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1A Earth > What's The Earth Made Of? (Lectures 9-14) > Flashcards

What's The Earth Made Of? (Lectures 9-14) Flashcards

1
Q

What are the 8 most abundant elements in the crust?

A 
Oxygen
Silicon
Aluminium
Iron
Calcium
Sodium
Potassium
Magnesium
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2
Q

Define a mineral

A

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

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3
Q

What are the main silicate minerals?

A 
Olivine
Pyroxene
Garnet
Quartz
Alkali/plagioclase feldspar
Biotie/muscovite mica
Amphibole
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4
Q

What is the general rule of thumb for when a wide range of substitution is possible for the ions in a mineral?

A

The size difference between ions is less than 15%

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5
Q

For some mineral groups, the extent of ion substitution depends on what and why?

A

Temperature

Higher T gives greater flexibility in the lattice

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6
Q

How are SiO4 tetrahedra joined and why?

A

Join by the corners using bridging oxygens

Minimises repulsion between Si(4+) ions

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7
Q

What are the different frameworks for silica tetrahedra?

A

Isolated tetrahedra
Single chains all pointing in one direction
Double chains
Continuous sheet
A 3D framework where each shares 4 corners

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8
Q

How are suitable cations chosen to balance the -ve charge from the oxygens in tetrahedra?
(4 points)

A

Ionic radii
Cation must touch all co-ordinating anions
Lowest is radius of anion x sqrt(2) for 2D
Lowest is radius of anion x sqrt(3) for 3D

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9
Q

What are the possible forms of cation coordination structure?

A 
Cuboctohedral: 12 fold
Cubic: 8 fold
Octahedral: 6 fold
Tetrahedral: 4 fold
Triangular: 3 fold
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10
Q

How are isolated silica tetrahedra linked together in olivine?

A

By Mg2+ and/or Fe2+ cations

Octahedral 6-fold coordinates sites between silica 4- tetrahedra

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11
Q

How are single-chain silicates balanced in charge?

A

Silica tetrahedra share two oxygens so repeating group [Si2O6]4-
Two chains linked together by Mg2+/Ca2+/Fe2+ in octahedral form

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12
Q

What cleavage is seen in single-chain silicates like pyroxenes in the basal section? Why?

A

90-degree cleavage

Bonds are easily broken between I-beams

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13
Q

How are double-chain silicates balanced in charge?

A

Repeating group of [Si8O22]12-

2+ cations join two chains in octahedral sites

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14
Q

What cleavage is seen in double-chain silicates like amphiboles in the basal section? Why is this different to that of single-chain silicates?

A

60-degree and 120-degree cleavage

I-beam is twice the size of that of pyroxene’s

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15
Q

How is charge balanced for sheet silicates?

A

Tetrahedra linked into sheets, each tetrahedron shares 3 oxygens
Repeating group of [Si4O10]4-
1/4 of tetrahedra occupied by aluminium
2 hydroxyl groups associated with the group
[AlSi3O10(OH)2]7- balanced by 2 Al3+/3 Mg2+ and a K+

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16
Q

What cleavage is seen in sheet silicates and why?

A

Perfect basal cleavage seen

Sheets weakly bonded together by large K+ or Na+

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17
Q

What is quartz?

A

SiO4 tetrahedra linked at all four corners

Gives neutral SiO2 repeating group

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18
Q

What is the difference between plagioclase and alkali feldspar?

A

Alkali involve the substitution of Na+ for K+

Plagioclase require the substitution of (CaAl)5+ for (NaSi)5+

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19
Q

What is translational symmetry?

A

A periodic arrangement such that a copy can be moved in a certain way for there to be overlap

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20
Q

What is the unit cell?

A

The repeating unit of the lattice

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21
Q

What is the difference between a primitive and non-primitive unit cell?

A

Primitive: lattice points at corners only

Non-primitive: lattice points at corners and inside the unit cell

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22
Q

If x, y and z axes are defined by the unit vectors a, b and c respectively, what are the lattice parameters?

