[Rockhounds] 18 sided beryl

[Kreigh Tomaszewski]

I found an 18 sided specimen on MinDat at https://www.mindat.org/gl/6805 (look
in the upper right corner) that is a better picture.


On Mon, Mar 11, 2019 at 8:04 PM Peter Richards <rpr at heidelberg.edu> wrote:

> 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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