Abstract: 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) Å3, Z = 3/8; Dobs 2.09 g/cm3.; Dcalc 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.