Extinction

Definition

In mineralogy, extinction refers to the phenomenon where a mineral grain becomes completely dark under polarized light during the rotation of the stage in a polarizing microscope. This behavior is crucial for identifying minerals as it reveals their optical properties, including crystal symmetry and orientation. The study of extinction helps geologists understand how minerals interact with light, which is essential for interpreting rock formations and their histories.

5 Must Know Facts For Your Next Test

1. Extinction occurs at specific angles during the rotation of the mineral stage in a polarizing microscope, which can vary depending on the mineral's crystal system.
2. The presence of extinction can indicate the orientation of a crystal and help in determining its symmetry class.
3. Some minerals exhibit distinct extinction patterns, such as parallel or inclined extinction, which are key to their identification.
4. The angle at which extinction occurs can be influenced by factors such as crystal habit and the wavelength of light used.
5. Recognizing extinction is critical for differentiating between similar-looking minerals in thin sections, aiding in accurate mineral identification.

Review Questions

• How does understanding extinction enhance the identification process of minerals under polarized light?
  • Understanding extinction is vital because it allows geologists to observe how a mineral interacts with polarized light. By rotating the stage of a polarizing microscope, one can determine at what angles a mineral appears dark, revealing its crystal orientation and symmetry. This information helps distinguish between minerals that may appear similar at first glance, leading to more accurate identification.
• Discuss how different extinction patterns can provide insights into a mineral's crystal structure and symmetry.
  • Different extinction patterns, such as parallel or inclined extinction, provide valuable insights into a mineral's internal structure and symmetry. For example, minerals belonging to the monoclinic system may show inclined extinction while those in the orthorhombic system typically exhibit parallel extinction. Analyzing these patterns can help geologists classify minerals and understand their crystallographic relationships.
• Evaluate the importance of extinction in interpreting geological formations and their histories.
  • Extinction plays a crucial role in interpreting geological formations as it provides essential data about mineral compositions and their optical properties. By analyzing extinction behaviors, geologists can make inferences about the conditions under which rocks formed, including temperature and pressure regimes. This understanding is essential for reconstructing geological histories and identifying processes such as metamorphism and sedimentation that have shaped the Earth's crust.

Related terms

Polarized Light: Light that oscillates in a single plane, which is used in microscopy to analyze the optical properties of minerals.

Birefringence: The difference in the refractive indices of a mineral, resulting in two different velocities of light through the material, often observed when viewed under polarized light.

Interference Colors: Colors produced by the interference of light waves when they pass through a birefringent mineral, providing valuable information about the mineral's properties.