[extropy-chat] Scientists Confront 'Weird Life' on Other Worlds

[Robert J. Bradbury]

On Sat, 15 May 2004, Alejandro Dubrovsky wrote:

> Pure diamond melts at 4100K, highest melting point of any substance.

Hmmmm....  The problem may be with the question of "pure".
Nanosystems, pg 150, derived from Hamza, discusses the fact that
hydrogenated diamond surfaces undergo rearrangement at 1275K
and my Handbook of Chemistry and Physics contains some interesting
notes about diamond transforming into graphite and subliming
at different temperatures at different pressures.  There would
seem to be some lack of clarity on this topic.

Re: CO2 vs. SiO2.

> CO2 doesn't but SiO2 does i think.  SiO2 in solid does not form a
> discrete molecule but is part of a covalent lattice.  Si does not form
> double bonds with its oxygen atoms, it forms four single bonds with four
> separate oxygens and shares each of those oxygens with another Si atom,
> so when it melts, or boils it "breaks".  Molecules of gaseous SiO2 don't
> boil away from a solid (i am not a chemist, and one will hopefully step
> in to clarify, but this is my best guess).  CO2 on the other hand
> remains in its small molecule form loosely held by non-covalent forces
> in a solid, so it melts (or sublimes) easily, but each CO2 molecule
> survives.

I will accept this as a plausible explanation.
(and thanks for contributing it!)
Though it still doesn't make me really happy as to *why* this is
the case -- but that may require my looking at some of the URLs
that Bill has posted.

Robert