Sedimentology
Grain Mineralogy
Reading Assignment: Boggs, Chapter 5

I.
Context of crust composition and surface rock composition
II. Quartz
III. Feldspar
IV. Rock fragments
V. Other grains
VI. Classification

Sedimentology
Grain Mineralogy (1): Crustal composition
Reading Assignment: Boggs, Chapter 5

Crust Composition: Elements
There are 92 naturally occurring elements, but most are rare.

Crust Composition: Minerals
Common Silicate Minerals:
Feldspar, Quartz, Olivine,
Pyroxene, Amphibole, Mica
Silicate Minerals

Crust Composition: Minerals
Common Silicate Minerals:
Feldspar, Quartz, Olivine,
Pyroxene, Amphibole, Mica
Common Non-Silicate Minerals:
Carbonates, Oxides,
Sulfides, Phosphates, Salts
Silicate Minerals

Crust Composition: Minerals
Common Silicate Minerals:
Feldspar, Quartz, Olivine,
Pyroxene, Amphibole, Mica
Common Non-Silicate Minerals:
Carbonates, Oxides,
Sulfides, Phosphates, Salts
There are ~ 3000 minerals
But 10 account for 90% of crust!
Silicate Minerals

Crust Composition: Rock Type
Continental Crust:
Granite ~ 60% Feldspar,
25% Quartz
10% Mica, Amphibole
Sedimentary Rocks:
Mudstones, Sandstones, Carbonates

Crust Composition: Rock Type
Continental Crust:
Granite ~ 60% Feldspar,
25% Quartz
10% Mica, Amphibole
Sedimentary Rocks:
Mudstones, Sandstones, Carbonates

Net Result: Grain Types
• Most common grains produced is some type of Feldspar (~60% of granitic crust)
• But Feldspar weathers rapidly
• Quartz, already abundant, is enhanced on Earth surface because it is resistant to weathering
• Rock Fragments – mostly sedimentary, but can be metamorphic or volcanic
• Note: Granite or Gneiss rock fragments are classified with Feldspar

Breakdown of Primary Grain Types
• Quartz
• Plutonic = “common quartz”
• Volcanic
• Metamorphic = composite/polycrystalline
• Recycled sediment
• Vein (from final magmatic stages)
• Feldspar
• Alkali Feldspar (Mostly K-Feldspar = K-spar)
• Plagioclase
• Intergrowths of K-spar & Plagioclase
• Rock fragments
• Sedimentary (SRF)
• Metamorphic (MRF)
• Volcanic (VRF)

Sedimentology
Grain Mineralogy (2): Quartz
Reading Assignment: Boggs, Chapter 5

Quartz Mineral Characteristics
• Hardness (7).
• Chemically stable SiO
2
• Can be present in: tetrahedron
• monocrystalline form- (most stable)
• composite grains
• Quartz is the most common sedimentary grain (65% of average sandstone)
• How do we identify it in thin section?
• Extinction under polarized light (straight to highly undulatory)
• Grain shape (subequant to bipyramidal)
• Inclusions & vacuoles
• Colors ranges from clear to a wide variety of colors caused by inclusions

https://www.youtube.com/watch?v=fn_9OXbcFY8
Quartz extinction in polarized light

https://www.youtube.com/watch?v=HpTENXNj8es
Extinction in granitic quartz

https://www.youtube.com/watch?v=J0eDfox-0i0
Undulose extinction in stressed quartz

Range of Extinction
Straight extinction

Range of Extinction Slightly Undulose Extinction

Range of Extinction
Strongly Undulose

Inclusions & Vacuoles
Inclusions
/vacuoles look like needles or bubbles within the quartz crystal

Plutonic Quartz
• Typically from granitic source
(continental crust)
• The ultimate weathering product from granite
• Characteristics
• monocrystalline
• Subequant shape
• Straight extinction with polarized light
• Few vacuoles or inclusions monocrystalline
Straight extinction
Sub-equant

Volcanic Quartz
• Volcanic source
• Distinctive bipyramidal shape
• Straight extinction
• Few inclusions or vacuoles bipyrimidal vacuoles

Metamorphic Quartz
• Composite/polycrystalline
• Interlocking crystals, sutured contacts
• Can be elongate (schist)
• Undulatory to strongly undulatory extinction polycrystalline
Sutured contacts
Undulatory extinction

Metamorphic
Quartz
• Composite/polycry stalline
• Interlocking crystals, sutured contacts
• Can be elongate
(schist)
• Undulatory to strongly undulatory extinction polycrystalline polycrystalline

Vein Quartz
• Forms in final magmatic stage (plutonic bodies) where volatiles are abundant
• Common vacuoles & inclusions (often called “milky” Quartz)

