Oyonite, Ag3Mn2Pb4Sb7As4S24, a New Member of the Lillianite Homologous Series from the Uchucchacua Base-Metal Deposit, Oyon District, Peru

The new mineral species oyonite, ideally Ag3Mn2Pb4Sb7As4S24, has been discovered in the Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, as very rare black metallic subhedral to anhedral crystals, up to 100 μm in length, associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix. Its Vickers hardness (VHN100) is 137 kg/mm2 (range 132–147). In reflected light, oyonite is weakly to moderately bireflectant and weakly pleochroic from dark grey to a dark green. Internal reflections are absent. Reflectance values for the four COM wavelengths [Rmin, Rmax (%) (λ in nm)] are: 33.9, 40.2 (471.1); 32.5, 38.9 (548.3), 31.6, 38.0 (586.6); and 29.8, 36.5 (652.3). Electron microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 0.76 (2), Ag 8.39 (10), Mn 3.02 (7), Pb 24.70 (25), As 9.54 (12), Sb 28.87 (21), S 24.30 (18), total 99.58 (23). Based on 20 cations per formula unit, the chemical formula of oyonite is Cu0.38Ag2.48Mn1.75Pb3.79Sb7.55As4.05S24.12. The main diffraction lines are (d in Å, hkl and relative intensity): 3.34 (−312; 40), 3.29 (−520; 100), 2.920 (−132; 40), 2.821 (−232; 70), 2.045 (004; 50). The crystal structure study revealed oyonite to be monoclinic, space group P21/n, with unit-cell parameters a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11)◦, V = 2004.1 (4) Å3, Z = 2. The crystal structure was refined to a final R1 = 0.032 for 6272 independent reflections. Oyonite belongs to the Sb-rich members of the andorite homeotypic sub-series within the lillianite homologous series. The name oyonite is after the Oyon district, Lima Department, Peru, the district where the type locality (Uchucchacua mine) is located.

The sample containing oyonite was not found in situ but originates from a sample given to one of the authors (FNK) by the mineral dealer John Veevaert. The material was found in October 2010 from the Nivel 890, Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, and it is the same material where menchettiite [4] was found. Geological and metallogenic data concerning this mining district have been reported by [2].
Oyonite is associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix.
The new mineral was named oyonite, after the Oyon district, Lima Department, Peru, where the type locality (Uchucchacua mine) is located. The mineral and its name have been approved by the IMA CNMNC, under the number 2018-002. The holotype specimen of oyonite is deposited in the mineralogical collections of the Museo di Storia Naturale, Università degli Studi di Firenze, Via G. La Pira 4, Florence, Italy, under catalogue number 3283/I. The mineralogical description of oyonite, as well as its crystal structure, are described in this paper.

Mineral Description and Physical Properties
Oyonite ( Figure 1) occurs as black subhedral to anhedral crystals, up to 100 µm in length. Streak is black, and the luster is metallic. In plane-polarized incident light, oyonite is weakly to moderately bireflectant and weakly pleochroic from dark grey to dark green. Internal reflections are absent. Between crossed polars, the mineral is anisotropic, without characteristic rotation tints.
Oyonite is brittle. Its Vickers hardness (VHN 100 ) is 137 kg/mm 2 (range 132-147), corresponding to a Mohs hardness of~3-3.5. Based on the empirical formula, the calculated density is 5.237 g/cm 3 . The density, calculated based on the ideal chemical formula (see below), is 5.275 g/cm 3 .

Chemical Data
A preliminary EDS (Energy Dispersive Spectrometry) analysis performed on the crystal grain used for the structural study did not indicate the presence of elements (Z > 9) other than Cu, Ag, Mn, Pb, As, Sb and S.

