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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 Mg6Fe~II(OH)16C12.4HzO. It is trigonal, R3m, a = 3.1183(9), c = 24.113(8) A, V = 203.1(2).~3, Z = 3/8; Dobs 2.09 g/cm3; D~c 2.04 g/cm3; hardness (Mohs)=21. The interlayer C1- ions are displaced from the threefold axis. It is uniaxial negative, with ~ = 1.561 _ 0.002, ~ = 1.543 __+ 0.002; coloured crystals are intensely pleochroic, due to intervalence charge transfer between the Fe 3 + and Fe z+ substituting for Mg z+, with O pale yellow, E deep blue-green. K~YwoRos: iowaite, pyroaurite, Palabora mine, Transvaa