10 results
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2. Letters.
- Subjects
LETTERS to the editor ,CURIOSITIES & wonders ,MINERAL collecting ,MINES & mineral resources ,GEMS & precious stones ,CONFERENCES & conventions - Abstract
Several letters to the editor are presented in response to articles including "Mineral Oddities" by Robert Cook in the January/February 2009 issue and "Spessartine from Navegadora Mine, Minas Gerais, Brazil" by John White in the January/February 2009 issue, and one which calls for papers for the Gem and Mineral Show on February 13, 2010.
- Published
- 2009
- Full Text
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3. U–Pb SHRIMP and Sm–Nd geochronology of granite–gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain
- Author
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Queiroz, Cláudia Lima, Jost, Hardy, da Silva, Luiz Carlos, and McNaughton, Neal J.
- Subjects
- *
IGNEOUS rocks , *GEMS & precious stones , *GEOLOGICAL time scales - Abstract
Abstract: The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goiás, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism, spanning ca. 140m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550Ma. One of the banded gneisses with a crystallization age of ca. 2700Ma (2σ) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590±10Ma (2σ). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses. [Copyright &y& Elsevier]
- Published
- 2008
- Full Text
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4. Quartz with Triphylite Inclusions from Brazil.
- Author
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Costanzo, Alessandra and Laurs, Brendan M.
- Subjects
GEMS & precious stones ,PHOSPHATE minerals ,QUARTZ ,DIFFRACTION gratings ,SILICA ,QUARTZ crystals - Published
- 2019
- Full Text
- View/download PDF
5. Purple to Reddish Purple Chrysoberyl from Brazil.
- Author
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Schmetzer, Karl, Hyršl, Jaroslav, Bernhardt, Heinz-Jürgen, and Hainschwang, Thomas
- Subjects
CHRYSOBERYL ,MINERAL analysis ,MINES & mineral resources ,GEMS & precious stones ,GEMOLOGY - Abstract
Mineralogical and gemmological data of two intensely coloured chrysoberyls from Brazil are presented. Both samples are twinned, and they appear violet-purple or purple in daylight, and reddish purple or red-purple in incandescent light. They have good transparency, but appear very dark in reflected light due to their enriched chromium contents. In one sample, an average of 1.54 wt.% Cr
2 O3 was measured, together with 0.60 wt.% Fe2 O3 . The high Cr content is responsible for a reduced transmission in the blue-green range of the visible spectrum, which causes a shift in the daylight coloration from blue-green for typical Cr-bearing chrysoberyl (alexandrite) to violet-purple or purple for such samples with distinctly higher Cr values. [ABSTRACT FROM AUTHOR]- Published
- 2014
- Full Text
- View/download PDF
6. Purplish-red almandine garnets with alexandrite-like effect: causes of colors and color-enhancing treatments.
- Author
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Krambrock, K., Guimarães, F., Pinheiro, M., Paniago, R., Righi, A., Persiano, A., Karfunkel, J., and Hoover, D.
- Subjects
GARNET ,ALEXANDRITE ,CRYSTALS ,ELECTRON probe microanalysis ,GEMS & precious stones ,COLORIMETRIC analysis - Abstract
Fine gem-quality, purplish-red garnets from the Tocantins State, Brazil, were investigated for their crystal chemistry and optical properties by several spectroscopic techniques, including electron microprobe analysis, Mössbauer, Raman spectroscopy and optical absorption. Although most garnets are purplish-red, some specimens show color zoning, with deep red color in the core and purple in the outer parts. Electron microprobe analysis showed that these garnets are principally almandine-pyrope solid solution at the rim. However, at the red core, they contain also up to 7 % of spessartine. Mössbauer spectroscopy reveals that the iron content is predominantly Fe (>99 %) in the natural garnets. The optical absorption spectra are dominated by spin-allowed and unusual high-intense spin-forbidden transitions from eightfold coordinated Fe(II) in the near infrared and visible spectral region, respectively. For the red core, in addition, three sharp bands centered in the blue part of the visible spectral range and a broad charge transfer band in the near-UV region are observed. All garnets with purplish colors show also a remarkable color-changing effect from purple in daylight light to red in incandescent light called alexandrite-like effect. Heat treatments in the 700-900 °C temperature range in oxidizing and reducing atmospheres lead to reversible and irreversible color changes which are discussed based on the microscopic changes in the Fe ion coordination and valence states. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
7. The History of Kunzite and the California Connection.
- Author
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Mauthner, Mark
- Subjects
SPODUMENE ,GEMS & precious stones ,HISTORY - Abstract
The article focuses on history of kunzite, a variety of spodumene that can be recognize for its pink and lilac to bluish-purple color. It states Fred M. Sickler is credited as the discovery of spodumene gem because he is the first to present an example of the mineral to George F. Kunz. It also mentions that kunzite was discovered at Simmons mine, Pala Chief Mine, and Vanderberg mine. It also mention that there were kunzite finds in California, Brazil, and Madagascar.
- Published
- 2011
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- View/download PDF
8. U–Pb zircon and 40Ar–39Ar K-feldspar dating of syn-sedimentary volcanism of the Neoproterozoic Maricá Formation: constraining the age of foreland basin inception and inversion in the Camaquã Basin of southern Brazil
- Author
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Borba, André W., Mizusaki, Ana Maria P., Santos, João Orestes S., McNaughton, Neal J., Onoe, Artur T., and Hartmann, Léo A.
