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Electron paramagnetic resonance spectroscopic study of carbonate-bearing fluorapatite: New defect centers and constraints on the incorporation of carbonate ions in apatites

¹ Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
² Illinois EPR Research Center, University of Illinois at Urbana-Champaign, Urbana, Illinois

Correspondence: ^* E-mail: yuanming.pan{at}sask.usask.ca

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: _FO^2–_F + _Ca2 + CO₃^2– F^–F^–F^–+ Ca²⁺ + PO₄^3–.

Key Words: EPR spectroscopy • fluorapatite • CO₂^– radical • new O^– center • carbonate ions

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