American Mineralogist, Volume 72, pages 80(-aI I, 1987

Cathodoluminescence and microprobe study of rare-earth elements in apatite

Pnrnn L. Rononn, DuNc.q.N MacAnrnun, Xrn-Pnr M4 Gnn-clo R. Pnr-vrcnQueen's University, Kingston, Ontario K7L 3N6, Canada

ANrnoNy N. MenuNo48 Page Brook Road, Carlisle, Massachusetts 01741, U.S.A.

Ansrnlcr

The cathodoluminescence (CL) spectrum was scanned from 400 to 700 nm for each offourteen apatite crystals from carbonatites, pegmatites, nelsonites, and granites. Peaks dueto activation by Srrrt*, Dy3+, Tb3*, Eu3+, Eu2+, and Mn2+ were identified in the CL spectra.Proton (rxn) and electron-microprobe analyses were used to determine the major (Ca, P,F) and minor elements (As, Si, Th, Mn, Fe, Sr, Na, Cl, REEs) in the apatite crystals. Themicroprobe analysis of two of the apatite samples (Durango and Huddersfield) comparefavorably with analyses reported in the literature for apatite samples from the same lo-calities. The rare-earth-element (REE) content varies from 0.2 to more than 15 wto/o andis generally proportional to Na * Si, suggesting charge compensation for substitution ofREE3* for Ca2+. The chondrite-normalized distribution of the REEs shows light REEenrichment for the majority of samples with the CelY ratio varying from 30 to 0. l. Therelative height of the Sm3+ and Dy3* peaks in the CL spectrum was found to be a goodguide to the relative light to heavy REE enrichment. Apatites from more alkaline envi-ronments tended to be richest in the light REEs. The CL spectrum for an apatite from theLlallagua tin deposit in Bolivia shows a broad Eu2* peak at a short wavelength and narrowEu3+ peaks superimposed on a broad Mn2* peak at a longer wavelength. The peak assign-ment was confirmed using an ultraviolet source and by comparison with spectra fromsynthetic apatite. The chondrite-normalized REE distribution for this sample shows a largepositive Eu anomaly, which is similar to the positive anomaly shown for an apatite fromthe Panasqueira tin deposit in Portugal.

A series ofglasses doped with Eu and heated at 1400"C at a series ofoxygen fugacitieswas used to show the effect ofoxygen fugacity on the relative height ofthe Eu2* and Eu3+peaks in the CL spectrum. It is concluded from these spectra that the positive Eu anomalyin the apatite from Llallagua and Panasqueira was not due to the preferential incorporationof Eu2* by the apatite structure, but rather to the crystallization of the apatite from areservoir strongly enriched in Eu.

INrnoouc:rIoN

Apatite is one of the most common accessory mineralsfound in crustal rocks, and its presence and distributionare often used to model geologic processes such as mantlemelting, differentiation in magma chambers, and hydro-thermal processes (e.g., Nash, 1972; Henderson, 1980;Kovalenko et al., 1982). The structure of apatite is suchthat it concentrates many elements that are found onlyin trace amounts in the Earth's crust, in particular therare-earth elements (REEs). Apatite and the REEs areoften concentrated in carbonatites and associated alkalinerocks that are important as ore deposits of phosphates,REEs, Nb, Y, IJ, etc. One very important tool in theexploration of carbonatites is cathodoluminescence(Mariano, 1978), which can be used to estimate the dis-tribution and kind of REE present in many minerals. Thepresent study involved the chemical analysis of the apa-tite for REEs using the electron and proton microprobes

and the correlation of the REE distribution to the cath-odoluminescent spectra of the apatite.

Naturally occurring apatites can be represented by theformula Ca,o(POo)uFr, with many substitutions possiblefor Ca, P, and F. Minor replacement of Ca in naturalapatites is mainly by Na, Sr, Mn, and the REE. The mostcommon substitution for P is by Si, coupled with a sub-stitution of Ca by trivalent REE ions. Smaller amountsof As, S, V, and also carbonate ion are possible. F isreplaced mainly by Cl and (OH)-. The fact that virtuallyall apatite shows cathodoluminescence indicates that verylimited amounts of Fe2* substitutes for Ca in the struc-ture, because Fe2+ is an effective quencher for cathodo-luminescence (CL) in most minerals.

The REEs have become very important in geochemicalmodeling. They act as a cohesive group of elements withvery similar chemical characteristics (Henderson, 1984),but there are subtle chemical differences between the light

0003-004x/87/0708-080 r $02.00 801

802 ROEDER ET AL.:REES IN APATITE

c

o

ooJ 2

X-RaY 5ng1gy11sY1

Fig. l. Proton-induced X-ray spectmm of the Durango apa-tite.

rare-earth elements (LREEs) and the heavy rare-earthelements (HREEs). The LREE ions in oxygen-dominatedstmctures tend to be larger than the HREE ions, and theelectronic configuration sometimes favors the 4+ state ofCe and the 2+ state for Eu. Apatite has often been usedas an example of a mineral that tends to concentrate theLREEs or the middle rare-earth elements relative to theHREEs, but the actual REE distribution in a particularapatite depends not only on the apatite structure, but uponthe REE distribution and chemical characteristics of themelt, rock, or fluid reservoir from which the apatite crys-tallized (Papike et al., 1984). Thus Watson and Green(198 l) have shown that apatite crystallizing from a gra-nitic melt has a fourfold increase in its ability to concen-trate REEs compared to apatite crystallizing from a ba-saltic melt. The oxidation state of the reservoir is alsovery important since elements such as Ce (Ce3*, Ce4")and Eu (Eu2+, Eu3+) are sensitive to the oxygen fugacityof the reservoir. The distribution of the REEs in apatitecrystallizing from residual melts or hydrothermal solu-tions may be sensitive to the stability of REE molecularcomplexes in the solutions or melts (Flynn and Burnham,1978; Taylor and Fryer, 1982). Subtle changes in acidityand alkalinity of the solution may be reflected in the REEdistribution of the crystallizing apatite, and apatite form-ing at different stages may have a diferent REE concen-tration and distribution (Shmakin and Shiryayeva, 1968).The complicated zoning in some apatite crystals (Knut-

son et al., 1985) reflects changing chemical conditionsduring growth. The zoning of REEs may be preservedbecause of the very low diffusion rate of REEs in theapatite structure (Watson et al., 1985). Thus the ubiqui-tous presence of small amounts of apatite may be usedto record chemical and physical changes in the environ-ment of crystallization and to correlate stages of growth.In order to use the REE distribution in apatite to its fullpotential, it is best to analyze this distribution using anondestructive technique with a high spatial resolution,a microprobe (ion, electron, or proton) or an optical tech-nique such as ultraviolet- or electron-excited lumines-cence.

