Magnesiohogbomite-2N4S: A new polysome from the central Sor Rondane Mountains, East Antarctica

Abstract

Hogbomite-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 magnesiohogbomite-2 N 4 S (IMA 2010–084) was found in the Sor Rondane Mountains, East Antarctica. It occurs in Mg-Al-rich, Si-poor skarns, characterized by a corundum-spinel-phlogopite-clinochlore assemblage. The new magnesiohogbomite polysome formed during the retrograde metamorphic stage. Magnesiohogbomite-2 N 4 S 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/cm 3 . The Mohs hardness is 6.5–7, and VHN 300 = 1020–1051, mean 1032 kg/mm 2 . The crystal structure of the new polysome magnesiohogbomite-2 N 4 S has been solved and refined ( R 1 = 2.74%) from single-crystal XRD data. The crystal chemical formula is T 10 M 24 O 46 (OH) 2 where T and M represent tetrahedral and octahedral sites. The mineral is hexagonal, space group P 6 3 mc (no. 186), a = 5.71050(10), c = 27.6760(4) A, Z = 1, V = 781.60(2) A 3 . The strongest lines in the powder XRD pattern [ d (A), I (%), hkl ] are: 2.8561(4), 37, 110; 2.6120(3), 39, 109; 2.42818(16), 100, 116; 2.4160(4), 39, 10 10 ; 2.01181(13), 50, 208; 1.54892(16), 35, 21 10 ; 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 SiO 2 0.05, TiO 2 7.08, SnO 2 0.15, Al 2 O 3 66.03, Cr 2 O 3 0.02, Fe 2 O 3 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, H 2 O 1.00, sum 99.51. The simplified formula is (Mg 8.2 Fe 1.2 Zn 0.2 ) 2+ (Al 22.7 Fe 0.1 ) 3+ Ti 4+ 1.6 O 46 (OH) 2 and ideal formula is Mg 10 Al 22 Ti 2 O 46 (OH) 2 . This mineral is a solid solution between the two ideal end-members, (Mg, Fe, Zn) 10 2+ (Al, Fe) 22 3+ Ti 2 4+ O 46 (OH) 2 and (Mg, Fe, Zn) 8 2+ (Al, Fe) 26 3+ O 46 (OH) 2 .