Synthesis of trace element bearing single crystals of Chlor-Apatite (Ca5(PO4)3Cl) using the flux growth method

  • Stephan Klemme1Email author,

    Affiliated with

    • Timm John1,

      Affiliated with

      • Mathias Wessels1,

        Affiliated with

        • Christof Kusebauch1,

          Affiliated with

          • Jasper Berndt1,

            Affiliated with

            • Arno Rohrbach1 and

              Affiliated with

              • Peter Schmid-Beurmann1

                Affiliated with

                Chemistry Central Journal20137:56

                DOI: 10.1186/1752-153X-7-56

                Received: 22 January 2013

                Accepted: 15 March 2013

                Published: 26 March 2013

                Abstract

                We present a new strategy on how to synthesize trace-element bearing (REE, Sr) chlorapatites Ca5(PO4)3Cl using the flux growth method. Synthetic apatites were up to several mm long, light blue in colour. The apatites were characterized using XRD, electron microprobe and laser ablation ICP-MS (LA-ICPMS) techniques and contained several hundred μg/g La, Ce, Pr, Sm, Gd and Lu and about 1700 μg/g Sr. The analyses indicate that apatites were homogenous (within the uncertainties) for major and trace elements.

                Introduction

                Apatite (Ca5(PO4)3(Cl,F,OH) is an ubiquitous accessory phase in igneous, metamorphic and sedimentary rocks. Natural apatites contain significant amounts of geologically relevant trace elements such as the rare earth elements (REE), high field strengths elements (HFSE) and large ion lithophile elements (LILE). Moreover, apatite is known to contain high concentrations of U and Th so that apatite formation can be established by conventional radioactive element decay dating or its thermal evolution can be reconstructed by investigating “fission tracks” caused by the decay of radioactive elements [15]. Furthermore, as human and animal bones consist of apatite, U-series dating of relatively young fossils is a new and exciting area of research in quaternary geosciences (e.g. [6]). To aid reliable analysis of trace element concentrations and isotopic ratios, matrix matched reference materials are needed. Single crystal homogeneous apatites that contain known amounts of trace elements would be ideal.

                Moreover, apatite weathering and replacement processes in low-grade metamorphic rocks have been in the focus of research recently both in our institution and elsewhere [710]. This is mainly, as apatite, when equilibrated with or growing from a super-critical fluid in low-grade to high-grade metamorphic rocks, may contain a “geochemical fingerprint”, that is a trace element signature from which one might be able to re-construct the composition of the fluid. In order to calibrate such a fingerprint, experiments are needed to investigate the partitioning of trace elements between apatite and fluids in a range of chemical compositions, pressures and temperatures. The experiments in turn need well-characterized starting materials, i.e. trace element bearing homogenous single crystals of apatite.

                Furthermore, phosphate ceramics have long been proposed as suitable materials for safe long-term nuclear waste storage [11, 12]. Experiments to simulate interaction of such apatite-based ceramics with water-rich fluids [11, 1315] need suitable actinide-bearing apatite crystals as starting materials [16].

                Here we report the high-temperature synthesis of mm-sized single crystal chlorapatites (Ca5(PO4)3Cl) using the so-called flux method. We tried several compositions, temperatures and synthesis routes and here we report on the most successful experiments, both in terms of crystal size as well as in terms of trace element homogeneity.

                Previous work

                Several studies report the synthesis of single crystal apatite, both fluorapatite, chlorapatite and hydroxyapatite [1723]. Most synthetic apatites contain no trace elements, only a few groups have synthesized apatites with high concentrations (ie. wt.%) of one or two REE [24, 25]. Most synthesis routes involve hydrothermal synthesis at high pressure [26], especially when hydroxyapatite is involved.

                Experiments

                Initial experiments in chemical compositions without trace elements confirmed the validity of previous experimental results [23]. Using the flux growth method pioneered by Prener and others, we could grow idiomorphic apatite single crystals up to ca. 6 mm in size. All experiments were conducted in Pt-crucibles in conventional vertical high-temperature furnaces at atmospheric pressure. The starting material consisted mainly of various mixtures of Ca3(PO4)2 and CaCl2, the latter of which acted as the flux. The experiments were heated to a temperature above the liquidus, they were held for a short time, and then slowly cooled to a final run temperature. During the cooling apatite crystals formed from the melt. After quenching, the experimental products were washed in water or diluted HCl for several hours. This effectively removes all the CaCl2 flux. Table 1 lists experimental run conditions of each individual experiment. Figure 1 shows some representative single crystal apatites grown in our laboratory.
                Table 1