The electron microprobe is the most convenient typeof microprobe to arllalyze the major elements in apatite.It combines a high spatial analyzing resoltuion of betterthan 5 pm, a very stable electron flux, and a very goodpeak resolution using a wavelength-dispersive spectrom-eter. The high bremsstrahlung background produced bythe electron flux makes it difficult to analyze REEs atlevels much lower than a few hundred ppm (Roeder,1985). The proton microprobe (nrxe) can, with suitableapertures and focusing magnets, produce a high-energysource of protons with a useful resolution of a few mi-crometers and a spectrum with a much lower backgroundlevel than that from the electron microprobe. Thus theREEs can, in principle, be measured at levels of a fewppm; and, in practice, levels of a few tens of ppm areachievable. A proton-induced X-ray emission (rrxn)spectrum of a Durango apatite using an energy-dispersivespectrometer is shown in Figure 1. The proton- and elec-tron-microprobe analysis for this sample (Probe) is com-pared in Table 2 to two published analyses.

Virtually all naturally occurring apatites show lumi-nescence in the visible spectrum when exposed to ultra-violet, laser, or electron excitation. The luminescence isdue to impurity elements that function as activators orcoactivators in the apatite host. A pure apatite crystalwithout activator impurities or vacancies exhibits no lu-minescence. The principal activators in natural apatitesare Mn2*, Sm3+, Dy3*, 15:*, Eu2* and Eu3+ (Marianoand Ring, 1975). The luminescence response of these ac-tivators is so efficient that crystals as small as 20 pm arevividly revealed, even for activator concentrations wellbelow 100 ppm. This characteristic of apatite is an assetto the petrographer because even though apatile is one ofthe most ubiquitous accessory minerals in rocks, it fre-quently occurs as small crystals whose recognition is oftenobscured due to association with other minerals.

Synthetic apatite is the main host material for phos-phors used in fluorescent lights, and rare-earth-activatedfluorapatite has been synthesized for solid-state lasercrystals. Emission spectra for most of the activators inapatite are well documented in the literature (e.g., Mari-ano, 1978), and interpretation of CL emission spectra inapatite has been made by correlation with work on syn-thetic apatites. The principal activator in natural apatitesis Mn2*, which produces a broad band in the yellow-

ROEDER ET AL.: REES IN APATITE 803

green part of the visible spectrum. The wavelength ofemission is not only determined by an electron transitionof the free ion but by the crystal-field influence of thehost, so that variations in the emission wavelength oftenoccur. The trivalent REE activators each show severalnarrow bands whose wavelength of emission is onlyslightly affected by the crystal field of the host. The wave-length of emission from Eu2*, however, is sensitive tostructural changes within a host crystal because the tran-sitions for this ion involve poorly shielded valence elec-trons (Palilla and O'Reilly, 1968). The peak positions andrelative intensities of the emissions for various REEs arevery complex and will be discussed in greater detail in alater section. Eu and, to a much lesser extent, Sm arenaturally occurring REEs that commonly occur in boththe 3+ and 2+ state and show emission peaks for bothoxidation states. These peaks for Eu occur both in thered and blue part of the spectrum, and thus the use ofultraviolet and cathodoluminescence can be a guide tothe oxidation state of Eu. Emission spectra (CL and UV)for apatite from various geologic environments has beenstudied by many workers including Portnov and Goro-bets (1969), Ovcharenko and Urev (1971), Mariano andRing (1975), Mariano (1978), and Huaxin (1980). Theidentification of the activators and their relative abun-dance in apatites serve as petrogenetic indicators, andthere have been many attempts to use the cathodolumi-nescence for this purpose. The use of CL has been mainlyas a qualitative tool because apatites found in nature arechemically too complex to determine the relative effectsofactivator, coactivator, and quenching elements on theemission spectra. However, it is the fairly dramatic changein the visible spectra produced by subtle changes in thetrace elements that make cathodoluminescence useful todiscriminate different stages of crystal growth.

ExpnnrvrBNTAL TECHNTeuES

The electron-microprobe analyses were done at Queen's Uni-versity using an ARL-sEMe microprobe at l5 and 25 kV. The Caand P analyses were carried out using energy-dispersive analysiswith a Si detector (155 eV FWHM resolution at Mn1(a) andsynthetic Ca.PrO, as a standard. The other elements (F, Na, Si,Cl, REEs) were analyzed by wavelength-dispersive analysis usingmethods similar to those described by Roeder (1985).

The prxe spectrum presented in Figure 1 was obtained withan external beam of 3.0-MeV protons from the Queen's Uni-versity Van de Graaff accelerator. The specimens, mounted inepoxy and polished, were all larger than 100 pm in size so thata final aperture of 25 pm was used. With a beam energy of 3MeV and a current of a few nanoamperes, spectra with sufrcientstatistical accuracy for analysis were obtained in - 1000 s. Pile-up of Ca and P signals, which would interfere with the REEspectra between 6 and 8 keV was reduced to an insignificantamount (0.2olo)with a 86 mg/cm, (650 pm) mylarabsorber, whichalso reduced the transmission of the La I X-rays to 50/0. Thiswas not a serious problem since the LREEs are normally thedominant ones.