Crystallography
For the X-ray single-crystal diffraction study, the intensity data were collected using an Oxford Diffraction Xcalibur 3 diffractometer, equipped with a Sapphire 2 CCD area detector, with MoKα radiation. The detector to crystal working distance was 6 cm. Intensity integration and standard Lorentz-polarization corrections were performed with the CrysAlis RED [5] software package. The program ABSPACK in CrysAlis RED [5] was used for the absorption correction. Tests on the distribution of |E| values agree with the occurrence of an inversion center (|E 2 − 1| = 0.895). This information, together with the systematic absences, suggested the space group P2 1 /n. We decided to keep this non-standard setting of the space group to make easier the comparison with the other members of the lillianite homologous series [6]. The refined unit-cell parameters are a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11) The crystal structure was refined with Shelxl-97 [7] starting from the atomic coordinates of menchettiite [4]. The occurrence of twinning on {100} was considered. The site occupancy factors (s.o.f.) were refined using the scattering curves for neutral atoms given in the International Tables for Crystallography [8]. After several cycles of anisotropic refinement, a final R 1 = 0.0227 for 2199 reflections with F o > 4σ(F o ) was achieved (0.0317 for all 6272 reflections). Crystal data and details of the intensity data collection and refinement are reported in Table 2.  Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters are given in Table 3. Selected bond distances are given in Table 4 and bond valence sums are given in Table 5. The crystallographic information file (CIF) is given as Supplementary Material.  2.887 (4) −S8 2.840 (4) −S10 3.053 (4) −S11 2.616 (4) −S8 3.054 (4) −S1 3.222 (4) −S1 3.181 (4) −S12 3.288 (4) X-ray powder diffraction data (Table 6) were obtained on the same fragment used for the single-crystal study with an Oxford Diffraction Excalibur PX Ultra diffractometer fitted with a 165 mm diagonal Onyx CCD detector and using copper radiation (CuKα, λ = 1.54138 Å). The working conditions were 40 kV and 40 nA with 1 hour of exposure; the detector-to-sample distance was 7 cm. The program Crysalis RED was used to convert the observed diffraction rings to a conventional powder diffraction pattern. The least squares refinement gave the following unit-cell values: a = 19.175 (1)

Crystal Structure Description
The crystal structure of oyonite ( Figure 2) agrees with those of the 4,4 L homologue in the lillianite homologous series [6]. There are 10 metal sites and 12 S sites in the unit cell.  Table 3 (for S sites, only the number is reported).
The general organization of oyonite, as seen down c, is shown in Figure 3. The crystal structure is formed by the alternation of (311) PbS slabs, four octahedra thick along (100) PbS and unit-cell twinned by reflection on (311) PbS planes. The crystal structure of oyonite shows a two-fold superstructure with respect to the short 4 Å axis of the substructure. Only one kind of (311) PbS slab occurs in the crystal structure of oyonite, formed by one kind of diagonal (100) PbS plane ( Figure 4). On the border of the plane, mixed (Ag,Cu) and (Mn,Ag)-centered octahedra (i.e., M4 and M9 sites, respectively) on one side, and Sb and (Sb,Mn)-centered sites alternate (M6 and M1 sites, respectively). In the center of the plane, a pure Sb site (M5) alternates with a pure As site (M10) along c in one column and mixed (Sb,As) and (As,Sb) sites in the other column (M7 and M2 sites, respectively). The composition of the diagonal (100) PbS plane is (Cu 0.20 Ag 1.25 Mn 0.86 Sb 3.65 As 2.04 S 10 ), that can be simplified as (Ag 1.5 MnSb 3.5 As 2 S 10 ). Thus, the simplified chemical composition of oyonite is given by 2 × Pb 2 S 2 + 2 × (Ag 1.5 MnSb 3.5 As 2 S 10 ) = Ag 3 Mn 2 Pb 4 Sb 7 As 4 S 24 .
The chemical formula, as obtained through the single-crystal X-ray diffraction study, is Cu 0.40 Ag 2.48 Mn 1.74 Pb 3.82 Sb 7.48 As 4.08 S 24 (Z = 2).

Relation to the Other Species
Oyonite belongs to the andorite sub-series of the Sb-rich homeotypic members within the lillianite homologous series ( [9]).
Following the calculation procedure for the order N and the Ag + + Me 3+ = 2 Pb 2+ substitution proposed by [6], considering minor cations, the homologue order from chemical analysis is N = 3.58, slightly smaller than the crystallographic value N = 4 indicated by the crystal structure refinement, whereas the substitution percentage is 83%. The idealized chemical composition corresponds to N = 4 and a substitution percentage of 75%.