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The Canadian Mineralogist; August 2001; v. 39; no. 4; p. 1065-1071; DOI: 10.2113/gscanmin.39.4.1065
© 2001 Mineralogical Association of Canada
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Articles

THE CRYSTAL STRUCTURE OF FENCOOPERITE: UNIQUE [Fe3+3O13] PINWHEELS CROSS-CONNECTED BY [Si8O22] ISLANDS

Joel D. Grice§

Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

§ E-mail address: jgrice{at}mus-nature.ca

The crystal structure of fencooperite, ideally Ba6Fe3+3Si8O23(CO3)2Cl3·H2O, has been solved in the trigonal space-group P3m1, with a 10.7409(5), c 7.0955(4) Å, V 708.9(1) Å3 and Z = 1. The final residual index for observed reflections is 0.038. The uniqueness of the fencooperite structure is a result of two previously unknown fundamental building blocks (FBB). The islands of silica tetrahedra form [Si8O22] units described as double open-branched triple tetrahedra sharing vertices along [001]. The other unique FBB consists of three (Fe3+O5) tetragonal pyramids having one of the O atoms common to all three polyhedra, forming a pinwheel trimer [Fe3O13]. The pronounced optical absorption is attributed to this coordination complex of iron. This trimer cross-links the [Si8O22] islands to form a framework structure. The framework supports intersecting tunnels in all three crystallographic directions that are filled by Ba and Cl atoms and by H2O and CO3 groups. The structure of fencooperite is compared to that of gillespite, pellyite, titantaramellite and orthoericssonite. In gillespite and pellyite, individual polyhedra of Fe2+ in four-fold coordination with oxygen cross-link sheets and chains of silica tetrahedra, whereas the chains of Fe3+ in (titan)taramellite having six-fold coordination with oxygen cross-links island rings of silica tetrahedra. In orthoericssonite, which has the same five-fold coordination of Fe3+ as fencooperite, the Fe3+ polyhedra cross-link [Si2O7] groups into sheets.

Keywords: fencooperite, unique crystal structure, hydrated barium–iron silicate–carbonate–chloride, ferric iron, five-fold coordination, pinwheel.




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