Composition Determination and Cathodoluminescenceof Natural Apatite from Different Phosphate Depositsin Northern China

QINGFENG GUO,1 LIBING LIAO,1,2,3 ZHIGUO XIA,1,4 and HAIKUN LIU1

1.—School of Materials Science and Technology, China University of Geosciences, Beijing 10083,People’s Republic of China. 2.—e-mail: clayl@cugb.edu.cn. 3.—e-mail: lbliao@cugb.edu.cn.4.—e-mail: xiazg@cugb.edu.cn

Two kinds of natural apatite minerals were obtained from two different phos-phate deposits in northern China, FanShan, Hebei province and HeiYingShan,Inner Mongolia province. Their chemical compositions, phase structure, andcathodoluminescence (CL) have been comparatively investigated. X-ray dif-fraction (XRD) analysis indicated that the raw phosphate minerals from Fan-Shan mainly are composed of FanShan apatite (FA) and FanShan mica,respectively, while the ore sample from HeiYingShan contained HeiYingshanapatite (HA) and HeiYingshan magnetite. The mineralogical characteristicsand crystal structure of FA and HA were further determined by XRD, scanningelectron microscopy, x-ray fluorescence spectrometer, and inductively coupledplasma-mass spectrometry, respectively. These results indicate that FAbelongs to fluorapatite, while the HA belongs to hydroxyapatite. The existenceof some trace elements has also been verified by the measured CL spectrum.The origin of the observed CL peaks has been discussed, and the relationship ofthe CL and the chemical composition has also been investigated.

INTRODUCTION

Apatite belongs to one of the accessory mineralsand is commonly found in igneous, metamorphic,and sedimentary rocks. Moreover, apatite mineralcrystals are one of the major constituents in boneand other mineralized vertebrate tissues.1 There-fore, as a kind of important mineral material, apa-tite and its derivatives can be widely used in manyfields, such as geochemistry analysis, optics devices,biomaterials, inorganic pigments, and so on.2–5

Generally, apatite minerals have three kinds ofimportant end members, such as fluorapatite,hydroxyapatite, and chlorapatite in the formula ofX5(ZO4)3(F, Cl, OH), which belongs to phosphatecompounds with a hexagonal system. However, theapatite mineral found in nature mainly belongs tothe fluorapatite, which is also the most stable andinsoluble form of apatite phase.6,7 Crystal chemistryanalysis indicates that the X positions can be occu-pied by different atoms (such as Ca, Na, Mg, and Sr)and that Z positions can also be substituted by theP, As, Si, and S atoms. The good structural tolera-

tion effect in the apatite crystal lattice allows vari-ous host substitutions without significant changesin symmetry with the P63/m space group;8,9 how-ever, those substitutions are accompanied by thevariation in the lattice parameters, physical andchemical properties, as well as the applicationproperties as the functional materials.10–16 There-fore, the detailed analysis on the mineralogicalcharacteristics and the chemical composition of theseparated natural apatite from the different depos-its plays an important role in the development of thenew inorganic mineral materials.

The Fanshan apatite (-magnetite) deposit (FAD)was discovered in northern China in the mid-1970s.Many Chinese geologists have paid much attentionto it since it was found. It was reported that thehighest apatite content can reach 95% in FPD as anapatite-rich deposit.17 However, Heiyingshan xeno-time-iron deposit (HXD) is famous for the high totalamount of the rare earth oxides in raw apatitemineral, which can reach 2.94–3.05% of the totaloxides according to the previous research.18 Thehigh rare earth content made this deposit different

JOM, Vol. 66, No. 6, 2014

DOI: 10.1007/s11837-014-0955-6� 2014 The Minerals, Metals & Materials Society

992 (Published online April 24, 2014)

from other apatite of China. However, littleresearch was reported on the mineralogical char-acteristics and cathodoluminescence (CL) spectrumof these apatite minerals. In the present work, westudy the mineralogical characteristics and CLspectrum of the separated natural apatite mineralsamples in FAD and HXD, and the final chemicalcomposition was also determined.

