Abstract

Jahnsite-(NaMnMg), (Na,Ca)(Mn2+,Fe3+)(Mg,Fe3+,Mn3+)2Fe3+2(PO4)4(OH)2(H2O)8, is a new mineral from the Sapucaia pegmatite, Conselheiro Pena pegmatite district, Minas Gerais, Brazil, and the White Rock No. 2 quarry, Bimbowrie Conservation Park, South Australia, Australia. At both localities, it is a low temperature, secondary mineral formed as the result of alteration of primary phosphates. The mineral occurs as light orange to orange-yellow prisms, elongate on [100], exhibiting the forms {100}, {001}, and {011} and twinned by reflection on {001}. The streak is white, the luster is vitreous, and crystals are transparent to translucent. The Mohs hardness is about 4. The tenacity is brittle, the fracture is irregular, stepped (splintery), and there is one very good cleavage on {001}. The measured density is 2.68(1) g/cm3 (Sapucaia). The mineral is slowly soluble in dilute HCl. Jahnsite-(NaMnMg) from Sapucaia is biaxial (–), with α 1.642(1), β 1.675(1), γ 1.677(1) (white light). The measured 2V is 29(1)°. Dispersion is very strong, r < v. The optical orientation is Z = b; X ^ c = 51° in obtuse β. Pleochroism is X = colorless, Y and Z = orange-yellow; YZ > X. Electron-microprobe analyses gave the empirical formulae (Na0.56Ca0.25Mn2+0.09)Σ0.90(Mn2+0.85Fe3+0.15)Σ1.00(Mg1.53Fe3+0.47)Σ2.00(Fe3+1.79Al0.21)Σ2(PO4)4(OH)1.83(H2O)8.17 for the Sapucaia material and (Na0.63Ca0.23Mn2+0.14)Σ1.00(Mn2+0.68Mn3+0.26Fe3+0.05Mg0.01)Σ1.00(Mg1.26Mn2+0.43Mn3+0.16Fe3+0.15)Σ2.00(Fe3+1.97Al0.02)Σ1.99(PO4)4(OH)1.98(H2O)8.02 for the White Rock material. The mineral is monoclinic, space group P2/a, with cell parameters (Sapucaia) a 15.1045(15), b 7.1629(2), c 9.8949(7) Å, β 110.640(7)°, V 1001.83(13) Å3, and Z = 2. The structure refinements of crystals from both localities were generally consistent and showed the mineral to be a member of the jahnsite group (Sapucaia: R1 = 3.19% for 1941 Io > 2σI reflections; White Rock: R1 = 6.94% for 13485 Io > 2σI reflections).

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