An Evaluation of the Analysis of Monazite and REE ... - ICDD

Copyright (C) JCPDS-International Centre for Diffraction Data 1999 

829 

An evaluation of the analysis of Monazite and REE compounds 

by WIXRFS: a spectroscopist’s nightmare (or challenge?) 

James P. Willis 

Department of Geological Sciences, University of Cape Town, 

Rondebosch, 7700, S. Africa 

and 

Edward B. McNew 

Analytical Laboratory, Molycorp, Inc. 

Mountain Pass, California 

Abstract 

This paper evaluates the WDXRFS analysis of rare earth element (REE) compounds and monazite, a 

radioactive phosphate mineral with very variable composition and high concentrations of rare earth 

elements (REEs) and thorium. The evaluation considers the use of K or L series REE, Nb and Y lines, 

L or M series lines for Th, U and Pb, and K series lines for common major elements. It is shown that 

the selection of the K or L line series for the REEs will determine whether powder briquettes (K series) 

or fusion disks (L series) must be used. For the light REEs (La through Gd) a Au target tube operated 

at 100 kV will give the best results, while a Au tube operated at 60 kV 45 mA will give the best results 

for all of the L series lines. It is suggested that a light REE, determined with the Ka line, could be 

used as a variable internal standard for the heavy REEs (L series lines), and thus solve particle size and 

mineralogical problems in powder briquettes. Since most users have only a single x-ray tube, Rh 

target, which is the third best tube for REE K lines and the fourth best for REE L lines, it is suggested 

that instrument suppliers consider the possibility of providing a Ge secondary transmission target on 

the tube filter support. This could increase considerably the excitation efficiency of the REE L lines, 

since GeKcl is ideally placed just short of the REE L series absorption edges. Problems of infinite 

thickness, mineralogical and particle size effects, spectral line overlap, crossing of major element 

absorption edges, lack of suitable of standards and lack of influence coefficient software to handle up 

to 32 elements in a single run are discussed. 

Introduction 

Monazite [(REE,Y,Th)PO,] is a radioactive mineral of very variable composition: 

45-57% REE oxides; l-20, commonly 3-9%) ThO, ; a few wt% U308, Y,O, and ZrO,. At 

certain locations mineral grains are often cemented with FeO, . 

REE, Y, Tb and U concentrations in three monazite “standards” are plotted in Figure 1, which 

clearly indicates the wide range of REE concentrations to be analysed for, namely -25% to 

10 ppm. 

WDXRFS analysis of monazite presents serious challenges to the analyst. Challenges include: 

choice of sample preparation, powder briquettes or fusions; the type of sample preparation 

determines the selection of analyte lines, and vice versa; choice of analyte lines, K, L or M 

series, can influence the choice of x-ray tube target and kV; choice of analyte lines will 

influence the choice of analysing crystal, collimator and detector; solving multiple spectral 

overlap problems. Most of the references in the literature (e.g. Gleisberg et al., 1991;


830 

Moriyasu et al., 1991) refer to the separation of the REEs prior to analysis by XRFS or other 

techniques. A few papers refer to the determination of REEs in powder samples by XRFS 

(Kim et al., 1986; Kim et al., 1987; La1 et al., 1989). 

1 - IGS-36 

----- 700K 

--- 800K 

0.001 

-I I 

La Ce Pr Nd (Prn)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U 

Element 

Figure 1. 

Some oxide concentrations in three monazite “standards”. The radioactive 

element Pm has long since decayed away, and is no longer present in the 

earth’s crust. 

Sample preparation 

Sample preparation is strongly influenced by the choice of analyte lines, K or L and M series. 

Samnle nrenaration for K series lines 

Use of Ka lines of very short wavelength (0.20-0.37 A) results in long path lengths within the 

specimen and large infiite thickness requirements. However, it also reduces considerably 

problems of particle size and mineralogical effects. 20 g 30 mm diameter or 40 g 40 mm 

diameter powder briquettes will be infinitely thick for K series lines. 

Samnle nreoaration for L and M series lines 

Relatively long wavelengths (1.7-4.1 A) will result in severe particle size and mineralogical 

effects in 2 g 30 mm diameter powder briquettes (infinitely thick for L series lines). This 

essentially forces the use of fusions. 

