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Phase Decomposition Upon Alteration of Radiation-Damaged Monazite–(Ce) from Moss, Østfold, Norway

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Phase Decomposition Upon Alteration of Radiation-Damaged Monazite–(Ce) from Moss, Østfold, Norway MINERALOGY CHIMIA 2010, 64, No. 10 705 doi:10.2533/chimia.2010.705 Chimia 64 (2010) 705–711 © Schweizerische Chemische Gesellschaft Phase Decomposition upon Alteration of Radiation-Damaged Monazite–(Ce) from Moss, Østfold, Norway Lutz Nasdala*a, Katja Ruschela, Dieter Rhedeb, Richard Wirthb, Ljuba Kerschhofer-Wallnerc, Allen K. Kennedyd, Peter D. Kinnye, Friedrich Fingerf, and Nora Groschopfg Abstract: The internal textures of crystals of moderately radiation-damaged monazite–(Ce) from Moss, Norway, indicate heavy, secondary chemical alteration. In fact, the cm-sized specimens are no longer mono-mineral monazite but rather a composite consisting of monazite–(Ce) and apatite pervaded by several generations of fractures filled with sulphides and a phase rich in Th, Y, and Si. This composite is virtually a ‘pseudomorph’ after primary euhedral monazite crystals whose faces are still well preserved. The chemical alteration has resulted in major reworking and decomposition of the primary crystals, with potentially uncontrolled elemental changes, including extensive release of Th from the primary monazite and local redeposition of radionuclides in fracture fillings. This seems to question the general alteration-resistance of orthophosphate phases in a low-temperature, ‘wet’ environment, and hence their suitability as potential host ceramics for the long-term immobilisation of ra- dioactive waste. Keywords: Chemical alteration · Monazite–(Ce) · Radiation damage · Thorium silicate 1. Introduction eventually to the formation of a non-crys- to undergo chemical alteration, and its in- talline form.[1,2] Such normally crystalline, crease with cumulative radiation damage, The accumulation of structural damage irradiation-amorphised minerals are com- ii) how exactly chemical alteration proc- generated by the corpuscular self-irra- monly described by the term ‘metamict’.[3] esses take place, and iii) as to which de- diation of minerals containing actinide The metamictisation process is controlled gree these materials (i.e. unaltered and/or elements has been studied widely in the strongly by the proportion of the rates of altered specimens) can resist the release of last decades. The bulk radiation damage damage accumulation and damage anneal- radionuclides. The investigation of chemi- is caused mainly by alpha-decay events: ing; the latter being strongly temperature- cally altered, radiation-damaged minerals Recoil of the heavy daughter nuclei upon dependent.[4,5] Whether or not a certain is, therefore, motivated strongly by the emission of a 4He core generate nm-sized mineral becomes metamict is consequent- question, how such materials perform in damage clusters, whose overlapping in- ly not only controlled by the mineral phase a low-temperature, ‘wet’ geological envi- ter-connection at high densities may lead itself and the amount of radioactivity it ex- ronment over extended periods of time. perienced since the time of its growth, but also by its thermal history. The metamictisation of minerals re- 2. Material and Methods sults in dramatic changes of their physi- cal properties, including volume swell- 2.1 Sample and Preparation ing and potentially subsequent frac- We have investigated monazite crystals turing,[6] a general decrease in elastic from a granite pegmatite located at the is- [7] *Correspondence: Prof. Dr. L. Nasdalaa properties and hardness, and a change land of Dillingøya (lake Vannsjø), just east Tel.: +43 1 4277 53220 in optical properties[8] (i.e. refraction of the city of Moss, Østfold district, south- Fax: +43 1 4277 9532 and birefringence). Further, the chemi- eastern Norway.[12] The area of origin be- E-mail: [email protected] [13] aInstitut für Mineralogie und Kristallographie, cal resistance of metamictised minerals longs to the Riphean (which, according Universität Wien is generally decreased, i.e. such materi- to recent timescales of the International Althanstrasse 14, A–1090 Wien, Austria als show enhanced solubility for instance Commission of Stratigraphy, corresponds bHelmholtz-Zentrum Potsdam, under conditions of near-surface weath- to the Meso- to Neoproterozoic). The mon- Deutsches GeoForschungsZentrum [9] Telegrafenberg, D–14473 Potsdam, Germany ering, and enhanced susceptibility to azite crystals are 2–2.5 cm large. They are cBWI Informationstechnik GmbH secondary loss of radiogenic isotopes.[10] medium to dark brownish, of thick-tabular Balanstr. 73, D–81541 München, Germany Knowledge of the self-irradiation behav- habit, and have well-shaped faces with dDepartment of Applied Physics, Curtin University of Technology iour of minerals and their associated prop- slightly rounded edges. Building 301, Kent Street, Bentley, WA 6102, Australia erty changes are hence of enormous rele- The monazite crystals were cut through eDepartment of Applied Geology, Curtin University of vance for the Earth sciences (e.g. petro-ge- the middle, along their longest dimension, Technology ∼ μ Building 312, Kent Street, Bentley, WA 6102, Australia ochemistry and U–Pb geochronology) and and polished thin sections ( 30 m thick- fFachbereich Materialforschung und Physik, the materials sciences (e.g. mineral-based ness) attached to a glass slide were pre- Universität Salzburg matrices for conditioning radionuclides in pared. These sections were used for optical Hellbrunnerstrasse 34, A–5020 Salzburg, Austria radioactive waste repositories).