lithium - a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several mineralsmetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. amblygonite - a white or gray mineral consisting of lithium aluminum phosphate; a source of lithium lepidolite - a mineral of the mica group; an important source of lithium spodumene - a pyroxene mineral consisting of lithium aluminum silicate; a source of lithium |