When double refraction is observed using a calcite crystal and when its rotated and tilted at one point the two images (the Ordinary image and the Extraordinary image) coincide and appears as one image. This direction in which no double refraction can be observed is the Optic Axis direction. While some minerals have only a single optical axis while others have two optic axes. Minerals are thus classified as follows
1. Uniaxial Minerals - one optic axis
2. Biaxial Minerals - two optic axes
Minerals that belong to the tetragonal and hexagonal crystal systems are uniaxial while minerals that belong to monoclinic, triclinic and orthorhombic crystal systems are biaxial.
eg. quartz, calcite - hexagonal - uniaxial pyroxene - orthorhombic and monoclinic - biaxial
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