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Uranyl-Evansites from Porto (Northwest Portugal) and Galicia (Northwest Spain): Structure and Assignment of Spectra Catholuminescence and Raman Bands a
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S. Sanchez-Moral , A. Fernandez-Cortes , S. Cuezva , J. C. Cañaveras , V. Correcher c
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Chaminé , M. Furio & J. Garcia-Guinea a
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, A. Z. Miller , A. Dionisio , J. M. Marques , C. Saiz-Jimenez , M. J. Afonso , H. I. a
Departamento de Geologia, MNCN-CSIC, Madrid, Spain
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Laboratorio de Petrología Aplicada, Universidad de Alicante, UA-CSIC, Alicante, Spain
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CIEMAT, Madrid, Spain
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Centro de Petrologia e Geoquímica, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal e
IRNAS-CSIC Sevilla, Spain
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Laboratório de Cartografia e Geologia Aplicada (DEG), Instituto Superior de Engenharia do Porto (ISEP) and Centro GeoBioTec|UA, Portugal Available online: 17 Oct 2011
To cite this article: S. Sanchez-Moral, A. Fernandez-Cortes, S. Cuezva, J. C. Cañaveras, V. Correcher, A. Z. Miller, A. Dionisio, J. M. Marques, C. Saiz-Jimenez, M. J. Afonso, H. I. Chaminé, M. Furio & J. Garcia-Guinea (2011): Uranyl-Evansites from Porto (Northwest Portugal) and Galicia (Northwest Spain): Structure and Assignment of Spectra Catholuminescence and Raman Bands, Spectroscopy Letters, 44:7-8, 511-515 To link to this article: http://dx.doi.org/10.1080/00387010.2011.610412
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Spectroscopy Letters, 44:511–515, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0038-7010 print=1532-2289 online DOI: 10.1080/00387010.2011.610412
Uranyl-Evansites from Porto (Northwest Portugal) and Galicia (Northwest Spain): Structure and Assignment of Spectra Catholuminescence and Raman Bands S. Sanchez-Moral1, A. Fernandez-Cortes1, S. Cuezva2, J. C. Can˜averas2, V. Correcher3, A. Z. Miller4, A. Dionisio4, J. M. Marques4, C. Saiz-Jimenez5, M. J. Afonso6, H. I. Chamine´6, M. Furio1, and J. Garcia-Guinea1 1
Departamento de Geologia, MNCN-CSIC, Madrid, Spain 2 Laboratorio de Petrologı´a Aplicada, Universidad de Alicante, UA-CSIC, Alicante, Spain 3
CIEMAT, Madrid, Spain
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Centro de Petrologia e Geoquı´mica, Instituto Superior Te´cnico, Technical University of Lisbon, Lisbon, Portugal 5
IRNAS-CSIC Sevilla, Spain Laborato´rio de Cartografia e Geologia Aplicada (DEG), Instituto Superior de Engenharia do Porto (ISEP) and Centro GeoBioTecjUA, Portugal
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This submission was presented during the CORALS-2 Meeting on Micro-Raman Spectroscopy and Luminescence Studies in the Earth and Planetary Sciences, which was held between May 19 and 21, 2011, in Madrid, Spain. This is an invited paper for a special CORALS-2 GEO-SPECTROSCOPY issue of Spectroscopy Letters. Received 23 June 2011; accepted 19 July 2011. Address correspondence to
S. Sanchez-Moral, Departamento de Geologia, Museo Nacional de Ciencias Naturales (CSIC), C=Jose´ Gutie´rrez Abascal, 2, 28006, Madrid, Spain. E-mail:
[email protected]
ABSTRACT Uranyl-evansites from Porto (Northwest Portugal) together with historical evansite standards from Galicia (Northwest Spain), Slovakia, and Congo were studied by Environmental Scanning Electron Microscopy (ESEM), Energy Dispersive Spectrometry (EDS), Back-Scattering (BS), Spectra Cathodoluminescence (CL), Micro-Raman and X-Ray Diffraction (XRD), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). This work mainly focused on Porto’s evansites collected from the urban underground, looking for uranyl groups and the subsequent air radon gas levels in hypogeal environments, for example, from 6000 to 7000 Bq=m3. Evansite (Al3(PO4)(OH)6 6H2O) comprises microlayers of amorphous hydrous aluminum-phosphate phases together with hydrolyzed uranyl groups and hydroxide UO2(OH)2 precipitates. The studied evansites contain uranyl groups reaching up to UO2 0.78% in the case of Kobokobo, as detected by EDS and CL, and 0.1% in the Porto samples, as detected by CL and ICP-MS. The CL spectra probe is a very fast tool in detecting uranyl groups. Raman spectra of evansites are homogeneous in samples from different localities and not previously published. Therefore, we suggest Zeleznik-type evansite as a new representative international Spectrum Raman pattern for evansite. KEYWORDS Portugal, Raman spectra, Spain, spectra cathodoluminescence, uranyl-evansites
