Download Lecture#3_Grain Mineralogy

Sedimentology
Grain Mineralogy
Reading Assignment: Boggs, Chapter 5
Key Concepts
I.
II.
III.
IV.
V.
VI.
Context of crust composition and surface rock composition
Quartz
Feldspar
Rock fragments
Other grains
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)
End of Part 1
Sedimentology
Grain Mineralogy (2): Quartz
Reading Assignment: Boggs, Chapter 5
Quartz Mineral Characteristics
• Hardness (7).
• Chemically stable SiO2 tetrahedron
• Can be present in:
• 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
Quartz
extinction in
polarized
light
https://www.youtube.com/watch?v=fn_9OXbcFY8
Extinction in
granitic
quartz
https://www.youtube.com/watch?v=HpTENXNj8es
Undulose
extinction in
stressed
quartz
https://www.youtube.com/watch?v=J0eDfox-0i0
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
Sub-equant
Straight extinction
Volcanic Quartz
• Volcanic source
• Distinctive bipyramidal
shape
• Straight extinction
• Few inclusions or vacuoles
vacuoles
bipyrimidal
Metamorphic Quartz
• Composite/polycrystalline
• Interlocking crystals,
sutured contacts
• Can be elongate (schist)
• Undulatory to strongly
undulatory extinction
Sutured
contacts
polycrystalline
Undulatory
extinction
Metamorphic
Quartz
polycrystalline
• Composite/polycry
stalline
• Interlocking
crystals, sutured
contacts
• Can be elongate
(schist)
• Undulatory to
strongly undulatory
extinction
polycrystalline
Vein Quartz
• Forms in final magmatic stage (plutonic bodies) where volatiles are abundant
• Common vacuoles & inclusions (often called “milky” Quartz)
End of Part 2
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
Feldspar Mineral Characteristics
• 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
End of Part 3
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
Plant
Red algae
Coal
Other Grains - Inorganic
• Heavy minerals (s.g. > 2.9)
• Most common zircon (very stable) ,rutile, magnetite (opague), pyroxene, amphibole,
mica
Other Grains - Inorganic
Muscovite
Opaque Mineral
Zircon
Amphibole
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?