Bannermanite, a new sodium-potassium vanadate isostructural with F-NaJeOrs

Bannermanite, (Na,K)*V1*V3-lOtr (0.90> x> 0.54), a natural vanadium oxide bronze compound, occurs as black subhedral to euhedral crystals in the oxide zone of a fumarole at Izalco volcano, El Salvador, Central America. The mineral is associated with shcherbin-aite, stoiberite, and ziesite. Bannermanite is monoclinic, space group C2lm, with a: 15.413(7), b:3.615(2),c = 10.066(E) A, p = tOg.Zg(8)",2 = 2, D(obs)- 3.5=0.Zglcm3, and D(calc) = 3.55 g/cm3. The crystal structure of bannermanite has been refined to wR = 0.074 from the vanadium oxide bronze structure model. The structure consists of a clinolattice of VO5 square-based pyramids and VO6 octahedra. This array of V-O polyhedra forms rectangular tunnels in which Na and K ions occur. The vanadium ions are in two charge states, Va+ and V5+, with the number of vanadium ions in the reduced state equal to the number of alkali ions in the tunnel sites; thus, charge balance is maintained. Bannermanite transmits light on thin edges, with a mean index of refraction of 2.2, on the basis of the rule of Gladstone and Dale. The mineral has two cleavages, (lr0l) and (0/rI), and a dark gray-black streak. It is named in honor of the late Dr. Harold M. Bannerman