Part B

Question 1

What is a rock?
What is a mineral and name their classes?
What is a crystal

Answer 1

A collection & mixtures of minerals or mineral-like substances.

A mineral is always naturally occurring, with characteristic composition and 3-D shape. Classes include, Silicates, Carbonates, Sulphides/ates, Native Elements, Ox/Hydroxides & Halides.
Silicates are by far the most abundant.

A crystal is a solid bounded by planar surface with definitive geometrical relationship to the atoms within, huge structures with repeated unit cells and often sites.

Question 2

Hexagonal & Cubic close packing.

Answer 2

Hexagonal: Layers A & B [1 & 2] are alternatively offset and a third layer A sits directly over layer 1.
Top view: Parallelogram/Rhombus
(ABABABABABAB)

Cubic: Three main layers A,B & C, with C sitting directly over layer 1 (A). 4th layer repeats and is over layer 1.
Oblique View: Cube
-Usually have a face-centred unit cell
close packed face (diagonals) is orientated [111]

74% Packing Efficiency

Question 3

Cation sites in close-packed anions.

Answer 3

Tetrahedral hole: Covered by second layer, 4 neighbours.

Octahedral hole: Not covered by second layer, 6 neighbours.

Both occurs in Hex/Cub packing.
Large cations can fit in Octahedral holes and only small cations can fit in Tetrahedral holes. r/R [Cations to Anion] for ratio can tell a lot about packing. r/R>0.4 is octahedral.

Question 4

Body Centred Cubes

Answer 4

r/R becomes too large (>0.73), the NaCl structure is abandoned for simple cubic packing sharing all six faces.

This is not close packing! It has an efficiency of 68%

Question 5

The silica tetrahedra

Answer 5

[SiO4]-4, r/R = 0.293, coordination = 4

The skeleton of all larger silicate structures. Polymerisation causes this.

Question 6

Nesosilicates, Island/Orthosilicates. General outline?

Answer 6

Share no corners with with other silica tetrahedra (Isolated), only cations. Means that Oxygens only bound with Silicon or Site atoms.

Question 7

Olivine?

Answer 7

Orthorhombic, Mafic mineral, Endmembers Mg2… and Fe2…. Occurs in 60% of the Earth’s upper mantle.
Pseudo HCP of oxygens.

Phase diagram to show state of Olivine at different endmember composition & temperatures. We have, solidus, melt, liquidus.
Olivine also transitions at varying densities & pressures.

Crystallisation: 100% magma before Olivine hits the liquidus line, from there on in, crystals of olivine form. Tie line, crystals become more likely iron rich as cooling occurs.

Olivine is anhydrous but unstable in water allowing for a breakdown (serpentinization) reaction.