Summary: Bideauxite is a new mineral found in specimens from the Mammoth-St. Anthony mine, Pinal County, Arizona. The composition is Pb₂AgCl₃(F,OH)₂ with Pb, Ag, Cl, F by duplicate wet analyses. Z = 16, G. calc. 6·256, meas. 6·274 (at 24·2 °C). Crystals are isometric hexoctahedral with a 14·132 Å. Space group Fd3m. Crystals show a, o, d, m {113}, μ {114}, n {112}, φ {116}, and {029} in decreasing order of importance. Colourless, transparent with white streak. Isotropic with n_D 2·192. le, almost sectile, withc onchoidal fracture. H = 3. Turns pale lavender in strong light. Low M.P. B.P. Decomposes in acids and warm NH₄OH. Bideauxite is an oxide zone mineral found with leadhillite, matlockite, and anglesite. May alter to cerussite.