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

Abstract

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 Aṏ n Taṏ ba, northwestern edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition IV (Fe 2+ 3 ZnMgAl) = 6 VI (Al 14 Fe 3+ Ti 4+ ) S = 16 O 30 (OH) 2 . The crystal structure was refined from single-crystal X-ray data. The mineral crystallizes in the acentric space group P6 3 mc 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 0.57 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 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.

β-alumina-14H and β-alumina-21R: Two chromic Na2-δ(Al,Mg,Cr)17O25 polysomes observed in slags from the production of low-carbon ferrochromium

“…It was noticed, that two sets of lattice parameters were found: Many crystals had a hexagonal P-lattice with a≈5.65 Å, c≈32.10 Å, and some crystals had an R-centred trigonal lattice with a≈5.65 Å, c≈48.07 Å. In comparison to related spinel-, β-alumina-type-, magnetoplumbite-and högbomite-structures, [1], [5], [6], [16], [17] …”

“…For reasons of charge balance it is not possible to have all tetrahedral and octahedral positions fully occupied by Al only. However, considering (1) that also Mg was found with electron microprobe analysis and (2) comparing the charge balance compensation mechanism for excess Na ions via Mg replacing Al in the β-alumina-type phases the following simplified composition can be given for β-alumina-14H and β-alumina-21R: Na 2-δ (Al, Cr, Mg) 17 O 25 . The Na-block is comparable to the so-called conduction plane of the β-alumina-type family.…”