Iowaite has been re-examined using new, well crystallized material from the Palabora open pit mine, Transvaal. Microprobe, carbon analyser and thermal analyses, powder and single-crystal X-ray diffraction, infrared, UV-visible and mass spectroscopic and optical studies were made. The ideal formula is Mg₆Fe2III(OH)₁₆Cl₂·4H₂O. It is trigonal, R3¯m, a = 3.1183(9), c = 24.113(8) Å, V = 203.1(2) ų, Z = 3/8; D_obs 2.09 g/cm^3.; D_calc 2.04 g/cm³; hardness (Mohs)=212;. The interlayer Cl⁻ ions are displaced from the threefold axis. It is uniaxial negative, with ω = 1.561 ± 0.002, ε = 1.543 ± 0.002; coloured crystals are intensely pleochroic, due to intervalence charge transfer between the Fe³⁺ and Fe²⁺ substituting for Mg²⁺, with O pale yellow, E deep blue-green.