[Rockhounds] Another extinction impact-13 KYA

[J Bryan Kramer]

That's interesting, wiki has a page on it:

<http://en.wikipedia.org/wiki/Lonsdaleite>

But that drawing of the crystal structure does not look like hexagonal close
packing to me. It looks more like some deformed cubic structure. Mindat
calls it Dihexagonal Dipyramidal, which as a chemist is greek to me.

BK

On Fri, Jan 2, 2009 at 18:03, R. Peter Richards <rpr at heidelberg.edu> wrote:

> 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:
>
>
>>
>>
>>
>>
>> &gt; The abstract mentions two
>> &gt; kinds of diamond particles found, and I think the terminology is a
>> little
>> &gt; 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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-- 

""It often seems to me that the night is much more alive and richly colored
than the day."

                      Vincent van Gogh
J Bryan Krämer
North Florida, USA
photos at:
http://pbase.com/photoburner


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