What's the Earth made of

Question 1

Main elements in Earth

Answer 1

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

Question 2

Main elements in core

Answer 2

Fe, Ni, Si, S

Question 3

Main elements in crust

Answer 3

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

Question 4

Olivine (with endmembers)

Answer 4

(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

Question 5

Calcite

Answer 5

CaCO[3]

Trigonal
Rhombohedral cleavage
DOuble refraction

Question 6

Feldspar

Answer 6

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

Question 7

Mica

Answer 7

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’.

Question 8

Amphibole

Answer 8

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

Question 9

Quartz

Answer 9

SiO[2]

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

Question 10

Why a crystal has low or high relief

Answer 10

Mounting glue has a refractive index of 1.54

Low relief minerals have refractive indices close to 1.54

Question 11

Becke line test

Answer 11

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

Question 12

Pleochroism

Answer 12

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

Question 13

Calcite diagnostic feature

Answer 13

Shows double refraction as highly anisotropic

Question 14

Straight extinction

Answer 14

Extinction parallel to a prominent feature e.g. cleavage

Question 15

Inclined extinction

Answer 15

Extinction at an angle with respect to a prominent feature

Question 16

Sensitive tint

Answer 16

Separates orders in Michel Levy chart

Question 17

Optical indicatrix

Answer 17

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

Question 18

Optical axis

Answer 18

View normal to an isotropic section of the indicatrix

Question 19

Omega

Answer 19

R.I. of isotropic section of a uniaxial indicatrix

Question 20

Uniaxial indicatrix

Answer 20

Tetragonal, hexagonal, trigonal

Positive uniaxial: epsilon > omega
E.g. quartz

Negative uniaxial: omega > epsilon
E.g. calcite

Question 21

Epsilon

Answer 21

R.I. along optical axis in a uniaxial indicatrix

Question 22

Biaxial indicatrix

Answer 22

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

Question 23

Orthopyroxene vs. Clinopyroxene

Answer 23

Orthopyroxene has straight extinction

Clinopyroxene has inclined extinction

Question 24

How iron is packed in the core

Answer 24

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

Question 25

Pressure at the inner core boundary

Answer 25

330GPa

Question 26

Temperature at inner core boundary

Answer 26

5400-5700K c. temp of the Sun

Question 27

Upper mantle composition

Answer 27

Olivine, Clinopyroxene, Orthopyroxene, Pyrope Garnet

Question 28

Lower / Upper mantle transition composition

Answer 28

Majorite garnet, wadsleyite, ringwoodite

Question 29

Lower mantle

Answer 29

Perovskite, ferropericlase

Question 30

Perovskite

Answer 30

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

Question 31

Ferropericlase

Answer 31

2 interpenetrating CCP lattices.

Question 32

Pyrope --> majorite garnet transition

Answer 32

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

Question 33

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

Answer 33

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.

Question 34

Aluminium storage in very upper mantle vs. deeper

Answer 34

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.

Question 35

Crystal systems examples

Answer 35

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

Question 36

Ca/Na content in plagioclase

Answer 36

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.

Question 37

Pyroxene

Answer 37

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.

Question 38

Crystalline

Answer 38

Structure has translational symmetry.

Question 39

Lattice

Answer 39

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

Question 40

Primitive lattice

Answer 40

1 lattice point per unit cell

Question 41

Non-primitive lattice

Answer 41

More than 1 lattice point per unit cell