Optical Mineralogy Pt.1- Plane & Cross Polarized Light, Birefringence, Pleochroism, etc. | GEO GIRL
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- Опубліковано 8 лип 2024
- Light slows down when traveling through thin sections, this is called retardation. The incident ray of light goes through one material into another and becomes refracted, which is why the ray traveling through the second material is called the refracted ray. The angle at which the light ray is refracted can help us figure out the refracted index (R.I.) of a mineral. But how can we figure out the R.I. of a mineral just by observing it in thin section? Relief. Relief, or relative contrast, which shows up on certain mineral grains in thin section as Becke lines (outlines of the crystal grains), helps us determine R.I. The more relief (the thicker and more apparent the Becke line), the higher the R.I. of the mineral relative to the R.I. of the epoxy that the thin section is mounted in.
In petrographic microscopes (or polarizing light microscopes), light is forced through one or more polarizers that cause the light to vibrate in specific directions. Observing a thin section in plane polarized light just means we are using one polarizer. Crossed polarized light just means we are observing the thin section with two polarizers inserted in the microscope.
Two perpendicularly vibrating light rays are produced when light travels through a thin section, and because these rays are out of phase, when they are recombined in the upper polarizer (or analyzer), they produce interference colors (or birefringence colors). Minerals with a high range of R.I. values have a high birefringence which causes them to produce beautiful, high order interference colors. Minerals with low birefringence produce duller, low order interference colors. Only anisotropic minerals produce interference colors. Isotropic minerals do not produce interference colors, they appear black under cross polarized light.
To measure extinction angles of anisotropic minerals you just need to rotate the microscope stage until your mineral goes into extinction (goes black) and record angle at which that occurs by using the ticks labeled on the stage.
Some minerals exhibit variations in color under plane polarized light as well. These is called pleochroism.
Reference: Earth Materials textbook: amzn.to/3DXOUnj
Color atlas of rocks & minerals in thin section: amzn.to/3JlbJDg
GEO GIRL Website: www.geogirlscience.com/ (visit my website to see all my courses, shop merch, learn more about me, and donate to support the channel if you'd like!)
Optical Mineralogy Part1:
0:00 light traveling through minerals
1:52 refractive index (R.I.)
4.35 relief & becke lines
6:09 plane & cross polarized light
10:33 isotropic vs. anisotropic minerals
10:47 birefringence & interference colors
13:01 orders of interference colors
14:02 accessory plates
18:00 extinction
19:10 extinction angles
21:05 length fast vs. length slow minerals
21:48 pleochroism
24:02 mineral identification chart
27:00 upcoming videos!
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Image sources:
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upload.wikimedia.org/wikipedi...
cdn1.byjus.com/wp-content/upl...
www.labotka.net/310/Atlas/Imag...
www.tulane.edu/~sanelson/image...
diffzi.com/wp-content/uploads...
www.bwoptics.com/html/UploadFi...
www.researchgate.net/publicat...
image1.slideserve.com/2693957...
upload.wikimedia.org/wikipedi...
www.mtholyoke.edu/~mpeterso/cl...
micro.magnet.fsu.edu/primer/i...
www.wgtn.ac.nz/sgees/images/e...
www.mtholyoke.edu/~mpeterso/c...
Other figures from book: www.amazon.com/Earth-Material...
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Hi there! If you'd like to see me do some mineral identification practice with thin section examples, you can watch this video: ua-cam.com/video/-FmZLS37QkE/v-deo.html
That video focuses on minerals in metamorphic rocks, but I will be coming out with an igneous version as well!
If there is anything else you'd like to see, let me know by replying to this comment ;D Thank you!!
Superb. I am doing a MSc GIS and have decided to utilise Geological mapping as my base data for my thesis: I’m refreshing my 35+ years redundant knowledge, and your videos are ideal!More of the same please. 😀👌
You really went over a lot of the details on how optical mineralogy works.
From what I can recall from my optical mineralogy labs, when I studied geology many many years ago, we students did not really use much beyond qualitative identification methods that are somewhat analogous to how one learns to identify hand samples.
God bless you. Been taking Mineralogy since my year 2 at college studying Geology, now i'm finishing (year 4) but still doesn't have much knowledge on this minerals identifications under thin sections until i came across this. Thank you!
Excellent summary! - I had to learn this 50 years ago.
