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Rare-earth elements in chlorapatite [Ca (sub 10) (PO (sub 4) ) (sub 6) Cl (sub 2) ]; uptake, site preference, and degradation of monoclinic structure

Michael E. Fleet, Xiaoyang Liu and Yuanming Pan
Rare-earth elements in chlorapatite [Ca (sub 10) (PO (sub 4) ) (sub 6) Cl (sub 2) ]; uptake, site preference, and degradation of monoclinic structure
American Mineralogist (October 2000) 85 (10): 1437-1446

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 REE-substituted ClAp [La-ClAp, Nd-ClAp, Sm-ClAp, Dy-ClAp; Ca (sub 10-2y) Na (sub y) REE (sub y) (PO (sub 4) ) (sub 6) Cl (sub 2) , with y = 0.05-0.09; space group P2 (sub 1) /b] have been grown from SiO (sub 2) -bearing, H (sub 2) O- and Na-rich phosphate-chloride melts, and their hexagonal (P6 (sub 3) /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 Ca2O (sub 6) X 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 Ca2O (sub 6) X polyhedron in Nd-ClAp, Nd-Fap, and Nd-OHAp. The P2 (sub 1) /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 P2 (sub 1) /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 (super -) groups) during the P6 (sub 3) /m-->P2 (sub 1) /b transition.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 85
Serial Issue: 10
Title: Rare-earth elements in chlorapatite [Ca (sub 10) (PO (sub 4) ) (sub 6) Cl (sub 2) ]; uptake, site preference, and degradation of monoclinic structure
Affiliation: University of Western Ontario, Department of Earth Sciences, London, ON, Canada
Pages: 1437-1446
Published: 200010
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 33
Accession Number: 2001-002752
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: University of Saskatchewan, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200101
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