The crystal structure of cyanotrichite

MILLS, STUART J.; CHRISTY, ANDREW. G.; COLOMBO, FERNANDO; PRICE, JASON

MINERALOGICAL MAGAZINE

MINERALOGICAL SOC

Lugar: Middlesex; Año: 2015 vol. 79 p. 321 - 335

0026-461X

We report the single crystalaverage structure of cyanotrichite, Cu4Al2[SO4](OH)12(H2O)2,from the Maid of Sunshine mine, Arizona, USA.Cyanotrichite crystallises in space group C2/m, with the unit cell parameters a = 12.625(3) Å, b = 2.8950(6) Å, c = 10.153(2) Å and β =92.17(3)o. All non-hydrogen atomswere located and refined to an R1= 0.0394 for all 252 observed reflections [Fo>4σFo] and 0.0424 for all 324unique reflections. The cyanotrichitestructure consists of a principal building unit of a three-wide [Cu2Al(OH)6]ribbon of edge-sharing Cu and Al polyhedra || b, similar to that foundfor camerolaite. The ribbons lie in layers || (001) and between these layers,SO4 tetrahedra and H2O molecules form rods, also running|| b. A hydrogen bondingscheme is also proposed. A sample of cyanotrichite from theCap Garonne mine, Le Pradet, France, showed a 4b superstructure with the following unit cell: space group P2/m,a = 12.611(2) Å, b = 11.584(16) = 4 × 2.896(4) Å, c = 10.190(1)Å and β = 92.29(6)o. The supercell could not be refined in detail,but nevertheless imposes constraints on the local structure in that whilethe space group symmetry prevents full order of SO4 and H2Oin the 4b supercell, it requires that the sequence of species along anygiven rod is [-SO4-SO4-(H2O)2-(H2O)2-]rather than [-SO4-(H2O)2-SO4-(H2O)2-].