Natural Monazite Research Articles

Partial resetting of the UPb isotope system in monazite through hydrothermal experiments: An SEM and UPb isotope study

Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their UPb isotope systematics measured before and after the runs. Runs were carried out with monazite grain size fractions ranging from 40 to 125 μm in diameter, and on < 1 to 15 μm fine powdered material at 400–750°C, 3 kbar and 28 days. UPb analyses of the starting material resulted in concordant UPb ages of 377 ± 2 Ma. U and Pb concentrations were ∼ 5500 and ∼ 800 ppm, respectively. All hydrothermally treated monazite grain size fractions yielded concordant UPb ages of ∼ 380 Ma and no loss of Pb and U. In contrast, monazite powder revealed temperature-dependent Pb loss. Pb concentrations decreased to ∼ 590 ppm at 750°C, ∼ 620 ppm at 650°C, and ∼ 710 ppm at 400°C, indicating Pb loss of ∼ 26%, ∼ 22% and ∼ 11%, respectively, of the initial Pb content. No fractionation of Pb isotopes occurred; all analyses yielded 207 Pb 206 Pb ages close to 380 Ma. U concentrations remained always unchanged. All recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379.3 ± 2.3 and −7 ± 18 Ma, the latter indicating the hydrothermally induced Pb loss at the time of the experiment. SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces. Run products from monazite powders showed recrystallization and grain growth induced by a dissolution-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650° and 750°C. A dissolution-precipitation process in presence of a fluid phase rather than Pb volume diffusion acts as a very efficient mechanism for the resetting of the UPb isotope system of monazites, even at low temperatures. This process is most important in shear zones where channelized fluid flow may occur.

XPS studies of natural monazite and relative compounds under ion bombardment

The natural monazite Ce x La y PO 4·(ThO 2) z , LaPO 4 and CePO 4 samples were analyzed by X-ray photoelectron spectroscopy (XPS). The criteria allowing one to distinguish between a mechanical mixture and a compound were established. These criteria are the set of Δ E i parameters (Δ E i : the differences of binding energies) and changes of the chemical composition after ion bombardment.

Leaching of uranium and thorium from monazite: I. Initial leaching

Interaction of three natural monazite specimens with a bicarbonate-carbonate solution was investigated for times up to 6.8 years. Dissolution was observed to be incongruent with respect to 238U and 232Th as well as their radiogenic daughters 234U, 230Th, and 228Th. Leaching was divided into a very rapid initial stage lasting a few hours and a slower process active for the remaining time. The initial stage was modeled as the sum of a contribution from a mechanico-chemically damaged portion of the specimen, which did not exhibit isotopic selectivity in leach properties, and a contribution from the selective removal of recoil daughter products from their recoil tracks in the surface of the otherwise undamaged bulk mineral. The latter effect is greater for short-lived 228Th compared to long-lived 234U. A correlation between the magnitude of the effect and the half-life of the radiogenic nuclide suggests an upper limit of ~ 10 6 years for the timescale of natural track annealing. After the initial dissolution stage, insoluble precipitates of the intermediate product in the Th chain, 228Ra, provide a supplementary source of 228Th by radioactive decay. Contributions to these precipitates come from dissolution-released and recoil-released 228Ra. This source is manifest as large apparent release rates of 228Th which begin after several weeks of leaching. Preannealing of a specimen at 800°C depresses the elemental Th leach rate but enhances the amount of 228Th 232Th fractionation. This enhancement is associated with rejection of Ra from the mineral during annealing of α-recoil damage.

Leaching of uranium and thorium from monazite: III. Leaching of radiogenic daughters

The solid-state diffusion model of actinide leaching developed in Part II of this series is applied to leaching of radiogenic daughters of the actinide decay chains. For an untreated natural monazite, the direct leaching component of 228Th release is larger than that for 232Th because of enhanced solid-state mobility for 228Th provided by 228Ra-recoil tracks. A significant portion of the 228Th which appears in the leachate, however, is attributed to decay of insoluble 228Ra which is continually released from the mineral by matrix dissolution and recoil ejection. For a monazite sample that was annealed at 800°C prior to leaching, the bulk of the 228Th in solution was supplied by decay of 228Ra rejected from the mineral matrix during annealing. The radiogenic 234U daughter of the 238U decay chain did not exhibit similarly enhanced leaching because the long half-life of 234U permitted local radiation damage to be annealed out at ambient temperature prior to 234U decay.

