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

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) ų, 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.