Erikjonssonite, (Pb32O21)[(V,Si,Mo,As)O-4](4)Cl-9, a new mineral from the Kombat mine and structural classification of layered lead oxychlorides related to litharge

摘要

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 0.3X 0.5 X 0.5 mm3. The new mineral is brittle, has Mohs hardness of 21/2 and an uneven fracture. Distinct cleavage on (010) is observed. D calc = 7.967 g/cm3. The reflectance values [R max, %/R min, % (?, 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 Pb31.50( V 1.45 5 + Si 1.09 Mo 0.90 6 + As 0.63 5 + )S4.07Cl8.89O37.11. 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 > 2s(I). Erikjonssonite is monoclinic, C2/c, a = 23.200(5), b = 22.708(5), c = 12.418(3) ,beta = 102.167(4)degrees, V = 6395(2) 3, 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 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 bond lengths are different and one site is exclusively occupied by As5+. The strongest lines of the powder X-ray diffraction pattern [d, (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 ), 1.9