Jahnsite-(NaMnMn), a New Jahnsite-Group Mineral from the Wiperaminga Hill West Quarry, Boolcoomatta Reserve, South Australia, Australia

Abstract Jahnsite-(NaMnMn), NaMn2+(Mn2+Fe3+)2Fe3+2(PO4)4(OH)2·8H2O, is a new mineral from the Wiperaminga Hill West Quarry, Boolcoomatta Reserve, Olary Province, South Australia, Australia. The mineral occurs in seams in a matrix comprising triplite, quartz, fluorapatite, and minor pyrite and formed as a result of hydrothermal alteration. Associated minerals are bermanite, phosphosiderite, strengite, and an unknown Mn-Fe phosphate. The mineral occurs as orange-yellow prismatic crystals, elongated parallel to [100], flattened on {001}, exhibiting the forms {001}, {100}, {011}, and {201} that are twinned on {001}. Crystals are up to 80 μm in length and 25 μm across. The Mohs hardness is about 4, the tenacity is brittle, the fracture is splintery, and there is one good cleavage on {001}. The calculated density is 2.774 g/cm3. Jahnsite-(NaMnMn) is biaxial (−) with α = 1.645(3), β = 1.671(3), and γ = 1.674(3) (white light). The measured 2V is 36(1)°. Dispersion could not be observed, and the optical orientation is Z = b. Pleochroism is X = pale yellow brown, Y and Z = light yellow brown; absorption X < Y ≈ Z. Electron probe microanalysis gave the empirical formula (Na0.61Ca0.38)Σ0.99Mn2+(Mn2+1.11Fe3+0.53Mg0.29Mn3+0.07)Σ2(Fe3+1.68Mn3+0.26Al0.06)Σ2(PO4)3.98(OH)2.03·8.05H2O based on 26 O atoms per formula unit. Jahnsite-(NaMnMn) is monoclinic, P2/a, a = 15.1359(10), b = 7.2035(3), c = 9.9876(6) Å, β = 110.361(5)°, V = 1020.92(11) Å3, and Z = 2. The crystal structure has been refined to R1 = 5.70% for 2406 observed reflections with I > 2σI. The new mineral is isostructural with other members of the jahnsite group.