Publication Date:
2014-05-17
Description:
Lead-containing (Pb- B - X )-2 H apatites encompass a number of [ A F ] 4 [ A T ] 6 [( B O 4 ) 6 ] X 2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb 2+ on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb 2+ at both the A F and A T results in substantial adjustments to the Pb F O 6 metaprism twist angle, . New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2 H (ideally Pb 10 (VO 4 ) 6 Cl 2 ), pyromorphite-2 H (Pb 10 (PO 4 ) 6 Cl 2 ), mimetite-2 H / M (Pb 10 (As 5+ O 4 ) 6 Cl 2 ) and finnemanite-2 H (Pb 10 (As 3+ O 3 ) 6 Cl 2 ). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s 2 lone-pair electrons on the Pb 2+ that occupy a volume similar to an O 2– anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lone-pair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the Pb F O 6 metaprism twist-angles () as the temperature changes. Finally, crystal-chemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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