Thanks so much for making these videos! It's nice that someone is actually explaining these topics on UA-cam so well! My only feedback would be that when you repeatedly say "we'll see later" it is distracting and takes my mind off of what we are seeing in the moment. But really great video. Nice length and easy to digest. This helped a lot to better understand the material I'm learning in class.
I’m so glad it helped you! :) I still consider myself very new to both teaching and UA-cam so I really appreciate both the praise and the feedback, thank you! 😊
Cross over to health care. Gouty crystals will be negative brirefringenet and pseudogout will be positively binrefringent. So much to learn.
as a photographer this is the greatest thing ever
Clear, to the point and so easy to understand.❤
At 11:59 "these blues!, these pink! and these yellows!" 🤣🤣🤣 So funny. 🔥🔥😆
This video was so needed!
This is excellent material thank you for taking the time to put this up. Concise and informative. I had been looking for an introduction lecture last year and so happy to find your lecture!
So glad you find it helpful and concise! Love this comment, made my day, thank you! :D
I've come back to this a few times now as i'm training for Bulk Analysis of Asbestos Fibres and this covers the basis of everything. Thank you for explaining in detail (and also putting it in to simple terms) that's easy to understand.
You are very welcome, so glad you find it helpful!
Well done. I haven't done optical work in over 30 years.
nice video on basics of microscopy of minerals
Gaaaah......! This was definitely the hardest subject I tackled during my geology studies.
that's so nice material. thank you so much!
Just start to work with optical mineralogy! And its a exciting phase of my life and challenging.
Thx for videos! So cool so Good!
Thanks! So glad you've found the videos helpful! ;D
Looks like some fantasying mineral content that I’m about to learn about.
#NerdAlert
I ❤️ GEO GIRL
Thank you for another great video. Awesome explanation. Now watching part-2 of Optical mineralogy. :)
Thanks so much for the comment, I am so glad you found it helpful, I hope you like the part 2 as well ;)
Oh Geogirl, so in love with you! Your video definitely was the best intro I could've got to Opt mineralogy. I'm in my 3rd semester of Geology career, greetings from Ecuador!!
Thank you so much! Best of luck with your geology career! ;D
Finally understand the difference and how PPL and XPL work!!
Yay! So glad this video could help ;D
I used to not watch foreigners lectures but today I did. U know why? Obviously the way of teaching
i love your videos, thank you!!
Thanks so much! So glad you love them :D
I'll have to watch this a couple of times to get it all. Thanks!
Of course! And trust, me I understand, it took me a while to understand these concepts as well ;) Glad you found the video helpful though! :)
Thanks for the video!
Thanks a ton! Extremely helpful. God bless!
You are very welcome, thanks for the comment, I am glad you found it helpful ;)
Thank you for the highly educational content.
Of course, so glad you like it! ;)
Hi and thank you so much for your excellent videos! I love the mineral identification chart at the end, but cannot seem to find it online - do you know a source from where I could download it for my own use?
You're amazing 💥❤️❤️❤️this helped me a lot to understand the concepts.
Thanks so much! I am so glad it helped you out, I had a difficult time learning concepts in optical mineralogy when I took the course, so hearing that I could teach them well enough now to help you is so rewarding :D
Keep continuing... GEO GIRL❤️it's so hard to find a perfect spot like this for geo studies. You're doing great. Sharing love..... 💞
Best explanation with simple words and images👏
Oh wow, thanks so much! That means so much to me given how many other optical min videos are out there, thanks for your support!
I'm from Mexico and I don't know much English, but your channel is very entertaining and also helps me learn English and geology.
Thanks
Thanks so much for the comment, I am so glad you are learning both english and geology from me haha! I wish you the best of luck! ;)
You are doing a great job 👏. Keep it up
Thank you so much, these kind of comments keep me going ;D
12:37. The recombination of the light rays that leads to the measurement of birefringence is a direct result of Einstein's General Theory of Relativity, showing conclusively that the speed of light is the same in all frames of reference: mind blown! 🎇
THANK YOU
I have an optics exam in 2 days and im last minute studying with your vids on them, thank u !!
Oh my gosh, GOOD LUCK! You can do it ;D
Hope my videos are helping!
