Xenotime composition

The extensive and detailed analytical results given in Table 1 of Forster (1998a) illustrate the extreme compositional diversity exhibited by natural monazites. In fact, Forster has even reported the existence of a monazite-group mineral that is intermediate between monazite and huttonite. Forster s analysis indicated that complete miscibility exists between common monazite-(Ce) and the phosphate mineral brabanite [Ca,Th,U(P04)2]- The monazite/xenotime compositional systematics have also been examined by Fleinrich et al. (1997). Additional data related to the compositional diversity of monazites can be found in the work of Bea (1996), Bea et al. (1994), and Hinton and Patterson (1994). 

Figure 7. Histograms of xenotime compositions (cations/4 oxygens), (a) Y, (b) Gd, (c) Dy, (d) Er, (e) Yb. Most metamorphic xenotimes are 75-80 mole % YPO4 with limited ranges of HREEs. Data sources as in Figure 3.

Figure 8. Ternary plots of (a) monazite and (b,c - next page) xenotime compositions. Note restricted range of monazite compositions, plus the nearly constant Ce values. Note the strong correlation in xenotime (b) between Dy and Yb. Data sources as in Figure 3.

Figure 12. Plots of xenotime composition illustrating chemical exchanges, (a) Th + U versus Si + Ca. (b) Th + U versus Ca. (c) Th + U versus Si. The lack of a strong...

Plots of xenotime compositions show much less obvious correlations (Fig. 12). Figure 12a reveals that there is an excess of Si + Ca over what is required for... 

Table 1. Ranked textural and compositional criteria for assumption of monazite-xenotime compositional equilibrium. From Pyle et al. (2001).

Figure 2. Results of a-ejection modeling, after Farley et al. (1996). Ft is fraction of a particles retained in an entire crystal. (A) shows resnlts calcnlated for a sphere (narrow lines, radins indicated at top) of apatite, monazite, or xenotime composition. Bold lines give Ft approximation for arbitrary geometries of these minerals based on sirrface to volirme ratio. (B) Ft for an apatite hexagonal prism as a function of half the distance between opposed apices, for two length/radius (L/R) ratios. Note the relative insensitivity to this qirantity. For apatite the mean stopping distance was assirmed,...

A limited number of rare-earth minerals are mined for large-scale rare-earth production mona2ite, bastnaesite, loparite [12173-83-OJ, xenotime [13817-22-6]. In addition, siace the 1980s rare-earth-containing clays called ionic ore are mined ia China. Table 4 shows the rare-earth composition of typical mineral concentrates. 

Rare Earths are produced primarily from three ores, monazite, xenotime, and bastnasite. Monazite is a phosphate mineral of essentially the cerium subgroup metals and thorium -(light rare Earths, Th) P04. The composition of monazite is reasonably constant throughout the world, with almost 50% of its rare Earth content as cerium and most of the remaining 50% as the other members of the cerium subgroup. Xenotime, like monazite, is a rare Earth orthophosphate but contains up to 63% yttrium oxide and also a markedly higher propor-... 

Euxenite is a titanotantalum/niobium-containing mineral and has a complex formula (Table 24.1) with variable chemical composition. It is usually found in sand deposits together with monazite, xenotime, zircon, beryl, columbite and other minerals. 

Several different flow sheets have been developed around DEHPA, which, as discussed earlier, vary according to the precise composition of the feed material. A typical process using a nitric acid leach of a xenotime ore follows as an example. 

Pyle J. M. and Spear F. S. (2003) Formation, consumption, and compositional zonation of pelitic monazite and xenotime metamorphic whole-rock reaction controls. J. Metamorph. Geol. (in review). 

The zircon structure-type (ZrSi04) [49] is also adopted occasionally by so-called ternary compositions (Table 3) [38], Zircon is isostructural with the xenotime series of phosphates, typified by xenotime-(Y), YPO4 [50, 51], In this structure-type, the eight-coordinate polyhedra (snub disphenoids) share edges to form a framework composed of cross-linked chains that extend along the [100] and [010] directions. The framework of polyhedra exhibits prominent voids along the [111] direction these are occupied by tetrahedra. The tetrahedra share edges with the polyhedra and thus serve to further connect the chains (Fig. 7). 

