Simultaneous determination of 14 lanthanides and yttrium in rare earth ores by inductively-coupled plasma atomic emission spectrometry

Analytica Chimica Acta, 183 (1986) 239-249 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SIMULTANEOUS DETERMINATION OF 14 LANTHANIDES AND Y’ITRIUM IN RARE EARTH ORES BY INDUCTIVELY-COUPLED PLASMA ATOMIC EMISSION SPECTROMETRY

KIYOSHI IWASAKI”, KEIICHIRO FUWA and HIROKI HARAGUCHI* Department of Chemistry, Tokyo 113 (Japan)

Faculty

of Science,

University

of Tokyo,

Bunkyo-ku,

(Received 6th June 1985)

SUMMARY A method is described for simultaneous determination of 14 lanthanides and yttrium in monazites and xenotimes by inductively-coupled plasma atomic emission spectrometry. The ore samples were decomposed by heating with nitric acid/perchloric acid or hydrofluoric/nitric/perchloric acids, or sulfuric acid, or by fusion with sodium carbonate, Among these methods, treatment of the samples with sulfuric acid, by evaporation to dryness followed by dissolution of the residue with hydrochloric acid is recommended; it provided complete decomposition for the five monazites and xenotimes examined. The accuracy and precision of the results were significantly influenced by spectral interferences of the rare earth elements themselves; correction methods and factors are given. The chondrite-normalized rare-earth patterns were examined to characterize the monazite and xenotime samples.

Accurate and convenient methods of determining rare earth elements (REE) are important for the development of geochemical knowledge and industrial applications [ 1, 21. Several instrumental methods have been successfully applied. Neutron activation [ 3-61 and isotope-dilution mass spectrometry [7, 81 provide suitable sensitivity, but are inconvenient because of complicated and sometimes prolonged experimental procedures. Neutron activation requires chemical group separation of the elements of interest from interfering elements [ 3, 51. Isotope-dilution mass spectrometry usually provides precise results, but it has an essential disadvantage in that four lanthanides (Pr, Tb, Ho and Tm) cannot be determined because they have no plural isotopes. In contrast, atomic absorption spectrometry [ 9-121 is easily applied for determining all REEs, but the sensitivities tend to be poor. Inductively-coupled plasma atomic emission spectrometry (i.c.p./a.e.s.) has been applied to the determination of REEs in ores and minerals [ 131, rare earth concentrates [ 14,151 and various geochemical materials [ 16-191. There are several advantages, i.e., wide dynamic ranges, freedom from chemi*On leave from Industrial Research Institute of Kanagawa Prefecture, Kanazawa-ku, Yokohama 236, Japan. 0003-2670/86/$03.50

0 1986 Elsevier Science Publishers B.V.

Showa-machi,

240

cal and ionization interferences, and capability of simultaneous or sequential multielement determination. In addition, i.c.p./a.e.s. provides much higher sensitivities for REEs than atomic absorption spectrometry [20]. The method, however, often suffers from spectral and physical interferences because of complicated sample matrices. In such cases, the method has been used in combination with ion-exchange [ 16-191 or high-performance liquid chromatography [21, 221 for the separation of REEs from interfering matrices. In the present work, i.c.p./ae.s. is utilized for the simple and rapid determination of REEs (14 lanthanides and yttrium) in the rare earth ores, monazite and xenotime. Rare earth elements which are present at the majorto-trace levels in these ores are simultaneously determined by using the digested sample solution without any separation for REEs. Decomposition methods and the correction of spectral interferences are examined in detail in order to evaluate the accuracy and precision of the results. EXPERIMENTAL

