Désorite, Pb2(Fe3+6Zn)O2(PO4)4(OH)8, a New Phosphate Mineral Isotypic with Jamesite

Abstract

The new mineral désorite, Pb2(Fe3+6Zn)O2(PO4)4(OH)8, is a secondary oxidation-zone mineral discovered on the dumps of the Schöne Aussicht mine, Dernbach, Westerwaldkreis, Rhineland-Palatinate, Germany. The crystals are irregular blades up to about 0.25 mm long, occurring in subparallel and radial aggregates. Désorite is brownish red. It has an orange streak, adamantine luster, brittle tenacity, splintery and curved fracture and two cleavages: {021¯} perfect and {001} good. It has a hardness (Mohs) of about 3 and is nonfluorescent in both long- and short-wave ultraviolet illumination. The calculated density is 4.633 g/cm3. Optically, crystals are biaxial (+) with α = 2.00(1), β = 2.02(calc) and γ = 2.10(calc) (white light). The 2V is 54(2)° and the optical orientation is Y ~⊥{021¯}, X ∧ a ≈ 13°. The mineral is pleochroic: X yellow brown, Y red brown, Z red brown; X < Y ≈ Z. The empirical formula from electron microprobe analysis is Pb1.93(Zn0.50Fe3+6.29☐0.21)Σ7.00(P3.90As0.10)Σ4.00O26H8.28. Désorite is triclinic, space group P1¯; the unit-cell parameters are a = 5.4389(7), b = 9.3242(13), c = 10.0927(12) Å, α = 109.024(8), β = 90.521(6), γ = 97.588(7)°, V = 478.90(11) Å3 and Z = 1. Désorite has a framework structure (R1 = 0.0487 for 937 I > 2σI reflections) assembled from Fe3+O6 octahedra and PO4 tetrahedra, with Pb occupying cavities in the framework. The Fe3+O6 octahedra in the framework occur in edge-sharing chains, edge-sharing trimers and individual octahedra, all sharing corners with each other and with PO4 tetrahedra. Désorite is isostructural with jamesite and lulzacite.