I'm posting this answer to consolidate some points raised in the original answers by various people, and expand/clarify points that could have been better-written answers.


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First of all, let's clarify what “silicon based” bio-molecules might entail.  It's well-known that a difference between Si and C is that Si has higher binding strength, and things like silicon grease are produced for high temperature use.  So, we conclude that Si analogues of Carbon-bearing molecules would need (and withstand) higher temperatures.

Well, that's not quite right.  Our grease and silly putty and caulk contain Si, but are not direct analogues to hydrocarbon molecules.  They contain alternating silicon and oxygen as a replacement for carbon in the main chain, and furthermore the side chains are conventional carbon-containing chains.

So, high temperature molecules would not be simply silicon based, and still needs carbon.  Freitas points out that the increased temperature tolerance is modest and probably not worth it.

On the flip side, pure silicon chains, analogous to carbon chains, are possible at cryogenic temperatures.  Furthermore, liquid nitrogen is a suitable solvent.  So rather than the Horta, we have Nadreck the Palainian.  But, this raises the issue that the energy levels for metabolism would be too high and you would blow everything apart instead of doing the combining and splitting of other molecules that being alive is all about.


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work in progress...