Chemicals The standard stock solutions of REEs (5.00 mg ml-‘) were prepared by dissolving their pure oxides (99.9-99.99%) in hot concentrated hydrochloric acid. For cerium, the oxide was dissolved by heating with nitric acid and 2-3 ml of 30% hydrogen peroxide, the excess of hydrogen peroxide being decomposed by boiling the solution. All oxides of REEs were used after heating at about 900°C for 3 h. Five groups of working standard solutions, in which each REE was 10.0 pg ml-’ in 1.2 M HCl, were prepared by mixing and diluting the stock solutions. The combinations of elements in the groups were as follows; (1) Sm; (2) Gd, Tb; (3) La, Ce, Tm, Yb; (4) Pr, Eu, Dy, Y; (5) Nd, Ho, Er, Lu. These combinations were selected after studying the spectral interference data given in Table 3; consequently, the mutual spectral interferences were negligibly small. Acids of analytical grade and distilled-deionized water were used for all preparations of the standard and sample solutions. Instrumentation The emission measurements were made with a Jarrell-Ash Plasma Atomcamp Model MK II. The operating conditions of the i.c.p. instrument were as shown in Table 1. The emission signals of all REEs were almost maximal at an observation height of 16 mm above load coil; the signal-to-background ratios increased at the higher observation position between 12 and 18 mm. The signal-to-background ratio also increased as the carrier argon pressure was increased in the range 17-20 psi, but the larger pressure caused significant decrease of the emission intensity. The analytical wavelengths given in Table 2 were chosen for minimal spectral interferences, as discussed by Crock and Lichte [ 171.

241 TABLE

1

Operating conditions I.c.p. source and sample introduction Forward r.f. power Reflected r.f. power Coolant argon flow Auxiliary argon flow Carrier argon flow Observation height Nebulizer Solution uptake rate Spectrometer ‘Me Focal length Grating Slit widths Integration time

system 1.1 kW <5w 19 1 min-’ 0.2 1 min-’ 0.52 1 min-* at 19 psi 16 mm above load coil Cross-flow type 1.6 ml min-* Paschen-Runge 75 cm 2400 grooves mm-’ Entrance 25 pm, exit 50 rrn 10s

Sample preparation for rare-earth ores Several techniques have been applied for the decomposition of rare-earth ores and minerals with satisfactory results. The following treatments with single or mixed acids or alkali fusion were examined as described below. Decomposition with concentrated sulfuric acid. Powdered sample (0.2 g) in a covered lOO-ml beaker was heated with 5 ml of concentrated sulfuric acid on a hot plate at 250-300°C for 3 h. Excess of the acid was evaporated to dryness by removing the watch glass. After cooling, the residue was dissolved by heating with 25 ml of concentrated hydrochloric acid. The solution was diluted with water, passed through a filter paper to remove any residue and then diluted to 100 ml with water. For the emission measurements, the solution was further diluted to an appropriate volume (a 4-ml aliquot to 25 ml for monazite and a 2-ml aliquot to 25 ml for xenotime) with 1.2 M HCl. At this stage, 10 pg ml-’ cadmium was added as an internal standard. Decomposition with nitric/perchloric acid or hydrofluoric/nitric/perchloric acid. The ore sample (0.2 g) was treated with 30 ml of nitric/perchloric acid mixture (4:1, v/v) in a covered lOO-ml beaker at about 120°C for 6 h. The watch glass was removed and the mixture was evaporated to dryness at about 150°C. Another mixed acid, HF/HN03/HC104 (2:2:1, v/v), was also tested for the decomposition of monazite and xenotime in a similar manner by using a teflon beaker. The subsequent procedures were similar to those described for the sulfuric acid decomposition. Fusion with sodium carbonate. The ore sample (0.2 g) was mixed well with 2 g of sodium carbonate in a platinum crucible, and fused by heating on a Meker burner. After cooling, the cake was transferred carefully with 3040 ml of 3 M HCl to a beaker and then completely dissolved by heating with

