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Earth 1A > What's the Earth made of > Flashcards

What's the Earth made of Flashcards

1
Q

Main elements in Earth

A

Fe, O, Si, Mg, Ni, Ca, Al

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2
Q

Main elements in core

A

Fe, Ni, Si, S

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3
Q

Main elements in crust

A

O, Si, Al, Fe, Ca, Na, K, Mg

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4
Q

Olivine (with endmembers)

A

(Mg, Fe)[2]SiO[4]
Mg endmember forsterite
Fe endmember fayalite

Orthorhombic
Medium/High Relief
Birefringence up to 3rd order
No cleavage
Curved cracks
Colourless
Alters to serpentine
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5
Q

Calcite

A

CaCO[3]

Trigonal
Rhombohedral cleavage
DOuble refraction

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6
Q

Feldspar

A

Alkali: (Na, K)AlSi[3]O[8]

K-feldspar: Orthoclase (monoclinic) or Microcline (triclinic)
Orthoclase: simple twinning

Plagioclase: NaAlSi[3]O[8], CaAl[2]Si[2]O[8]

Triclinic
Multiple laminar twinning
Low relief
First order grey/white birefringence
Colourless
Extinction angle depends on the Ca content
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7
Q

Mica

A

Biotite KMg[3]AlSi[3]O10[2]

Monoclinic
Birefringence up to 3rd order colours
Straight extinction
Pleochroic browns/greens
1 perfect cleavage

Muscovite KAl[2]AlSi[3]O10[2]

Monoclinic
Colourless
Straight extinction
Birefringence up to 3rd order
1 perfect cleavage

Pairs of sheets are held together with Al or Mg ions to form ‘sandwiches’. Sandwiches are help together far more weakly with large, K ions. Therefore, there is a single cleavage parallel to the ‘sandwiches’.

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8
Q

Amphibole

A

Mg[7]Si[8]O22[2]

Polarisation up to 2nd order
2 cleavages at 56 degrees

Pairs of double chains of silica tetrahedra held together with cations. Form wide I-beams

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9
Q

Quartz

A

SiO[2]

Trigonal
Low relief
Colourless
No cleavage
1st order grey/white birefringence
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10
Q

Why a crystal has low or high relief

A

Mounting glue has a refractive index of 1.54

Low relief minerals have refractive indices close to 1.54

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11
Q

Becke line test

A

Slightly close aperture
Becke line is bright fringe around a crystal
When stage is moved up away from the lens, it moves in the direction of greater refractive index

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12
Q

Pleochroism

A

As stage is rotated under PPL. Anisotropic minerals change colour since different wavelengths are absorbed

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13
Q

Calcite diagnostic feature

A

Shows double refraction as highly anisotropic

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14
Q

Straight extinction

A

Extinction parallel to a prominent feature e.g. cleavage

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15
Q

Inclined extinction

A

Extinction at an angle with respect to a prominent feature

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16
Q

Sensitive tint

A

Separates orders in Michel Levy chart

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17
Q

Optical indicatrix

A

Ellipsoid that has dimensions equivalent to the magnitude of R.I. in any direction

18
Q

Optical axis

A

View normal to an isotropic section of the indicatrix

19
Q

Omega

A

R.I. of isotropic section of a uniaxial indicatrix

20
Q

Uniaxial indicatrix

A

Tetragonal, hexagonal, trigonal

Positive uniaxial: epsilon > omega
E.g. quartz

Negative uniaxial: omega > epsilon
E.g. calcite

21
Q

Epsilon

A

R.I. along optical axis in a uniaxial indicatrix

22
Q

Biaxial indicatrix

A

Can be cut in 2 slices such that it will be isotropic
Orthorhombic, monoclinic, triclinic

In orthorhombic, the principal axes of the indicatrix must lie along the orthogonal crystallographic axes.

In monoclinic, 1 of the principal axes must be along the y -axis

In triclinic, indicatrix can be in any orientation

23
Q

Orthopyroxene vs. Clinopyroxene

A

Orthopyroxene has straight extinction

Clinopyroxene has inclined extinction

24
Q

How iron is packed in the core

A

Large uncertanties in measurements at high temperatures and pressures mean there is no consensus yet where it’s hcp or bcc

25
Q

Pressure at the inner core boundary

A

330GPa

26
Q

Temperature at inner core boundary

A

5400-5700K c. temp of the Sun

27
Q

Upper mantle composition

A

Olivine, Clinopyroxene, Orthopyroxene, Pyrope Garnet

28
Q

Lower / Upper mantle transition composition

A

Majorite garnet, wadsleyite, ringwoodite

29
Q

Lower mantle

A

Perovskite, ferropericlase

30
Q

Perovskite

A

c. 86% of lower mantle, so is most prevalent silicate.

CCP
Si in 6 fold coordination due to increased need for packing efficiency at high pressure

31
Q

Ferropericlase

A

2 interpenetrating CCP lattices.

32
Q

Pyrope –> majorite garnet transition

A

Pyrope garnet has isolated silicate tetrahedra.
To transition to majorite, pyroxene ‘dissolves’ into garnet.
Silicon in majorite is in tetrahedral and octahedral interstices. Therefore, some silicon is in its preferred coordination

33
Q

Olivine –> wadsleyite –> ringwoodite –> peroskite and magnesium oxide

A

Olivine –> wadsleyite at 410km (jump in seismic velocity as density increases)

wadsleyite (HCP) –> ringwoodite (CCP) at 525km

ringwoodite –> peroskite and magnesium oxide at 660km (jump in seismic velocity as density increases)

ALL of Olivine, wadsleyite and ringwoodite have Si in tetrahedral coordination

Wadsleyite contains c. 3% water as free oxygens can be hydrated.

34
Q

Aluminium storage in very upper mantle vs. deeper

A

At lower/upper mantle transition, majorite contains the majority of the aluminium.
At very shallow depths, (less than 50km), Plagioclase and spinel store most of the aluminium.

35
Q

Crystal systems examples

A 
Cubic: Garnet, halite, magnetite
Tetragonal: zircon
Trigonal: quartz, calcite
Orthorhombic: pyroxene
Monoclinic: orthoclase, pyroxene, hornblende, chlorite
36
Q

Ca/Na content in plagioclase

A

Straight extinction relative to twins at 20% Ca. Generally, high extinction angle for greater percentage calcium: up to 70 degrees for Ca-end member.
Max extinction angle for Na-end member is 20 degreed.

37
Q

Pyroxene

A

Clinopyroxene: monoclinic, contains Ca
Orthopyroxene: orthorhombic

Pairs of silica tetrahedra chains with Mg octahedra in between form I-beams.
Bonds between I-beams are far easier to break, therefore basal sections have characteristic 90 degree cleavage.

38
Q

Crystalline

A

Structure has translational symmetry.

39
Q

Lattice

A

A series of points that radiates out infinitely in all directions where the view from all lattice points is identical.

40
Q

Primitive lattice

A

1 lattice point per unit cell

41
Q

Non-primitive lattice

A

More than 1 lattice point per unit cell

Earth 1A (17 decks)

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