A 
α = b ^ c (angle between b and c)
β = a ^ c (angle between a and c)
γ = a ^ b (angle between a and b)
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23
Q

What is a lattice vector?

A

A vector joining any two lattice points

Written as a combination of the unit cell vectors

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24
Q

What is the shorthand for a lattice vector if it was: t = U x a + V x b + W x c?
How are negative values written?

A

t = [UVW]

Bar over the top

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25
Q

What are the 5 lattice types?

A

Primitive - all points at corners
Body centred - lattice point in the centre
Face centred - lattice points in the centres of each face
C face centred - only two opposite faces have lattice points in the centre
Rhombohedral - a=b=c and α=β=γ=/=90

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26
Q

What are the forms of mirror symmetry?

A

2 fold = diad
3 fold = triad
4 fold = tetrad
6 fold = hexad

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27
Q

What are the 7 crystal systems?

A 
Cubic
Tetragonal
Hexagonal
Trigonal
Orthorhombic
Monoclinic
Triclinic
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28
Q

What symmetry is seen for each crystal system?

A 
Cubic: 4 triads
Hexagonal: 1 hexad
Trigonal: 1 triad
Tetragonal: 1 tetrad
Orthorhombic: 3 diads
Monoclinic: 1 diad
Triclinic: no symmetry
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29
Q

Define a lattice plane

A

A plane which passes through any three lattice points which are not in a straight line

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30
Q

A set of parallel lattice planes are characterised by what?

A

Miller indices (hkl)

31
Q

If a plane is parallel to an axis, what is the corresponding miller index?

A

Zero

32
Q

Define form

A

Sets of related planes or faces in a crystal

33
Q

Why are curly brackets used?

A

To distinguish forms from individual planes

34
Q

Define the habit of a crystal

A

Describes the overall shape of the crystal

35
Q

What is twinning in a crystal?

A

Two or more adjacent parts in which the crystal structure is differently oriented

36
Q

How does twinning arise?

A

Mistakes during crystal growth from:
mechanical deformation
crystal structure changing on cooling to a more stable, lower T structure

37
Q

Define an isotropic material

Define an anisotropic material

A

The same refractive index in all directions

Refractive index is not the same for all directions

38
Q

What are the four things that can be seen in a thin section?

A

Habit
Microstructure (twinning, cleavage, zoning)
Colour
Relief

39
Q

Define relief

A

How clearly the edge of a grain or crystal can be seen

40
Q

What does relief depend on?

A

The difference in refractive indices of the grain and the embedding medium

41
Q

Outline the Becke Line Test

A

Becke Line is a bright fringe often observed just outside/inside the crystal outline
Raising the focal plane moves the Becke Line into the medium of higher RI

42
Q

What is pleochroism?

How does it work?

A

Colour change with orientation

Anisotropic minerals absorb different wavelengths of light in different vibration directions

43
Q

Polarised light in an anisotropic medium is only permitted to vibrate along two perpendicular what?

A

Permitted vibration directions

44
Q

What constrains the permitted vibration directions?

A

Crystal’s symmetry

45
Q

Why are the two permitted vibration directions different?

A

One interacts with fewer atoms and so has a lower RI and is called “fast”
Other interacts with more atoms and so has a higher RI and is called “slow”

46
Q

What is double refraction?

A

Light travels through an anisotropic material as two separate rays, vibrating in perpendicular directions

47
Q

Define birefringence

A

The difference between the RIs of the two rays brought about by double refraction

48
Q

What are the observations under crossed polars?

3 points

A

Isotropic materials let no light through and appear black
Anisotropic materials let light of a specific colour through
Rotating an anisotropic material such that one of its permitted vibration directions is parallel to the polariser, no light gets through (extinction position)

49
Q

How often do extinction positions occur?

A

Every 90 degrees

50
Q

Why are isotropic materials black under crossed polars?

