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 Post subject: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 4:54 pm 
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Hey guys.

I am just trying to understand crystal symmetry still (I think I need a thorough video presentation for my brain!) I understand now and can visualise all the axes of symmetry except 4 three fold axes. Now I understand that these go from corner to corner of a cube - 8 corners, 4 axes...... but how can it only be 3 fold - wouldn't a cube look the same 4 times it if was rotated around this axes?

Thank you to anyone who can explain this in blonde ;)

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 5:15 pm 
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Just a wee beginning


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 5:24 pm 
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Thanks Barbra.....

I'm still confused about the 4 three fold axis though - if you rotated the cube around the four corner to corner axes - wouldn't it be four fold?

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 5:33 pm 
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I'm a blonde... 8)

Lucy... try a dodecahedron rather than a cube and it will be immediately clear what they mean with 4 3-fold axes.

May I suggest you download google sketchup and load this model into it. Read its description. Figure out how to work with sketchup here. (its REALLY EASY)

All other crystal systems can be downloaded via Google's 3D warehouse.

I'm sure you'll figure it out...


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 6:02 pm 
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You can try this page too:
http://www.uned.es/cristamine/cristal/s ... nglish.htm

Go to "isometric system", choose "cube (hexahedron)" and then click on the lable "3-fold axes" in the picture...

Interactive stereographic projections are curious too.

I had a lot of fun making these pictures and flash animations 10 years ago... :)

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 6:05 pm 
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Egor, that is great!!!!


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Mon Jun 25, 2012 6:07 pm 
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Here Lucy, just in case you don't have much time...


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 4:56 am 
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Ha! thanks Tim and Egore.

Amazing what you mind gets fixated on - cube, must therefore be be square, must therefore be 4!!!

Urgh - that was a serious blonde moment!

Cheers from England!

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 6:17 am 
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You just need to see it once... :D

Anybody running into the same problem could always download the paper models provided for every crystal form over at webmineral.com and create a cardboard reference set as an alternative to digital 3D modelling.


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 10:00 am 
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Thanks again Tim!

Now onto the rest!

It just seems to me, that the Gem-A course notes don't want to expand, they just want you to remember the numbers - but if I don't understand why, I can't remember them! All I want is diagrams of each system, detailing where all these damn axes and planes are :) as sometimes they aren't so obvious :( My tutor isn't so on the ball unfortunately!

I will post any decent links I find here for anyone else confused in the future :)

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 10:11 am 
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The older notes had such diagrams...

This may help too: http://www.gemologyproject.com/wiki/ind ... e=Symmetry


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 10:26 am 
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Alain made those diagrams a million years ago. They are very good.


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 11:39 am 
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yeah they are great diagrams, I have referred to them a million times too :) I wonder why they got rid of the diagrams in the coursework :(

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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 3:38 pm 
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I thought they had stopped teaching essential symmetry axes for the several systems. Glad to see I was wrong


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 Post subject: Re: Crystal Systems - confused with the axes
PostPosted: Tue Jun 26, 2012 5:21 pm 
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Its not actually in the course notes in any detail frank. Just to be able to answer the questions in the exam I like to fully understand and not just learn the answer parrot fashion :(

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