The proton probe used an energy-dispersive system with aresolution of 160 eV FWHM (Mn Ka), and the quantity of eachREE (plus Fe and Mn) was determined with a specrrum-strip-

Tnele 1. REEs (wt%) in glass 5-253

Neutron activation analyses

CePrNdSmEuGdTbDyHoElTmYbLu

0.0730.o720.0760.0760 0680.0700.0710.0770.0950.0750.0800.0760.0760.070

0.08 0.060.08 0.060.08 0.080.08 0.080.08 0.060.08 0 080.08 0.080.08 0.070.08 0.060.08 0 070.08 0 060.08 0.050.08 0.060.08 0 06

0.075 0.090 0.1000.070 0.079 0.100

0.067 0.1000.07s 0.087 0.0970 074 0.077 0.088

0.0950.078 0.080 0.088

0.0800.097

0.0770.074 0.080 0.0880 075 0.080 0.086

Note.'Columns are as follows: Nom : nominal wto/" from initial weighing(Roeder, 1 985); eve : electron-microprobe analysis; PtxE : external beamproton microprobe, Queen's University; 1 : analysis by N Eby, Universityof Lowell, Lowell, Massachusetts, U S.A. (interlaboratory standard was"BASALT"); 2 : analysis of GINA (Universit6 de Montr6al, Montreal,Quebec); 3 : analysis by Neutron Activation Services, McMaster NuclearReactor. Hamilton. Ontario

ping procedure (Ma et al., 1987) utilizing an accurately deter-mined response function for a single element in a matrix similarto that ofthe actual specimens. These functions were obtainedas mathematical expressions from experimentally measuredspectra of a single REE in a phosphate crystal. The expressionsconsisted of a number of Gaussian peaks positioned at the ex-pected energies ofthe strong Z X-rays, with the ratio ofthe peaksadjusted slightly from the values given by Mitchell and Ziegler(1977) to match the observed spectra. Eleven I X-ray lines wereneeded to accurately reproduce a spectrum.

The prxe spectra were calibrated with a glass standard (5-254)containing l0lo ofeach REE as described by Roeder (1985). Thespectra from this standard were analyzed in exactly the sameway as those from the specimens. Thus the ratio of the countsfor an element in a sample to the counts in the standard gavethe concentration ofthe element directly. For those analyses onsmall samples where some doubt existed as to whether the totalproton beam hit the target, the Ca concentration determinedwith the proton microprobe was made equal to that determinedwith the electron microprobe, thus efectively normalizing theconcentrations for the REEs.

The reproductibility of the analyses of the Durango apatitewas determined to be better than 100/o for reasonably strong lines,e.9., La, Ce, Nd, and well-resolved lines such as As and Sr, de-creasing to 25o/o for elements such as Pr, Gd, and Dy. The resultsofthe analyses on the various apatites are given in Table 2. Sinceseveral samples of the same apatite were usually available, theanalyses are usually the average of two or three measurementson the different samples.

A glass (5-253, available from Roeder) containing a nominal0.08 wt0/0 of each REE (Roeder, 1985) was analyzed for REEsby electron (rrvrr) and proton (erxr) microprobes. These analysesare shown in Table I together with three neutron-activationanalyses of this glass. Although there is considerable variation ofthe REEs, the average rxr analysis is close to the nominal 0.08wto/0.

A glass having the approximate composition CaO : 9, MgO :

22, A12O3: 16, SiO' : 52o/owas doped with 10/o europium oxidein order to determine the effect of oxygen fugacity on the CLspectrum ofEu. Each glass sample was heated at l400Cfor 24

804

TneLe 2. Analyses (wt%) of apatite

ROEDER ET AL.: REES IN APATITE

1-Durango 2-Huddersfield J_

Australia 4-Blue River 5-Snarum

Micro-prooe

Younget al., Rogers et1969 a l . , 1984

Micro-prooe

Trzcienskiet a l . , 1974

Micro-probe

Micro-probe

Micro-prooe

Crufte t a l ,1 965

P.o.As.Ousio,Tho,CaOMnOFeOSrONaroFcl

40.90 40.780.133 0.090.040.024

54.0s 54.020.011 0 .010.028 0.050.0s9 0.070 10 0.23e R 2 e E z

0 36 0.41

0.072 0.0970 414 0.4930 505 0.5510 069 0.0940.151 0 .2330 019 0.0350.006 0.0020.04s 0.0230.027 0.0120.021 0.017

<0 005 0.003<0 005 0.011<0.004 0.001

0.006 0.0060 004 0.001

99.01

10.0665 9592 0 1 90 0400.085

38.320.0081.070.083

53.440.0150.0340.2070.04o . Y /

n.d.

0 . 1 1 40.3280.8110.0940 4050.040o 0120 0780.0330.029

<0.005<0.005<0 004

0.0080.003

97.47

38.37

1.07

38.26

1.76

53.91

o . 1 20.280283.92

0.15o.441.23

0.001

35.25<0 001

0.050.004

50.420.0030.0310.0511.043.440.01

0.2750.0010.0210.0150 0070.0140.0010.0420.0040.0260.0080.0290.0060.0310 001

89.33

10.8005.7332.089U.JVO

0.041

53.980.010.040.27

3.300.20

41.77 41 .98<0.001 0.001

0.060 0.060.001 0.004

54.14 51.560.035 0.0100.112 0.0840.429 0.028012 0 .30253 0.710 01 4.79

Y.o.LarO"Ce.O"PrrO.Ndro3SmrO.Eur03Gd,o3Tbro3Dyro.HorO"ErrO"TmrO.Ybro3LurO.

Total'

Ca siteP siteF + C lN a + S iY + REES

0 .4160 .5170.0s00.1 50

<0.0060.0120.0300.0440.0260.005

<0.0020.0050.011

0 . 1 80.350.96

0.3950.0980.2980.0620.2550 0790.0100.0940.0290.0750.0340.0550.0160.0540.010

99.71

9.7536.1 411.7870 . 1 1 00.106

0.0140.094 0.1070.193 0.2320.0180.079 0.1120.021 0.0200.003 0.0100.0140.001 0.0020.007

<0.009<0.006<0.006<0.005 0.001<0.00498.58

1 0 . 1 0 16.0881.3780.0500.028

0.0940.281

0.2440.0870.011

0.020

l o n s p e r 2 6 O + F + C l10 223s.8832.2030.2010.127

Nofe: Sample numbers correspond to those given in the text. Microprobe analyses were conducted using the electron and proton microprobes. Theneutron-activation analyses (NAA) were conducted by Nelson Eby, University ot Lowell, Lowell, Massachusetts, U S A

" Total is corrected for oxygen equivalent of fluorine.

h in air or at an oxgen fugacity controlled by mixing CO, andH, (Jamieson and Roeder, 1984).

