Structure cristalline de la langite, Cu4[SO4(OH)6H2O].H2O

Abstract

The blue prisms of langite are biaxe (—) Ng = 1.715, Nn = 1.700 and Np = 1.650. The formula is Cu4[SO4(OH)6(H2O)].H2O. It crystallizes in the monoclinic system, space group Pc with the parameters : a = 7.118(5), b = 6.034(2), c = 11.209(7) Å and β = 90.02(3)°, ρexp = 3.28(2) g/cm3, ρx = 3.37 g/cm3, Z = 2. The mineral found belongs to Mazega, Aveyron, France. The structure has been refined to R = 0.028. The network is built with [Cu4O(OH)6H2O]n layers on which are grafted SO4 sulfate groups ; an extra water molecule is inserted and packs with the oxygens of the trigonal base of (SO4). All the copper atoms are octahedrally surrounded ; these CuO6 octahedra are rather distorted. The order of O--, OH- and H2O has been determined by crystal chemical considerations. The network of langite, [Cu4(SO4)(OH)6(H2O)], is strongly related with that of posnjakite.