Mineralogical Spectroscopy Of Apatites

Apatite is the most widespread calcium phosphate mineral. Extensive isomorphic substitutions may greatly affect the properties of this mineral. Therefore, study on apatite crystal chemistry and mineralogical spectroscopy has both scientific significance and application values. Chemistry, structure and spectroscopy of natural apatites from igneous, sedimentary, metamorphic environments and synthetic apatites obtained by hydrothermal, sol-gel, solid reaction and Czochralski methods, were investigated. Electron Microprobe Analysis (EMPA), Scanning Electron Microscopy (SEM), X-Ray Powder Diffraction (XRPD), X-ray Single Crystal Diffraction (XRSCD), FTIR, Raman and Single Crystal Polarized Raman Spectroscopy(SCPRS), and ³¹P,¹H,¹³C solid magic angle spinning nuclear magnetic resonance (NMR) were used. Particularly, XRSCD with diamond-anvil cell was applied to investigate the structural compressibility and vibrational behaviors of F-apatite up to 7 GPa; The thermal expansion behaviors of F-apatite, OH-apatite and C-F-apatite were investigated with XRPD up to 950℃. The main ions that can substitute Ca²⁺ are Sr²⁺and REE³⁺ and main anion that can substitute PO₄^3- are [SiO₄]^4-, [SO₄]^2- and [CO₃]^2-. The isomorphic substitutions depend on genesis, which can be distinguished by a Tetrahedral Substitution Index (TSI=100×[Si+S+C]/P, atom/cell). Apatite from extrusive alkaline rock yielded TSI generally below 1, while apatite from intrusive carbonatite yielded TSI = 3-4. TSI ranges widely for apatite from intrusive alkaline rocks (3-14) and apatite from extrusive carbonatite (10-50). The tetrahedral distortion can be described by Quadratic Elongation (QE) and Tetrahedral Angle Variance (TAV). The substitution of the ions [SiO₄]^4-, [SO₄]^2- and [CO₃]^2- for PO₄^3- resulted in a marked tetrahedral distortion, an increase in TAV and QE. Meanwhile, the Raman peaks broadened and shifted to the lower-frequency region with substitutions for phosphate. According to group theoretical analysis, there are 9 IR active and 15 Raman active vibration modes for the phosphate tetrahedral group, and 3 IR-active and 4 Raman-active modes for the hydroxyl group in apatite. All the theoretically predicted optical-active modes of phosphate were detected in our IR and Raman spectra. Some clear lattice Raman-modes were also observed and assigned, using SCPRS. In natural apatite, structural carbonate is generally in B-position, but in apatite from an extrusive carbonatite, a A-B mixing carbonate substitution was found. Under pressure, the frequencies of all internal and external Raman modes shifted linearly to the higher-frequency region as pressure increased. The Gruneisen parameters (γi) range from 0.232 to 0.800 for tetrahedral modes, and from 0.99 to 2.59 for lattice modes. Chemical shift of 31P NMR peak occurs at δ=1.9±0.1ppm, and an asymmetrical resonance at δ=170.5ppm was found for the 13C NMR spectrum of natural carbonate-F-apatite. In 1H NMR spectra, a resonance at δ=5.0ppm by absorbed H2O can be distinguished from that in δ=-0.04—+1.61ppm by structural hydroxyl, which weakens and extinguishes as F content increases. In Fluor-hydroxylapatite, a new resonance at δ=0.90-1.61ppm occurs and its chemical shift and intensity increase with F content, implying the influence of F substitution on the hydrogen bonds in structural channel. Under 7 Gpa pressure, there was no phase change for F-apatite. Lattice parameters of varied linearly as pressure increased, with βa=3.26×10-3Gpa-1, βc=2.38×10-3Gpa-1 and average volume compressibility=8.77×10-3Gpa-1. Structural channel was much more compressible than tetrahedron. Below 950oC, lattice parameters a, c, V increased with increasing temperature and expansion along the a axis was more significant than along the c axis.