[Rockhounds] 18 sided beryl

[Peter Richards]

Sorry for the delay, all - we have been in New York for most of a week.  The drawing shows a hypothetical beryl with an 18-sided prism.  The pinacoid and bipyramid are entirely speculative, since the actual specimens seem always to lack terminations.  But you gotta put some sort of top on a crystal drawing!

Beryl can have three kinds of prisms, two of which have six faces and occur in special positions relative to the crystal axes, with one rotated 60° about the six-fold c-axis relative to the other.  The third kind of prism is much rarer and more general in the positions of its faces, and without going into extreme detail, it consequently has 12 faces.  To get a prism with 18 faces, you have to combine one of the six-faced prisms with the 12-faced prism.  Usually, the faces of the 12-faced prism are narrow compared to those of the six-faced prism.  That’s the way I drew it. One sees a face of the 6-prism, then two faces of the 12-prism, and this is repeated all around the crystal.  On the actual crystals, the faces of the 12-sided prism may be much narrower than those of the six-sided prism.

Hope this is clearer than mud….

Pete
___________________________
R. Peter Richards, Ph.D.
rpr at heidelberg.edu
Morphological Crystallographer

> On Mar 5, 2019, at 8:20 PM, <larryrush at att.net> <larryrush at att.net> wrote:
> 
> Pete asked me to post a picture of his drawing of an example .....maybe he
> will explain this a bit for us..
> 
> https://www.irista.com/gallery/sxqqhua9bflr 
> 
> 
> 
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