Plumboperloffite, PbMn2+2Fe3+2(PO4)3(OH)3, a new member of the bjarebyite group from Wiperaminga Hill, South Australia, Australia

Abstract

AbstractPlumboperloffite, PbMn2+2Fe3+2(PO4)3(OH)3, is a new mineral and member of the bjarebyite group from Wiperaminga Hill West Quarry, Boolcoomatta Reserve, Olary Province, South Australia, Australia. The mineral was found in a single cavity in triplite–barbosalite matrix associated with fluorapatite, phosphosiderite, natrodufrénite and fluorite. The mineral forms intergrowths of subparallel, thin tabular to bladed crystals. Individual crystals are up to 40 μm in length. Plumboperloffite is brownish orange in colour with a vitreous lustre. The mineral has brittle tenacity, an excellent cleavage on {100} and uneven fracture. The calculated density is 4.416 g/cm3. Plumboperloffite is biaxial (+), α = 1.87(1), β = 1.88(1) and γ = 1.89(1) as measured in white light. The measured 2V is 88(1)°. Dispersion is apparently strong, based on extinction colours and the orientation is Y = b. The pleochroism in shades of yellow brown is X < Z < Y. Electron microprobe analysis gave the empirical formula (based on 15 O apfu) (Pb0.92Ca0.04Ba0.01K0.01)Σ0.98(Mn2+1.84Fe2+0.13)Σ1.97(Fe3+1.97Al0.03)Σ2.00(P3.01O11.94)(OH)Σ3.06. Plumboperloffite is monoclinic, space group P21/m with a = 9.1765(18), b = 12.340(3), c = 5.0092(10) Å, β = 101.01(3)°, V = 556.8(2) Å3 and Z = 2. The crystal structure has been refined using X-ray single-crystal data to a final R1 = 0.0207 on the basis of 1417 reflections with Fo > 4σ(Fo). The mineral is isostructural with members of the bjarebyite-group minerals.