When the vibration directions of the ordinary and extraordinary rays of an anisotropic mineral are parallel to the vibration directions of the nicols in a petrological microscope, no light reaches the eye and the mineral is said to be in extinction. This is because of the fact that light which can pass the polarizer also passes the mineral but is stopped by the analyser, as it has a vibration direction perpendicular to that of the polarizer. This phenomenon occurs four times during the rotation of the stage of the microscope (the thin section) through an angle of 350° under crossed-nicols position.
An isotropic mineral is always in the position of extinction between crossed nicols. In case of anisotropic mineral, the two adjacent positions of extinction are separated from each other by 90°.
There are four types of extinction as follows:
(i) Straight extinction:
If the section becomes darkest when its length (which is usually parallel to c-axis) or a prominent cleavage direction is parallel to any of the cross-wires, the extinction is straight or '0°'.
(ii) Oblique or Inclined extinction:
When a mineral is in extinction with the cleavage or crystal boundaries lying oblique to the planes of vibration of the two nicols, i.e., inclined to the crossfire the extinction is said to be 'Inclined or oblique.' All the Biaxial minerals (Orthorhombic, monoclinic, triclinic) show oblique extinction.
(iii) Symmetrical extinction:
This type of extinction is shown by minerals with squares outline or rhombic cross-section. In this case the mineral section extinguishes with the planes of vibration of the nicols being parallel 10 the diagonals of the rhombic pattern.
(iv) Wavy or Undulose extinction:
In this case it appears as a band or series of bands of darkness crossing a single crystal unit daring rotation. It may owe to strain in mineral that the mineral fail to extinguish completely. Quartz often shows wavy extinction.
Extinction angle:
The angle between a crystallographic direction and the position of maximum extinction is called the extinction angle. Ordinarily the angle is measured with reference to cleavage direction but in the absence of cleavage, crystal outline, prominent crack, twin plane etc. are taken into account for the measurement of extinction angle.
Interference colour:
Anisotropic minerals in positions intermediate between the positions of extinction show some colours which is the outcome of the interference of the two rays of light having the same vibration plane (after their emergence from the analyzer) but one is retarded with reference to the other ; accordingly these colours are known as interference colour.
These colours depend upon the thickness of the mineral section, birefringence of the section, crystallographic orientation of the section etc.
An isotropic mineral is always in the position of extinction between crossed nicols. In case of anisotropic mineral, the two adjacent positions of extinction are separated from each other by 90°.
There are four types of extinction as follows:
(i) Straight extinction:
If the section becomes darkest when its length (which is usually parallel to c-axis) or a prominent cleavage direction is parallel to any of the cross-wires, the extinction is straight or '0°'.
(ii) Oblique or Inclined extinction:
When a mineral is in extinction with the cleavage or crystal boundaries lying oblique to the planes of vibration of the two nicols, i.e., inclined to the crossfire the extinction is said to be 'Inclined or oblique.' All the Biaxial minerals (Orthorhombic, monoclinic, triclinic) show oblique extinction.
(iii) Symmetrical extinction:
This type of extinction is shown by minerals with squares outline or rhombic cross-section. In this case the mineral section extinguishes with the planes of vibration of the nicols being parallel 10 the diagonals of the rhombic pattern.
(iv) Wavy or Undulose extinction:
In this case it appears as a band or series of bands of darkness crossing a single crystal unit daring rotation. It may owe to strain in mineral that the mineral fail to extinguish completely. Quartz often shows wavy extinction.
Extinction angle:
The angle between a crystallographic direction and the position of maximum extinction is called the extinction angle. Ordinarily the angle is measured with reference to cleavage direction but in the absence of cleavage, crystal outline, prominent crack, twin plane etc. are taken into account for the measurement of extinction angle.
Interference colour:
Anisotropic minerals in positions intermediate between the positions of extinction show some colours which is the outcome of the interference of the two rays of light having the same vibration plane (after their emergence from the analyzer) but one is retarded with reference to the other ; accordingly these colours are known as interference colour.
These colours depend upon the thickness of the mineral section, birefringence of the section, crystallographic orientation of the section etc.
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