Monazite Structure Research Articles

The aqueous geochemistry of the rare earth elements. Part XIV. The solubility of rare earth element phosphates from 23 to 150 °C

Rare earth element (REE) phosphates such as monazite and xenotime are important as ore minerals, potential hosts of radioactive waste, and target phases for isotopic dating. However, there are still insufficient thermodynamic data with which to model dissolution and precipitation of these phases in crustal fluids quantitatively. Therefore, the solubilities of end-member La(III)-, Nd(III)-, Sm(III)-, and Y(III)- phosphates were determined at 23 and 50 °C in NaCl–HCl and NaClO 4–HClO 4 solutions with pH m from 0 to 2 and ionic strengths of 0.1, 0.5, 1.0, and 5.0 m. The solubility of Nd(III)-phosphate was also determined in chloride solutions at 150 °C. The La(III)- and Nd(III)-phosphates had the monazite structure, and Sm(III)-phosphate and Y(III)-phosphate had the rhabdophane and xenotime structures, respectively. The dependence of solubility on pH and chloride concentration, together with data from the literature, indicated that H 3PO 4 0 and Ln 3+ (where Ln 3+ represents any free, hydrated trivalent REE ion) were the predominant species in our experimental solutions. At each ionic strength and temperature investigated, conditional equilibrium constants ( Q s3) were determined for reactions of the following type: LnPO 4+3H +↔Ln 3++H 3PO 4 0 The conditional equilibrium constants determined at various ionic strengths were extrapolated empirically to obtain the equilibrium constants at infinite dilution ( K s3). These constants were then converted to solubility products ( K s0) for the following reaction: LnPO 4(s)↔Ln 3++PO 4 3− using acid dissociation constants for H 3PO 4 0 available in the literature. The values of log K s0 so obtained are: 23°C 50 °C 150 °C La −25.7 −25.4 – Nd −25.8 −26.6 −30.8 Sm −24.6 −24.8 – Y −27.9 −27.8 – These values are in reasonable agreement with the majority of those in the literature, with the exception of the values for Y(III)-phosphate, which are substantially lower. Our results, combined with data in the literature, suggest that the solubility products of REE phosphates are retrograde (i.e., decrease with increasing temperature) up to at least 300 °C. Moreover, the solubility of REE phosphate is quite low up to 300 °C, even at low pH and high chloride concentrations, confirming the robustness of these phases as hosts for radionuclides.

Evidence for monazite-, barite-, and AgMnO4(distorted barite)-type structures of CaSO4at high pressure and temperature

Using laser-heated diamond-anvil cells, we have observed CaSO4 undergoing phase transitions from its ambient anhydrite structure to the monazite type, and at highest pressure and temperature to crystallize in the barite-type structure. On cooling, the barite structure distorts from an orthorhombic to a monoclinic lattice to produce the AgMnO 4 -type structure. The barite-structured form of CaSO 4 that we encounter at high pressure and temperature has been, in particular, long expected as a highpressure phase of CaSO 4 -anhydrite from systematic trends of similar A II B VI O 4 -type sulfates, selenates, and tellurates, but has not been observed before. Similarly, the monoclinic distortion of the barite structure has itself been proposed as an intermediate phase between anhydrite and barite types through comparison with the phase diagrams of NaBF 4 and NaClO 4 . This result has important consequences for identifying structural trends between different ABO 4 -type phases of Group II sulfates, selenates, tellurates, chromates, molybdates and tungstates that crystallize in anhydrite, zircon, monazite, barite and scheelite-type structures at ambient and high pressures.

Synthesis and structure of new thorium phosphate CaGdTh(PO4)3

Phosphate CaGdTh(PO4)3 was prepared by thermal treatment of a mixture of oxides. The final temperature was 1400°C. The phosphate was characterized by powder X-ray diffraction analysis and IR spectroscopy. The crystal structure was studied by the Rietveld method. The compound crystallizes in the monazite structure type (sp. gr. P21/n). A comparative analysis of the structures of this phosphate and cerium orthophosphate CePO4 was carried out.

