extinction angle
A student measures the extinction angle of a calcite crystal under a polarizing microscope.
Noun: - The angle from its axis that a crystal must be rotated before appearing maximally dark when viewed in polarized light: This is a specific optical property measured in mineralogy and crystallography. When a crystal is placed under a polarizing microscope and rotated, it will become darkest at a specific orientation relative to its crystallographic axes; this orientation is the extinction angle.
The term is used in scientific contexts, particularly in geology, mineralogy, and materials science, to describe and identify minerals based on their optical characteristics. - The mineral's identity was confirmed by measuring its extinction angle under cross-polarized light. - A key diagnostic feature for plagioclase feldspar is its inclined extinction angle.
- Symmetrical vs. Inclined Extinction: Crystals can exhibit different types of extinction. "Parallel extinction" or "symmetrical extinction" occurs when the crystal appears dark when its cleavage or crystal edges are parallel to the vibration directions of the polarizers. "Inclined extinction" occurs when the crystal must be rotated to a specific angle (the extinction angle) from these reference lines to achieve maximum darkness.
- Measurement Context: The extinction angle is always measured relative to a recognizable crystallographic direction, such as a cleavage trace or crystal face. It is reported in degrees.
- Extinction: (Noun) In this optical context, the phenomenon where light is blocked when a crystal is in a specific orientation under crossed polarizers.
- Optic Angle: (Noun) A different but related optical property referring to the angle between the two optic axes in a biaxial crystal.
- Birefringence: (Noun) The difference in refractive indices within a crystal, which is related to the interference colors observed and which changes until extinction occurs.
- Angle of Extinction: A less common but equivalent phrase.
- Extinction Position: Refers to the rotational orientation itself rather than the numerical angle.
- Undulose Extinction: (Noun Phrase) A wavy or patchy pattern of extinction in a crystal grain, often indicating strain or deformation.
- Crossed Polars / Crossed Nicols: (Noun Phrase) The microscope setup (with two polarizing filters perpendicular to each other) required to observe extinction phenomena.
A student measures the extinction angle of a calcite crystal under a polarizing microscope.
- the angle from its axis that a crystal must be rotated before appearing maximally dark when viewed in polarized light