Rankamaite from the Urubu pegmatite, Itinga, Minas Gerais, Brazil: Crystal chemistry and Rietveld refinement

A new occurrence of rankamaite is here described at the Urubu pegmatite, Itinga municipality, Minas Gerais, Brazil. The mineral forms cream-white botryoidal aggregates of acicular to fibrous crystals, intimately associated with simpsonite, thoreaulite, cassiterite, quartz, elbaite, albite, and muscovite. The average of six chemical analyses obtained by electron microprobe is (range in parentheses, wt%): Na2O 2.08 (1.95-2.13), K2O 2.61 (2.52-2.74), Al2O31.96 (1.89-2.00), Fe2O30.01 (0.00-0.03), TiO20.02 (0.00-0.06), Ta2O581.04 (79.12-85.18), Nb2O59.49 (8.58-9.86), total 97.21 (95.95-101.50). The chemical formula derived from this analysis is (Na1.55K1.28)Σ2.83(Ta8.45Nb1.64Al0.89Fe3+0.01Ti0.01)Σ11.00[O25.02(OH)5.98]Σ31.00. Rankamaite is an orthorhombic “tungsten bronze” (OTB), crystallizing in the space group Cmmm. Its unit-cell parameters refined from X‑ray diffraction powder data are: a = 17.224(3), b = 17.687(3), c = 3.9361(7) Å, V = 1199.1(3) Å3, Z = 2. Rietveld refinement of the powder data was undertaken using the structure of LaTa5O14as a starting model for the rankamaite structure. The structural formula obtained with the Rietveld analyses is: (Na2.21K1.26)Σ3.37(Ta9.12Nb1.30Al0.59)Σ11.00[O26.29(OH)4.71]Σ31.00. The tantalum atoms are coordinated by six and seven oxygen atoms in the form of distorted TaO6octahedra and TaO7pentagonal bipyramids, respectively. Every pentagonal bipyramid shares edges with four octahedra, thus forming Ta5O14units. The potassium atom is in an 11-fold coordination, whereas one sodium atom is in a 10-fold and the other is in a 12-fold coordination. Raman and infrared spectroscopy were used to investigate the room-temperature spectra of rankamaite.