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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
What are the 5 lattice types?
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
26
What are the forms of mirror symmetry?
2 fold = diad 3 fold = triad 4 fold = tetrad 6 fold = hexad
27
What are the 7 crystal systems?
``` Cubic Tetragonal Hexagonal Trigonal Orthorhombic Monoclinic Triclinic ```
28
What symmetry is seen for each crystal system?
``` Cubic: 4 triads Hexagonal: 1 hexad Trigonal: 1 triad Tetragonal: 1 tetrad Orthorhombic: 3 diads Monoclinic: 1 diad Triclinic: no symmetry ```
29
Define a lattice plane
A plane which passes through any three lattice points which are not in a straight line
30
A set of parallel lattice planes are characterised by what?
Miller indices (hkl)
31
If a plane is parallel to an axis, what is the corresponding miller index?
Zero
32
Define form
Sets of related planes or faces in a crystal
33
Why are curly brackets used?
To distinguish forms from individual planes
34
Define the habit of a crystal
Describes the overall shape of the crystal
35
What is twinning in a crystal?
Two or more adjacent parts in which the crystal structure is differently oriented
36
How does twinning arise?
Mistakes during crystal growth from: mechanical deformation crystal structure changing on cooling to a more stable, lower T structure
37
Define an isotropic material | Define an anisotropic material
The same refractive index in all directions | Refractive index is not the same for all directions
38
What are the four things that can be seen in a thin section?
Habit Microstructure (twinning, cleavage, zoning) Colour Relief
39
Define relief
How clearly the edge of a grain or crystal can be seen
40
What does relief depend on?
The difference in refractive indices of the grain and the embedding medium
41
Outline the Becke Line Test
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
What is pleochroism? | How does it work?
Colour change with orientation | Anisotropic minerals absorb different wavelengths of light in different vibration directions
43
Polarised light in an anisotropic medium is only permitted to vibrate along two perpendicular what?
Permitted vibration directions
44
What constrains the permitted vibration directions?
Crystal's symmetry
45
Why are the two permitted vibration directions different?
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
What is double refraction?
Light travels through an anisotropic material as two separate rays, vibrating in perpendicular directions
47
Define birefringence
The difference between the RIs of the two rays brought about by double refraction
48
What are the observations under crossed polars? | 3 points
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
How often do extinction positions occur?
Every 90 degrees
50
Why are isotropic materials black under crossed polars?
Every direction is a permitted vibration direction | So all light is absorbed by the E-W and N-S polariser/analyser
51
What causes straight extinction?
Permitted vibration direction with the larger RI is parallel to the length of the crystal (length slow)
52
What happens if light is sent through a crystal vibrating in a non-permitted direction? (3 points)
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
What is the equation to figure out the optical path difference?
Δ = t(n1 - n2)
54
What happens if the two components of light exit in phase or out of phase?
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
When does the light pass through the analyser, in or out of phase?
In phase does not | Out of phase does
56
What is the optical indicatrix?
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
What is the significance of an isotopic section?
A circular cross-section through the indicatrix | Normal to the optic axis
58
Why do cubic crystal systems give a spherical optical indicatrix?
3-fold symmetry means the RI is always the same
59
Which crystal systems give a uniaxial indicatrix? | What is the difference between the two forms of a uniaxial indicatrix?
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
What are ε and ω with respect to the optical indicatrix?
ε is the value of RI parallel to the z-axis | ω is the value of RI within the plane perpendicular to z
61
When a mineral is cut in the thin section, when is the anisotropy at a maximum and zero?
Maximum: cut parallel to the optic axis Zero: cut perpendicular to the optic axis
62
Which crystal systems have a biaxial indicatrix?
Orthorhombic, monoclinic and triclinic
63
What is a biaxial indicatrix?
Two isotropic sections and two optic axes
64
How is the biaxial indicatrix different for the crystal systems that can have it?
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
Close packed layers of identical spheres can be stacked to form which two structure types?
Hexagonal close-packed (hcp) | Cubic close-packed (ccp)
66
What is the difference between hcp and ccp structures? | What is the similarity?
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
What is the body centred cubic structure (bcc)?
A cubic unit cell with atoms at the corners and one at the centre of the unit cell Coordination number 8
68
What are the packing efficiencies for hcp, ccp and bcc?
hcp and ccp 74% | bcc 68%
69
Which minerals are there in the lower mantle?
Perovskite | Ferropericlase
70
Which minerals are there in the transition zone?
Ringwoodite Wadsleyite Majorite garnet
71
Which minerals are there in the upper mantle?
Olivine Orthopyroxene Clinopyroxene Pyrope garnet
72
What shows that perovskite is under very high pressure?
Mg has 12-fold coordination and normally has 6 | Si is in 6-fold octahedral coordination
73
What is a feature of majorite garnet that shows it is under high pressure?
Si is in both tetrahedral and octahedral sites

1A Earth (15 decks)

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