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Questions and Answers
How is grain size connected to sedimentary rock name?
How is grain size connected to sedimentary rock name?
2 mm: conglomerate (rounded clasts), breccia (angular clasts). 0.625-2 mm: sandstone. 0.0098-0.0625: mudstone.
What is the difference between a conglomerate and a breccia?
What is the difference between a conglomerate and a breccia?
Conglomerates have rounded clasts while breccias have angular clasts.
What is the volume fraction of gravel in a conglomerate?
What is the volume fraction of gravel in a conglomerate?
30% or greater.
What is the fraction of mud in an arenite?
What is the fraction of mud in an arenite?
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What is the grain size associated with an arenite?
What is the grain size associated with an arenite?
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What is a Wacke?
What is a Wacke?
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What is the definition of a Mudstone?
What is the definition of a Mudstone?
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What are the definitions for Lithic Arenite, Feldspathic Arenite, Quartz Arenite?
What are the definitions for Lithic Arenite, Feldspathic Arenite, Quartz Arenite?
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Why is a compositional classification for sandstones useful?
Why is a compositional classification for sandstones useful?
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What is the phi scale?
What is the phi scale?
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What minerals can occur as clay-sized grains?
What minerals can occur as clay-sized grains?
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What is mud, Mudstone, Silt, Siltstone, Clay, Claystone, Shale? What does fissile mean?
What is mud, Mudstone, Silt, Siltstone, Clay, Claystone, Shale? What does fissile mean?
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How do clay minerals form and why are they so common?
How do clay minerals form and why are they so common?
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What is the basic mineralogic description of clay?
What is the basic mineralogic description of clay?
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What is the role of surface area in clay mineralogy?
What is the role of surface area in clay mineralogy?
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How stable are clay minerals?
How stable are clay minerals?
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Why are clay-rich deposits common hydrocarbon source rocks?
Why are clay-rich deposits common hydrocarbon source rocks?
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For the Kaolin Clay Group, what is the most common mineral and its basic formula?
For the Kaolin Clay Group, what is the most common mineral and its basic formula?
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For the Illite Clay Group, what are the most common minerals and their basic formula?
For the Illite Clay Group, what are the most common minerals and their basic formula?
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For the Chlorite Clay Group, what is the basic formula?
For the Chlorite Clay Group, what is the basic formula?
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For the Smectite Clay Group, what are the most common minerals and their basic structure?
For the Smectite Clay Group, what are the most common minerals and their basic structure?
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Study Notes
Grain Size and Sedimentary Rock Naming
- Grain size classification:
-
2 mm: conglomerate (rounded clasts), breccia (angular clasts)
- 0.0625-2 mm: sandstone
- < 0.0625 mm: mudstone
-
Conglomerate vs. Breccia
- Conglomerate consists of rounded clasts, while breccia contains angular clasts.
Volume Fraction in Conglomerates
- A conglomerate is characterized by having 30% or greater gravel content.
Arenite Characteristics
- Arenite contains 0-5% mud.
Grain Size of Arenite
- Arenite corresponds to a grain size of 0.0625-2 mm.
Definition of Wacke
- Wacke is a type of sandstone that incorporates 5-50% mud.
Mudstone Definition
- Mudstone is a fine-grained, siliciclastic sedimentary rock with grains smaller than 0.0625 mm and contains more than 50% mud.
Types of Arenite
- Lithic Arenite: High in unstable rock fragments, with a lithic to quartz ratio of 3:1, containing less than 75% quartz.
- Lithic Arkose: Lithic to feldspar ratio between 3:1 and 1:1, with less than 75% quartz.
- Feldspathic Litharenite: Feldspar to lithic ratio between 1:1 and 1:3, with less than 75% quartz.
- Litarenite: Less than 1:3 feldspar to lithic ratio, with more than 75% quartz and greater than 1:1 feldspar to lithic ratio.
Compositional Classification of Sandstones
- Useful for identifying and differentiating various sandstone types based on mineral composition.
Phi Scale Overview
- The phi scale measures grain size of clastic sediments, ranging from -12 (boulders) to 14 (clays).
Clay-Sized Grains Composition
- Clay-sized grains primarily contain feldspar, along with micas, pyroxene, and amphibole.
Definitions of Key Terms
- Clay: Refers to grain size only, not mineral composition.
- Mud: Comprised of clay-sized particles (33-65%), can be bedded or laminated.
- Siltstone: Contains fine sand and clay minerals.
- Fissile: A property of rock that allows easy splitting along pale layers.
Formation of Clay Minerals
- Clay minerals form through chemical weathering, characterized by their high reactivity due to large surface area.
Stability of Clay Minerals
- Clay minerals are unstable, containing dissolved cations and possessing a large surface area.
Clay-Rich Deposits and Hydrocarbons
- Clay-rich deposits serve as common hydrocarbon source rocks because organic material can adhere to clay.
Kaolin Clay Group
- Most common mineral: Kaolinite (Al2Si2O5(OH)).
- Formed through weathering in highly leached tropical soils.
- Uses include ceramics, medicine, coated paper, toothpaste, food additives, and cosmetics.
Illite Clay Group
- Common minerals: Illite and Glauconite.
- Basic formula: (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2(H2O)].
- Weathers from feldspar and muscovite.
- Presence of glauconite indicates an iron-rich environment.
Chlorite Clay Group
- Basic formula: (Mg,Fe)3(Si,Al)4O10(OH)2(Mg,Fe)3(OH)6.
- Weathers from mafic minerals like pyroxene and amphibole.
Smectite Clay Group
- Common minerals: Montmorillonite and Bentonite.
- Basic structure: (Na,Ca)0.3(Al,Mg)2(Si4O10)(OH)2·nH2O.
- "Swelling clays" due to their ability to hydrate and dehydrate; used in drilling muds, landfill liners, and pond plugs.
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