                Experimental run conditions

                Experiment

                ST

                RR

                PT

                H

                CR

                ET

                Trace elements

                /

                /

                /

                /

                /

                /

                 

                °C

                °/h

                °C

                h

                °/h

                °C

                 

                SynCLAP3

                800

                70

                1300

                10

                6

                1100

                No

                SynCLAP5

                800

                70

                1300

                12

                6

                1025

                No

                SynCLAP6

                800

                70

                1300

                10

                6

                850

                Yes

                SynCLAP8

                800

                70

                1420

                10

                6

                800

                Yes

                SynCLAP9

                800

                70

                1320

                10

                6

                800

                Yes

                SynCLAP10

                800

                70

                1370

                20

                6

                800

                Yes

                SynCLAP11

                800

                70

                1370

                20

                6

                800

                Yes

                SynCLAP12

                800

                70

                1370

                20

                6

                800

                Yes

                ST: starting temperature, RR: ramp rate during heating to PT, PT: Peak temperature, H: hours at PT, CR: cooling rate down to ET, ET: final run temperature.

                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Fig1_HTML.jpg
                Figure 1

                Chlorapatite crystals grown with the flux method; crystals from experiment SynCLAP6.

                X-ray powder diffraction (XRPD)

                For phase characterization an X-ray powder diffraction pattern was recorded using a PHILIPS X´PERT PW 9430 diffractometer with Cu-K α1 radiation and a primary Ge-(111) monochromator of Johansson Type. The operating conditions were 45 kV and 40 mA. Rietveld refinement was performed using the FULLPROF SUITE 2005 [27]. As starting parameters lattice parameters and crystal structural data including isotropic temperature factors for apatite-(CaCl) were taken from the literature [28]. The parameters which were varied for the refinement included the scale factor, the lattice parameters a and c, 4 background parameters, the sample displacement, two asymmetry parameters as well as the shape parameters w and Y of the Thompson-Cox-Hastings pseudo-Voigt profile function. The refinement converged to an Rwp = 12.4% (Rexp = 9.4%). No significant line broadening could be detected with respect to the Si-640a NIST-Standard which was used to determine the resolution function of the diffractometer. As can be seen from Figure 2 one weak reflection at 25.41°(2θ) remained unexplained which is therefore assumed to belong to an additional unidentified phase. As its intensity is about 0.7% of that of the most intense apatite reflexion we assume that the amount of that phase is about 1% by weight. The results are given in Figure 2 and Table 2 together with recent literature data. In conclusion our apatite sample can be characterised as nearly pure chlor-apatite with very good crystallinity.
                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Fig2_HTML.jpg
                Figure 2

                X-ray diffraction: Observed, calculated and difference intensity powder patterns of synthetic chlor-apatite.

                Table 2

                Unit-cell parameters of synthetic chlorapatites (space group P6/3m )

                 

                Sample

                a [Å]

                c [Å]

                Chlor-Apatite

                   

                This work

                SynCLAP-3

                9.6397(2)

                6.7693(1)

                García-Tuñón et al. 2012 [28]

                Clap

                9.6452(2)

                6.7636(2)

                Luo et al. 2009 [42]

                THClAP

                9.6330(2)

                6.7834(2)

                Luo et al. 2009 [42]

                UClAP

                9.6233(2)

                6.7784(3)

                Synthesis of trace element bearing apatites

                Once we were satisfied which the flux growth itself (SynCLAP3 and SynCLAP5, see Table 1), we conducted further experiments where the starting material contained a number of geochemically relevant trace elements. However, although we added relatively large amounts of trace elements (e.g., SynCLAP 6, 300 μg/g of each trace element, see Table 3) to the initial starting material mixture, we found that the resulting flux-grown apatites did not contain high concentrations of trace elements (generally well below 10 ppm of each trace element). We believe that the overall low concentrations of trace elements in the synthetic apatite crystals was caused by the fact that most of these trace elements, many of which are trivalent rare earth elements, are incorporated into apatites by a coupled substitution which involves incorporation of Na+ which replaces Ca2+ or of Si4+ which replaces P5+ in the apatite structure. Below we show two possible exchange mechanisms for the incorporation of trivalent rare earth elements (REE) into the apatite structure [29, 30].
                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Equ1_HTML.gif
                (1)
                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Equ2_HTML.gif
                (2)
                Table 3

                Starting materials

                Experiment

                Ca3(PO4)2

                CaCl2

                Trace elements

                /

                /

                 

                g

                g

                 