242 xo ml of concentrated hydrochloric acid. The refractory residue was removed by filtration. To separate the REEs from the large amount of sodium salt in the sample solution, in order to reduce the salt effect on physical interferences, coprecipitation with hydrated iron(II1) oxide was adopted. To a l/10 aliquot of the above solution, 20 mg of Fe(II1) (as a chloride solution) was added, and diluted ammonia liquor was added. The solution was boiled for a few minutes, and the precipitate was filtered, washed with water, and then dissolved again in 30 ml of 6 M HCl. The iron carrier was removed by liquidliquid extraction using methyl isobutyl ketone. The aqueous phase containing the REEs was evaporated to near dryness twice with addition of nitric acid, and finally diluted with 1.2 M HCl to the appropriate concentrations of REEs. RESULTS

AND DISCUSSION

Analytical figures of merit for the determination of rare earth elements and correction 0 f spectral interferences The wavelengths, signal-to-background ratios for 1 Mg ml-‘, and detection limits are summarized in Table 2. The detection limit is defined as the concentration corresponding to the emission intensity equivalent to twice the standard deviation of the background intensity. The values obtained were similar to those reported by Crock and Lichte [17] except for that of thulium. TABLE 2 Analytical wavelengths, earth elements Metal

La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Y

signal-to-background

Wavelength (nm)

SIBa

398.852 418.660 422.293 430.358 442.434 381.967 303.284 367.635 340.780 345.600 369.265 313.126 328.937 261.542 377.433

4.3 0.77 0.98 1.0 0.49 22 1.1 1.1 1.4 4.6 3.6 0.46 31 39 16

ratios (S/B)

and detection

limits for rare

Detection limit (rg 1-i) Present work

Publishedb

2.1 12 16 8.6 92 0.9 17 9.2 15 2.4 3.1 69 0.4 0.8 0.7

4.3 31 64 22 30 0.9 18 58 25 3.6 8.0 5.1 2.2 1.0 -

sThe S/B values were estimated by measuring the emission intensities of 1 @g ml-’ AUtions for all REEs. bCited from Crock and Lichte [ 17 1.

243

Spectral interferences were corrected on the basis of the equation, Ci = Ai ZZjkijCi,where Ci is the concentration of the analyte i, Ai the apparent concentration of the analyte i, Cj the concentration of interfering element i, and

-

k, the interference coefficient of the element i for the element i. The ki, values were determined for 14 REEs and Y by measuring the emission intensities at the specified 15 wavelengths by using the single element solutions. The kij values found are shown in Table 3; values are also given for thorium interference which cannot be neglected in analyses of monazite. The accuracy and precision in the correction of spectral interferences were checked by analyzing two artificial solutions which were prepared by mixing single element solution of the REEs; one was similar to monazite composition and the other to xenotime composition. For the monazite-type solution, trace constituents such as Eu, Tm and Lu were added in much larger amounts than the abundance in real rare-earth ores. The correction was made by using all kij values larger than 0.5 given in Table 3. As shown in Table 4, the results obtained after correction were in good agreement with the prepared composition within an error of 0.6% for the major elements at 15 pg ml-’ or more, and 4% for most of the minor and trace elements. The large error found for thulium is probably due to insufficient correction of spectral interferences. It can be concluded that the accuracy and precision of the present corrections were satisfactory for practical analysis of rare-earth ores. TABLE 3 Interelement spectral interferences of rare earth elements Element

Interfering element and interference coefficient ko (ng ml-’ analyte/pg ml-’ interferent)