The cathodoluminescence (CL) spectra were collected usingelecron excitation from a Nuclide Luminoscope (n) u-u-za,which consists ofa cold cathode discharge that provided energiesvariable between 5 and 15 keV and beam currents up to 800nA. The CL spectra of the luminescing minerals were recordedwith a Gamma Scientific model 3000 AR scanning spectrora-diometer and a more recent model NM-3 monochromator sys-tem, which also includes an SC-l scanning control; a modelD-43 extended-spectral-range (250-930 nm) sideJooking pho-tomultiplier detector; and a model DR-1 digital radiometer. Theexperimental conditions are similar to those described by Mar-iano and Ring (1975). For the generation ofthe CL spectra, anelectron-beam diameter of approximately 2 mm was employed.

Rnsur,rsThe fourteen apatites that have been studied are de-

scribed in Appendix l. The electron microprobe (rvr)and proton microprobe (erxE) analyses of the apatites areshown in Table 2 in the columns labeled "Probe." Theelements P, Si, Ca, Na, F, and Cl were analyzed by nvre,

and the other elements were analyzed by rIxe. The oneexception is sample 6, which contains a major amount ofREEs, and it was thought that the EMp results for La, Ce,Pr, Sm, and Gd were more accurate. The Durango andHuddersfield apatites are well known and have been stud-ied in considerable detail. The microprobe analyses ofthese two samples are compared in Table 2 with analysesby other workers on different samples from the same lo-calities. The microprobe analysis of the Durango samplecompares favorably w'ith the wet-chemical and spectro-graphic analysis of Young et al. (1969) and with the pIxE

analysis of Rogers et al. (1984). The Huddersfield apatitewas very carefully analyzed by cruft et al. (1965), usingwet-chemical analysis and was analyzed again byTrzcienski et al. (1974). The microprobe analysis com-pares favorably with both ofthese earlier studies, and anydifferences may be explained by natural heterogeneity.The REEs for a few apatite samples were also analyzedby N. Eby using neutron activation (uea) and are shownin a separate column. The results of the microprobe andneutron-activation analltical techniques agreed quite well,

ROEDER ET AL.: REES IN APATITE 805

TneLe 2. Continued

6-Paja-rito 7-Timmins 8-Kerimasi

Micro-proDe

9-Llallaqua1o-Mt

PaSs 1 1-Tapira 1 2-Khibina 1 3-Mineville 14-Oka

Micro-NAA prooe

Micro-prooe

Micro-prooe NAA

Micro- Micro-probe probe

Micro-probe

Micro-probe

Micro-prooe

36.550.0031 .710.01038 350 0650 1690.200263J O J

0.01

0.3064.488.500.933.690.50

<0.0590 3 30 477

<0.033<0.082<0.024<0.005<0 .017< 0 0 1 51 0 1 . 1 1

9.8696 .0172.130| . z c c

1.299

38.620.0130.600.1 15

51 .130.0370.0773.0190.26J-52

0 . 1 6

0 0900.4420.9100 1 1 1o.4290.0800.0200.0800.052

<0.013<0.010<0.009

0.0080.0060 005

98.25

't0.2675.8972.0190.1950.144

40.84 40.090.003 <0.0010.56 0.600.007 0.005

54 43 54.110 044 0.0180.549 0.1580.17s 0.939

<0.01 0.013 99 2380.02 0.04

0.337 0 0210.096 0.1360.130 0.2750.007 0 0490.084 0.0680.036 0.0160.002 <0 0020 065 0.0240.020 0.0140.083 0.0200.015 <0.0060 046 <0.0040.028 <0.0040.044 0.007

<0 005 <0.00199.93 97.97

10.053 10.2905.954 6.0322.144 1.3260.094 0.1080 068 0.040

l o n s p e r 2 6 O + F + C l

39.04 34.89 37 19<0.001 0.330 0.003

0.24 3.30 1.790.009 0.258 0.030

51 .95 47.93 51 .760.013 0.0s1 0.0480.009 0.347 0.0913.058 0 057 0.9060.01 0.02 0.073.33 3.20 2.650.01 0 04 0.01

0.032 1.888 0.0750.273 1.614 1.208 07710.41 6 3.182 2.691 1 .5000.027 0.366 0.2460.143 1 .585 1 .186 0 .5030.028 0.304 0.240 0.0580.008 0.037 0.021 0.0190.033 0.361 0.0890.019 0.194 0.049 0.069

<0.006 0.189 0.003<0.006 0 062 <0 011<0.005 0 167 <0.009<0.005 0.051 <0.008<0.005 0.174 0j23 <0.007

0.001 0.059 <0.00697 25 99.30 96.76

10.272 10 089 10.3395.912 5.968 5.9861 .873 1 .841 1 .5100.046 0.603 0.3460.064 0720 0.219

42.35<0.001

0.14<0.00254 630 1390.0320.046

<0.013.74U.U5

0.0160.020 0.0060.027 0 0100.0070.0170.014 0.0030 1 4 6 0 1 1 4

<0.005<0.004 0.001<0.004

0.001<0 004<0.004

0.003 0.001<0 00599.79

9 8806 0482.0010.0230.015

40.35<0.001

u c c<0.00253.740.0070.0261.029

<0,011.770.02

0.0200.109 0 .1180 176 0 .1590.0450.053 0.0710 006 0.0120.003 0.0040.0180.009 0.0010.008

<0 003<0 003<0.001

0.009 0.0010.005

97.20

10.270o . t u /n oon0.0960.029

and usually when there was a significant difference be-tween techniques, the results for all the elements wouldbe lower or higher (sample l3).

