INTRODUCTION
In optical mineralogy, mineral extinction refers to the complete loss of visible light transmission through a mineral when it is viewed under a polarizing microscope with crossed polarisers. This occurs when the mineral is oriented at an angle that is perpendicular to the plane of polarization of the transmitted light, which causes the mineral to appear completely dark against a bright background.
Mineral extinction is an important optical property of minerals that is used in the identification and characterization of minerals. It can provide information about the crystallographic orientation and structural features of the mineral, as well as its optical properties such as birefringence and Pleochroism.
In optical mineralogy, the angle of mineral extinction is measured relative to the vibration direction of the polarized light. The angle of extinction can be used to determine the crystallographic orientation of the mineral within the sample, as well as its symmetry and optical behavior.
Mineral extinction can occur in different directions based on the mineral's crystal structure and the direction of extinction can provide important information about the crystallographic axes and symmetry of the mineral.
Overall, mineral extinction is an important property of minerals in optical mineralogy that provides valuable information about their crystallographic orientation, structure, and optical properties.
IMPORTANCE :
Mineral extinction is an important concept in optical mineralogy because it is one of the key optical properties which used to identify minerals under a polarizing microscope. When a thin section of a rock is placed under crossed polarisers, the minerals in the rock interact with the light in specific ways, producing a range of colors and patterns.
Mineral extinction occurs when a mineral is oriented at an angle that is perpendicular to the plane of polarization, causing it to appear dark or black against a bright background. This effect is useful in identifying minerals because it is a characteristic property of each mineral that can be used to distinguish it from other minerals.
By observing the angle and direction of mineral extinction under a polarizing microscope, petrologists can identify minerals in rocks and determine their crystallographic orientation and structural features. This information is then used to interpret the geological history of the rocks and the conditions under which they were formed. In addition to aiding in mineral identification and interpretation of geological history, mineral extinction can also be used to determine the composition and texture of rocks, as well as to investigate the physical and chemical properties of minerals.
Overall, mineral extinction is an essential concept in optical mineralogy, providing valuable information about the properties and characteristics of minerals that are essential for the study of rocks and the Earth's geology.
EXTINCTION ANGLE AND TYPES :
In optical mineralogy, mineral extinction angle refers to the angle between the crystallographic axis of a mineral and the direction of vibration of the polarized light used in the microscope. When a mineral is placed in a polarizing microscope, it appears bright or dark depending on the angle of the crystal orientation relative to the direction of the light polarization.
When a mineral is oriented such that the crystallographic axis is parallel to the vibration direction of the polarized light, it will appear bright, indicating that it is transmitting light. However, when the mineral is oriented at an angle that is perpendicular to the vibration direction of the polarized light, it will appear dark, indicating that it is not transmitting light. This angle at which the mineral appears dark is known as the extinction angle.
There are four categories of mineral extinction angles:
- Parallel Extinction
- Inclined Extinction
- Symmetrical Extinction
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Fig : Different kinds of extinction angles |
1. Parallel Extinction
With parallel extinction the mineral is extinct when the cleavage or length is aligned parallel to one of the cross hairs.
The extinction angle is 0°.
Either the slow ray or fast ray vibration direction is parallel to the trace of cleavage or length of the mineral.
2. Inclined Extinction
With inclined extinction the mineral is extinct when the cleavage of length is aligned parallel to one of the cross hairs.
The extinction angle will be greater than 0°.
Neither vibration direction is aligned parallel to the trace of the cleavage of the length of the mineral.
If the slow ray vibration direction is closest to the length or trace of cleavage, the mineral is length slow. If the fast ray is closest the mineral is length fast.
3. Symmetrical Extinction
Symmetrical extinction may be observed in minerals that display either two cleavages or two distinct crystal faces.
If the extinction angles EA1 and EA2 measured from the two cleavage or crystal faces to the same vibration direction, are the same, extinction is symmetrical.
4. No Extinction Angle
Many minerals lack distinct cleavages or do not display an elongation or crystal faces.
Although they go extinct once every 90° of stage rotation, there is no cleavage, elongation or crystal face from which to measure an extinction angle.
In these situation we say that the mineral has no extinction angle.
5.Other Types of Extinction
In addition to the before mentioned types of extinction, some minerals may not go totally extinct at any stage position.
Usually this is the result of strain in the crystal lattice chemical zoning.
In many deformed rocks, the mineral grains are bent or distorted so that different parts of the grain go extinct at different times. If the extinction follows an irregular or wavy pattern it is called undulose extinction. Because in the monoclinic and triclinic minerals, extinction angles can change with composition, causing core and rim of a grain to show different angles due to zoning.
CONCLUSION :
Mineral extinction in petrology refers to the disappearance of a mineral phase due to changes in physical or chemical conditions, such as during metamorphism or weathering. It helps geologists interpret a rock’s formation environment (pressure, temperature) and affects its properties—e.g., loss of mica may reduce strength. This concept is key to understanding rock evolution and Earth’s crustal processes.
