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Atoms to volcanoes - 33739 > Felsic minerals and classification > Flashcards

Felsic minerals and classification Flashcards

1
Q

What are the most common continental felsic rocks?

A

granite
rhyolite

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2
Q

What is the bulk geo-chemistry of felsic like compared to mafic?

A

Much more silica rich (70% for F, 50% for M)
Rich in alkalis
Poor Mg, Fe and Ca oxides

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3
Q

What process is responsible for the difference in bulk Geo-chemistry of felsic and mafic?

A

Fractional crystallisation

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4
Q

What are alkalis that felsic can be rich in?

A

Na2O- sodium oxide
K2O- potassium oxide

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5
Q

How can the composition of felsic magma also be referred to as?

A

Evolved

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6
Q

What does it mean to say felsic magma is evolved?

A

magma experienced modification from primitive mafic mainly via fractional crystallisation

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7
Q

What is fractional crystallisation?

A

early formed crystals removed- thus the minerals that form them are used creating a dilution affect of liquid melt

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8
Q

What are the first minerals to form in a cooling basalt?

A

olivine
pyroxene
Ca-rich plagioclase

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9
Q

What will the the same and what will be variable in the evolved rocks?

A

Not all same silica content
Rich be rich in felsic minerals

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10
Q

What elements typically make up felsic minerals?

A

Al, Si, K, Na

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11
Q

What are some examples of minerals rich in felsic minerals ?

A

Feldspars (alkali feldspars)
Quartz or feldspathoids

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12
Q

What is the general reason rocks will become evolved?

A

as the magma crystallises the minerals forming have a chemistry that doesn’t match liquid melt

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13
Q

What are the first minerals to form from cooling basalt usually composed of?

A

rich in- Mg Fe Ca
Poor in silica

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14
Q

What are compatible elements?

A

elements that go into a specific mineral

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15
Q

What are incompatible elements?

A

those which dont go into phases

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16
Q

What will happen to the liquid melt during crystallisation involving incompatible?

A

liquid melt will become enriched in incompatible elements

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17
Q

What can fractional crystallisation produce graphically?

A

a continuous spectrum of real magma compositions from mafic starting to felsic end

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18
Q

What are the common minerals in felsic igneous?

A

Quartz
Feldspars
Micas- muscovite and biotite
Hornblende
Magnetite (opaque oxide phases)

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19
Q

What is the equation for quartz?

A

SiO2

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20
Q

Where is quartz commonly found?

A

widespread in upper continental crust

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21
Q

What is quartz common in?

A

felsic igneous rocks
metamorphic rock
clastic sediments

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22
Q

What will the framework of quartz be like?

A

SiO2 tetrahedra

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23
Q

How many polymorphs of silica are there?

A

6

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24
Q

What is the stability of the silica polymorphs like?

A

all stable at different pressure and temperature

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25
Q

What is the stability of quartz like?

A

stable at low temp and pressure (upper continental crust)
highly resistant to weathering
Very hard

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26
Q

What will quartz appear like in igneous hand specimen?

A

grey glassy looking mineral
no cleavage instead glassy conchoidal fractures

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27
Q

What is quartz like in thin section?

A

colourless
low relief
low birefringence
no twinning
no cleavage
Clean even when around altered minerals
Undulose extinction

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28
Q

What is undulose extinction and why does it occur in quartz?

A

occurs when structure is strained
it is extinction sweeping across grain as platform is rotated

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29
Q

What elements will you typically find in feldspars?

A

(Ca, Na, K)
Al
(Al, Si)
Si2O8

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30
Q

Where are feldspars dominant?

A

the crust- continental and oceanic

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31
Q

What is the framework of feldspars like?

A

alumino-silicates
Si and Al in tetrahedral framework

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32
Q

What 2 series exist for feldspar in solid solution?

A

Plagioclase feldspars
Alkali feldspars

33
Q

What ion exchange occurs in plagioclase feldspars?

A

Na-Si
Ca-Al

34
Q

What do you transfer to and from with plagioclase feldspar?

A

Albite (Na) to Anorthite (Ca)

35
Q

What endmembers is there no natural transfer between? (feldspars)

A

Potassium and Calcium (orthoclase and Anorthite)

36
Q

What can alkali feldspars transition inbetween?

A

Orthoclase
Albite

37
Q

What ion exchange takes place with alkali feldspars?

A

Na and K (sodium and potassium)

38
Q

What are the common elements found in plagioclase feldspar?