I have exam tomorrow thanks for the class
How is it possible to get a 30um sample of rock? Which techniques are used to obtain such a thin sample? Thank you.
hey, how can i get a copy of the mineral identification chart you used in your video?
hey from brazillll!!!! love your videos :) currently in my 2rd semester of Geology, greetings from unicamp!!
Hey! So glad to hear that! :D Best of luck! And thank you so much for the support!!
thank you so much for explaining this topic. I have some problem that while representing the fast and slow ray in a mineral, we make a cross sign with one line longer than the other. does long line represents fast ray or slow ray? (in sign elongation topic figure)
I am not sure I totally understand your question, but it may help out if you check out the second optical mineralogy video: ua-cam.com/video/OuXBLIIs0LA/v-deo.html because in that video we go over the fast and slow ray directions in much more detail. :)
Great it was really helpful thanks alot
Thanks, I am so glad to hear that ;D Let me know if there are any other topics you would like me to cover in future videos, thanks :)
My favorite class in college was optical mineralogy ❤
Good for you! Embrace the optical physics concepts and you will be top of the class!
Beautifully explained..👌👌
Thanks! I am glad it came off that way!🙌🏼😊
nice video
Isotropic can be clear like a glass fiber too. They dont always appear black.
Very interesting
thanks
Where can you find that chart to purchase? I cannot find it online so far.
It's a figure in this book: amzn.to/38Lz14Z
I tried to find a way to just buy the chart by itself, but had no luck, I am sorry :/ But if you are looking for a good way to identify minerals in thin section I recommend this book: amzn.to/3CLkWlD rather than the one with the almighty chart haha. This one is geared toward identifying minerals in thin section and includes great pictures and illustrations, whereas the first one focuses on mineralogy & petrology very broadly with only one chapter on optical mineralogy. Hope that helps you out. Sorry I couldn't find you that chart!
Very helful. Thank you
Of course, glad you found it helpful, thanks for the comment!
really helpful.🙂
Thanks, so glad you found it helpful!
If possible please provide the mineral identification chart...
Figure in: amzn.to/3ymGN2T :)
awesome explanation! thanks for sharing this and I ask from you if you can make extra lessons in optical mineralogy in addition to practical optical mineralogy! i will be thankful for you.....
awsome lecture
Thank you! Glad you enjoyed it:)
Osmmm video, love from India💝💝
Thank you! So glad you liked it :D
thanks a lot for sharing this fantastic video..Please, I have a short question..why does the polarized light from the lower polarizer turn into 2 directional vibrating light after going through a thin section.
thanks
So I thought this was a great question so I was just reading about it and it sounds to me like anisotropic minerals exhibit 'double refraction' (in specific orientations) that causes this split of one light ray into two when it passes through, but in other orientations, the mineral will not show this double refraction, and instead the light ray will continue vibrating the way it was before it hit the mineral (this orientation in which the mineral does not refract the light is its extinction position). That is why only isotropic minerals are always in extinction, they never refract the light the way that anisotropic minerals do.
So I hope that at least partially answers your question (in short, the mineral double refracts the light), and I am sure there is a more in depth & physicsy reason why they do this double refraction, but I do not know it haha!
@@GEOGIRL thanks for having time to answer my question... and the explanation seems quiet reasonable.
Loved this mineralogy playlist. Helped me a lot in my researchs
So glad you found this playlist helpful! Thanks for the comment ;D
Thankyou very much mam. I'm from India. I'm preparing for competitive exams, and geology is my optional subject. I can't find any good youtube lectures on geology . Even if there are some videos, they will be in the language apart from English. Your explanation is very clear and it helped me to understand the concepts well. Thank you again, Mam❤️.
So glad to hear you found my video helpful! Best of luck in your exams! :D
So sweet of you mam 😇💫❤️.I'm sure these lectures are very useful to us, who took geology as their optional subject.
Bhaut bahut dhanya bad guruji 🙏🙏🙏🙏
You are very welcome! ;)
Thanks so much from France / Merci 🙏🔬〰️💎〰️🔦 😁
You are so welcome! (de rien) ;)
where can I get this chart, I don't have one in my book.
Unfortunately, I do not know of a way to get the figure without buying the textbook: amzn.to/3DXOUnj. I have looked elsewhere online for the figure, but haven't been able to find it :/
@@GEOGIRL thank you very much.☺️
Good ❤ xpl explain
So light 💡 travels 2.5X slower through your wedding ring 💍 Diamond 💎 than through air?
