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POSSIBLE STRUCTURAL AND CHEMICAL RELATIONSHIPS IN THE CYANOTRICHITE GROUP

Simon L. Hager, Peter Leverett and Peter A. Williams

School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia

E-mail address: p.williams{at}uws.edu.au

The members of the cyanotrichite group, cyanotrichite, carbonate-cyanotrichite, camerolaite and khaidarkanite, are closely related in terms of their structures, although the single-crystal structure of the latter is the only one to have been determined. Powder X-ray-diffraction data for all members can be indexed on the basis of a common monoclinic cell with a 10.2, b 2.9, c 12.6 Å and β in the range 92.3–97.9° (a and c are interchanged in khaidarkanite owing to C-centering). The structures of cyanotrichite, carbonate-cyanotrichite and camerolaite can be rationalized in terms of that of khaidarkanite, with sulfate and carbonate taking the positions of AlF₃ groups. Ordering and disordering of the sites yields P- and C-centered cells, respectively, depending on composition. The analytical data originally reported for carbonate-cyanotrichite are re-interpreted in terms of Al(OH)₃ taking the place of AlF₃ groups in khaidarkanite. There is no evidence for solid solution between cyanotrichite and carbonate-cyanotrichite; it is evident that some carbonate-cyanotrichite specimens contain no sulfate. A range of other stoichiometries may be accommodated by the archetypical structural motif, and these remain to be discovered in nature. Other related species, including a Co-rich member of the group from the Grandview mine, Arizona, and a cyanotrichite-like phase from the Clara mine, Germany, have also been examined. These too conform to the structural relations that have been derived. Final resolution of the nature of individual members of the group awaits single-crystal structural studies, if and when suitable crystals become available, or if structure solution using powder X-ray diffraction data or ab initio methods becomes possible.

Keywords: cyanotrichite, carbonate-cyanotrichite, camerolaite, khaidarkanite, relationships, structure, composition, powder X-ray-diffraction data.