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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Vaibhav Singh posted an Question
July 14, 2021 • 20:03 pm
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30 points
30 points
- IIT JAM
- Geology (GG)
206. inclined extinction is shown by: a calcite b. basal section of augite c. basal section of hornblende d. none of the above (bhu-2010)
206. Inclined extinction is shown by: A Calcite B. Basal section of augite C. Basal section of hornblende D. None of the above (BHU-2010) How to solve this type of Q in which sections and category of extinction is related?? I have encountered in theory that Extinction angle measured on a specific mineral in thin section or grain mount depends exactly on grain orientation!! PLEASE CLARIFY ALL THESE confusion of HOW section of minerals RELATED TO EXTINCTION!
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Mayank vedwal Vedwal Best Answer
Here option D is correct.... For answering this question I want to clear few points :- 1) Only monoclinic and triclinic minerals show inclined extinction not hexagonal, orthorhombic, cubic and tertagonal minerals. 2) Not all planes of monoclinic and triclinic minerals show inclined extinction. Now calcite comes under hexagonal system so it can't show inclined extinction. Then augite and hornblende comes under monoclinic system. Now in monoclinic crystal system plane parallel to 010 shows maximum inclined angle, plane parallel to 100 shows parallel extinction and plane parallel to 001 shows symmetrical extinction. So options B and C are incorrect because augite and hornblende are monoclinic minerals and their basal section will show symmetrical extinction. so for this question option D is correct. You can check following images which I took from nesse
![cropped5320081845414707005.jpg]()
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Vaibhav singh
thanks mayank bhaiya..Actually this page of Nesse was unclear to me..You made it clear. .