lepidolite, the most common lithium mineral, basic potassium and lithium aluminosilicate; a member of the common mica group. It is economically important as a major source of lithium. Because it is one of the few minerals containing appreciable amounts of rubidium, it is useful in determining geological age according to strontium–rubidium ratios. Lepidolite occurs almost exclusively in granite pegmatites. For chemical formula and detailed physical properties, see mica (table).