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 [Pb.sub.2][P.sub.2][O.sub.7] flux that is derived from the thermal decomposition of Pb[HPO.sub.4] The lead pyrophosphate flux is used to dissolve the oxide starting materials at elevated temperatures ([approximately equal to] 1360 [degrees]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 excelle nt materials.

Key words: EPMA; microanalysis; orthophosphates; quantitative analysis; rare earth elements; rare earth phosphates; REE; standards.

1. Introduction

Highly accurate analyses from the electron microprobe analyzer (EMPA) are only (but not solely) obtainable through the use of well-characterized and stable standards containing a major and/or known concentration of the element in question. For the rare earth elements (REE) this goal has, until recently, been elusive due to the lack of specimens exhibiting these vital properties.

The lanthanide orthophosphates, consisting of compounds with the stoichiometry Ln[PO.sub.4] where Ln represents any of the REE in the series extending from La to Lu (plus the related compounds [YPO.sub.4] and Sc[PO.sub.4]), are chemically durable and radiation resistant refractory materials. During the early 1980s a variety of single crystal rare earth orthophosphate samples were synthesized at Oak Ridge National Laboratory and the structures determined from x-ray refinements [1, 2, 3, 4, 5, and 6]. The primary purposes of these studies were varied, but they included nuclear and actinide waste disposal and scintillator material research as well as fundamental materials characterization investigations. The crystals were synthesized using a high-temperature solvent (flux-growth) technique, the details of which are available from the original papers, and a good overview of the development of these orthophosphates is discussed in Boatner and Sales (7), and references therein.

One interesting fact is that although the starting materials were carefully selected to be free from REE impurities, they were grown in a lead pyrophosphate (Pb[HPO.sub.4]) flux. Pb contamination was not a concern for the original purposes of those experiments, however its presence was detected early on, and the solid state chemistry (but not the concentration) of Pb in the orthophosphate was characterized by means of electron paramagnetic resonance spectroscopy (EPR) (8). Subsequently, these materials were investigated for possible use as standards for EPMA by the Smithsonian Institution (9), and put through a series of tests. These included homogeneity testing and a comparison to the commonly used REE doped aluminum silicate glass standards of Drake and Weill (10) using the EPMA, and a check of 10 selected REE contaminants on 7 of the compounds using instrumental neutron activation analysis. The materials appeared to be robust under electron bombardment, did not oxidize or seem hygroscopic, and no serious contamination or inhomogeneities were noted at the time and these efforts were followed by a general distribution of the material to interested parties.

In the late 1990s it was reported to one of us (JJD) that at least one investigator (E. J. Essene, University of Michigan, personal communication) had raised the issue of the role of the Pb impurity in some of the REE phosphate standards. The Pb impurity is especially significant in the [CePO.sub.4] crystals whose black coloration is consistent with possible mixed valence ([Ce.sup.3+] - [Ce.sup.4+]) effects--the presence of which could alter the high-temperature solid-state chemical properties and lead to an enhanced incorporation of Pb during the crystal-growth process. Subsequent investigations of the materials revealed Pb ranging in concentration from less than 0.01 mass fraction to more than 0.04 mass fraction in the [CePO.sub.4], depending on the specific grains analyzed. It is the intent of this paper to characterize the extent of the Pb contamination in these otherwise extremely useful standards for EPMA.

2. Experimental Methods

Quantitative wavelength dispersive spectrometry (WDS) analyses for the REEs Sc, Y, and Pb in each of the 16 orthophosphate samples were done using a Cameca SX-[51.sup.1] electron microprobe at 20 key, 20 nA (2.0 X [10.sup.-8] A), using a 10 [mu]m beam diameter at UC Berkeley. In addition, one of the Drake and Weill RiEE glasses (10), and two other REE doped calcium aluminum silicate discussed in Roeder (11) and Roeder et al. (12) were analyzed. For quantitative analyses, the [K.sub.[alpha]] x-ray line was used for Sc, [L.sub.[alpha]] lines for Y and the other REE elements, and the [M.sub.[alpha]] line was used for Pb. Count times were 20 s on peak and 10 s on each off-peak position except for Pb where the count times were doubled, respectively.

A complete description of the analytical setup and secondary standard accuracy for the analyzed elements (the composition of the REE phosphate primary standards in these cases had been previously adjusted for average Pb concentrations) is presented in Table 1. Secondary standards included synthetic yttrium-aluminum garnet (YAG) and alamosite (Pb[SiO.sub.3]) from Tsumeb, Namibia and were assumed to be stoichiometric for Y and Pb, respectively. The Roeder REE glass S-254 (12) was assumed to have a nominal concentration (1.04 X [10.sup.-2] mass fraction) for La, Ce, Pr, Nd, Sm, Dy, Ho, Er, Yb, and Lu, and the Drake and Weill REE-1 glass was used based on published concentrations for Eu, Gd, Tb, and Tm (10). For all rare-earth elements, the relative differences obtained when comparing the secondary standards to the primary standard is better than 10 % at the 0.01 mass fraction to 0.04 mass fraction concentration levels and better than 6 % in all but three cases (Pr, Sm and Lu).

The difficulty of dealing with interfering elements for REE analyses using the [L.sub.[alpha]] x-ray lines is painfully evident in even cursory WDS spectral scans on these samples and can only be overcome by careful and consistent application of an automatic correction scheme. Table 2 shows the REEs that interfere with the analyzed elements. These were interferences quantitatively corrected for using the iteration method of Donovan et al. (13), that is especially well suited for using large magnitude interferences for trace element determinations. For the Pb analyses, the [M.sub.[alpha]] line was used with a quantitative interference correction for Y (possible high order interferences from La and Tb were not observed). Standard and background intensities along with the calculated P/B (peak to background) for each line in its associated primary standard are shown in Table 3.

Under the analytical conditions which were utilized at Berkeley, the minimum detection limits for both single analyses calculated from Love and Scott (14), and for the average of 10 replicate analyses based on Goldstein et al. (15), are shown in Table 4. Minimum detection limits for 10 replicate analyses based on the actual measured standard deviation are about 3.0 X [10.sup.-4] mass fraction to 6.0 X [10.sup.-4] mass fraction for all elements in all matrices although only values for Ce[PO.sub.4] or Gd[PO.sub.4] are shown in Table 4. A measured detection limit of 3 X [10.sup.-4] mass fraction to 6 X [10.sup.-4] mass fraction for the average of 10 replicates at 99 % a confidence level was typical for the REE analyses under these conditions. The Pb detection limit at a 99 % confidence level was about 4.5 X [10.sup.-4] mass fraction.

Another set of measurements, for the analysis of Pb homogeneity only, were also done on the same grains, but in a different area from the REE and Pb measurements done at UC Berkeley. These measurements were made for each REE orthophosphate using a JEOL 8900 Superprobe at the University of Maryland-College Park. X-ray intensities of Pb were obtained using an accelerating voltage of 20 keV, and a beam current of 150 nA. Count times were 60 s on peak, and 30 s for backgrounds on each side of the peak. Pb was analyzed using a PETH (which utilizes a smaller diameter Rowland Circle allowing for higher count rates, but has poorer wavelength resolution) crystal, and background positions of +4 mm (L = 173.307 mm or 5.4013 A) and -- 3 mm (L = 166.307 mm or 5.1828 A). Natural cerussite (Pb[CO.sub.3]) from Tsumeb, Namibia, was used as a standard for Pb (0.8393 mass fraction PbO). It should be noted that although cerussite is a carbonate mineral it did not appear to degrade under the electron beam during the analyses. The Pb [M.sub.[alpha]] x-ray line was used for all analyses, with the exception of [YPO.sub.4], where [M.sub.[beta]] was used due to an interference from [Y.sub.1[gamma]3] on Pb [M.sub.[alpha]]. For these Pb homogeneity measurements, the REE and phosphate concentrations were not measured but were incorporated as stoichiometric proportions into the ZAF algorithm in order to approximately account for matrix effects. The single analysis detection limit at a 99 % confidence level for Pb under these analytical conditions was about 1.4 X [10.sup.-4] mass fraction Pb based on a standard count rate of 263.9 cps/nA and a background of 0.8 cps/nA measured on Ce[PO.sub.4].

Measurements were done on two different sets of REE orthophosphate samples. The first set consists of material for 16 orthophosphates, including Sc and Y obtained from one of us (JMH) and mounted along with primary and secondary standards for analysis and interference corrections. These materials were mounted in a 25 mm diameter acrylic mount approximately 1.5 cm deep using a cold set epoxy and circulated to both laboratories. This sample will be referred to as the "Round Robin" mount in the discussion that follows.

The "Round Robin" mount was carefully analyzed for Pb at both Berkeley and College Park to check for inter-laboratory differences since the analytical results of trace element measurements are extremely sensitive to differences in spectrometer resolution and placement of off-peak intensity measurement positions. Homogeneity measurements were also done on this mount at College Park to check for possible Pb variations within this material itself.

Additional Pb measurements were performed at UC Berkeley on other material that was originally resident in the laboratory standard collection to check for possible inter-batch differences in Pb contamination some of the material had been produced in several runs at Oak Ridge under possibly different growth conditions. Analyses on this material will be referred to as the "Berkeley" REE mount.

3. Results and Discussion

3.1 REE Impurities in the Orthophosphate Standards

Table 5 shows the trace REE elements measured in each of the orthophosphates at UC Berkeley on the "Round Robin" mount. One can see that as stated in the original paper by Jarosewich and Boatner [19], the material is generally very pure based on quantitative results from instrumental neutron activation analysis (INAA). The only statistically significant REE contamination anomalies we observed were the presence of approximately 9 X [10.sup.-4] mass fraction Eu in [GdPO.sub.4] (Jarosewich and Boatner reported 1.9 X [10.sup.-5] mass fraction Eu in [GdPO.sub.4] using INAA), 1.1 X [10.sup.-3] mass fraction Ho and 7 X [10.sup.-4] mass fraction Y in the [DyPO.sub.4] (Jarosewich and Boatner reported 2.47 X [10.sup.-3] Ho in [DyPO.sub.4] using INAA, Y was not analyzed by INAA), and approximately 1.1 X [10.sup.-3] mass fraction Er in the [TmPO.sub.4] (Er was not analyzed by Jarosewich and Boatner with INAA). It is difficult to obtain commercially available REE oxide materials that are completely free of other REE impur ities due to the nature of the starting materials (REE-rich phosphate and carbonate minerals) that must be processed to extract individual REEs. The apparent concentration of 0.0009 [+ or -] 0.0007 mass fraction Lu in [GdPO.sub.4] is possibly due to an interference of Gd [L.sub.[gamma]1] at 1.5928 [Angstrom] and the 0.0006 [+ or -] 0.0005 mass fraction Dy in [YbPO.sub.4] is possibly due to an interference of Yb L1 at 1.8946 [Angstrom] and finally the 0.0004 [+ or -] 0.0003 mass fraction Yb in [YPO.sub.4] is possibly due to an interference of Y K[[alpha].sub.1] (II) at 1.658 [Angstrom]. No other interferences could be invoked to explain the other apparent REE concentrations shown in bold in the table.

3.2 Pb Impurities in the Orthophosphate Standards

The results for Pb in the last row of Table 5 reveal that Pb is present from almost 0.02 mass fraction down to about 0.005 mass fraction element in seven of the REE orthophosphates in the "Round Robin" mount (in order of decreasing concentration: [CePO.sub.4], [LaPO.sub.4], [SmPO.sub.4], [PrPO.sub.4], [NdPO.sub.4], [EuPO.sub.4], and [GdPO.sub.4]. The remaining REE orthophosphates did not contain Pb concentrations above the UC Berkeley detection limit of 4.5 X [10.sup.-4] mass fraction. These measurements consisted of a 10-point traverse on a single grain of each REE orthophosphate. Table 6 shows the Pb homogeneity measurements on the same "Round Robin" mount but performed in College Park with increased sensitivity (longer count times and higher beam currents). The two data sets agree well considering the apparent inhomogeneity of the Pb contaminated materials.

What is striking is that the Pb content varies considerably not only within each grain, but even more so from grain to grain, as seen in Table 7 where a number of Pb measurements (13-16) over the face of the four [CePO.sub.4] grains in the "Berkeley" mount show tremendous variation between grains from about 0.015 mass fraction to 0.045 mass fraction element.

3.3 Crystal Structure and Pb Contamination

Lead is present in significant amounts only in the monoclinic, high-temperature, monazite-structure orthophosphates ([LaPO.sub.4] through [GdPO.sub.4]), and is absent, or nearly so, in the tetragonal, xenotime-structure, compounds ([TbPO.sub.4] through [LuPO.sub.4] and [ScPO.sub.4] and [YPO.sub.4]) as can be seen in Fig. 1, where Pb concentration is plotted as a function of REE atomic number. Boatner and Sales (7) showed that there is a distinct structural change (monoclinic to tetragonal) between [GdPO.sub.4] and [TbPO.sub.4] which suggests that the incorporation of Pb in the monazite structure, and the lack of Pb incorporation in the xenotime structure orthophosphates, is related to this change in structure. The so-called lanthanide contraction is a continuous decrease in size across the REEs, and may also play a role in this, however, there are no abrupt decreases in the trivalent ionic radii across the REE series (including from Gd to Tb). Our data suggest that the exclusion of the large (e.g., 1.29 [Angstrom] in eight coordination, (16) divalent lead cation is limited by the space available in the heavy [REEO.sub.8] ([HREEO.sub.8]) polyhedra and that the divalent Pb ion, or the trivalent HREEs, will not fit easily into the xenotime structure. For the monoclinic orthophosphates, the light [REEO.sub.9] ([LREEO.sub.9]) polyhedra is much larger and can accommodate the divalent [Pb.sup.2+] ion into the xenotime structure (16).

In examining the [REEPO.sub.4] structures, it is evident that when the REE cation radius contracts beyond a certain point (empirically 1.05 [Angstrom]), the REE cation becomes too small to maintain the monoclinic structure type, and the structure distorts to a lower density, lower energy, tetragonal structure type. Once this change from monoclinic to tetragonal symmetry has occurred, the divalent lead cation can no longer fit into this confined [HREEO.sub.8] polyhedra. The tetragonal orthophosphates are all of the same structure type, with a slight contraction of unit cell volume with increasing atomic number. The same holds true for the monoclinic orthophosphates. There is a dramatic jump in the cell volumes between Gd (276 [[Angstrom].sup.3]) and Tb (292 [[Angstrom].sup.3]) with the phase change.

The tetravalent lead cation with an ionic radius of 0.94 [Angstrom] (16), would appear to fit better into the tetragonal xenotime structure orthophosphates with the smaller BREE cations (1.04 [Angstrom] to 0.87 [Angstrom], (16)), but significant Pb was not observed in those samples. Abraham et al. (8), did find some trivalent Pb in their EPR experiments, but other valence states of Pb such as tetravalent lead could have been present since they are not observable by means of EPR spectroscopy (8). The flux used for crystal growth, [Pb.sub.2][P.sub.2][O.sub.7], derived from the decomposition of [PbHPO.sub.4], contains divalent lead thus, it seems more likely that the Pb was in the divalent state under the conditions of synthesis.

Characterizing the exact Pb contamination within a given orthophosphate is problematic because of the degree to which the Pb concentrations vary, not only within a single grain but also from grain to grain. For this reason it is recommended that each laboratory perform systematic x-ray mapping for Pb of their "in house" REE orthophosphates grains to determine the actual extent and variation of Pb contamination in their own mounts. As can be seen in Table 7 (e.g., grain #3), it may be that the Pb contamination is homogeneous enough that some portion or another of the material may be suitable for use as a quantitative standard for major element concentrations of the REE in question. Once the Pb concentration for a homogeneous grain is known and the position noted, the measured Pb can be proportionally subtracted from the ideal [REEPO.sub.4] composition and entered into the laboratory's standard compositional database for general use.

Regarding which [REEPO.sub.4] material should be used for P as an EMPA standard, we suggest that one of the tetragonal orthophosphates should be used to minimize any nonstoichiometry introduced by Pb impurities.

4. Conclusions

Due to their qualities of robustness under the electron beam, resistance to oxidation, and REE purity, the REE orthophosphate standards remain a valuable set of standards for EPMA despite significant Pb contamination in at least 7 of the 16 samples examined. Of those with measurable Pb contamination, only the monoclinic [CePO.sub.4] and possibly the [LaPO.sub.4] and [SmPO.sub.4] contain enough Pb to noticeably affect the stoichiometry for use as a primary standard for major element quantitative analysis (approximately 2 % to 4 % relative differences from their theoretical compositions). None of the tetragonal, xenotime structure orthophosphates (Gd-[LuPO.sub.4] and [ScPO.sub.4] and [YPO.sub.4]) contain appreciable Pb.

[FIGURE 1 OMITTED]

Table 1

Analytical setup and measured differences between the secondary
standards and the primary standard for REE quantitative analysis (a)



Element             Spect. setup    Primary standard
Sc [K.sub.[alpha]]  LiF (FPC-2)   Sc[PO.sub.4] (syn.)

Y[L.sub.[alpha]]    PET (FPC-1)    [YPO.sub.4] (syn.)
La[L.sub.[alpha]]   LiF (FPC-2)   La[PO.sub.4] (syn.)
Ce[L.sub.[alpha]]   LiF (FPC-2)   Ce[PO.sub.4] (syn.)
Pr[L.sub.[alpha]]   LiF (FPC-2)   Pr[PO.sub.4] (syn.)
Nd[L.sub.[alpha]]   LiF (FPC-2)   Nd[PO.sub.4] (syn.)
Sm[L.sub.[alpha]]   LiF (FPC-2)   Sm[PO.sub.4] (syn.)
Eu[L.sub.[alpha]]   LiF (FPC-2)   Eu[PO.sub.4] (syn.)
Gd[L.sub.[alpha]]   LiF (FPC-2)   Gd[PO.sub.4] (syn.)
Tb[L.sub.[alpha]]   LiF (FPC-2)   Tb[PO.sub.4] (syn.)
Dy[L.sub.[alpha]]   LiF (FPC-2)   Dy[PO.sub.4] (syn.)
Ho[L.sub.[alpha]]   LiF (FPC-2)   Ho[PO.sub.4] (syn.)
Er[L.sub.[alpha]]   LiF (FPC-2)   Er[PO.sub.4] (syn.)
Tm[L.sub.[alpha]]   LiF (FPC-2)   Tm[PO.sub.4] (syn.)
Yb[L.sub.[alpha]]   LiF (FPC-2)   Yb[PO.sub.4] (syn.)
Lu[L.sub.[alpha]]   LiF (FPC-2)   Lu[PO.sub.4] (syn.)
Pb[M.sub.[alpha]]   PET (FPC-1)   Pb[CO.sub.3]
                                  (Tsumeb)

                    Secondary standard
                       Conc in mass
Element                 fraction X      Relative diff.
Sc [K.sub.[alpha]]      [10.sup.2]

[YL.sub.[alpha]]       YAG (stoic.)     +0.368, +0.82 %
La[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.020, -1.92 %
Ce[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.010, -0.95 %
Pr[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.103, -9.95 %
Nd[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.007, -0.70 %
Sm[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.055, -5.27 %
Eu[L.sub.[alpha]]   REE-1 (3.63 pub.)   +0.069, +1.90 %
Gd[L.sub.[alpha]]   REE-1 (3.87 pub.)   -0.012, -0.31 %
Tb[L.sub.[alpha]]   REE-1 (3.78 pub.)   -0.116, -3.08 %
Dy[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.035, -3.35 %
Ho[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.041, -3.92 %
Er[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.047, -4.51 %
Tm[L.sub.[alpha]]   REE-1 (3.81 pub.)   -0.127, -3.33 %
Yb[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.047, -4.53 %
Lu[L.sub.[alpha]]   S-254 (1.04 nom.)   -0.103, -9.94 %
Pb[M.sub.[alpha]]   PbSi[O.sub.3]       +0.550, +0.75 %
                    (stoic.)

(a)Analytical spectrometer setup (flow proportional detectors: FPC-1
indicates 1 atm P-10 and FPC-2 indicates 2 atm P-10) for REE elements
(plus Sc, Y, and Pb) and results of secondary stanard measurements
(algebraic difference and relative difference) performed at UC Berkeley.
All elements were measured at 20 KeV, 20 nA (150 nA for the four grain
map), 10 [micro]m beam diameter, 20 s on-peak integration time and 10 s
on each off-peak except for Pb which was counted for 40 s on-peak and 20
s on each off-peak position (240 s on-peak and 120 s on each off-peak
position for the four grain map in Fig. 5). Each result shown is the
average of 10 measurements.

Table 2

Quantitative interferences. (a) Also listed are the wavelengths (in
[angstrom] of the x-ray lines

Element [angstrom]           On peak interferences [angstrom]

Sc[K.sub.[alpha]] at 3.0320  Er[L.sub.[beta]2] (II) at 3.0284
Y[L.sub.[alpha]] at 2.6657   La[L.sub.[gamma]1] (III) at 6.4260
                             (not observed)
La[L.sub.[alpha]] at 2.6657  NdL1 (I) at 2.6766
Ce[L.sub.[alpha]] at 2.5615
Pr[L.sub.[alpha]] at 2.4630  La[L.sub.[beta]1,4] (I) at 2.4595, 2.4595
                             SML1 (I) at 2.4826
Nd[L.sub.[alpha]] at 2.3704  Ce[L.sub.[beta]1,4] (I) at 2.3566, 2.3499
                             Pb[L.sub.[alpha]1,2] (II) at 2.3504, 2.3732
Sm[L.sub.[alpha]] at 2.1998  Ce[L.sub.[beta]2] (I) at 2.2092
                             Pr[L.sub.[beta]3] (I) at 2.2175
                             (not observed)
Eu[L.sub.[alpha]] at 2.1209  Nd[L.sub.[beta]3] (I) at 2.1273
                             Pr[L.sub.[beta]2] (I) at 2.1197
Gd[L.sub.[alpha]] at 2.0468  Ce[L.sub.[gamma]1] (I) at 2.0489
                             La[L.sub.[gamma]2,3] (I) at 2.0462, 2.0415
                             Nd[L.sub.[beta]2] (I) at 2.0365
Tb[L.sub.[alpha]] at 1.9765  La[L.sub.[gamma]4] (I) at 1.9834
                             Pr[L.sub.[gamma]1] (I) at 1.9614
                             (not observed)
                             Sm[L.sub.[beta]3] (I) at 1.9627
                             (not observed)
                             Pb[L.sub.[beta]1,2] (II) at 1.9660, 1.9650
                             (not observed)
Dy[L.sub.[alpha]] at 1.9088  Eu[L.sub.[beta]1,4] (I) at 1.9207, 1.9258
                             YbL1 (I) at 1.8946 (possibly observed)
Ho[L.sub.[alpha]] at 1.8450  Gd[L.sub.[beta]1,4] (I) at 1.8472, 1.8543
                             LuL1 (I) at 1.8362 (not observed)
Er[L.sub.[alpha]] at 1.7842  Tb[L.sub.[beta]1,4] (I) at 1.7770, 1.7867
                             Nd[L.sub.[gamma]2,3] (I) at 1.8015, 1.7968
Tm[L.sub.[alpha]] at 1.7268  Dy[L.sub.[beta]1,4] (I) at 1.7110, 1.7212
                             Gd[L.sub.[beta]2] (I) at 1.7457
                             Sm[L.sub.[gamma]1] (I) at 1.7275
Yb[L.sub.[alpha]] at 1.6718  Eu[L.sub.[gamma]1] (I) at 1.6577
                             Sm[L.sub.[gamma]2] (I) at 1.6608
                             Tb[L.sub.[beta]2] (I) at 1.6834
                             Y[K.sub.[alpha]1] (II) at 1.6580
                             (possibly observed)
                             Ho[L.sub.[beta]4] (I) at 1.6597
                             (not observed)
Lu[L.sub.[alpha]] at 1.6195  Ho[L.sub.[beta]3] (I) at 1.6207
                             Dy[L.sub.[beta]2] (I) at 1.6241
                             Gd[L.sub.[gamma]1] (I) at 1.5928
                             (possibly observed)
Pb[M.sub.[alpha]] at 5.2860  Y[L.sub.[gamma]3] (I) at 5.2848
                             La[L.sub.[alpha]1] (II) at 5.3326
                             (not observed)
                             Tb[L.sub.[beta]1] (III) 5.3310
                             (not observed)

(a)Analyzed elements and interfering elements were quantitatively
corrected by using the iteration method of Donovan et al. (12). Many of
these interferences are 1st order interferences and therefore are the
same energy as the interfering line, and hence, cannot be reduced by the
use of pulse height analysis (PHA). Selection of alternative (beta)
lines is sometimes possible, but the resulting reduction in intensity
will also reduce sensitivity.

Table 3

Standard peak and background intensities (linear interpolation  method)
(a)

Element               Peak intensity     Background intensity
                         (cps/nA)              (cps/nA)

Sc[K.sub.[alpha]]  49.3 (ScP[O.sub.4])           0.2
Y[L.sub.[alpha]]   68.3 (YP[O.sub.4])            0.5
La[L.sub.[alpha]]  38.5 (LaP[O.sub.4])           0.3
Ce[L.sub.[alpha]]  45.4 (CeP[O.sub.4])           0.5
Pr[L.sub.[alpha]]  55.1 (PrP[O.sub.4])           0.6
Nd[L.sub.[alpha]]  64.9 (NdP[O.sub.4])           0.6
Sm[L.sub.[alpha]]  80.8 (SmP[O.sub.4])           1.3
Eu[L.sub.[alpha]]  89.6 (EuP[O.sub.4])           1.1
Gd[L.sub.[alpha]]  95.2 (GdP[O.sub.4])           1.2
Tb[L.sub.[alpha]]  101.9 (TbP[O.sub.4])          1.3
Dy[L.sub.[alpha]]  107.8 (DyP[O.sub.4])          1.5
Ho[L.sub.[alpha]]  113.6 (HoP[O.sub.4])          2.2
Er[L.sub.[alpha]]  119.5 (ErP[O.sub.4])          2.1
Tm[L.sub.[alpha]]  122.9 (TmP[O.sub.4])          2.5
Yb[L.sub.[alpha]]  128.0 (YbP[O.sub.4])          2.6
Lu[L.sub.[alpha]]  131.3 (LuP[O.sub.4])          3.4
Pb[M.sub.[alpha]]  72.0 (PbC[O.sub.3])           0.6

Element            Peak/Background


Sc[K.sub.[alpha]]       246.5
Y[L.sub.[alpha]]        136.6
La[L.sub.[alpha]]       128.3
Ce[L.sub.[alpha]]        90.8
Pr[L.sub.[alpha]]        91.8
Nd[L.sub.[alpha]]       108.1
Sm[L.sub.[alpha]]        62.2
Eu[L.sub.[alpha]]        81.5
Gd[L.sub.[alpha]]        79.3
Tb[L.sub.[alpha]]        78.4
Dy[L.sub.[alpha]]        71.9
Ho[L.sub.[alpha]]        51.6
Er[L.sub.[alpha]]        56.9
Tm[L.sub.[alpha]]        49.2
Yb[L.sub.[alpha]]        49.2
Lu[L.sub.[alpha]]        38.6
Pb[M.sub.[alpha]]       120.0

(a)Average peak and background intensities measured on the primary
standards for the analyzed elements along with calculated peak to
background ratios. Off-peak positions were based on high-resolution
spectral scans of the low to high off-peak regions of each REE element
and Pb in each of the REE phosphates. The purpose was to avoid off-peak
interferences as much as possible.

Table 4

Typical single analysis and average (replicate) detection limits (a)

Element            Detection limit (single point)
                   (.99 CL)(mass fraction X
                   [10.sup.2] in CeP[O.sub.4])

Sc[K.sub.[alpha]]    0.058
Y[L.sub.[alpha]]     0.103
La[L.sub.[alpha]]    0.187
Ce[L.sub.[alpha]]    0.147 (in GdP[O.sub.4])
Pr[L.sub.[alpha]]    0.104
Nd[L.sub.[alpha]]    0.111
Sm[L.sub.[alpha]]    0.103
Eu[L.sub.[alpha]]    0.137
Gd[L.sub.[alpha]]    0.097
Tb[L.sub.[alpha]]    0.139
Dy[L.sub.[alpha]]    0.100
Ho[L.sub.[alpha]]    0.140
Er[L.sub.[alpha]]    0.097
Tm[L.sub.[alpha]]    0.139
Yb[L.sub.[alpha]]    0.139
Lu[L.sub.[alpha]]    0.142
Pb[M.sub.[alpha]]    0.077 (in GdP[O.sub.4])

Element            Detection limit (avg. of 10)
                   (.99 CL)(mass fraction X
                   [10.sup.2] in CeP[O.sub.4])

Sc[K.sub.[alpha]]    0.018
Y[L.sub.[alpha]]     0.024
La[L.sub.[alpha]]    0.045
Ce[L.sub.[alpha]]    0.050 (in GdP[O.sub.4])
Pr[L.sub.[alpha]]    0.058
Nd[L.sub.[alpha]]    0.068
Sm[L.sub.[alpha]]    0.042
Eu[L.sub.[alpha]]    0.052
Gd[L.sub.[alpha]]    0.125 (b)
Tb[L.sub.[alpha]]    0.046
Dy[L.sub.[alpha]]    0.033
Ho[L.sub.[alpha]]    0.042
Er[L.sub.[alpha]]    0.042
Tm[L.sub.[alpha]]    0.033
Yb[L.sub.[alpha]]    0.038
Lu[L.sub.[alpha]]    0.043
Pb[M.sub.[alpha]]    0.045 (in GdP[O.sub.4])

(a)Single point analysis detection limits in a matrix of CeP[O.sub.4]
at a 99% confidence level (CL). A GdP[O.sub.4] matrix for Ce and Pb
was used since Ce is a major element in CeP[O.sub.4] and Pb was
determined to be inhomogeneous in the CeP[O.sub.4]. CL and averaged
detection limits for the same matrices at 99% confidence interval based
on the actual measured standard deviation of 10 measurements on
each standard are reported.

(b)Gd is possibly present as very small, widely dispersed concentrations
in the CeP[O.sub.4] which could explain this unusually high calculated
detection limit (for example the calculated averages detection limit for
Gd[L.sub.[alpha]] in DyP[O.sub.4] is 0.07 mass fraction X [10.sup.2]).

Table 5

Trace Pb and REE concentrations in the [REEPO.sub.4] standards (a)
(concentrations and uncertainties in mass fraction X [10.sup.2])

                     [ScPO.sub.4]      [YPO.sub.4]
USNM #                  168495            168499

[ScK.sub.[alpha]]                    .01 [+ or -] .01
[YL.sub.[alpha]]   .01 [+ or -] .02
[LaL.sub.[alpha]]  .01 [+ or -] .01  .02 [+ or -] .03
[CeL.sub.[alpha]]  .00 [+ or -] .01  .05 [+ or -] .06
[PrL.sub.[alpha]]  .03 [+ or -] .03  .01 [+ or -] .02
[NdL.sub.[alpha]]  .00 [+ or -] .00  .01 [+ or -] .02
[SmL.sub.[alpha]]  .02 [+ or -] .03  .01 [+ or -] .02
[EuL.sub.[alpha]]  .02 [+ or -] .03  .01 [+ or -] .02
[GdL.sub.[alpha]]  .02 [+ or -] .03  .02 [+ or -] .03
[TbL.sub.[alpha]]  .01 [+ or -] .01  .02 [+ or -] .03
[DyL.sub.[alpha]]  .03 [+ or -] .02  .02 [+ or -] .02
[HoL.sub.[alpha]]  .01 [+ or -] .02  .01 [+ or -] .02
[ErL.sub.[alpha]]  .03 [+ or -] .03  .02 [+ or -] .02
[TmL.sub.[alpha]]  .02 [+ or -] .02  .01 [+ or -] .02
[YbL.sub.[alpha]]  .00 [+ or -] .00  .03 [+ or -] .04
[LuL.sub.[alpha]]  .02 [+ or -] .03  .01 [+ or -] .03
[PbM.sub.[alpha]]  .00 [+ or -] .00  .01 [+ or -] .01

                     [LaPO.sub.4]           [CePO.sub.4]
USNM #                  168490                 168484

[ScK.sub.[alpha]]   .01 [+ or -] .02       .01 [+ or -] .01
[YL.sub.[alpha]]    .01 [+ or -] .01       .01 [+ or -] .01
[LaL.sub.[alpha]]                          .00 [+ or -] .00
[CeL.sub.[alpha]]   .01 [+ or -] .01
[PrL.sub.[alpha]]   .07 [+ or -] .13 (b)   .02 [+ or -] .03
[NdL.sub.[alpha]]   .00 [+ or -] .01       .01 [+ or -] .02
[SmL.sub.[alpha]]   .01 [+ or -] .02       .02 [+ or -] .04
[EuL.sub.[alpha]]   .03 [+ or -] .03       .00 [+ or -] .01
[GdL.sub.[alpha]]   .03 [+ or -] .06       .04 [+ or -] .05
[TbL.sub.[alpha]]   .02 [+ or -] .03       .00 [+ or -] .01
[DyL.sub.[alpha]]   .03 [+ or -] .04       .02 [+ or -] .03
[HoL.sub.[alpha]]   .02 [+ or -] .03       .02 [+ or -] .03
[ErL.sub.[alpha]]   .02 [+ or -] .03       .03 [+ or -] .04
[TmL.sub.[alpha]]   .02 [+ or -] .02       .01 [+ or -] .02
[YbL.sub.[alpha]]   .02 [+ or -] .03       .04 [+ or -] .05
[LuL.sub.[alpha]]   .01 [+ or -] .02       .03 [+ or -] .04
[PbM.sub.[alpha]]  1.05 [+ or -] .17      1.68 [+ or -] .07

                     [PrPO.sub.4]          [NdPO.sub.4]
USNM #                  168493                168492

[ScK.sub.[alpha]]  .01 [+ or -] .01      .01 [+ or -] .01
[YL.sub.[alpha]]   .00 [+ or -] .01      .02 [+ or -] .03
[LaL.sub.[alpha]]  .03 [+ or -] .05      .02 [+ or -] .04
[CeL.sub.[alpha]]  .03 [+ or -] .04      .03 [+ or -] .04
[PrL.sub.[alpha]]                        .00 [+ or -] .01
[NdL.sub.[alpha]]  .01 [+ or -] .03
[SmL.sub.[alpha]]  .01 [+ or -] .02      .00 [+ or -] .00
[EuL.sub.[alpha]]  .08 [+ or -] .11 (b)  .03 [+ or -] .05
[GdL.sub.[alpha]]  .01 [+ or -] .03      .02 [+ or -] .03
[TbL.sub.[alpha]]  .03 [+ or -] .04      .00 [+ or -] .00
[DyL.sub.[alpha]]  .00 [+ or -] .00      .00 [+ or -] .00
[HoL.sub.[alpha]]  .00 [+ or -] .00      .00 [+ or -] .01
[ErL.sub.[alpha]]  .00 [+ or -] .00      .02 [+ or -] .03
[TmL.sub.[alpha]]  .02 [+ or -] .03      .02 [+ or -] .02
[YbL.sub.[alpha]]  .04 [+ or -] .04      .03 [+ or -] .03
[LuL.sub.[alpha]]  .02 [+ or -] .03      .04 [+ or -] .03
[PbM.sub.[alpha]]  .77 [+ or -] .04      .60 [+ or -] .03

                     [SmPO.sub.4]      [EuPO.sub.4]      [GdPO.sub.4]
USNM #                  168494            168487            168488

[ScK.sub.[alpha]]  .01 [+ or -] .01  .00 [+ or -] .00  .01 [+ or -] .01
[YL.sub.[alpha]]   .01 [+ or -] .02  .00 [+ or -] .00  .01 [+ or -] .02
[LaL.sub.[alpha]]  .03 [+ or -] .03  .01 [+ or -] .01  .02 [+ or -] .04
[CeL.sub.[alpha]]  .03 [+ or -] .04  .02 [+ or -] .02  .03 [+ or -] .04
[PrL.sub.[alpha]]  .01 [+ or -] .01  .02 [+ or -] .03  .01 [+ or -] .02
[NdL.sub.[alpha]]  .00 [+ or -] .01  .04 [+ or -] .04  .01 [+ or -] .02
[SmL.sub.[alpha]]                    .01 [+ or -] .02  .02 [+ or -] .03
[EuL.sub.[alpha]]  .03 [+ or -] .02                    .09 [+ or -] .06
[GdL.sub.[alpha]]  .00 [+ or -] .00  .02 [+ or -] .05
[TbL.sub.[alpha]]  .03 [+ or -] .05  .00 [+ or -] .01  .01 [+ or -] .02
[DyL.sub.[alpha]]  .00 [+ or -] .00  .02 [+ or -] .03  .00 [+ or -] .00
[HoL.sub.[alpha]]  .00 [+ or -] .00  .00 [+ or -] .00  .14 [+ or -] .21b
[ErL.sub.[alpha]]  .07 [+ or -] .04  .00 [+ or -] .00  .00 [+ or -] .00
[TmL.sub.[alpha]]  .06 [+ or -] .07  .06 [+ or -] .06  .01 [+ or -] .02
[YbL.sub.[alpha]]  .02 [+ or -] .03  .02 [+ or -] .03  .01 [+ or -] .01
[LuL.sub.[alpha]]  .00 [+ or -] .00  .00 [+ or -] .01  .09 [+ or -] .07
[PbM.sub.[alpha]]  .99 [+ or -] .07  .52 [+ or -] .06  .49 [+ or -] .07

                     [TbPO.sub.4]      [DyPO.sub.4]      [HoPO.sub.4]
USNM #                  168496            168485            168489

[ScK.sub.[alpha]]  .00 [+ or -] .00  .01 [+ or -] .01  .01 [+ or -] .02
[YL.sub.[alpha]]   .01 [+ or -] .02  .07 [+ or -] .05  .02 [+ or -] .03
[LaL.sub.[alpha]]  .01 [+ or -] .02  .02 [+ or -] .04  .03 [+ or -] .04
[CeL.sub.[alpha]]  .01 [+ or -] .02  .02 [+ or -] .02  .01 [+ or -] .03
[PrL.sub.[alpha]]  .01 [+ or -] .03  .01 [+ or -] .02  .02 [+ or -] .03
[NdL.sub.[alpha]]  .02 [+ or -] .03  .01 [+ or -] .02  .01 [+ or -] .03
[SmL.sub.[alpha]]  .00 [+ or -] .01  .03 [+ or -] .04  .00 [+ or -] .01
[EuL.sub.[alpha]]  .03 [+ or -] .03  .00 [+ or -] .01  .00 [+ or -] .01
[GdL.sub.[alpha]]  .01 [+ or -] .02  .01 [+ or -] .02  .00 [+ or -] .01
[TbL.sub.[alpha]]                    .02 [+ or -] .03  .01 [+ or -] .01
[DyL.sub.[alpha]]  .00 [+ or -] .00                    .00 [+ or -] .00
[HoL.sub.[alpha]]  .01 [+ or -] .02  .11 [+ or -] .06
[ErL.sub.[alpha]]  .05 [+ or -] .09  .01 [+ or -] .02  .00 [+ or -] .00
[TmL.sub.[alpha]]  .00 [+ or -] .00  .03 [+ or -] .05  .03 [+ or -] .03
[YbL.sub.[alpha]]  .02 [+ or -] .03  .00 [+ or -] .01  .00 [+ or -] .00
[LuL.sub.[alpha]]  .02 [+ or -] .03  .02 [+ or -] .05  .05 [+ or -] .07
[PbM.sub.[alpha]]  .02 [+ or -] .02  .02 [+ or -] .03  .02 [+ or -] .03

                     [ErPO.sub.4]      [TmPO.sub.4]      [YbPO.sub.4]
USNM #                  168486            168497            168498

[ScK.sub.[alpha]]  .01 [+ or -] .02  .00 [+ or -] .00  .00 [+ or -] .01
[YL.sub.[alpha]]   .02 [+ or -] .03  .01 [+ or -] .01  .04 [+ or -] .03
[LaL.sub.[alpha]]  .01 [+ or -] .02  .01 [+ or -] .01  .03 [+ or -] .05
[CeL.sub.[alpha]]  .03 [+ or -] .04  .01 [+ or -] .01  .01 [+ or -] .02
[PrL.sub.[alpha]]  .02 [+ or -] .04  .01 [+ or -] .03  .02 [+ or -] .05
[NdL.sub.[alpha]]  .02 [+ or -] .04  .03 [+ or -] .04  .00 [+ or -] .00
[SmL.sub.[alpha]]  .02 [+ or -] .02  .03 [+ or -] .03  .03 [+ or -] .03
[EuL.sub.[alpha]]  .01 [+ or -] .03  .01 [+ or -] .01  .01 [+ or -] .02
[GdL.sub.[alpha]]  .00 [+ or -] .00  .02 [+ or -] .02  .02 [+ or -] .03
[TbL.sub.[alpha]]  .00 [+ or -] .00  .02 [+ or -] .03  .02 [+ or -] .03
[DyL.sub.[alpha]]  .02 [+ or -] .04  .00 [+ or -] .00  .06 [+ or -] .05
[HoL.sub.[alpha]]  .01 [+ or -] .01  .01 [+ or -] .03  .02 [+ or -] .04
[ErL.sub.[alpha]]                    .11 [+ or -] .07  .02 [+ or -] .02
[TmL.sub.[alpha]]  .00 [+ or -] .00                    .00 [+ or -] .01
[YbL.sub.[alpha]]  .03 [+ or -] .04  .00 [+ or -] .00
[LuL.sub.[alpha]]  .00 [+ or -] .00  .01 [+ or -] .04  .00 [+ or -] .00
[PbM.sub.[alpha]]  .02 [+ or -] .02  .01 [+ or -] .02  .02 [+ or -] .03

