Melcherite, trigonal Ba2Na2Mg[Nb6O19]center dot 6H(2)O, the second natural hexaniobate, from Cajati, Sao Paulo, Brazil: Description and crystal structure

Autor(es)

Andrade, Marcelo B. ^[1] ; Atencio, Daniel ^[2] ; Menezes Filho, Luiz A. D. ^[3] ; Spratt, John ^[4]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Sao Carlos Inst Phys, Dept Phys & Interdisciplinary Sci, Caixa Postal 369, BR-13560970 Sao Carlos, SP - Brazil

^[2] Univ Sao Paulo, Inst Geociencias, Dept Mineral & Geotecton, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

^[3] Univ Fed Minas Gerais, Inst Geociencias, Ave Antonio Carlos 6627, BR-31270901 Belo Horizonte, MG - Brazil

^[4] Nat Hist Museum, Core Res Labs, Cromwell Rd, London SW7 5BD - England

• Tipo de documento: Artigo Científico
• Fonte: MINERALOGICAL MAGAZINE; v. 82, n. 1, p. 111-120, FEB 2018.
• Citações Web of Science: 1

Resumo

Melcherite (IMA2015-018), ideally Ba2Na2Mg{[}Nb6O19]center dot 6H(2)O, occurs as a vug mineral in the carbonatite of the Jacupiranga mine, Cajati county, Sao Paulo state, Brazil, associated with dolomite, calcite, magnetite, pyrrhotite, tochilinite, ` pyrochlore' and fluorapatite. This is also the type locality for zirkelite, quintinite, menezesite and pauloabibite. The mineral forms irregular, tabular crystals up to 200 mu m in maximum dimension. Melcherite is transparent and displays a vitreous lustre; it is beige with a white streak. It is nonfluorescent. The mineral displays perfect cleavage on [001]. Chemical composition varies from Ba2Na2Mg{[}Nb6O19]center dot 6H(2)O to (BaK)(NaCa) Mg{[}Nb6O19]center dot 6H(2)O. Empirical formulae for the first and the second compositions are: (Ba1.75K0.19)(Sigma 1.94)(Na1.80Ca0.19)(Sigma 1.99)(Mg0.96Mn0.02Al0.02)(Sigma 1.00)Nb6.02O19.00 center dot 6H(2)O and (Ba0.99K1.00)(Sigma 1.99)(Na1.02Ca0.96)(Sigma 1.98)(Mg0.95Mn0.05)(Sigma 1.00)Nb6.02O19.00 center dot 6H(2)O, respectively. Data for a single crystal with the second composition are: trigonal, R (3) over bar, a = 9.0117(6) angstrom, c = 23.3986(16) angstrom, V = 1645.64(19) angstrom(3) and Z = 3. Calculated density for this formula is 3.733 g/cm(3), and the calculated mean refractive index is 1.924. Melcherite is a hexaniobate that has structural layers parallel to the xy plane that stack along the c axis with simultaneous 1/3 {[}1 (1) over bar0] displacement so as to produce an R lattice. The melcherite structure is built by layers of {[}(Ba,K)(O,H2O)(9)] polyhedra and the {[}Nb6O19](8-) super-octahedron (Lindqvist anion) interconnected by {[}(Na,Ca)O-6] polyhedra. Cations of Mg2+ are bonded to six water molecules each and are not associated with Lindqvist oxygen ions. The mineral is named in honour of Geraldo Conrado Melcher (1924-2011), a pioneer in Jacupiranga carbonatite studies. (AU)

• Processo FAPESP: 11/22407-0 - Caracterização de espécies minerais brasileiras novas e raras
• Beneficiário: Daniel Atencio
• Modalidade de apoio: Auxílio à Pesquisa - Regular
• Processo FAPESP: 13/03487-8 - Centro de caracterização de novas espécies minerais: espectroscopia Raman, difração por raios X e nêutrons, e microssonda eletrônica
• Beneficiário: Marcelo Barbosa de Andrade
• Modalidade de apoio: Auxílio à Pesquisa - Jovens Pesquisadores