                SynCLAP3

                4.65

                15.35

                None

                SynCLAP5

                4.65

                15.35

                None

                SynCLAP6

                4.65

                15.35

                300 μg/g of REE, Sr, Y. Th, U, Pb, Ba, Rb, Li, B using the solution K-M1

                SynCLAP8

                1.16

                3.84

                3000 μg/g Sm added as Sm2O3, 2 wt.% Si added as SiO2 and 2 wt.% Na added as NaCl

                SynCLAP9

                1.16

                3.84

                Identical to SynCLAP8

                SynCLAP10

                1.16

                3.84

                3000 μg/g of La, Ce, Pr, Sm, Gd, Lu, Hf, Zr, Ta, Ti, Sc each (added as oxides) and 2 wt.% Si added as SiO2 and 2 wt.% Na added as NaCl

                SynCLAP11

                1.16

                3.84

                2000 μg/g of La, Ce, Pr, Sm, Gd, Lu each (added as oxides) and 2 wt.% Si added as SiO2 and 2 wt.% Na added as NaCl

                SynCLAP12

                1.16

                3.84

                1500 μg/g of La, Ce, Pr, Sm, Gd, Lu, Sr each (added as oxides) and 0.8 wt.% Si added as SiO2

                K-M1: solution containing 1000 μg/g of several REE (La, Ce, Gd, Nd, Sm, Yb, Lu), Sr, Y, Th, U, Pb, Ba, Rb, Li, and B.

                We believe that the lack of Na+ and Si4+ in apatites grown in SynCLAP 6 strictly limited the incorporation of trivalent trace elements. Consequently, when we added some Na and Si (2 wt.%, SynCLAP 8, see Table 1 for details) to the starting material, we found that the flux-grown apatites contained significant amounts of Si and also significantly higher amounts of trace elements. This shows that incorporation mechanism (1) is more important than mechanism (2). Experiments SynCLAP 9 and 10 were similar to SynCLAP 8. The latter experiments yielded large and trace element bearing apatite but due to high SiO2 contents of the melt lots of other acicular, needle-like, Ca-silicates formed in the melt. It was difficult to separate apatite crystals from the quench-crystallized matrix after the flux had been washed out. Figure 3 shows typical textures observed in the experiments SynCLAP 8-10.
                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Fig3_HTML.jpg
                Figure 3

                SynCLAP10: Ideomorphic apatite crystals (lighter grey) in a matrix of acicular Ca-silicate crystals, most of it wollastonite (CaSiO 3 ), after washing with HCl solutions. The fine intergrowth of apatite with wollastonite needles makes physical recovery of apatite single crystals difficult.

                Consequently, SynCLAP 11 and 12 contained less REE and less Na and Si (Table 3). In conclusion, the apatite single crystal synthesis is best-done following procedures and compositions like in experiment SynCLAP 12. The apatite crystals grown in these experiments are large (see Figure 3), they contain high concentrations of trace elements (Table 4) and the apatite crystals can be easily removed from the matrix.
                Table 4

                Trace element concentrations in Apatites (SynCLAP12)

                 

                1-1*

                1-2

                1-3

                1-4

                1-5

                1-6

                1-7

                1-8

                /

                /

                /

                /

                /

                /

                /

                /

                 

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                Mg

                41

                40

                43

                41

                42

                43

                43

                42

                Si

                10374

                10186

                10819

                10118

                9868

                10847

                10797

                10630

                Fe

                4

                4

                6

                7

                8

                5

                9

                4

                Sr

                1790

                1696

                1780

                1660

                1717

                1674

                1806

                1669

                La

                867

                844

                887

                845

                868

                1232

                908

                1085

                Ce

                16

                16

                17

                16

                16

                20

                17

                19

                Pr

                574

                573

                598

                567

                583

                823

                599

                717

                Sm

                552

                552

                595

                546

                556

                824

                584

                722

                Gd

                548

                548

                594

                537

                547

                832

                584

                725

                Lu

                100

                98

                108

                98

                96

                125

                107

                119

                 

                2-1

                2-2

                2-3

                2-4

                2-5

                2-6

                2-7

                2-8

                /

                /

                /

                /

                /

                /

                /

                /

                 

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                Mg

                46

                42

                41

                43

                42

                41

                42

                41

                Si

                10698

                10377

                10532

                10679

                10552

                10600

                11535

                10605

                Fe

                18

                3

                6

                7

                7

                7

                8

                9

                Sr

                1785

                1722

                1774

                1666

                1740

                1705

                1710

                1760

                La

                888

                816

                853

                831

                814

                844

                838

                1037

                Ce

                17

                16

                17

                17

                17

                17

                18

                19

                Pr

                570

                540

                556

                557

                551

                575

                547

                711

                Sm

                589

                501

                529

                522

                511

                532

                527

                685

                Gd

                599

                495

                523

                520

                515

                534

                531

                685

                Lu

                115

                94

                101

                100

                96

                100

                108

                113

                 

