Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Ferrofettelite, ideally [Ag<jats:sub>6</jats:sub>As<jats:sub>2</jats:sub>S<jats:sub>7</jats:sub>][Ag<jats:sub>10</jats:sub>FeAs<jats:sub>2</jats:sub>S<jats:sub>8</jats:sub>], 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 (VHN<jats:sub>20</jats:sub>) is 122 kg/mm<jats:sup>2</jats:sup> (range 111–131). The calculated density is 5.74 g/cm<jats:sup>3</jats:sup> (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 Ag<jats:sub>16.04</jats:sub>(Fe<jats:sub>0.55</jats:sub>Hg<jats:sub>0.40</jats:sub>Cu<jats:sub>0.02</jats:sub>)<jats:sub>Σ0.97</jats:sub>(As<jats:sub>3.94</jats:sub>Sb<jats:sub>0.03</jats:sub>)<jats:sub>Σ3.97</jats:sub>S<jats:sub>15.02</jats:sub> on the basis of total atoms = 36.</jats:p><jats:p>Ferrofettelite is monoclinic, space group <jats:italic>C</jats:italic>2, with <jats:italic>a</jats:italic> = 26.011(2), <jats:italic>b</jats:italic> = 15.048(1), <jats:italic>c</jats:italic> = 15.513(1) Å, β = 90.40(1)° and <jats:italic>V</jats:italic> = 6071.9(7) Å<jats:sup>3</jats:sup> for <jats:italic>Z</jats:italic> = 8. The six strongest Bragg peaks in the powder X-ray diffraction pattern (<jats:italic>d</jats:italic>, Å (<jats:italic>I</jats:italic>, %) (<jats:italic>hkl</jats:italic>) are: 3.18 (50) (<jats:inline-formula><jats:alternatives><jats:tex-math>$\bar{8}$</jats:tex-math><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X22000287_inline1.png" /></jats:alternatives></jats:inline-formula>01), 3.104 (100) (005), 3.004 (60) (<jats:inline-formula><jats:alternatives><jats:tex-math>$\bar{8}$</jats:tex-math><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X22000287_inline2.png" /></jats:alternatives></jats:inline-formula>02), 2.755 (40) (<jats:inline-formula><jats:alternatives><jats:tex-math>$\bar{4}$</jats:tex-math><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X22000287_inline3.png" /></jats:alternatives></jats:inline-formula>43), 2.501 (30) (<jats:inline-formula><jats:alternatives><jats:tex-math>$\bar{4}$</jats:tex-math><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X22000287_inline4.png" /></jats:alternatives></jats:inline-formula>44) and 1.880 (30) (<jats:inline-formula><jats:alternatives><jats:tex-math>$\overline {12}$</jats:tex-math><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X22000287_inline5.png" /></jats:alternatives></jats:inline-formula>40). The crystal structure can be described as the alternation of two kinds of layers along the <jats:bold>c</jats:bold>-axis: layer <jats:italic>A</jats:italic> with general composition [Ag<jats:sub>6</jats:sub>As<jats:sub>2</jats:sub>S<jats:sub>7</jats:sub>]<jats:sup>2–</jats:sup> and layer <jats:italic>B</jats:italic> with a general composition of [Ag<jats:sub>10</jats:sub>FeAs<jats:sub>2</jats:sub>S<jats:sub>8</jats:sub>]<jats:sup>2+</jats:sup>. In the structure, the Ag atoms adopt various coordinations extending from quasi linear to quasi tetrahedral, the AsS<jats:sub>3</jats:sub> 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 Fe<jats:sup>2+</jats:sup>. The high-temperature behaviour of ferrofettelite was studied up to 410 K and compared to that of fettelite.</jats:p>