La Ce

Ce 0.7, Pr 3.7, Nd 3.1, Sm 1.9, Eu 2.5, Yb 5.8, (Th 2.8). La 0.9, Pr 11, Nd 14, Sm 6.8, Eu 1.8, Gd 2.4, Tb 11, Dy 187, Er 15, Y 0.7, (Th 1.6). Ce 18, Nd 27, Sm 12, Gd 13, Tb 12, Dy 2.5, (Th 2.0). Ce 1.0, Pr 94, Sm 2.9, Eu 2.0, Gd 6.0, Tb 3.1, Dy 3.8, Er 14. La 1.9, Ce 10, Pr 29, Nd 5.7, Gd 3.8, Tb 4.9, HO 2.7, Er 2.6. Pr 0.7, Nd 3.7, Sm 0.5, Gd 0.5. Ce 1.5, Pr 5.7, Nd 4.1, Sm 8.2, Eu 1.7, Tb 28, Dy 11, Ho 18, Er 8.4, Tm 33, Yb 12, Y 0.6, (Th 172). La 0.8, Ce 5.9,Pr 11, Nd 8.1,Sm 11, EU 1.4, Gd 1.9, Dy 102, Ho 3.9, Er 34, (Th 3.4). Ce 1.6, Pr 3.9, Nd 3.9, Sm 19, Gd 49, Tb 16, Ho 24, Er 13, Tm 4.6, (Th 6.0). Pr 2.7, Nd 2.0, Sm 3.4, Gd 5.4, Tb 7.4, Dy 11, Tm 8.1. Pr 3.3, Nd 3.7, Sm 10, Eu 2.6, Gd 1.2, Tb 4.9, Dv 2.2, Yb 60, Y 1.2. Pr 2.7, Nd 13, Sm 17, Eu 3.9, Gd 114, Tb 50, Dy 14, Ho 18, Er 13, Yb 36, (Th 28). Nd 0.5, Sm 0.6, Tb 0.6, Dy 0.9, Ho 1.3, Tm 1.5. Yb 0.5. Pr 1.2, Nd 0.7, Sm 4.9, Gd 3.8, Ho 1.2.

Pr Nd Sm Eu Gd Tb DY

Ho Er Tm Yb Lu Y

TABLE 4 Accuracy and precision of the correction of rare-earth ore compositions

Found (rg ml-‘)*

Composition (Irg ml-‘)

Monazite La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Y Xenotime La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb LU

Y

for spectral interference for artificial solutions

Without correction

With correction

30.0 80.0 8.00 30.0 6.00 0.50 5.00 0.50 1.00 0.50 0.50 0.10 0.30 0.10 5.00

30.2 f 0.2 80.2 i 1.1 10.3 f 0.2 30.8 * 1.0 7.16 i 0.08 0.64 i 0.01 5.15 * 0.06 1.48 * 0.03 1.69 * 0.03 0.69 f 0.02 0.76 * 0.02 1.30 f 0.04 0.34 f 0.00 0.10 f 0.02 5.11 f 0.05

30.0 f 79.6 i 8.16 29.9 f 5.88 0.50 4.80 0.49 0.98 0.49 0.48 0.07 0.30 0.10 5.01

2.00 5.00 1.00 5.00 2.00 0.10 5.00 2.00 15.0 2.00 15.0 2.00 15.0 2.00 90.0

2.21 f 0.02 8.16 f 0.05 1.45 f 0.04 5.29 f 0.05 2.31 * 0.03 0.14 f 0.00 5.68 i 0.06 4.28 * 0.04 15.5 f 0.1 2.33 * 0.02 16.0 * 0.1 3.74 f 0.09 15.3 f 0.1 2.08 f 0.06 89.8 i 0.5

2.08 f 0.02 4.95 f 0.03 1.07 f 0.04 4.87 f 0.04 2.07 * 0.02 0.11 * 0.00 4.95 * 0.06 2.08 f 0.03 14.9 f 0.1 2.02 f 0.01 14.9 f 0.1 1.99 * 0.08 15.2 f 0.1 2.08 * 0.06 89.8 * 0.5

type solution 0.2 1.1 f 0.12 1.0 * 0.07 f 0.01 f 0.05 f 0.02 f 0.03 t 0.02 f 0.02 * 0.06 f 0.00 f 0.02 * 0.05

type solution

*Mean of 5 determinations with standard deviation.

