CRYSTALS

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Crystals
Axis of symmetry
D
Make a triangle, square, and hexagon with a small hole in the centre. These are placed on a
drawing pin on a overhead projector or large versions on the white board and turned to show 3,
4, and 6 fold symmetry
Mirror planes and axes
A P F 20 min
Students are given sheets with the letters of the alphabet in capitals. For each letter they
draw on lines to show the mirror planes and give the axis of symmetry, e.g. twofold, fourfold
etc.
Symmetry rules
D 3 min
This activity is to show how the symmetry rules mean that existence of one crystal face
determines the presence of a set of other faces. Students look down a kaleidoscope at a
single bold line or a pencil (= one face) on white paper.
Symmetry of the cube
A P and then D 10 min
Students are given wooden cubes and told to work out the position and number of all the axes
of symmetry and all the mirror planes. A large cardboard cube is used by the teacher to
demonstrate the symmetry of the cube.
The relationship between unit cells and crystal faces
D
Buy some planed wood with a square section of about 2cm. Cut it into many cubes using a
circular saw with a fine blade or band saw. The cubes can be stacked and may be glued to give a
cube, a rectangle, an octahedron, and a pyritohedron. It is also possible to cut and use
tetragonal and orthorhombic shapes to show the relation of the unit cells to the crystal faces.
Law of constancy of angles
A P 5 min
Students are given several "cuboids" of pyrite and a goniometer. For each they measure the
length of the edges and the angles. The angle is always 90o but the lengths vary. Repeat with
small quartz crystals or diagrams of cross sections of them with varying width of faces.
Identifying crystal class 1
A P 1 min per sample
Student work out the crystal class of a variety of objects, match box, pencil, Toblerone bar
etc. or hand made simple wooded shapes.
Identifying crystal class 2
A P 1 min per sample
Students examine wooden models of crystal forms and work out their class.
Identifying crystal class 3
A P 1 min per sample
Students examine real crystals and work out their class.
Unit cells 1
D
Use a molecular model of halite to show a unit cell.
Unit cells 2
A I F 5m
Wall paper or wrapping paper with coloured dots on is used to represent the atoms in a mineral.
Students have to work out the repeat pattern which is equivalent to the unit cell of a mineral.
Unit cells and crystal faces 1
D
Picture of stacked cans in a supermarket
Unit cells and crystal faces 2
Pa I 15 m
Students work out the angle of a face using graph paper. On the paper each square represents
one unit cell. Students draw faces in which the unit cells go up one and along one, up one and
along two, and up one and along three and then measure or calculate the angle. Also try using
rectangular unit cells.

Earth Science Activities and Demonstrations