Model Answer
0 min readIntroduction
Pleochroism, derived from the Greek words ‘pleo’ (more) and ‘chroa’ (color), refers to the property exhibited by certain minerals where they display different colors when viewed from different crystallographic directions. This optical phenomenon arises due to the anisotropic nature of light absorption within the mineral’s crystal structure. Unlike isotropic minerals which absorb light equally in all directions, anisotropic minerals have varying absorption coefficients depending on the direction of light propagation. This difference in absorption leads to the perception of different colors as the mineral is rotated, making pleochroism a valuable tool in mineral identification.
Understanding Pleochroism
Pleochroism is fundamentally linked to the crystal structure and chemical composition of minerals. Minerals exhibiting pleochroism are typically anisotropic, meaning their optical properties vary with direction. This anisotropy stems from differences in the arrangement of atoms within the crystal lattice.
Causes of Pleochroism
1. Anisotropic Absorption of Light:
The primary cause of pleochroism is the differing absorption of light waves vibrating in different directions within the crystal. This occurs because the electronic structure and bonding arrangements within the mineral are not symmetrical in all directions. Light interacting with these asymmetrical structures is absorbed to varying degrees depending on its polarization and direction of travel.
2. Chemical Composition and Transition Metal Ions:
The presence of transition metal ions (like iron, chromium, vanadium, and titanium) within the mineral’s structure plays a crucial role. These ions have partially filled d-orbitals, which can absorb specific wavelengths of light, resulting in color. The absorption spectra, and therefore the colors observed, are dependent on the coordination environment and oxidation state of these ions. Different crystallographic directions expose these ions to different ligand fields, leading to variations in absorption and thus, pleochroism.
3. Crystal Structure:
The crystal system significantly influences pleochroism. Minerals belonging to the orthorhombic, monoclinic, and triclinic systems are more likely to exhibit pleochroism than those belonging to the cubic system, which are isotropic. The lower symmetry of the former systems allows for greater anisotropy in light absorption.
Examples of Pleochroic Minerals
- Tourmaline: Exhibits strong pleochroism, often displaying different shades of green, brown, or pink depending on the viewing angle.
- Cordierite: Commonly shows pleochroism ranging from colorless to blue or violet.
- Andalusite: Displays pleochroism with colors varying from yellowish-green to reddish-brown.
- Biotite Mica: Shows pleochroism, typically with shades of brown or green.
- Epidote: Often displays pleochroism ranging from yellowish-green to brownish-green.
Determining Pleochroism
Pleochroism is typically observed using a rotating stage and a polarizing microscope. By rotating the mineral sample under crossed polarizers, the different colors become apparent. The number of colors observed indicates the degree of pleochroism – dichroism (two colors) is common, while trichroism (three colors) is less frequent.
Conclusion
In conclusion, pleochroism is a valuable optical property arising from the anisotropic absorption of light in minerals, primarily due to their crystal structure and the presence of transition metal ions. The varying absorption coefficients along different crystallographic axes result in the perception of different colors when the mineral is rotated. This phenomenon serves as a crucial diagnostic tool in mineral identification and provides insights into the mineral’s chemical composition and internal structure.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.