Crystals belonging to orthorhombic, monoclinic, and triclinic systems have two optic axes and therefore they are called ”biaxial”. These optic axes intersect each other to form acute and obtuse angles. The acute angle between the two optic axes is called the ”optic axial angle” and is commonly designated as 2v. The plane containing the two optic axes is called the ”optic axial plane”. The important optical characters of the biaxial crystals are as follows.
- When light passes through a biaxial crystal in a direction other than an optic axis, it splits into two rays with mutually perpendicular vibrations. The velocities of the rays differ from each other and change with the change in the crystallographic direction. Thus in biaxial crystals, there are three vibration directions: (i) X, the vibration direction of the fastest ray, (ii) Z, the vibration direction of the slowest ray, and (iii) Y, the vibration direction of the ray of intermediate velocity. These X, Y, and Z directions are at right angles to one another.
- There are three refractive indices, α (alpha), β (beta), and γ (gamma), corresponding to the fast, medium, and slow rays in the X, Y, and Z vibration directions respectively. The value of α is lowest, γ is highest and β is intermediate between the two. The difference between the greatest and least refractive indices is called ”birefringence”.
- The X and Z vibration directions lie in the optic axial plane. The Y-direction that lies at a right angle to this plane is called the ”optic normal”.
- The X and Z vibration directions bisect the acute and obtuse angles between the two optic axes. The direction which bisects the acute angle is called ”acute bisectrix” and the one which bisects the obtuse angle, is called ”obtuse bisectrix”.
- A biaxial mineral is said to be ”positive” when Z is the acute bisectrix and ”negative” when X is the acute bisectrix.