Jahnsite-(MnMnMg), Mn2+Mn2+Mg2+2Fe3+2(PO4)4(OH)2·8H2O, is a new phosphate mineral belonging to the jahnsite subgroup contained in the jahnsite group of minerals. It was found in the dumps of the Sapucaia pegmatite, Sapucaia do Norte, Galiléia, Minas Gerais, Brazil by one of the authors (SV). The mineral occurs on the surfaces of hydrothermal cavities and is associated with frondelite, mangangordonite, and leucophosphite. It forms prisms weakly elongated along a, flattened on [010], and twinned by reflection on {001} up to 200 μm in length. The new species is a late stage alteration product of primary phosphates such as triphylite. Its color is yellow to honey-colored or greenish-yellow and it is partially transparent with a vitreous luster. The streak is very pale yellow. Jahnsite-(MnMnMg) is brittle with good cleavage on {001} and irregular fracture. Its inferred Mohs hardness is 4 and the calculated density is 2.625 g/cm3. The mineral is optically biaxial (–) with α = 1.616, β = 1.619, γ = 1.656 and 2V (calc.) = 74° and non-fluorescent under 254 nm (short wave) and 366 nm (long wave). The empirical formula is: (Mn0.50Ca0.25Na0.20)Σ0.95Mn(Mg1.26Mn0.52Fe2+0.21Zn0.01)Σ2(Fe3+1.63Al0.37)Σ2(PO4)4(OH)1.70·8H2O. The endmember formula is Mn2+Mn2+Mg2+2Fe3+2(PO4)4(OH)2·8H2O. Jahnsite-(MnMnMg) is monoclinic, with space group P2/a and unit-cell parameters a 15.177(2) Å, b 7.176(1) Å, c 10.006(3) Å, and β 110.01(2)° with V 1017.3(2) Å3 for Z = 2. Its crystal structure was refined and the mineral is isostructural with other members of the jahnsite group.

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