EXPERIMENTAL

FAD and HXD samples in this study wereobtained from ZhuoLu County, Hebei province andHeiYingShan country, Inner Mongolia province,respectively. As-obtained FanShan apatite (FA) andHeiYingShan apatite (HA) were selected from theraw mineral samples under XTL-1 stereomicro-scope. After that, FA and HA were placed into twobeakers filled with distilled water to be washed viathe ultrasonic treatment because we can obtain theclean samples for the following measurement.Finally, the obtained apatite and mica were dried inthe oven at 50�C.

Phase structures of the as-obtained and selectedmineral samples were all checked by x-ray powderdiffractometer (D/max-rA 12kw, Japan) with Cu Karadiation (k = 1.5418 A) from 3� to 70� (2h). Thesurface morphology and micro-area analysis of FAand HA samples were observed by scanning electronmicroscope (Hitachi S-520, Shimadzu). Elementcontent in FA and HA were detected by x-ray fluo-rescence (XRF) carried out with the XRF Spec-trometer (PW2400, PANalytical), operating at40 kV, 50 mA. The content of rare earth elements ofFA and HA were obtained using an inductivelycoupled plasma-mass spectrometry (ICP-MS) (SeikoSPQ8000H, Seiko Instruments Inc.). The CL mea-surements were carried out in CL Miriamin(RELION CL 6, United States) with a vacuumchamber (about 6 9 10�4 Pa), where the apatitewas excited by an electron beam of 4 kV with fila-ment current of 100 lA, and the emission spectrawas recorded using an fiber spectrometer (OceanOptics QEB0388) with a charge coupled device(CCD) camera at room temperature.

RESULTS AND DISCUSSION

FAD occurs in the Fanshan complex, a potassicultramafic-syenite layered intrusion located innorthern China. The Fanshan complex intrudedinto Proterozoic sedimentary rocks in the earlyTriassic period. The complex comprises first-to-thirdphase intrusives.17 The subrhythmic layering con-sists of five typical rocks: biotite clinopyroxenite,biotite rock, biotite-apatite rock, alternation biotite-apatite rock, and apatite rock.17 Figure 1a illus-trates the digital images of the raw ore samplesobtained from FanShan deposit, and the enlargedimage of the FanShan raw ore after being crushed is

shown in Fig. 1b (A1-apatite; B1-mica). It is foundthat the raw ore sample of FanShan mainly con-tained the yellow-green apatite and the black micawith sheet shape. We can also find from Fig. 1b thatFA presents a transparent characteristic, whichindicated that the raw ore belongs to biotite-apatiterock. HXD is located 270 km north of Jiuquan City,which was discovered in August 1957, and HXDbelongs to volcanic rock.18 It has aroused muchinterest from geologists home and abroad. Figure 1cand d show the digital images of the raw ore sam-ples obtained from HeiYingshan and the enlargedimage of the HeiYingshan raw ore after being cru-shed (C1-apatite; D1-magnetite). The HeiYingShansamples are composed of red apatite and blackmagnetite samples. To further verify the phasestructure of the contained mineral composition,Fig. 2 illustrates the x-ray diffraction (XRD) pat-terns of the raw ore obtained from FAD and HXD.Among them, ‘‘A’’ indicates the diffraction peaks ofapatite; ‘‘M’’ indicates the diffraction peaks of mica;‘‘Ma’’ indicates the diffraction peaks of magnetite.As shown in Fig. 2, all of the diffraction peaks ofFAD can be assigned to the phase of mica or apatitewithout any other impurity diffraction peaks, whichis also verified by the observed images via themicroscope. Therefore, the main phases of the rawore are mica and apatite, respectively. It is alsofound that all the diffraction peaks of HXD can beassigned to the phase of magnetite or apatite.

