Erikjonssonite, (Pb32O21)[(V,Si,Mo,As)O4]4Cl9, a new mineral from the Kombat mine and structural classification of layered lead oxychlorides related to litharge

The new lead oxychloride vanadate mineral erikjonssonite was discovered at the abandoned Kombat copper mine, Grootfontein district, Namibia, in association with hausmannite, calcite, glaucochroite, baryte, cerussite, and an insufficiently studied mineral chemically related to kombatite. Erikjonssonite forms thick tabular orange-red grains up to 0.3 × 0.5 × 0.5 mm³. The new mineral is brittle, has Mohs’ hardness of 2½ and an uneven fracture. Distinct cleavage on (010) is observed. D_calc = 7.967 g/cm³. The reflectance values [R_max, %/R_min, % (λ, nm)] are: 20.4/18.9 (470), 19.0/17.6 (540), 18.5/17.1 (589), 18.0/16.6 (650). The infrared spectrum shows the absence of H-, C- and B-bearing groups. The chemical composition is (wavelength-dispersive mode electron microprobe, wt%): SiO₂ 0.86, V₂O₅ 1.73, As₂O₅ 0.96, MoO₃ 1.70, PbO 92.54, Cl 4.15, –O=Cl −0.94, total 101.00. The empirical formula based on 46 O + Cl atoms per formula unit (apfu) is Pb_31.50(⁠$V1.455+Si1.09Mo0.906+As0.635+$⁠)_Σ4.07Cl_8.89O_37.11. The simplified formula is {(Pb₃₂O₂₁)[(V,Si,Mo,As)O₄]₄}Cl₉. The crystal structure has been refined to R = 4.3% based on 7730 independent reflections with I > 2σ(I). Erikjonssonite is monoclinic, C2/c, a = 23.200(5), b = 22.708(5), c = 12.418(3) Å, β = 102.167(4)°, V = 6395(2) ų, and Z = 4. The structure of erikjonssonite is very similar to that of hereroite: in both minerals two [Pb₃₂O₂₁]²²⁺ oxocentred blocks formed by OPb₄ tetrahedra with inserted TO₄ tetrahedral groups alternate with one chloride sheet. Average <T–O> bond lengths in the tetrahedral groups demonstrate very similar values which is interpreted as both T1 and T2 sites having mixed occupancies of V⁵⁺, Si⁴⁺, Mo⁶⁺ and As⁵⁺, unlike hereroite where <T–O> bond lengths are different and one site is exclusively occupied by As⁵⁺. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.501 (24) (531, $2¯61$⁠), 2.980 (100) (551, $2¯24$⁠), 2.794 (45) (⁠$8¯02$⁠, $5¯14$⁠), 1.990 (24) (⁠$8¯82$⁠), 1.977 (21) (⁠$6¯06$⁠), 1.762 (20) (715, $8¯.10.2$⁠, 10.6.2), 1.648 (33) (⁠$1¯1.5.5$⁠). The mineral is named in honour of the Swedish mineralogist Erik Jonsson (b. 1967). The holotype material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia. The concept of a “defect number (DN)” which is the number of O^2– ions removed from the ideal [PbO] block per cavity area is suggested in order to classify the layered Pb oxychloride minerals structurally related to litharge.