The structure of cyanotrichite: a combined analysis of Automated Electron Diffraction Tomography and Synchrotron Powder X-ray Diffraction

The hydrated copper-aluminium sulphate cyanotrichite, ideally Cu4Al2(SO4)(OH)122H2O, often occurs in sky blue clumps or aggregates of sub-millimeter sized fibrous crystals. The problem of indistinguishable admixing of variable amounts of carbonate-cyanotrichite with cyanotrichite, the close association with other copper sulphates (chalcoalumite, brochantite) and the very small size of the acicular crystals hampered to date an ab initio structure determination from conventional X-ray diffraction. In light of these difficulties, we have taken advantage of the recent development of precessed automated electron diffraction tomography (ADT) combined with synchrotron powder X-ray diffraction to investigate the crystal structure of cyanotrichite. Through ADT investigation, two similar monoclinic cell were determined, corresponding to cyanotrichite (a = 10.16, b = 2.90, c = 12.64 Å and β = 92.4°) and carbonatecyanotrichite (a = 10.16, b = 2.91, c = 12.42Å and β = 98.4°). A structure model was obtained ab initio by direct methods in space group C2 from electron diffraction data and tested with the Rietveld method against X-ray powder diffraction profiles. All reflections in the powder pattern were indexed with the two cyanotrichite-like phases, according to electron diffraction data. The Rietveld analysis, consistently with electron diffraction investigations, indicates that the refined structural model has based on Al(OH)6 octahedra interconnected through common edges to build infinite columns running along b. Each Al-columns is coupled by sharing the remaining edges to two Cu-columns based on Cu distorted octahedra giving rise to ribbons along b. These ribbons are linked by SO4 tetrahedra to form corrugated layers.