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 [1–5]. 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. ). 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 [7–10]. 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, 13–15] need suitable actinide-bearing apatite crystals as starting materials .
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.
Several studies report the synthesis of single crystal apatite, both fluorapatite, chlorapatite and hydroxyapatite [17–23]. 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 , especially when hydroxyapatite is involved.