INTRODUCTION The evansite mineralization associated to Porto granite (Northwest Portugal) vein infill was first described in the 1930s[1] as layers up to 2 cm thick of amorphous Al2(PO4)3-5 nH2O precipitated from hydrothermal fluids, suggesting a chemical composition for the Porto evansite, of P2O5 % 12.20; Al2O3 % 40.62; H2O % 40.03, which is roughly correct. Previously, in the 1920s, evansite specimens were studied in several areas of Spanish Northwest.[2] Concurrently, the same mineral was described, also in Galicia, 511
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as an amorphous hydrated phosphate from Campo Lameiro, like pale greenish yellow fracture-fillings up to 1 cm thick in granite but renamed as ‘‘bolivarite’’ with formula Al2(PO4)3 5H2O. In the 1970s, a new uranium-bolivarite sample was from Kobokobo (Congo).[3] Later, in the 1990s, we demonstrated that the bolivarite-type (Campo Lameiro, Spain) and the evansite-type (Mt. Zeleznik, Slovakia) specimens are the same mineral phase.[4] Therefore, the International Mineralogical Association (IMA) discarded the bolivarite.[5] Later, VanWambeke wrote us supporting the bolivarite mineral denomination arguing that Kobokobo bolivarite has circa 0.3% uranium. In those years, we lacked analytical facilities for analyzing uranium content of evansite specimens from Porto, Zeleznik, Kobokobo, and Galicia outcrops. In July 2010, new fresh evansite samples were collected in the underground historical water galleries excavated in Porto urban.[6] Meanwhile radon gas measurements were performed in Porto groundwater values ranging from 2 up to 800 Bq=L.[7] The nondestructive modern techniques (ESEM-CL) together with the strong CL-spectral visibility of the uranyl water group placed in ‘‘white insulators’’ such as in hydrous AlPO4 compounds offer an excellent system to analyze uranium in order to increase knowledge of evansite.
OBJECTIVES Objectives of this work are to study evansites from Porto (Northwest Portugal) together with historical evansites from Zeleznik-Slovakia (type-locality), Kobokobo (Kivu, Rep. Congo), and Galicia (Northwest Spain). This study was mainly focused to determine uranyl contents by ICP-mass and spectra CL, to assign Raman modes not previously published, and to characterize a whole set of historical evansites from the structural, molecular, and luminescent points of view. Furthermore, the studies on the uranyl groups coupled to the evansite phases are interesting in the Porto underground areas for understanding the origin of the high concentrations of radon gas.
performed by environmental scanning electron microscopy and energy dispersive X-ray spectroscopy (EDS-ESEM) using a FEI Inspect ESEM (FEI, Netherlands) and an Inca Penta Fetxs (Oxford Instruments, UK). CL Spectra were collected on evansite chips, at low-vacuum mode without coating to keep an open way out for the CL emission, using a Gatan MonoCL3 detector (Gatan, UK) with a PA-3 photomultiplier attached to the ESEM. The Photomultiplier Tube (PMT) covers a spectral range of 185–850 nm and is most sensitive in the blue parts of the spectrum. A retractable parabolic diamond mirror and a photomultiplier tube are used to collect and amplify luminescence. Samples were positioned 15 mm beneath the bottom of the CL mirror assembly. The excitation for CL measurements was provided at 25 kV electron beam. The automatic hyper-spectral line-scans of evansite specimens were performed by Raman Microscopy using a new Thermo Fischer DXR Raman Microscope (West Palm Beach, FL 33407, USA), which has a point-and-shoot Raman capability of 1 mm spatial resolution using a laser source at 532 nm. We select the 20 objective of the confocal microscope together with the laser source 532 nm at 6 mW in mode laser power at 100%. The average spectral resolution in the Raman shift ranging from 100 to 3600 cm1 was 4 cm1, that is, grating 900 lines=mm and spot size 2 mm. The system was operated under OMNIC 1.0 software fitting working conditions such as pinhole aperture of 25 mm, bleaching time 30 s; four exposures of average time 10 s each. The XRD analyses of evansite specimens were performed using XPOWDER software, which also allows a full duplex control of the Philips PW-1710=00 diffractometer using the CuKa radiation with a Ni filter and a setting of 40 kV and 40 mA. Patterns were obtained by step scanning, from 3 to 65 2h, with a count for 0.5 s=step exploration speeds of 7 =min in the X-ray tube. The qualitative search-matching procedure was based on the ICDDPDF2 database and the DIFDATA free database; we utilized Boolean searching and chemical restraints to the initial P and Si. Evansite samples were analyzed for major and trace elements by ICP-MS at ACME Anal. Lab. (ISO 9002 Accredited Co.).