XRD patterns of the selected FA and HA areshown in Fig. 3. The standard XRD data for Ca5

(PO4)3F (JCPDS card no. 56313) is shown as a ref-erence. Both of the observed XRD patterns verifiedthe pure phase character of apatite. The XRD peaksof FA and HA are very sharp, indicating the highcrystalline degree of the samples. In addition, it isobvious that all the diffraction peaks of FA and HAcan be exactly assigned to the pure hexagonal phaseof Ca5(PO4)3F [space group: P63/m] (JCPDS card no.56313). In an isomorphous substitution, it is gen-erally observed that the unit cell parameters (andhence the unit cell volume) change smoothly withthe substitution. Table I illustrates cell parametersof FA and HA.

The standard cell parameters for Ca5(PO4)3F(JCPDS card no. 56313) are shown as a reference. Theunit-cell parameters of FA (a = 9.399 A, c = 6.887 A,V = 526.82 A3, c/a = 0.733) and HA (a = 9.383 A,c = 6.876 A, V = 524.31 A3, c/a = 0.732) were calcu-lated by using jade5 software. It is found that theobtained values of FA and HA are merely slightlydifferent from that of Ca5(PO4)3F (JCPDS card no.56313). Therefore, the detailed chemical compositionanalysis will be performed in the following section.

Figure 4 presents an scanning electron micros-copy (SEM) image of HA (a), SEM image of FA (b),energy-dispersive spectroscopy (EDS) element ana-lysis (surface points within the rectangle area) ofHA sample (c), and EDS analysis of FA sample (d).

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Both samples from FPD and HPD appear as thecolumn shape, which is consistent with the hexag-onal columnar structure of the apatite phase. Thecompositional elements of the surface points within

the rectangle area, as identified by EDS primaryelemental peaks illustrated in Fig. 4c, indicate thepresence of Ca, Mg, Al, Si, P, C, O, and F elementsfor the HA sample. According to the EDS analysis in

Fig. 1. The digital (a) and enlarged (b) images of the raw ore samples obtained from FanShan (A1-apatite; B1- mica); the digital (c) and enlarged(d) images of the raw ore samples obtained from HeiYingshan (C1-apatite; D1-magnetite).

Fig. 2. XRD pattern of the raw ore obtained from FAD and HXD. Allpeaks can be assigned to apatite or mica. All peaks can be assignedto apatite or magnetite. (‘‘A’’ indicates the diffraction peaks of apatite;‘‘M’’ indicates the diffraction peaks of mica; ‘‘Ma’’ indicates the dif-fraction peaks of magnetite).

Fig. 3. XRD pattern of FA and HA, and all diffraction peaks markedwith crystal plane index correspond to apatite with the hexagonalsystem. The standard data for Ca5(PO4)3F (JCPDS card no. 56313)are shown as a reference.

Guo, Liao, Xia, and Liu994

Fig. 4d, it contains Ca, Si, P, C, O, and F elementsfor the FA sample.

As is known to us, a lot of articles to characterizethe composition of minerals using XRF have beenreported all over the world. For instance, an XRFmethod describes the bulk composition analysis ofmagnetite and ilmenite grains.19 It is reported thatlignite and intraseam nonlignite rock samples fromthe Achlada open-cut mine in northern Greece werecharacterized by XRF techniques.20 The detailed

and systematic elements investigations of the FAand HA samples were analyzed by XRF, and thecorresponding data are shown in Table II.

The atomic ratio of Ca to P is 1.67 for the idealfluorapatite with the chemical formula of Ca5

(PO4)3F, while the ratios of Ca/P for FA and HAwere calculated as 1.78 and 2.18, respectively, basedon the data in Table II, indicating that ismorphoussubstitution existed in the FA and HA phases. Apartfrom Ca ions, other cations, such as Na, K, Al, Mg,

Table I. Cell parameters of FA and HA

Samples a (A) c (A) V (A3) c/a

FA 9.399 6.887 527.07 0.733HA 9.383 6.876 524.31 0.732Ca5(PO4)3F(JCPDS 15-0876) 9.368 6.884 523.25 0.735

The standard cell parameters for Ca5(PO4)3F (JCPDS Card No. 56313) are shown as a reference.