Mechanisms of Actinide Isotope Leaching from Monazite

The remarkable geological stability of the actinide-containing rare-earth orthophosphate mineral monazite has led to consideration of synthetic analogs of this material as the wasteform for high-level nuclear wastes (1). Monazite is resistant to metamictization in spite of the alpha radiation from decaying actinides to which it has been subjected during its lifetime. This material is also very insoluble in most groundwaters. Consequently, dissolution of the mineral and leaching of its constituent actinides are very small. However, leaching experiments on natural monazite specimens, presented here, show that radioactive decay and radiation damage produce significant differences in the release rates of several isotopes into aqueous solutions. Within the duration of the present experiments, 6.3 to 6.8 years, these effects are particularly prominent in the nuclides in the decay chain of 232-Th. This series includes the alpha-recoil 228-Ra atom (a 5.8-year fT emitter formed directly by 232-Th decay), and its daughter 228-Th (a 1.9-year a emitter, produced by 3 decay). The present experiments exhibit preferential release into solution of 228-Th relative to 232-Th by a factor of 1.3 to 21. This highly enhanced 228-Th release is explained by four different isotope incongruency effects that operate during leaching. One mechanism involves the preferential etching of alpha-recoil tracks. The three other mechanisms depend upon the behaviour of 228-Ra. In particular, dissolution-released, recoil-released and diffusion-released 228-Ra atoms (the latter process is applicable to a high-temperature anneal) are observed to remain with the solid as an insoluble precipitate, possibly loosely attached to the mineral surface. Subsequent decay of this released insoluble 228-Ra immediately leads to the formation of readily dissolved 226-Th. The phenomena may be quite general, so potential wasteforms should be examined for these isotope fractionation effects in order to insure satisfactory prediction of the long-term leaching of actinides from the material.

Melting Temperatures of Monazite and Xenotime

The melting temperatures of natural and synthetic monazite and xenotime (rare‐earth ortnophosphates) were measured, using a heliostat‐type solar furnace. The results obtained are as follows: natural monazite from Japan (2057°40°C), synthetic monazite RPO4 (R=La, 2072°20°C; R=Ce, 2045°20°C; R=Pr, 1938°20°C; R=Nd, 1975°20°C; R=Sm, 1916°20°C), and synthetic xenotime RPO4(R=Y, 1995°20°C; R=Er, 1896°20°C).

Synthesis and characterization of actinide phosphates

The immobilization of actinides by incorporation into synthetic monazite is an attractive concept for the long term storage of actinides. This stems from the known stability, over geological age, of natural monazites (which contain uranium and thorium) toward radiation and sea water. In seeking methods for the preparation of metal phosphates from insoluble metal compounds by simple reactions, we have identified both BPO 4 and SiP 2O 7 as convenient reagents for such a task. a large variety of reactions, involving compounds of Th, U, Np, Pu, Am and all the lanthanides were studied. The results obtained, based mainly on Raman spectroscopy and X-ray powder diffraction, allowed for postulating reaction mechanism and for optimizing conditions for the preparation of ortho-, pyro- and trimeta-phosphates and actinide oxide phosphates. Some of the ternary phosphates thus prepared were used to prepare other phosphate, e.g., Pu(PO 3) 3 and sodium -containing quaternary phosphates, e.g. Na 3Pu(PO 4) 2 and Na 3Ln(PO 4) 2 Ln is a lanthanide element). Raman spectroscopy proved to be a very sensitive technique for identifying most of the compounds and for establishing their purity.

Synthesis of nuclear waste monazites, ideal actinide hosts for geologic disposal

Monazite, an orthophosphate mineral of the lanthanides (Ln) and the actinides (An) U and Th, is a model for an ideal synthetic mineral waste form for geologic disposal of long-lived nuclear waste actinides. Natural monazites are known to have survived many of the conditions that might be inflicted on a nuclear waste repository by geological disruptions. High Th and U monazites with compositions typical of nuclear wastes have been synthesized with a routine calcination-pelletization-crystallization procedure. Charge balance for the Th 4+ → Ln 3+ substitution can be provided by either an equimolar Ca 2+ → Ln 3+ or Si 4+ → P 5+ substitution. For U 4+ → Ln 3+, only the Ca 2+ → Ln 3+ substitution resulted in a phase-pure monazite. Unit cell parameter data were obtained for each nuclear waste monazite phase.

An X-ray study of natural monazite

Monazite is commonly believed to be a nonmetamict mineral. The results of the published X-ray examination of monazite do not unambiguously support this belief. The results of the present investigation on four different samples of monazite of known thorium content show that the thorium-containing monazite occur in a fairly high degree of metamictization. The results of the petrological examination also support this conclusion.

An X-ray study of natural monazite: I

An X-ray study of natural monazite: I