Wet chemical analysis (for MnO2, MnO, and H2O(+)) and electron microprobe analysis (for Fe2O3 and PbO) give MnO2 74.91, MnO 11.33, Fe2O3 (total Fe) 4.19, PbO 0.03, H2O (+) 9.46, sum 99.92%. 'Available oxygen determined by oxalate titration method is allotted to MnO2 from total Mn, and the remaining Mn is calculated as MnO. Traces of Ba, Ca, Mg, K, Cu, Zn, and Al were found. Li and Na were not found. The existence of (OH) is verified from the infrared absorption spectra. The analysis corresponds to the formula Mn4+4.85(Mn2+0.90Fe3+0.30)1.20O8.09(OH)5.91, on the basis of O=14, 'or ideally Mn4+5−x(Mn2+,Fe3+)1+xO8(OH)6 (x≈0.2). X-ray single crystal study could not be made because of the distortion of single crystals. But the x-ray powder pattern is satisfactorily indexed by an orthorhombic cell with a 9.324, b 14.05, c 7.956\AA., Z=4. The indexed powder diffraction lines are 9.34(s) (100), 7.09(s) (020), 4.62(m) (200, 121), 4.17(m) (130), 3.547(s) (112), 3.212(vw) (041), 3.101(s) (300), 2.597(w) (013), 2.469(m) (331), 2.214(vw)(420), 2.098(vw) (260), 2.014 (vw) (402), 1.863(w) (500), 1.664(w) (314), 1.554(vw) (600), 1.525(m) (601), 1.405(m) (0.10.0). DTA curve shows the endothermic peaks at 250−370∘C and 955∘C. The former is due to the dehydration: and oxidation forming(Mn,Fe)2O3(cubic, a 9.417\AA), and the latter is interpreted as the formation of a hausmannite-type oxide (tetragonal, a 5.76, c 9.51\AA) from (Mn,Fe)2O3. Infrared absorption spectral curve shows Mn-O stretching vibrations at 515cm−1 and 545cm−1, O-H bending vibration at 1025cm−1 and O-H stretching vibration at 3225cm−1. Opaque. Reflectance 13-15%. Bireflectance distinct in air and strong in oil. Reflection pleochroism changes from whitish to light grey. Between crossed nicols, color changes from yellowish brown with bluish tint to grey in air and yellowish brown to grey through bluish brown in oil. No internal reflections. Etching reactions: HCl(conc.) and H2SO4+H2O2-grey tarnish; SnCl2(sat.)-dark color; HNO3(conc.)-grey color; H2O2-tarnish with effervescence. It is black in color. Luster dull. Cleavage one direction perfect. Streak brownish black to dark brown. H. (Mohs) 2-3, very fragile. Specific gravity 3.59(obs.), 3.57(calc.). It occurs as radiating groups of flakes, flower-like aggregates, colloform bands, dendritic or arborescent masses composed of fine grains in the cementation zone of the supergene manganese oxide deposits of the Janggun mine, Bonghwa-gun, southeastern Korea. Associated minerals are calcite, nsutite, todorokite, and some undetermined manganese dioxide minerals. The name is for the mine, the first locality. The mineral and name were approved before publication by the Commission on New Minerals and Mineral Names, I.M.A.