Optical Mineralogy Pt.2- Optical Indicatrix, Uniaxial vs Biaxial, Interference Figures | GEO GIRL

Ever get to the optical indicatrix part of optical mineralogy and just close the book because it made no sense? Well this video covers how to use the optical indicatrix and its properties to identify minerals. How does the orientation of a mineral grain under thin section affect its optical properties? All minerals have an optical indicatrix, which is just a fancy word for an ellipsoid we use to simplify the mineral grain and its orientation compared to the light passing through it. We can use special lenses, such as the upper swing-in condensing lens and the Bertrand lens, on petrographic microscopes to tilt mineral grains under thin section to determine the optical indicatrix orientation. What can understanding the optical indicatrix do for us when identifying minerals? Well it can allow us to determine the optic axis or axes, the optic angle, the optic sign, and the interference figures of mineral and use these optical properties for identification. At the end of this video, we summarize all the diagnostic properties useful for identifying minerals that we have gone over in this mineralogy playlist of videos. Enjoy!
References: Earth Materials textbook: amzn.to/3l5xbSf
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!)
0:00 Review & what this video will cover
2:17 How crystal system affects optical properties of minerals
4:28 Intro to optical indicatrix
5:15 The uniaxial optical indicatrix
7:12 Uniaxial optic axis
9:40 The biaxial indicatrix and optic axes
12:03 Biaxial optic angle and optic sign
12:33 How to apply this to minerals under thin section?
13:30 Bertrand lens & condensing lens
16:14 Uniaxial interference figures
17:33 Uniaxial flash figures
18:46 How to determine optic sign of uniaxial minerals
21:57 Biaxial interference figures
22:35 Biaxial optic angle or 2V angle
25:15 Acute bisectrix figure & obtuse bisectrix figures
26:18 Biaxial optic sign
27:57 Review (mineral identification chart)
29:52 Upcoming content (& NEW PLAYLIST!)
31:07 Bloopers!
Directly offset your carbon footprint with Wren: shrsl.com/3d0t2
Non-textbook books I recommend:
Oxygen by D. Canfield: amzn.to/3gffbCL
Brief history of Earth by A. Knoll: amzn.to/3w3hC1I
Life on young planet by A. Knoll: amzn.to/2RBMpny
Your inner fish by N. Shubin: amzn.to/3cpw3Wb
Oxygen by N. Lane: amzn.to/3z4FgwZ
Alien Oceans by K. Hand: amzn.to/3clMx1l
Tools I use as a geologist/teacher/student:
Geology field notebook: amzn.to/3lb6dJf
Geology rock hammer: amzn.to/3DZw8MA
Geological compass: amzn.to/3hfbdLu
Geological hand lens: amzn.to/3jXysM5
Camera: amzn.to/3l6fGRT
Carbon-neutral backpack: shrsl.com/3cvkc
Image sources:
4.bp.blogspot.com/-E3PN_Nk8ud...
amzn.to/3wWF2Wg
i.pinimg.com/474x/c8/f4/c0/c8...
amzn.to/3wWF2Wg
upload.wikimedia.org/wikipedi...
www.mtholyoke.edu/~mpeterso/cl...
www.researchgate.net/publicat...
www.wgtn.ac.nz/sgees/images/e...
image3.slideserve.com/6569585...
viva.pressbooks.pub/app/uploa...
amzn.to/3wWF2Wg
www.tulane.edu/~sanelson/imag...
viva.pressbooks.pub/app/uploa...
opengeology.org/Mineralogy/wp...
viva.pressbooks.pub/app/uploa...
opengeology.org/Mineralogy/wp...
opengeology.org/Mineralogy/wp...
Disclaimer: Links in this description might be affiliate links. If you purchase a product or service with the links that I provide I may receive a small commission, but there is no additional charge to you! Thank you so much for supporting my channel!

[@GEOGIRL]

Hi there! If you'd like to see me do some mineral identification practice with thin section examples, you can watch this video: kzbin.info/www/bejne/Y3fQi3-JaJyEoac 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!!

