[Rockhounds] Another extinction impact-13 KYA

[R. Peter Richards]

I think the reference is to a diamond-like mineral with a hexagonal  
atomic structure, not a hexagon-shaped diamond.  Normal diamond  
crystallizes in the isometric (cubic) symmetry system.  Look up  
lonsdaleite in a mineral reference book; it's the analog for diamond  
in the hexagonal symmetry system.

Since graphite, diamond, and lonsdaleite are pure carbon, they can be  
considered structures formed from spheres (atoms) of one size only.   
There are two different ways to put together spheres as densely as  
possible (i.e. with as little open space among the atoms as  
possible); these are referred to as "closest packing" structures.   
Diamond structure has one form - cubic closest packing - and  
lonsdaleite has the other - hexagonal closest packing.  Graphite has  
a lot of open space - it is not a close-packed structure.

Dana's System Volume 8 mentions that lonsdaleite is often associated  
with diamonds and with meteorites or impact sites, and is generally  
microscopic.

I think this is what is being referred to as "hexagonal diamonds".

Pete Richards


On Jan 2, 2009, at 4:05 PM, donhalterman at verizon.net wrote:

>
>
>
>
>
> > The abstract mentions two
> > kinds of diamond particles found, and I think the terminology  
> is a little
> > more technical than six-sided diamonds.
>
>
> Hi,
>
> Unfortunately
> I don't have time to read these linked articles, but I've seen some
> posted references to "hexagonal diamonds," and I'm thinking the
> following might help (or maybe not...)
>
> A hexagon is a
> 2-dimensional figure, which is Greek for "six sides." On the other
> hand, a six-faced, equilateral, 3-dimensional solid is--a cube! Take a
> look at a cube and count the faces. The crystallographic name for a
> cube is a hexahedron. There are cubic diamonds; in fact, former list
> member John Betts sells cubic diamond crystals on his website. These
> form on earth as well.
>
> I don't know if that helps with the discussion here, but at least  
> it should be a fun fact.
>
> Best,
> Don
>
>
>
>
>
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___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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