Model Answer
0 min readIntroduction
Pleochroism, derived from the Greek words ‘pleo’ (more) and ‘chroa’ (color), is an optical property exhibited by anisotropic minerals where the mineral appears to change color when viewed under polarized light at different orientations. This phenomenon arises due to the differing absorption of light waves vibrating in different crystallographic directions within the mineral. Biotite, a common phyllosilicate mineral belonging to the mica group, is a classic example demonstrating pleochroism. Understanding why biotite exhibits pleochroism under certain orientations and not others is crucial for its identification and understanding its crystallographic properties.
Understanding Pleochroism
Pleochroism occurs in minerals that are anisotropic, meaning their optical properties vary with direction. This anisotropy is a direct consequence of the mineral’s crystal structure. Light, being a wave, vibrates in all directions perpendicular to its path. In isotropic minerals, the refractive index is the same in all directions, and therefore, light is not split or affected differently based on its vibration direction. However, in anisotropic minerals like biotite, the refractive index varies with direction, leading to birefringence and pleochroism.
Biotite: Structure and Optical Properties
Biotite has a layered structure, belonging to the monoclinic crystal system. Its chemical formula is generally represented as K(Mg,Fe)3AlSi3O10(F,OH)2. The layers are held together by weak Van der Waals forces, resulting in perfect basal cleavage – meaning it cleaves easily along the {001} plane. This cleavage plane is parallel to the sheet-like structure of the mineral.
Pleochroism and Cleavage Orientation
The reason a biotite section cut parallel to cleavage does not show pleochroism lies in the orientation of its absorption axes relative to the direction of light propagation. Biotite is strongly pleochroic, typically exhibiting shades of brown, green, and yellow. It possesses two main absorption axes (X and Y) and a third axis (Z) perpendicular to the cleavage plane.
When a section is cut parallel to the cleavage plane ({001}), the light travels essentially parallel to the Z-axis. In this orientation, the light is vibrating predominantly along the Z-axis, and the absorption along this axis is relatively constant. Therefore, regardless of the rotation of the section under polarized light, the mineral appears to have the same color. The absorption is isotropic in this plane.
However, when a section is cut oblique to the cleavage plane, the light now travels at an angle to the Z-axis and interacts with the X and Y absorption axes. As the section is rotated under polarized light, the angle between the light’s vibration direction and the X and Y axes changes. This varying angle causes different amounts of light to be absorbed along these axes, resulting in a change in color – the manifestation of pleochroism. The color observed depends on which absorption axis is aligned more closely with the direction of the vibrating light.
Illustrative Example
Imagine a section cut at 45 degrees to the cleavage. As you rotate the stage, at one point the light might be vibrating more along the X-axis (absorbing more light, appearing darker), and at another point, more along the Y-axis (absorbing less light, appearing brighter). This difference in absorption is perceived as a change in color.
Factors Influencing Pleochroism
- Chemical Composition: The presence of transition metal ions (like Fe2+, Fe3+, Mg2+) in the biotite structure significantly influences its pleochroism.
- Crystal Structure: The arrangement of atoms within the crystal lattice dictates the direction of absorption axes.
- Thickness of Section: Thicker sections generally exhibit more pronounced pleochroism.
Conclusion
In conclusion, pleochroism in biotite is a direct consequence of its anisotropic nature and the orientation of its absorption axes relative to the direction of polarized light. A section parallel to cleavage doesn’t show pleochroism because light travels along an axis with relatively constant absorption, while an oblique section reveals pleochroism as the light interacts with varying absorption along different crystallographic directions. Understanding this relationship is fundamental to mineral identification and interpreting the optical properties of anisotropic minerals.
Answer Length
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