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

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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
Figure 1
figure1

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-Kα1 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.

Figure 2
figure2

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 )

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].

C a 2 + + P 5 + = RE E 3 + + S i 4 +
(1)
2 C a 2 + = RE E 3 + + N a +
(2)
Table 3 Starting materials

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.

Figure 3
figure3

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)
Figure 4
figure4

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.

References

  1. 1.

    Brandon MT, Roden-Tice MK, Garver JI: Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State. Geol Soc Am Bull. 1998, 110: 985-1009. 10.1130/0016-7606(1998)110<0985:LCEOTC>2.3.CO;2.

  2. 2.

    Gallagher K, Brown R, Johnson C: Fission track analysis and its applications to geological problems. Annu Rev Earth Planet Sci. 1998, 26: 519-572. 10.1146/annurev.earth.26.1.519.

  3. 3.

    Hasebe N, Barbarand J, Jarvis K, Carter A, Hurford AJ: Apatite fission-track chronometry using laser ablation ICP-MS. Chem Geol. 2004, 207: 135-145. 10.1016/j.chemgeo.2004.01.007.

  4. 4.

    Soderlund U, Patchett JP, Vervoort JD, Isachsen CE: The Lu-176 decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions. Earth Planet Sci Lett. 2004, 219: 311-324. 10.1016/S0012-821X(04)00012-3.

  5. 5.

    Wagner GA: Fission track dating of apatites. Earth Planet Sci Lett. 1968, 4: 411-415. 10.1016/0012-821X(68)90072-1.

  6. 6.

    Eggins S, Grun R, Pike AWG, Shelley M, Taylor L: U-238, Th-232 profiling and U-series isotope analysis of fossil teeth by laser ablation-ICPMS. Quat Sci Rev. 2003, 22: 1373-1382. 10.1016/S0277-3791(03)00064-7.

  7. 7.

    Engvik AK, Golla-Schindler U, Berndt J, Austrheim H, Putnis A: Intragranular replacement of chlorapatite by hydroxy-fluor-apatite during metasomatism. Lithos. 2009, 112: 236-246.

  8. 8.

    Putnis A: Thermodynamics and Kinetics of Water-Rock Interaction. Edited by: Oelkers EH, Schott J. 2009, 70: 87-124. Mineral Replacement Reactions, Reviews in Mineralogy & Geochemistry

  9. 9.

    Rendon-Angeles JC, Yanagisawa K, Ishizawa N, Oishi S: Effect of metal ions of chlorapatites on the topotaxial replacement by hydroxyapatite under hydrothermal conditions. J Solid State Chem. 2000, 154: 569-578. 10.1006/jssc.2000.8888.

  10. 10.

    Yanagisawa K, Rendon-Angeles JC, Ishizawa N, Oishi S: Topotaxial replacement of chlorapatite by hydroxyapatite during hydrothermal ion exchange. Am Mineral. 1999, 84: 1861-1869.

  11. 11.

    Oelkers EH, Montel JM: Phosphates and nuclear waste storage. Elements. 2008, 4: 113-116. 10.2113/GSELEMENTS.4.2.113.

  12. 12.

    Weber WJ, Roberts FP: A review of radiation effects in solid nuclear waste forms. Nucl Technol. 1983, 60: 178-198.

  13. 13.

    de Kerdaniel ED, Clavier N, Dacheux N, Terra O, Podor R: Actinide solubility-controlling phases during the dissolution of phosphate ceramics. J Nucl Mater. 2007, 362: 451-458. 10.1016/j.jnucmat.2007.01.132.

  14. 14.

    Terra O, Dacheux N, Audubert F, Podor R: Immobilization of tetravalent actinides in phosphate ceramics. J Nucl Mater. 2006, 352: 224-232. 10.1016/j.jnucmat.2006.02.058.

  15. 15.

    Vance ER, Ball CJ, Begg BD, Carter ML, Day RA, Thorogood GJ: Pu, U, and Hf incorporation in Gd silicate apatite. J Am Ceram Soc. 2003, 86: 1223-1225. 10.1111/j.1151-2916.2003.tb03455.x.

  16. 16.

    Frei D, Harlov D, Dulski P, Ronsbo J: Apatite from Durango (Mexico) - A potential standard for in situ trace element analysis of phosphates. Geochim Cosmochim Acta. 2005, 69: A794-A794.

  17. 17.

    Baumer A, Argiolas R: Hydrothermal synthesis and characterization by X-ray of chlor, fluor or hydroxyapatite crystals. N Jahrb Mineralogie-Monatsh. 1981, 8: 344-348.

  18. 18.

    Garcia-Tunon E, Couceiro R, Franco J, Saiz E, Guitian F: Synthesis and characterisation of large chlorapatite single-crystals with controlled morphology and surface roughness. J Mater Sci Mater Med. 2012, 23: 2471-2482. 10.1007/s10856-012-4717-0.

  19. 19.

    Koutsopoulos S: Synthesis and characterization of hydroxyapatite crystals: A review study on the analytical methods. J Biomed Mater Res. 2002, 62: 600-612. 10.1002/jbm.10280.

  20. 20.

    Oishi S, Kamiya T: Flux growth of fluorapatite crystals. Nippon Kagaku Kaishi. 1994, 9: 800-804.

  21. 21.

    Oishi S, Sugiura I: Growth of chlorapatite crystals from a sodium chloride flux. Bull Chem Soc Jpn. 1997, 70: 2483-2487. 10.1246/bcsj.70.2483.

