Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: Uptake, site preference, and degradation of monoclinic structure
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Michael E. Fleet
,
Xiaoyang Liu
Abstract
Differences in the inter- and intracrystalline partitioning behavior of rare earth elements (REE) between chlorapatite (ClAp), fluorapatite (FAp), and hydroxylapatite (OHAp) are directly or indirectly related to substitution mechanism and spatial accommodation. The substitution of REE for Ca is charge compensated by Na in ClAp, Na and Si in FAp, and Si in OHAp. Twinned crystals of REEsubstituted ClAp [La-ClAp, Nd-ClAp, Sm-ClAp, Dy-ClAp; Ca10-2yNayREEy(PO4)6Cl2, with y = 0.05- 0.09; space group P21/b] have been grown from SiO2-bearing, H2O- and Na-rich phosphate-chloride melts, and their hexagonal (P63/m) subcell structures refined at room temperature with single-crystal X-ray intensities to R = 0.020-0.023. The crystal/melt partition coefficients for La, Nd, Sm, and Dy are 0.073, 0.128, 0.122, and 0.101. Thus, uptake of REE is up to two orders of magnitude lower in ClAp than in REE-substituted FAp and OHAp crystallized under equivalent conditions, but remains peaked at Nd. REE site occupancy ratios [(REE-Ca2)/(REE-Ca1)] obtained from direct refinement of electron densities are 0.71, 1.11, 0.21, and 0.09 for La-, Nd-, Sm-, and Dy-ClAp, respectively, and are consistent with relative change in Ca-O bond distances and sizes of Ca polyhedra. Thus, La, Sm, and Dy favor the Ca1 position of ClAp, not Ca2, as in FAp and OHAp; this unusual site preference is attributed to the large increase in size (6-8%) and distortion of the Ca2O6X polyhedron on substitution of Cl for (F,OH). The slight preference of Nd for Ca2 and the peaking of REE uptake at Nd are attributed to a 4f crystal-field contribution, which is revealed by an anomalous decrease in the Ca2-O1 bond length and volume of the Ca2O6X polyhedron in Nd-ClAp, Nd-Fap, and Nd-OHAp. The P21/b structure of La-ClAp has been refined by detwinning the diffraction pattern, confirming that this monoclinic superstructure largely represents ordering of Cl atoms displaced along [001]. The P21/b structure of Dy-OHAp has been refined similarly. The intensity of superstructure reflections decreases abruptly with increasing substitution of REE for Ca, showing that REE cations interfere with ordering of Cl atoms(and OH- groups) during the P63/m → P21/b transition.
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- Nucleation and growth kinetics of analcime from precursor Na-clinoptilolite
- Viscosities of granitic (sensu lato) melts: Influence of the anorthite component
- Geochemistry of pyroxene inclusions from the Warrumbungle Volcano, New South Wales, Australia
- Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan
- Experimental study of intracrystalline Fe2+-Mg exchange in three augite crystals: Effect of composition on geothermometric calibration
- Transmission electron microscopy and differential thermal studies of lazurite polymorphs
- Re-equilibration of glass and CO2 inclusions in xenolith olivine: A TEM study
- Origin and structural character of haüyness in spinel dunite xenoliths from La Palma, Canary Islands
- Pyrrhotite varieties from the 9.1 km deep borehole of the KTB project
- Mineralogy and magnetic behavior of pyrrhotite from a 260 °C section at the KTB drilling site, Germany
- Surface defects and self-diffusion on pyrite {100}: An ultra-high vacuum scanning tunneling microscopy and theoretical modeling study
- Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: Uptake, site preference, and degradation of monoclinic structure
- Thermal pressure in MgO and MgSiO3 perovskite at lower mantle conditions
- Electron energy-loss spectroscopy of silicate perovskite-magnesiowüstite high-pressure assemblages
- Magnesium coordination environments in glasses and minerals: New insight from high-field magnesium-25 MAS NMR
- In situ powder diffraction study of titanite (CaTiOSiO4) at high pressure and high temperature
- Surinamite: A high-temperature metamorphic beryllosilicate from Lewisian sapphirine-bearing kyanite-orthopyroxene-quartz-potassium feldspar gneiss at South Harris, N.W. Scotland
- Single-crystal X-ray diffraction study of FeGeO3 high-P clinopyroxene (C2/c) up to 8.2 GPa
- High-pressure Ca4Al6O13: An example of a calcium aluminate with three different types of coordination polyhedra for aluminum
- Cation distribution in synthetic zinc ferrite (Zn0.97Fe2.02O4) from in situ high-temperature neutron powder diffraction
- Infrared study of OH sites in tourmaline from the elbaite-schorl series
- Second occurrence of okayamalite, Ca2SiB2O7: chemical and TEM characterization
- Rietveld refinement of okayamalite, Ca2SiB2O7: Structural evidence for the B/Si ordered distribution
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