Extinction angle

Definition

The extinction angle is the angle at which a mineral appears dark under polarized light when it is rotated between crossed polarizers. This angle is essential in identifying minerals because it provides valuable information about their optical properties, such as their crystallographic orientation and symmetry. Understanding extinction angles helps to analyze interference figures and optical indicatrix, which are key concepts in mineralogy.

5 Must Know Facts For Your Next Test

1. Extinction angles vary among different mineral types and can be used as a diagnostic tool in identifying them under polarized light.
2. The extinction angle is measured between the direction of polarization and the crystallographic axes of the mineral.
3. In uniaxial minerals, extinction angles can be either parallel or perpendicular to the crystal's optic axis, influencing their identification.
4. For biaxial minerals, extinction angles can appear more complex, often requiring detailed analysis through interference figures.
5. The extinction angle is often represented in degrees and can be calculated using the formula: $$ ext{Extinction Angle} = ext{atan}igg(rac{ ext{b}}{ ext{a}}igg)$$, where 'a' and 'b' refer to specific crystallographic axes.

Review Questions

• How does the extinction angle assist in identifying minerals under polarized light?
  • The extinction angle aids in mineral identification by indicating the orientation of the crystal structure in relation to polarized light. By measuring the angle at which a mineral goes dark between crossed polarizers, geologists can determine its optical properties and infer its crystallographic axes. Different minerals have characteristic extinction angles that help distinguish them from one another during microscopic analysis.
• Discuss the significance of understanding both extinction angles and birefringence in analyzing minerals.
  • Understanding extinction angles alongside birefringence is crucial for a comprehensive analysis of minerals. While the extinction angle gives insights into the orientation of a crystal and helps identify specific minerals, birefringence reveals how a mineral interacts with polarized light by splitting it into two rays. Together, these properties help characterize a mineral’s optical behavior, enhancing accuracy in identification and study of their crystalline structures.
• Evaluate how variations in extinction angles among different mineral groups impact their classification and identification in optical mineralogy.
  • Variations in extinction angles among different mineral groups significantly impact their classification and identification by providing key optical signatures unique to each group. For instance, uniaxial minerals exhibit specific patterns in their extinction angles depending on their optical axis orientation, while biaxial minerals may show complex behaviors due to multiple axes. Analyzing these variations allows geologists to classify minerals accurately, assess their geological history, and predict their behavior under various conditions, making extinction angles a vital aspect of optical mineralogy.

Related terms

Polarized Light: Light that vibrates in a single plane, used in microscopy to enhance the visibility of minerals.

Birefringence: The property of a mineral to refract light into two different paths, leading to variations in optical properties depending on the orientation of the mineral.

Interference Figure: A pattern created by the interference of light waves passing through a mineral under polarized light, revealing information about its optical characteristics.