Cleavage of large crystals
Practical Activity for 14-16
This impressive demonstration shows how easily the crystal comes apart if the blade is accurately aimed between planes of atoms.
Apparatus and Materials
- Calcite crystal, large
- Single-edged razor blade or sharp knife
- Hammer, light
Health & Safety and Technical Notes
Razor blades must be handled with care and disposed of so that the sharp edge cannot harm anyone. The demonstrator should wear eye protection.
Private practice develops the skill needed to ensure success in front of students as well as helping to make the demonstration safer for the teacher!
If obtainable, single-edge razor blades are particularly good because they usually fail to break the crystal if held at the wrong angle. A fine chisel will also work.
The less brute force that is used, the more impressive the demonstration.
- Although not essential, most teachers prefer to place the crystal on a bed of Blu-tac.
- As shown in the diagram, the plane of the blade needs to be parallel to a face of the crystal.
- Give the blade a light tap with the small hammer.
- If a student asks why the right orientation of the blade is so important, show what happens if the orientation is wrong - no clean cleavage is obtained.
- If a ready-made polystyrene sphere model of a crystal is available, this could be used to show what is happening with the calcite.
- Seeing this demonstration should help students appreciate the skills of those who cleave gem diamonds!
This experiment was safety-tested in May 2004