PLANE POLARIZED LIGHT (polarizer in; analyzer out).

Relief--conspicuousness of a mineral, how the mineral "stands out" in its surrounding medium. Relief is determined by the amout of difference between the refractive index of the mineral and its surroundings. Minerals that stand out strongly have high relief. If the mineral does not appear to stand out from the surrounding medium, its relief is low. Relief is a subjective value, not easily quantified.

Habit--shape of the mineral grain. Shape is not always a diagnostic characteristic of minerals in soils. Physical and chemical weathering and different modes of diagenesis may often alter the original morphology of many minerals. Minerals may appear euhedral (well defined crystal faces) or anhedral (without crystal faces). Other descriptive terms include prismatic, acicular (needle-like), fibrous, tabular, bladed, equant, lath- like, or platy.

Cleavage--The tendency of certain minerals to cleave (fracture) along specific, smooth crystallographic planes of weakness. these cleavage planes are usually parallel, but may intersect other cleavage planes at an angle. Crystals may have cleavage in one direction (mica), two directions (pyroxenes and amphiboles), rhombohedral cleavage in three direction (calcite), or even four (fluorite) or six (garnet) directions. Many minerals (quartez) have no cleavage.

Color--Minerals exhibit a wide range of colors in plane light, ranging form colorless (e.g. quartez, feldspars, gypsum), highly colored (biotite, hornblend), to opaque (Fe-Mn nodules, some heavey minerals). Since the light used in microscopy is polychromatic, selective absorption of certain wavelengths may take place which results in a colored mineral. The color is based on the wavelengths transmitted by the mineral. Ionic substitutions within the structure may cause variations in color.

Pleochrosim--expression of degree to which light is bent or refracted as it enters a mineral grain from air. An isotropic mineral has only one characteristic index regardless of orientation of the mineral in th elight source. Depending on mineral orientation, one may obtain minimum and maximum indexes for uniaxial minerals and mimum, maximum, and intermediate indexes (corresponding to beam transmitted alo9ng the two optic axes) for biaxial minerals. In other words, an isotropic mineral has one characteristic index, a uniaxial mineral has two characteristic indices, and a biaxial mineral has three characteristic indices.

Becke line--a white border that appears when the mineral grain is just out of focus. If the line moves inward over the mineral grain as the barrel of the microscope is raised (or stage lowered), the mineral has a higher index than the liquid oil or mounting medium. Conversely, if the line moves outward, the oil has higher index than the mineral. The Becke line is used to determine refractive index.

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