A

(Ca, Na) Al (Al, Si) Si2O8

39
Q

What can the more complex exchange in solid solution of plagioclase feldspar lead to?

A

reaction to changing magmatic conditions can be slow and lead to compositional zoning

40
Q

What is zoning in crystallisation?

A

bands of light and dark crystal contained inside crystal border almost radiating from a central nucleus

41
Q

How will plagioclase feldspar appear in hand specimen?

A

white blocky grains

42
Q

What will plagioclase feldspar be like in thin section?

A

low relief
colourless
low birefringence
Unclear cleavage - can show 2 (90*- cubic)
Polysynthetic twinning (multiple or lamellar)
Altered (dusty)

43
Q

What is K feldspar?

A

K-rich alkali feldspar

44
Q

What will K feldspar in hand specimen?

A

pinkish colour can also be white

45
Q

What will K feldspar be like in thin section?

A

Similar to plagioclase
but instead simple twinning

46
Q

What will twinning be like in a low T-form microcline?

A

might be cross-hatched

47
Q

What can the texture of coarsely-crystalline alkali feldspars be called?

A

perthite- “flame”, “patchy” and “lamellar”

48
Q

What are the 2 common mica minerals in igneous rocks?

A

Biotite
Muscovite

49
Q

What are the important bits to remember from biotite formula?

A

Hydrous- OH- anion
Range of cations- Al, K, Fe, Mg

50
Q

What is biotite more commonly referred to as?

A

Dark mica

51
Q

What is the general thing to take away from muscovite formula?

A

Al rich (different from biotite as absence of Fe and Mg)

52
Q

What is muscovite more commonly referred to as?

A

white mica

53
Q

What type of silicates are micas?

A

sheet silicates

54
Q

What is the structure of micas like?

A

tetrahedra link in sheet form (perfect cleavage between sheets)
Thin platy grains break with basal cleavage
Grain shape hexagonal in other plane

55
Q

How can micas be recognised in hand specimen?

A

strong cleavage
reflective surface- biotite: black, muscovite: silvery

56
Q

What will biotite be like in thin section for ppl?

A

Pleochroic brown
Moderate relief
One very good cleavage
Alteration prone

57
Q

What can happen to the cleavage in thin section for biotite?

A

kinked due to mineral weakness causing irregular crystal ends

58
Q

What will alteration look like in biotite thin section?

A

patchy

59
Q

What is biotite like in xpl thin section?

A

high birefringence (third order)
Extinction grains have speckled texture

60
Q

What is muscovite like in thin section ppl?

A

Colourless
One very good cleavage

61
Q

What can the kinking of the cleavage in muscovite lead to?

A

irregular crystal and development of woody texture

62
Q

What is muscovite like in thin section xpl?

A

high birefringence (upto 3rd order)

63
Q

What does crystal size relate to in igneous rocks?

A

cooling rate

64
Q

What is the factor that distinguishes basalts from gabbro due to chemical and mineralogical sameness?

A

texture- crystal size

65
Q

What will a slow cooling rate do to crystallisation?

A

coarser crystals

66
Q

Where will extrusive rocks form?

A

on the surface

67
Q

What does the surface cooling environment mean for extrusive rocks?

A

heat lost rapidly
liquid magma will crystallise quickly
finer grained

68
Q

Where do intrusive rocks cool and solidify?

A

in the crust

69
Q

What are some examples of shallow intrusions?

A

Sills
Dykes

70
Q

What can shallow intrusions also be called?

A

hypabyssal rocks

71
Q

What is cooling rate like with hypabyssal rocks?

A

small size lose heat relatively quickly

72
Q

What is the typical grain size of hypabyssal rocks?

A

medium to fine grained

73
Q

What 2 broad types of intrusive body?

A

hypabyssal
plutonic

74
Q

What are plutonic intrusive bodies?

A

larger
greater depth (few km)

75
Q

What is cooing rate and crystal size like in plutonic bodies?

A

generally slower forming coarser crystals

76
Q

What will the hand specimen of plutonic rocks be like?

A

granular texture - formed due to large interlocking crystals

77
Q

How will crystal formation and nucleation be affected by rapid cooling?

A

many crystals nucleate but dont have time to grow into larger crystals

78
Q

What is nucleation for crystallisation?

A

initial process of crystal formation from solution

Atoms to volcanoes - 33739 (16 decks)

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