Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Mark Cooper, Frank C. Hawthorne, Petr Černý

Refinement of the crystal structure of cyrilovite from Cyrilov, Western Moravia, Czech Republic

Journal of the Czech Geological Society, volume 45 (2000), issue 1-2, 95 - 100


  Abstract

The crystal structure of cyrilovite from Cyrilov, western Moravia, Czech Republic, ideally Na [Fe3+3 (PO4)2 (OH)4 (H2O)2], tetragonal, a 7.3255(4), c 19.328(2) Å, V 1037.2(2) Å3, Z = 4, was refined to an R-index of 1.4 % for 1532 unique reflections measured with MoKα X-radiation on an automated four-circle diffractometer. Cyrilovite crystallizes in the enantiomorphic space group P41212 and refinement of a twin model shows that both enantiomers, P41212 and P43212, are present as a 1:1 mixture; thus cyrilovite is racemically twinned. The structure consists of intersecting corner-sharing chains of Fe3+Φ6 (Φ = unspecified anion) octahedra that form sheets parallel to (001). These sheets are linked into a framework of octahedra and tetrahedra by PO4 tetrahedra, with [8]-coordinated Na and H2O in the interstices of the framework. Consideration of local bond-valence requirements suggests that a small amount of Ca is incorporated into the cyrilovite structure via the substitution NaCa + M(1)Fe2+NaNa + M(1)Fe3+. Cyrilovite is isostructural with wardite, Na [Al3 (PO4)2 (OH)4 (H2O)2]. Cyrilovite has no cleavage, as suggested by the pattern of bond valence throughout the structure. However, wardite is listed in standard reference works as having perfect {001} cleavage. Examination of wardite shows it to have no cleavage, as suggested by its structural arrangement.

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ISSN: 1802-6222

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