The ionic proportions were calculated for each analy-sis; however, only a summary of the ionic proportions isshown at the bottom of each analysis in Table 2 (com-plete ionic proportions are available from P. L. Roeder).The ionic proportions generally f it the formulaCa,o(POo)uFr. It has been assumed that Ass+ and Si4+substitute for Ps*, whereas Cl- substitutes for F , andthe other cations substitute for Ca2*. It is believed thatmost of the major species in the apatite have been ana-lyzed, except for carbon dioxide and water. The sampleshowing the greatest deviation from the proportions l0:6:2 is the Australian sample (3), which gave a very pooranalytical total; this may be because the surface of thepolished section had many small holes, which may reflectthe original crystal growth in the lateritic weathering pro-file. Thus the texture of this sample may not have beenideal for normal microprobe techniques; however, the lowanalytical total may also be due to the presence of other

species such as carbon dioxide and water. It has beennoted by many workers (e.g., Watson and Green, l98l)that most of the REEs may substitute in apatite as tri-valent cations with the concomitant substitution of Sio*for P5+ to provide a charge balance. We have found thatthis is generally true; however, we found an even bettercorrelation of the Y + REE substitution with the substi-tution of Na'+ plus Sia*. This can be seen by comparingthe Na + Si and Y + REE columns at the bottom ofTable 2. Thus the charge-balanced substitution may beshown as follows: REE3+ + Na+ ) Ca2+ * Ca'z+ andREE3+ + Si4+ + Ca2+ * P5+. One of the factors that maycontrol whether Na or Si substitutes with the REEs is thedegree of silica saturation (or the alkalinity) of the res-ervoir from which the apatite crystallized.

The total REEs are quite variable between samples,and so also is the distribution of individual REEs. A con-venient method of discriminating between the degree ofLREE (Ce group) relative to HREE (Y group) enrichmentis to use the CelY ratio. The REE distribution relative tothe concentration of the REEs in chondritic meteorites is

806 ROEDER ET AL.: REES IN APATITE

o. I

o

_cO

LULUE

a b c

Ce/ \Ce / \

3 0 . 1

2 1 . 6

1 . 8

o .8o .4o.o8

t 4 . 1

r 14.3

o

14.8

1 0 3

plotted in Figure 2. Only those REEs with values over100 ppm and only the even atomic-numbered REE (in-cluding La) have been plotted, because ofthe general lowlevels of odd-numbered REEs and thus the increased an-alytical uncertainty. Figures 2a, 2b,2c are shown in theorder of degrees of LREE to HREE enrichment. The Llal-lagua sample (9) has been omitted from Figure 2 becauseof the low level of most of the REEs and the unique Euanomaly, which will be discussed in greater detail in alater section.

Many workers (e.g., Amli, 1975) have pointed out thegeneral enrichment of LREEs in apatite. However, someof the apatite samples shown in Figure 2c do not showLREE enrichment, and the partition coemcient data ofWatson and Green (1981) suggest that the apatite struc-ture may not concentrate LREES relative to the HREEs.The LREE enrichment often found in apatite is probablydue to the crystallization of apatite from a reservoir withLREE enrichment. Apatites that have grown in alkalineenvironments, such as carbonatites, have often been cited(e.9., Fleischer and Altschuler, 1982) as showing a strongLREE enrichment. This has been confirmed in the pres-ent study. The apatite samples (2, 4,6,8, 10, I l, 12, 14)that can be identified as from a very alkaline environ-

1 0 5

1 0 4

102Sm cd Dy lc" Nd sm Gd Dy Er Yb

La

Fig. 2. REEs in apatite normalized to the distribution found in a chondritic meteorite. The sample number is shown in boldtype to the left ofeach curve and the CelY ratio is shown to the right ofeach curve.

l c r N dLa

l c " Nd Sm Gd DyL A

ment have CelY ratios ranging from 30. I to 7 .7. ThePajarito apatite (6) has a very high total REE content andthe highest Ce/Y ratio and still maintains the apatitestructure, as determined by X-ray diffraction (R. C. Pe-terson, pers. comm., 1986). The compilations of Fleisch-er and Altschuler (1982, 1986) of more than 900 REEanalyses in apatite showed that the general provenanceof the apatite could be determined by the REE distribu-tion. The present data on the carbonatite apatite are con-sistent with the observations that apatite from carbona-tites are LREE enriched with a low Y/REE ratio and hightotal REE content.

The visible luminescence spectrum for each of the ana-lyzed apatites was measured using electron excitation (CL),and these spectra are shown in Figure 3. The major peakshave been labeled with the most probable element andoxidation state that produced the peak. The element as-signment is consistent with most of the earlier work (e.g.,Mariano and Ring, 1975; Langurwey, 1977; Marfunin,1979) and with the spectra collected in the present studyusing separate REE phosphates synthesized by L. Boat-ner. The most prominent peak in apatites showing LREEenrichment (high CelY ratio) is due to Stnr* 1\ : 595nm), whereas the major Dy3* peak (I : 568 nm) only

Sm3*

ROEDER ET AL.: REES IN APATITE 807

Fig. 3. Cathodoluminescence spectra of apatite with the wavelength plotted from 400 to 700 nm. The sample number is to theleft and the CelY ratio to the right of each curve. The most probable element and oxidation state for the peaks are shown.

becomes prominent in apatites with a low CelY ratio orslight LREE enrichment. Thus the relative height of theSm3+ (595 nm) and Dy3+ (568 nm) peaks can be used asa guide to the relative degree of LREE enrichment. Themajor Sm3+ (595 nm) and Dy3+ (568 nm) peaks some-times seem to coalesce (sample 7) because a very broadMn'?+ peak becomes prominent. The broad Mn2+ peak isshown in a Mn-doped synthetic apatite in Figure 4d, andthe broad Mn peak (560 nm) dominates the Llallaguaapatite in Figure 4a. Sample 3 from a lateritic weatheringprofile has the lowest Mn content of all the apatites andshows (Fig. 3) excellent resoltuion of the Sm3+ and Dy:+peaks due to the absence of the broad Mn2+ peak. TheMn2+ peak is often absent in samples from oxidizing en-vironments. In these environments, there may be abun-dant manganese oxides with Mn4+, but there is little Mnr*in the minerals. The spectrum of sample 3 (Fig. 3) alsoshows a well-resolved peak at 615 nm that is probablydue to Eu3+. The absence ofa Eu2* peak may reflect theoxidizing conditions during crystallization. The peak at480 nm has been assigned to Dy3*, but for some samplesthis may be questionable because the ratio of the heightof the Dy3+ peak at 568 nm to the other Dy peak is quitevariable. Although Tb is usually present in only traceamounts, the major Tb3+ peak (I : 540 nm) is oftenfound [Tapira (l l), Kerimasi (8), Snarum (5), Australian(3)1. Most of the major peaks (Fig. 3) are in the yellow-

orange (560-600 nm) part of the spectrum because thedominant activators are Sm3+, Dy3*, and Mn2*. The Du-rango (l) spectrum has a strong blue component becauseof a peak that is tentatively identified as Eu2*. Many ofthe spectra show a peak in the 400-450-nm range, whichhas been shown at the position of Eu2* but is likely dueto some other unknown activator (e.g., Ce3* or a crystal-defect mechanism).

