There is a close relationship between optical properties of crystals and crystal structure. A crystal is a solid composed of atoms arranged in an orderly repetitive array. Each atom in a crystal is where it is due to the balance of attractive and repulsive forces that each atom in the array exert on each other. The outward morphology and symmetry (crystal structure) are a reflection of the inner ordered arrangement of atoms, the nature of their bonding, and their relaitonship to each other. Opticla properties are therefore closely related to a mineral's crystallographic symmetry.
The optical properties of a mineral are determined by the kinds and proportions of atoms present (chemical composition) and the arrangement and density of these atoms with respect to each other (crystal structure and bonding). It is the interaction of the electric vectors of light waves with the electrical character of the atoms and the chemical bonds in minerals that affects the behavior of light in minerals. In all optically anisotropic minerals, light finds a different electronic environment depending on direction of propagation, and the elctron cloud about each atom will vibrate at different resonant frequencies in different crystallographic directions. Thus, light traveling in any direction except along the optic axis may be divided into two mutually perpendicular rays traveling at different rates and along different paths. If the two rays travel at different velocities within the crystal, one gradually gets ahead of the other. The amount one ray is ahead or behind the other will depend on their difference in velocities and the distance they have traveled with these velocities.
Amorphous and isometric minerals are isotropic; tetragonal, hexagonal and rhombohedral (trigonal) minerals are uniaxial; orthorhombic, monoclinic and triclinic minerals are biaxial. As the symmetry of minerals decrease, the minerals optically become more complex, and are characterized by an increasing number of measurable optical properties (see following table).
| CRYSTAL and OPTICAL SYSTEMS | ||
|---|---|---|
| Crystal System | Axis Lengths and Angles | Optical System |
| Cubic | Three equal axes at right angles a=b=c; | Isometric |
| Tetragonal | Three axes at right angles, two equal a=b | Uniaxial |
| Orthorhombic | Three unequal axes at right angles | Biaxial |
| Rhombohedral (Triginal) |
Three unequal axes, equally inclines | Uniaxial |
| Hexagonal | Two equal coplanar axes at 120° third axis at
right angles | Uniaxial |
| Monoclinic | Three unequal axes, one pair not at right
angles | Biaxial |
| Triclinic | Three unequal axes, unequally inclined and none
at right angles | Biaxial |
This page
(http://www.ces.ncsu.edu/plymouth/programs/light.html)
created by
Vera MacConnell,
Research Technician, I
on October 26, 1997.
Last Updated on November 11, 1997.