Hidden mineral treasures in rust samples of the Muonionalusta iron (IVA) meteorite

Exceptionally well-developed crystals of akaganeite, (Fe3+,Ni2+)8(OH,O)16Cl1.25⋅nH2O, were observed during the investigation of rust samples from the Muonionalusta iron meteorite, constituting ideal candidates for the first single-crystal X-ray diffraction investigation carried out on this mineral. Other techniques here employed to study akaganeite include SEM-EDS and Raman spectroscopy. The structure refinement (R1 = 2.23%) confirmed akaganeite to be monoclinic in symmetry (space group I2/m), with a = 10.560(4) Å, b = 3.0268(12) Å, c = 10.512(4) Å, β = 90.050(15)∘ and V = 336.0(2) Å3. The mineral is also confirmed to be isostructural with monoclinic members of the hollandite supergroup, with 2 × 2 tunnels parallel to the b axis constituted by edge-linked Fe-octahedral chains. Chemical analyses resulted in a Cl range of 2.8–5.6 wt.% and an average mole Fe/Cl ratio of 7.6, with trace amounts of Si, Al and S (<0.1 wt.%), and no detectable Ni or Co. The combination of structural and chemical data yielded the stoichiometric formula Fe8O7(OH)9Cl. The Raman spectrum of the Muonionalusta akaganeite is comparable with Raman spectra from synthetic akaganeite, showing several peaks between 138 and 1390 cm–1 and the O–H stretching band at 3510 cm–1; no peaks are observed in the H2O bending-mode area of the spectrum, in keeping with the structural data. Taking into account all the collected data, we propose two possible new formulae for akaganeite (Z = 8): FeO1–x(OH)1+xClx (0.01 < x < 0.20) or, taking Ni into account, (Fe1–xNix)O1–x–y(OH)1+x+yCly (0 < x < 0.19 and 0.01 < y < 0.20). In the Muonionalusta corrosion rust, in addition to akaganeite, nickel-bearing humboldtine [Fe(C2O4)⋅2H2O] was also identified through Raman spectroscopy, powder X-ray diffraction and chemical analyses. It possibly represents the first occurrence of an oxalate mineral as a product of terrestrial weathering of a meteorite.