Fusions 

Consistent preparation of homogeneous REE fusion disks can be difficult. Commonly 

observed problems are undissolved material remaining at the end of fusing, bubbles remaining 

in the crucible after cooling, and crystallization of the molten mixture during annealing.


831 

Insufficient annealing time causes frequent shattering of disks. It has also been observed that 

airborne dusts contacting the sample at the pouring stage can lead to an increase in the 

frequency of crystallization. A clean, dust-free environment is essential. 

Mixtures of lithium tetraborate and lithium metaborate (90: 10 or 100:0) have been used very 

successfully. Higher proportions of lithium metaborate have caused more frequent 

crystallization problems. A sample to flux weight ratio of 0.5:9.5 in log fusion disks, with 

a few drops of LiBr (25 g/l00 ml) as releasing agent, is a reasonable compromise between 

sample dilution and melt viscosity. Greater sample loading causes the melt viscosity to 

increase and residual melt to remain in the crucible after pouring. Samples containing 

carbonates or sulphides must be fused for sufficient time to end gas evolution, and bubbles 

must be removed before casting, otherwise the disks tend to crack. 

Instrumental conditions for K Series Lines 

K excitation potentials for the BEES are 63-39 kV. Different excitation efficiencies between 

Au and Rh side-window tubes at different kV settings are shown in Figure 2. A Rh tube at 

60 kV gives poor excitation and will be virtually useless at low concentrations (Figure 2). The 

maximum kV on end-window tubes is 60-70 kV. The best excitation conditions are achieved 

with a 1OOkV generator and Au side-window X-ray tube (Figures 2 and 3). The K excitation 

potential for W is 70 kV and for Au 81 kV. 100 kV on a Au or W tube will, therefore, 

generate some Au and W K line intensity. Au K lines are well-placed to excite the BEE K 

lines (Figure 3). However, most of the excitation will be by tube continuum, and the highest 

atomic number target material gives the maximum continuum intensity and the maximum 

excitation efficiency. Therefore, a Au tube will be best, followed by a W tube. Even at 60kV 

a Au tube gives sufficient excitation to allow the determination of La, Ce, Pr, Nd, Sm and 

Gd(?) using their Ka lines (Figure 2). Analyte lines and problems of spectral overlap are 

clearly demonstrated in Figure 4. 

100 

- Autube,lOOkV 

80 ---a Rhtube,lOOkV 

- Autube,GOkV 

iz. -- Rhtube,GOkV 

-!i 60 

s 

.= 

g 40 

E 

0 

12 13 14 15 

‘20 LiF(220) 

6 

Figure 2. 

Wavelength scans of a monazite specimen at 100 kV and 60 kV with Au and 

Rh side window tubes.


832 

AuKa 

RhKa MoKa 

RhKP 

I I 

8 8 

I I 

b..,, :. . . . . . . . . . ,““““‘,.“....‘.,..‘.‘....r ..,...*.. ..,....*. . I- 

MoKP 1 

I 

I I 

i -I 

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 

Wavelength, 

A 

Figure 3. Relative positions of REE K absorption edges and Au, Rh and MO tube lines, 

100 - 

80 - 

- Au tube, 100kV 

CeKa 

. 

x 

2 60- 

>; 

C 

2 

CD 40- 

E 

LaKa 

20 - 

0-r 

12 13 14 

‘20 LiF(220) 

16 

Figure 4. 

Wavelength scan of a monazite specimen with a Au tube at 1OOkV. The 

analyte lines used for the LREE La through Sm (and Gd?)] are 

underlined.


833 

Summary for K Series lines 

K series lines of the REEs are usable for La, Ce, Pr, Nd, Sm, and possibly Gd, with Au, W 

or Rh tubes at 100 kV. The use of K series lines is probably not viable with a Rh tube at 

60 kV. A fine primary collimator is essential and a secondary collimator is a big advantage 

(it clips peak “wings”). A LiF(220) analysing crystal gives insufficient spectral resolution, 

and a LiF(420) should be better, despite a loss of -50% intensity. There is some doubt as to 

whether the NaI crystals in conventional scintillation detectors are infinitely thick for the high 

energy REE K lines. Even if they are not infinitely thick, the problems would be the same 

for specimens and standards and should not provide serious difficulties. The very high 

background from the continuum and poor excitation efficiency, combined with low 

concentrations, make the K lines unsuitable for the HREE. 