  22. 22.

    Perloff A, Posner AS: Preparation of pure hydroxyapatite crystals. Science. 1956, 124: 583-584.

  23. 23.

    Prener JS: Growth and crystallographic properties of calcium fluor- and chlorapatite crystals. J Electrochem Soc. 1967, 114: 77-83. 10.1149/1.2426512.

  24. 24.

    Fleet ME, Pan YM: Site preference of rare-earth elements in fluorapatite. Am Mineral. 1995, 80: 329-335.

  25. 25.

    Mitchell RH, Xiong J, Mariano AN, Fleet ME: Rare-earth-element-activated cathodoluminescence in apatite. Can Mineral. 1997, 35: 979-998.

  26. 26.

    Hughes JM, Rakovan J: Phosphates: Geochemical, Geobiological, and Materials Importance. Edited by: Kohn MJ, Rakovan J, Hughes JM. 2002, 48: 1-12. The crystal structure of apatite, Ca-5(PO4)(3)(F,OH,Cl), Reviews in Mineralogy & Geochemistry

  27. 27.

    Rodriguez-Carvajal J: Full Prof Suite 2005, Lab. de Léon Brillouin (CEA-CNRS). 2005, France: CEA/Saclay

  28. 28.

    García-Tuñón E, Dacuña B, Zaragoza G, Franco J, Guitián F: OH ion-exchanging process in chlorapatite Ca5(PO4)3Clx (OH)1-x – a deep insight. Acta Crystallogr Sect B Struct Crystallogr Cryst Chem. 2012, 68: 467-479. 10.1107/S0108768112019520.

  29. 29.

    Prowatke S, Klemme S: Trace element partitioning between apatite and silicate melts. Geochim Cosmochim Acta. 2006, 70: 4513-4527. 10.1016/j.gca.2006.06.162.

  30. 30.

    Ronsbo JG: Coupled substitutions involving REE and Na and Si in apatites in alkaline rocks from the Ilimaussaq intrusion, South-Greenland, and the petrological implications. Am Mineral. 1989, 74: 896-901.

  31. 31.

    Cherniak DJ: Rare earth element diffusion in apatite. Geochim Cosmochim Acta. 2000, 64: 3871-3885. 10.1016/S0016-7037(00)00467-1.

  32. 32.

    Cherniak DJ: Uranium and manganese diffusion in apatite. Chem Geol. 2005, 219: 297-308. 10.1016/j.chemgeo.2005.02.014.

  33. 33.

    Cherniak DJ, Lanford WA, Ryerson FJ: Lead diffusion in apatite and zircon using ion-implantation and rutherford backscattering techniques. Geochim Cosmochim Acta. 1991, 55: 1663-1673. 10.1016/0016-7037(91)90137-T.

  34. 34.

    Chazot G, Menzies MA, Harte B: Determination of partition coefficients between apatite, clinopyroxene, amphibole, and melt in natural spinel lherzolites from Yemen: Implications for wet melting of the lithospheric mantle. Geochim Cosmochim Acta. 1996, 60: 423-437. 10.1016/0016-7037(95)00412-2.

  35. 35.

    Fujimaki H: Partition-coefficients of Hf, Zr, and REE between zircon, apatite, and liquid. Contrib Mineral Petrol. 1986, 94: 42-45. 10.1007/BF00371224.

  36. 36.

    Klemme S, Dalpe C: Trace-element partitioning between apatite and carbonatite melt. Am Mineral. 2003, 88: 639-646.

  37. 37.

    Watson EB, Green TH: Apatite liquid partition-coefficients for the rare-earth elements and strontium. Earth Planet Sci Lett. 1981, 56: 405-421.

  38. 38.

    Heinrich CA, Pettke T, Halter WE, Aigner-Torres M, Audetat A, Gunther D, Hattendorf B, Bleiner D, Guillong M, Horn I: Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry. Geochim Cosmochim Acta. 2003, 67: 3473-3497. 10.1016/S0016-7037(03)00084-X.

  39. 39.

    Longerich HP, Jackson SE, Gunther D: Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. J Anal Atom Spectrom. 1996, 11: 899-904. 10.1039/ja9961100899.

  40. 40.

    Klemme S, Meyer HP: Trace element partitioning between baddeleyite and carbonatite melt at high pressures and high temperatures. Chem Geol. 2003, 199: 233-242. 10.1016/S0009-2541(03)00081-0.

  41. 41.

    Beyer C, Klemme S, Wiedenbeck M, Stracke A, Vollmer C: Fluorine in nominally fluorine-free mantle minerals: Experimental partitioning of F between olivine, orthopyroxene and silicate melts with implications for magmatic processes. Earth Planet Sci Lett. 2013, 337: 1-9.

  42. 42.

    Luo Y, Hughes JM, Rakovan J, Pan Y: Site preference of U and Th in Cl, F, and Sr apatites. Am Mineral. 2009, 94: 345-351. 10.2138/am.2009.3026.

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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.

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Correspondence to Stephan Klemme.

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Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

MW synthesized the samples, and together with JB and CK, performed the data analysis. SK drafted the manuscript; PSB carried out the XRD measurements and participated in the design of the experiments and helped to draft the manuscript. TJ, AR, and CK participated in the experimental design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.

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Keywords

  • Apatite
  • Fluorapatite
  • Apatite Crystal
  • High Field Strength Element
  • Rare Earth Element