Evaluation of sample decomposition methods The results obtained for REEs in three monazites and two xenotimes are given in Table 5, where the four kinds of the decomposition method examined here are compared. The insoluble residues were also analyzed qualitatively by x-ray fluorescence spectrometry to check if they contained REEs. The results gave detailed information about the effectiveness of decomposition for the rare-earth ores.

246 TABLE 5 Results obtained for rare earth elements in rare earth ores treated by various decomposition methods (wt. %) Metal Acid decomposition HNO,/ HCIO, Monaxite I La 4 Ce 4 PI 4 Nd 4 Sm 2 EU Gd 4 Tb 4 DY -b Ho 4 E? =: Tm Yb -b -b-d LU Y 4 Mona&e La Ce Pr Nd Sm EU Gd Tb DY

Ho Er Tm Yb Lu Y

Na,CO, fusion

HF/HNO,/ HClO,

H,SO,a

8.19 21.8 2.54 9.16 1.79 0.015 1.04 0.14 0.45 0.070 0.13 -c 0.055

8.13 f 0.03 8.18 21.5 f 0.1 21.9 2.49 f 0.01 2.59 8.57 f 0.02 8.94 1.77 f 0.14 1.74 0.015 f 0.003 0.015 1.12 f 0.04 1.11 0.14 0.14 f 0.01 0.54 0.54 f 0.02 0.077 f 0.01 0.081 0.17 ye'8 f 0.01 -c 0.11 -d 1.86 * 0.02 1.86

HNO,/ HClO,

HF/HNO,/ HClO,

I¶,SO,a

NazCGa fuelon

MonaxiteII (yellowmonazite)

1.43

5F * 0*01

III(black monm :ite) 6.58 6.53 6.69 6.49 f 0.06 19.6 19.8 19.0 f 0.2 16.6 2.24 2.33 2.29 f 0.01 2.28 8.16 8.48 8.46 8.43 f 0.03 1.74 1.71 1.57 f 0.09 1.66 0.24 0.24 0.23 0.25 f 0.01 0.77 0.18 0.73 f 0.01 0.76 0.049 0.10 0.056 0.064 * 0.008 0.21 0.18 0.20 f 0.02 0.18 0.018 0.016 0.019 f 0.003 0.015 0.015 0.030 0.015 0.020 f 0.007 _c -c -c -C 0206 4 0.0°4 soo3 "2O" * 0*Oo2 0.31 f 0.01 0.36 0.36 0.36

XenotimeII La 0.38 Ce 0.93 Pr 0.14 Nd 0.48 Sm 0.17 EU 0.003 Gd 0.49 Tb 0.09 DY 0.51 Ho 0.11 Er 0.41 Tm 0.071 Yb 0.45 LU 0.10 Y 3.60

Acid decomposition

0.43 1.15 0.14 0.60 0.34 0.006 0.75 0.18 1.26 0.32 1.07 0.18 1.14 0.20 8.79

9.02

9.46

23.2 23.0 2.57 2.64 9.14 9.85 1.84 1.71 0.071 0.068 1.04 1.06 0.15 0.12 0.40 0.35 0.043 0.049 0.074 0.080 -c -.c

o*023%o22

-d

1.01

1.06

XenotimeI 0.65 0.66 1.77 1.85 0.24 0.22 0.81 0.79 0.29 0.40 0.006 0.003 0.57 0.73 0.14 0.18 0.87 1.44 0.18 0.34 0.69 1.13 0.19 0.22 1.25 0.76 0.16 0.22 5.89 9.69