[11] In view microscopy, electron probe micro-analyser gInstitut für Geowissenschaften, Universität Mainz Johann-Joachim-Becher-Weg 21, D–55099 Mainz, of the latter, key problems to be studied (EPMA) investigation, and micro-Raman Germany include i) the susceptibility of materials spectroscopy. Sections were coated with 706 CHIMIA 2010, 64, No. 10 MINERALOGY carbon prior to EPMA imaging and analy- X-ray spectroscopy (WDS) analysis in a Curtin University of Technology, Perth.[16] sis. For Sensitive High mass-Resolution JEOL JXA 8900 RL EPMA. The accel- The monazite surface was sputtered with − Ion MicroProbe (SHRIMP) analysis, small erating voltage was 15 kV and the beam a primary, mass-filtered (O2) beam with chips of the sample were, together with the current was 50 nA. The focal spot area ∼1 nA current, focused to a ∼7–10 μm SHRIMP reference MAD–1, embedded in of the electron beam had a diameter of spot. The SHRIMP was operated with a araldite epoxy, and flat polished sample <1 μm. Calibration standards used were mass resolution (M/ΔM) better than 5000. mounts were prepared and coated with well-characterized natural and synthetic The sensitivity for Pb isotopes was about gold. For transmission electron microsco- materials, including YAG (Al, Y), wol- 20 counts per second per ppm, per nA. A py (TEM), electron-transparent foils were lastonite (Si), monazite (P), Fe2O3 (Fe), single analysis consisted of seven scans. prepared by conventional hand-polishing CeAl2 (Ce), REE silicate glasses (lantha- Data for each spot were collected in sets and Ar ion milling at 5 kV. The TEM foils nides except Ce), crocoite (Pb), Th metals of seven scans through the mass range 202 203 204 were then coated with carbon. (Th), and UO2 (U). The CITZAF routine of LaPO2, CePO2, Pb, background in the JEOL software, which is based on near 204Pb, 206Pb, 207Pb, 208Pb, 232Th, 238U, Φ ρ [14] 248 270 2.2 Experimental Details the ( Z) method, was used for data ThO2, and UO2. The total analytical The thin sections were first examined processing. The results were corrected for time was ca. 16 min per spot. Results were and imaged under an optical binocular, in rare-earth element (REE) peak overlaps. calibrated against MAD–1, a 514 Ma old plane-polarised and cross-polarised trans- Back-scattered electron (BSE) imaging, reference monazite. The 204Pb method was mitted light. Raman spectra were obtained and high-resolution element mapping, employed for the correction for non-radio- in quasi-backscatter geometry using an were done using a JEOL JXA–8500F ther- genic Pb.[17,18] edge filter-based Renishaw RM1000 sys- mal field emission-type EPMA. The ele- tem equipped with Leica DMLM optical ment distribution maps[15] were obtained in microscope (50 objective, numerical ap- WDS mode with an acceleration voltage of 3. Results and Discussion erture 0.75) and Peltier-cooled, Si-based 6 kV, a probe current of 40–50 nA, and a charge-coupled device (CCD) detector. dwell time of 0.25−0.40 s per step (stage 3.1 Alteration Textures and Spectra were excited with the 632.8 nm step intervals 0.1−0.3 μm). Chemical Composition emission of a He–Ne laser. The laser Transmission electron microscopy Transmitted light and BSE images re- power at the sample surface was ∼8 mW, – including electron diffraction, bright veal that the monazite crystals have a re- which is well below the threshold for any field (BF) and dark field (DF) imaging, markably heterogeneous internal texture local sample changes due to intense light and high-resolution electron microscopy (Fig. 1). In contrast to the macroscopic ap- absorption. The system was operated in (HREM) imaging – was done using a JEOL pearance of a single-crystal, the material the quasi-confocal mode, resulting in a 3010 system equipped with LaB6 cathode, consists of several phases. The specimens lateral resolution of ~4−5 μm. Band posi- and a PHILIPS CM200 system equipped are apparent ‘pseudomorphs’ after prima- tions were calibrated using the Rayleigh with EDAX X-ray analyser and GATAN ry monazite crystals whose macroscopic line and neon lamp emission lines. The imaging filter. The systems were operated crystal shapes are still well-preserved (Fig. wavenumber accuracy was better than 1 at a voltage of 300 kV (JEOL) and 200 kV 1a), even though they are actually a very cm−1, and the spectral resolution was ~3− (PHILIPS), respectively. heterogeneous composite of phases. The 4 cm−1. Analyses of the U−Th−Pb isotopic dominant monazite (transparent with pale The chemical composition was deter- composition were done using a SHRIMP brownish colour, medium BSE intensity) is mined by means of wavelength-dispersive II at the Department of Applied Physics, inter-grown closely with patches of apatite Fig.
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