Microscopic Mineralogy: How to measure the angle of extinction?

How to measure the angle of extinction in a mineral under crossed Nicol’s
Optical properties of the mineral under crossed Nicols or Analyser has been explained with suitable examples. The properties like Isotropism and Anisotropism, Extinction, Interference colours or Double Refraction colours or Birefringence or Polarisation colour, Twinning and Cross Hatching are described systemarically.
PROPERTIES BETWEEN ANALYSER /
CROSSED NICOLS (BXN)
(1) Isotropism and Anisotropism:-
A mineral is said to be isotropic if it remains dark or extinguished during the complete rotation of the stage and the phenomenon is called as Isotropism. e.g. Garnet
A mineral is said to be anisotropic when it becomes dark 4 times and bright 4 times during a complete rotation of the microscopic stage and the phenomenon is called as anisotropism. e.g. Biotite, Hornblende, Augite, etc. The colourless minerals are called as anisotropic. e.g. Quartz
(2) Extinction:-
The position at which the anisotropic mineral becomes dark is called extinction position.
There are three types of extinction:
(a) Straight or Parallel extinction:- Mineral becomes dark in a position when one of the cross wire is parallel to the cleavage or to the length of the mineral. This is called as straight or parallel extinction e.g. Biotite.
(b) Oblique or Inclined extinction:- If the cleavage of the mineral or the length of the mineral makes an angle with one of the cross wire in the position of the extinction, then this position of extinction is described as the oblique / inclined extinction. e.g. L.S. of Hornblende. In this case one has to find out the extinction angle
(c) Symmetrical extinction:- If in a mineral the vertical cross wire bisects the angle between the 2 sets of cleavages in the position of extinction, then the mineral is said to show symmetrical extinction e.g. Calcite.
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[@ayubjamadar3460]

Amazing

[@pcgeokimberlite2947]

Jazakallah khaire Ayub Jamadar

[@bibiayeshahundekar5334]

Informative and definately help students

[@pcgeokimberlite2947]

Jazakallah khaire

[@GLBKhaleeqMahmood]

Thank you sir was very confused and searched on youtube, fingd ur video doubt cleared

[@pcgeokimberlite2947]

Jazakallah khaire Khaleeq. Thanks for compliments. Videos will your basic geology related to minerals,ore minerals,Crystallography ,microscopic properties of minerals , texture structure , structural and paleontology micro and mega geology,petrology. All with theory as well as practical.

[@srashtidubey7638]

Thank u sir.......best explaination 😃

[@pcgeokimberlite2947]

Most welcome

[@babukurmi5844]

Good way of explanation sir 👍

[@pcgeokimberlite2947]

Thanks for compliments

[@nvsuhail]

Thank you so much sir❤️

[@pcgeokimberlite2947]

Most welcome

[@studywithadarsh2019]

Sir how can determine the uniaxial positive or uniaxial negative.by sign of elongation... please sir make a video... please hindi me bataye sir

[@pcgeokimberlite2947]

I already uploaded a video on Uniaxial mineral Positive and Negative. Soon biaxial and sign of elongation will be prepared and upload.

[@studywithadarsh2019]

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[@pcgeokimberlite2947]

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[@studywithadarsh2019]

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[@nvsuhail]

Sir, please don't forget to explain in English also.. There is only a little videos are available in youtube about this topic. So, please don't limit only in hindi sir.

[@pcgeokimberlite2947]

Thank you NV Suhail.I will take your suggestions in future