UNIQUE CRYSTAL CHEMICAL ASPECTS OF FLUORAPATITE FROM MONT SAINT-HILAIRE

The apatite crystal structure has been demonstrated to accommodate nearly half of the elements on the periodic table, including many of the harmful byproducts of nuclear fuel generation such as uranium (U) and thorium (Th). Therefore, apatite has been proposed as a solid nuclear waste form. This concept is largely predicated on the observation of naturally occurring apatite bearing elevated levels of U and Th. While numerous works suggest apatite can effectively uptake and retain U, Th, and other radionuclides over short timescales, it is still unclear if the apatite structure is stable while hosting high concentrations of these radionuclides over geologically relevant timescales. The current study has analyzed three suites of fluorapatites from the Mont Saint-Hilaire pluton, Quebec, Canada, via electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), and Fourier transform infrared spectroscopy (FTIR).

EPMA analyses revealed significant Th content, reaching multiple weight percent; the highest recorded Th contents yet found in natural fluorapatite by an order of magnitude. Further, the crystals are at least 120 Ma in age and show no structural deterioration from Th alpha decay. EPMA results also showed highly elevated Na and Y contents. SCXRD results indicate at least two of the three suites of crystals are desymmetrized from the classic P6₃/m apatite space group to the P-3 space group, likely because of Na and Y substituting for Ca in the metal sites, thus splitting the Ca1 site into two nonequivalent sites. FTIR spectra clearly show the presence of CO₃^2- in the fluorapatite structure, specifically substituting for the PO₄^3- anion at the tetrahedral site, and not for F^- along the anion column. From EPMA, SCXRD, and FTIR the estimated carbonate content ranges from 3 – 6 wt%. FTIR spectra also suggest the presence of the debated CO₃F^3- anion, with a small amount of absorbance seen at ~1200cm^-1; within the known range of absorbance for this anion. These fluorapatites contain many unique crystal chemical characteristics which bolster the idea of the structurally robust and chemically diverse nature of apatite.