Abstract

Högbomite-group minerals are complex Fe-Mg-Zn-Al-Ti oxides related to the spinel group. Their polysomatic structure is composed of spinel (S) and nolanite (N) modules. The new polysome magnesiohögbomite-2N4S (IMA 2010–084) was found in the Sør Rondane Mountains, East Antarctica. It occurs in Mg-Al-rich, Si-poor skarns, characterized by a corundum-spinel-phlogopite-clinochlore assemblage. The new magnesiohögbomite polysome formed during the retrograde metamorphic stage.

Magnesiohögbomite-2N4S appears macroscopically orange red, the streak is light orange colored. Euhedral crystals are hexagonal plates or prisms with cleavage planes on {001}. The mineral is optically uniaxial (−) and pleochroic with O = reddish brown and E = pale brown. The mean refractive index calculated from reflectance data in air at 589 nm is 1.85(3). The calculated density is 3.702(2) g/cm3. The Mohs hardness is 6.5–7, and VHN300 = 1020–1051, mean 1032 kg/mm2.

The crystal structure of the new polysome magnesiohögbomite-2N4S has been solved and refined (R1 = 2.74%) from single-crystal XRD data. The crystal chemical formula is T10M24O46(OH)2 where T and M represent tetrahedral and octahedral sites. The mineral is hexagonal, space group P63mc (no. 186), a = 5.71050(10), c = 27.6760(4) Å, Z = 1, V = 781.60(2) Å3. The strongest lines in the powder XRD pattern [d (Å), I (%), hkl] are: 2.8561(4), 37, 110; 2.6120(3), 39, 109; 2.42818(16), 100, 116; 2.4160(4), 39, 1010; 2.01181(13), 50, 208; 1.54892(16), 35, 2110; 1.42785(6), 57, 220. Strongest peaks in Raman spectra are at 302, 419, 479, 498, 709, 780, and 872 cm−1, with a broad OH-characteristic absorption around 3400 cm−1. The mean chemical composition (wt%) is SiO2 0.05, TiO2 7.08, SnO2 0.15, Al2O3 66.03, Cr2O3 0.02, Fe2O3 0.50, FeO 4.87, MnO 0.06, MgO 18.71, CaO 0.01, ZnO 0.96, NiO 0.01, CoO 0.02, F 0.06, Cl 0.01, H2O 1.00, sum 99.51. The simplified formula is (Mg8.2Fe1.2Zn0.2)2+(Al22.7Fe0.1)3+ Ti4+1.6O46(OH)2 and ideal formula is Mg10Al22Ti2O46(OH)2. This mineral is a solid solution between the two ideal end-members, (Mg, Fe, Zn)102+(Al, Fe)223+Ti24+O46(OH)2 and (Mg, Fe, Zn)82+(Al, Fe)263+O46(OH)2.

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