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The crystal structure of the new mineral dyrnaesite-(La),Na8CeIVREE2(PO4)6

Published online by Cambridge University Press:  02 January 2018

Tonči Balić-Žunić
Tonči Balić-Žunić*
Affiliation:
Natural History Museum, University of Copenhagen
*
*E-mail: [email protected]
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Abstract

Dyrnaesite-(La), Na7.89(Ce0.94Ca0.06)∑1.00(La1.14Ce0.40Pr0.10Nd0.24Ca0.12)∑2.00(PO4)6 is orthorhombic, Pnma, a = 18.4662(7), b= 16.0106(5), c = 7.0274(2) Å, V = 2077.7(1) Å3, Z = 4. The crystal structure is related to the group of Na3REE(XO4)2 compounds (with X = P, V, As), based on the aphthitalite/glaserite structural type. Dyrnaesite is distinct in having ordered Na vacancies, and a rare-earth element (REE) site occupied preferentially by Ce4+. This also distinguishes it from closely related vitusite-(Ce) [Na3REE(PO4)2]. The relation of their unit cells is: ad= bv, bd = 3av, cd = 1/2 cv. The distinct Ce4+ site in dyrnaesite-(La) has smaller coordination with shorter bond lengths than the other REE site in the same structure or the REE sites in vitusite-(Ce). It is adjacent to the predominately vacant Na site, which in its turn has the largest coordination of all Na sites in the structure. REE sites, or Na sites in a [010] row (similar to [100] in vitusite(Ce)) assume two types of coordinations with and without mirror symmetry and two different configurations of surrounding PO4 tetrahedra. This summarizes the topological difference to vitusite-(Ce) where the corresponding coordinations are similar in the same row and intermediate in character to the two types in dyrnaesite-(La).

Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 1 , February 2017 , pp. 199 - 208
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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