Jahnsite-(NaFeMg), a new mineral from the Tip Top mine, Custer County, South Dakota: Description and crystal structure

Abstract

Jahnsite-(NaFeMg), NaFe 3+ Mg 2 Fe 2 3+ (PO 4 ) 4 (OH) 2 ·8H 2 O, space group P 2/ a , a = 15.0811(16), b = 7.1403(8), c = 9.8299(11) A, β = 110.445(1)°, V = 991.8(2) A 3 , 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/cm 3 , and the calculated density is 2.608 g/cm 3 . It is optically biaxial (–), with α = 1.632(1), β = 1.669(1), γ = 1.671(1) (589 nm); 2 V (meas.) = 25(5)°, 2 V (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: Na 2 O 2.82, CaO 0.34, MnO 0.32, MgO 10.27, Fe 2 O 3 27.35, P 2 O 5 35.93, H 2 O 21.58 (calc.), total 98.61, which in terms of the general formula for the whiteite-jahnsite group XM1M2 2 M3 2 (PO 4 ) 4 (OH) 2 ·8H 2 O, yields the empirical formula: (Na 0.72 Ca 0.05 Mn 0.04 2+ )(Fe 0.72 3+ Mg 0.01 )Mg 2.00 Fe 2.00 3+ (PO 3.77 OH 0.23 ) 4 (OH) 2 ·8H 2 O. Crystal structure refinement ( R 1 = 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 (Fe 3+ ) 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.