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

Exceptionally well-developed crystals of akaganeite, (Fe³⁺,Ni²⁺)₈(OH,O)₁₆Cl_1.25·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) ų. 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): FeO_1–x(OH)_1+xCl _x (0.01 < x < 0.20) or, taking Ni into account, (Fe_1–xNi _x )O_1–x–y(OH)_1+x+yCl _y (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.