Earth Materials Lecture 4: Introduction to Crystallography

Question 1

What are the 7 crystal systems?

Answer 1

-Cubic (isometric).
– Tetragonal.
– Orthorhombic.
– Trigonal.
– Hexagonal (rhombohedral = trigonal).
– Monoclinic.
– Triclinic.

Question 2

How many crystal lattice shapes are there and who theorised them?

Answer 2

August Bravais (1811 -1863)

14 Bravais lattices/unit cells

Question 3

What are the names of the 14 bravais lattices?

Answer 3

Simple cubic
Face centred cubic
Body-centred cubic
Simple tetragonal
Body-centered tetragonal
Hexagonal
Simple orthorhombic
Body-centered orthorhombic
Base-centered orthorhombic
Face-centered orthorhombic
Rhombohedral
Simple Monoclinic
Base centered monoclinic
Triclinic

Question 4

Describe the three axis of a crystal and how to tell them apart

Answer 4

The C axis looks like the height of the crystal

The B axis looks like the width of the crystal

The A axis looks like the depth of the crystal

Question 5

Define Plane symmetry

Answer 5

A plane across which one half of the crystal is the
mirror image of the other.

Question 6

Define Rotational symmetry

Answer 6

An axis about which the crystal is rotated to
produce the same view.

Question 7

Define the centre symmetry

Answer 7

A crystal has a centre of symmetry if each face
has a parallel opposite face.