Silica substituted carbonate apatite: synthesis and analytical challenges

We use a high temperature experiment to demonstrate a coupled substitution mechanism for carbonate and silica in apatite, namely 2PO₄^3- → SiO₄^4- + CO₃^2-, with carbonate substituting for phosphate (type-B substitution). The carbonate anion group occupies a crystallographically distinct site as one of two side faces of a now vacant T site phosphate tetrahedron, and an oxygen site vacancy is formed. In our experiment, apatite is synthesised using a high-pressure carbonate flux method, resulting in large crystals amenable to a range of analytical techniques which are otherwise not feasible on the more commonly synthesised nanoscale material. The apatite is analysed with wavelength dispersive spectrometry (WDS) using an electron probe microanalyser (EPMA), secondary ion mass spectrometry (SIMS), Fourier-transform infrared spectroscopy (FTIR) using both transmission and attenuated total reflectance (ATR) techniques, and single crystal X-ray diffraction (SCXRD). There is no agreement on total carbonate contents between the analytical methods with EPMA-WDS and FTIR-ATR indicating ∼5 wt% CO₂, SIMS suggesting roughly 2.6 wt% CO₂, and SCXRD unable to conclusively support one or the other. Both estimates are sufficient to account for phosphate substitution by type-B carbonate and orthosilicate (SiO₄^4-), but the higher 5 wt% estimate raises the possibility of additional carbonate hosted in the X channel site as type-A carbonate. The bioactivity of this type of substitution relative to other vectors (such as Na-Si) is currently unknown and requires further research. As our apatite was synthesised under geologically reasonable conditions, it also raises the possibility that this substitution is present in CO₂-rich environments in the deep Earth, such as carbonic hydrothermal fluids and carbonatite magma systems, from the mantle to the crust.