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

Abstract

Ferrohogbomite-2 N 2 S is a polysomatic hogbomite-group mineral composed of two nolanite modules ( N ) and two spinel modules ( S ) where the spinel modules have ideally hercynite composition. Ferrohogbomite has been described in the literature for a long time but was not distinguished from magnesiohogbomite and simply named hogbomite. The revised nomenclature of hogbomite minerals required definition of ferrohogbomite polysomes as new mineral species. The sample from Ain Taiba, northwestern edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition IV (Fe 2+ 3 ZnMgAl) Σ = 6 VI (Al 14 Fe 3+ Ti 4+ ) Σ = 16 O 30 (OH) 2 . The crystal structure was refined from single-crystal X-ray data. The mineral crystallizes in the acentric space group P 6 3 mc with a = 5.712(1), c = 18.317(7) A, and shows twinning by merohedry with the inversion centre as twinning operation. This ferrohogbomite-2 N 2 S is associated with ilmenite, hematite, minor magnetite, and pseudorutile. Some of the ferrohogbomite-2 N 2 S grains are rimmed by hercynite, Fe 0.57 Zn 0.26 Mg 0.18 Al 2 O 4 , 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 ferrohogbomite 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 ferrohogbomite-2 N 2 S with minor planar faults increasing towards the grain boundaries. The observed orientation relation between ferrohogbomite-2 N 2 S and hematite suggests that ferrohogbomite-2 N 2 S formed topotactically from a primary spinel or magnetite phase that had been subsequently oxidized to hematite.