9.26

f 0.07

9.44

23.1 f 0.2 23.0 2.59 * 0.03 2.52 9.20* 0.12 8.89 1.72 f 0.09 1.65 0.010 f 0.002 0.066 1.09 f 0.04 1.01 0.12 f 0.01 0.11 0.39 f 0.02 0.34 0.047 f 0.004 0.045 0.088 f 0.005 0.078 .-c -c 0.029 f 0.001 yz27 4 1.11 f 0.02 1.08 0.66 0.63 f 0.02 1.88 f 0.05 1.86 0.22 0.23 f 0.01 0.93 f 0.01 0.91 0.58 f 0.02 0.54 0.008 f 0,002 0.009 1.60 f 0.01 1.52 0.41* 0.01 0.47 3.66 f 0.05 3.68 0.85 f 0.05 0.86 2.97 f 0.04 2.86 0.56 f 0.02 0.52 3.25 3.22 * 0.01 0.45 0.47 f 0.02 25.2 f 0.2 25.2

0.41 0.42 + 0.01 1.15 * 0.05 1.15 0.18 f 0.02 0.15 0.74 f 0.02 0.72 0.50t 0.04 0.54 0.012 f 0.00'1 0.012 1.89 f 0.04 1.79 0.49 f 0.02 0.45 3.87 f 0.03 3.82 0.85 * 0.08 0.96 3.19 3.15 f 0.05 0.55 0.58 f 0.02 3.52 f 0.04 3.52 0.52 0.58 f 0.02 27.8 28.2 f 0.2

aDetermined 3 times separately. bNot analyzed because of shortage of sample. CNot evaluated because of large spectral interferences. dNot evaluated because of poor reproducibility.

246

For the decomposition with the mixed nitric and perchloric acids, the x-ray fluorescence spectra indicated that the amount of REEs remaining in the insoluble residues of the monazite samples were negligibly small, i.e., the monazites were almost completely decomposed. In contrast, decomposition of the xenotimes was difficult; about 60--80% of Y and heavy REEs remained in the insoluble residues, while La, Ce and Pr were dissolved almost completely. The mixed hydrofluoric, nitric and perchloric acids provided similar results to the nitric/perchloric acid mixture. The addition of hydrofluoric acid did not improve the decomposition although it might help to digest minerals such as niobatetantalate or silicate present in the samples. There was little possibility of the existence of REEs as difficultly soluble fluorides because the fluorides were easily decomposed on heating with perchloric acid. Monazites, which mainly consists of orthophosphates of the cerium group members with thorium silicate, could be decomposed with the mixed acid including perchloric acid as reported by Ooghe and Verbeek [9]. In contrast, xenotimes were hardly decomposed with the triple acid mixture even on prolonged heating treatment although they also contain the orthophosphates. Concentrated sulfuric acid was very effective for the decomposition of both monazites and xenotimes. All 14 lanthanides and yttrium in the samples were dissolved quantitatively by heating with sulfuric acid to dryness and subsequently dissolving the residues with hydrochloric acid. The insoluble materials were also analyzed for their major constituents. The results are shown in Table 6. The REEs remaining in the insoluble materials were only trace levels of Ce and Nd for monazites and traces of Y, Dy, Er and Yb for xenotimes. Fusion of the ore samples with sodium carbonate followed by dissolution with hydrochloric acid was also effective for the dissolution of REEs in these monazites and xenotimes. The alkali fusion method, however, is timeconsuming when further chemical separation of REEs is required in the analyses. It is concluded from the results described above that the heating of the sample to dryness with concentrated sulfuric acid and subsequent dissolution of the residue in hydrochloric acid is to be recommended for dissolution of monazites and xenotimes in REE determinations. TABLE 6 Constituents in insoluble residues of rare-earth ores after decomposition sulfuric acid

with concentrated

Rare earth ore

Residue (%)

Major constituents

Monazite Monazite Monazite Xenotime Xenotime

4.4 2.6 21 f 7.6 4.7

Zr, Zr, Zr, Zr, Zr,

I II III I II

f f 1 f f

0.2 0.1 0.3 0.4

Nb, Ta, Sn, Si. Si. Si. Nb, Ta, Fe, Ti, Si. Nb, Ta, Si.

