Pleochroism

Pleochroism is the variations in the colour of certain minerals in non-analysed polarised light ("natural light") observed when the orientation of a thin section is changed relative to the plane of polarisation. Minerals with this property are called pleochroic (e.g. biotite - normal pleochroism, the dark sections lying in the east-west position, and tourmaline - inverse pleochroism, the dark sections lying in the north-south position). Relief pleochroism is an identical but non-coloured phenomenon, which, in high relief minerals (carbonates), results in a variation in the thickness of the contours of the crystal.