Crystals, Minerals and Rocks Flashcards
Define MINERAL
They are naturally occurring homogenous solids with definite chemical composition and an ordered, repeating physical structure at the atomic/molecular scale.
Define ENDMEMBER, using Olivine as an example
End members are the 100% of one composition or the other, e.g. for Olivine, Fe2SiO4 is fayalite and Mg2SiO4 is forsterite. A solid solution is anything in between.
Define CRYSTAL and GLASS
A repeating mineral structure is a crystal, glasses form where structures are not able to.
Define POLYMORPH
Two minerals with the same composition but different structures, like diamond and graphite.
How can bond length be found?
Where Coulombic attractive force F_A ∝(q+)(q−)/d2, and the Born repulsion F_R∝−n/d^(n−1) are equal.
What is the difference between hexagonal and cubic close packing?
hcp - ababab
ccp - abcabc
CN of tetrahedra?
4
CN of octahedra?
6
CN of cubes?
8
CN of cuboctahedra?
12
Define LATTICE and UNIT CELL
A lattice is a set of points, described with a combination of vectors
A unit cell breaks down the lattice into repeating patterns.
What are the 14 types of 3D Bravais Lattice (split into 6 groups)?
Triclinic - no sides or angles equal
Monoclinic (primitive and body centred) - 2 90o angles, no sides equal
Orthorhombic (primitive, base centred, body centred and face centred) - all 3 angles 90o, no sides equal
Tetragonal (primitive and body centred) - two sides equal, all angles 90o
Hexagonal (primitive and base centred) - 2 angles 90o, 1 angle 120o, two sides equal
Isometric (primitive, body centred and face centred) - all sides equal, all angles 90o
Define CONVOLUTION in terms of MOTIFS
Convolution involves replacing the lattice points with the minerals, or motifs, hence creating a crystal.
What are MILLER INDICES?
Numbers that describe lattice planes in 3D. The three numbers are the ratios of the reciprocals of the distance in each direction from the origin before you cross the plane.
Define HABIT
How well-formed the mineral is - euhedral vs anhedral
Define FORM
The shape the crystals form e.g. fibrous, tabular
Define PLEOCHROISM
Pleochroism if the effect where the colour of a mineral changes depending on its orientation/rotation in PPL
This occurs since the polarised light interacts with the bonds in different ways.
Define RELIEF
How strongly a mineral stands out. If the RI of the glue is very different to the mineral then the light gets refracted, making the boundaries thicker.
Define BIREFRINGENCE
Birefringence is optical property where differences in refractive index along different permitted vibration directions in a crystal cause the splitting and variable recombination of light. This gives rise to colour in crossed polars.
When plane polarised light enters a crystal, it is split into two permitted vibration directions. These interact with different bonds, so travel at different speeds.
These two waves recombine as they leave the crystal, which can produce a phase shift. This phase shift has the effecting of rotating the plane of polarisation, such that some light can now pass through the analyser (which is perpendicular to the polariser).
Cubic minerals stay black since the bonds are the same in both directions and so the two waves have no phase shift.
Name the types of silicate, and an example for each.
- [SiO4]4– Orthosilicates - Olivine
- [Si2O6]4- - Chain Silicates/Inosilicates - each silica has 2 bridging oxygens, forming a chain. Pyroxene is an example.
- [Si4O11]6- - Double Chain Silicates/Inosilicates - each silica has 3 bridging oxygens. Amphibole is an example.
- [Si2O5]2- - Sheet Silicates/Phyllosilicates - each silica also has 3 bridging oxygens but form sheets rather than chains. An example is Mica
- [SiO2] - 3D Framework Silicates - each silica has 4 bridging oxygens, forming a 3D structure. Quartz is this.
Olivine
Orthosilicate
Formula: (Fe,Mg)2 SiO4
Calcium can replace but only in small amounts due to its size.
Garnet
Orthosilicate
Formula: A3 B2 (SiO4)3 where A is 8CN and B is 6CN.
Cubic, so isotropic
Example is Pyralspire series, showing extent of metamorphism with which form appears
Pyroxene
Inosilicate - double chain
Formula: X Y Z2 O6 where X is large, Y is small and Z is Al or Si.
Structure is flexible so many elements can be incorporated, e.g.
○ Diopside: CaMgSi2O6
○ Hedenbergite: CaFeSi2O6
Amphiboles
Inosilicate - double chain
Formula: (W) X2 Y5 Z8 O22 (OH,F)2 where W is large if present, X is 1+ or 2+, Y is 2+ or 3+ and Z is Si or Al.
Explain the layering in Phyllosilicates
There are two types of layers:
- Tetrahedral silicate layers, often with OH- or other anions incorporated
- Octahedral layers to balance out the negative charge.
There are three principal groups of layering:
- 1:1 layer - T-O T-O T-O … e.g. Kaolinite
- 2:1 layer - T-O-T T-O-T T-O-T … e.g. Muscovite
- 2:1:1 layer- T-O-T O T-O-T O T-O-T O …
Quartz
Framework Silicate
Formula: SiO2
○ Structure can be changed by temperature and pressure, which higher symmetry with temperature.
○ High quartz is hexagonal as opposed to trigonal in low quartz in a displacive phase transition as no bonds broken. Other transitions where bonds break are reconstructive.
Feldspars
Framework Silicate
Formula: X Y4 O8 where X is a large cation and Y is Si or Al.
○ K-Na feldspars, known as alkali feldspars
§ KAlSi3O8 - NaAlSi3O8
§ Orthoclase - albite
○ Ca-Na feldspars, known as plagioclase feldspars
§ NaAlSi3O8 — CaAl2Si2O8
§ Albite - anorthite
What is the core’s composition?
90% iron and nickel, closely packed (data is bcc but this is not the most dense)
What is the lower mantle’s composition?
Perovskite (mostly bridgemanite) with fcc structure, where Si is in an octahedra because of pressure.
What is the transition zone’s (410-660km) composition?
Olivine from the upper mantle converts to wadsleyite then ringwoodite, then bridgemanite in the lower mantle. These are all polymorphs.
Garnet increasingly becomes majorite as 2 aluminium ions substitute for magnesium and silicon.
What is the difference between clino and orthopyroxenes?
Clinopyroxenes have 5-50% Ca, Orthopyroxenes have <5%.
How does feldspar weather?
Chemical weathering to clay minerals from rainfall
