Crystal chemistry of the polysome ferrohögbomite-2N2S, a long-known but newly defined mineral species

Ferrohögbomite-2N2S is a polysomatic högbomite-group mineral composed of two nolanite modules (N) and two spinel modules (S) where the spinel modules have ideally hercynite composition. Ferrohögbomite has been described in the literature for a long time but was not distinguished from magnesiohögbomite and simply named högbomite. The revised nomenclature of högbomite minerals required definition of ferrohögbomite polysomes as new mineral species.

The sample from Aïn Taïba, northwestern edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition ^IV(Fe²⁺₃ZnMgAl)_{Σ = 6}^VI(Al₁₄Fe³⁺Ti⁴⁺)_{Σ = 16}O₃₀(OH)₂. The crystal structure was refined from single-crystal X-ray data. The mineral crystallizes in the acentric space group P6₃mc with a = 5.712(1), c = 18.317(7) Å, and shows twinning by merohedry with the inversion centre as twinning operation. This ferrohögbomite-2N2S is associated with ilmenite, hematite, minor magnetite, and pseudorutile. Some of the ferrohögbomite-2N2S grains are rimmed by hercynite, Fe_0.57Zn_0.26Mg_0.18Al₂O₄, which separates them from hematite. The original assemblage probably consisted of ilmenite and Al-bearing magnetite or another Ti-bearing spinel phase. During oxidation ilmenite was replaced by ferrohögbomite on its rims and magnetite was oxidized to hematite. Finally, residual ilmenite was partly transformed along cracks and grain boundaries to pseudorutile. A HRTEM study showed homogeneous ferrohögbomite-2N2S with minor planar faults increasing towards the grain boundaries. The observed orientation relation between ferrohögbomite-2N2S and hematite suggests that ferrohögbomite-2N2S formed topotactically from a primary spinel or magnetite phase that had been subsequently oxidized to hematite.