Electrons in crystals, especially valence electrons, will respond to the changing electric field associated with light waves. The degree of polarization depends on the degree that the outer electrons are bound to one another an dhow free they are to vibrate due to external forces. The pattern of polarization depends on the nature of the electron's closest neighbor where a dipole can be developed. The pattern of polarization is thus strongly tied to the crystal structure of minerals. The plane of maximum polarization is usually the plane of maximum atom density, and the plane of minimum polarization is perpendicular to it.
Polarization strongly effects the velocity of light waves.
The stronger the polarization, the slower the velocity of light and the greater the refractive index. Depending on th edegree of polarization in the crystal, each light wave will be transmitted with a different velocity, and will have a different refractive index. The ions are held tightly together by strong, largely covalent bonds that are much stronger than any external bond and are highly polarized in the plane of the atom. Light waves transmitted parallel to the c-crystallographic axis vibrate parallel to the planes of the carbonate ions, increasing polarization already present. Calcite therefore shows a relatively high index of refraction and a corresponding low velocity for waves advancing parallel to c, regardless of their vibrational direction normal to the axis.