Interference color--a property of anisotrophic minerals resulting from differences between refractive indices. It is the result of the mineral dividing a light wave into two mutually perpendicular components. The two waves travel through the mineral with unequal velocities and unequal wavelengths. The amount of light transmitted through the upper analyzer is function of the phase differnce between the two transmitted waves. This phase difference is determined by crystal thickness, mineral orientation, birefringence, and the nature of the mineral. Interference color is often a key diagnostic criteria for mineral identification. In thin section , the thickness is uniform, thus the interference color is dependent on the birefringence and orientation of the mineral.

Twinning--In many minerals, crystal faces are not parallel. The result if that portions of the mineral are in different orientation. Twins consist of segments of the mineral that usually go extinct at different positions with stage rotation. Twinning is a diagnostic feature used to identify many minerals. Feldspars are the most commonly observed minerals in soils that are twinned.

Carlsbad twinning--large twins, common in orthoclase.&nsp;

Polysynthetic (albite) twinning--Fine parallel striations. Common in albite, plagioclase feldspars, sometimes thin lamellar twins seen in large calcite and dolomite crystals, but rarely in pedogenic calcite.

Pericline (grid) twinning--A distinctive cross-hatched or "tartan plaid" pattern. Characteristic of microcline.

Perthite--not a twinning pattern but an intergrowth of albite and other sodic plagioclase in orthoclase or microcline. The perthite pattern may take several forms.

Zoning--not true twinning but forms during crystallization which forms solid solutions. Imperfect reaction between the solid phase and liquid phase may result in zoning. Zoning may take several forms. Chemical zoning is common in volcanic intrusive feldspars.

Extinction--isotropic minerals are always extinct. Anisotropic minerals go extinct (dark) every 90° of rotation (4 times in complete rotation of stage). Brightness is at a maximum when mineral vibration directions are in 45° positions and at a minimum when N-S or E-W. The change in intensity from extinction to maximum intensity follows a sin function, not a linear relationship. As light intensity approaches zero, interference colors do not change; the intensity will change, but not the hue. The angle between a linear crystallographic feature (cleavage traces, crystal face) and vibration direction of analyzer and polarizer is called the extinction angle. In the tetragonal, hexagonal and orthorhombic systems, all the principal vibration directions of light are parallel to the crystallographic axes. For monoclinic minerals, on ly one vibrational direction coincides with a crystallographic axis (b axis), and the other will be inclined to the crystallographic axis. In the triclinic syste, it is possible for only one vibration direction to be parallel to a crystallographic axis, but usually all vibration directions are inclined to all the axes.

Parallel--mineral is dark when the crystal face or cleavage face is parallel to the crosshairs of microscope. An example is biotite. All uniaxial minerals have parallel extinction, but so do orthorhombic biaxial minerals (olivine, orth-pyroxenes).

Inclined--mineral is dark when the crystal face of cleavage face forms an angle with the crosshairs of the microscope. The extinction angle is a diagnostic property of minerals. Hornblend is an example. All biaxial minerals excluding orthorhombic minerals have inclined extinction.

Symmetrical--a special case of inclined; mineral is dark when crosshairs bisect cleavage angles.

No extinction angle--Many minerals do not have a habit or cleavage face to exhibit an extinction angle, or weathering may have altered the mineral. Quartz has no extinction angle.

Elongation--anisotropic minerals of linear habit or fragments produced by prismatic or platy cleavages exhibit either positive or negative elongation. In uniaxial minerals, such habits and cleavage exhibit parallel extinction and polarized light will vibrate in planes parallel and perpendicular to the elongation direction. We can determine through the use of proper crystal orientation (NE-SW direction) and accessories (commonly a gypsum plate) whether the elongation direction parallels the plane of the fast or slow wave. If the slow wave is parallel to the elongation, the mineral is said to be lengthslow, or to have positive elongation. Conversely, if the fast wave is parallel to the elongation, the crystal is said to be length fast, or to have negative elongation. Elongation of biaxial minerals with parallel or near parallel extinction is analogous to uniaxial minerals. Biaxial minerals with inclined extinction make the elongation sign more difficult to determine.

The gypsum plate is normally inserted in a NW-Se direction such that its slow direction is oriented NE-SW. The plate is normally marked with an arrow showing its slow direction. The gypsum plate is cut to a thickness that reults in 550 mµ retardation. If the slow wave from the mineral is parallel to the slow wave of the gypsum plate, there is a further retardation of the wave. The sum of the two retardations results in higher colors. On an interference color chart, the lower x-axis label is retardation, or the difference in mµ between the two waves. For example, if the initial retardation from the mineral was 200 mµ (gray-yellow) and there is an additional retardation of 550 mµ, the sum is 750 mµ (green). this is termed increased color. If the fast wave of the crystal is parallel to the slow wave of the gypsum plate, the color subtracts (550-200=350 mµ) (yellow gold) resulting in a color decrease.

Determining which is the fast or slow wave has practical utility in mineral identification and determining orientation. Clay minerals are too small to clearly identify individual particles. However, they often combine into domains or units that appear to act like a larger entity. Clays often are oriented around pedological features such as voids, mineral grains or structural units. But it may be important to determine whether the clays are oriented parallel or normal to the feature. Clay minerals have positive elongation (length slow), so by using the gypsum plate the orientation of the clays around a feature can be determined.

Interference figure--optical pattern, obtained using the Bertrand lens above the stage and condensing lens below the stage. Useful for determining optical orientation and optic sign of a mineral grain. Interference figure is a useful diagnostic property for mineral identification.

This page (http://www.ces.ncsu.edu/plymouth/programs/cross.html) created by
Vera MacConnell, Research Technician, I on November 13, 1997.
Last Updated on November 14, 1997.