[@emma11421]

Would you please help send a link for that mineral identification chart, I've tried looking for it but I can't find it

[@TaylorSwift1201]

understood more watching these two videos than in 2 weeks of lecture :)

[@GEOGIRL]

So glad these videos were helpful to you! :)

[@Siqipilaci]

I made my Bachelor in Geoscience in 21. I am so unlucky that you haven’t been on KZbin much before I finished it. Your Videos are Great! Keep going on!

[@GEOGIRL]

Thank you so much! ;D

[@vihanga4141]

Thank god I found you. Explained better than the lecturer 😅❤

[@NeuronToNeuron]

I am just digging back into mineralogy/geosciences and uncovered you! Your videos are like a synaptic duster clearing neural pathways I forgot I had. The curation and delivery of your content makes my brain feel like pegmatite for the first time in a long time, what with how crystallized you make these concepts floating around in it seem. 🪨🤓 I am an instant-fan and will be playing your videos on repeat.❤🌎🪨⛏🔬💎❤

[@GEOGIRL]

I am so happy to hear that! Thanks so much for the kind words and support

[@sib007]

you Summerised my 6 months....thanks love from India

[@GEOGIRL]

Haha! So glad you liked it :) Thanks for the comment, I am glad it came across as information packed because my brain nearly exploded after the research I did for that video hahaha

[@shaakirmahdi]

Pls can u assist me written summery about this video .. ​@@GEOGIRL thnks❤

[@gneissnicebaby]

Like others have said, THANK YOU! I've been doing well at identifying minerals but I was struggling to understand what it means when I report that a mineral is unixial negative or what it means when I'm viewing a flash figure, and what's going on with the extraordinary and ordinary rays. Mineralogy has been the hardest class so far, but I know from others that it gets easier with time and practice. For anyone else struggling with mineralogy, you CAN do it! Just think about it like learning a new language; you're going to struggle, you're going to get it wrong, but everyone did and does at one point or another, just like everyone gets their own language wrong from time to time.

[@GEOGIRL]

Thank you for sending out this message to everyone! It is so true, it comes with time and usage. Also, I love your username! ;)

[@lawarlkila]

I am doing an oral minerals identification test in some few hours today, thats the reason i came looking for this 😅. Thanks, wish me well!

[@GEOGIRL]

Good luck! You got this :D

[@dvk751]

Thank you for making these videos. Thanks to your structured video I actually understand what the professor is trying to explain in class

[@sehran5509]

I had some confusions regarding these topics ive been searching everywhere. My concepts are clear now. Thank you so much.🙏

[@GEOGIRL]

So glad to hear that! ;D Thanks for the comment!

[@nivaanlobo2165]

Just subscribed to your channel, you explained this a lot clearer compared to my professors during undergrad. Thank you and keep up the great work.

[@jackdavis3563]

Wish I found your channel sooner, nice videos.

[@GEOGIRL]

Thanks! Glad you have found them helpful :D

[@abhraacharya135]

It really good to see how you enjoyed while understanding the concepts to us.you're too sweet! Great informative video btw!

[@GEOGIRL]

Aw, thank you so much, I am so glad you think so! I am always trying to make the concepts as well as my personality fun to watch ;)

[@nopassage123]

one of the best lectures. So many topics covered in a short video. loved it❤️❤️

[@GEOGIRL]

So glad you liked it, thanks for the comment! ;D

[@somrinzi]

Hi, Your videos deserve more views. i really like it and in case you ever think of not making any more videos, you are wrong, it is really helpful.

[@GEOGIRL]

Wow, this is such a nice comment to read because I 100% always second guess whether I should keep doing this, and whether my videos really make an impact if there are other videos on the same topics out there, so hearing this really motivates me to continue, thank YOU! ;)

[@rathaiahyv3469]

really useful video with good graphics and examples

[@bradyfox2345]

I’m taking mineralogy right now and this was sooo helpful, thank you!!

[@GEOGIRL]

OMG! I am so glad to hear that! I was worried it did not come across that great, I am so glad you understood it and it helped you :D Thanks for the comment

[@Curious_as_cats]

I like how you say epsilloid instead of ellipsoid 😊. Fantastic videos, I’m learning so much from you. Thank you!