                3-1

                3-2

                3-3

                3-4

                3-5

                3-6

                  

                /

                /

                /

                /

                /

                /

                  
                 

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                μg/g

                  

                Mg

                41

                44

                44

                42

                42

                39

                  

                Si

                10508

                12256

                11634

                11562

                10897

                10951

                  

                Fe

                <8.52

                9

                <8.17

                <8.65

                11

                12

                  

                Sr

                1804

                1819

                1841

                1851

                1843

                1706

                  

                La

                1008

                884

                851

                869

                915

                1378

                  

                Ce

                20

                21

                19

                18

                19

                23

                  

                Pr

                710

                616

                595

                603

                640

                981

                  

                Sm

                706

                598

                567

                585

                623

                1041

                  

                Gd

                681

                561

                533

                548

                584

                985

                  

                Lu

                111

                114

                104

                104

                102

                134

                  

                Trace element analyses performed using Laser Ablation ICP-MS. Analytical uncertainties are in the order of 15%. *: The individual analysis numbers (e.g., 1-1 stand for crystal 1 analysis 1) correspond with numbers in white circles (SEM images) in Figure 3.

                http://static-content.springer.com/image/art%3A10.1186%2F1752-153X-7-56/MediaObjects/13065_2013_581_Fig4_HTML.jpg
                Figure 4

                Synthetic chlorapatite crystals from experiments SynCLAP12. First row: back scattered electron images taken with an analytical scanning electron microscope (SEM). Crystals 1, 2 and 3 were analysed for major elements (Ca, P, Cl, Si) with electron microprobe analyzer (EMPA) and the black lines in the SEM pictures mark the line scans where EMPA Analyses were undertaken. The second row diagrams show the major element composition of the apatite crystals along the line scans. The third row diagrams show trace element concentrations of the apatite crystals which were analysed with Laser Ablation ICP-MS techniques at Münster University. The analyses are numbered (purple circles) and the analysis sites are given in the SEM pics in the first row.

                Trace element concentrations in synthetic apatites

                When single crystals are grown from a melt (or flux), trace elements will be incorporated into the crystals. The concentration of the trace elements in the crystals depends on their equilibrium partition coefficients (if equilibrium is attained) and the bulk concentration of the trace element. If diffusion rates of trace elements are low in the crystal (and this is the case for all geologically relevant trace elements in apatite [3133], crystals may be zoned, at least in elements which are compatible, that is elements with a crystal/melt partition coefficient >1. This is due to the fact that the first crystals formed will contain comparatively high concentrations of this compatible trace element and the coexisting melt will be consequently depleted in this element. Crystals that form later, or layers of the crystal which form later during cooling will contain significantly lower concentrations of the trace element. As it is well known that many REE, Sr and other important trace elements are compatible in apatite [29, 3437] we were concerned initially that our synthetic apatites may be significantly zoned. However, analytical results using in-house laser ablation ICP-MS techniques [7, 36, 3841] show that the apatites synthesized in SynCLAP12 are rather homogeneous in terms of major and trace elements, surely within the analytical uncertainties. The homogeneity surprised us initially but this is probably due to the fact that the partition coefficients between apatite and CaCl2-rich flux are probably very different from the published apatite/silicate melt partition coefficients (e.g., [29]). Moreover, the flux/crystal ratio employed in our study is high which further minimizes potential zoning during crystal growth. Figure 4 shows major and trace element concentrations of some representative apatite crystals from SynCLAP12.

                In summary, we present an effective procedure to synthesize mm-sized single crystals of chlorapatite that contain a variety of geochemically relevant trace elements. These crystals may be used as starting materials for further experiments or used as reference materials for geochemical analysis.

                Declarations

                Acknowledgements

                Our thanks go to M. Feldhaus, H. Heying, M. Trogisch, and U. Böcker for their sterling efforts in the Mineralogy workshops at Münster University. We also thank A. Breit for help with the XRD measurements and V. Rapelius with help in the chemical laboratories. The manuscript benefitted from careful and helpful reviews by Dr K-D Grevel and an anonymous reviewer. We acknowledge funding by the DFG (DFG grant No. JO 349/3-1). We further acknowledge support by the Open Access Publication Fund of the University of Münster.

                Authors’ Affiliations

                (1)
                Institut für Mineralogie, Westfälische Wilhelms Universität Münster

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