Protonic conduction in Sr-doped (La 1− xSm x)PO 4

High temperature protonic conduction in 1 mol% Sr-doped (La 1− x Sm x )PO 4 ( x=0.3, 0.5 and 0.7) with the monazite structure (S. G.: P2 1/ n) was investigated. The conductivities of the materials under wet atmosphere were considerably higher than those under dry atmosphere. From the conductivity measurements under H 2O- and D 2O-containing atmospheres, it was found that the magnitude of H/D isotope effect was 1.2–1.4. These results indicated that protonic conduction was dominant in the materials under wet atmosphere. The p(H 2O) dependencies of the conductivities were discussed in terms of the defect equilibria, and, as a result, higher Sm content in 1 mol% Sr-doped (La 1− x Sm x )PO 4 seemed more preferable for proton dissolution.

Synthesis of REE and Y phosphates by Pb-free flux methods and their utilization as standards for electron microprobe analysis and in design of monazite chemical U-Th-Pb dating protocol

(REE,Y) phosphates were synthesized in a 1 atm furnace by flux-growth methods involving Pb-free fluxes. Microcrystalline (REE,Y) phosphate was precipitated from a solution of (REE,Y) chlorides or nitrates plus ammonium dihydrogen phosphate, and mixed with a M2CO3 (M = Li or Na)-MoO3 flux (75:25:2 molar ratio M2CO3:MoO3:REEPO4). Crystal growth was achieved over the temperature range 1350–870 °C, with extended (15 h) high- T soaking, and cooling rates of 3 °C/h. Crystals grown by this method are clear, generally inclusion-free, up to several mm in length, and are easily extracted from the water-soluble flux. Crystal size and habit are influenced by the alkali component of the flux. Successful LREE phosphate synthesis is favored by both Li-and Na-bearing fluxes, whereas better results for Y and MREE-HREE phosphates were achieved with Na-bearing fluxes. Li-bearing fluxes produced LREE phosphates with an overall platy habit, in contrast to more prismatic-to-equant LREE-MREE phosphates produced with Na-bearing fluxes. Structural refinements of single-crystal X-ray diffraction data show that LREEPO4 (La-Gd) crystallize with the monoclinic monazite structure, and HREEPO4 (Tb-Lu) plus YPO4 crystallize with the tetragonal xenotime structure. Element distribution maps of synthetic REEPO4 reveal homogeneous distribution of REE and P at grain-size scale and below, and both wavelength-dispersive (WD) spectral scans and quantitative electron microprobe analyses show no other elements (e.g., flux inclusions) present at significant levels. The synthetic phosphates grown with this method are suitable for use as electron microprobe standards and as compositionally simplified monazite analogs for use in the design of U-Th-Pb chemical dating protocols. WD scans of synthetic unary and ternary (La-Ce-Nd) phosphates reveal that Ar X-ray detectors produce non-filterable LREE escape peaks with pulse-height analyzers optimized for Pb analysis. Such LREE escape peaks complicate collection of both U and Pb peak and background counts. Due to the larger energy difference between LREE peaks and Xe (relative to Ar), LREE escape peaks produced in Xe detectors are filterable with pulse height analyzers, resulting in LREE-free spectra over the wavelength range sampled for Pb peak and background collection.

Orthophosphates of Tetravalent Ce, Th, U, Np, and Pu with the Monazite Structure

The problem of development of a ceramic phosphate matrix for solidification of the lanthanideactinide fraction of radioactive wastes is tackled from the crystal-chemical viewpoint. New double orthophosphates of tetravalent f elements (Ce, Th, U, Np, Pu) with alkaline-earth elements and cadmium of the formulas B0.5M2(PO4)3 and B1.5M1.5(PO4)3 (B = Mg, Ca, Sr, Ba, Cd; M = Ce, Th, U, Np, Pu) were prepared. The compounds were characterized by X-ray diffraction (including high-temperature measurements) and IR spectroscopy. They crystallize in the monazite structural type (space group P21/n). The thermal behavior of the compounds was studied. The unit cell parameters of the phases synthesized were determined. The influence of the cationic composition on the structure formation is analyzed.