[@joshh2317]

This really helped me thank you!!

[@fazlurrahmanaffiq7968]

Thank you so much, i'm now studying mineralogy and get help by watching your videos

[@GEOGIRL]

You are so welcome! I am so glad my videos could help you :)

[@JoesFirewoodVideos]

This is going to be great video I❤️GEO GIRL

[@Vinevenom]

I wish I found this at the beginning of my semester ! ur videos are great

[@GEOGIRL]

Thank you! And best of luck with the rest of your semester ;D

[@piousshimphrui9475]

I'm learning both English and optical mineralogy 🤭🤭🤭 Thank you so very much, the video really help in my preparation 🙏

[@GEOGIRL]

Haha, killing two birds with one stone I see! So glad you found my video helpful ;) Best of luck learning English & geology! ;D

[@oliverweeweepie3132]

I agree, a video where you show a thin section analysis through a scope would’ve good. Are there any videos like this on KZbin that you can recommend?

[@GEOGIRL]

Not to my knowledge, hopefully I can make some soon enough ;)

[@guezeiabdelali8547]

As always good content, we are waiting for the new playlist 😁

[@GEOGIRL]

Thanks! Glad to hear you are exciting for the upcoming playlist! :D

[@mishkosimonovski23]

Cool stuff.

[@GEOGIRL]

Thanks! So glad you like it :D

[@biswajitrout4575]

Mam plz tell how birefringence of uniaxial mineral depends on crystallographic orientation of mineral thin section when studied under microscope

[@snigdhabhattacharya3214]

Nice explaination👍...but please check the biaxial positive and negative figures

[@ayushkrishnaifs8653]

Great 🎉

[@ella.willmott]

Your Videos really help me! However at my university (ETH) in our script it says: When you add the accessory plate while looking at a biaxial mineral under the bertrand lens, the optic sign should be exactly the other way round than what it says in your video 27:31

[@ella.willmott]

Maybe it’s a mistake from my end, however could you maybe clarify this? :)

[@serhatkoksal9462]

Hi Rachel, thank you very much for your videos. You've put a lot of work into it and it's very useful. I'm waiting for the continuation. In the part about the biaxial optic sign (26:18), the positive and negative ones should be reversed, as far as I can remember. Can you have a look? I wish you healthy and beautiful days.

[@GEOGIRL]

It depends on which way the fast and slow directions are defined on your accessory plate. So here, our accessory plate has the slow direction defined as perpendicular to the length of the plate, but if that had been switched to parallel (as it is in many accessory plates), then yes, the positive & negative here would be opposite. Hope that makes sense ;) Thanks so much for the kind words and support :)

[@serhatkoksal9462]

@@GEOGIRL Thank you very much for the explanation. I'm looking forward to new videos. Regards.

[@TylerHnatiuk]

@@GEOGIRL I want to start by saying this is a great video thank you for taking the time to make it. However, I have to side with the original commenter. On an acute biaxial IF (in NESW orientation) if the area between the melatope subtracts (is yellow) then the parallel plane to the accessory plate must be the fast ray making this mineral optically negative.

[@patrice-or3xx]

Thank you for making these extremly helpfull videos :)

[@GEOGIRL]

Of course! Thanks for your support :D

[@patrice-or3xx]

@@GEOGIRL Do you also use your slides for presentations at your university or only for your KZbin Channel?

[@GEOGIRL]

@@patrice-or3xx At first the historical geology lectures I made and posted on KZbin were reused from lectures I had given at my university, but most of my KZbin content now is purely for KZbin :)

[@hamzakhallaf1295]

thank you so much please where I can get the mineral identification chart

[@GEOGIRL]

Thanks! The identification chart is a figure in this textbook: Earth Materials: amzn.to/3l5xbSf

[@DeRailShow]

You don't have a playlist on structural geology?

[@GEOGIRL]

Nope, I plan to eventually make one, but it will have to be in the far future since I don't currently have proficient knowledge in structural to teach it and I don't currently research that or have time to research it in my free time :/ But hopefully some day!! :D

[@darkgeologist]

how can i get this mineral identification chart?