- Subjects
ROCK-forming minerals ,GEMS & precious stones ,STRUCTURAL geology ,ROCKS ,SEDIMENTARY rocks ,SEDIMENTOLOGY ,GEOLOGICAL basins ,SEDIMENTARY basins - Abstract
New U–Pb zircon and
40 Ar–39 Ar K-feldspar data are presented for syn-sedimentary volcanogenic rocks from the Neoproterozoic Maricá Formation, located in the southern Brazilian shield. Seven (of nine) U–Pb sensitive high-resolution ion microprobe analyses of zircons from pyroclastic cobbles yield an age of 630.2±3.4 Ma (2σ), interpreted as the age of syn-sedimentary volcanism, and thus of the deposition itself. This result indicates that the Maricá Formation was deposited during the main collisional phase (640–620 Ma) of the Brasiliano II orogenic system, probably as a forebulge or back-bulge, craton-derived foreland succession. Thus, this unit is possibly correlative of younger portions of the Porongos, Brusque, Passo Feio, Abapã (Itaiacoca) and Lavalleja (Fuente del Puma) metamorphic complexes. Well-defined, step-heating40 Ar–39 Ar K-feldspar plateau ages obtained from volcanogenic beds and pyroclastic cobbles of the lower and upper successions of the Maricá Formation yielded 507.3±1.8 Ma and 506.7±1.4 Ma (2σ), respectively. These data are interpreted to reflect total isotopic resetting during deep burial and thermal effects related to magmatic events. Late Middle Cambrian cooling below ca. 200 °C, probably related to uplift, is tentatively associated with intraplate effects of the Rio Doce and/or Pampean orogenies (Brasiliano III system). In the southern Brazilian shield, these intraplate stresses are possibly related to the dominantly extensional opening of a rift or a pull-apart basin, where sedimentary rocks of the Camaquã Group (Santa Bárbara and Guaritas Formations) accumulated. [ABSTRACT FROM AUTHOR]- Published
- 2008
- Full Text
- View/download PDF
9. Formation Conditions and 40Ar/39Ar Age of the Gem-Bearing Boqueirão Granitic Pegmatite, Parelhas, Rio Grande do Norte, Brazil.
- Author
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Strmić Palinkaš, Sabina, Palinkaš, Ladislav, Neubauer, Franz, Scholz, Ricardo, Borojević Šoštarić, Sibila, and Bermanec, Vladimir
- Subjects
TANTALUM ,SPODUMENE ,FLUID inclusions ,MUSCOVITE ,GEMS & precious stones ,ARGON-argon dating - Abstract
The Boqueirão granitic pegmatite, alias Alto da Cabeça pegmatite, is situated in Borborema Pegmatitic Province (BPP) in Northeast Brazil. This pegmatitic province hosts globally important reserves of tantalum and beryllium, as well as significant quantities of gemstones, including aquamarine, morganite, and the high-quality turquoise-blue "Paraíba Elbaite". The studied lithium-cesium-tantalum Boqueirão granitic pegmatite intruded meta-conglomerates of the Equador Formation during the late Cambrian (502.1 ± 5.8 Ma;
40 Ar/39 Ar plateau age of muscovite). The pegmatite exhibits a typical zonal mineral pattern with four defined zones (Zone I: muscovite, tourmaline, albite, and quartz; Zone II: K-feldspar (microcline), quartz, and albite; Zone III: perthite crystals (blocky feldspar zone); Zone IV: massive quartz). Huge individual beryl, spodumene, tantalite, and cassiterite crystals are common as well. Microscopic examinations revealed that melt inclusions were entrapped simultaneously with fluid inclusions, suggesting the magmatic–hydrothermal transition. The magmatic–hydrothermal transition affected the evolution of the pegmatite, segregating volatile compounds (H2 O, CO2 , N2 ) and elements that preferentially partition into a fluid phase from the viscous silicate melt. Fluid inclusion studies on microcline and associated quartz combined with microthermometry and Raman spectroscopy gave an insight into the P-T-X characteristics of entrapped fluids. The presence of spodumene without other LiAl(SiO3 )2 polymorphs and constructed fluid inclusion isochores limited the magmatic–hydrothermal transition at the gem-bearing Boqueirão granitic pegmatite to the temperature range between 300 and 415 °C at a pressure from 1.8 to 3 kbar. [ABSTRACT FROM AUTHOR]- Published
- 2019
- Full Text
- View/download PDF
10. Cartografia da Promessa: Potosi e o Brasil em um continente chamado Peruana.
- Author
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Alves de Oliveira, Antonio José
- Subjects
HISTORY of cartography ,GEMS & precious stones ,SIXTEENTH century - Abstract
In the final years of the 1590s, the Dutch cartographer Petrus Plancius (1552-1622) had represented not only South America but also the ocean to the west of it, known now as the Pacific, as I America Peruana i and I Oceanus Peruvianus i respectively, metonymically claiming the entire continent for the Vice-Royalty of Peru, an interesting hint followed by Doré to understand the expectations created by the Europeans about the continent. In I Cartografia da Promessa: Potosi e o Brasil em um continente chamado Peruana i (Cartography of Promise: Potosi and Brazil in a Continent called Peruana), the historian Andréa Doré (Universidade Federal do Paraná) makes an important study of South American cartography during the sixteenth and seventeenth centuries. Doré's approach is to track the expectations and to analyse the accounts of those who had experienced the space and narrated what they saw to show us how these narratives were portrayed on the maps. [Extracted from the article]
- Published
- 2021
- Full Text
- View/download PDF
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