# Uniaxial minerals

The crystals of the tetragonal and hexagonal systems possess only one optic axis, which is parallel to the c-axis. For this reason, they are called ”uniaxial”. The optical characters of the uniaxial minerals may be summarized as follows:

1. When a ray of light passes through a uniaxial crystal in any direction other than the optic axis, two polarized rays are produced due to double refraction. These are O-ray and E-ray, which vibrated in mutually perpendicular planes and travel with different velocities. The O-ray vibrates in the basal plane and E-ray vibrates in the plane that includes the c-axis.
2. The O-ray has a single and constant refractive index ω (omega) whereas the re-

fractive index of the E-ray varies with the direction of the crystal section.The refractive index of E-ray becomes maximum in the crystal section. The refractive index of E-ray becomes maximum in crystal sections parallel to the c-axis and this maximum value is denoted by the symbol γ (gamma). In sections making oblique angles with the c-axis the refractive index of E-rays varies between limits ω (omega) and γ (gamma)The difference between the refractive indices of the O-ray and E-ray is called ”birefringence”.

3. In a section normal to the optic axis, there is no double refraction and the light has only one value of refractive index which is ω. In this direction, the crystal behaves as an isotropic substance.

### Positive and Negative Crystals:

As the O-ray travels with the same velocity in all directions, its surface can be represented by a sphere as shown in the above figure. The surface of E-ray is an ellipsoid of revolution because its velocity varies with the direction. Uniaxial crystals are said to be ”positive” if the velocity of O-ray is greater than E-ray, and ”negative” if the velocity of E-ray is greater.