[@DeRailShow]

Have you made video on different mineral group? Or silica group?

[@GEOGIRL]

I have a few: One really old and simplified video: kzbin.info/www/bejne/jHzKYntpprmFf68 One about general mineral classification: kzbin.info/www/bejne/Y5mypZmqg8adrsU One song about the mineral groups: kzbin.info/www/bejne/qnvCaKilZaiXmJY One song about the crystal systems: kzbin.info/www/bejne/onvImGSeaqtlp9E I am sure I am forgetting some, but this is what I can think of off the top of my head ;)

[@siyacer]

Interesting

[@torbjornskarsgard]

Hi there. Regarding biax optical sign: I believe that the picture and text are incorrect with the set-up as shown - with the Y-axis parallel to the fast axis of the accessory plate. If I’m correct pls keep the setup but change the “signs” as that will give a logical connection to the Uniax examples (I.e. blue & blue = optical negative).

[@torbjornskarsgard]

P.S: I have found a mnemonic for uniax optic sign: ‘NE, NE, NE’ 1) NorthEast (where to look) 2) + NEgative delay (e.g. yellow) 3) = NEgative optic sign This works also for biax - as long as you have the optical plane/axises also NE (y-axis parallel to the insertion slate).

[@ankit5820]

Could you pls share the ppts in pdf or any form for study using them as notes

[@venkatasrikanth245]

you rock girl 🤘😆.

[@GEOGIRL]

Thanks! You do too! :D

[@bread3088]

You saved me

[@muadzfarizul1873]

Can you send me that mineral identification chart please!

[@samyjoseph4798]

You should put spanish subtitles to your videos, they're very well explained

[@GEOGIRL]

Thanks for the input, I will add them ;)

[@fl4sh812]

There is mistake in biaxial interference figures please do check and correct as it can mislead to many

[@SaurabhKumar-dp1rj]

Can u provide us the charts which u had prepared

[@GEOGIRL]

All the images and figures are linked in the video description. The mineral identification chart is a figure in this book: amzn.to/38Lz14Z :) Unfortunately, I am not able to distribute the figures from the book individually (without adding value, like I do in my lectures) because I am not the copyright owner. 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 :)

[@SaurabhKumar-dp1rj]

Thanku for your support

[@barbaradurfee645]

@@GEOGIRL Wow from reading the comments in your two Optical Mineraology videos its surprising how many students have not been given the optical mineralogy identification chart, I would have thought every optical class on the planet would provide that chart. I would also have thought the chart would be available without buying the textbook. Hmmmm

[@samyjoseph4798]

Also, it would be really cool and helpful if you did a microscope instructive video to like see all of this applied 👉👈

[@GEOGIRL]

Haha, I know, unfortunately, I no longer have access to such a microscope, but I have made a video in which I identify minerals in thin section: kzbin.info/www/bejne/Y3fQi3-JaJyEoac so check it out and let me know if you would like me to do more videos like this ;)

[@samyjoseph4798]

@@GEOGIRL Of course, I would love that, I'm coursing mineralogy 2 and optic mineralogy has me dying

[@GEOGIRL]

@@samyjoseph4798 Yea, when I took mineralogy it was my least favorite thing ever, I have a lot more appreciation for it now looking back, but man it can be really difficult!

[@Worldkids136]

can we communicate by privet just i want to talk

[@muhammadpanezai9720]

From yesterday i have started watching your vedios. These are quit helpful and the hardest one in mineralogy huhhhhhh. I am not sure whether i should concentrate on your text or words because you speak too fast and i have to download your vedios to pause and think and then play it again.

[@GEOGIRL]

Haha I know I am sorry it's a bit fast paced, but you can pause as much as you need to ;) Even I have to pause and think when I watch this one back too! lol

[@gabeesp9654]

Hi

[@GEOGIRL]

Hi there, hope you enjoyed the video!

[@gabeesp9654]

@@GEOGIRL (●♡∀♡)