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 crystal average 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). All non-hydrogen atoms were located and refined to an R1 = 0.0394 for all 252 observed reflections [F>4σ] and 0.0424 for all 324 unique reflections. The cyanotrichite structure 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 found for camerolaite. The ribbons lie in layers || (001) and between these layers, SO4 tetrahedra and H2O molecules form rods, also running || b. A hydrogen bonding scheme is also proposed.A sample of cyanotrichite from the Cap Garonne mine, Le Pradet, France, showed a 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). The supercell could not be refined in detail,but nevertheless imposes constraints on the local structure in that the space group symmetry prevents full order of SO4 and H2O in the 4 b 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].