potassium - a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvitemetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. carnallite - a white or reddish mineral consisting of hydrous chlorides of potassium and magnesium; used as a fertilizer and as a source of potassium and magnesium kainite - a white mineral consisting of magnesium sulphate and potassium chloride; a source of potassium salts; used as a fertilizer langbeinite - a mineral consisting of potassium magnesium double sulphate; used as a fertilizer sylvine, sylvite - a mineral consisting of native potassium chloride; an important ore of potassium that is found in sedimentary beds |