When vibration directions of an anisotropic mineral coincide with those of the polarizer and analyzer, the mineral appears dark. This position of an anisotropic mineral is called ”position of extinction”. During a complete rotation of the microscope stage, the position of extinction occurs four times at 90° intervals. In extinction position, the light from the polarizer passes through the crystal vibrating parallel to the plane of the polarizer and is eliminated on the analyzer.
The extinction position helps in locating the vibration directions of the crystal section under study. In this position the vibration directions of the crystal coincide with the cross-hairs of the microscope eyepiece, which are set parallel to the planes of the polarizer and analyser.
Extinction Angle: The extinction angle is the angle between the extinction position and some crystallographic direction of a crystal. Since extinction positions are always 90° apart, usually the angle between the cleavage planes or crystal boundary and the nearest extinction position is measured.
Types of Extinctions:
There are three types of extinction namely;
(i) Parallel Extinction, (ii) Symmetrical Extinction, and (iii) Inclined Extinction.
- Parallel Extinction: When the direction of extinction of a crystal is parallel to its crystallographic axis or to the trace of an axial plane, it is said to show ”parallel extinction”. Generally, the physical directions of a crystal, such as straight edges, cleavage cracks, and length of the crystal, represent crystallographic directions. In uniaxial minerals, since vibration directions always lie in some crystallographic axial plane, all sections show parallel extinction. Orthorhombic crystals also show parallel extinction.
- Symmetrical Extinction: When the direction of extinction bisects the angle between two sets of cleavage, the crystal is said to show ”symmetrical extinction”. For example, diamond-shaped sections of hornblende and augite, cut normal to c-axis, show symmetrical extinction. Symmetrical extinction is characteristic of orthorhombic crystals.
- Inclined Extinction: When the direction of extinction is not parallel to the crystallographic directions, the crystal is said to show ”inclined extinction”. Minerals belonging to monoclinic and triclinic systems show inclined extinctions.
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