[Rockhounds] "Hexagonal" Diamond and other mineral structures, was: Another extinction impact-13 KYA

[R. Peter Richards]

We always push "Send" too soon...

Two clarifications:  I meant to say "once you lay ouT the first  
layer" but you probably figured that out.  The other relates to the  
change from one carbon structure to the other - I started out talking  
about changing lonsdaleite into diamond, but wound up pondering the  
fact that diamonds survive the trip to the surface of the earth  
without being converted to graphite (in fact some are converted,  
apparently, but not all).

The point is that all three structures are quite different, so  
changing one into another is a disruptive, or "reconstructive" phase  
change.  This is in contrast, for example, to the change from high- 
temperature beta quartz to standard low-temperature alpha quartz,  
which only involves a kinking of the structure.  This happens almost  
immediately when the right temperature is reached, and can be undone  
and done again just by appropriate heating and cooling. By contrast,  
the change from tridymite or crystobalite to quartz is  
reconstructive, and is slow to happen if it happens at all.  As a  
consequence, we find crystals of tridymite and cristobalite at earth  
surface conditions with their structures basically intact, but quartz  
that started out as high quartz is invariably found as complexly  
twinned low quartz.

There, now I'm done!

Pete Richards



On Jan 2, 2009, at 7:40 PM, R. Peter Richards wrote:
...snip...
>
> 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....

...snip again...
>
> 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?
>

___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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