DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Resultados da Pesquisa
Item Formation conditions and 40Ar/39Ar age of the Gem-Bearing Boqueirão Granitic Pegmatite, Parelhas, Rio Grande do Norte, Brazil.(2019) Palinkaš, Sabina Strmic; Palinkaš, Ladislav; Neubauer, Franz; Cipriano, Ricardo Augusto Scholz; Šoštaric, Sibila Borojevic; Bermanec, VladimirThe 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; 40Ar/39Ar 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 (H2O, 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. View Full-TextItem Metavivianite, Fe2+Fe3+2(PO4)2(OH)2 6H2O : new data and formula revision.(2012) Chukanov, Nikita V.; Cipriano, Ricardo Augusto Scholz; Aksenov, Sergey M.; Rastsvetaeva, Ramiza K.; Pekov, Igor V.; Belakovskiy, Dmitriy I.; Krambrock, Klaus Wilhelm Heinrich; Paniago, Roberto Magalhães; Righi, Ariete; Martins, R. F.; Belotti, Ricardo Augusto; Bermanec, VladimirThe composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mo¨ssbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galile´ia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with a = 1.600(3), b = 1.640(3), g = 1.685(3) and 2Vmeas = 85(5)º. The measured and calculated densities are Dmeas = 2.56(2) and Dcalc = 2.579 g cm 3. The chemical composition, based on electronmicroprobe analyses, Mo¨ssbauer spectroscopy (to determine the Fe2+:Fe3+ ratio) and gas chromatography (to determine H2O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe2O3 26.60, P2O5 28.62, H2O 26.5; total 101.32 wt.%. The empirical formula is (Fe3+ 1.64Fe2+ 1.23Mg0.085Mn0.06)S3.015(PO4)1.98(OH)1.72·6.36H2O. Metavivianite is triclinic, P1¯ , a = 7.989(1), b = 9.321(2), c = 4.629(1) A ˚ , a = 97.34(1), b = 95.96(1), g = 108.59(2)º, V = 320.18(11) A ˚ 3 and Z = 1. The crystal structure was solved using a single-crystal techniques to an agreement index R = 6.0%. The dominant cations in the independent sites are Fe2+ and Fe3+, with multiplicities of 1 and 2, respectively. The simplified crystal-chemical formula for metavivianite is Fe2+(Fe3+,Fe2+)2(PO4)2(OH,H2O)2·6H2O; the endmember formula is Fe2+Fe3+ 2 (PO4)2(OH)2·6H2O, which is dimorphous with ferrostrunzite.Item Pseudomorphs of low microcline adularia fourlings from the Alto da Cabeça (Boqueirão) and Morro Redondo pegmatites, Brazil.(2012) Bermanec, Vladimir; Horvat, Marua; Gobac, Željka Žigovečki; Zebec, Vladimir; Cipriano, Ricardo Augusto Scholz; Škoda, Radek; Wegner, Reinhard; Barreto, Sandra de Brito; Dodony, IstvanPotassium feldspar crystals from the Alto da Cabei;:a (Boqueiriio), Rio Grande do Norte (sample BQ) and the Morro Redondo, Minas Gerais (sample MR) pegmatites in Brazil bave a typical adularia fourling habit consisting of a combination of Baveno and Manebach twins. Crystal morphology suggests a monoclinic symmetry, but XRPD measurements and TEM experiments show that the samples are low microcline. Calculated values of triclinicity [A = 0.915 (BQ) and A = 0.9225 (MR)], cell parameters [a8.587(4), b 12.969(4), c 7.223(2) A, o: 90.66(3) 11 115.97(4), 'I 87.69(3)0 for BQ and a 8.584(3), b 12.970(3), c 7.225(2) A, o: 90.66(3) 11 115.97(3), 'I 87.74(3)0 for MR), t1o occupancy of A! [0.99 (BQ) and 0.98 (MR)], order index [dsM = 0.97 (BQ) and dsM= 0.99 (MR)] and IAI at the T1 sites [98.6% (BQ) and 99.5% (MR)] reveal a triclinic syrnmetry and a high state of order for the sarnples of potassium feldspar. Transmission electron microscopy and selected-area electron-diffraction investigations of the crystals show that they are compositionally homogeneous, without exsolution larnellae of other phases. The electron-diffraction pattems show no splitting of any refiection and no streaking, and thus indicate ideal order in both sarnples. According to crystal morphology and the results on submicroscopic scale, we conclude that in this case adularia sarnples bave a completely ordered structure. The measurements and calculations demonstrate that they are highly ordered, i.e„ maximum microcline. Adularia pseudomorphs are highly ordered triclinic feldspar developed through transformation of a monoclinic pareni.Item Crystal morphology and xrd peculiarities of brazilianite from different localities.(2011) Čobić, Andrea; Zebec, Vladimir; Cipriano, Ricardo Augusto Scholz; Bermanec, Vladimir; Barreto, Sandra de BritoForty four brazilianite crystals from several localities in Brazil, Rwanda and Canada were measured on a two-circle goniometer to determine brazilianite morphology. Twenty forms were recorded; six of them have not been recorded before. All faces in the [001] zone are striated along crystallographic axis c. All striated forms in the [001] zone exhibit multiple signals. Two of the signals observed on the form {110} are always very clear. There is an exception on one crystal where just one face, (110), exhibits only one clear signal. Five groups of habits were recorded, two of them new to this mineral species. Eleven samples were examined by X-ray diffraction for calculation of the unit cell parameters yielding a=11.201(1)–11.255(2) Å, b=10.1415(5)–10.155(1) Å, c=7.0885(7)–7.119(2) Å and b=97.431(7)–97.34(1) °. All X-ray diffraction patterns show a peculiarity: some diffraction lines are widened or doubled with the appearance of additional diffraction lines systematically on lower °2Q. These diffraction lines have smaller intensities and cannot be indexed in accordance with brazilianite crystal structure.