Muonionalustaite, Ni3(OH)4Cl2·4H2O, a new mineral formed by terrestrial weathering of the Muonionalusta iron (IVA) meteorite, Pajala, Norrbotten, Sweden

ABSTRACT Muonionalustaite, ideally Ni3(OH)4Cl2 · 4H2O, is a new mineral species (IMA 2020–010), found as a terrestrial weathering product of the Muonionalusta iron meteorite, in a fragment excavated 1.5 km NE of Lake Kitkiöjärvi in northernmost Sweden. Muonionalustaite occurs in cavities of corrosion crust, associated with taenite, goethite, maghemite, amorphous Fe-Ni oxy-hydroxides and soil mineral particles. The mineral is green in colour and transparent. It occurs as lath-like crystals up to ca. 5 μm, elongated along [010] and flattened on {001}, forming aggregates and thin crusts. The calculated density and overall refractive index are 2.67(1) g·cm−3 and 1.68, respectively. An empirical formula from point analyses is (Ni2.88Fe0.02S0.02Al0.01Si0.01)Σ2.94(OH3.73Cl2.27)Σ6.00 · 4H2O. The crystal structure was refined in the space group C2/m from powder X-ray diffraction data to R Bragg = 3.55%. The monoclinic unit-cell parameters are a = 15.018(3) Å, b = 3.1490(6) Å, c = 10.502(3) Å, β = 101.535(15)º and V = 486.62(19) Å3 for Z = 2. Muonionalustaite is isostructural with the synthetic compounds Ni3(OH)3.9Cl2.1 · 4H2O and Mg3(OH)4Cl2 · 4H2O. The strongest X-ray diffraction lines are [I(%), d(Å), hkl]: 100, 10.30, 001; 67, 5.49, 201; 31, 3.868, 202; 30, 7.36, 200 and 25, 2.409, 6‾02. Raman spectra show prominent bands at 3624, 3612, 3571 and 3507 cm−1, related to O–H-stretching vibrations of OH− groups, and in the region 450–530 cm−1 representing metal–O(H) vibration modes.