Crystal chemistry of Th in fluorapatite

Abstract

Through the complementary use of single-crystal X-ray diffraction and X-ray absorption spectroscopy, we present in this paper the first direct results on the site occupancy of thorium in the fluorapatite structure and the structural distortion created by its substitution. Structure refinements based on single-crystal X-ray diffraction data from synthetic Th-doped fluorapatite indicates that Th substitutes almost exclusively in the M2 site. A single-crystal X-ray study of natural fluorapatite from Mineville, New York, also indicated that substituting heavy scatterers (including Th) are concentrated in the apatite M2 site, but definitive site assignments of specific elements were not possible. Extended X-ray absorption fine-structure spectroscopy (EXAFS) was used to probe the local structure of Th in the synthetic fluorapatite (single-crystal form) with a Th concentration of roughly 20 000 ppm, as well as Th in the natural Mineville fluorapatite (powder form) with a Th concentration of ~2000 ppm. The EXAFS fitting results also indicate that Th partitions into the M2 site and yield a ~0.05-0.08 Å decrease of average M2-O bond distances associated with local structure distortions that are not obtainable from single-crystal X-ray diffraction studies.

Crystal-chemical and structural characterization of fluorapatites in ejecta from Somma-Vesuvius volcanic complex

“…The unusual Th content can explain some structural vacancies previously reported, likely due to the following substitutions: (a) Th4 + + o = 2Ca 2+ , (b) U 4+ + o = 2Ca 2+ (Pan and Fleet 2002), (c) Th 4+ + 2Na + = 3Ca 2+ , and (d) Th 4+ +2Si 4+ = Ca 2+ + 2P 5+ (Luo et al 2011). In the Z site, P 5+ (5.484-5.885 apfu), S 6+ (0.125-0.181 apfu), Si 4+ (0.101-0.160 apfu), and V 5+ (0.030-0.050 apfu) coexist.…”

Britholite, monazite, REE carbonates, and calcite: Products of hydrothermal alteration of allanite and apatite in A-type granite from Stupné, Western Carpathians, Slovakia

“…While thorium concentrations range from 2.5 to 5.2 wt.% ThO 2 (0.07 to 0.15 apfu), uranium reaches only 0.4 to 1.0 wt.% UO 2 (0.01 to 0.03 apfu U), with maximum Th and U concentrations denoting fluorbritholite-(Y) composition. Several possible substitution mechanisms explain the entry of tetravalent Th and U cations into the apatite-britholite structure: (1) (Th,U) 4+ + Si 4+ = (Y,REE) 3+ + P 5+ (Pekov et al, 2011), (2) (Th,U) 4+ + 2Si 4+ = Ca 2+ + 2P 5+ (Luo et al, 2011;MacDonald et al, 2013) (Oberti et al, 2001). Positive correlation between (Th,U) and Si but negative relationship between (Th,U) and P indicate that mechanism (2) which, together with domi- 4E).…”

Numerical Modeling of REE Fractionation Patterns in Fluorapatite from the Olympic Dam Deposit (South Australia)

“…In instances where an elements valance state is different to that of the position it is substituting, such as in the case of REE 3+ substituting for Ca 2+ , overall charge balance is maintained through a variety of different coupled charge-compensated substitutions [35,36]. [38]). The different grey shadings of the PO4 tetrahedra show their orientation relative to projection planes.…”

“…In contrast, REE-trends in hydrothermal apatite could be more varied since the fluids capable of crystallizing apatite can carry variable concentrations of REE as a range of soluble complexes (e.g., [7]). These factors, combined with [37]); (b,c) Environment of Me1 and Me2 sites, respectively, (from Luo et al [38]). The different grey shadings of the PO 4 tetrahedra show their orientation relative to projection planes.…”