MATERIALS AND METHODS Microscopy observation, chemical analyses, and CL measurements of evansite specimens, such as spectral curves, panchromatic, and monochromatic plots were S. Sanchez-Moral et al.
RESULTS AND DISCUSSION The probable different amounts of water and uranium together with different Al=P ratios could 512
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explain the observed different layers under the BSE probe in the ESEM microscope (Fig. 1a). Concerning Fig. 1b, ascribed to an evansite sample from Porto, the EDS chemical data point to similar components, but the panchromatic cathodoluminescence image suggests uranyl groups’ presence, showing strong CL emission, while other phases appear in darkness. We have performed chemical analyses of evansite samples by EDS, being mainly composed by Al2O3 and P2O5. Probably an interesting oddity is the Kobokobo-1 evansite, that is, Al2O3 49.21%, P2O5 50.01%, and UO2 0.78% (by EDS), since the uranium contents are normally under the detection threshold of the EDS probe. Other Kobokobo-2 evansite specimens do not contain uranium (Fig. 1c). Detecting uranium amounts was under this threshold observed in the Kobokobo-1 sample, that is, 0.78% UO2. CL spectra or CL panchromatic plots are much more accurate in detecting uranyl than the EDS probe in both chemical spot analysis and line-scan plot methods. We also perform ICP-MS analyses of the Porto evansite samples to check the uranium amounts since they were a main focus of this study. An average chemical analysis by ICP-mass of a standard evansite from Porto could display the following
FIGURE 1 ESEM-CL analyses of evansite specimens. (a) Back-scattering image of an evansite from Outeiro do Casal (Spain) with botryoidal textures. (b) Panchromatic cathodoluminescence image of a Porto evansite. (c) CL Spectra curves of historical evansites from porto (Northwest Portugal), galicia (Northwest Spain), and evansite patterns from zeleznik (Slovakia) (evansite-type) and kobokovo (Congo), displaying the characteristic luminescence spectrum of the uranyl groups. 513
major components: loss on ignition 36%, Al2O3 37%, P2O5 24%, SiO2 2%, Fe2O3 0.3%, CaO 0.5%, UO2 0.11%. Assuming the great importance of the presence of uranyl groups and radon gas in granite, water, and air of Porto underground areas, we analyzed Porto samples by ICP-mass in the ACME analytical laboratories of Vancouver, Canada. A resultant average amount of uranium in the Porto evansite, analyzed by ICP-Mass, is 0.11%, being undetectable by EDS, but displaying a large CL spectrum of uranium. Kobokovo-1 evansite specimen presents detectable uranium by EDS and a more intense CL spectrum characteristic of uranium. Other evansite specimens do not exhibit detectable uranium by EDS but display clear uranyl CL spectra. Both contents of uranium, for example, Kobokobo 0.78% (EDS), and Porto 0.11% (ICP-Mass), explain the larger intensities of their CL spectra in Fig. 1c. Porto evansite displays four maxima CL peaks at 515, 535, 558, and 582 nm with an unambiguous structure of uranyl group extensively described in the bibliography. In addition, we have also analyzed one single grain of quartz placed close to the evansite in one of the Porto samples, displaying the same spectrum CL emission peaked at 515, 535, 558, and 582 nm with less intensity together with the intrinsic CL emissions of the quartz. It seems that both CL spectra taken from evansite and from quartz are independent from their host minerals, since the uranyl-water complexes give the impression of emitting CL emission independently from the host minerals. Raman spectra of evansites measured from different outcrops and countries are very similar to each other, as observed by analyzing specimens of botryoidal textures and translucent appearance. Moreover, these spectra roughly match also on the Infrared ATR spectra of other hydrous aluminum phosphates with structural and chemical similarity such as Kobokoboite, Afmite, and Planerite[8] (Fig. 2). Because of the well-studied Raman spectroscopy of other crystalline hydrous aluminum phosphates,[9] such as Variscite (AlPO4 2H2O), for the evansite spectra we infer that the common observed modes at circa 1007 and 1047 cm1 can be attributed to PO stretching vibrations, that is, the oxyanion (PO4)3 showing an antisymmetric mode (v3) at 1017 cm1 (Fig. 2). Accordingly, the evansite Raman mode assignations could be as follows: 1645 cm1 produced by water bending, 1020 cm1 produced
Uranyl-Evansites from Porto (Northwest Portugal) and Galicia (Northwest Spain)
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FIGURE 2 Raman spectra of historical evansites such as Zeleznik (locality-type), Lameiro (bolivarite locality-type), Porto (Northwest Portugal), and Outeiro (Northwest Spain). For comparison, we also include Kobokoboite and Afmite Raman spectra with other AlPO4-hydrated phases with crystalline structures.
