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

Bederite, ideally square Ca2Mn22+Fe23+Mn22+(PO4)(6)(H2O)(2), orthorhombic, a = 12.559(2), b = 12.834(1), c = 11.714(2) Angstrom, V = 1887.8(4) Angstro m(3), Z = 4, space group Pcab, is a new mineral from the EI Penon pegmatite , Nevados de Palermo, Salts Province, Republica Argentina. The mineral occu rs as rare ellipsoidal nodules (similar to 5 cm in diameter) enclosed in po tassium feldspar or quartz at the core-margin zone of a beryl-type rare-ele ment pegmatite. Associated minerals are quartz, potassium feldspar, muscovi te, beryl, columbite, possibly heterosite, and powdery coatings of Mn- and Fe-oxides; in the dumps of the pegmatite, there are numerous other phosphat es including altered triphylite-lithiophyllite, arrojadite, eosphorite,laue ite, brazilianite, and fairfieldite. Bederite is very dark brown to black w ith 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/cm(3), 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 c ross-polarized light, it is biaxial negative with strong dispersion, v > r, 2V(obs) = 54 degrees and 2V(calc) = 60 degrees. Refractive indices are as follows: alpha = 1.729(3), beta = 1.738(3), gamma = 1.741(3). Chemical anal ysis by electron microprobe plus the Penfield method and thermogravimetry g ave 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 Fe 2O3 and FeO contents were derived from the refined crystal structure. The f ive strongest lines in the X-ray powder diffraction pattern are as follows: d (Angstrom), 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 (>5 si gma F)reflections measured with MoK alpha X-radiation. Bederite is isostruc tral with wicksite, grischunite, and an unnamed wicksite-like phase; it is related to wicksite by the substitutions (Na)square + Fe-M2(3+) --> Na-Na Mg-M2, Mn-M1(2+) --> Fe-M1(2+) and Mn-M3(2+) --> Fe-M3(2+).