What's the Earth made of Flashcards
Main elements in Earth
Fe, O, Si, Mg, Ni, Ca, Al
Main elements in core
Fe, Ni, Si, S
Main elements in crust
O, Si, Al, Fe, Ca, Na, K, Mg
Olivine (with endmembers)
(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
Calcite
CaCO[3]
Trigonal
Rhombohedral cleavage
DOuble refraction
Feldspar
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
Mica
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’.
Amphibole
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
Quartz
SiO[2]
Trigonal Low relief Colourless No cleavage 1st order grey/white birefringence
Why a crystal has low or high relief
Mounting glue has a refractive index of 1.54
Low relief minerals have refractive indices close to 1.54
Becke line test
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
Pleochroism
As stage is rotated under PPL. Anisotropic minerals change colour since different wavelengths are absorbed
Calcite diagnostic feature
Shows double refraction as highly anisotropic
Straight extinction
Extinction parallel to a prominent feature e.g. cleavage
Inclined extinction
Extinction at an angle with respect to a prominent feature
Sensitive tint
Separates orders in Michel Levy chart
Optical indicatrix
Ellipsoid that has dimensions equivalent to the magnitude of R.I. in any direction
Optical axis
View normal to an isotropic section of the indicatrix
Omega
R.I. of isotropic section of a uniaxial indicatrix
Uniaxial indicatrix
Tetragonal, hexagonal, trigonal
Positive uniaxial: epsilon > omega
E.g. quartz
Negative uniaxial: omega > epsilon
E.g. calcite
Epsilon
R.I. along optical axis in a uniaxial indicatrix
Biaxial indicatrix
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
Orthopyroxene vs. Clinopyroxene
Orthopyroxene has straight extinction
Clinopyroxene has inclined extinction
How iron is packed in the core
Large uncertanties in measurements at high temperatures and pressures mean there is no consensus yet where it’s hcp or bcc
Pressure at the inner core boundary
330GPa
Temperature at inner core boundary
5400-5700K c. temp of the Sun
Upper mantle composition
Olivine, Clinopyroxene, Orthopyroxene, Pyrope Garnet
Lower / Upper mantle transition composition
Majorite garnet, wadsleyite, ringwoodite
Lower mantle
Perovskite, ferropericlase
Perovskite
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
Ferropericlase
2 interpenetrating CCP lattices.
Pyrope –> majorite garnet transition
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
Olivine –> wadsleyite –> ringwoodite –> peroskite and magnesium oxide
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.
Aluminium storage in very upper mantle vs. deeper
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.
Crystal systems examples
Cubic: Garnet, halite, magnetite Tetragonal: zircon Trigonal: quartz, calcite Orthorhombic: pyroxene Monoclinic: orthoclase, pyroxene, hornblende, chlorite
Ca/Na content in plagioclase
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.
Pyroxene
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.
Crystalline
Structure has translational symmetry.
Lattice
A series of points that radiates out infinitely in all directions where the view from all lattice points is identical.
Primitive lattice
1 lattice point per unit cell
Non-primitive lattice
More than 1 lattice point per unit cell