                     [LuPO.sub.4]
USNM #                  168491

[ScK.sub.[alpha]]  .01 [+ or -] .02
[YL.sub.[alpha]]   .03 [+ or -] .03
[LaL.sub.[alpha]]  .02 [+ or -] .03
[CeL.sub.[alpha]]  .01 [+ or -] .02
[PrL.sub.[alpha]]  .01 [+ or -] .02
[NdL.sub.[alpha]]  .03 [+ or -] .04
[SmL.sub.[alpha]]  .02 [+ or -] .03
[EuL.sub.[alpha]]  .01 [+ or -] .01
[GdL.sub.[alpha]]  .01 [+ or -] .01
[TbL.sub.[alpha]]  .01 [+ or -] .03
[DyL.sub.[alpha]]  .01 [+ or -] .03
[HoL.sub.[alpha]]  .03 [+ or -] .03
[ErL.sub.[alpha]]  .00 [+ or -] .01
[TmL.sub.[alpha]]  .04 [+ or -] .04
[YbL.sub.[alpha]]  .00 [+ or -] .00
[LuL.sub.[alpha]]
[PbM.sub.[alpha]]  .04 [+ or -] .04

(a)Average trace analyses of REE elements plus Sc, Y, and Pb for the
USNM REE phosphates in the "Round Robin" mount measured at Berkeley. The
quoted uncertainty is the measured one standard deviation value for 10
measurements.

(b)Large magnitude interference corrections resulting in increasing
uncertainty at trace levels. The apparent for these three cases could be
greatly reduced by using longer acquisition times on the unknown and the
standard used for the interference correction.

Table 6

Pb (mass fraction x [10.sup.2]) in the "round robin" mount measured in
College Park (a)

                       [ScPO.sub.4]    [YPO.sub.4] (b)

[PbM.sub.[alpha]]    .00 [+ or -] .00  .00 [+ or -] .00

                     [LaPO.sub.4]      [CePO.sub.4]       [PrPO.sub.4]

[PbM.sub.[alpha]]  .90 [+ or -] .32  1.90 [+ or -] 0.7  .92 [+ or -].04

                     [NdPO.sub.4]      [SmPO.sub.4]      [EuPO.sub.4]

[PbM.sub.[alpha]]  .86 [+ or -] .17  .86 [+ or -] .13  .64 [+ or -].16

                     [GdPO.sub.4]      [TbPO.sub.4]      [DyPO.sub.4]

[PbM.sub.[alpha]]  .39 [+ or -] .16  .00 [+ or -] .00  .00 [+ or -].00

                     [HoPO.sub.4]      [ErPO.sub.4]      [TmPO.sub.4]

[PbM.sub.[alpha]]  .00 [+ or -] .00  .00 [+ or -] .00  .00 [+ or -].00

                     [YbPO.sub.4]      [LuPO.sub.4]

[PbM.sub.[alpha]]  .00 [+ or -] .00  .00 [+ or -].00

(a)Averaged mass fraction x [10.sup.2] results of Pb contamination
measurements performed in College Park on the "round robin" mount.
The mass fraction detection limit (99 % confidence level) was approxi-
mately 140 x [10.sup.-6]. Note that the measured Pb standard
deviations for the uncontaminated materials are significantly smaller
than the measurements performed at Berkeley. These results are due
to the increased beam current and counting time used at College Park.

(b)[PbM.sub.[beta]] was used to avoid the [Y.sub.1[gamma]3] line.

Table 7

Pb grain to grain variation within the [CePO.sub.4] material in the
"Berkeley" mount (a)

          Average (concentrations
            in mass fraction x
                [10.sup.2])        Standard deviation  Minimum  Maximum

Grain #1           2.68                   0.45          2.04     3.47
Grain #2           2.55                   0.16          2.33     2.83
Grain #3           1.54                   0.04          1.48     1.59
Grain #4           3.64                   0.46          3.08     4.50

(a)Average and standard deviations (13-16 points over the face of each
grain) of four grains from the "Berkeley" mount mapped in Fig. 1 in
elemental mass fraction x [10.sup.2]. Analytical conditions were 20 keV,
150 nA, and a 10 [micro]m diameter beam. Each analysis is the average of
13 to 16 measurements distributed over the face of each grain. Only #3
was relatively homogeneous in Pb.

Acknowledgments

Thanks to Tim Teague at the UC Berkeley Petrographic Laboratory for meticulous sample preparation and to all other researchers who pointed out the presence of Pb in these materials.

Accepted: August 22, 2002

(1.) NIST disclaimer: Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

5. References

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(2.) W. O. Milligan, D. F. Mullica, G. W. Beall, and L. A. Boatner, Structural Investigations of Er[PO.sub.4], Tm[PO.sub.4], and Yb[PO.sub.4]. Acta Crystallog. C39, 23-24 (1983).

(3.) W. O. Milligan, D. F. Mullica, G. W. Beall, and L. A. Boatner, Structural Investigations of Three Lanthanide Orthophosphates. Inorg. Chim. Acta 70, 133-136 (1983).

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About the authors: John J. Donovan is a research assistant with the University of Oregon, John M. Hanchar is a assistant professor of geochemistry at The George Washington University, Phillip M. Picolli is a research scientist at the University of Maryland, Department of Geology, Marc D. Schrier is as staff scientist at Quantum Dot Corporation, Lynn A. Boatner is a corporate fellow in the Solid State Division at Oak Ridge National Laboratory and Eugene Jarosewich is a research chemist (emeritus) at the Department of Mineral Sciences, the Museum of Natural History, Smithsonian Institution.