Ferrofettelite, ideally [Ag6As2S7][Ag10FeAs2S8], is a new mineral (IMA2021-094) from the Glasberg quarry, Nieder-Beerbach, Odenwald, south-western Germany. It occurs as anhedral to subhedral flakes and grains up to 80 μm, associated with proustite and xanthoconite, on arsenolite, calcite and prehnite. Ferrofettelite is opaque with a metallic lustre and possesses a dark reddish-grey streak. It is brittle with an uneven fracture; the Vickers microhardness (VHN20) is 122 kg/mm2 (range 111–131). The calculated density is 5.74 g/cm3 (on the basis of the empirical formula). In plane-polarised reflected light, ferrofettelite is greyish white. Between crossed polars it is weakly anisotropic with red internal reflections. Electron-microprobe analyses give the chemical formula Ag16.04(Fe0.55Hg0.40Cu0.02)Σ0.97(As3.94Sb0.03)Σ3.97S15.02 on the basis of total atoms = 36.

Ferrofettelite is monoclinic, space group C2, with a = 26.011(2), b = 15.048(1), c = 15.513(1) Å, β = 90.40(1)° and V = 6071.9(7) Å3 for Z = 8. The six strongest Bragg peaks in the powder X-ray diffraction pattern (d, Å (I, %) (hkl) are: 3.18 (50) (⁠|$\bar{8}$|forumla01), 3.104 (100) (005), 3.004 (60) (⁠|$\bar{8}$|forumla02), 2.755 (40) (⁠|$\bar{4}$|forumla43), 2.501 (30) (⁠|$\bar{4}$|forumla44) and 1.880 (30) (⁠|$\overline {12}$|forumla40). The crystal structure can be described as the alternation of two kinds of layers along the c-axis: layer A with general composition [Ag6As2S7]2– and layer B with a general composition of [Ag10FeAs2S8]2+. In the structure, the Ag atoms adopt various coordinations extending from quasi linear to quasi tetrahedral, the AsS3 groups form pyramids as are typically observed in sulfosalts, and mixed (Fe,Hg) links two sulfur atoms in a linear coordination. Ferrofettelite is the first reported inorganic phase showing a linear coordination for Fe2+. The high-temperature behaviour of ferrofettelite was studied up to 410 K and compared to that of fettelite.

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