Abstract

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 Mg3.8Fe3.2Zn1.6Ti1.0 Al18.3O38 (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 Mg13.5Fe5.6(Zn,Mn,Ni)0.2Ti4.7Al41.7(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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