A

Every direction is a permitted vibration direction

So all light is absorbed by the E-W and N-S polariser/analyser

51
Q

What causes straight extinction?

A

Permitted vibration direction with the larger RI is parallel to the length of the crystal (length slow)

52
Q

What happens if light is sent through a crystal vibrating in a non-permitted direction?
(3 points)

A

Light resolved into two components along the permitted vibration directions
Component along the direction of smaller RI travels faster than component along the direction of larger RI
When slower component exits material, the fast component has travelled an extra distance called optical path difference

53
Q

What is the equation to figure out the optical path difference?

A

Δ = t(n1 - n2)

54
Q

What happens if the two components of light exit in phase or out of phase?

A

In phase: resulting light vibrates in same direction as the incident light
Out of phase: resulting light vibrates at 90 degrees to the incident light

55
Q

When does the light pass through the analyser, in or out of phase?

A

In phase does not

Out of phase does

56
Q

What is the optical indicatrix?

A

A radius vector parallel to each vibration direction with length proportional to the RI for the crystal with that VD
Always forms an ellipsoid

57
Q

What is the significance of an isotopic section?

A

A circular cross-section through the indicatrix

Normal to the optic axis

58
Q

Why do cubic crystal systems give a spherical optical indicatrix?

A

3-fold symmetry means the RI is always the same

59
Q

Which crystal systems give a uniaxial indicatrix?

What is the difference between the two forms of a uniaxial indicatrix?

A

Tetragonal, hexagonal and trigonal
Indicatrix contains one isotropic section and one optic axis, isotropic section always parallel to (001) or z
Positive uniaxial ε > ω (rugby ball shape)
Negative uniaxial ε < ω (burger shape)

60
Q

What are ε and ω with respect to the optical indicatrix?

A

ε is the value of RI parallel to the z-axis

ω is the value of RI within the plane perpendicular to z

61
Q

When a mineral is cut in the thin section, when is the anisotropy at a maximum and zero?

A

Maximum: cut parallel to the optic axis
Zero: cut perpendicular to the optic axis

62
Q

Which crystal systems have a biaxial indicatrix?

A

Orthorhombic, monoclinic and triclinic

63
Q

What is a biaxial indicatrix?

A

Two isotropic sections and two optic axes

64
Q

How is the biaxial indicatrix different for the crystal systems that can have it?

A

Orthorhombic: diad symmetry along x, y and z axes so principle axes must lie along x, y and z
Monoclinic: diad symmetry along the y-axis so one of the principle axes lies along the axis
Triclinic: no symmetry so indicatrix can adopt any orientation

65
Q

Close packed layers of identical spheres can be stacked to form which two structure types?

A

Hexagonal close-packed (hcp)

Cubic close-packed (ccp)

66
Q

What is the difference between hcp and ccp structures?

What is the similarity?

A

hcp has two kinds of atoms, A and B, and stacks ABABAB
ccp has three kinds of atoms, A, B and C, and stacks ABCABC
Both have a coordination number 12

67
Q

What is the body centred cubic structure (bcc)?

A

A cubic unit cell with atoms at the corners and one at the centre of the unit cell
Coordination number 8

68
Q

What are the packing efficiencies for hcp, ccp and bcc?

A

hcp and ccp 74%

bcc 68%

69
Q

Which minerals are there in the lower mantle?

A

Perovskite

Ferropericlase

70
Q

Which minerals are there in the transition zone?

A

Ringwoodite
Wadsleyite
Majorite garnet

71
Q

Which minerals are there in the upper mantle?

A

Olivine
Orthopyroxene
Clinopyroxene
Pyrope garnet

72
Q

What shows that perovskite is under very high pressure?

A

Mg has 12-fold coordination and normally has 6

Si is in 6-fold octahedral coordination

73
Q

What is a feature of majorite garnet that shows it is under high pressure?

A

Si is in both tetrahedral and octahedral sites

1A Earth (15 decks)

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