L and M Series Lines 

A Au tube is also the best choice for all REE L series lines (Figure 5), providing good 

excitation and minimal tube line spectral overlap. Ideally, for both Au and Rh tubes, it would 

be best to use La, lines for Ba and for all the REEs except Ce, Pr, Nd, Sm, for which the L/3, 

lines should be used. However, with a Au tube it is necessary to use the NdLa, line, despite 

line overlap from CeLcl, , because the AuLq(2) overlaps NdLP, (otherwise free of spectral 

interference). Ma lines can be used for U and Th (in fusions), and Lcq for Nb and Y. 

AuLa, ,WLq 

WLYl I I 

i Lu~a, 

LaLa, 

RhKa 

I 

I 

I I I 

I 

iA Ul q 1 

L-L-L-L 

1 _ 

I 

I 

I 

II I, 

I 

I 

. 

0.5 1tr.s 2 

GeKa 

Wavelength, A 

WI 

RhLa, 

RhLP, ; 

. I 

I I 

I I 

I 

I I 

I I 

I 

I 

I 

i i 

Figure 5. 

Relative positions of the REE Lcl, lines and corresponding L,,, absorption 

edges and Au, W and Rh tube lines.


834 

One possible solution to the poor excitation of the REE L series lines by Rh tubes (used in the 

vast majority of modern XRF spectrometers) is shown in Figure 6. Essentially it involves 

placing in the tube filter holder a thin foil of a metal to be used as a secondary transmission 

target. The best choice for the REE L series lines would be germanium. The GeK lines are 

situated ideally just short of the HREE L series absorption edges (Figure 5). 

Secondary k~ 

transmissiontarget 

2? 

SPECIMEN 

I 

eKa$ 

radiation 

Thin foil mounted in 

tube filter carousel 

/ 

End-window 

X-ray 

tube 

/ 

(or Au, W, Cu, Ni, Co) 

Figure 6. 

A proposed secondary transmission target (located on the filter disk holder 

and very closely coupled with the specimen) to provide better excitation of 

the REE (Ge target) and transition elements when using a Rh tube. 

Spectral line overlaps are a nightmare for the L series lines, as are absorption edges 

(Figure 7). It is also necessary to correct for spectral overlap from K series lines of the 

transition elements (Cu, Ni, Co, Fe, Mn and Cr) on REE lines (Figure 8). 

Keith Norrish @ers. comm., 1996) uses a LiF(2OO) crystal for the REE L series lines . 

(Table 1). A LiF(220) crystal gives much better spectral resolution but less intensity. It may 

be the preferred analyzing crystal but figure of merit tests need to be conducted. 

For adequate matrix corrections it is necessary also to measure other major elements (Fe, Ca, 

S, P, Si, Al and Mg) (Table 2). Thirty-two element peaks in all have to be measured. 

Norrish uses a “no background” procedure which considerably reduces counting time. He 

determines backgrounds at peak positions using factors measured from single element 

interference standards, and simultaneously corrects for background and spectral overlap. The 

method still needs to be thoroughly tested. If background positions must be counted in 

addition to peaks, it will increase considerably the number of counting positions and the total 

counting time. 

Thirty-two single element spectral interference standards are required to determine accurately 

the required correction factors. The chemicals used need to be spectrally pure but not 

necessarily stoichiometric, since they are not used as standards.


835 

Zn Cu Ni Fe Mn Cr v Ti 

I I I I I I I I 

I I I I I I I I 

I 

I 

I 

I I LuibTm Er Ho Dy’ Tb Gd Lu Sm 

I 

I : I:IIIll:lI~ I : I Ii) 

I 

Nd Pr I Ce La ;a 

LuYbTm Er Ho Dy Tb Gd Eu Sm Nd Pr Ce La Ba 

I: I I: I I I11 II II I II: I I I 1 I I I : II 1 : 1 

Nd Pr Ce La Ba 

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 

Wavelength, A 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

Figure 7. 

