Vitor Paulo Pereira

Mineralogical characterization of a highly-weathered soil by the Rietveld Method

The mineralogical characterization through mineral quantification of Brazilian soils by X-ray diffraction data using the Rietveld Method is not common. A mineralogical quantification of an Acric Ferralsol from the Ponta Grossa region, state of Parana, Brazil, was carried out using this Method with X-Ray Diffraction data to verify if this method was suitable for mineral quantification of a highly-weathered soil. The A, AB and B 3 horizons were fractioned to separate the different particle sizes: clay, silt, fine sand (by Stokes Law) and coarse sand fractions (by sieving), with the procedure free of chemical treatments. X-ray Fluorescence, Inductively Coupled Plasma Atomic Emission Spectrometry, Infrared Spectroscopy and Mossbauer Spectroscopy were used in order to assist the mineral identification and quantification. The Rietveld Method enabled the quantification of the present minerals. In a general way, the quantitative mineralogical characterization by the Rietveld Method revealed that quartz, gibbsite, rutile, hematite, goethite, kaolinite and halloysite were present in the clay and silt fractions of all horizons. The silt fractions of the deeper horizons were different from the more superficial ones due to the presence of large amounts of quartz. The fine and the coarse sand fractions are constituted mainly by quartz. Therefore, a mineralogical quantification of the finer fraction (clay and silt) by the Rietveld Method was successful.

Waimirite-(Y), orthorhombic YF3, a new mineral from the Pitinga mine, Presidente Figueiredo, Amazonas, Brazil and from Jabal Tawlah, Saudi Arabia: description and crystal structure

Abstract Waimirite-(Y) (IMA 2013-108), orthorhombic YF3, occurs associated with halloysite, in hydrothermal veins (up to 30 mm thick) cross-cutting the albite-enriched facies of the A-type Madeira granite (~1820 Ma), at the Pitinga mine, Presidente Figueiredo Co., Amazonas State, Brazil. Minerals in the granite are ‘K-feldspar’, albite, quartz, riebeckite, ‘biotite’, muscovite, cryolite, zircon, polylithionite, cassiterite, pyrochlore-group minerals, ‘columbite’, thorite, native lead, hematite, galena, fluorite, xenotime-(Y), gagarinite-(Y), fluocerite-(Ce), genthelvite–helvite, topaz, ‘illite’, kaolinite and ‘chlorite’. The mineral occurs as massive aggregates of platy crystals up to ~1 μm in size. Forms are not determined, but synthetic YF3 displays pinacoids, prisms and bipyramids. Colour: pale pink. Streak: white. Lustre: non-metallic. Transparent to translucent. Density (calc.) = 5.586 g/cm3 using the empirical formula. Waimirite-(Y) is biaxial, mean n = 1.54-1.56. The chemical composition is (average of 24 wavelength dispersive spectroscopy mode electron microprobe analyses, O calculated for charge balance): F 29.27, Ca 0.83, Y 37.25, La 0.19, Ce 0.30, Pr 0.15, Nd 0.65, Sm 0.74, Gd 1.86, Tb 0.78, Dy 8.06, Ho 1.85, Er 6.38, Tm 1.00, Yb 5.52, Lu 0.65, O (2.05), total (97.53) wt.%. The empirical formula (based on 1 cation) is (Y0.69Dy0.08Er0.06Yb0.05Ca0.03Gd0.02Ho0.02Nd0.01Sm0.01Tb0.01Tm0.01Lu0.01)∑1.00[F2.54⃞0.25O0.21]∑3.00. Orthorhombic, Pnma, a = 6.386(1), b = 6.877(1), c = 4.401(1) Å, V = 193.28(7) Å3, Z = 4 (powder data). Powder X-ray diffraction (XRD) data [d in Å (I) (hkl)]: 3.707 (26) (011), 3.623 (78) (101), 3.438 (99) (020), 3.205 (100) (111), 2.894 (59) (210), 1.937 (33) (131), 1.916 (24) (301), 1.862 (27) (230). The name is for the Waimiri-Atroari Indian people of Roraima and Amazonas. A second occurrence of waimirite-(Y) is described from the hydrothermally altered quartz-rich microgranite at Jabal Tawlah, Saudi Arabia. Electron microprobe analyses gave the empirical formula (Y0.79Dy0.08Er0.05Gd0.03Ho0.02Tb0.01 Tm0.01Yb0.01)∑1.00[F2.85O0.08⃞0.07]∑3.00. The crystal structure was determined with a single crystal from Saudi Arabia. Unit-cell parameters refined from single-crystal XRD data are a = 6.38270(12), b = 6.86727(12), c = 4.39168(8) Å, V = 192.495(6) Å3, Z = 4. The refinement converged to R1 = 0.0173 and wR2 = 0.0388 for 193 independent reflections. Waimirite-(Y) is isomorphous with synthetic SmF3, HoF3 and YbF3. The Y atom forms a 9-coordinated YF9 tricapped trigonal prism in the crystal structure. The substitution of Y for Dy, as well as for other lanthanoids, causes no notable deviations in the crystallographic values, such as unit-cell parameters and interatomic distances, from those of pure YF3.

