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

Autor(es)

Atencio, Daniel ^[1] ; Contreira Filho, Reynaldo R. ; Mills, Stuart J. ^[2] ; Coutinho, Jose M. V. ^[1] ; Honorato, Sara B. ^[3] ; Ayala, Alejandro P. ^[3] ; Ellena, Javier ^[4] ; de Andrade, Marcelo B. ^[4]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Inst Geociencias, Dept Mineral & Geotecton, BR-05508080 Sao Paulo - Brazil

^[2] Univ British Columbia, Dept Earth & Ocean Sci, Vancouver, BC V6T 1Z4 - Canada

^[3] Univ Fed Ceara, Dept Fis, BR-60455900 Fortaleza, Ceara - Brazil

^[4] Univ Sao Paulo, Inst Fis Sao Carlos, Dept Fis & Informat, BR-13560970 Sao Carlos, SP - Brazil

• Tipo de documento: Artigo Científico
• Fonte: AMERICAN MINERALOGIST; v. 96, n. 10, p. 1455-1460, OCT 2011.
• Citações Web of Science: 2

Resumo

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%): Na(2)O 2.08 (1.95-2.13), K(2)O 2.61 (2.52-2.74), Al(2)O(3) 1.96 (1.89-2.00), Fe(2)O(3) 0.01 (0.00-0.03), TiO(2) 0.02 (0.00-0.06), Ta(2)O(5) 81.04 (79.12-85.18), Nb(2)O(5) 9.49 (8.58-9.86), total 97.21 (95.95-101.50). The chemical formula derived from this analysis is (Na(1.55)K(1.28))(Sigma 2.83)(Ta(8.45)Nb(1.64)Al(0.89)Fe(0.01)(3+)Ti(0.01))(Sigma 11.00){[}O(25.02)(OH)(5.98)](Sigma 31.00). 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) angstrom, V = 1199.1(3) angstrom(3), Z = 2. Rietveld refinement of the powder data was undertaken using the structure of LaTa(5)O(14) as a starting model for the rankamaite structure. The structural formula obtained with the Rietveld analyses is: (Na(2.21)K(1.26))Sigma(3.37)(Ta(9.12)NB(1.30) Al(0.59))(Sigma 11.00){[}O(26.29)(OH)(4.71)](Sigma 31.00). The tantalum atoms are coordinated by six and seven oxygen atoms in the form of distorted TaO(6) octahedra and TaO(2) pentagonal bipyramids, respectively. Every pentagonal bipyramid shares edges with four octahedra, thus forming Ta(5)O(14) units. 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. (AU)

• Processo FAPESP: 09/09125-5 - Minerais novos do Brasil: caracterização cristaloquímica e síntese
• Beneficiário: Daniel Atencio
• Modalidade de apoio: Auxílio à Pesquisa - Regular
• Processo FAPESP: 08/04984-7 - Determinação e refinamento da estrutura cristalina de minerais do grupo de pirocloro
• Beneficiário: Marcelo Barbosa de Andrade
• Modalidade de apoio: Bolsas no Brasil - Pós-Doutorado