Abstract

Bernalite is a new iron hydroxide from the Proprietary mine at Broken Hill, New South Wales, Australia. The mineral occurs as flattened pyramidal to pseudo-octahedral crystals up to 3 mm on edge with concretionary goethite and coronadite. The crystals are dark bottle green, with a vitreous to adamantine luster. Bernalite has a pale green streak and is brittle with no cleavage but an uneven to conchoidal fracture and a Mohs hardness of 4. Optical data are incomplete due to the effects of twinning; the indices of refraction are in the range 1.92–1.94. Chemical analysis gave Fe2O3, 65.53; SiO2, 2.99; ZnO, 1.13; PbO, 2.70; H2O, 25.2; CO2, 1.0; total 98.55 wt%. The simplified formula is very close to Fe(OH)3. Bernalite is pseudocubic but single-crystal studies gave an orthorhombic cell with a = 7.544(2), b = 7.560(4), c = 7.558(2) Å, V = 431.0(3) Å3. The density is Dmeas = 3.32(2) g/cm3, Dcalc = 3.35 g/cm3. The crystal structure was solved by Patterson methods, but full refinement was not possible because of the effects of polysynthetic twinning. The best refinement in space grotp Immm with Z = 8 gave R = 0.106, Rw = 0.l12, using a set of 785 reflections, of which 462 were considered observed [I > 3σ(I)]. Bernalite has a distorted ReO3-like structure, consisting of a three-dimensional network of corner-connected Fe(OH)6. octahedra.

The strongest lines in the X-ray powder pattern are [dobs(Å), Iobs, hkl] 3.784 (100) (200, 020, 002); 2.676 (15) (220, 202, 022); 2.393 (16) (310, 130, 013, 031, 301, 103); 1.892 (10) (400, 040, 004); 1.692 (17) (420, 240, 204, 402, 024, 042); 1.545 (9) (422, 242, 224). The name is for the British crystallographer J. D. Bernal (1901–1971).

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