ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPIC STUDY OF CARBONATE-BEARING FLUORAPATITE: NEW DEFECT CENTERS AND CONSTRAINTS ON THE INCORPORATION OF CARBONATE IONS IN APATITES

Abstract

X-band electron paramagnetic resonance (EPR) spectra of gamma-irradiated crystals of carbonate-bearing fluorapatite from the Levant mine, Cornwall, England, revealed the presence of two previously characterized centers (i.e., an O- defect and an O-□ F defect, where □F represents a vacancy at the F site), a CO2- radical, and a new oxygen-associated hole-like center in the anion column. The O-□F center in carbonate-bearing fluorapatite is stable at room temperature, whereas in carbonate-free fluorapatite the stability of this radical is shifted to lower temperatures (<225 k). the co2- radical herewith first reported in carbonate-bearing fluorapatite is characterized by an axial symmetry at room temperature but a weakly orthorhombic symmetry at 77 K, similar to its counterpart in carbonate-bearing hydroxylapatite. This CO2- radical most likely formed from Type A carbonate ions by the loss of an O atom and trapping of an electron during gamma irradiation. The single-crystal EPR spectra of the new hole-like center are characterized by the absence of any hyperfine interactions and a strongly orthorhombic symmetry. The spin Hamiltonian parameters of this new center suggest a structural model involving the trapping of a hole by a substitutional oxygen ion sandwiched between two fluorine ion vacancies in the anion column and strongly disturbed by vacancies at the neighboring Ca2 and O3 sites, suggesting a complex substitution of the type: □FO2- □F + □Ca2 + CO2-3 → F-F-F- + Ca2+ PO43-.