The spectra for the Llallagua (9) apatite are shown inFigures 4a and 4b together with other samples showingprominent Mn2* and Eu peaks. The Llallagua CL spec-trum (Fig. 4a) is dominated by the broad Mn2+ peak onwhich are superimposed four Eu3* peaks (585, 613,645,690 nm). These Eu3+ peaks show up much better using aUV source (Fig. ab) because the UV is less efficient atgenerating the broad Mn2* peak. The peak at 450 nm isidentified as due to Eu2+. This peak is better seen in Fig-ure 4c, which is from a synthetic apatite doped with Eu,and thought to have been synthesized at the low oxygenfugacity produced by H, gas. Silicate glasses doped withl0lo Eu were prepared at diferent oxygen fugacities at1400"C in order to confirm the assignment of Eu'?+ andEu3* lines and to show the relative peak heights as afunction ofoxygen fugacity. Figure 5 shows the CL spec-tra of these glasses together with the logarithm of theoxygen fugacity. The glass prepared in air (log/", : -0.7)

was run at an oxygen fugacity higher than that expected

Eu2t

ro3'

sm3*

808 ROEDER ET AL.: REES IN APATITE

M n 2 ' Eu3t

Fig. 4. (a) Cathodoluminescence and (b) ultraviolet-inducedspectra for the Llallagua apatite (sample 9) plotted from 400 to700 nm. Spectrum (c) is of an apatite doped with Eu and syn-thesized at low oxygen fugacity. Spectrum (d) is of a syntheticapatite doped with Mn.

in nature, whereas the sample run at log fo, : -9.9 ismore reduced for the temperature of synthesis than mostconditions expected in nature and is close to the iron-wiistite bufer. The lo9for: -6.9 sample was run at anoxygen fugacity close to the extrapolated value for thequartz-fayalite-magnetite buffer. The four Eu3* peaks ofthe glasses are at the same wavelength and have the samerelative peak height as those in the Llallagua (9) apatite.The broad peak at short wavelengths in the glass samplesrun at the lower oxygen fugacities is due to Eu2* and isat a higher wavelength (I : 500 nm) than the Eu2t peak(I : 450 nm) in the Lalllagua and synthetic apatite. Otherworkers have shown that the Eu2+ peak position is sen-sitive to the structure of the host (Palilla and O'Reilly,1968). The relative size of the Eu3* and Eu2+ peaks inthe glasses and calculations using the equation of Drake(1975) suggest that most of the Eu in the Llallagua apatiteis in the 3+ state. The relative height of the Eu3* andEu2* peaks ofthe Llallagua apatite is closest to the ratioof the peaks for the glass run at log fo, : -6.9. No con-clusions can be made, however, as to the relative oxidiz-ing conditions under which the Llallagua apatite crystal-

400 500 600 700

Fig. 5. Cathodoluminescence spectra of glasses doped withEu and synthesized at a series of oxygen fugacities shown to theright of each spectrum.

lized, because we do not know what structural control, ifany, there is on the Eu3+/Eu2+ ratio in the apatite as com-pared to the glass. We are suspicious, however, of theassignment of Eu2+ to some of the 400-450 nm peaks

shown in Figure 3 because of the absence of Eu3* peaksand the very low oxygen fugacities that would apparentlybe necessary to produce an Eu2+ peak such as that in thespectrum ofthe Durango (l) apatite.

The chondrite-normalized plot of the rrxe-analyzedREEs for the Llallagua apatite is shown in Figure 6 bythe circles. The plxe analyses for all the REEs, except forEu, were very low and so a Llallagua sample was sent forneutron-activation analysis (Nea) in order to veriry thatthe very strong Eu anomaly was indeed significant. Thechondrite-normalized trends in Figure 6 for the exn (opencircles) and Naa (crosses) analyses are very similar inshape. The neutron-activation analysis of a sample ofapatite from the Panasqueira tin deposit in Spain wastaken from Knutson et al. (1985) and plotted (solid stars)in Figure 6 for comparison. The very strong positive Euenrichment for the two apatite samples from separate tindeposits is quite remarkable. Both the prxe and Na-e anal-

ROEDER ET AL.:REES IN APATITE 809

yses of the Llallagua apatite show a small positive Smanomaly that is consistent with Sm, like Eu, being ableto sometimes exist in the 2+ oxidation state.

The CL and UV spectra for the Llallagua apatite showmainly Eu3+, and there is no apparent structural reasonwhy apatite should concentrate Eu3* relative to the otherREEs. The UV-activated spectrum for the Panasqueiraapatite (not shown) shows predominantly Mn2+ activa-tion with weaker activation for both Eu2* and Eu3+. ThePanasqueria apatite shows quite variable luminescence,both as a mottled pattern and as distinct zonation as re-ported by Knutson et al. (1985). There are some studiesthat show a negative Eu anomaly in apatite (Puchelt andEmmermann,1976; Brunfelt and Roelandts, 1974), andEby (1975) has suggested that Eu'?+ may be excluded fromapatite because of its size. Thus it seems that the Llalla-gua apatite, and by inference the Panasqueira apatite, musthave crystallized from a reservoir which was strongly en-riched in Eu relative to the other REEs. This could havecome about by the hydrothermal alteration of plagioclasethat had concentrated Eu at some earlier stage. This kindof process is considered in some detail by Graf (1977).Alderton et al. (1980) have shown that the seritization ofgranite in southwest England produced a negative Euanomaly and suggested that secondary muscovite can takeup REE3* more easily than Eu2*. Thus the resulting hy-drothermal fluid may be enriched in Eu relative to theother REEs. There may also be properties of hydrother-mal solutions (Flynn and Burnham, 1978) that enhancethe transport of Eu2* relative to the trivalent REEs. Thedifference in field strength of 2+ and 3+ ions may leadto stronger chloride complexes for 2+ ions (e.g., EuCl+)and fluoride complexes for 3+ ions. Thus the characterof the transporting medium may be as important as theREE composition of the medium in determining the kindof REE distribution in a mineral.