REE and TRU Incorporation into Monazite Structure Ceramics

AbstractOne of the high levels of actinide, and in particular Cm, waste streams at the Russian radiochemical Production Association (PA) Mayak was generated during spent fuel reprocessing. Using oxalate precipitation, the rare earth elements (REE) and transuranic elements (TRU) settled out in the form of oxalate residues. Due to in high REE contents in this residue, the mineral-like matrix based on (REE)PO4 solid solution, with monlclinic monazite structure have been proposed to use as a suitable ceramics form for final actinide immobilization. For this purpose the synthetic REE oxalates were first transformed into REE orthophosphates in a thin-film evaporator (TFE). Then the (REE)PO4 powder was compacted both by either hot uniaxial pressing (HUP) or cold uniaxial pressing followed by sintering (CUP). This ceramic with the monazite structure has a high density and exhibits chemical durability by leaching.

Correlation of Formation Enthalpies with Critical Amorphization Temperature for Pyrochlore and Monazite

AbstractSystematic studies of a series of RE-titanate pyrochlore single crystals determined the formation enthalpies, ΔH0f-ox, and the critical amorphization temperature, Tc, the temperature above which the crystal can no longer be amorphized. A negative linear correlation was observed between ΔH0f-ox and Tc. In general as the cation radius ratio, RA/RB, decreases the formation enthalpy becomes less exothermic, i.e. the pyrochlore structure becomes less thermodynamically stable while the susceptibilty to ion beam-induced amorphization decreases. The opposite relation, i.e. a positive linear correlation between ΔH0f-ox and Tc was observed for a series of REPO4 single crystals with both the monazite and xenotime structures. Both relations can be understood in terms of pyrochlore and orthophosphate crystal chemistry, their formation of point defects, and the role of ion-irradiation induced phase transformations.

M.J. Kohn, J. Rakovan and J.M. Hughes (Eds) Phosphates - Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48, Washington, D.C. (Mineralogical Society of America and Geochemical Society), 2002. 742 pp. Price US$40. ISBN 0-939950-60-X.

M.J. Kohn, J. Rakovan and J.M. Hughes (Eds) Phosphates - Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48, Washington, D.C. (Mineralogical Society of America and Geochemical Society), 2002. 742 pp. Price US$40. ISBN 0-939950-60-X. - Volume 67 Issue 4

Protonic conduction in rare earth orthophosphates with the monazite structure

Electrical conduction in rare earth orthophosphates, LnPO 4 (Ln=La, Pr, Nd and Sm), with the monazite structure ( P2 1/ n) was investigated by using conductivity measurements at 500–925 °C. From the conductivities of undoped and 1 mol% Sr-doped LnPO 4 under wet (H 2O and D 2O) and dry atmospheres, it was found that LnPO 4 began to conduct protons under wet atmosphere by substituting Sr for Ln. The conductivity behavior of 1 mol% Sr-doped LnPO 4 versus p(H 2O) and p(O 2) was discussed in terms of the defect equilibria. It was concluded that protonic conduction was dominant in the materials though electron holes contributed slightly to the total conductivity as temperature increased. All the 1 mol% Sr-doped LnPO 4 investigated in this study exhibited similar electrical conduction regardless of rare earth element used.

Phosphate monazite- and NaZr2(PO4)3 (NZP)-like ceramics containing uranium and plutonium

The synthesis and results of the X-ray diffraction examination of complex orthophosphates involving the cations Ca2+, Cd2+, Gd3+, Ti4+, Hf4+, Ce4+, U4+, and Pu4+ are described. Crystalline products containing either cerium or plutonium or these two elements together were prepared. Cerium was used as plutonium simulator. Phosphates containing cations Ti4+ and Hf4+ were found to form two-phase products. One of these phases was assumed to be of the NaZr2(PO4)3 (NZP) type, the other one of the CePO4 type (monazite). Formation of single-phase products with a monazite structure was found in the specimens that did not contain Ti and Hf. X-ray diffraction characteristics of synthesised phases were identified. An effect of the cation composition on the phase formation and also the effect of the cation size on the crystal lattice parameters of the formed phases were established.