Table II. Major and minor elements of FA and HA Analyzed by XRF (wt%; BDL means below detection limit(0.01 wt%))

Element C F O Na Mg Al Si P S Cl Ca Fe Y Sr Ca Ce Pr Nd

FA 0.99 2.54 37.05 0.12 0.09 0.44 0.46 16.61 0.36 0.05 38.14 0.04 0.02 0.81 BDL BDL BDL BDLHA 1.50 1.46 36.80 0.20 0.04 1.08 0.67 14.2 0.04 0.34 40.00 0.93 0.34 0.04 0.49 0.90 0.13 0.48

Fig. 4. (a) SEM image of HA. (b) SEM image of FA. (c) Energy-dispersive spectroscopy (EDS) element analysis of the surface points within therectangle area in (a). (d) Energy dispersive spectroscopy (EDS) element analysis of the surface points within the rectangle area in (b).

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Sr, Y, and Fe, can also occupy the position of Ca inFA. The positions of P of FA should be occupied bySi, C, and S. Anionic sites are possibly occupied by Fand Cl. The result also shows that HA containsmany elements including P, C, Cl, F, O, S, Ca, Na,Mg, Fe, Al, Sr, Si, La, Ce, Nd, Pr, and Y and showsdifferent characteristics for their origins. Apatitesfrom HeiYingshan contain Cl, Al, Si La, Ce, Nd, Pr,and Fe with higher content, but the content of C, F,and P is lower than that of the apatite from Fan-Shan. Based on the measured elements and corre-sponding contents, the chemical compositions of thetwo kinds of apatite samples have been determined,

and the valence balance in apatite phase was real-ized by using OH occupying the rest position ofanions. Therefore, the chemical formula of the FAand HA were calculated as (Ca0.9687Na0.0041K0.0003

Sr0.0094Y0.0003Fe0.0007Mg0.0007Al0.0165)5(P0.8791Si0.2779

S0.0189O4)3F0.6786Cl0.0076(OH)0.3138 and (Ca4.5835

Na0.0407Mg0.0083Al0.0831K0.0015Mn0.0059Fe0.0761Sr0.0023

Y0.0174La0.0162Ce0.0295Pr0.0041Nd0.0153Sm0.0036)4.9875

[(C0.5472Si0.1100P2.1066S0.0065)2.9652O12]F0.3531Cl0.0443

(OH)0.6026, respectively.Figure 5a presents the content distribution of

rare earth contained in FA and HA, which aredetermined by the ICP-MS. It is found that somerare earth elements can be both detected in FA andHA, while apatite mineral samples from these twodistricts have a great difference in rare earth ele-ment contents. Both HA and FA show a right-in-clined pattern with strong light REE enrichment.FA contained Ce, Pr, Nd, Sm, and Yb; however,more rare earth elements could be detected in HA,such as Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,and Yb. In addition, the content of rare earth ele-ments in HA is much higher than that in FA, whichis consistent with the previous report.17 It isreported that LREE/HREE values of the HA are8.31–9.18;18 however, the value of the HA studied inthis paper is 4.30, which is a little different from thevalues of former studies, which may be caused bydifferent methods used. And the LREE/HREE val-ues of the HA are much higher than other apatitesin China. It is believed that the apatite ofHeiYingshan might have been formed by a mixedsource of depleted mantle- and crustal-derivedmagma or fluids.

Fig. 5. (a) The content of rare earth contained in FA. (b) The contentof rare earth contained in HA. (All values are in mg/kg).

Fig. 6. (a) The CL spectra of FanShan apatite. (b) The CL spectra of HeiYingshan apatite (accelerating voltage = 4.0 kV; filament cur-rent = 100 lA). The spectrum is labeled according to the element activator, which causes the luminescence represented by each peak.

