Abstract

Jahnsite-(NaFeMg), NaFe3+Mg2Fe23+(PO4)4(OH)2·8H2O, space group P2/a, a = 15.0811(16), b = 7.1403(8), c = 9.8299(11) Å, β = 110.445(1)°, V = 991.8(2) Å3, and Z = 2, is a new mineral from the Tip Top mine, Custer County, South Dakota. It occurs as the dominant chemical zone in transparent pseudo-orthorhombic twinned prisms up to 1 mm long with shallow wedge-like terminations, both as isolated twinned crystals and in subparallel to divergent intergrowths. The crystals are yellow with orange-red bands just below the terminations. The mineral has a white streak, vitreous luster, Mohs hardness of 4, and good {001} cleavage. The measured density is 2.58(2) g/cm3, and the calculated density is 2.608 g/cm3. It is optically biaxial (–), with α = 1.632(1), β = 1.669(1), γ = 1.671(1) (589 nm); 2V (meas.) = 25(5)°, 2V (calc.) = 26°; dispersion: r > v, very strong; orientation: Y = b, Z ^ a = +28° (in β obtuse); pleochroism: X = colorless, Y and Z = beige; Y = Z > X. Electron microprobe analyses provided: Na2O 2.82, CaO 0.34, MnO 0.32, MgO 10.27, Fe2O3 27.35, P2O5 35.93, H2O 21.58 (calc.), total 98.61, which in terms of the general formula for the whiteite-jahnsite group XM1M22M32(PO4)4 (OH)2·8H2O, yields the empirical formula: (Na0.72Ca0.05Mn0.042+)(Fe0.723+Mg0.01)Mg2.00Fe2.003+(PO3.77OH0.23)4 (OH)2·8H2O. Crystal structure refinement (R1 = 0.041) supports these cation site assignments. Typical crystals have as many as five chemical zones from base to termination corresponding to the following jahnsite species: jahnsite-(NaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMgMg). The characterization of the new mineral jahnsite-(NaFeMg) is based upon the second zone listed, which corresponds to roughly 80% of the crystal volume. Jahnsite-(NaMnMg) and jahnsite-(CaMgMg) also represent new species; however, the very small volume of these zones is insufficient to permit complete characterization of these species. Jahnsite-(NaFeMg) is the first member of the whiteite-jahnsite group in which a monovalent cation (Na+) predominantly occupies the X site and a trivalent cation (Fe3+) the M1 site. The structure differs from that determined for jahnsite-(CaMnMg) in that the X site is [8]-coordinated and the M1 and P1 polyhedra exhibit greater distortions caused by cation-cation repulsion across their shared edges. The determination of H positions allows the elucidation of a definitive H-bonding scheme.

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