A wavelength plot of REE L series absorption edges and La, and Lb, lines 

togther with the Ka and KP lines of some of the transition elements and Ba 

and Ti. 

Ka Zn Cu Ni Co Fe Mn Cr V Ti 

I I I I I I I I I 

La, : 

I I I I I I I I I 

I Lu’;bTmE:&Qy’lq@j J& : lid : (Au) k i3E 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

~ 

I !; I 

I; i 

i: i 

,I iii i 

11 I I:’ ,I1 I 

II 

: I 

!, I 

II 

I’ I 

:: 

T-k 

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 

:; II 

Wavelength, A - - Au L series (2) 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

Figure 8. 

A wavelength plot of REE L series analyte lines with some Ka and KP lines 

of transition elements which could cause spectral interference problems.


836 

Table 1. 

Instrumental conditions and counting times for REE analysis at the 

CSIRO, Australia (K. Norrish, pen. comm.). Rh tube 80 kV, 35 mA, 

flowcounter. 

Element 

Line 

Collimator 

Crystal 

Time (s) 

Lu 

La1 

F 

LiF(200) 80 

Yb 

La1 

F 

LiF(200) 60 

Tm 

La1 

F 

LiF(200) 80 

Er 

La1 

F 

LiF(200) 60 

Ho 

La1 

F 

LiF(200) 80 

DY 

La1 

F 

LiF(200) 30 

Tb 

La1 

F 

LiF(200) 80 

Gd 

La1 

F 

LiF(200) 30 

EU 

La1 

F 

LiF(200) 50 

Sm 

w 

F 

LiF(200) 50 

Nd 

La1 

F 

LiF(200) 20 

Pr 

LP1 

F 

LiF(200) 30 

Ce 

W 

F 

LiF(200) 16 

La 

La1 

F 

LiF(200) 20 

Analytical program for L series lines 

The analytical program (element lines, collimators, crystals and counting times) used for 

monazite and REE mineral analysis by Keith Norrish at the CSIRO in Australia is given in 

Tables 1 and 2. Other instrumental conditions are: Rh tube; 80 kV 35 mA; 40 kV 70 mA 

for Nb, Y, Th, U, Fe, Ca, S, P, Si, Al, Mg; flowcounter; 32 peak positions. Total counting 

time: 1026 seconds or 18 minutes. Selection of a coarse collimator for Y La, and the Ka 

lines of S, P, Si, Al and Mg increases intensities but still provides adequate spectral resolution 

with the particular analysing crystals used for these lines. 

A wavelength scan of a typical monazite specimen is given in Figure 9. The two parts of the 

scan separated by the dashed line are shown separately in Figures 10 and 11.


837 

8 

22 

.= 

s 

2 

a 

- E 

40 

30 

20 

10 


BaLP, 

, FeKa 

CeLa, 

0 

‘28 LiF(220) 

Figure 9. 

A wavelength scan of the REE L series lines from a monazite specimen. A 

Au tube was used at 60 kV 45 mA with a LiF(220) analysing crystal, fine 

collimator and flowcounter. 

20 

15 

10 

5 

0 

100 110 120 130 140 

‘243 LiF(220) 

Figure 10. 

An expanded view of part of Figure 9, showing details of the wavelength 

scan of the LREE L series analyte lines from a monazite specimen.


838 

u) 

a 

6 

x 4 

s 

C 

E 

a 

- E 2 


2 

9 

IF 

0 

0 

70 75 80 85 

‘20 LiF(220) 

Figure 11. 

An expanded view of part of Figure 9, showing details of the wavelength 

scan of the HREE L series analyte lines from a monazite specimen. 

Standards 

Very few CRMs are available for high REE mineral specimens and none for high purity REE 

compounds, a major problem in developing and testing accurate analytical methods. Using 

fusions for L and M series lines should allow preparation of standards from pure chemicals. 

The problem is to get pure a stoichiometric elemental oxides. Chemical suppliers guarantee 

spectral purity. However, due to moisture or CO, absorption and variations in redox state, 

they do not guarantee to a sufficient accuracy the stoichiometric purity of REE compounds. 

Spectrally pure REE oxides can be fired at 1000°C for 30 minutes and the theoretical oxide 

to element conversion factors used. This may lead to systematic errors that are difficult to 

overcome. The analyst is forced to rely on other chemical methods to determine the actual 

stoichiometry of purchased pure oxides. The analytical uncertainty (l-2%) of ICP-AES and 

ICP-MS is much greater than that required for obtaining element to oxide conversion factors 

adequate for accurate preparation of artificial standards. Therefore, the analyst must use 

considerable judgement when preparing synthetic standards for REE analysis. 

A round-robin program for establishing a number of suitable standard minerals or oxides for 

REE analysis is required urgently. Sufficiently large quantities of REE concentrates and ores 

are available for this purpose.


839 

Table 2. 

Instrumental conditions and counting times for elements other than the 

REEs in monazite samples at the CSIRO, Australia (K. Norrish, pen. 

comm.). Rh tube, see text for kV-mA settings, flowcounter. 

CU Ku F LiF(200) 20 

Ni Ku F LiF(200) 20 

co Ku F LiF(200) 30 

Fe Ku F LiF(200) 20 

Mn Ku F LiF(200) 10 

Ca Ku F LiF(200) 10 

S Ku C Ge 20 

P Ku C Ge 20 

Si Ku C PE 20 

Al Ku C PE 20 

m Ku C TlAP 20 

Software for data reduction 

It is essential to make corrections for inter-element matrix effects. Most, if not all, influence 

coefficient software packages cannot process up to 32 elements in a single run (20-2 1 elements 

is the current maximum). One solution is to dilute specimens sufficiently that algorithms such 

as Lachance-Traill, using single, constant influence coefficients for each inter-element effect, 

would be appropriate. It would certainly simplify the calculations. The problem in using high 

dilutions is the loss of sensitivity for HPEE elements, which usually occur at low 

concentrations in REE minerals.


840 

Crossing major or minor element absorption edges 

For the determination of only Nb and Y (Kcr lines) and Th and U (La, lines) in powder 

briquettes, mass absorption coefficients (MACs) obtained using the Rh or MO Compton peak 

intensities can be used to correct for matrix effects due to absorption. Great care must be 

exercised in crossing major or minor element absorption edges. Ignoring the presence of 

major or minor element absorption edges between tube Compton peaks and analyte 

wavelengths (Figure 12) can ‘result in large systematic errors (Table 3), when using mass 

absorption coefficients determined from tube Compton peak intensities. Ignoring absorption 

edges of minor elements at concentrations greater than -2000 mg/kg can lead to significant 

errors. The use of the Nesbitt ef al. (1976) method for crossing major element absorption 

edges easily solves the problem (Figure 13). 

RhKaC(T) 

MoKaC(T) 

ThLa, 

Figure 12. 

0.5 0.6 0.7 018 

Wavelength, A 

Problems (crossing major element absorption edges) in using tube Compton 

peaks to determine MACs for Th and Y analyses, when Th is present at 

minor to major concentrations. 

1 

-2 - 0.5109 + 5.3353.10-"(Th kcps) Y 

20 40 60 

intensity ThLa, 

(kcps) 

60 

Figure 13. 

Plot of ThLcl, intensity versus the ratio of c(M,,Kcrcl~~Kcr for estimating from 

MoKaC the vahte of ,.&r&KS across the ThL, absorption edge. Data points 

are blank specimens of known composition to which different, known 

amounts of ThOl have been added.


841 

Table 3. 

Errors in Th data in monazite-rich sands when using mass absorption 

coefficients obtained by measuring the MO tube Ka Compton peak intensity 

while ignoring major element absorption edges. The MAC at RbKa was 

estimated from the MAC at. MoKaC using the Nesbitt et al. (1976) method 

(Figure 13). 

Sample 

Th @w/kg) 

using MAC at MoKaC 

Th (mg/kg)* 

using MAC at RbKcl 

% difference 

ll2A 31380 27780 13.0 

1/3A 27650 24670 12.1 

1/4A 9460 8870 10.8 

l/5 4900 4710 4.0 

l/12 943 942 0.1 

l/6 497 500 0.6 

l/l 311 314 1.0 

l/14 105 106 1.0 

l/13 23 23 0.0 

* correct value 

Conclusions 

Rare earth element compounds are becoming an important industrial product and good quality 

control in their manufacture is essential. WDXRFS is a rapid and reliable technique for 

routine analysis of “normal” minerals and oxides. However, WDXRFS analysis of REEs in 

minerals and high purity REE oxides is a daunting and complicated task. 

Particle size and mineralogical effects for all REEs can be eliminated by use of fusion disks. 

L and M (Th and U) series lines m be used with fusion disks, because of infinite thickness 

limitations. Using powder briquettes, K series lines can be measured for La, Ce, Pr, Nd, Sm, 

Gd(?), Nb, and Y, and L series lines for Th and U. Particle size and mineralogical effects will 

be minimal due to relatively large penetration depths. 

The possibility needs to be investigated of using L lines of La, Ce, Pr, Nd or Sm (whose 

concentration data were obtained from K series lines) as variable internal standards for HREE 

(Eu, and Gd through Lu) L series lines on powder briquettes. The use of powder briquettes 

for rapid, routine and reasonably accurate analyses of all REEs would eliminate the need for 

a fusion process, with considerable savings in time and costs.


842 

New or modified software programs are needed to process the large number of spectral lines 

(30-40) required for the accurate analysis of REEs in minerals. 

Since suppliers of spectrally pure REE oxides do not guarantee the stoichiometry of the 

oxides, it is difficult and a potential source of systematic error to use these compounds in 

making up artificial standards. It is necessary to analyse them accurately by some other 

analytical technique to determine their stoichiometry. ICP-MS is a useful analytical technique 

for REE determination, but it is not always easy to get ALL the REEs into solution and 

generally one should fuse before dissolution. Very high concentrations (20-30%) of some of 

the LREEs make accurate determination of some of the HREEs at very low concentrations 

(0.001% or 10 ppm) difficult by ICP-MS. A round-robin program for establishing a number 

of suitable standard minerals or oxides for REE analysis is urgently required. 

A simple and reliable WDXRPS method for the routine determination of REEs at high and low 

concentrations in minerals and chemicals would be a valuable addition to any geological 

laboratory. 

References 

Gleisberg, B., Ly, B.M. and Gorski, B. (1991) Investigations of the separation of neptunium, 

protactinium, uranium, thorium and REE [rare-earth elements] in geological samples. 

J. Radioanal. Nucl. Chem., 147, 95-107. 

Kim, Y .M., Kim, Y .S., Park, Y .C. and Lee, C. W. (1986) Study on the correction of spectral 

line overlaps for the determination of rare-earth elements by X-ray fluorescence 

spectrometry. Taehan Hwakakhoe Chi, 30, 538-547. 

Kim, Y .M., Choi, B.S., Kim, S.T. and Lee, C.W. (1987) Determination of rare-earth oxides 

by X-ray fluorescence spectrometry using empirical coefficient method. Taehan 

Hwakukhoe Chi, 31, 64-70. 

Lal, M., Choudhury, R.K., Joseph, D., Bajpal, H.N. and Iyer, C.S.P. (1989) Elemental 

analysis of selected Indian monazite ores by energy-dispersive X-ray fluorescence 

(EDXRF) spectroscopy. J. Radioanal. Nucl. Chem., 137, 127-133. 

Moriyasu, K., Mizuta, H. and Nishikawa, Y. (1991) Determination of rareearth-metal ions 

by XRF with hydroxyiminodiacetic acid chelating resin. Bunseki Kaguku, 40, 

175-179. 

Nesbitt, R.W., Mastins, H., Stolz, G.W. and Bruce, D.R. (1976) Matrix corrections in trace 

element analysis by X-ray fluorescence: an extension of the Compton-scattering 

technique to long wavelengths. Chem. Geol., 18, 203. 

Norrish, K. (1976). Personal communication. CSIRO, Division of Soils, Private Bag No. 

2, Glen Osmond, South Australia, 5064, Australia.

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