Isotropic minerals--those which exert no control over vibration direction of light passing through them. Minerals of the isometric (cubic) system, or non-crystalline materials fit into this category. Isometric crystals are highly symmetric and display the same chemical bonding and crystal structure in all directions. Garnets, fluorite, spinels, and lucite are examples. These minerals appear black in the field of a microscope when both polarizer and analyzer are in the light path. The will appear transparent or colored when the analyzer is removed from the light path.

Anisotropic minerals--those which exert some control over vibration direction o flight as it passes through the mineral.

Uniaxial--minerals in the hexagonal, tetragonal, and rhombohedral (trigonal) crystal systems containing one optic axis (c crystallographic axis). They have a high degree of symmetry about the c crystallographic axis. There is uniform chemical bonding in all directions within the (001) or (0001) plane, which is at right angles to the c axis. Light passing through the mineral (other than along the optic axis) is broken into two mutually perpendicular rays vibrating parallel and normal to the c axis. These rays travel with different velocities yielding two indices of refraction. The fact that light vibrates intwo directions is called double refraction. The degree to which these two vibration directions differ is expressed as birefringence. Quartz, carbonates, apatite, and zircon are examples.

Biaxial--minerals in the orthorhombic, monoclinic, and triclinic crystal systems. They are less symmetric than the uniaxial minerals, and vary in crystal structure and chemical bonding in all directions. Light passing through these minerals vibrates in two directions, both of which are different from that of the incident light. In addition, there are two axes (optic axes) along which light may be propagated with the vibration direction unchanged from that of the incident light. The acute angle between these axes is the 2V of the mineral. Uniaxial minerals have a 2V = O° and the optic axes coincide, i.e., there is in effect only one optic axes. Gypsum, feldspars, pyroxenes (augite-enstitite), amphiboles (hornblende), olivine and clay minerals are examples of biaxial minerals.

Opaque minerals--those through which light does not pass. Many Fe, Mn and Ti minerals fit into such a category. These appear dark when observed under the microscope, whether the analyzer is in or out. Reflected light from Ti minerals often produce a shiny, silvery mineral surface to the eye. Pyrite is brassy with a metallic luster. Impinge light on opaque minerals from above the stage to differentiate between iron and titanium minerals.

This page (http://www.ces.ncsu.edu/plymouth/programs/optical.html) created by
Vera MacConnell, Research Technician, I on November 13, 1997.
Last Updated on November 13, 1997.