The crystal chemistry of Li in gadolinite
-
Fernando Cámara
,
Roberta Oberti
Abstract
This paper describes a multi-technique approach to the complete crystal-chemical characterization of a gadolinite-(Y) sample found in a volcanic holocrystalline ejectum near the Vico lake (Latium, Italy). Gadolinite-(Y) occurs as poly-twinned crystals forming rounded short-prismatic aggregates (generally 0.1-0.3 mm in size, with the largest ever found >1 mm), associated with zircon, thorite, danburite, betafite, and tourmaline. Both the chemical and the structural characterization of gadolinite-(Y) from Vico required nonstandard procedures. After correction for (100) twinning, the structure of a crystal with unit-cell dimensions a = 4.7708(4) Å, b = 7.6229(7) Å, c = 9.8975(9) Å, β = 90.017(7)°, and V = 359.95(6) Å3 was refined in the P21/c space group down to R = 2.3%. Electron microprobe (EMP) analyses failed to give accurate quantification of major elements, due to the presence of light and volatile elements as well as of rare earth elements (REE) and actinides. Secondary ion-mass spectrometry (SIMS) analysis done with accurate calibrations on well-characterized minerals allowed quantification of light, volatile, REE, and actinide elements, and also of Ca and Si. The derived chemical composition was interpreted with reference to the site-scattering values obtained from single-crystal structure refinement. The resulting unit formula is (Ca0.81REE0.66Y0.39Th0.13U0.02)Σ2.01(Fe2+0.29Li0.14 Fe3+0.12Mn0.02Mg0.01)Σ0.58 (Si1.98Be1.09B0.81Li0.12)Σ4.00 O8(O1.20F0.51OH0.29)Σ2.00, which yields a calculated density of 4.267 g cm-3. Fourier transform infrared spectroscopy (FTIR) single-crystal spectrum of gadolinite-(Y) shows several absorptions in the OH-stretching region that can be assigned to the different local configurations involving Ca and (REE,Y) at the A site and Be, B, and Li at the Z site. Lithium incorporation in gadolinite-group minerals is proposed to occur according to the exchange vectors: (1) XFe2+ + AY → XLi + A(Th +U) and (2) ZBe + XFe2+ → ZLi + XFe3+; the maximum amount of Li allowed in the gadolinite structure is 1.0 apfu. This work provides the first evidence that Li is a significant component in gadolinite-group minerals, particularly in geochemical environments enriched in actinides. This conclusion suggests that materials having the composition of Li-rich gadolinite may be considered as possible forms for radioactive waste disposal.
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
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Articles in the same Issue
- Rietveld structure refinement of MgGeO3 post-perovskite phase to 1 Mbar
- High-temperature ammonium white mica from the Betic Cordillera (Spain)
- Synthesis and crystal structure of the feldspathoid CsAlSiO4: An open-framework silicate and potential nuclear waste disposal phase
- The crystal chemistry of Li in gadolinite
- The effect of the hedenbergitic substitution on the compressibility of jadeite
- The structure of monohydrocalcite and the phase composition of the beachrock deposits of Lake Butler and Lake Fellmongery, South Australia
- First-principles calculation of the infrared spectrum of hematite
- In situ observations of muscovite dissolution under alkaline conditions at 25–50 °C by AFM with an air/fluid heater system
- Ramanite-(Cs) and ramanite-(Rb): New cesium and rubidium pentaborate tetrahydrate minerals identified with Raman spectroscopy
- Pressure-temperature studies of talc plus water using X-ray diffraction
- Hematite and magnetite precipitates in olivine from the Sulu peridotite: A result of dehydrogenation-oxidation reaction of mantle olivine?
- Arsenide in a metasomatized peridotite xenolith as a constraint on arsenic behavior in the mantle wedge
- Multiple oxygen sites in synthetic phyllosilicates with expandable layers: 17O solid-state NMR study
- On the existence of a Na-deficient monoclinic trinepheline with composition Na7.85Al7.85Si8.15O32
- Chlorite and biotite weathering, Fe2+-rich corrensite formation, and Fe behavior under low Po₂ conditions and their implication for Precambrian weathering
- Structural characterization of natural UO2 at pressures up to 82 GPa and temperatures up to 2200 K
- Dense hydrous magnesium silicates, phase D, and superhydrous B: New structural constraints from one- and two-dimensional 29Si and 1H NMR
- Mid-IR bands of synthetic calcic amphiboles of tremolite-pargasite series and of natural calcic amphiboles
- Probing the site occupancies of Co-, Ni-, and Mn-substituted biogenic magnetite using XAS and XMCD
- Origin of titanite in metarodingite from the Zagros Thrust Zone, Iraq
- Experimental calibration of aluminum partitioning between olivine and spinel as a geothermometer
- Single-crystal X-ray diffraction study of CaIrO3
- Orthorhombic polymorph of rengeite from Ohmi region, central Japan
- New insights into the crystal chemistry of epididymite and eudidymite from Malosa, Malawi: A single-crystal neutron diffraction study
- Mechanisms of rhyolitic glass hydration below the glass transition
- Heat capacity and entropy of melanophlogite: Molecule-containing porosils in nature
- Crystal structure of glaucodot, (Co,Fe)AsS, and its relationships to marcasite and arsenopyrite
- Modification of gas speciation in quartz-hosted fluid inclusions by stray laser radiation during LA-ICPMS analysis
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