This is an interesting question, particularly considered in the
context that Cairns-Smith (1985) even suggested that clays
(silicates in solution) may have had some sort of early
selection acting on them due to their surface chemistries.
However, there are a number of major problems with Silicon. Some
are chemical and some are astrophysical in nature. For example:
- Silicon has a lower electronegativity than carbon and a longer
bond length. Silicon can polymerize, but many conformations
(such as rings) are highly reactive or unstable. - Silicon lacks
chirality. Since biochemical reactions are very specific this
may present a fundamental problem for alien biochemistries. - We
don't see silicon macromolecules in nature. Large carbon
molecules are seen in space such a polycyclic aromatic
hydrocarbon rings. The largest silicon molecule seen in space is
a chain of SiC_3 (and maybe SiC_4). - On reacting with oxygen
(which it does readily) silicon likes to form solids like sand.
- Silicon is much less common than Ca