The monazite structure is dimorphous with xenotime (zircon stmcture-t5q)e), and is one of the major natural hosts of Th and U in cmstal rocks the tetravalent actinides can be accommodated in conjunction with a large divalent cation in a coupled substitution for lanthanide elements, e.g., complete solid solution is found between brabantite, (Cao sTho 5)P04, and monazite-(Ce), CeP04 [51]. Ternary compositions, and partial occupancy are also mechanisms of actinide substitution (Table 5). 

Apparently, formation of actinide (IV) compounds with more complex cationic and anionic compositions with the monazite or zircon (xenotime) structure types is possible. These compounds can be considered to be solid solutions. The possibility of forming this kind of compound is realized in the minerals mentioned above. These minerals are characterized by the complex cation compositions monazite - (La, Ce, other lanthanides, Y, Ca, Th)(P, Si)04, xenotime - (Y, lanthanides. Sc, Zr, Th, U)(Si, P)04 zircon - (Zr, Hf, Th, U, lanthanides, Ca, Fe, Nb, Ta)(Si,P)04 [71]. The ionic radii and cationic proportions, anionic sizes and synthesis conditions affect the formation of each type considered. 

Given the variety of important actinide oxidation states, what naturally occurring anion would best effect actinide mineralization The lanthanides, chemically analogous to the trivalent actinides, occur naturally in three commercially important forms monazite and xenotime, which are orthophosphate minerals, and bastnasite, which has the approximate composition LnFCOs. Uranium ores may be divided into... 

Hydrothermal methods have been used by Gratz and Heinrich (1997) to synthesize solid solutions of (Ce,Y)P04 at temperatures in the range of 300 to 1000°C and at pressures between two and 15 kbar. These workers reported the formation of two immiscible phases (monazite and xenotime) over a wide range of compositions. 

Forster H-J (1998b) The ehemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany. Part II Xenotime. Am Mineral 83 ... 

Morgan PED and Marshall DB (1996) Ceramic composites having a weak bond material selected from monazites and xenotimes. U.S. patent 5,514,474... 

Figure 3. REE plots of apatite (a, d), monazite (b, e) and xenotime (c, f). REE plots in a, b, and c are from sources indicated and represent a restricted suite of rock compositions (taken from Grauch 1989). REE plots in d, e, and f are compilations of analyses from the literature and show a wider total spread of values. Lined fields in d, e, and f encompass all tabulated analyses. Stippled fields in e and f are the same data with suspected spurious points eliminated. Data on monazite and xenotime from Pyle (2001), Franz et al. (1996), Finger et al. (1998), Forster (1998), and Zhu and O Nions (1999b). Data for apatite from Bea and Montero (1999), Finger et al. (1998), Bingen et al. (1996), Amli (1975), Jamtveit et al. (1997), Bea (1996), Lee and Bastron (1967), Puchelt and Emmerman (1976), Ayers and Harris (1997), Cruft (1996), and Pyle (2001).

Xenotime has the nominal composition YPO4, and Y comprises 75-85% of the total (Fig. 7a) with the HREEs comprising the bulk of the remainder (Figs. 7b-e 8b,c 10c,d). The range of HREE contents is actually rather restricted with Gd, Dy, Er, and Yb all... 

Physical contact of monazite and xenotime grains Both grains in compositional equilibrium 1... 

Discontinuously Y-zoned monazite included in specific garnet, and xenotime included in different grain of garnet, and both inclusions present in same type of "compositional domain" within garnets Only high-Y portion of monazite in equilibrium with xenotime 6... 

Figure 22. Composition plots of coexisting monazite and xenotime. (a) LREE-HREE-Y ternary showing tie lines between coexisting monazite and xenotime. (b) LREE-HREE+Y-(Th+U+Pb) ternary, x = biotite + chlorite zone, = garnet zone, A = staurolite zone, O = sillimanite zone, diamonds = migmatite zone. From Pyle et al. (2001).

Accuracy of the monazite-xenotime thermometer is difficult to assess at this time. It is clear that HREEs and XhSi04 affect the position of the gap relative to the pure CeP04-YPO4 system. The brabantite component [CaXh(P04)2], which is primarily responsible for Th and U substitution into monazite, may also be significant, but its effect has not been examined. A solution model for monazite that incorporates the major compositional... 

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