Browsing by Author "Stern, Willem B."

Now showing 1 - 4 of 4
  • No Thumbnail Available
    Item
    Characterization of beryl (aquamarine variety) from pegmatites of Minas Gerais, Brazil.
    (2002) Viana, Rúbia Ribeiro; Evangelista, Hanna Jordt; Costa, Geraldo Magela da; Stern, Willem B.
    Eight samples of the beryl variety aquamarine were selected from four pegmatites in the Governador Valadares and Arac¸uaı´ regions in northeastern Minas Gerais State, Brazil. These samples were fully characterized by chemical analysis, infrared and UV-visible spectroscopy, thermal analyses, and high-temperature X-ray diffraction (from room temperature up to 800 C). Several physical and chemical properties of beryl were found to depend on the amount of water and ions residing in the structural channels. The thermal expansion coefficients from room temperature to about 800 C are temperature-independent, with aa @ )3.2 · 10)6 C)1 and ac @ )8.7 · 10)6 C)1. The contraction of both a and c unit-cell parameters with increasing temperature and the shift of the infrared band centered at about 1200 cm)1 were tentatively ascribed to interactions between channel water and the silicate rings. The color of beryl seems to be dictated by the relative proportions of Fe3+ in the octahedral sites and of fe2+ in the channels. Thus, deep-blue samples have little Fe3+, whereas greener samples have more Fe3+ or less channel Fe2+.
  • No Thumbnail Available
    Item
    Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province.
    (2006) Viana, Rúbia Ribeiro; Evangelista, Hanna Jordt; Stern, Willem B.
    Um grande cristal de muscovita, macroscopicamente homogêneo, procedente do Pegmatito Cruzeiro, localizado na Província Pegmatítica Oriental, em Minas Gerais, exibe padrão de distribuição complexa para alguns elementos traços. Em estudos geocronológicos e petrológicos, como, por exemplo, na separação entre micas magmáticas e pós-magmáticas, a causa de zoneamento deve ser levada em consideração. O complexo zoneamento químico no cristal de mica estudado é melhor explicado pelo crescimento em um magma evoluído, seguido pela alteração, proveniente da percolação de fluidos hidrotermais. O enriquecimento de Rb nas bordas é interpretado como resultado da evolução química do magma residual durante o crescimento do cristal. A diminuição em (IVAl+VIAl), bem como o aumento de (Fe+Mg) e Si ao longo da fratura é explicado pela substituição hidrotermal celadonítica da muscovita. A alteração hidrotermal causou, também, a diminuição nos conteúdos de Rb, Ga, Y, Nb, Sn e Zn ao longo desta fratura, além da concentração residual de Ti. Elementos tais como, Ga, Y, Nb, Sn, e Zn, pouco considerados em discussão de diferenciação ou processos de alteração, mostraram significância tanto quanto os elementos alcalinos.
  • No Thumbnail Available
    Item
    Geochemistry of muscovite from pegmatites of the Eastern Brazilian pegmatite province : a clue to petrogenesis and mineralization potential.
    (2007) Viana, Rúbia Ribeiro; Evangelista, Hanna Jordt; Stern, Willem B.
    Single crystals of muscovite (N = 143) from different zones of selected granitic pegmatites belonging to two wellknown gem-producing districts of the Eastern Brazilian Pegmatite Province (northern Araçuai and western Governador Valadares in Minas Gerais) were analyzed for major, minor and trace elements. Structural formulae display considerable (Fe+Mg) variation within the octahedral site, coupled with tetrahedral Si-Al substitution (Tschermak substitution – phengite component). Tetrahedral Si varies from 3.06 to 3.21 and AlVI from 1.71 to 1.90 apfu. Trace elements in muscovite are significant indicators for the economic potential of pegmatites as well as for the differentiation degree and origin of the magma. Muscovite with higher Li and B contents is characteristic for gem-tourmaline-bearing pegmatites. Lower concentrations in K/Rb, Ti and Mg and higher F contents are found in muscovites from higher differentiated zones within the pegmatite. In comparison with anatectic pegmatites, relatively low K/Rb ratios of muscovite from the studied pegmatites are indicative of derivation by fractional crystallization of granite magma. Zn, Ga and Y, elements rarely analyzed in micas, tend to increase with decreasing K/Rb ratios, showing them to be good indicators of the fractionation degree of magmas.
  • No Thumbnail Available
    Item
    The unusual Mössbauer spectrum of beryl.
    (2001) Viana, Rúbia Ribeiro; Costa, Geraldo Magela da; Evangelista, Hanna Jordt; Stern, Willem B.
    The Mössbauer spectra of several aquamarine samples have been obtained in the temperature range of 4.2–500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe2+ doublet ( EQ ∼ 2.7 mm/s, δ ∼ 1.1 mm/s), with a very broad lowvelocity peak. This asymmetry is not caused by preferred orientation since the spectrum collected under the magic angle did not show any difference in the line intensities, nor is it caused by the superposition of a Fe3+ doublet. At 4.2 K the spectrum of a deep-blue beryl could be well fitted with three symmetrical doublets, with the major Fe2+ doublet accounting for 87% of the total spectral area. At 14 K the symmetry remains, but at 30 K the low-velocity peak is again broad. Surprisingly, the spectrum at 500 K also shows a broad, but symmetrical doublet, with a clear splitting of the lines indicating the presence of at least two Fe2+ components. The room-temperature spectrum obtained after the 500 K run shows the same features as before the heating. A meaningful fit for the roomtemperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed.