Wikipedia: Carbon chauvinism

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Carbon chauvinism is a term used in xenobiology to describe the viewpoint of those who assert that because carbon is the basis of all life on Earth, it must also be the basis of all possible forms of life.

The most common other proposed basis is silicon, since silicon has many similar chemical properties to carbon. Silicon has a number of handicaps as a carbon analogue, however. Silane?s (hydrogen-silicon compounds analogous to the alkane hydrocarbons) are highly reactive, with long-chain silanes spontaneously decomposing. Also, [silicon dioxide]? (the analogue of carbon dioxide) is a non-soluble solid at the temperature range where liquid water is possible, making it difficult for silicon to be introduced into water-based biochemical systems. It is possible that silicon compounds may be biologically useful under more exotic environmental conditions, however, in a role less directly analogous to carbon.

One such possible application is as a component of machine life. It is possible, in principle, to construct a machine or a system of machines that is capable of replicating itself from raw ores and natural energy sources. Such a machine system could be considered alive, in that it is capable of evolution through mutational errors in its inherited design patterns, but is in no way required to be composed of carbon-based compounds. The most detailed proposition for machine life made so far considered self-replicating lunar factories, for example, an the Earth's moon is extremely carbon-poor.

Related to machine life is the concept of self-replicating nanotechnology, sometimes referred to as "grey goo" when it is operating without programmed limitations. Nanotechnology, like larger scale machines, could potentially be made of non-carbon-containing materials. Although both diamondoid? and buckytube?s are commonly proposed materials for use in nanomachines, neither of these forms of carbon is used by life as it is currently known, and furthermore it is often proposed that nanotech devices will operate without the water environment that life as it is currently known requires.

Chlorine is sometimes proposed as a biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones.