• journal_title：American Mineralogist
• Contributor：Toshiaki Shimura ; Junji Akai ; Biljana Lazic ; Thomas Armbruster ; Masaaki Shimizu ; Atsushi Kamei ; Kazuhiro Tsukada ; Masaaki Owada ; Masaki Yuhara
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.3827
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：2-3
• firstpage：268
• section：Articles

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) ų. 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 (Mg_8.2Fe_1.2Zn_0.2)²⁺(Al_22.7Fe_0.1)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.