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}$01), 3.104 (100) (005), 3.004 (60) ($\bar{8}$02), 2.755 (40) ($\bar{4}$43), 2.501 (30) ($\bar{4}$44) and 1.880 (30) ($\overline {12}$40). 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.

The crystal structure and chemical composition of two samples of fettelite from the type locality, including a portion of the holotype material, was investigated to verify if a previously proposed revision of the chemical formula was applicable, and to study the role of cation substitution for Hg that would suggest new members of the fettelite family. The crystal structure of fettelite from the type locality was found to be equivalent to that reported previously for the Chilean occurrence, and consists of an alternation of two kinds of layers along c: layer A with general composition [Ag6As2S7]2– and layer B with general composition [Ag10HgAs2S8]2+. In this structure, the Ag atoms occur in various coordination configurations, varying from quasi-linear to quasi-tetrahedral, the AsS3 groups form pyramids as are typically observed in sulfosalts, and Hg links two sulfur atoms in a linear coordination. The refined compositions for the crystals in this study, [Ag6As2S7][Ag10(Fe0.53Hg0.47)As2S8] (R100124) and [Ag6As2S7][Ag10(Hg0.79Cu0.21)As2S8] (R110042), clearly indicate that new mineral species related to fettelite are likely to be found in nature.

La Colorada, in Zacatecas State, Mexico is an epithermal Ag-Zn-Pb system hosted in Mesozoic calcareous sedimentary rocks overlain by Tertiary volcanic rocks. The dominant vein is associated with a fault system that accommodates Tertiary normal and strike-slip faulting. The ore consists of fahlore [(Cu,Ag)10(Zn,Fe)2(Sb,As)4S13], polybasite [(Ag,Cu)16Sb2S11]–pearceite [(Ag,Cu)16As2S11] solid solution, pyrargyrite [Ag3SbS3]–proustite [Ag3AsS3] solid solution, acanthite-argentite [Ag2S], and native silver; associated sulphides include galena, sphalerite, chalcopyrite and pyrite. The Ag:Sb of the bulk concentrate from the mine is 1.076 and the Ag:Pb is 0.088. Compositions of the assemblages fahlore + pyrargyrite-proustite + sphalerite, and fahlore + polybasite-pearceite solid solution + (Ag,Cu)2S solid solution + sphalerite encapsulated in quartz and sphalerite indicate a primary depositional temperature of ∼325°C for a depth between 725 and 960 m below the inferred palaeosurface, which is in accord with fluid-inclusion data for higher elevations in the mine.

The determination of the crystal structure of benleonardite (P3m1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag15.00Cu1.00Sb1.58As0.42S7.03Te3.97) obtained using data from a gem-quality, untwinned crystal recovered from the type material, revealed that benleonardite exhibits the structure observed for minerals of the pearceite-polybasite group. The structure consists of the stacking of [Ag6(Sb,As)2S6Te]2– A and [Ag9Cu(S,Te)2Te2]2+B layer modules in which (Sb, As) forms isolated SbS3 pyramids typically occurring in sulfosalts; Cu links two (S,Te) atoms with linear coordination, and Ag occupies sites with coordination geometries ranging from quasi-linear to almost triangular. The silver ions are found in the B layer module along two-dimensional diffusion paths and their electron densities are evidenced by means of a combination of a Gram-Charlier development of the atom displacement factors and a split model. In the structure, two S positions are completely replaced by Te (i.e. Te3 and Te4) and one is half occupied [S1: S0.514(9)Te0.486], whereas S2 is completely filled by sulfur. This distribution reflects the crystal-chemical environments of the different cations. On the basis of information gained from this characterization, the crystal-chemical formula of benleonardite was revised according to the structural results, yielding Ag15Cu(Sb,As)2S7Te4 (Z = 1) instead of Ag8(Sb,As)Te2S3(Z = 2) as previously reported. Thus, the mineral must be considered a member of the pearceite-polybasite group. A recalculation of the chemical data listed in the scientific literature for benleonardite according to the structural results obtained here leads to excellent agreement.