Lookup NU author(s): Dr Stevin Pramana
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A gem-grade apatite from Brazil of general composition (Ca, Na) 10[(P, Si, S)O 4] 6(F, Cl, OH) 2 has been studied using single-crystal X-ray and neutron diffraction together with synchrotron powder X-ray diffraction. Earlier electron microscopy studies had shown the nominally single-phase apatite contains an abundant fluorapatite (F-Ap) host, together with chloro-hydroxylapatites (Cl/OH-Ap) guest phases that encapsulate hydroxylellestadite (OH-El) nanocrystals. While the latter features appear as small (200-400 nm) chemically distinct regions by transmission electron microscopy, and can be identified as separate phases by synchrotron powder X-ray diffraction, these could not be detected by singlecrystal X-ray and neutron analysis. The observations using neutron, X-ray and electron probes are however consistent and complementary. After refinement in the space group P6 3/m the tunnel anions F-are fixed at z = 1/4 along <001>, while the anions Cl - and OH - are disordered, with the suggestion that O-H···O-H··· hydrogen-bonded chains form in localized regions, such that no net poling results. The major cations are located in the 4f A FO 6 metaprism (Ca+Na), 6h A TO 6X tunnel site (Ca only), and 6h BO 4 tetrahedron (P+Si+S). The structural intricacy of this gem stone provides further evidence that apatite microstructures display a nano-phase separation that is generally unrecognized, with the implication that such complexity may impact upon the functionality of technological analogues.
Author(s): Baikie T, Schreyer MK, Wong CL, Pramana SS, Klooster WT, Ferraris C, McIntyre GJ, White TJ
Publication type: Article
Publication status: Published
Journal: American Mineralogist
Print publication date: 01/10/2012
Online publication date: 24/09/2012
ISSN (print): 0003-004X
ISSN (electronic): 1945-3027
Publisher: Mineralogical Society of America
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