Yep! Pretty cool huh?!😊
Great 😁😁 actually on instagram I suggest stratigraphy but I forgo that I am much weak in optical thanks for reminding me 😁😁😁😁
Haha, yes! More stratigraphy will come, but I make content months in advance, so it may take a while for me to get to that, I am sorry! I just have soooo many topic ideas it's so hard to pick which ones to make first 🤣
But I am glad to hear you appreciate the optical mineralogy topic, hope the video is helpful for you! 😁
🔥🔥🔥🔥
you haven't explain how we measure the extinction from the accessory plate.
Could you explain that.... The isotropic and anisotropic properties of a mineral depends on which factors....
Whether a mineral is isotropic or anisotropic is just a matter of which crystal system it belongs to. Minerals that belong to the isometric crystal system are isotropic. All others are anisotropic :)
@@GEOGIRL thank you so much....
You are amazing.. and please keeps video uploading
Where can I get the mineral identification chart
It's a figure in this book: amzn.to/38Lz14Z :)
❤️❤️❤️
hi Miss Geo girl, i want a thank you for your explaination, i was great. just i want a ask you if you have some references? (from morocco)
Of course! My references for my videos are always listed and linked in the description of the video. For this video, I used Chapter 6 of a book called Earth Materials by Cornelis Klein & Anthony Philpotts: amzn.to/3DXOUnj :)
As a lapidary artist, this is important to study. Labradorite is one of my more important stones. Frustrating and satisfying. If I cut at an incorrect angle, it is worthless. If I find the correct plane, the colours are intense. I am SO curious how you would make these cutting decisions!
WOW! This is so funny because I go rock hunting with my brother a lot and he's not a geologist so he's always asking me what the rocks are and stuff, but then sometimes he'll ask me how to cut them or polish them to look nice and I am just like.... no idea, you need to talk to a lapidary!😂And now here you are commenting on a video that I never even thought to reference when it comes to cutting rocks lol
Well anyway, I appreciate the comment, but unfortunately the knowledge I shared in this video hasn't gotten me any closer to cutting rocks along their most shiny plane ;) If you have any tips for cutting/polishing rocks yourself, PLEASE let me and my brother know haha, thanks!!
Also, I just found this webpage that has a small section on cutting labradorite that you may find helpful (but you may already know all this) -> geology.com/gemstones/labradorite/
😳. I was thinking that I found a person with some magical scientific recipe 😂. That article was Fascinating, thanks! I will cut some today with these new insights. Sometimes I get it Very right, but certainly not always
@@davejackson9819 Yea, I mean the premise makes sense, like yes, I know what plane to cut to make it most shiny, but no I don't know how to predict whether those planes are in the right orientation when I am cutting haha. Best of luck! Thanks for the comment again, very cool to hear from your perspective :)
@@GEOGIRL it’s a difficult stone. The planes change. A lot.
i please need your assistance on the notes
Could you please tell why unpolarized light cannot be used while working with petrographic microscope
I think it's because when the light is unpolarized (going in any/every which direction), we cannot understand anything useful from the color of the mineral at that point because we don't know the direction at which the wavelengths of light are vibrating. Does that make sense? I am not 100% sure this is the reason, but it's my best guess. Hope it is helpful ;)
@@GEOGIRL thank you so much . My teacher said to find the answer of it😂
@@samakshgupta8750 Haha! Well I hope I am right 😅
@@GEOGIRL thanks could you plz make video on igneous textures
@@samakshgupta8750 I actually already have one! ua-cam.com/video/UJSz2Z7IEXY/v-deo.html :D
Your shirt matches the subject
I got very bad grades in my mineralogy paper and I'm trying to learn the subject again.
Well you are starting off strong by taking the initiative to watch these videos! I hope they help, and if there is anything specific you struggle with that I haven't covered, let me know and maybe I can make a video on it! :D
Thanks for sharing your knowledge 💯.. Very interesting 🐵
Thank YOU for watching and commenting! I am so glad you found the video interesting! :D
The quantum explanation of lighting passing through different densities is way more complicated. Fyi.
Oh yes, for sure, quantum stuff is always wayyy more complicated LOL I don't think I'll ever touch on that hahaha
Awesome coloured twinning on your t-shirt.. does not look like plagioclase though
lol yes! I was wondering when someone was going to mention my shirt haha ;)
#I love you too much...💕💖💝🌼
Isn't the angle of extinction based on it's relation to cleavage, not the elongation direction?
