

3) disaggregation of peloids;4) disintegration of calcareous algaeneedles precipitated in loose array to keep plant uprightdeath algae, needles released;

5) direct chemical precipitation of calcium carbonate.

For many years 5) was thought to be the origin of most micrite,but more recent studies indicate that 4) is the most importantorigin of micrite (modern and ancient).


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Carbonate particle framework types and proportion of carbonatematrix provide useful information about carbonate sedimentaryenvironments.

As we shall see in Part 4, the understanding of carbonate particle

associations allows a means by which carbonate rocks can beclassified.


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3C. Carbonate Minerals and Particles.1. Carbonate Minerals.2. Basic Carbonate Chemistry.

3. Carbonate Particles.



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3D. Clay Minerals.

In sedimentary petrology, the term clay is used in 2 ways:

1) a texture grain size term (


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3D. Clay Minerals.

Clay minerals are important, for example:

-- solids in well drilling fluids;-- ultramicroscopic filters;-- medicines (e.g. Kaopectate);-- food additives;

-- paper coatings; and-- soil binders and sealants.

Some biochemist argue that clay crystal structures may haveacting as an inorganic template for the first DNA structures.

Clay mineralogy is a complex subject.

Requires advanced knowledge of crystallography.

Identification of specific clay minerals involves detailed labpreparation and analysis, and x-ray difractometry.

P d M l d P l


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3D. Clay Minerals.

3D. 1. Common Clay Minerals and Their Crystal Structure.

Clay minerals have 2 ideal basic components:

tetrahedral sheet(abbreviation = t or t-)

Si surrounded by O

octahedral sheet

(abbreviation = o or o-)Al surrounded by OH

Note the sheet crystal structure clay phyllosilicates.

P 3 di Mi l d P i l


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3D. Clay Minerals.3D. 1. Common Clay Minerals and Their Crystal Structure.

Various clay minerals are defined by combinations of

t- and o-sheets, and ionic substitutionswith in and between the sheet structures.

Gibbsite:

single octahedral sheet ( o )

Kaolinite:

octahedral + tetrahedral sheet (o-t)sheets linked by sharing O ions

P 3 S di Mi l d P i l


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Smectite, which also known as Montmorillonite:

2 tetrahedral sheets,octahedral sheet andexchangable cations

t-o-t-(cations+H2O)

exhangable cations satisfycharge deficiencies frommetal substitutions in the

t and o sheets

why H2O?

notorious for swelling (can double in volume)

P t 3 S di t Mi l d P ti l


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Illite:

similar to smectite butdifferent cations betweensheets

t-o-t-(70-80% K++cations)

strong K bonds; lower ionexchange capacity than smectite

related to muscovite

t-o-t-( K+ )



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Chlorite:

although we think of chloriteas a metamorphic mineral, itis common clay mineral in

sedimentary rocks

similar structure as smectiteand illite, but differentintersheet composition

t-o-t-( [Mg+2

, Fe+2

]o )



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mixed-layer clays:

mixed-layer clays are random, quasi-regular or regularcrystalline interlayering of the basic clay types

although almost all types are possible, the most common are:

smectite-illite; and

illite-chlorite



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3D. Clay Minerals.

3D. 2. Stability and Properties of Clays.

At low pH, such as in tropical soils or acid diagenetic pore waters:

kaolinite and gibbsite develop;

kaolinite is stable under many diagenetic conditions(common alteration of feldspar);

illite converts to kaolinite.

At high pH, such as in semiarid and arid soils and basic diagenetic pore

waters:

illite stable; and

kaolinite converts to illite.


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3D. Clay Minerals.

3D. 3. Clay Mineral Sources.

As we have been discussing, clastic particles are important byproductsof the weathering of pre-existing rocks.

Clays are byproducts of chemical weathering processes, thus they

are associated to a certain extent with climate:

kaolinitetropical weathering

illitetemperate weathering

chloritepolar climate and volcanic rock weathering

smectitevolcanic rock weathering

Sedimentary Minerals and Particles - Clastic Minerals and Particles

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