Sedimentology
Grain Mineralogy (3): Feldspars
Reading Assignment: Boggs, Chapter 5

Feldspar Family
Alkali feldspar –
• Solid solution series from KAlSi3O8
→ (K, Na)AlSi3O8 → NaAlSi3O8
• Most common are K-feldspar
• Orthoclase (KAlSi3O8)
• Microcline (NaAlSi3O8)

Feldspar Family
Alkali feldspar –
• Solid solution series from KAlSi3O8
→ (K, Na)AlSi3O8 → NaAlSi3O8
• Most common are K-feldspar
• Orthoclase (KAlSi3O8)
• Microcline (KAlSi3O8)
Plagioclase feldspar-
• Solid solution series from
NaAlSi3O8 (Albite) → CaAl2Si2O8
(Anorthite)
• General formula (Na,Ca)(Al,
Si)Si2O8

• Most common mineral in Earth crust
• Hardness (6) = less than Quartz
• Chemically unstable, commonly altered, weathers to clay minerals
• Pink, white, gray, brown in color
• Untwinned to distinctive twinning habit in Plagioclase &
Microcline
• K-spar appears similar to Quartz, identified by staining techniques

K-Feldspar – Staining to differentiate from quartz

K-Feldspar – Grid Twinning in Microcline

K-Feldspar – Grid Twinning in Microcline

K-Feldspar – Dissolution & Alteration

Plagioclase - Twinning

Plagioclase – Alteration & Dissolution

Perthite – Intergrowth of K-Spar & Plagioclase

Sedimentology
Grain Mineralogy (4): Quartz
Reading Assignment: Boggs, Chapter 5

Rock Fragments
• Sedimentary rock fragments (SRF)
• Chert – microcrystalline quartz
• Glauconite – marine, typically from fecal pellets or organic material
• Shale, siltstone, mudstone, sandstone
• Metamorphic rock fragments (MRF)
• Slate, schist (gneiss is considered Feldspar for classification)
• Volcanic rock fragments (VRF)
• Glass, tuff

Rock Fragments- SRF

SRF – Claystone & Shale

SRF - Siltstone

SRF - Sandstone

SRF – Limestone, Fossil, Dolomite

SRF - Chert

Rock Fragments- MRF

MRF - Slate

MRF - Schist

Rock Fragments- VRF
Glass, dissolution Tuff w/ Glass shards

Rock Fragments- Plutonic

Other Grains - Biogenic
Glauconite
Coal
Plant
Red algae

Other Grains - Inorganic
• Heavy minerals (s.g. > 2.9)
• Most common zircon (very stable) ,rutile, magnetite (opague), pyroxene, amphibole, mica

Other Grains - Inorganic
Opaque Mineral
Amphibole
Zircon
Muscovite

Classification – Primary Tool for Determining Source Area/Provenance

Questions You Should be Able to Answer
1.
How many naturally occurring elements? Which are most common in the Earth’s crust? What are the 2 most common?
2.
What are silicate minerals? Why are they so common in the Earth’s crust? What are the common silicate minerals? Which two are most common?
3.
What are common groups of non-silicate minerals?
4.
What is the general mineral composition of granite?
5.
What rock types compose the crust by volume? By surface cover?
6.
What is the most common sedimentary rock?
7.
What are the 3 main types of sedimentary grains?
8.
What are the characteristics of the mineral Quartz?
9.
What are the origins of Quartz grains? What are the distinguishing characteristics of each type? Which type is most common? Why?
10. What does the extinction pattern in Quartz tell you?
11. What are inclusions in grains? Vacuoles? How do they form?

12. What are the characteristics of Feldspars?
13. What is the overall structure of the family of Feldspar minerals? How do these plot on a tertiary diagram?
14. What are Alkali Feldspars? What are the most common types? What is the general chemical composition?
What are the distinguishing characteristics that allow you to identify these?
15. What are Plagioclase Feldspars? What are the most common types? What is the spectrum in chemical composition across this solid-state series? What are the distinguishing characteristics that allow you to identify these?
16. How do you identify mineral alteration to clay and dissolution in Feldspars?
17. What types of intergrowths of Feldspar minerals are common?
18. What are the main types of rock fragments seen as sedimentary grains?
19. What is chert?
20. What is glauconite?
21. What features allow you to identify a sedimentary rock fragment?

22. What are the types of metamorphic rock fragments? How would you identify these?
23. How do you identify igneous rock fragments?
24. What is the basis for classification of grains on the standard tertiary diagram? You need to be able to reproduce this diagram, and know the composition of Quartzarenite, Arkose, Litharenite, etc…
25. What is the purpose of the classification of grains by mineralogy/type?