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Earth Sciences > Lectures 15-20 > Flashcards

Lectures 15-20 Flashcards

1
Q

How much of the earth is the core?

A

roughly 1/3

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2
Q

What is the main composition of the core?

A

Fe-Ni alloy

  • some other siderophile elements (iron loving)
  • 10% of a lighter material—>unknown but most likely O, S, Si
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3
Q

Crust composition

A

75% = Si and O

8 elements make up 99%

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4
Q

What is a mineral?

A

An element or chemical compound that is normally crystalline and that has been formed as a result geological processes

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5
Q

What does crystalline mean?

A

Regular repeating pattern of atoms

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6
Q

What are the 7 most common mineral types?

A

1) Silicates (SiO4 4-)
2) Oxides/hydroxides (O2-/OH-)
3) Sulfides (S2-)
4) Halides (Cl-, F-)
5) Carbonates (CO3 2-)
6) Phosphates (PO4 3-)
7) Sulphates (SO4 2-)

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7
Q

What is the chemical formula of olivine?

A

(Mg, Fe)2 SiO4

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8
Q

What is the chemical composition of Pyroxene?

A

Ca(Mg, Fe) Si2O6

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9
Q

What is the chemical composition of Garnet?

A

(Mg, Fe, Ca)3 Al2Si3O12

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10
Q

What is the chemical formula of Quartz?

A

SiO2

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11
Q

What is the chemical formula of Alkali Feldspar?

A

NaAlSi3O8-KAlSi3O8

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12
Q

Chemical formula of plagioclase Feldspar

A

NaAlSi3-CaAl2Si2O8

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13
Q

Chemical formula of Mica

1) Biotite
2) Muscovite

A

1) K(Mg,Fe)3AlSi3O102

2) KAl2AlSi3O102

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14
Q

Chemical formula of amphibole

A

Ca2(Mg,Fe))5Si8O222

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15
Q

Chemical formula of calcite

A

CaCO3

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16
Q

What is a solid solution

A

A series formed when there are intermediary minerals
- so 2 possible forms so 1 rock has mix

  • fully one = endmember
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17
Q

What are the endmembers of Olivine?

A

Forsterite (Mg2)

Fayalite (Fe2)

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18
Q

What does the ability of a cation to be substituted by another depend on?

A

1) Ionic radius
2) Ionic charge (lesser extent)

Size within 15%
Charge within 1

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19
Q

What is the basic unit of silicate structure?

A

SiO4 4- tetrahedron

  • so angles of 109.5 degrees
  • sp3 hybrid orbitals of Si4+ atoms form a mixed ionic covalent bond with oxygens
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20
Q

What are the types of oxygens in a SiO4 tetrahedra structure?

A

bridging oxygens and non-bridging oxygens

  • non count 1 towards total number of oxygens
  • bridging count 1/2
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21
Q

What allows a lot of different structural topologies to be created ion SiO4 tetrahedron?

A

Si-O-Si bond is relatively flexible

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22
Q

What are orthosilicates?

A

Isolated tetrahedra closely packed in regularly-spaced array and linked by metal cations

MOST IMPORTANT IS OLIVINE
- [SiO4]4-

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23
Q

What is the most important orthosilicate?

A

Olivine

  • Mg and/or Fe cations occupy 6-fold coordinated octahedral sites between the tetrahedra
  • simple substitution between the two as same size (roughly) and same charge
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24
Q

What are two examples of isolated tetrahedra SiO4 other than olivine?

A

1) Garnet

2) Ringwoodite (high pressure form of olivine)

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25
Q

What are single chain silicates?

Give example

A

[Si2O6]4-

  • silica tetrahedra linked into chains
  • each group shares 2 oxygens with neighbours

-PYROXENES

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26
Q

What are the two main types of pyroxenes?

A

1) Clinopyroxene (monoclinic)
- Ca(Mg, Fe) Si2O6

2) Orthopyroxene (orthorhombic)
- (Mg, Fe) Si2O6

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27
Q

How to the single chain silicates form large crystals?

A

Cations cross link chains together

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28
Q

How are the pair of chains in single chain silicates joined?

A

held by Mg octohedra form a tightly-bonded unit = I-beam

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29
Q

What gives pyroxenes their 90 degree cleavage plane?

A

The fact it is easier to break bonds between I beams

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30
Q

What are the essential constituents of basalt?

