Bederite, a new pegmatite phosphate mineral from Nevados de Palermo, Argentina: Description and crystal structure

Abstract

Bederite, ideally []Ca2Mn22+Fe23+Mn22+(PO4)6(H2O)2, orthorhombic, a = 12.559(2), b = 12.834(1), c = 11.714(2) Å, V = 1887.8(4) Å3, Z = 4, space group Pcab, is a new mineral from the El Penon pegmatite, Nevados de Palermo, Salta Province, Republica Argentina. The mineral occurs as rare ellipsoidal nodules (~5 cm in diameter) enclosed in potassium feldspar or quartz at the core-margin zone of a beryl-type rare-element pegmatite. Associated minerals are quartz, potassium feldspar, muscovite, beryl, columbite, possibly heterosite, and powdery coatings of Mn- and Fe-oxides; in the dumps of the pegmatite, there are numerous other phosphates including altered triphylite-lithiophyllite, arrojadite, eosphorite, laueite, brazilianite, and fairfieldite. Bederite is very dark brown to black with a dark olive-green streak and a vitreous luster. It is brittle with an irregular fracture and a good cleavage parallel to {100}, Mohs hardness is 5, and the observed and calculated densities are 3.48(1) and 3.50 g/cm3, respectively. In transmitted plane-polarized light, bederite is pleochroic X = Y = olive green, Z = brown with X = Y > Z and X = a, Y = c, Z = b. In cross-polarized light, it is biaxial negative with strong dispersion, v > r, 2V(obs) = 54°and 2V(calc) = 60°. Refractive indices are as follows: α = 1.729(3), β = 1.738(3), γ = 1.741(3). Chemical analysis by electron microprobe plus the Penfield method and thermogravimetry gave P2O5 41.76, Al2O3 0.82, Fe2O3 12.00, FeO 2.25, MnO 20.59, MgO 3.45, ZnO 0.40, CaO 10.91, SrO 0.43, Na2O 0.63, H2O 3.52, sum 96.76 wt% where the Fe2O3 and FeO contents were derived from the refined crystal structure. The five strongest lines in the X-ray powder diffraction pattern are as follows: d (Å), I, (h k l): 2.768, 100, (4 0 2); 2.927, 78, (0 0 4); 3.006, 67, (1 4 1); 2.814, 35, (0 4 2); 2.110, 33, (1 6 0). The crystal structure of bederite was refined to an R index of 2.8% based on 2530 observed (>5σF) reflections measured with MoKα X-radiation. Bederite is isostructural with wicksite, grischunite, and an unnamed wicksite-like phase; it is related to wicksite by the substitutions (Na)[] + (M2)Fe3+ → (Na)Na + (M2)Mg, (M1)Mn2+ → (M1)Fe2+ and (M3)Mn2+ → (M3)Fe2+.