Monazite composition

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.

Table 7. Effect of ZAP correction on Trebilcock monazite composition. 

Standards. REE phosphates should be used as standards for the REE. Silicates or phosphates of U, Th, and Pb are also suggested as standards for these elements. For EMP dating of monazite, use of lead silicate or phosphate results in smaller ZAF corrections for typical monazite composition than does the use of lead chromate or sulfide. Lead-doped (500-5000 ppm) alkaline earth aluminosilicate glass produces the smallest ZAF corrections for typical monazite compositions, but may generate insufficient X-ray counts due to low lead concentration, compromising its use as a standard. 

ZAP corrections (Tables 5-7). ZAP corrections for certain elements (Si, Ca, Y, U, Pb) are inherently large due to overvoltage requirements and typical monazite compositions resulting in large [A] factors for unknowns. However, variation in compositions resulting from application of different ZAF models is equal to or less than the compositional variation associated with analytical uncertainty. 

With respect to the actinides, monazite tends to concentrate thorium, whereas xenotime tends to concentrate uranium, but can take up also appreciable amounts of thorium. According to Deer et al. (2013), common varieties of monazite have 4-12 mol% of Th02, whereas uranium occurs in minor amounts. Van Emden et al. (1997) mention Th02 contents in monazite ranging 1.2-21.9 wt%, whereas UO2 contents are from detection limit up to 0.75 wt%. Xenotime analyses show UO2 contents ranging from detection limit to 5.82 wt%, while Th02 varies from detection limit to 8.44 wt%. Watt (1995) lists monazite compositions showing wt% of Th to vary from 5.17-21.41 wt%, and UO2 from 0.22-3.17 wt%. 

An interesting variant of Group I is the determination of thorium in monazite concentrates.73 Here the variations that may occur in the chemical composition of the matrix leave its x-ray absorbance virtually unaltered. This simplicity is possible because the principal individual rare-earth elements present in the samples lie in the range of atomic numbers from 57 to 60, a range so small as to preclude marked variations in the over-all mass absorption coefficient. 

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. 

India has very large deposits of monazite on the coastal shores of Kerala and Chennai. A typical mineral composition of this type of deposit is 60% ilmenite, 1.2% rutile, 5% zircon, 6.4% garnet, 4% silinanite, 16% quartz, 2.5-5% monazite and 1-7% shell. Research work involved different anionic collectors and pH during monazite flotation, along with the level of sodium silicate used as depressant. 

Chondrite-normalized REE profiles (Fig. 3) display a distinct enrichment in LREE with prominent negative Eu anomalies, which are broadly consistent with known profiles of monazite in metamorphosed terrains (Spear Pyle 2002). The LREE contents of monazite have fairly restricted compositions dominated by Ce averaging 29.8% Ce203 and behaving sym-... 

Rock samples collected from archived core, mine workings and outcrop were pulverized, homogenized, then analyzed using four-acid dissolution (SGS Lab, Toronto) to determine the near-total lithogeochemical composition (cassiterite, rutile, monazite, zircon, sphene, gahnite, chromite and barite are partially dissolved). Gold analyses were done by Fire Assay with Atomic Absorption finish on 30g samples and have a detection limit of 5 ppb. 

Forster, H-J. 1998. The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebvirge-Fichtelgebirge region, Germany, Part I The monazite-(Ce)-brabantite solid solution series. American Mineralogist, 83, 259-272. 

Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%. 

Comment More accurate calculation requires adjustment of Pb atomic mass using the calculated Pb isotopic composition based on Th and U abundance and the age, and then calculating the number of moles of Pb per kilogram of monazite. Iterate until the age converges. The result is still 382 Ma. Error calculation is relatively complicated and not discussed here. 

The chemical treatment used depends upon the composition of the mineral. The sodium hydroxide treatment of monazite. Figure 8, is preferably used since it has the advantage of removing phosphates more readily than the sulfuric acid procedure. 

The LaRochelle capacity for ore cracking is over 5000 tons as rare earths oxides per year and in Table VIII we show the average production capabilities for the heavier rare earths. It should be noted that the above figures are average values as the rare earths composition of monazite varies somewhat depending on the source. 

Wronkiewicz, D. L., Wolf, S. F. DiSanto, T. S. 1996. Apatite- and monazite-bearing glass-crystal composites for the immobilization of low-level nuclear and hazardous wastes. Materials Research Society Symposium Proceedings, 412, 345-352. 

A limited number of rare-earth minerals are mined for large-scale rare-earth production monazite, bastnaesite, loparite [12173-83-0], xenotime [13817-22-6]. In addition, since the 1980s rare-earth-containing days called ionic ore are mined in China. Table 4 shows the rare-earth composition of typical mineral concentrates. 

Davis, J.B., et al., Influence of interfacial roughness on fiber sliding in oxide composites with La-monazite interphases, J. Am. Ceram. Soc., 86(2) 305-316 (2003). 

There are only a few minerals where thorium occurs as a significant constituent. The commercially important ore is the golden-brown, lanthanide phosphate, monazite [13064-1 -8/, LnPO where Ln = Ce, La, or Nd, in which thorium is generally present in a 1—15% elemental composition (7,8). Monazite is widely distributed around the world. Some deposits are quite large. Beach sands from Australia and India contain monazite from which concentrates of lanthanides, titanium, zirconium, and thorium are produced (7). The Travancore deposits in India are the most famous, and have been perhaps one of the most significant sources of commercial thorium. Additional information on the occurrence of thorium in minerals can be found in the literature (7). A review of the mineralogy of thorium is also available (9). 

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