Sunau.l.nv AND coNcLUsroNS

The present study was undertaken in order to deter-mine the relationship between the the color produced bycathodoluminescence and the chemical composition ofapatite, in particular the REE distribution. A combina-tion of proton- and electron-microprobe analyses havebeen used to determine the major- and minor-elementcontent of fourteen apatite samples from various geologicenvironments. The microprobe analyses of the Durangoand Huddersfield apatites compare favorably with pub-lished analyses of apatite from the same localities.

The total REE content of the apatites varies from lessthan 0.2 to more than 15 wto/0, and the total number ofREE ions is generally proportional to the total number ofNa plus Si ions, indicating a charge compensation in thesubstitution of the REE3+ for Ca2+. The amount of Narelative to Si may depend on the alkalinity of the envi-ronment of crystallization. Most of the investigated apa-tites are LREE-enriched with a Ce/Y ratio varying from30 to less than 0.1. The apatites from alkaline environ-ments tend to show the ereatest enrichment in LREEs.

I

I

IIIIIIIII

Sm Dy Lu

Fig. 6. REEs normalized to the REE content found in a chon-dritic meteorite for the Llallagua apatite as determined by elxr(circles) and by neutron activation (crosses) and for the Pana-sequeira apatite (stars) as reported by Knutson et al. (1985).

The cathodoluminescence spectrum of the apatites wasscanned from 400 to 700 nm, and peaks due to Mn2+,Sm3*, Dy3+, Tb3*, Eu'*, and Eu3+ were identified. Theheight of the Sm peaks relative to the Dy peaks was foundto be a good indicator of the degree of LREE to HREEenrichment. The resolution of the sharp Sm and Dy peaksin the 600-nm spectral region is diminished in the pres-ence of a broad peak for Mn2+. For example, an apatitefrom a granite pegmatite (Timmins, Ontario) has a lowMn concentration, but the Mn content is fairly high rel-ative to the REEs, and the CL spectrum shows poor re-soltuion of the Sm and Dy peaks because of the broadMn'?+ peak. This is contrasted with an apatite from aweathering profile in Australia that contains abundantoxidized manganese oxides (Mno+), but whose CL spec-trum shows good resolution of the Sm and Dy peaks, thusindicating little Mn2+ in the apatite structure owing to theoxidizing conditions.

The CL spectrum of an apatite associated with tin min-eralization (Llallagua, Bolivia) has a very distinctive CLspectrum with prominent Mn2+, Eu2*, and Eu3+ peaks.The chondrite-normalized REE distribution for this sam-ple has a positive Eu anomaly of almost one hundredtimes the other REEs. The relative height of the Eu2+ andEu3* peaks in the CL and UV-excited spectra were com-pared to the CL spectra of Eu-doped glasses preparedwith various oxygen fugacities at 1400'C. It is concludedthat most of the Eu in the Llallagua apatite is Eu3+ andthat the apatite crystallized from a solution that wasstrongly enriched in Eu. The apatite structure is thoughtto concentrate Eu3+ relative to Eu2*, and thus no conclu-sion can be made on the oxidation state of the Eu in the

10

= t u€o

uJtI|E

102

8 1 0 ROEDER ET AL.: REES IN APATITE

solution from which the apatite crystallized. Sverjensky(1984) has suggested, however, that hydrothermal solu-tions at temperatures above 250"C almost certainly con-tain Eu in the Eu2+ state.

The present study has helped to demonstrate the use-fulness of cathodoluminescence as a nondestructive geo-chemical indicator of the environment of crystallizationofapatite. This technique provides the petrographer withnot only a colorful, but a useful guide to the crystalliza-tion history.

AcrNowr,nncMENTS

We are grateful to John S. White, Jr., of the Smithsonian Institutionfor the Llallagua apatite (sample no. ll4l75), to T. L. Barry of GTESylvania for the Euz*-, Eu3+-, and Mn'?*-doped synthetic apatites, to LA. Boatner of the Oak Ridge National Laboratory for the REE phos-phates, and to C. A. Francis and W. Metropolis of the Harvard Mu-seum for the Panasqueira apatite (sample no. l09l 17). We gratefully ac-knowledge the assistance of David Kempson, Larke Zarichny, Ann MacDonald, and Christopher Peck. Financial support was provided by theNatural Sciences and Engineering Research Council ofCanada.

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Bandy, M.C. (1976) Mineralogy of Llallagua, Boliva. Tucson [Arizona]Gem and Mineral Society Special Paper 1

Brunfelt, A.O., and Roelandts, l. (1974) Determination ofrare earth andthorium in apatites by thermal and epithermal neutron-actiyation anal-ysis. Talanta, 21, 513-521

Cruft, E.F, Ingamells, C.O., and Muysson, J. (1965) Chemical analysisand the stoichiometry of apatite. Geochimica et Cosmochimica Acta,29, s8t-597 .

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Eby, G.N. (1975) Abundance and distribution ofthe rare-ear1h elementsand yttrium in the rocks and minerals ofthe Oka carbonatite complex,

Quebec Geochimica et Cosmochimica Acta, 39,597-620Fleischer, M, and Althschuler, ZS. (1982) The lanthamdes and yttrium

in minerals of the apatite group: A review. U.S. Geological SurveyOpen-File Report 82-783, l-153.

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Gold, D P. (1963) The relationship between the limestones and the al-kaline igneous rocks of Oka and St Hilaire, Quebec. Ph D thesis, McGillUniversity, Montreal, Quebec

Graf, J L., Jr. (1977) Rare earth elements as hydrothermal tracers duringthe formation ofmassive sulfide deposits in volcanic rocks. EconomicGeology, 72,527-548

Grossi Sad, J.H., and Torres, N. (1976) Mineral exploration in the TapiraComplex, Minas Gerais, Brazil. GEOSOL-Geologia e Sondagens Ltda.Belo Horizonte, M.G., Brazil

Henderson, P. (1980) Rare earth element partition between sphene, apa-tite and other coexisting minerals of the Kangerdlugssuag intrusion, E.Greenland. Contributions to Mineralogy and Petrology, 72, 8l-85.

