[Rockhounds] Another extinction impact-13 KYA
R. Peter Richards
rpr at heidelberg.edu
Fri Jan 2 16:40:58 PST 2009
Dihexagonal dipyramidal is the name of the symmetry class within the
hexagonal symmetry system. It simply defines all the symmetry
elements (axes of rotation, planes of mirror symmetry, etc) possessed
by minerals with structures that fall within that symmetry class (of
which there are 32, and only 32). I cannot comment authoratitively
on the claim in wikipedia that lonsdaleite forms by a distortion of a
graphite structure, but I have my doubts. Graphite has carbon bonds
only in the planes parallel to its sheets, and much weaker bonds in
between; diamond and lonsdaleite have carbon-carbon bonds in three
dimensions. Something profound would have to be undone and done up
again to turn graphite into lonsdaleite (maybe that's what Superman
really did...) This is analogous (though maybe somewhat crudely) to
the difference between mica, with sheets of linked silicate
tetrahedra, and quartz, with a 3-D network of linked silicate
tetrahedra. The similarity in properties is not coincidental.
The structure shown in the Wiki article does not look close-packed
because the atoms are shown essentially as points, not at their
effective sizes in the structure.
To get really technical, the difference between the cubic and
hexagonal closest packed structures is that, if you form layers of
spheres (pennies work OK for this analogy), once you lay our the
first layer, with all the spheres (pennies) touching each other, the
place for the next layer to go (to be closest-packed) is over the
triangular holes between the first-layer spheres or pennies. There
are two choices; you can choose one or the other. Which ever one you
choose, it's called choice B. B because the orientation of the first
layer is called A; this does not matter now but it will later. So
now we get to build the third layer. We can either choose the holes
in the second layer (B) that are right under the centers of the atoms
in layer A, or we can choose the as-yet unused position C. Then we
repeat this layering gazillions of times to build a crystal. The
first alternative has the layer sequence ABABABABAB... The second
has the sequence ABCABCABCABCABC... The first creates a hexagonal
closest-packed structure, the second creates a cubic closest-packed
structure. These layers are laid up perpendicular to the c-axis of a
hexagonal mineral, or parallel to the body diagonal, the [111] axis,
of an isometric mineral. See http://en.wikipedia.org/wiki/Close-
packing_of_spheres.
Several interesting points arise from this...
1. How does a growing crystal "remember" what sequence it is using
to build itself? A closest-packed structure cannot repeat a
particular layer twice in a row, i.e. AAABBBABBAA is not closest-
packed, but why don't we get ABABCBCABCBACABABCA or other comparable
garbage?
2. It must be really destructive (in terms of disturbing strong
chemical bonds) to try to change an ABABABAB layering sequence into
an ABCABCABC layering sequence. How can this happen? Does it ever
happen? Is this part of the reason that diamond can get shot up from
the depths of the earth and not get changed in to graphite?
There's even more, but by now most everybody is looking at another
message or something....
Happy New Year,
Pete Richards
On Jan 2, 2009, at 6:16 PM, J Bryan Kramer wrote:
> 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:
>>
>>
>>>
>>>
>>>
>>>
>>> > 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
>>>
>>>
>>>
>>>
>>>
>>> --- StripMime Report -- processed MIME parts ---
>>> text/html (html body -- converted)
>>> ---
>>> --
>>> _______________________________________________
>>> Rockhounds at drizzle Mailing List
>>> Subscription Services:
>>> http://lists.drizzle.com/mailman/listinfo/rockhounds
>>> List Home Page, with a link to the List Usage Policy:
>>> http://www.eclecticlapidary.com/Rockhounds/index.html
>>>
>>
>> ___________________________________
>> R. Peter Richards
>> rpr at heidelberg.edu
>> Morphological crystallographer
>>
>>
>>
>>
>>
>> --- StripMime Report -- processed MIME parts ---
>> multipart/alternative
>> text/plain (text body -- kept)
>> text/html
>> ---
>> --
>> _______________________________________________
>> Rockhounds at drizzle Mailing List
>> Subscription Services:
>> http://lists.drizzle.com/mailman/listinfo/rockhounds
>> List Home Page, with a link to the List Usage Policy:
>> http://www.eclecticlapidary.com/Rockhounds/index.html
>>
>
>
>
> --
>
> ""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
>
>
> --- StripMime Report -- processed MIME parts ---
> multipart/alternative
> text/plain (text body -- kept)
> text/html
> ---
> --
> _______________________________________________
> Rockhounds at drizzle Mailing List
> Subscription Services:
> http://lists.drizzle.com/mailman/listinfo/rockhounds
> List Home Page, with a link to the List Usage Policy:
> http://www.eclecticlapidary.com/Rockhounds/index.html
___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer
--- StripMime Report -- processed MIME parts ---
multipart/alternative
text/plain (text body -- kept)
text/html
---
More information about the Rockhounds
mailing list