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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)