In general, when light passes from one medium such as air to another such as a transparent mineral, it is bent or refracted. The deflection, caused by a change in the velocity of light, relates to the density of the constituent atoms in the substance encountered. Velocity goes down with increasing density.
Minerals crystallizing in the cubic system contain atoms packed similarly in all directions, and the refraction is therefore independent of direction. In minerals with lower symmetry, the arrangement of atoms varies with direction, and so does refraction. Two values pertain for minerals crystalizing in the tetragonal and hexagonal systems and three values for minerals crystalizing in the orthorhombic, monoclinic, and triclinic systems.
The refractive index measures velocity of light in a given substance relative to the velocity of light in air. Values of refractive index are well known for most minerals. By measuring the refraction under standard conditions for an unknown mineral, its refractive index (indices) can be determined and the mineral identified through comparison with known values.
73. Halite (Refraction)