by PO4 antisymmetric, 634 cm1 produced by vibrational water, 534 cm1 produced by PO4 out-ofplane bends, and 367 cm1 produced by PO4 in-plane bends. The multiple bands observed in the 1000–1050 cm1 region are explained by the reduced symmetry of the PO4 groups in the amorphous structure of the evansite. The observed bands in both the symmetric and antisymmetric stretching regions of the PO4 units are an indication of multiple PO4 species, such as nonhydrogen-bonded PO4 and strongly hydrogen-bonded PO4 units together with (Al(OH)2)þ (H2PO4) type species, including phosphate, dihydrogenphosphate, and monohydrogenphosphate species.[9] Concerning the structural data of the historical evansite specimens analyzed by XRD (powder method), we obtain amorphous XRD profiles very similar among themselves together with variable impurities of quartz (peak ´˚ ) and Na-feldspar at 3.33 A´˚ ), K-feldspar (peak at 3.23 A (peak at 3.18 A´˚ ). In addition to the Porto samples (Northwest Portugal), we also recorded experimental XRD from evansite specimens from Zeleznik (Slovakia) and Lameiro, Mourete, Geve, Outeiro, Louro, Magdalena, and Arcade (Galicia, Northwest Spain) (Fig. 3). The S. Sanchez-Moral et al.
FIGURE 3 X-Ray diffraction patterns of the historical collection of evansite specimens of the Museo Nacional Ciencias Naturales (Madrid, Spain). Note that the evansite mineral is an amorphous phase.
amorphous structure of the evansite phase is an important characterization key, since there are other hydrous aluminum phosphates with similar formulae, for example, AlPO4 2H2O with crystalline structure such as variscite or augelite.
CONCLUSIONS Botrioidal evansite masses are composed by different microlayers deposited around former nuclei since the observed dissimilarities by BSE can be attributed to diverse amorphous hydrous aluminumphosphate phases, for example, with varied amounts of water and dissimilar Al=P ratios, AlO(OH) or Al(OH)3 phases, FeO(OH) and Fe(OH)3 phases, hydrolyzed uranyl groups, and hydroxide UO2(OH)2 precipitates. The studied evansites contain uranyl groups reaching up to UO2 0.78%, as the case of Kobokobo detected directly by EDS. From the structural point of view, all evansite specimens analyzed are amorphous as observed by X-ray diffraction. From the molecular point of view, it is interesting to note a large homogeneity of the Raman spectra of evansite samples (not previously published) from 514
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different localities. Therefore we suggest that Zeleznik evansite-type could be a new representative international Spectrum Raman pattern for evansite. The experimental CL spectra plots of historical evansites such as Porto, Lameiro (bolivarite-type site), Zeleznik (evansite-type site), Kobokobo, Outeiro, Redondela, and many others cases include a spectral detection of uranyl groups. For that reason, we suggest to the International Mineralogical Association that the uranyl presence, detected by CL spectra, could be included in the evansite definition. The CL spectra probe is a very fast and useful tool for detecting uranyl groups not detectable by the EDS probe. A practical result of the analyses of uranium in Porto evansites could help to explain the high levels of radon gas from 6000 to 7000 Bq=m3 detected in the Porto underground areas, coming very probably from the natural endogenous evansite in fill veins throughout the well-known sequence U-Th-Rn.
ACKNOWLEDGMENTS We are grateful to the Spanish project CGL2010-17108=BTE and the project ‘‘Microclimatologia, geoquimica, mineralogia y geomicrobiologia de ambientes subterraneos’’ for financial support. This study was also made possible by the support
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of Centro de Petrologia e Geoquı´mica of Instituto Superior Te´cnico, Technical University of Lisbon, Portugal. We thank Prof. Alcides Pereira of the Natural Radioactivity Laboratory (University of Coimbra) for the radon gas measurements.
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Uranyl-Evansites from Porto (Northwest Portugal) and Galicia (Northwest Spain)
