by Richard Drees
In thin section and detrital grain mounts some minerals may appear opaque. They are opaque because light does not pass through them due to composition or coatings. In thin section the grains are sufficiently thin that opaqueness is due to adsorption of light by elements such as Fe. In such instances it is best to examine the minerals under reflected light rather than transmitted light as the petrographic microscope is not well suited to the study of opaque minerals.
Some microscopes are designed for relection analysis by passing a light beam through the microscope objective lens onto the specimen. In these microscopes the light beam may be polarized and focused onto the specimen. These microscopes are ideal for this type examination because they can be used in crossed and uncrossed polarized light modes. Lacking a reflected light microscope, lamps can be positiioned around the microscope stage to illuminate the specimen. The lamp for transmitted light should be turned off.
Because images in reflected light lack the optical information available in transmitted light, identification is usually based on color an dmorphology. However, in many soils, morphology is not always diagnostic because of pedogeni processes. Below is a list of common opaque minerals and color:
| MINERAL | COMPOSITION | COLO R IN REFLECTED LIGHT |
|---|---|---|
| Pyrite | FeS2 | Pale brassy yellow |
| Chalcopyrite | CuFeS2 | Dark brass yellow to yellow gold |
| Ilmenite | FeO*TiO2 | Metallic gray to black |
| Leucoxene | (secondary Ti minerals) | White gray, porcelain like color |
| Hematite | Fe2O3 | Metallic black with red highlights to blood red |
| Magnetite | Fe3O4 | Dull gray to steel blue |
| Graphite | C | Variable, from metallic black to silvery gray |
This page
(http://www.ces.ncsu.edu/plymouth/programs/reflected.html)
created by
Vera MacConnell,
Research Technician, I
on November 17, 1997.
Last Updated on November 17, 1997.