A mineralogist was a type of geologist that specialised in the study of minerals.
John was a mineralogist on Captain Maitland's ship. (TV: The Sensorites)
Jeff Stone (COMIC: The Humanoids) was a mineralogist from New Paris, Venus. (COMIC: Invasion of the Daleks)
Ethan Kalwell was a mineralogist who went to Jegg-Sau to investigate the minerals there. (AUDIO: The Relics of Jegg-Sau)