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An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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2.5 Learning outcomes for Section 2

Now you have completed this section, you should be able to:

  • suggest different ways in which light passing through a crystalline material is modified
  • define the terms refractive index and birefringence
  • explain the meaning of the terms isotropic and anisotropic, permitted vibration direction, double refraction and pleochroism
  • describe how plane-polarised light passes through an anisotropic crystal on the stage of a polarising microscope
  • explain the origin of interference colours in anisotropic materials
  • describe how to identify an unknown mineral (in thin section) using a polarising microscope.

Now try the following questions to test your understanding of Section 2.


A slice of an unknown mineral has been examined with a polarising microscope. In plane-polarised light the mineral appeared colourless, but when viewed between crossed polars it appeared dark, and remained dark as the mineral was rotated. To what crystal system is this mineral likely to belong?


In plane-polarised light, a thin section of a mineral shows very high relief, but as the stage is rotated through 90°, this changes to very low relief. Why does the relief change with orientation? What prediction can you make about the interference colour for this mineral?


In Section 1, you were introduced to the crystal structures of two polymorphs of carbon: diamond and graphite. Based on your knowledge of these structures, what prediction(s) can you make about the optical properties of diamond and graphite?