Occurs as black subhedral to euhedral crystals in the oxide zone of a fumarole at Izalco Volcano, El Salvador. Bannermanite is monoclinic, space group C2/m, with a = 15.413(7), b = 3.615(2), c = 10.066(8) angstrom, beta = 109.29(8) degrees , Z = 2,D(obs) = 3.5+ or -0.2 g/cm 3 , and D(calc) = 3.55 g/cm 3 . The crystal structure of bannermanite has been refined to wR = 0.074. Bannermanite transmits light on thin edges, with a mean index of refraction of 2.2. The mineral has two cleavages, (h0l) and (0kl), and a dark gray-black streak.--Modified journal abstract.