Understanding Optical Properties of Minerals
When studying minerals under crossed nicols, various optical properties can be observed, including extinction, interference color, interference figure, and pleochroism. Among these, one property stands out that is not typically observed under crossed nicols.
Property Not Observed: Pleochroism
- Definition: Pleochroism refers to the property of certain minerals to display different colors when viewed at different angles, particularly under polarized light.
- Observation Context: While pleochroism is an essential optical characteristic, it is more effectively demonstrated under plane-polarized light rather than crossed nicols. When using crossed nicols, the optical effects related to color changes due to pleochroism become less pronounced.
- Crossed Nicols Setup: In a crossed nicols setup, the two polarizers are oriented perpendicular to each other, effectively extinguishing the light from the sample when it is in extinction positions. This setup primarily highlights interference colors, extinction, and interference figures, which are related to the crystal structure and orientation of the mineral.
Key Observations Under Crossed Nicols
- Extinction: The mineral becomes dark when aligned correctly with the polarizers, allowing geologists to determine the crystallographic orientation.
- Interference Color: This color results from the interaction of light with the mineral's internal structure, visible as the sample rotates through its optic axes.
- Interference Figure: This is a pattern observed in birefringent minerals, providing insights into the optical orientation and character of the mineral.
Conclusion
In summary, while pleochroism is an important optical property of minerals, it is not effectively observed under crossed nicols due to the nature of light interaction in this specific setup.