Synthesis and Crystal-Chemical Properties of Phosphates BIIRIIIMIV(PO4)3 Containing f, d, and Alkaline-Earth Elements

New triple phosphates containing f, d, and alkaline-earth elements or cadmium of the general formula BIIRIIIMIV(PO4)3 (B = Mg, Ca, Sr, Ba, Cd; R = Nd, Sm, Eu, Gd; M = Zr, Ce, Th, U) were prepared. The compounds were characterized by X-ray phase analysis, IR spectroscopy, and high-temperature X-ray diffraction. The compounds crystallize in the monazite and kosnarite structures (space groups P21/n and R-3c or R-3, respectively). The X-ray diffraction characteristics of the synthesized phases were determined. The influence of the cationic composition on the structure formation was analyzed.

Dating diagenetic monazite in mudrocks: constraining the oil window?

Authigenic monazite nodules, increasingly recognized in Pre-Mesozoic mudrock successions, provide a means for dating diagenesis by exploiting trace uranium within the monazite structure. But, dating is difficult in practice because of zonation and abundant host rock inclusions. We report here a refined technique that has yielded a date of 419 ± 14 Ma 2 σ MSWD = 1.2 for a 17-point Pb–Pb regression age from a Telychian (Silurian: late Llandovery) mudrock from central Wales. SHRIMP analyses of monazite nodules from the same horizon are consistent with this, and additionally demonstrate concordance of the U–Pb and Th–Pb systems. These results open the door to providing robust dates for a phase of burial-related diagenesis which, in Wales, seems approximately coincident with, and perhaps was related to, hydrocarbon expulsion.

Contamination in the Rare-Earth Element Orthophosphate Reference Samples.

Several of the fourteen rare-earth element (plus Sc and Y) orthophosphate standards grown at Oak Ridge National Laboratory in the 1980s and widely distributed by the Smithsonian Institution’s Department of Mineral Sciences, are significantly contaminated by Pb. The origin of this impurity is the Pb2P2O7 flux that is derived from the thermal decomposition of PbHPO4. The lead pyrophosphate flux is used to dissolve the oxide starting materials at elevated temperatures (≈1360 °C) prior to the crystal synthesis. Because these rare-earth element standards are extremely stable under the electron beam and considered homogenous, they have been of enormous value to electron probe micro-analysis (EPMA). The monoclinic, monazite structure, orthophosphates show a higher degree of Pb incorporation than the tetragonal xenotime structure, orthophosphates. This paper will attempt to describe and rationalize the extent of the Pb contamination in these otherwise excellent materials.

Cerium arsenate, CeAsO4

CeAsO4 was synthesized by the high-temperature solid-state reaction between CeO2, K2CO3 and (NH4)H2AsO4. The compound is isostructural with compounds of the monazite structure type. The structure is built up from CeO8 bis-disphenoids and AsO4 tetrahedra, joined by corners and edges.

Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 °C and 1.8–3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at ≤550 °C, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu*<0.35), while most of the metamorphic monazites have moderately negative Eu anomalies (Eu/Eu*>0.4). Small differences have been observed in Y and HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+↔Th 4++Ca 2+. In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Y and Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO 2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Y or HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE.