247

Evaluation of results for the REEs in monazite and xenotime The results obtained by the recommended procedure were very reproducible, as can be seen in Table 5. It was difficult to determine simultaneously very small amounts of thulium and lutetium in the monazite samples together with the other REEs. In particular, the determination of thulium was nearly impossible because of serious spectral interferences from the other REEs. Besides the mutual spectral interferences of REEs, interferences caused by other constituents in the ore samples must be taken into consideration. Qualitative analysis by x-ray fluorescence spectrometry indicated that the monazite and xenotime samples contained Th, U, Zr, Nb, Ta, Sn, Ti, Fe, Si and P as major or minor constituents. In the recommended decomposition with concentrated sulfuric acid, most of the Zr, Nb and Ta were left in the insoluble residues, and they were not found in the sample solutions except for about 0.3% of niobium in xenotime II. The partly dissolved iron (0.42.0%) and titanium (*0.9%) did not interfere with the REE determination. Small amounts of uranium, which provided poor emission sensitivity, also caused negligible interferences. Only the interference of thorium was found in the analysis of the monazite samples. As can be seen in Table 3, 1 pg ml-’ thorium gave a signal equivalent to 0.172 pg ml-’ gadolinium. When this value was used along with the concentration of thorium in the sample solutions, it was estimated that thorium caused ca. 90,70 and 9% positive errors for gadolinium in monazites I, II and III, respectively. For xenotimes, however, the errors caused by thorium were not significant because the contents of thorium in xenotimes are much lower than those in monazites. Characterization of rare-earth ores based on distribution pattern of rare earth elements An attempt was made to characterize the rare-earth ores by normalizing their rare earth abundances with those of chondrite, as has been proposed by Masuda [ 231 and Coryell et al. [24]. The normalized rare-earth patterns for the three monazite samples analyzed here are shown in Fig. 1 along with those for the monazite 71aG (England) and a Taiwan black monazite. The absolute rare earth contents for the latter two were reported by Ooghe et al. [ 91 and Hwang et al. [ 251, respectively. The rare earth abundance of Leedey chondrite used for the present normalization was taken from the values reported by Masuda et al. [8, 261. For the monazite samples, the normalized abundances generally decreased with increasing atomic number, and the rare earth patterns obtained were composed of some rectilinear segments with europium anomaly. The rare earth pattern of monazite III (a black monazite of unknown origin, M III in Fig. 1) was nearly identical to that of the Taiwan black monazite (R I). The rare earth patterns of monazite II (M II) and 71aG (R II) were also similar to each other. In the rare earth pattern of monazite I (M I), the linear line was broken at the holmium position. Such simple and rectilinear relationships as those seen for monazites were not obtained for xenotimes.

248

102

0

Lacc Pr

Nd

Sm Eu Gd lb

ATOMIC

Gy Ho Er Tm Vb Lu

NUMBER

Fig. 1. Chondrite-normalized rare-earth patterns for monazites. M I, monazite I; M II, monazite II (yellow monazite); M III, monazite III (black monazite); R I, Taiwan black monazite [ 25 ] ; R II, 7 1 aG from the Bureau of Analysed Samples, England [ 91. Thulium and lutetium in monazites I, II and III were determined separately by using a higher-resolution spectrometer during the present work.

Conclusion In spite of the difference of three orders of magnitude in concentration between the major and trace REE constituents of rare-earth ores, the REEs could be determined accurately, easily, and rapidly without any separation and preconcentration of the elements of interest. Only thulium and lutetium in monazites were subject to large errors because of their low contents as well as severe spectral interferences. When the analytical wavelengths selected here were used, thorium interfered with the determination of gadolinium. The results for 14 lanthanides can be used to illustrate simple and rectilinear rare earth patterns for monazites by normalization with the rare earth contents in chondrite. This research was supported by a Grant-in-Aid of Scientific Research (No. 59470024) from the Ministry of Education, Culture and Science, Japan.

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