Guo, Liao, Xia, and Liu996

The existence of the rare earth elements in somemineral samples will generally produce lumines-cence phenomenon under the different excitationconditions, such as UV light, mechanical force,electric field and cathode radiation, and so on.Therefore, some functional application from thenatural mineral can be expected, and people canalso build up the relationship of the structure andluminescence properties of the natural mineral. Asfor the apatite mineral, rare earth ion will sub-stitute the Ca sites, and some luminescence behav-ior can be observed. To know the luminescence ofFA and HA, the CL spectrum of the two samples hasbeen measured. As is shown in Fig. 6a, severalobvious emission lines peaked at 613 nm and648 nm can be observed, which could be signed to1D2-3H4 and 3P0-3H4 transitions of Pr3+. The broadband at 400 nm in the CL spectrum is signed to the4f–5d transitions of Ce3+, and the two bands cen-tered at 599 nm and 704 nm are a result of the3G5/2-6H7/2 and 3G5/2-6H9/2 transitions of Sm3+. Inaddition, the broad band centered at 885 nm isassigned to the 4F3/2-4I9/2 transition of Nd3+. Mean-while, it is found in Fig. 6b that several obviousemission bands centered at 602 nm and 647 nm canbe observed, which could be assigned to 1D2-

3H4 and3P0-

3H4 transitions of Pr3+, and the two bands cen-tered at 489 nm and 578 nm are a result of the4F4/2-

6H15/2 and 4F9/2-6H13/2 transitions of Dy3+. It is

also found that the two bands centered at 706 nm,755 nm, and 807 nm are a result of the 3G5/2-

6H9/2 and3G5/2-

6H11/2 transitions of Sm3+. The 4f–5d transitionsof Ce3+ centered at 400 are also found in the CLspectrum of HA. In addition, the broad band centeredat 887 nm in the long wavelength is assigned to the4F3/2-

4I9/2 transition of Nd3+. As discussed earlier, theluminescence of HA and FA was caused by the REEelements contained in the corresponding composi-tions. All of these results are in good agreement withthe chemical compositions of FA and HA.

CONCLUSION

The unit cell parameters of FA (a = 9.399 A,c = 6.887 A, V = 526.82 A3, c/a = 0.733) andHA (a = 9.383 A, c = 6.876 A, V = 524.31 A3,c/a = 0.732) were determined for the natural apatiteminerals from FanShan and HeiYingShan, respec-tively. It is found that ismorphous substitution existsin FA and HA mineral. The chemical formulas of theFA and HA were calculated as (Ca0.9687Na0.0041

K0.0003Sr0.0094Y0.0003Fe0.0007Mg0.0007Al0.0165)5(P0.8791

Si0.2779S0.0189O4)3F0.6786Cl0.0076(OH)0.3138 and (Ca4.5835

Na0.0407Mg0.0083Al0.0831K0.0015Mn0.0059Fe0.0761Sr0.0023

Y0.0174La0.0162Ce0.0295Pr0.0041Nd0.0153Sm0.0036)4.9875

[(C0.5472Si0.1100P2.1066S0.0065)2.9652O12]F0.3531Cl0.0443

(OH)0.6026. The luminescence of FA is caused by somecations (such as Ce3+, Pr3+, Sm3+, and Nd3+), whileluminescence of HA is caused by Ce3+, Dy3+, Pr3+,Sm3+, and Nd3+.

ACKNOWLEDGEMENTS

This present work is supported by the NationalNatural Science Foundations of China (Grant No.41172053, No. 51002146, and No. 51172242), NaturalScience Foundations of Beijing (2132050), the Pro-gram for New Century Excellent Talents in Univer-sity of Ministry of Education of China (NCET-12-0950), and the Fundamental Research Funds for theCentral Universities (53200959580, 2011YYL131).

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