A mineral was a naturally occurring substance that was solid, inorganic and had an ordered atomic structure.
Minerals could be purposed in a variety of ways. Some functioned as power sources, such as bernalium or genelliums. (TV: The Wheel in Space, AUDIO: The Invasion of E-Space)
Some minerals, like duralinium, were used in construction. (TV: The Monster of Peladon) They could be components, as Jethrik was for certain types of warp drive engines. (TV: The Ribos Operation)
Others had stranger effects. Cerulium (found on Ceres) had the property of inhibiting free will. (TV: Sirens of Ceres) Radioactive thaesium was used by Solonians to cause the "mutations" that were part of their life cycle. (TV: The Mutants)
Dark planets could support life, despite the lack of adequate light, as radioactive minerals were present. (PROSE: The Dark Planet)