spot the error II
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Author:  Doos [ Fri Oct 05, 2007 1:57 pm ]
Post subject:  spot the error II


During the last gem-a exam a topaz crystal was handed out and needed to be identified by looks alone.
A sample of such a crystal (taken from the internet):


In cross-section (looking down the c-axis) it looks like a kite:


Now spot the error(s) if any (still looking down the c-axis) in the following image:


Please motivate.
Again for eternal glory.

Author:  HME [ Fri Oct 05, 2007 2:07 pm ]
Post subject: 

Topaz forms in the orthorhombic crystal system, usually dipyramidal class if I remember correctly (2/m 2/m 2/m). Therefore all 3 axes should be at 90 degrees to each other rather than at whatever they are in the illustration.

Author:  Doos [ Fri Oct 05, 2007 2:09 pm ]
Post subject: 

Hi Kevin,

Very good (and fast)!
There is another error aswell.

Author:  HME [ Fri Oct 05, 2007 2:21 pm ]
Post subject: 

I didn't think the obvious lenght of the lines was at issue. If lines a and b are taken to be relative lenghts, then they should not be equal. Orthorhombic crystals have their 3 axes of unequal lengths and at 90 degrees to each other.

At last I get to use that expensive degree for something.... :)

Author:  Doos [ Fri Oct 05, 2007 2:27 pm ]
Post subject: 

Hot dang! You're good.

Here is the correct image:


Eternal glory for Kevin.

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