Crystal chemistry of the polysome ferrohogbomite-2N2S, a long-known but newly defined mineral species
Hejny, Clivia; Gnos, Edwin; Grobety, Bernard; Armbruster, Thomas
European Journal of Mineralogy Volume 14 Number 5 (2002), p. 957 - 967
published: Sep 27, 2002
ArtNo. ESP147051405010, Price: 29.00 €
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=6VI(Al14Fe3+Ti4+)Σ=16O30(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, Fe0.57Zn0.26Mg0.18Al2O4, 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.