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

Ferrohoegbomite-2N2S is a polysomatic hoegbomite-group mineral composed of two nolanite modules (N) and two spinel modules (S) where the spinel modules have ideally hercynite composition. A sample from Ain Taiba, on the NW edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition (super [4]) (Fe (super 2+) (sub 3) ZnMgAl) (super [6]) (Al (sub 14) Fe (super 3+) Ti (super 4+) )O (sub 30) (OH) (sub 2) . The crystal structure was refined in the acentric space group P6 (sub 3) mc with a 5.712, c 18.317 Aa and shows twinning by merohedry with the inversion centre as twinning operation. This ferrohoegbomite-2N2S is associated with ilmenite, hematite, minor magnetite and pseudorutile. Some of the ferrohoegbomite-2N2S are rimmed by hercynite, Fe (sub 0.57) Zn (sub 0.26) Mg (sub 0.18) Al (sub 2) O (sub 4) , which separates them from hematite. The origin assemblage probably consisted of ilmenite and Al-bearing magnetite or another Ti-bearing spinel phase; during oxidation ilmenite was replaced by ferrohoegbomite on its rims and magnetite was oxidized to hematite.