Melcherite, trigonal Ba2Na2Mg[Nb6O19]·6H2O, the second natural hexaniobate, from Cajati, São Paulo, Brazil: Description and crystal structure
| Autor: | Marcelo B. Andrade, Luiz A.D. Menezes Filho, John Spratt, Daniel Atencio |
|---|---|
| Rok vydání: | 2018 |
| Předmět: |
Materials science
Fluorapatite Pyrochlore Crystal structure engineering.material 010502 geochemistry & geophysics 010403 inorganic & nuclear chemistry 01 natural sciences 0104 chemical sciences Crystallography Geochemistry and Petrology Quintinite CRISTALOGRAFIA engineering Pyrrhotite Single crystal Chemical composition 0105 earth and related environmental sciences Zirkelite |
| Zdroj: | Repositório Institucional da USP (Biblioteca Digital da Produção Intelectual) Universidade de São Paulo (USP) instacron:USP |
| ISSN: | 1471-8022 0026-461X |
| DOI: | 10.1180/minmag.2017.081.026 |
| Popis: | Melcherite (IMA2015-018), ideally Ba2Na2Mg[Nb6O19]·6H2O, occurs as a vug mineral in the carbonatite of the Jacupiranga mine, Cajati county, São 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 µm in maximum dimension. Melcherite is transparent and displays a vitreous lustre; it is beige with a white streak. It is non-fluorescent. The mineral displays perfect cleavage on {001}. Chemical composition varies from Ba2Na2Mg[Nb6O19].6H2O to (BaK)(NaCa)Mg[Nb6O19].6H2O. Empirical formulae for the first and the second compositions are: (Ba1.75K0.19)Σ1.94(Na1.80Ca0.19)Σ1.99(Mg0.96Mn0.02Al0.02)Σ1.00Nb6.02O19.00·6H2O and (Ba0.99K1.00)Σ1.99(Na1.02Ca0.96)Σ1.98(Mg0.95Mn0.05)Σ1.00Nb6.02O19.00·6H2O, respectively. Data for a single crystal with the second composition are: trigonal, R$\bar 3$, a = 9.0117(6) Å, c = 23.3986(16) Å, V = 1645.64(19) Å3 and Z = 3. Calculated density for this formula is 3.733 g/cm3, 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 [110] 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)O6] 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. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|