The chemical that is added to reduce the melting point of the reaction mixture during the extraction of aluminium is:

In the extraction of aluminium by the Hall-Heroult process, alumina (Al$$_2$$O$$_3$$) is dissolved in molten cryolite (Na$$_3$$AlF$$_6$$) before electrolysis. The purpose of adding cryolite is to lower the melting point of the mixture from about 2072°C (melting point of pure alumina) to around 950°C, making the process economically feasible.

Bauxite is the ore from which alumina is obtained, not an additive to reduce melting point. Calamine is an ore of zinc. Kaolite (kaolinite) is a clay mineral.

The answer is $$\boxed{\text{Cryolite}}$$.