X-ray diffraction study of brabantite–monazite solid solutions

We synthesized compounds with stoichiometry of natural brabantite M 2+Th(PO 4) 2 with M 2+=Ca, Cd, Sr, Pb, Ba at 1 bar, 1200 °C and 2.5 kbar, 700 °C. Those compounds were studied by powder X-ray diffraction and electron microprobe. For Ca, Sr, and Pb, we obtained always crystals with the monazite structure. For Cd, the monazite structure is obtained at 1 bar, but not at 2.5 kbar. For Ba, we obtained the monazite structure only at 2.5 kbar. The unit-cell parameters of the compounds with monazite structure vary regularly with the size of the M 2+ ion. We studied also the solid-solution LaPO 4–M 2+Th(PO 4) 2, at 1200 °C, 1 bar. The solid solutions for Cd, Ca, and Sr are continuous, and the unit-cell parameters vary linearly with the degree of substitution. For Pb, the solid solution is continuous, but the unit-cell parameters do not vary linearly. For Ba, there is a miscibility gap, with a maximum of about 50 mol% of BaTh(PO 4) 2 in LaPO 4. These results have several consequences for U–Th–Pb geochronology of monazite, and for utilization of monazite as a nuclear-waste ceramic.

Synthesis, Structure, and Properties of Monazite, Pretulite, and Xenotime

### General characteristics Orthophosphate compounds of the type A(PO4) include the minerals monazite [RE(PO4) with RE = the light rare-earth ions, e.g., La, Ce, Nd…], xenotime [Y(PO4), and also incorporating the heavy RE ions], and pretulite [Sc(PO4)]. The name of the mineral monazite is derived from the Greek word monazein , “to be solitary,” whereas that of xenotime is derived from the Greek words xenos , meaning “foreign” and time , meaning “honor.” The mineral pretulite is named for Pretul Mountain, located in the Fischbacher Alps in Styria, Austria where the mineral was discovered (Bernhard et al. 1998). Pretulite is only the sixth mineral to have been found in which scandium is a major constituent. The anhydrous rare-earth (RE) orthophosphates can be structurally divided between the light RE-element compounds with the monoclinic monazite structure (space group P 21/ n , Z = 4) and the heavier RE compounds with the tetragonal (zircon-type) xenotime structure (space group I 41/ amd , Z = 4). In terms of the crystal chemistry of these two groups of compounds, a primary distinguishing structural feature is the coordination of the RE, Y, or Sc ions. In the monoclinic monazite structure, the RE ion is located in a polyhedron in which it is coordinated with nine oxygen ions. In the tetragonal xenotime structure, the heavier REs, Y, or Sc are located in a polyhedron in which they are coordinated with eight oxygen ions. Scandium orthophosphate is the basic constituent of the recently identified mineral, pretulite, which belongs to the orthophosphate group with the tetragonal xenotime structure (Bernhard et al. 1998). Both monazite and xenotime are relatively widely distributed as microcrystalline (or small crystalline) accessory inclusions in granitic rocks, rhyolites, pegmatites, and gneisses. Monazite also occurs in carbonatites, charnockites, migmatites, and …

Studies on triple orthovanadates: VIII. Synthesis and spectrostructural characterization of triple orthovanadates BaLnTh(VO 4) 3 (Ln=La or Pr) and BaLnCe(VO 4) 3 (Ln=La, Pr, Nd or Sm)

To study the effect of substitution at the tetravalent ion, some barium lanthanide containing triple orthovanadates have been synthesised and structurally examined. The X-ray diffraction and the infrared spectroscopic data are analysed and interpretations are reported. It is observed that thorium containing compounds crystallise with a monoclinic monazite structure while those containing cerium are tetragonal superstructured zircon.

Double Phosphates of Ce(IV) and Some Mono- and Bivalent Elements

A series of double Ce(IV) orthophosphates NaCe2(PO4)3 and B0.5Ce2(PO4)3 (B = Mg, Ca, Sr, Cd) was prepared by heat treatment of gels with the stoichiometric content of the components. These phosphates were characterized by powder X-ray diffraction, IR spectroscopy, and electron spin resonance. The com- pounds have the monazite structure. The unit cell parameters of the phosphates were determined. The crystal structure of the phosphates is stable at temperatures from 20 to 1600°C. The crystallographic parameters of the double Ce(IV) orthophosphates were compared with those of simple Ce(III) phosphate CePO4 which is artificial analog of monazite. Crystal chemical features of formation of double phosphates of tetravalent f elements and mono- and bivalent elements were discussed.