Symmetry Operations, Types of Twinning, & Miller Indices of Crystal Planes- Mineralogy | GEO GIRL

Understanding symmetry elements and operations, twinning in minerals, and miller indices of planes is important in mineralogy and crystallography. This video covers the 4 major symmetry operations (mirrors, rotation axes, centers of symmetry or inversion, and rotoinversion axes), twinning (twin elements such as twin planes, twin axes, and twin centers, and types of twinning such as contact twins, penetrated twins, and multiple twins), and miller indices of cleavage planes (first I go over what a mineral lattice and unit cell is then a miller index, how to figure out a miller index, then we do practice!), lastly we go over the importance of miller indices (determining d-spacing with miller indices and x-ray diffraction (XRD), as well as Bragg's Law!). This lecture leads into the upcoming lectures on XRD and optical mineralogy.

0:00 4 symmetry operations
0:38 mirrors and rotation axes
5:06 centers of symmetry or inversion
6:17 rotoinversion axes
7:01 twinning crystals
10:14 cleavage planes & miller indices
11:41 unit cells in crystal lattices
14:06 miller indices explained
18:15 miller indices practice
27:54 why do miller indices matter?
30:37 upcoming content!
32:31 bloopers

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[larsfredriksson]

For the upper orange one, move the origin to the front down right corner of the cube. For the upper middle one, move the origin to either of the down right corners. Upper right: Move origin to either of the down back corners. One is always free to move the origin so that the indices can be found. The reason for this being possible is that these planes repeat themselves over and over again; they are fixed in the individual unit cell but not fixed between the unit cells. Alternatively and equivalently, one may hence move the plane itself but keep the origin constant.

[green_tuber]

At 22:54 so genuine to say "we don't know" 😆😆. That is really great. At least we know that we have something to figure out ourselves. Instead of just believing whatever we have told.

[williamnajera]

I watched the full video and laughed at the end when you said you were explaining the fundamentals of Miller indices without really understanding it, and I guess my understanding is that the light of a crystal or jewel diffracts these indices onto each other; I have a jewel tray that I like to put on the windowsill so the sunlight will shine celestial light beams onto the ceiling and the light signatures seem to mix together more than they would shine greater amounts of speckles; I also noticed that sunlight grows a stronger current in a limited spectrum like when it's shining through the window shutter blinds and this seems like it could be measured with the intensity of the light refracting or in this case diffracting off of a jewel. Thank you for making this video!

[lawrencekosinski-jackson]

I’ve been stumbling over learning this section in my Gem Identification Course and your video just came over my recommended and I can honestly say I’m so grateful and found a new song I can listen to that helps me learn what’s needed to pass my courses… I wish my grandfather was still here so I could have shown him this. Thank you 🙏 ❤

[binhaowang]

Great video! A minor comment for better understanding the indices for some of the "weird" cases. The Miller indices show the normal vector (pole) of the plane and are scaled so that each component is an integer. Calculating the reciprocals of the intercepts is just one specific way to get the normal vector of a plane. Hope this helps!

[JoesFirewoodVideos]

YAY! I get to learn about crystals tonight from my favorite geologist
I ❤️ GEO GIRL!

[babyblossom]

Hey geo girl! I love love watching you videos to learn geology. you make it so easy to grasp the concepts which i used to find so hard, with your fun and entertaining videos. now these topics dont feel like a chore to me .for that you are a blessing indeed. I am really waiting for you to cover braggs law and some more indepth mineralogy videos.... Thank you soo much and much love to you

[williamphillips]

Bragg's Law OMG!!!! not many youtube videos on this!

[Anonymous-vnik]

For the negative values for the indices, I think of it as shifting the plane being observed along the axes till it forms a polygon with the origin being the polygon's one vertex not touching the plane.

[emmah]

You are single handedly helping my mineralogy grade so much

[yvonnejansen]

Bless your heart! Super helpful! I was super confused in my geology class. I appreciate it <3

[burninsmoke]

Thank you so much I really appreciate your hard work

[maleehakhan]

Thank you sooo much for posting this! It’s
is SO helpful for my biophysics class!

[ruatfelahrahsel]

your videos are so good, i can understand them easily

[abhilashsundaram]

You’ve a very delightful personality and the content is great as well. A little bit of work on the animation bit will thrust your channel into the elite league.
Ps: I’ve been watching your videos for a geology exam that i’d be writing. I had no clue since I majored in physics, but your videos have been super helpful. Thanks!

[BorisNVM]

Apparently the miller indices show components of the vector normal/perpendicular to the plane. I think it's a basic concept to get, I'd argue it is enough to learn what it is a vector and its components and what is a normal vector to a plane. It could confuse at first because (111) and (222) would represent the same thing.

[murph_mustela]

I can't believe I didn't find your channel earlier; please improve your audio quality as the echo is really frustrating - otherwise so helpful, don't stop please!

[SpottedWeasel]

You make learning very enjoyable! Thanks for that, keep it up :)

[toughenupfluffy]

20:46. The issue I see with the bar 110 is that the origin is set at the forward face of the cube, with the a2 axis going into the paper; whereas the normal placement that I've ever seen has the origin at the rear face, with the a2 axis coming out of the paper. This orientation, I think, is why the a1 is negative instead of positive. All other resources I've ever seen regarding Miller indices use the second orientation, giving this plane a 110 without the negative (bar 1).

[Enkaptaton]

Did you know that you have to rotate an electron 720 degrees before it looks the same again?

I think the example for when you need to do Miller Indices is needed in the beginning of the video. You need every bit of motivation if you want to get through this stuff Hahaha

Very very cool channel btw. I study medicine, but I am interested in stones sice i was a child. Now learning the basics feels great, thank you for this :)