2:30 PM - 2:45 PM
[SMP42-02] OH defects in hydrothermally synthesized monazite (LaPO4) and xenotime (YPO4) single crystals
Keywords:Monazite, Xenotime, Hydrothermal syntheses, Nominally Anhydrous Minerals, Polarized infrared spectroscopy
The X-ray analyses for these minerals has been conducted by Ni et al. (1995). Monazite has monoclinic symmetry with space group P21/n, and preferentially incorporates LREE (light rare earth elements, here La~Gd). On the other hand, xenotime has tetragonal symmetry with space group I41/amd (isostructure with zircon), and preferentially incorporates HREE (heavy rare earth elements, here Tb~Lu + Y). Monazite and xenotime can contain trace amount of hydrogen as nominally anhydrous minerals (NAMs). However, only few studies have been reported on infrared spectroscopic measurements of monazite and xenotime. In this study, we conducted polarized infrared spectroscopic observation on hydrothermally synthesized monazite (LaPO4) and xenotime (YPO4) single crystals.
The single crystals of monazite and xenotime, synthesized with a piston-cylinder apparatus under a pressure of 1.0 GPa and cut perpendicular and parallel to the elongated direction, are mounted and doubly polished (i.e., monazite: E is nealy perpendicular to Y or Z, xenotime: E⊥c or a). The polarized infrared spectra of monazite and xenotime single crystals were measured in the transmission mode by Nicolet iN10 (Thermo Scientific Inc.).
Monazite crystals show two pleochroic absorption bands at 3164 cm-1 (FWHM = ca. 50 cm-1) and around 3330 cm-1 (FWHM = ca. 140 cm-1). On the other hand, xenotime crystals show a pleochroic absorption band at 3302 cm-1 (FWHM = ca. 10 cm-1). According to obtained absorption figures, the OH dipole in monazite is weakly oriented to , while in xenotime, it is strongly oriented perpendicular to c-axis. Based on these results, we suggest the OH incorporation models as shown in Figure 1. Two incorporation models must be considered on monazite due to the n glide symmetric element. The charge balancing mechanism in monazite is expressed as REE3+ U REE + 3H+. In contrast, the charge balance in xenotime is maintained by the reaction of REE3+ + O2- U REE + O + H+. When the Libowitzky (1999) correlation is applied to the obtained band positions (monazite: 3164 cm-1 and 3330 cm-1, xenotime: 3302 cm-1), the O-O distance of 2.69, 2.76 and 2.74 Å is estimated, respectively. Considering the estimated inter-atomic distance and our OH incorporation models, we could suggest the formation of vacant REE site cause the slight deformation of monazite and xenotime crystal lattices.