Crystals, Minerals and Rocks

Question 1

Define MINERAL

Answer 1

They are naturally occurring homogenous solids with definite chemical composition and an ordered, repeating physical structure at the atomic/molecular scale.

Question 2

Define ENDMEMBER, using Olivine as an example

Answer 2

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.

Question 3

Define CRYSTAL and GLASS

Answer 3

A repeating mineral structure is a crystal, glasses form where structures are not able to.

Question 4

Define POLYMORPH

Answer 4

Two minerals with the same composition but different structures, like diamond and graphite.

Question 5

How can bond length be found?

Answer 5

Where Coulombic attractive force F_A ∝(q+)(q−)/d2, and the Born repulsion F_R∝−n/d^(n−1) are equal.

Question 6

What is the difference between hexagonal and cubic close packing?

Answer 6

hcp - ababab
ccp - abcabc

Question 7

CN of tetrahedra?

Answer 7

4

Question 8

CN of octahedra?

Answer 8

6

Question 9

CN of cubes?

Answer 9

8

Question 10

CN of cuboctahedra?

Answer 10

12

Question 11

Define LATTICE and UNIT CELL

Answer 11

A lattice is a set of points, described with a combination of vectors
A unit cell breaks down the lattice into repeating patterns.

Question 12

What are the 14 types of 3D Bravais Lattice (split into 6 groups)?

Answer 12

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

Question 13

Define CONVOLUTION in terms of MOTIFS

Answer 13

Convolution involves replacing the lattice points with the minerals, or motifs, hence creating a crystal.

Question 14

What are MILLER INDICES?

Answer 14

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.

Question 15

Define HABIT

Answer 15

How well-formed the mineral is - euhedral vs anhedral

Question 16

Define FORM

Answer 16

The shape the crystals form e.g. fibrous, tabular

Question 17

Define PLEOCHROISM

Answer 17

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.

Question 18

Define RELIEF

Answer 18

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.

Question 19

Define BIREFRINGENCE

Answer 19

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.

Question 20

Name the types of silicate, and an example for each.

Answer 20

• [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.

Question 21

Olivine

Answer 21

Orthosilicate
Formula: (Fe,Mg)2 SiO4
Calcium can replace but only in small amounts due to its size.

Question 22

Garnet

Answer 22

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

Question 23

Pyroxene

Answer 23

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

Question 24

Amphiboles

Answer 24

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.

Question 25

Explain the layering in Phyllosilicates

Answer 25

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 …

Question 26

Quartz

Answer 26

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.

Question 27

Feldspars

Answer 27

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

Question 28

What is the core’s composition?

Answer 28

90% iron and nickel, closely packed (data is bcc but this is not the most dense)

Question 29

What is the lower mantle’s composition?

Answer 29

Perovskite (mostly bridgemanite) with fcc structure, where Si is in an octahedra because of pressure.

Question 30

What is the transition zone’s (410-660km) composition?

Answer 30

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.

Question 31

What is the difference between clino and orthopyroxenes?

Answer 31

Clinopyroxenes have 5-50% Ca, Orthopyroxenes have <5%.

Question 32

How does feldspar weather?

Answer 32

Chemical weathering to clay minerals from rainfall