What is an optical axis in mineralogy ?

Optical axis : definition

An optical axis is a direction in which a crystal behaves isotropically. In practice, this is a direction in which there is an absence of pleochroism, birefringence and where the crystal is completely black in analyzed polarized light. The optical axes are only present in anisotropic minerals (non-cubic and non-amorphous). Uniaxial crystals (trigonal, tetragonal/quadratic or hexagonal systems) have a single optical axis while biaxial crystals (triclinic, monoclinic or orthorhombic systems) have two, the angle formed by these two axes is called 2V.