A

1) pyroxenes

2) Plagioclase feldspar

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31
Q

What are double chain silicates

A

two single chains joined together, side by side
= AMPHIBOLES
= [Si8O22]12-

  • chains linked by cations to make up charge deficit and make stable
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32
Q

What gives amphiboles the distinctive 56 degree cleavage?

A

double width of I-beams

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33
Q

What are the difference between tremolite and hornblende

- what can they tell us about the magma that cooled?

A

Tremolite = Ca2Mg5Si8O222

Hornblende = Ca2(Mg, Fe, Al)5Si7AlO222

  • hydrous which means water present in the magma
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34
Q

What are sheet silicates?

A

[SiO10]4-

  • tetrahedra groups linked into sheets
  • each one shares 3 oxygens with neighbours
  • stacked sheets
  • MICAS
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35
Q

What are the two types of micas and their chemical formulas?

A

1) Muscovite = KAl2AlSi3O102
- white

2) Biotite = KMg3AlSi3O102
- can have iron in which is why it is darker (replace Mg)

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36
Q

How often is one of the tetrahedra occupied by aluminium in muscovite and biotite?

A

every 4

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37
Q

How are sheet silicates held together?

A

Weakly bonded by large K+ or Na+ ions

- perfect basal cleavage

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38
Q

What are tectosilicates?

A

Framework silicates = [Si4O8]

  • all 4 oxygens are bonded
  • like quartz = SiO2
  • charge neutral so no other ions
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39
Q

Which framework silicates (tectosilicates) need ions?

A

Alumnosilicates

  • like feldspars
  • Al3+ in places of Si4+ (only 1 or 2)
  • so accomodate K+, Al+ or Ca2+
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40
Q

What are the three endmembers of Feldspars?- chemical formula?

A

1) orthoclase = KAlSi3O8
2) Albite = NaAlSi3O8
3) Anorthite = CaAl2Si2O8

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41
Q

What is the name of the feldspars that are a solid solution of orthoclase and albite?

A

Alkali feldspars

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42
Q

What is the name of the solid solution feldspars between albite and anorthite?

A

Plagioclase feldspars

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43
Q

Why is ionic radii extremely useful in deciding suitability of cation given structural site?

A
  • cation must not rattle around in polyhedron
  • must touch all

LIMITING CASE=
Ra/Rb = 0.414 for squares (4 anions)
- a is anion and b is cation

Ra/Rb = 0.732 fro 8 anions

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44
Q

What are the common coordination numbers for the common cations?

A

GRAPH

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45
Q

What is the type of structure for Ra/Ro of 1

A

12=coordination number

  • cuboctohedral
  • like Fe in core
46
Q

Ra/Ro of 1-0.732

A

8 = cubic

- garnet [FeO8]

47
Q

Ra/Ro = 0.732-0.414

A

6 = octahedral

[MgO6] in olivine

48
Q

Ra/Ro =0.414-0.225

A

4 = tetrahedral

[SiO4] in feldspar

49
Q

Ra/Ro = 0.225-0.155

A

3 = triangular

[CO3] in calcite

50
Q

Is the earth’s core isotropic or anisotropic?

A

The solid inner core is anisotropic

51
Q

What is the most efficient way of packing identical spheres in two dimensions?

A

packed layer with 6 contacts with neighbouring atoms - in a hexagon

  • layers can then be stacked on top
  • various packing structure
  • HCP and CCP
52
Q

What is HCP packing?

A

HEXAGONAL CLOSE PACKING

  • so in 3D each atom has 12 neighbouring = 12 coordination number - 3 above and 3 below and 6 in own layer
  • typical if packing same size atoms
  • ABABABA layers
53
Q

What is CCP?

A 
CUBIC CLOSE PACKING
- ABCABCABC layering
12 fold coordination
- similar to but not identical to hcp structures
- same packing but different symmetry
- fcc is the face centred cubic
- bcc is the body centred cubic
54
Q

What is FCC?

A

FACE CENTRED CUBIC

- cubic with 12 coord

55
Q

What is BCC?

A

BODY CENTRED CUBIC

  • 8 coord
  • lower density than hcp or fcc structures
56
Q

What is the most suitable packing for iron?

A
  • dont know as not completely pure iron in core
  • using high pressure phase diagram GRAPH
  • most stable at certain temp and pressure
  • good at low temp
  • and high pressures low temp
  • harder after
57
Q

What is the packing efficiency of HCP, fcc and bcc?

