HIBBINGITE, GAMMA-FE2(OH)3CL, A NEW MINERAL FROM THE DULUTH COMPLEX, MINNESOTA, WITH IMPLICATIONS FOR THE OXIDATION OF FE-BEARING COMPOUNDS AND THE TRANSPORT OF METALS

Hibbingite is a new divalent iron hydroxychloride in the atacamite family from the Duluth Complex, Minnesota. It occurs as vein fillings in drill core of partially serpentinized, troctolitic rocks. It is associated with serpentine, olivine, plagioclase, biotite, and secondary magnetite or goethite. Grain sizes can be up to 700 mum long and 100 mum wide. Electron microprobe analysis yielded an empirical formula (based on O + Cl = 4 and OH by difference) of (Fe1.72Mg0.21Mn0.06Si0.01)SIGMA2.00(OH)3.00[Cl0.87(OH)0.12]SIGMA0.99. The simplified formula is gamma-Fe2(OH)3Cl. Its d(calc) is 3.04 g/cm3; it is soluble in H2O and ethanol. The mineral is slightly pleochroic and colorless to pale green when unoxidized but becomes increasingly reddish with oxidation. It may manifest a parting or cleavage perpendicular to vein walls. The approximate index of refraction is 1.6-1.7. It has first-order gray birefringence and is length-fast with parallel extinction. In reflected light, bireflectance is observable under crossed polars. Fresh samples show greenish internal reflections, which become reddish with oxidation. The IR spectrum shows a strong peak at 3552 cm-1 due to the OH- stretching vibration. The crystal field band of Fe2+ occurs at 12550 cm-1. Oxidized samples show a Fe2+ --> Fe3+ intervalence charge transfer band at about 20000 cm-1. Electron diffraction data agree with X-ray diffraction data of similar, unnamed specimens from the literature. The d(obs) (hkl) of some electron diffraction lines, in Angstroms, are 7.08 (001), 5.68 (011), 5.07 (110), 4.60 (020), 4.20 (111), 3.70 (120), 3.55 (002), 2.93 (112), 2.37 (202), 2.30 (040), 2.14 (113), 1.90 (203), 1.65 (242). The data were indexed on an orthorhombic unit cell by analogy with beta-Mg2(OH)3Cl, giving a = 6.31(6), b = 9.20(4), c = 7.10(7) angstrom, V = 412.17 angstrom3, and Z = 4. The probable space group is Pnam, which is also by analogy with atacamite-type compounds. The occurrence of hibbingite has implications for serpentinization processes, sea-floor alteration by hydrothermal fluids, and metal transport in Cl-bearing solutions. In the Duluth Complex, it probably formed as a result of Cl-rich fluids participating in serpentinization reactions. It may be a previously unrecognized intermediate phase during such processes as the corrosion of steel in brines and the terrestrial weathering of meteorites. Hibbingite is named after the city of Hibbing, Minnesota, and type material is preserved in the Drill Core Library of the Department of Natural Resources, Hibbing, Minnesota.