A variabilidade de composição da apatita associada a carbonatitos

Apatite, one of the most abundant non-silicate minerals, is practically the only ore mineral in phosphatic mines over the world. The simplified structural formula of fluorapatite (the commonest type) is Ca1O (P0(4))6 F2. These elements may be substituted by others, in all crystallochemical sites, in apatite from all sorts of geological environments, even from organic and synthetic origins. Many of the natural and inorganic examples, mainly associated with carbonatites, are discussed here. The occupation ofthe monovalent site gives the official names to the species: fluorapatite, hydroxylapatite and chlorapatite. Among the substituents for calcium, can be mentioned rare earth elements and others of environmental importance, such as cadmium, which may be incorporated into fertilizers, leading to contamination problems. There are two possible positions for calcium substituents; several recent studies explore this aspect, evincing interacting factors, controlling the Ca1 or Ca2 site preference of each substituent cation, depending on their characteristics. ln the substitution for phosphate anions, carbonate is the most important anion, and its entry in apatite produces structural changes and requires mechanisms to keep the charge equilibrium neutral, -such as coupled substitutions or creation of vacancies in the structure. This question has not yet been solved, and has been argued for decades; it is considered fundamental to the understanding of solubility, reactivity, clistalinity, and themal stability.

Caracterização Petrográfica e Geoquímica da Parte Leste do Granito Europa, Distrito Mineiro de Pitinga, AM

The important mineral deposits of the Pitinga Mine, in the Amazonian region are related to A-type granites intruded in the Iricoume Group. The Europa granite is one of these A-type rocks, intruded in the Iricoume Group, which is represented by subaerial vulcanoclastic rocks (crystal-rich ignimbrites, thin massive tuffs and siltic tufaceous arenites) and minor hipabissal rhyolites. The volcanic rocks were probably generated in a caldera environment. The Europa granite is an alkali-feldspar peralkaline granite (hipersolvus) without genetic relationship with to the volcanic rocks of the Iricoume Group, but it could have been generated during the resurgence stages. The petrographic and geochemical data attest that fractional crystallization process was the principal mechanism during the crystallization, which led to the generation of two different granitic facies. The Nb soil anomalies overprinted on the more differentiated facies are related to the astrophillite weathering.

Evaluation of biological absorption coefficient of trace elements in plants from the pitinga mine district, amazonian region

Specimens of Ampelozizyphus amazonicus and Adiantum sp., together with adjacent soils, were sampled in the Pitinga Mine District, Amazonian region, in order to investigate the distribution of some trace elements in plants and soils, and their relation to the presence of mineral deposits. The Pitinga Mine contains large deposits of tin, with high concentrations of niobium and zirconium, hosted by the Madeira Granite, which is intrusive into a volcanic sequence named the Iricoume group, all of them with Paleoproterozoic age. Our results point to the potential use, for both plants, of the Biological Absorption Coefficient (BAC) as an indicator of mineral deposits when the elements involved in this process have moderate to high mobility in the supergene environment. The high BAC for gold indicates that this element can be used as an indicator of gold deposits. The presence of sulfide deposits is indicated by high BAC for Cu, Zn and Pb, whereas tin deposits are indicated by increasing BAC for Y and Sn. This suggests that the BAC of some trace elements in both plants is a good indicator of geochemical enrichment associated with mineral deposits. The importance of biogeochemistry for mineral exploration is confirmed for areas with thick vegetal cover.

Late sodic metasomatism evidences in bimodal volcanic rocks of the Acampamento Velho Alloformation, Neoproterozoic III, southern Brazil

A mineralogical study was carried out in mafic and felsic volcanic rocks of the Acampamento Velho Alloformation at Cerro do Bugio, Perau and Serra de Santa Barbara areas (Camaqua Basin) in southern Brazil. The Acampamento Velho bimodal event consists of two associations: lower mafic at the base and upper felsic at the top. Plagioclase and alkali-feldspar were studied using an electronic microprobe, and magnetite, ilmenite, rutile, illite and alkali-feldspar were investigated through scanning electron microscopy. The rocks were affected by a process of late sodic autometasomatism. In mafic rocks, Ca-plagioclase was transformed to albite and pyroxenes were altered. In felsic rocks, sanidine was partially pseudomorphosed, generating heterogeneous alkali-feldspar. In this association, unstable Ti-rich magnetite was replaced by rutile and ilmenite. In mafic rocks, the crystallization sequence was: (1) Ti-rich magnetite (?), (2) pyroxene and Ca-plagioclase, (3) albite (alteration to Ca-plagioclase), (4) sericite, chlorite and calcite (alteration to pyroxene), and kaolinite (alteration to plagioclase/albite). In felsic rocks: (1) zircon, (2) Ti-rich magnetite, (3) sanidine, (4) quartz. The introduction of late Na-rich fluids, generated the formation of (5) heterogeneous alkali-feldspar, (6) ilmenite and rutile from the Ti-rich magnetite, (7) albite in the spherulites. Finally, alteration of sanidine, vitroclasts and pumice to (8) illite.

Variações mineralógicas e petrográficas na porção central do albita-granito Madeira, Pitinga, AM

O albita-granito de Pitinga e uma facies porfiritica do granito paleoproterozoico Madeira (~1,83Ga) que contem concentracoes importantes de criolita disseminada e macica na porcao central de sua facies de nucleo. Os fenocristais, principalmente de quartzo, mas tambem feldspato potassico, mica e anfibolio, associados a zircao, criolita, polilitionita, pirocloro, cassiterita e minerais opacos, sao corroidos por uma matriz composta por albita, feldspato potassico, quartzo, cassiterita, criolita e minerais opacos, alem de fluorita secundaria. As variacoes mineralogicas e petrograficas observadas nos tres furos de sondagem estudados, localizados na porcao central do granito, sugerem que solucoes salinas ricas em agua exsolvidas a partir do mesmo corpo de magma permitiram a ocorrencia de reacoes de oxidacao e processos de criolitizacao, argilizacao e silicificacao que corroeram e substituiram os minerais magmaticos em um estagio tardio de sua historia de resfriamento. Palavras-chave: albita-granito, criolita, Pitinga, Granito Madeira, Proterozoico.