STRUCTURAL ENVIRONMENT OF FLUORIDE IONS IN APATITES OF CHEMICAL COMPOSITION AND ORIGIN: 19F MAS NMR STUDY

Abstract

Apatites of markedly different chemical composition and origin were studied using 19F magic-angle spinning nuclear magnetic resonance. Synthetic carbonate fluorapatites (CFAp) containing 2.6 to 4.7 wt% F, 0 to 4 wt% CO2 and 0 to 1 wt% Na2О, hydroxylfluorapatite (OH:F ≈ 1:1) containing about 3 wt% Y (Y-HFAp), natural REE-apatites, and CFAp and Y-HFAp heated at temperature from 700 to 1000 oC were researched. The spectra of apatites with isomorphic substitutions show the signals (chemical shift δ) caused by fluoride ions in fluorapatite structure and, possibly, near defects in Ca sites (from −102.5 to −100 ppm), near water molecules (H2Os) incorporated in the channels (about −96.5 ppm), and one or two signals with δх from −91 to −86 ppm. The spectra of synthetic CFAp and Y-НFAр heated up to 900 oС and original natural REE-apatites show two components, δх1 and δх2 shifted with 2 − 3 ppm, in this range. It is shown that the component δx2 is new, it hasn’t been observed in the spectra previously. Signals in the δх range are caused by Fх ions (up to 12% F) whose structural environment is different substantially from the "ideal" fluorapatite structure. It is found that the contents of Fх ions and CO2 in synthetic CFAp correlate linearly. It is shown that Fx (Fх1) ions can occupied sites in the channels near single vacancies Ca, and Fх2 ions — near double vacancies, Ca and anionic those in the channels, in CFAp with the F content not higher than stoichiometric and Y-НFАр. These vacancies can form through different heterovalent isomorphism mechanisms such as РО43− → СО32− and/or Са2+ → M3+ (М = REE, Al, Fe), vacancies in the channels of heated apatites with partial substitutions F → H2Os, OH — through dehydration and/or dehydroxylation.