Henderson, P., Ed (l 984) Rare earth element geochemistry Elsevier Sci-ence Publications, B.V.

Huaxin, D. (1980) Cathodoluminescence study of apatite. Geochimica,4,368-314

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Knutson, C., Peacor, D.R., and Kelly, W.C. (1985) Luminescence, colorand fission track zoning in apatite crystals of the Panasqueira tin-tung-sten deposit, Beira-Baixa, Portugal. American Mineralogist, 70, 829-837 .

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Mariano, A N., and Ring, P J ( I 975) Europium-activated cathodolumi-nescence in minerals. Geochimica et Cosmochimica Acta,39, 649-660

Mariano, A.N., and Roeder, P.L. (1983) Kerimasi: A neglected carbon-atite volcano. Journal of Geolo gy, 9 l, 449 -4 5 5.

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Nash, W.P. (1972) Apatite chemistry and phosphorus fugacity in a dif-ferentiated igneous intrusion. American Mineralogist, 57, 877-886.

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Taylor, R.P., and Fryer, B.J (1982) Rare earth element geochemistry asan aid to interpreting hydrothermal ore deposits. In A.M Evans, Ed.,Metallization associated with acid magnetism, p.357-365. Wiley, NewYork.

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Mm'uscnrr.r RECETvED Ocrosen 14, 1986MnNuscnrrr AccEprED Apnrr- 3. 1987

AppnNorx 1. SaMpr,s DESCRrprroN oF ApATTTES

l. Durango apatite (5-226), Cerro de Mercado mine, Mexico(Young et a1., 1969). The famous Durango gem-quality apatitesare reported to be generated by hydrothermal reworking of aprimary magnetite-apatite igneous body.

2. Huddersfield apatite (S-2 12), Huddersfield Township, Que-bec, Canada (Trzcienski et al., 1974). The apatite occurs m askarn zone where crystals of apatite, phlogopite, sphene, micro-cline, and fluorite are in a matrix ofpink calcite.

3. Australian supergene apatite (1-G-7) that occurs as botryoi-dal masses in laterite overlying carbonatite found during a min-eral exploration program in Western Australia (collected by A.N. Mariano).

4. Blue river apatite (52-C-6), Bone Creek area of Blue River,British Columbia (collected by A. N. Mariano for the AnschutzMining Corp., 1982). The apatite occurs as ovoid masses in anapatite beforsite that is predominantly femrginous dolomite,richterite, and apatite with accessory Ta-rich pyrochlore and zir-on. The beforsite is part ofthe Blue River carbonatite that occursas concordant sills in the Precambrian Shuswap terrane. Theage of the richterite in the carbonatite was found by K-Ar to be2 1 1 + l 0 M a .

5. Snarum chlorapatite (28-G-1), region between Lallesard andBamle, southeastern Norway. The chlorapatite is from veins ingabbroic rocks believed to have formed from hydrothermal so-

lutions generated by the intrusion ofgabbroic masses into a sed-imentary sequence.

6. Pajarito apatite (T-654-0), Otero County, New Mexico (E.

E. Foord, S. L. Moore, R. F. Marvin, J. E. Taggart, Jr., unpub.,1983). This apatite is from Pajarito Mountain and is an acces-sory mineral along with elpidite, zircon, thorite, and allanite ina peralkaline riebeckite syenite. The syenite is part ofa series ofigneous Proterozoic basement rocks that includes syenites andalkali syenite pegmatites.

7. Timmins (2-E-36), Ontario, Canada. Collected from a gran-

ite pegmatite prospect north of Timmins. Apatite occurs withquartz, augite, microcline, sodic plagioclase, calcite, allanite, andtrace amounts of thorite, samarskite, hematite, rutile, and fluo-rite.

8. Kerimasi (3-B-10) volcano, Eastern Rift zone, Tanzania(Mariano and Roeder, 1983). The apatite occurs as euhedralcolorless prisms together with magnetite and salite in a melanite-melteigite xenolith in nephelinite and carbonatite agglomerates.

9. Llallagua (8-K-7), Bolivia (Bandy,1976). The apatite occursas transparent to translucent pale-pink crystals in the Contactovein, Siglo XX mine, associated with stannite, cassiterite, jean-

bandyite, and crandallite.10. Mountain Pass (6-E-5), California (Olson et al., 1954)'

Apatite occurs as euhedral to subhedral colorless prisms that areattached to, or as inclusions in augite, phlogopite, and olivine inshonkinite associated with the Mountain Pass carbonatite.

11. Tapira (29-K-12), Minas Gerais, Brazil (Grossi Sad, andTorres, I 976). The apatite occurs in an early-crystallizing pyrox-

enite of the Tapira carbonatite complex. The other minerals ofthe pyroxenite include salite, magnetite, perovskite, and biotite.

12. Khibina (30-D-8) massif, Kirovsk, Kola Peninsula, USSR(J. Gittins, pers. comm., 1985; Notholt, 1979). The Khibinamassif consists of an alkaline ring complex with nepheline sye-nite, foyaites, and carbonatites. A zone oflayered ijolite includeshigh-grade apatite, ijolites, and urtites. The Khibina massif isone of the world's largest sources of phosphate rock.

13. Mineville (l-E-24), New York (McKeown and Klemic,1956; Lindberg and Ingram, 1964). The apatite occurs with mas-sive magnetite together with gabbro and syenite in Grenvillemetasedimentary rocks. The mineralogy and texture resemblethose of magnetite-apatite rocks (nelsonites) that are often as-sociated with gabbroic anorthosites. The fluoroapatite has beencharacterized as having anomalous REEs, Y, Th, and U.

14. Oka (5-E-11), Quebec, Canada (Gold, 1963). The apatiteoccurs as primary colorless prisms associated with calcite, salite,phlogopite, and magnetite in an apatite sovite located near theBritholite zone ofthe Oka carbonatite.