A mineral of variable composition which is also known as lithium mica and lithionite, K₂(Li,Al)₅₋₆- (Si₆₋₇,Al₂₋₁)O₂₀₋₂₁(F,OH)₃₋₄. Rubidium (Rb) and cesium (Cs) may replace potassium (K); small amounts of Mn, Mg, Fe(II), and Fe(III) normally are present; and the OH/F ratio varies considerably. Polithionite is a silicon- and lithium-rich, and thus aluminum-poor, variety of lepidolite.