A
  1. HCP = 74%
  2. FCC = 74%
  3. BCC = 68%
  • extremely hot so vibrate more - higher entropy vs efficient packing
  • only know that if pure Fe then hcp under core conditions
58
Q

How hot is the core?

A

From phse diagram, know it is 330GPa at boundary

  • so if pure Fe then 6300K
  • but not pure - presence of light elements reduces temperature
  • 5400-5700K
59
Q

Do we know the mineral composition of the mantle?

A

Yes
- easier to do pressure and temperature experiments

GRAPH

60
Q

What 2 minerals make up the lower mantle?

A

Perovskite

Ferropericlase

61
Q

What makes up the transition zone?

A

ringwoodite (γ)
wadsleyite
majorite garnet

62
Q

What is the upper mantle made of?

A

Olivine
Orthopyroxene
clinopyroxene
pyrope garnet

63
Q

What silicate coordination is favoured in the upper mantle?

A

tetrahedral in all phases

64
Q

What characterises a crystalline material

A

regular atomic structure

  • repeats itself in 3 dimensions
  • translation symmetry
65
Q

What is a lattice

A
  • generated from any repeating pattern
  • each point which can be superimposed is a lattice point
  • not all atoms sit on lattice points
66
Q

What is the unit cell?

A

The repeating unit of the lattice

  • in two dimensions it is a parallelogram with lattice points at the corners
  • primitive unit cell has no lattice points contained
  • non-primitive if they do
  • IN 3D = parallelepiped
67
Q

Diagram of primitive unit cell and angles

A

DIAGRAM

68
Q

What are lattice parameters

A
  • defining the 3 dimension
  • choose axes x,y,z
  • point along edges of unit cell
  • The lattice parameters are the lengths along the axes and then the angles between them
69
Q

Body centred vs face centred vs primitive lattice

A

Body centred = I = one lattice point also in the centre

Face centred = F = lattice points on corners and also centre of each face
- c face centred - only two on opposite faces

Rhombohedral
- a = b = c
α = β = γ not equal to 90 degrees

70
Q

What is a lattice vector?

A

vector joining lattice points
- written =

t = Ua + Vb + Wc
t written as [UVW]
- negative values have bar on top

71
Q

Rotational symmetry

A

An n fold rotational symmetry operation

  • rotates by 360/n
  • only 1,2,3,4,6 in periodic structure
72
Q

Mirror symmetry

A

reflection of the object in a plane = itself

73
Q

What are crystal systems?

A

the rotational symmetry of a crystal constrains the shape of the conventional unit cell we describe the structure with
- 7 possible

74
Q

Lattice parameters of crystal systems - table

A

TABLE

75
Q

What is a lattice plane?

A

plane which passes through any three lattice points

- not in a straight line

76
Q

What is a set of parallel lattice planes?

A

Miller indices indicate
(hkl)
- define where first plane away from origin intersects the x,y,z crystallographic axes
- if parallel to axis then zero

INTERSECTIONS
- at a/h along x axis and then b/k on y and c/l on z

77
Q

What does form mean?

A

set of related planes or faces in a crystal

  • denoted {hkl}
  • related to (hkl) by symmetry
78
Q

What does habit mean?

A

Described overall shape of crystal (or group of crystal)

  • depends on relative development of faces in the various forms present
  • a substance can take on more than one habit depending on the growth conditions - not always diagnostic feature
79
Q

What is twinning?

A

a twinned crystal consists of two or more adjacent parts in which the crystal structure is differently oriented

  • separated by twin boundary - related by a symmetry element like mirror plane or rotation axis
80
Q

How might twinning arise?

A

mistakes during crystal growth

1. mechanical deformation

81
Q

What type of microscope is used to examine thin rock slices?

A

Petrographic

82
Q

What are the properties of light?

A

Transverse electromagnetic wave

  • perpendicular electric (E) and magnetic field (B)
  • vibration always taken as parallel to E and perpendicular to travel direction of the wave

Unpolarised light vibrates in all directions perpendicular to the travel direction
- polarised means only in one vibration direction

83
Q

What is the speed of light

A

c = C0/n

C0 is speed of light in a vacuum
- The higher the refractive index, the slower the light travels

84
Q

What is an isotropic material

A

Same refractive index in all directions

85
Q

What is an anisotropic material?

A

Different directions have different refractive indexes

86
Q

What is the habit/shape of the thin section?

A

The order/shape of the minerals

87
Q

What is the microstructure of thin section?