In fact it is both! Cleavage planes are related to the elongate/long direction of the mineral grain and vice versa. Here is a quote from a website I was just reading about how to find extinction angle: "simply line up the cleavage lines/long direction with one of the crosshairs in the microscope, and turn the mineral until the extinction occurs." See how they use cleavage lines and long direction interchangeably? It's because they are dependent on each other. If you line up the cleavage lines with the crosshairs you are also lining up the long direction with the cross hairs, it is just how crystal habit works. If it is easier for you to go by cleavage, then by all means use that, you will still get the right answer :) What a great question, thanks for bringing that up in the comments! I should have mentioned that in the video ;) Anyway, if you want to check out the website I used for that quote here's the link: rehabilitationrobotics.net/how-do-you-find-the-extinction-angle/
@@GEOGIRL Thanks! Great video, I just took my exam today and this helped me study! Curious about igneous petrology, so I'll be checking those videos out soon!
@@jeremiahfunke4801 That's wonderful to hear! I bet you aced it! And I hope the you enjoy the igneous petrology videos ;)
Mineral identification chart link give me!
here, it's a figure in this book: amzn.to/38Lz14Z :)
@@GEOGIRLyou not have pic formats? I am not able to buy that book because I am middle class.
@@Selva..yb3iv I am not able to distribute the figure without adding value (like I do in my lectures) because I am not the copyright owner of the figure. My videos fall under 'fair use' because I add intellectual value, but if I just send it out, I not longer fall under fair use. I apologize. Feel free to try and pause the video and screenshot, it may not be the best quality but it might work :)
@@GEOGIRL ok no problem
@@GEOGIRL thank you for mentioned my name
love this, but i think I might have an attention defect because even in class, i can't focus on anything :( Commented on 5:06
Don't worry, even I have to watch this video in stages haha! It's a lot to take in all at once ;)
It's MINERALS marie, not rocks! - Hank Schrader
With your permission, you can do the translation
wow discover you only now, are you real?:)
Very good, but she does talk so quickly.
How else am I supposed to get through it all? Lol Just kidding! Thanks for the input, I have tried to get better at this in my newer videos, but it's a work in progress ;) Thanks for the comment! I am glad you enjoyed it despite the quick pace :)
@@GEOGIRL seriously I always enjoy and find interesting your videos. Maybe I find your speech a bit rapid because I am an old Englishman 🙂
Please edit the comments and add paragraphs to break it up into smaller bites of information. And it would be better if you slowed down a bit. Give the audience a few seconds to digest what you have said before adding more new information.
Thanks for the input! Yea so I typically do smaller amounts of info on each slide & speak slower for in person talks, but I figure since UA-cam videos can be paused, I should put more info and speak quickly in case people want to get through it fast and those that want it slower can either pause periodcally or slow the speed down with the settings tab. But I appreciate the input and it isn't the first time I have been asked to slow down so I will certainly work on it, thanks again! ;)
@@GEOGIRL Thanks for the information. I'm 71 years old and studied this material 30 years ago when I did asbestos analysis for a laboratory. I'm just waking up early and figured I'd review some PLM minerology this morning. You covered a lot of great information and explain it well. I did pause the video a few times to read the text and then listen to you. Keep up the good work. I subscribed to your channel. I'll be checking your other videos out and will be looking for a video on central stop dispersion staining using different RI liquids and different asbestos fibers. And I want to review setting up the PLM from scratch. I used an Olympus PLM in the past but that was 30 years ago.
@@Moveon10001 Wow, that is very cool! Nice to have someone as knowledgable as you watching my videos ;)
I don't have one on that topic. I am not an expert and I tend to try and stay in the introductory yet still in depth range of conversation so I can cover a wider range of topics. But I hope in any case you will still enjoy some of my other videos. Thanks so much for the support and feedback! :)
And best of luck with your work!
Could u please speak a bit slow... Not able to get ur words
I will try! Thanks for the feedback :)
unlike for no caps, shame on ya
Geological mapping has nothing in particular to .............geology ?
I cant watch the video the constantly moving cursor is dizzying and distracts from the information being conveyed.
Really doesn't unless you're some sort of spastic