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Polysomatism in högbomite: The crystal structures of 10T, 12H, 14T, and 24R polysomes

Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, reiestr. 3, CH-3012 Bern, Switzerland

Correspondence: ^* E-mail: armbruster{at}krist.unibe.ch

Högbomite is a closest-packed polysomatic mineral composed of spinel, T₂M₄O₈, and nolanite-like, TM₄O₇(OH), modules where T stands for tetrahedrally and M for octahedrally coordinated cations. The modules are stacked in an ordered fashion in various ratios. Single-crystal X-ray diffraction for a 24R and a 10T polysome and structure modeling for a 12H and 14T polysome have been applied to characterize different stacking variants.

Högbomite from a spinel-phlogopite schist at Corundum Creek (South Australia) with composition Mg_3.8Fe_3.2Zn_1.6Ti_1.0 Al_18.3O₃₈ (OH)₂ is a 10T polysome with a = 5.723(1), c = 23.026(4) Å, space group Pm1, Z = 1. This polysome with the general formula T₈M₂₀O₃₈(OH)₂ is composed of an alternation of spinel (S) and nolanite-like (N) blocks stacked in the sequence NSSNS.

Högbomite from a Fe-Ti deposit at Liganga (Tanzania) with composition Mg_13.5Fe_5.6(Zn,Mn,Ni)_0.2Ti_4.7Al_41.7(Cr,Ga)_0.2O₉₀(OH)₆ is a 24R polysome with a = 5.7145(7), c = 55.056(5) Å, space group Rm, Z = 1. The structure with the general formula T₁₈M₄₈O₉₀(OH)₆ is composed of a periodic alternation of two S and two N blocks.

The crystal structures of högbomite-12H, T₁₀M₂₄O₄₆(OH)₂, a = 5.7, c = 27.6 Å, space group P6₃mc, Z = 1, and högbomite-14T, T₁₂M₂₈O₅₄(OH)₂, a = 5.7, c = 32.2 Å, space group Pm1, Z = 1, were modeled from the stacking principles of the known 6T, 8H, 10T, and 16H polysomes. The 12H and the 14T polysomes have stacking sequences NSSNSS and NSSSNSS, respectively.

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