A

The cleavage, twinning, zoning

- how the mineral differs across the crystal

88
Q

What is the relief of a mineral?

A

term used to describe how clearly the edge of a grain/crystal can be seen

  • depends between the RIs of grain and bedding medium = 1.54
  • if same then invisible

Low relief = RI close to mounting
- little refraction by surface topography

High relief = RI very different from medium
- large surface topography

89
Q

What is the Becke line test

A

Test to see if RI is greater than or less than RI of medium

  • Lower stage = raise focal plane
  • Line moves into higher RI medium
90
Q

What does anisotropy in plane polarised light?

A

In petrographic microscope electic field vibrates E-W

  1. Variation in Relief
    - relief changes as rotate
  2. Pleochroism
    - Colour variation as rotate
    - specific, diagnostic in many cases
91
Q

What are the slow and fast directions?

A

The directions where
- Small n = fast

  • large n = slow
92
Q

What is double refraction

A

Through anisotropic material - two separate rays, one fast and other slow, perpendicular directions
- called double refraction
Ordinary = unrefracted
Extraordinary = Slight angle to ordinary ray

93
Q

What is it meant when ‘crossed polars’

A

Crossed polarisers

  • in this isotropic materials always dark (no light through)
  • In anisotropic - extinction positions at 90 degrees from one another and colours at other points
94
Q

Why do isotropic materials appear black in crossed polars?

A

In these materials, every direction is a permitted vibration direction

  • so straight through without changing polarisation state
  • absorbed by analyser so appears black

identify isotropic materials like cubic minerals or glass

95
Q

Why do anisotropic materials go extinct every 90 degrees?

A

If one of the permitted vibration directions of an anisotropic material is parallel to polariser

  • then same as isotropic
  • in extinction position

Straight and inclined extinction

96
Q

What is the difference between straight extinction or inclined extinction?

A

Crystals have distinct microstructure

- straight extinction is when vibration length slow

97
Q

How to measure extinction angle?

A
  1. rotate the stage so feature of interest is oriented N-S
    - note angular position of the graduated stage
  2. Rotate crystal clockwise to extinction position
    - note angle of stage
  3. Repeat 1 and 2 but rotate anticlockwise to another extinction position
  4. Calculate angle between feature of interest and which vibration direction was the closest
98
Q

What happens if we try and send light through crystal vibrating in a non-permitted direction

A
  • light resolved into two components vibrating in one of the permitted vibration directions
  • The two components then travel at different speeds
  • causing optical path difference/retardation
99
Q

What is the birefringence of a material?

A

difference in refractive indices of two vibration directions

100
Q

How to work out retardation (Δ)?

A

t (n1-n2)

t = thickness of crystal

n1 = refractive index of slow direction
n2 = RI of fast direction
101
Q

Phase difference between fast and slow components

A

the difference between the components when they recombine when leave crystal

if in phase then does not pass through analyser
- if out of phase does pass

102
Q

Fringe spacing of red vs green

A

Red larger spacing than green as t increases uniformally

103
Q

White light in quartz of varying lengths?

A

no dark as in single wavelength

  • violet instead at each order
  • called sensitive tint
104
Q

What is the Michel-Levy chart?

A

Shows colours superimposed on a plot of thickness against retardation

GRAPH

105
Q

What is the optical indicatrix?

A

A 3D ellipsoid which describes how the RI varies in all directions in a crystal

106
Q

What is the optical axis?

A

The point that if we look down then an anisotropic material looks isotropic

107
Q

Uniaxial vs Biaxial

A

Uniaxial = one optic axis

Biaxial = 2

108
Q

What minerals have uniaxial indicatrix?

A
  1. Tetragonal
  2. Hexagonal
  3. Trigonal
109
Q

Optical indicatrix of cubic crystals

A

Perfect sphere

- so isotropic

110
Q

What are the two types of uniaxial indicatrix?

A
  1. Positive uniaxial ε > ω
    - Like quartz, like a rugby ball
  2. Negative ω > ε
    - like calcite = burger
111
Q

Which minerals have a biaxial indicatrix?

A
  1. Orthorhombic
  2. Monoclinic
  3. Triclinic
112
Q

Orientation of the biaxial indicatrix

A

Constrained by symmetry

  • Orthorhombic = principle axes along x, y, z
  • Monoclinic = principle axes, one must be on y axis of crystal
  • Triclinic have no symmetry so no constraint on indicatrix orientation

Earth Sciences (12 decks)

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