An Apatite-Group Praseodymium Carbonate Fluoroxybritholite: Hydrothermal Synthesis, Crystal Structure, and Implications for Natural and Synthetic Britholites

Britholites are the lanthanide-silica-rich end-members of the apatite group, commonly studied for their optical properties. Here, we show ∼50-100 μm single crystals synthesized hydrothermally at 650-500 °C and 500-300 MPa composed of a solid solution between Ca₂Pr₃(SiO₄)₃F-fluorbritholite and CaPr₄(SiO₄)₃O-oxybritholite, with a significant carbonate component substitution, via C⁴⁺ replacing Si⁴⁺. Single-crystal X-ray diffraction and density functional theory computations show that a planar carbonate group occupies the face of a now-vacant silica tetrahedron. This modifies Pr-O bond lengths, diversifying lanthanide optical emission wavelengths. Our britholite was synthesized in geologically reasonable conditions and compositions, suggesting that carbonated oxybritholites could exist as yet-unrecognized natural minerals.